Mechanical, Thermal, and Environmental Energy Harvesting Solutions in Fully Electric and Hybrid Vehicles: Innovative Approaches and Commercial Systems

Abstract

Energy harvesting in the automotive sector is a rapidly growing field aimed at improving vehicle efficiency and sustainability by recovering wasted energy. Various technologies have been developed to convert mechanical, thermal, and environmental energy into electrical power, reducing dependency on traditional energy sources. This manuscript provides a comprehensive review of energy harvesting applications/methodologies, aiming to trace the research lines and future developments. This work identifies the main categories of harvesting solutions, namely mechanical, thermal, and hybrid/environmental solar–wind systems; each section includes a detailed review of the technical and scientific state of the art and a comparative analysis with detailed tables, allowing the state of the art to be mapped for identification of the strengths of each solution, as well as the challenges and future developments needed to enhance the technological level. These improvements focus on energy conversion efficiency, material innovation, vehicle integration, energy savings, and environmental sustainability. The mechanical harvesting section focuses on energy recovery from vehicle vibrations, with emphasis on regenerative suspensions and piezoelectric-based solutions. Specifically, solutions applied to suspensions with electric generators can achieve power outputs of around 1 kW, while piezoelectric-based suspension systems can generate up to tens of watts. The thermal harvesting section, instead, explores methods for converting waste heat from an internal combustion engine (ICE) into electrical power, including thermoelectric generators (TEGs) and organic Rankine cycle systems (ORC). Notably, ICEs with TEGs can recover above 1 kW of power, while ICE-based ORC systems can generate tens of watts. On the other hand, TEGs integrated into braking systems can harvest a few watts of power. Then, hybrid solutions are discussed, focusing on integrated mechanical and thermal energy recovery systems, as well as solar and wind energy harvesting. Hybrid solutions can achieve power outputs above 1 kW, with the main contribution from TEGs (≈1 kW), compared to piezoelectric systems (hundreds of W). Lastly, a section on commercial solutions highlights how current scientific research meets the automotive sector’s needs, providing significant insights for future development. For these reasons, the research results aim to be guidelines for a better understanding of where future studies should focus to improve the technological level and efficiency of energy harvesting solutions in the automotive sector.

1. Introduction

The transport sector is one of the largest sources of greenhouse gas emissions in the European Union, showing little progress in reducing emissions in recent decades [1]. In Italy, in 2022, the transport sector contributed 26.6% of the national total, of which road transport contributed 91.5% of total transport emissions, of which 63.8% are from cars. With the world’s growing population, rising wages, and the fact that more people can afford to travel via cars, trains, and planes, the demand for transport is expected to expand globally in the coming decades [2]. The electrification of automotive transport is certainly a solution for reducing air pollution, and the interest in replacing the internal combustion engine (ICE) of vehicles has encouraged the rapid evolution of electric motors’ performance [3]. These considerations have promoted the spread of electric and hybrid vehicles, which do not depend on fossil fuels or only depend on them in part. Electric vehicles (EVs) represent the foreseeable future of the automotive industry thanks to their greater energy efficiency and zero exhaust emissions [4,5]. This consideration opens up new challenges related to the EVs’ autonomy [6,7], an aspect that hybrid vehicles overcome thanks to the dual propulsion mode, comprising an electric motor and an ICE, increasing the battery’s efficiency [8]. Energy efficiency and sustainability have become key aspects in developing next-generation vehicles, pushing the automotive industry to explore innovative solutions for energy recovery. Energy harvesting in the automotive sector aims to convert energy lost during vehicle operation through vibrations, friction, heat, and movements to power the battery and auxiliary systems and improve the vehicle’s overall efficiency, as shown in Figure 1 [9]. The scientific literature offers numerous harvesting solutions with excellent performance and capable of recovering energy. Mechanical energy recovery is the main methodology adopted in the automotive sector; there are many ways to recover it based on physical principles that allow the kinetic energy dissipated during vehicle operation to be transformed into electrical energy. Among the most commonly used systems, energy recovery during braking, known as regenerative braking, converts energy generated by deceleration into electricity, recharging the vehicle’s batteries. Regenerative braking is the first energy recovery methodology that manufacturers have adopted [10]. Energy recovery from the suspensions’ vibrations due to the asphalt’s irregularity promises excellent results, as demonstrated by numerous publications. During the vehicle’s journey, the suspensions attenuate the road’s irregularity by cushioning the impact and dissipating kinetic energy; energy recovery from the suspension focuses on exploiting the movement of the spring–damper system through electromechanical systems [11,12]. In some cases, regenerative suspensions adopt systems for converting linear motion to rotary motion to operate an electric generator [13]. Other cases preserve the linear movement of the suspension as they exploit the electromagnetic interaction to produce electrical energy [14]. In addition, some advanced technologies use piezoelectric materials that convert vibrations and mechanical deformations into electrical energy through the direct piezoelectric effect. Such systems have a wide range of uses; in fact, they can be used in regenerative suspensions [15], on the exhaust pipe [16,17], on the engine block [18], and at other points where vibration sources are present. Piezoelectric elements have a high power density, and this feature has enabled the creation of intelligent tires [19,20]. These tires are characterized by piezoelectric elements that exploit the deformation of the tread to recover electrical energy and power small IoT sensors.

Thermal energy harvesting is mainly used in ICE-based or hybrid vehicles by exploiting the wasted heat from the combustion engine (about 70% of total energy) and converting it into electrical energy via thermoelectric generators (TEGs). More than half of this thermal energy loss is caused by waste heat from exhaust gases alone [21]. A TEG consisting of n- and p-type modules enclosed in a heat exchanger assembly is a bidirectional energy converter that converts heat into electrical energy using the Seebeck principle [22].

Besides the development of innovative energy harvesting technologies, the strategies to synergistically recover energy play a crucial role; intelligent control algorithms can dynamically manage the distribution of recovered energy across multiple sources to maximize system performance. In this context, advanced optimization techniques, such as the two-stage accelerated asynchronous decentralized alternating direction method of the multipliers (TSA-AD-ADMM) algorithm proposed in [23], can optimize energy distribution in electricity and heat systems while reducing operational risks. Integrating energy management approaches can enhance the overall efficiency of energy recovery systems, ensuring a more reliable and adaptive energy flow across different energy sources.

The harvesting solutions adopted for energy harvesting in the automotive sector can be classified according to the type of produced power; in fact, there are harvesting devices generating direct current (DC) and others producing alternating current (AC), with different management methods for the produced power. Harvesters that generate DC power are characterized by simpler conditioning to make power suitable to be stored in a storage device or power auxiliary devices. Solar and thermal harvesters such as TEGs belong to this class. Instead, due to their transduction mechanism, harvesters that generate AC power require a rectification section (AC/DC conversion), followed by DC/DC regulators to make generated power suitable for recharging batteries or powering auxiliary devices. For example, piezoelectric, triboelectric, and electromagnetic harvesters belong to this class. In both cases, impedance matching is fundamental to ensure maximum power transfer from the harvester to the conditioning section.

Another important aspect of refining energy harvesting technologies is a better understanding of the impact of operating condition uncertainty on energy recovery efficiency; these conditions can include climate factors (such as wind and sunlight), the state of road maintenance, vehicle speed, and load conditions. In this regard, an effective approach is using probabilistic modeling tools to quantify efficiency variations in regenerative braking systems due to varying operational conditions [24].

Starting from the described scenario, an overview of energy harvesting solutions is provided with this document by dividing harvesting systems into three macro categories:

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Mechanical harvesting: various harvesting methodologies that exploit vehicle vibrations to recover electrical energy are presented, placing particular attention on regenerative suspensions and piezoelectric harvesting and presenting innovative solutions with excellent performance (Section 2).

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Thermal harvesting: some harvesting solutions have been reported that can convert heat waste from ICEs into electrical energy to power the battery or small devices. Energy recovery solutions that exploit the organic Rankine cycle by recovering the heat produced by the ICE are also presented (Section 3).

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Hybrid and environmental harvesting (solar–wind): an overview of solutions capable of simultaneously exploiting a vehicle’s mechanical and thermal energy source is provided. In addition, some harvesting solutions that exploit solar and wind energy are comprehensively described (Section 4).

Each section contains a comparative analysis of the evaluated papers and summary tables emphasizing their salient features to improve the publication’s scientific contribution. Furthermore, Section 5 offers cutting-edge commercial solutions created by multinational corporations for mechanical, thermal, and solar energy harvesting.

The structure of this review is as follows. Section 1 introduces the energy harvesting paradigm while illustrating the methodology for selecting the reviewed manuscripts. Section 2 provides an overview of mechanical energy harvesting solutions addressing energy harvesting via electromechanical and piezoelectric systems. Section 3 reviews thermal harvesting solutions by identifying architectures that employ TEG solutions, organic Rankine cycle systems, and systems capable of recovering waste heat from the braking system. Section 4 explores hybrid mechanical–thermal energy harvesting solutions and innovative onboard vehicle solutions that recover energy from solar and wind power. Section 5 reviews innovative commercial applications related to the discussed topics. Finally, Section 6 presents comments and conclusions, highlighting potential future applications.

Selection Method of the Analyzed Articles Based on PRISMA Methodologies

This section describes the selection criteria for scientific articles, emphasizing important factors such as publication year, redundancy with other studies, and relevance to the topics covered. This review aims to give readers a thorough, up-to-date understanding of energy harvesting applications. The PRISMA methodology is used in the paper selection process, guaranteeing the validity and relevance of the selected strategy [25,26]. The first step in the selection process is analyzing each prospective article’s title to find relevant keywords. The abstract is then examined to determine how well it fits the topics covered in this study. The article is carefully read and examined if it seems pertinent. For articles with uncertainty issues, ambiguities are resolved by consulting additional sources. The article will be deleted if further research reveals that the information is unclear.

The selection procedure, which includes evaluating the title’s relevance, the abstract’s affinity, and the scientific worth of the manuscript, is depicted in Figure 2a. The primary terms used to filter the literature are shown in Figure 2b. For all the topics covered, the same methodology has been used. The authors examined 114 documents, including research and review articles, conference papers, and websites, to accurately analyze the subjects discussed in this manuscript (Figure 3).

2. Electromechanical Solutions for Mechanical Energy Harvesting to Power Auxiliary Devices and Batteries

Mechanical harvesting is based on recovering the kinetic energy dissipated by the vehicle while driving; for instance, the earliest and still most widely used energy recovery technology harnesses a vehicle’s kinetic energy during braking by operating the electric motor in generator mode, thereby converting mechanical inertia into electrical energy while decelerating the vehicle. The energy dissipated by the suspension is an excellent source of energy for recovery; in fact, the suspension dissipates energy by absorbing shocks due to the roads’ irregularity. In this case, the recovery principle uses the movement of the spring–damper system to activate a mechanism capable of generating electricity. Another available energy source is the vibrations that propagate through the vehicle’s structure; this method exploits the nature of piezoelectric materials, which generate electricity that can power auxiliary devices or recharge batteries when stressed.

Electromechanical and piezoelectric energy harvesting represents an innovative frontier in the automotive sector, thanks to its ability to convert mechanical energy into electrical energy. This technology is based on physical principles that exploit the vibrations and deformations generated by the movement of vehicles, allowing them to power onboard electronic devices without the need for traditional batteries. The integration of energy harvesting systems can help improve the energy efficiency of vehicles, reduce weight, and increase sustainability, making it possible to power sensors and monitoring devices in real time. Furthermore, adopting these solutions aligns with current trends toward increasingly intelligent and connected vehicles, opening up new opportunities for technological innovation in the automotive sector.

2.1. Electromechanical Energy Harvesting Applications

The conversion of vibrations generated by vehicle suspension has produced countless applications that exploit different technologies to convert mechanical energy into electrical energy, thus introducing the regenerative shock absorber (RSA) concept [27]. Principal models adopt electric motors as generators by requiring the design of systems that can convert the linear motion of suspensions into rotary motion through the coupling of racks and gear wheels. Models that exploit the principle of electromagnetism require interaction between magnets and coils, and these systems are well-suited to the linear motion of the suspension, thus producing a leaner system capable of being integrated within the suspension. Other models use an evolving fluid that, moved by a hydraulic actuator driven by the movement of the suspension, expands into a hydraulic motor directly connected to an electric generator, producing electrical power. These systems can be combined with models that take advantage of the principle of electromagnetism through a hybrid system, using a hydraulic actuator whose plunger consists of permanent magnets flowing inside coils placed on the outer jacket of the hydraulic actuator.

Building upon these advancements, Abdelkareem et al. propose a robust performance analysis framework for regenerative shock absorbers, leveraging probabilistic modeling and Monte Carlo simulations to evaluate energy harvesting efficiency under stochastic operating conditions [28]. It considers how suspension parameters, such as the damping coefficient and tire stiffness, impact energy recovery efficiency. A two-DOF quarter-car model is employed, with excitation frequencies ranging from 0 to 30 Hz. The results indicate that the damping rate and tire stiffness significantly influence energy harvesting, while sprung mass and spring stiffness have a lesser effect.

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Energy harvesting using electric generators

The continuous demand for solutions capable of recovering energy to partially regenerate the battery of an electric and hybrid vehicle has pushed research to redesign vehicle shock absorbers to exploit the system’s oscillations related to the irregularity of the asphalt. Salam et al. proposed an innovative RSA designed for EVs with in-wheel motors capable of capturing the energy of the vibrations generated by the rough road surface while driving [13]. The system combines modules for motion capture, vibration transmission in unidirectional rotary motion, and conversion into electrical energy via generators, gears, and mechanical clutches, as shown in Figure 4. An auxiliary generator optimizes energy generation and damping control, while the inclusion of a supercapacitor and voltage regulator ensures stable charging. Experimental tests conducted on an Instron electromechanical test bench confirmed the validity of the design, with power generation exceeding 380 W and an efficiency of 62%. This system proves to be a promising solution for improving the range of EVs by exploiting renewable energy.

Abdelrahman et al. developed an energy recovery shock absorber (ERSA) for electric buses to harvest kinetic energy from vibrations caused by rough roads and improve the vehicle’s energy efficiency [29]. The solution, consisting of four modules (energy input, motion converter, generator, and storage) is based on a splined link conversion mechanism, which transforms the linear motion of the suspension into unidirectional rotations to drive an electric generator made by a brushless DC motor. After developing the theoretical model and MATLAB 2022a simulations, the full-scale prototype was tested on a mechanical test bench, achieving an average power of 6.591 W and a maximum efficiency of 67.7% with sinusoidal excitations of 7.5 mm and 2.5 Hz. Some energy recovery systems use energy collectors consisting of electromagnetic shock absorbers, replacing traditional ones.

Techalimsakul et al. developed a suspension-based harvesting vibration absorber (HVA) for two-ton EVs by combining a supercapacitor–lithium battery hybrid energy storage paradigm (SCB-HESP) [30]. This architecture is integrated with a regenerative braking system (SCB-HESP-RBS) controlled by an artificial neural network, which optimizes the switching waveform of the three-phase inverter to improve the transfer of braking energy to the storage device. The conversion mechanism is a system that transforms the bidirectional linear motion into a unidirectional rotation of the generator shaft. The HVA proposed by the authors consists of two cylinders that slide into each other and two racks that engage a pinion. The axle on which the pinions are mounted is equipped with a freewheel clutch to transmit the motion in only one direction. The results show that the combined use of the SCB-HESP-RBS and an HVA increases the maximum energy harvesting efficiency by up to 50.58%, which is much higher than using the HVA or SCB-HESP-RBS alone, with an obtained electrical power of 8.253 W for the vibration frequency and amplitudes of 4 Hz and 10.5 mm, respectively. In addition, the MEV with the full system achieved a range of 247.34 km (22.5 driving cycles), which is higher than 214.40 km (19.5 cycles) with the SCB-HESP-RBS and 164.25 km (15 cycles) with HVA alone, demonstrating the improvement in performance and energy efficiency. With a similar system, Li et al. in [31] designed an RSA capable of converting the bidirectional linear motion of the suspension into unidirectional rotary motion to drive an electric generator. The collected energy is then stored in a supercapacitor. Bench tests demonstrated that, with a sinusoidal displacement of 7 mm and a frequency of 2.5 Hz, the system generates an average power of 4.25 W, with maximum and average efficiencies of 65.02% and 39.46%, respectively. Wang et al. presented a novel RSA for EVs [32] based on a system divided into four modules: vibration input, transmission, generator, and energy storage. Irregular road-induced suspension vibrations are captured by the input module and transformed into unidirectional rotary motion by a pair of ball screws and gear systems in the transmission module. This configuration optimizes vehicle comfort and damper efficiency by using different screw pitches to tailor damping coefficients and improve vibration absorption. Bench test results showed that the prototype achieved an average output power of 3.701 W with a peak efficiency of 51.1% and an average efficiency of 36.4%. Furthermore, the regenerative damper can extend the range of EVs by approximately 1 mile for every 100 miles driven on Class B roads at 60 km/h.

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Energy harvesting using electromagnetic systems

The oscillations produced by the suspensions can be used by models that use the electromagnetic principle to recover electrical energy. By deriving the relative movement between coil and permanent magnets, it is possible to recover the energy needed to power small devices or recharge the battery. In this regard, Zhou et al. developed a magnetic energy-harvesting suspension (MEHS) designed to power a wireless sensor onboard the system [14]. The MEHS, illustrated in Figure 5a,b, consists of a stator on which the permanent magnets are mounted and a slider on which the coils are present. The key parameters affecting energy harvesting, including excitation frequency, amplitude, and external load resistance, were identified through numerical analysis. Experimental tests showed that the maximum output power is achieved when the excitation frequency matches the system’s natural frequency, reaching a peak of 0.34 W at 3.3 Hz.

Hajidavalloo et al. developed a novel ball screw-based energy-harvesting shock absorber (EHSA), which integrates an inert pendulum vibration absorber (IPVA) with an electromagnetic rotating EHSA [33]. This design exploits the nonlinear effects of the pendulum to simultaneously improve ride comfort and harvesting efficiency, overcoming the limitations of traditional EHSAs that only respond to narrow-band vibrations. To further optimize performance, a new stochastic linearization-based predictive control (SL-MPC) approach was developed, which improves modeling accuracy compared to standard linearization. This method ensures system stability and reduces the computational cost of predictive control without compromising performance. Simulations demonstrated the superiority of the new nonlinear EHSA compared to traditional systems, highlighting the effectiveness of SL-MPC in improving vibration control and energy efficiency, showing that the harvested power can potentially be increased by 60%.

Jiang et al. studied a dual-mode magnetic suspension with high safety and a compact structure to analyze its energy-harvesting performance [34]. The work examined how the actuator’s structural parameters affect energy regeneration, using the finite element method to evaluate factors such as the magnetic ring, cap thickness, heat dissipation, and air gap. The electromagnetic actuator consists of a fixed part on which the permanent magnets are positioned and a moving part consisting of the winding (Figure 6).

In [35], Hu et al. developed a hybrid generator that combines a sliding-mode triboelectric nanogenerator (S-TENG) [36] and an electromagnetic generator (EMG) to harvest energy from suspension vibrations without replacing the original shock absorber or compromising suspension performance. This technology solution uses a stator and slider produced by 3D prototyping and includes patterned polyimide (PI) films to improve the durability of the S-TENG, reducing wear by up to 63% compared to smooth surfaces and optimizing the patterning parameters to maximize energy generation. The hybrid generator was demonstrated to charge a 4.7 μF capacitor to 18.05 V in 60 s, with the contribution of the S-TENG component increasing over time. Additionally, integration with the original shock absorber occurred without altering the acceleration of the sprung mass, allowing for effective energy harvesting even on roads with random vibrations.

Song et al. [37] focused their studies on the development of an electromagnetic-pneumatic regenerative shock absorber (EP-RSA) designed for lunar and Mars rovers to harvest energy from vibrations generated during their movement on rough terrain. The EP-RSA’s structure comprises three modules: one for vibration input, one for transmission, and one for the generator. An innovative aspect is the inclusion of a pneumatic damper in the transmission module, which increases the damping force and improves the rover’s ride comfort. Mathematical analyses and simulations showed that the damper provides superior performance, with the damping force increased by nearly 50 times by implementing the pneumatic system. Experimental tests confirmed that the system can generate an average power of 1.26 W, which is consistent with simulation predictions. The main advantages of the proposed technological solution include significant energy savings and improved rover driving dynamics, making the EP-RSA a promising solution for future space exploration missions.

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Energy harvesting through hydraulic systems

Other systems instead exploit the kinetic energy produced by the vehicle’s suspension to set in motion a flow that evolves in a hydraulic circuit. Zou et al. developed an innovative hydraulic interconnected integrated regenerative suspension (HIIRS) system, which can harvest energy from suspension vibrations and improve vehicle handling by decoupling the four vehicle vibration modes (bounce, roll, pitch, and deformation) [38].

The authors proposed a system for improving vehicle dynamics and recovering energy by combining the advantages of an EHSA and linked hydraulic suspension. The main parts include a hydraulic motor generator unit, four hydraulic cylinders, four hydraulic rectifiers, and a high- and low-pressure accumulator. Experimental results showed that the HIIRS significantly improved vehicle stability: the roll angle was reduced by 34.42% in a lane change maneuver and the pitch angle by 28.91% in straight-line braking, with an initial pressure of 40 bar and a load resistance of 10 Ohm. Furthermore, the system generated 87.69 W and 147.86 W of regenerative power in the two scenarios, respectively, highlighting the potential of the HIIRS to improve vehicle safety and energy efficiency. With the same approach, Zhang et al. in [39] developed a novel mechanical–electric–hydraulic regenerative suspension system (MEH-RSS) designed for high-speed tracked vehicles capable of kilowatt-level energy recovery under off-road conditions. This technology relies on a hydraulic motor generator (HMG) to efficiently convert hydraulic energy into electricity, even in limited spaces. The system proposed by the authors is shown in Figure 7. It consists of a vane damper, a hydraulic switch, and an HMG. When the tracked vehicle drives on rough roads, a connecting rod transmits the vibration to the hydraulic damper, allowing the oil inside to circulate, which expands into a unidirectional hydraulic motor that turns an electric generator. The operating environment of high-speed tracked vehicles is complex and variable. A typical stochastic road solicitation is employed to examine the energy recovery characteristics of the MEH-RSS under various road conditions and driving speeds. In developing the random road excitation model, the power spectral density (PSD) characterizes the statistical properties of road surface roughness, while Gaussian white noise synthesis generates the corresponding time-domain excitation signal. The power generated is converted from unstable alternating current to stable direct current to charge the battery pack. Tests have shown that the energy recovery module is highly efficient, achieving hydraulic–electric conversion with a maximum efficiency of 40.4%, an external resistance of 5 Ω, and an electrical power of 1104 W.

Zhang et al. proposed an inflatable hydraulic–electric regenerative suspension (IHERS) system to improve ride comfort and energy harvesting in heavy-duty vehicles (HDVs) [40]. The objective is to mitigate vibrations and recover dissipated energy from the suspension system. A mathematical model of the IHERS is developed and integrated into a half-car model with five degrees of freedom for performance analysis. Simulations show that the IHERS provides tunable damping forces, enhancing ride comfort by reducing vertical acceleration by 23.3% and achieving an energy recovery efficiency of 41.9% on rough roads. The combined oil–gas design ensures stable energy conversion while protecting the hydraulic motor and generator. Comparative analysis confirms that the IHERS outperforms traditional suspensions in comfort, stability, and energy recovery.

2.2. Piezoelectric Energy Harvesting Applications

In recent years, the scientific community has paid considerable attention to electromechanical harvesting [41] because it offers the possibility of collecting energy from mechanical sources to power devices onboard the vehicle and, simultaneously, to power the battery. In this context, the main characteristics of piezoelectric devices are their power density, compact size, and ease of use [11]. In this regard, Zhang et al. developed a novel no-contact magnetic force-based piezoelectric EHSA designed to exploit the suspension vibrations of vehicles, especially light trucks [15]. The system, mounted in parallel to the rear suspension system, converts vibrations into rotation via a ball screw, with a unidirectional high-speed rotor supported by a bearing and a planetary gear mechanism. The EHSA system, highlighted in Figure 8, consists of a rotor with alternating polarity, permanent magnets, and a stator with piezoelectric laminations, on the center of which magnets are positioned. Through magnetic attraction and repulsion, the piezoelectric unit undertakes frequent deformations, generating electricity. In tests, the EHSA achieved a maximum power of 7.51 W and an average power of 1.89 W with sinusoidal excitation (40 mm, 0.4 Hz). Under random vibrations, the device powered 824 LED lights and a temperature and humidity sensor, demonstrating the ability to power self-powered appliances onboard.

Yu et al. propose a rotating piezoelectric energy harvester (PEH) to harness automotive motion and convert vibrations into electrical energy [42]. Their system consists of a circular base and two piezoelectric cantilever beams. Two opposed supports are placed on the base to hold two piezoelectric beams. Figure 9 illustrates the piezoelectric harvesting device proposed by the authors. The supports can be rotated to vary the angle between the piezoelectric beam and the diameter. A synchronizer is placed on the free end of the piezoelectric cantilever to act as the tip mass of the beam. To validate the system, the authors conducted an experiment using a stepper motor with a pulse frequency of 1700 Hz, a point mass of 5.6 g, a counterweight mass of 31.6 g, and a load resistance of 1 kΩ, obtaining a maximum output power of 6.25 mW.

Li et al. studied mechanical and electrical topology optimization to improve the efficiency of electrical energy harvesting by piezoelectric cantilever beams applied to vehicle suspensions [43]. Their work focused on the theoretical analysis of the stress of the piezoelectric elements and the design of optimal configurations to maximize energy conversion. Through a simulation in ANSYS, the authors determined that the placement of connection and excitation points significantly affects the stress of piezoelectric elements. In particular, when these points are placed at the ends of the free element, the average stress increases by 92.9% compared to a central position. From the electrical point of view, the most efficient configuration was the one in which the driven elements are connected in parallel and then in series with the excited element, ensuring a more stable output power as the load resistance varies. Under optimal conditions, with a load of 26 kΩ, a set of four piezoelectric elements produced an output power of 86.4 mW. Hazeri et al. studied the use of piezoelectric materials in tires to harvest mechanical waste energy generated by tread deformations during driving [19]. To test this solution, they designed an experimental setup that simulated the movement and pressure inside the tire by applying piezoelectric elements on its outer surface. On the tire’s tread, five pairs of equidistant piezoelectric elements are connected in parallel. The microcontroller, a rectifier for each pair of piezoelectric elements, the electrical circuit, and a power bank are placed on the wheel’s side. The experiment results showed that this technology can produce up to 2.31 W using 56 piezoelectric elements. Furthermore, the energy harvested is directly proportional to the overall stress on the tread, the vehicle speed, and the applied weight, suggesting that the system could recover even more energy in more demanding driving conditions. In addition, Surya Dadi et al. explored electrical energy harvesting using piezoelectric actuators by conveniently placing them on the tire’s circumference [20]. The authors have performed an analysis and simulation to determine the optimal actuator position, resonant frequency, and impedance that allow for maximum energy harvesting. Through a simulation based on the hypotheses considered, the authors found that at a speed of 60 km/h, the system of 37 piezoelectric actuators provided an energy power of 0.004346 W, corresponding to a current of 0.002112 A and a voltage of 2.056 V. On the other hand, at a speed of 80 km/h, the system provided a power of 0.007 W with a current of 0.002116 and a voltage of 3.318 V, which is suitable for supplying auxiliary devices or charging batteries. Ikbal et al. developed a novel piezoelectric energy harvester based on polyvinylidene fluoride (PVDF) for integration into vehicle tires [44]. This technology uses tire deformation during rotation to power wireless sensors for the real-time monitoring of vehicle conditions. The system uses thermoplastic polyurethane (TPU) terminals, which provide high resistance to deformation and extreme temperatures, improving its reliability. The research analyzed the influence of load and rotation speed on the harvester’s performance, obtaining an instantaneous output voltage between 41 and 63 V. The maximum power recorded was 0.203 mW RMS and 3.42 mW peak, with a power density of 3.11 μW/mm³. Additionally, the system demonstrated the ability to charge a 22 μF capacitor up to a saturation voltage of 14 V. A similar approach has been adopted by Suganya Devi et al., who have developed an innovative system for optimizing electrical energy in EVs, exploiting piezoelectric sensors and IoT technology [45]. The proposed system converts mechanical energy generated by the vibrations, pressure variations, and deformations of tires into electrical energy, helping to reduce dependence on fossil fuels. In standard driving conditions, a constant power of 5 W was recorded with a maximum peak of 12 W, which shows that the system is able to adapt to strong mechanical stress. The energy stored reached 1500 mAh, with a conversion efficiency from mechanical stress to electrical energy of 65% and a charging efficiency of 75%. A key aspect of the research is the real-time monitoring and advanced analysis of data collected by piezoelectric sensors through an IoT framework. The data are pre-processed to eliminate noise and inconsistencies, then segmented to identify key features that allow for estimating the energy generation potential of different vehicle components. Advanced machine learning algorithms are also applied to identify patterns and optimize energy production.

Koo et al. studied the performance of a dual piezoelectric generator designed to harvest energy from the vibrations of a small car engine [18]. The main problem addressed is the sensitivity of piezoelectric cantilever beams to the resonant frequency; in fact, a system with multiple piezoelectric generators and a variable mass box was developed, allowing dynamic adaptation of the resonant frequency. The box consists of two masses with different weights connected in series. Experimental results show that the generator installed in the engine block can produce 0.038 mW at 2200 rpm and up to 0.357 mW at 3200 rpm.

Alhumaid et al. developed a system for harvesting energy from the vibrations of vehicle shock absorbers using piezoelectric transducers [46]. The proposed solution is a rotational piezoelectric harvester, which uses a magnetic coupling mechanism to ensure stable performance over a wide frequency range. The device is driven by a unidirectional suspension system that converts linear motion into rotary motion (Figure 10). By coupling two gear wheels, the rotation is transferred to a cylinder whose circumference is positioned with four permanent magnets with alternating poles, which interact with magnets directly connected to the piezoelectric plates. Piezoelectric plates deform during the cylinder’s rotation thanks to the magnetic interaction, producing an electric current whose peak power is 14.86 mW with an oscillation amplitude of 9 mm at a frequency of 2.5 Hz.

Zhao et al. developed a PEH to harness the vibrations of the vehicle suspension system and improve overall energy efficiency [47]. The system consists of two main elements: a linear-to-rotary motion conversion component via screw–nut coupling and an energy conversion component, in which the rotary motion is converted into electrical energy through a combination of non-contact magnetic forces and the piezoelectric effect. Field tests showed significant results: the PEH generated up to 24.28 W at 60 km/h on a random road with the vehicle loaded. On a pulsed road, the harvested power reached 3346 W at 30 km/h under unloaded conditions.

Huang et al. studied vibration energy recycling in exhaust systems and proposed a self-powered intelligent device (SPID) based on a piezoelectric energy generator [16]. The apparatus comprises a piezoelectric generator installed at the end of the exhaust system and a sensor unit, including diode lights and low-power sensors (Figure 11). The generator showed a peak power of 23.4 μW through simulated excitation experiments. The device’s self-powered and signal-monitoring functions were successfully tested, suggesting that the SPID could play a significant role in intelligent driving and automotive intelligence.

An example of an application of energy harvesting for powering small auxiliary systems is given by Madaro et al., whose work contributed to the realization of a piezoelectric energy recovery system exploiting the kinetic energy of exhaust gases inside the exhaust pipe [17]. The energy recovered by the piezoelectric device is used to power an IoT (Internet of Things) remote sensor to characterize the exhaust gases by measuring temperature, pressure, flow rate, and velocity.

Morad et al. aimed to harvest electrical energy from vehicle vibrations using layered and block-structured piezoelectric plates [48]. The primary objective is to convert the mechanical energy dissipated in suspension systems into usable electrical energy. A novel columnar damper structure was developed using 3D printing, integrating 16 piezoelectric units arranged in four layers. Each block of four units uses TPU dampers for vibration absorption, while diode bridges rectify the voltage output into DC, enhancing system efficiency and achieving a peak output of 146 volts. This approach presents a promising solution for enhancing renewable energy systems and reducing reliance on traditional power sources through vehicle-based energy recovery.

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Electromechanical/piezoelectric hybrid energy harvesting solutions

Tang et al. developed an innovative kinetic energy harvesting (KEH) system for driverless electric buses to recover the inertial kinetic energy usually wasted during driving [49]. The proposed technological solution is based on an irregular turntable that collects and amplifies the energy generated by the omnidirectional accelerations of the vehicle, and the system is shown in Figure 12. The KEH system is composed of three main modules: the energy input module, which collects and amplifies vibrations via a gear train and a magnet array; the energy conversion module, which converts kinetic energy into electricity by combining an electromagnetic generator and a piezoelectric oscillator; and the energy management module, which stores the collected energy in capacitors to power low-power devices. Experimental tests showed that the KEH system can generate an average power of 8.31 mW at an acceleration of 8 m/s². Furthermore, the hybrid system achieved a charging voltage of 3.72 V with a 220 μF capacitor, showing an improvement of 481% compared to the electromagnetic system alone and 11% compared to the piezo system alone; thus, the KEH system was able to power 54 LEDs, confirming its potential for self-powered applications.

Li et al. studied the vehicle body vibrations caused by road roughness and evaluated the theoretical model in a wind tunnel [50]. The authors’ proposed system combines two energy harvesting technologies: a flutter piezoelectric energy harvester (FPEH) and an electromagnetic vibration energy harvester (EVEH). The impact of road roughness (under different road conditions) on system performance was analyzed by simulating the vehicle’s vibrations; the results showed that the optimal operating speed is 32 km/h, as the vibrations produced at higher speeds reduce the system’s efficiency. On a Class E road, the system starts producing energy at 20 km/h, and by 57 km/h, the FPEH and EVEH generate 1.74 mW and 2.51 mW, respectively (on a smoother Class A road, the power increases to 2.88 mW and 3.25 mW).

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Investigation of materials for piezoelectric devices

Piezoelectric materials, essential to many aspects of life in the modern world, can increase electro- and photo-chemical activity when mechanical deformation occurs [51]; when subjected to external stress, they convert mechanical energy into electrical energy. Based on their physical structure and chemical composition, these materials are divided into four groups: piezoelectric ceramics, polymers, monocrystals, and composite materials [52].

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Piezoelectric ceramics are mechanically robust and can withstand high static stresses, promote the accumulation of electrical charges, increase the sensitivity of devices, and convert mechanical energy into electrical energy with high efficiency [53,54]. The most common piezoelectric ceramics are based on complex oxides, such as PZT (lead zirconate titanate, Pb(Zr,Ti)O₃), thanks to high piezoelectric efficiency and the possibility of being polarized to improve performance [15]; however, because of the lead, it is toxic. BaTiO₃ (barium titanate) is less widely used than PZT and in less demanding applications. KNN (potassium sodium niobate, K_0.5Na_0.5NbO₃) is an ecological alternative to PZT since it is lead-free. As they are made of ceramic materials, they are fragile and can break under high mechanical stress. Finally, their piezoelectric properties can vary with temperature, limiting some applications.

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Polymer-based piezoelectric materials are more flexible and lightweight and can be bent and shaped, which is ideal for wearable devices or for applying to irregular surfaces [55]. Unlike ceramics, they are not brittle and can withstand greater loads without breaking. They have high chemical stability and corrosion resistance but produce a lower electric charge than ceramic materials with the same deformation level and have lower thermal stability. The most common polymer-based piezo-materials are PVDF (polyvinylidene fluoride (C₂H₂F₂)_n), which has good piezoelectricity and chemical stability [42,44], PVDF copolymers (PVDF-TrFE), which have better piezoelectricity than pure PVDF, and polymers loaded with piezoelectric ceramic nanoparticles (PZT and BaTiO₃) [56].

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Monocrystalline piezoelectric materials have a highly ordered and continuous crystalline structure without the grain boundaries or defects typical of polycrystalline ceramics, giving them superior properties compared to ceramics and polymers. These materials have a very high piezoelectric coefficient, high thermal and chemical stability, and greater electromechanical coupling. On the other hand, they are more difficult to produce, are more fragile than polycrystalline materials, and are expensive [57].

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Piezoelectric composite materials are engineered materials combining the properties of traditional piezo-materials (ceramic or polymeric) with those of a support matrix (often polymer), optimizing electromechanical properties for specific applications [58].

The direct piezoelectric effect is the simplest form of energy harvesting using piezo-materials; the efficiency depends on the location and amount of material used, as well as the level of stress the device receives. Advances have enabled the development of more resilient and effective piezo-materials that can withstand frequent and varied stresses, making the direct piezoelectric effect increasingly feasible for harvesting applications.

Vibration-based energy harvesting is most relevant in environments with constant and predictable vibrations, such as automotive or industrial fields, by installing the piezoelectric elements where vibrations occur. These elements capture the energy from the vibrations and convert it into electrical power to feed monitoring sensors, auxiliary lighting, or continuously recharge the battery. The effectiveness of vibration-based harvesting depends on the frequency and amplitude of vibrations, as well as the optimization of piezoelectric properties to maximize efficiency.

When a piezoelectric material is subjected to mechanical stress (such as compression or tension), a change in the distribution of electric charges within the material occurs, generating a separation of electric charges and, therefore, an electric polarization. This phenomenon creates a potential difference, and if the deformations are periodic, as in the case of vibrations, an AC power. The relationship between mechanical strain and electrical potential is described by the material’s piezoelectric constants (denoted by the term d_ij), which quantify the amount of electric charge generated per unit of mechanical strain. They are generally measured in picocoulombs per Newton (pC/N) and can vary depending on stress and the direction of polarization. When subjected to cyclic deformations, piezo-materials can exhibit a hysteresis loop, where the electrical response is not perfectly linear and lags in relation to the applied strain. The hysteresis loop is important to consider in practical applications as it can affect the efficiency and response of the device. Piezoelectric materials also have a dielectric constant, determining their ability to store electrical energy. This property is important for designing piezoelectric devices as it affects the amount of energy that can be stored and the electrical response of the material.

Engineers are continually refining the designs of these systems to optimize efficiency and adaptability, which is enabling broader applications in numerous industries. Energy harvesting using piezoelectric materials is an active and growing area of research, with the potential to significantly contribute to renewable energy generation and environmental sustainability. Continued innovation in materials and technologies promises to improve these systems’ efficiency and applicability.

2.3. Comparative Analysis and Limitations Related to Energy Harvesting Applications Using Mechanical Energy

In Section 2, numerous applications related to electromechanical harvesting have been reported, some of which are ready to become industrial products and be installed in cars. In recent years, the scientific community has shown considerable interest in energy recovery from vehicles, exploiting the vibrations of suspensions due to the asphalt’s irregularity to extract electrical energy for powering small auxiliary devices or increasing the hybrid or electric cars’ autonomy by recharging the battery. Different applications have emerged that can convert mechanical energy into electrical energy through different conversion methods, such as electric generators, electromagnetic systems, and applications that exploit the direct piezoelectric effect through piezo-elements.

Table 1. Main comparison features for electromechanical harvesting systems.

Reference	Developed System	Harvesting Technology	Application Point	Specific Design Features	Recovered Power
W. Salam et al. [13]	RSA for EVs	Electric generator	Suspension inside the wheel	Racks, bevel pinions, and freewheel clutch	380 W
M. Abdelrahman et al. [29]	Energy recovery shock absorber for electric buses	Electric generator	Inside the suspension	Slotted link mechanism, bevel pinions, and freewheel clutch	6.591 W
P. Techalimsakul et al. [30]	Suspension-based vibration absorber for 2-ton EVs	Electric generator	Inside the suspension	Racks, bevel pinions, and freewheel clutch	8.253 W
H. Li et al. [31]	RSA	Electric generator	Inside the suspension	Racks, bevel pinions, and freewheel clutch	4.25 W
Z. Wang et al. [32]	RSA for EVs	Electric generator	Inside the suspension	Twin ball screws, cylindrical gear, and freewheel clutch	3.701 W
R. Zhou et al. [14]	Magnetic energy-harvesting suspension	Electromagnetic generator	Harvester device in the suspension spring–damper system	Stator with two magnets and mover with three coils	0.34 W
M.R. Hajidavalloo et al. [33]	Ball screw-based energy-harvesting shock absorber	Electromagnetic generator	Device included in the suspension assembly	Ball screws, planet gear, sun gear, and inert pendulum vibration absorber	~70 to ~133 W (1)
W. Jiang et al. [34]	Dual-mode magnetic suspension	Electromagnetic generator	Device integrated into the suspension spring–damper system	Stator with a permanent magnet ring and mover with three coils	7.6 V (excitation 2.8 mm and 21 Hz)
Y. Hu et al. [35]	Hybrid generator with an S-TENG and EMG	Electromagnetic generator	Device integrated into the suspension spring–damper system	One slider with an S-TENG system (CU foils and PI films) and coils for EMG; one stator with a Halbach magnet array	from 2.791 V to 3.176 V in 30 min (2)
J. Zou et al. [38]	Hydraulic interconnected integrated regenerative suspension	Electric generator	Energy harvesting via hydraulic cylinders that replace the dumper	Hydraulic motor generator group, a high- and low-pressure accumulator, 4 actuators, and 4 hydraulic rectifiers.	87.69 W to 147.86 W
W. Zhang et al. [39]	Mechanical–electric–hydraulic regenerative suspension	Electric generator	Device positioned at a point adjacent to the vehicle’s suspension.	Device with a vane damper, hydraulic commutator, and HMG, driven by a rod directly connected to the suspension	1104 W
B. Zhang et al. [15]	Piezoelectric energy-harvesting shock absorber	Direct piezoelectric effect	Device included in the suspension assembly	Ball screw, freewheel, and planetary mechanism for one-way rotation and speed amplifier. Rotor with alternating permanent magnets that interact with those in the piezo module of the stator	Max: 7.51 W, Average: 1.89 W
G. Yu et al. [42]	Rotating piezoelectric energy harvester for automotive motion	Direct piezoelectric effect	Base disc with piezoelectric cantilever beams on the vehicle wheel	Piezoelectric cantilever beams with a tip mass constrained to a circular base connected to the vehicle wheel	6.25 mW
Y. Li et al. [43]	Piezoelectric harvesting from suspension vibrations	Direct piezoelectric effect	Piezoelectric cantilever beams applied to vehicle suspensions	Gear wheel driven by a rack that deforms a vector of piezoelectric elements during rotation	86.4 mW
S. Hazeri et al. [19]	Energy harvesting by tire deformations	Direct piezoelectric effect	Piezoelectric elements on the tire surface	56 circular piezoelectric elements attached to the central tire’s surface	2.31 W
P.S. Surya Dadi et al. [20]	Energy harvesting by tire deformations	Direct piezoelectric effect	Piezoelectric elements on the tire surface	75 piezo elements on a 600 mm diameter wheel	4.346 mW (60 km/h), 7 mW (80 km/h)
M. Ikbal et al. [44]	Energy harvesting by tire deformations	Direct piezoelectric effect	Piezoelectric elements inside the tires	PVDF-based piezo-elements integrated into a sandwich made from PTFE and rubber on a TPU and cap	max: 3.42 mW, med: 0.20 mW
K.S. Devi et al. [45]	Harvesting by vibration, pressure variations, and deformations of tires	Direct piezoelectric effect	Piezoelectric elements on the suspension system	Piezoelectric patches mounted onto the suspensions to recover energy and to provide information regarding mechanical stress and vibration variations	max: 12 W average: 5 W
B.-G. Koo et al. [18]	Piezoelectric harvesting by engine vibrations	Direct piezoelectric effect	Cantilever piezoelectric beam on the engine block	Piezoelectric cantilever beams with the tip load mounted on the top part of the engine	0.038 mW at 2200 rpm; 0.357 mW at 3200 rpm
S. Alhumaid et al. [46]	Rotational piezoelectric harvester	Direct piezoelectric effect	Piezoelectric elements in a cylindrical stator integrated in the suspension	Racks and pinions for motion conversion, freewheel clutches for one-way rotation; the deformation of piezo-elements obtained by magnetic interaction	14.86 mW
Z. Zhao et al. [47]	Piezoelectric harvesting by suspension vibrations	Direct piezoelectric effect	Piezoelectric patch in a rotary drum included in the suspension assembly	Ball screw for motion conversion and magnetic interaction for mechanical stress of piezoelectric elements	24.28 W (60 km/h), 3346 W (30 km/h) (3)
J. Huang et al. [16]	Vibration energy recycling in the exhaust pipe	Direct piezoelectric effect	Piezoelectric elements around the exhaust pipe	Piezoelectric beams constrained on one side to a fixed frame and on the other to a support attached to the exhaust pipe	23.4 μW
F. Madaro et al. [17]	Piezoelectric harvester to convert the kinetic energy of exhaust gases	Direct piezoelectric effect	Piezoelectric elements inside the exhaust pipe	Piezoelectric converter positioned on an aluminum body in correspondence with the silencer	Max.: 42 μW, Average: 33 nW
Song et al. [37]	Electromagnetic energy recovery from vibration	Electromagnetic-pneumatic generator	System applied on vehicle suspension	The EP-RSA’s structure comprises three modules: one for vibration input, one for transmission, and one for the generator	Average: 1.26 W
Zhang et al. [40]	Hydraulic–electric regenerative suspension	Electric generator	System applied on vehicle suspension	Inflatable hydraulic–electric regenerative suspension	124 W (5 m/s), 566 W (20 m/s)
Morad et al. [48]	Piezoelectric harvesting by suspension vibrations	Direct piezoelectric effect	System applied on vehicle suspension	16 piezoelectric units arranged in four layers. Each block of four units uses TPU dampers for vibration absorption, while diode bridges rectify the voltage output into DC	Peak output of 146 V (4)

⁽¹⁾ Dissipated power per damper. ⁽²⁾ Charging voltage variation of the lithium battery in 30 min. ⁽³⁾ Driving on a random road or on a pulse road, respectively. ⁽⁴⁾ The peak voltage obtained when the outputs of all four layers are connected in series.

highlights the main characteristics that have emerged from analyzing the scientific works.

Energy recovery through electric generators has certainly produced the best results as they provide greater electric power values of 380 W, 87.69 W, and 147.86 W (in the roll and pitch direction), and 1104 W (Salam et al. [13], Zou et al. [38], and Zhang et al. [39], respectively). The design of such systems requires converting the linear motion of the suspension oscillations into the rotary motion to operate the electric generator, which implies the creation of mechanisms with non-negligible dimensions. Some systems require the use of a system of racks, gear wheels, and one-way clutches that allow the rotation of the electric generator in a single direction (Salam et al. [13], Techalimsakul et al. [30], Li et al. [31]). Other systems, however, use reversible ball screws, constituting a more compact system that is easy to implement in car suspensions (Wang et al. [32]).

Of considerable interest are hydraulic systems, which use a fluid moved by the suspensions’ oscillations that expand inside hydraulic motors directly connected to electric generators ([38,39]). In addition to the recovery of electrical energy, these systems can improve driving comfort on irregular roads; the system proposed in [38] is even able to recover energy through the pitch and roll oscillations of the vehicle, making it more stable. Systems using the electromagnetic principle are much more compact and simple (Zhou et al. [14], Jiang et al. [34], Hu et al. [35]). Such systems can be installed directly in the spring–damper system of the vehicle’s suspension, making it easy for the operator to access during maintenance. This technology certainly has several advantages, including simplicity of construction and ease of installation, and it does not require systems that convert motion since its operation adapts optimally to the linear movement of the suspension.

Harvesting solutions using piezoelectric materials technology are certainly the models with the highest power density; if we compare the power–volume ratio with the other systems analyzed so far, it is certainly higher. Electric energy from piezoelectric materials is obtained when they are subjected to mechanical stress. Among the systems studied, those of particular importance are the models using piezoelectric beams in which the magnetic interaction between a fixed element and a rotating element induces mechanical stress (Zhang et al. [15], Alhumaid et al. [46]). An equally interesting solution is positioning piezoelectric elements on the tire’s surface to recover energy from tread deformations (S. Hazeri et al. [19], Surya Dadi et al. [20], Ikbal et al. [44]). These solutions are very useful for powering small devices, such as wireless sensors, to monitor tire pressure and wear.

The combination of multiple recovery techniques, both piezoelectric and electromagnetic, is an excellent collection solution as it allows for the maximization of recovered energy (Tang et al. [49], Li et al. [50]). These systems optimize energy recovery by exploiting electromagnetic generators and the piezoelectric principle. In fact, it would be very interesting to adopt electromagnetic systems directly integrated into the suspension spring–damper system in order to exploit its oscillations and the piezoelectric elements positioned on the tire tread. In this way, it will be possible to simultaneously power low-consumption auxiliary devices and the battery in the case of hybrid vehicles or EVs.

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Discussion of limitations related to mechanical energy-based harvesting applications

Electric harvesting systems from automobile suspensions exploit suspension vibrations to convert mechanical energy into electrical energy to power auxiliary devices or recharge the battery. Although mature enough to be implemented inside the suspensions, the systems analyzed still have room for improvement. Systems based on the conversion of linear motion to rotary motion are subject to wear, requiring careful design and continuous maintenance of the moving mechanical parts; otherwise, they can be damaged, compromising the correct functioning of the system. Furthermore, not all systems analyzed in Section 2 have been tested in real operating conditions, i.e., simulations on different road conditions, which enable the verification of the harvesting device’s stability. Moreover, systems based on piezoelectric elements, despite having a higher power density than other systems, have a limited lifetime as they tend to degrade over time due to repeated mechanical stress; furthermore, they tend to convert only a small fraction of the mechanical energy into electrical energy due to the reduced electromechanical coupling with the solicitation source, making the power produced insufficient to satisfy high energy demands. In addition, the piezoelectric harvesters are frequency-sensitive; thus, they provide optimal energy harvesting only in a narrow frequency band, which is usually mismatched with the broadband, stochastic vibrations of real-world road conditions. In addition, their performance fluctuates under extreme automotive temperatures, affecting the material’s properties, such as piezoelectric coefficients. Possible future developments for improving electromechanical harvesting systems include combining multiple harvesting methodologies to maximize the recovered power.

An example would be to combine harvesting systems integrated into the suspensions (using electric generators or electromagnetic or piezoelectric systems) with hydraulic harvesting systems. Hydraulic harvesting systems use actuators that act as vibration dampers; in this way, driving comfort is increased, and electrical energy is recovered.

Electromagnetic harvesting from suspensions has strong growth potential, especially when integrated with other sustainable mobility technologies. Success will depend on factors such as material evolution, electronic optimization, and integration with future active suspension and energy management systems in electric and autonomous vehicles.

3. Energy Harvesting Solutions That Exploit the Wasted Thermal Energy in Vehicles

Like mechanical harvesting, which recovers kinetic energy from vibrations or movements, thermal harvesting collects the heat generated by thermal engines to produce electrical energy. In most cases, these applications aim to exploit the heat in the exhaust gases of ICE vehicles. The systems are based on devices that convert thermal energy into electricity and are installed directly on the exhaust pipes or through intermediate heat exchangers capable of recovering much more heat from the gases.

Thermal harvesting improves vehicle efficiency, reducing pollution and increasing energy sustainability and performance. The automotive market offers a wide range of powertrain solutions optimized for reducing fuel consumption and environmental impact. Among these, ICE vehicles find greater application because they generate significant waste heat, which can be recovered and converted into useful energy. However, ICE, electric, and hydrogen vehicles benefit from thermal energy harvesting. In electric solutions, electrical resistance affects component performance, and thermal energy recovery improves the system’s efficiency. Similarly, the fuel cell systems produce heat as a byproduct of the electrochemical reaction for hydrogen vehicles. The scientific community addresses this topic by studying various systems, mainly based on implementing organic Rankine cycles (ORCs) or TEGs.

TEGs, known as thermoelectric power generators or thermoelectric generation systems, convert heat into electrical energy through the Seebeck effect. According to this, a temperature difference between two conductive materials generates a potential difference proportional to the temperature difference between the two sides of the device. The output voltage can vary depending on the configuration and used materials, the most common materials being bismuth telluride (Bi₂Te₃) and lead selenite (PbSe). The current generated is DC and depends on the connected electrical load, the internal resistance, and the TEG’s design. The generated power depends on the following:

• The temperature difference;
• The materials used in the generator, which must feature good thermoelectric properties, such as a high Seebeck coefficient and low thermal conductivity;
• Operating conditions, which must be stable to avoid fluctuations in power production.

TEGs, although sensitive to temperature variations, are known for their robustness and reliability, as they have no moving parts that require maintenance compared to other harvesting technologies; furthermore, they are characterized by an internal resistance that affects the amount of electrical power that can be extracted. In fact, a high internal resistance can limit the output current and reduce the overall efficiency of the system.

3.1. Energy Harvesting Using Different Propulsion Systems

Thermal energy harvesting is focused on studying and developing different solutions for powertrain systems that differ depending on the engine class (ICE, electrical, or hydrogen); thus, this section is organized by taking into account the developed systems and methodologies for the different engine classes.

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Vehicles with an Internal Combustion Engine

In an ICE, Zhao et al. [47] developed a thermodynamic model for exhaust TEGs and conducted energy and exergy analyses to evaluate their performance and irreversibility losses. The research identified convective heat transfer and the thermal conductivity of the P–N junction as the primary sources of exergy loss, accounting for over 52% of the total. The study showed that increasing the exhaust temperature improves system performance but reduces the available energy recoverable by the cooling water. Enhancing the exhaust heat transfer coefficient reduces convective exergy losses while increasing the exhaust mass flow rate, which boosts output power. The findings highlighted the importance of optimizing exhaust parameters to improve the efficiency of exhaust TEG systems. The study provides valuable insights for developing high-efficiency exhaust power generators and lays the foundation for future research on integrated systems involving ICEs and TEGs.

Rijpkema et al. [59] analyzed solutions for waste heat recovery (WHR) using an ORC to enhance engine efficiency, reducing CO₂ emissions in heavy-duty transport. Figure 13a shows the thermodynamic circuit related to a Rankine cycle, using water to recover waste heat from the diesel engine’s exhaust. The results were used to calibrate and validate component models: the pump, evaporator, expander, and condenser (Figure 13b). Simulations were performed with three working fluids—water, cyclopentane, and ethanol—under various engine operating conditions and a typical long-haul truck driving cycle. The evaporator that allows the heat exchange between the exhaust gases and the ORC’s fluid is shown in Figure 13c. Figure 13d highlights the evaporator installed in the ORC circuit. The study found that cyclopentane and ethanol outperformed water, offering higher net power outputs and greater energy recovery. The total recovered energy over the driving cycle was 11.2 MJ for cyclopentane, 8.2 MJ for ethanol, and 5.2 MJ for water, corresponding to recoveries of 3.4%, 2.5%, and 1.6%, respectively. The results provide valuable insights for optimizing WHR systems in heavy-duty applications.

Sok et al. [60] aimed to optimize the use of TEGs for waste heat recovery in a next-generation 2.2 L diesel engine. The technical solution for harvesting is shown in Figure 14. The primary goal was to maximize the engine’s brake thermal efficiency (BTE) and power by integrating a TEG system, considering the trade-off between pump losses and the effective power generated by the TEG. A high-fidelity 1D TEG model was developed and calibrated with experimental heat transfer and flow friction data. The engine and TEG models were integrated to optimize performance at peak BTE conditions. The results showed that a 9 × 10 thermoelectric module arrangement could generate 1.1 kW of effective power and improve BTE by 1.1% without power loss. This work has demonstrated the potential of a TEG to boost engine efficiency, enabling its integration into hybridized diesel engines, and sets the foundation for future studies on vehicle-level simulations and system optimization.

Kumar et al. [61] focused on optimizing exhaust gas heat recovery (EGHR) systems in gasoline engines to improve fuel economy through effective thermal management. The aim was to evaluate the performance of different heat exchanger (HE) designs for exhaust gas-to-oil and exhaust gas-to-coolant heat transfer. The study performed by the authors combines physical testing and simulations to assess the impact of various EGHR designs on fuel efficiency, particularly during the worldwide harmonized light vehicles test cycles (WLTC). Results show that integrating exhaust gas-to-coolant and exhaust gas-to-oil HEs can improve fuel efficiency by 0.5% and 0.8%, respectively. The study highlighted the importance of selecting the right HE, such as those with bypass valves and valve-controlled oil coolers, to optimize coolant and oil heating performance. Di Battista et al. [62] investigated the recovery of exhaust waste heat in a turbocharged diesel engine using two recovery stages to improve engine efficiency and reduce fuel consumption (Figure 15). The first stage utilizes turbo-compounding to recover energy from exhaust gases through a parallel turbine, generating up to 3 kW. The second stage uses an ORC-based unit to recover additional energy from the high-temperature exhaust gases, generating up to 3.5 kW. Together, these methods result in up to 10% of the engine’s brake power being recovered, with a fuel consumption reduction of 5–7% at medium to low loads and up to 8% at maximum power. The study also evaluated the negative effects of increased backpressure and additional vehicle weight. The results suggest that these recovery technologies can be applied effectively to heavy-duty engines, improving fuel efficiency, reducing CO₂ emissions, and offering both environmental and operational benefits.

Khayum et al. [63] explored the integration of a TEG with a heat pipe to recover waste heat from a stationary diesel engine operating in dual-fuel mode, using waste cooking oil methyl ester and biogas. The performance was evaluated under different engine loads (1, 2, and 3 kW). The results highlighted the importance of incorporating a heat pipe, as it significantly increased the temperature difference (TD) across TEG modules, enhancing the thermoelectric conversion efficiency. The TEG generated limited power without a heat pipe due to a small TD; instead, the TD improved by 680% with the heat pipe, leading to a maximum power of 6.1 W and a conversion efficiency of 2.9%. Similarly, Huang et al. [64] evaluated the performance of automotive exhaust annular thermoelectric generators (AEATEGs) for waste heat recovery (Figure 16). A theoretical model was developed using the finite element method to account for temperature variations and temperature-dependent properties. AEATEGs were compared with conventional flat-plate thermoelectric generators (AEFTEGs) regarding net power, efficiency, and operating parameters. The study identified key factors such as flow arrangements, cooling medium flow rates, and exhaust temperature on AEATEG performance. The results showed that AEATEGs provide a 1.1% increase in net power compared to AEFTEGs. Using a novel strategy of inserting a hollow cylinder into the heat exchanger, a maximum power increase of 214.0% was recorded.

Sharma et al. [65] evaluated the effectiveness of TEG technology for recovering waste heat from the diesel engine’s exhaust. The TEG modules were mounted on a stainless steel pipe with a square cross-section. The experimental results showed that the power output of the TEG increased with the engine load, with a maximum electrical power output of 37 W at a load of 6 kg. The study showed that the conversion efficiency of the TEG system increased with engine load, further demonstrating that waste heat recovery improves the overall thermal efficiency of the diesel engine compared to conventional ones.

Roeinfard et al. [66] investigated the possibility of improving energy efficiency and reducing fuel consumption using an ORC and a high-temperature Kalina cycle (HTKC) in a bi-fuel engine by recovering waste heat from exhaust gases. A thermodynamic model based on experimental data was developed to analyze and optimize these cycles under different operating conditions. The results showed that the HTKC significantly outperforms the ORC, recovering 10–25 kW of power compared to the ORC’s 2–7 kW and achieving higher efficiency (25–40% vs. 8–13%). Moreover, the HTKC reduces fuel consumption by 20–30% compared to the ORC’s 8–10%. Despite its superior performance, the HTKC poses greater technical challenges for integration, whereas the ORC is easier to implement. Burnete et al. [67] explored the potential of using TEGs for waste heat recovery in ICEs, aiming to improve vehicle efficiency by capturing and converting excess thermal energy into electricity. The study outlined the significant challenge of reducing emissions and improving efficiency in ICEs, especially with the rise of electrified vehicles. The paper highlighted that around 65–70% of the energy input in ICEs is lost as heat, primarily through the exhaust gas system and the radiator. By partially recovering this wasted heat, TEGs can enhance overall thermal efficiency.

Liu et al. [21] introduced an innovative automobile exhaust flexible thermoelectric generator (IAE-FTEG) that leverages a liquid metal (LM)-based stretchable heatsink to capture waste heat and power multiple vehicle-mounted sensors efficiently. The primary objective was to enhance the performance of TEGs in capturing energy from cylindrical and curved exhaust surfaces, which is challenging due to the need for flexible and effective heat transfer solutions. The IAE-FTEG system demonstrated significant advancements by incorporating porous sandwich-based soft electrode films and LM-based thermal interface materials, achieving a 25.7% increase in output power. The flexible heatsink, created by embedding LM droplets and copper particles into an elastic matrix, provides excellent thermal conductivity (2.40 W/mK) and high-temperature resistance while maintaining flexibility for deformation (up to 200% stretch). Simulation results revealed a maximum power density of 244 kW/m³, with real-world tests under urban conditions showing stable output at 117 kW/m³. The IAE-FTEG system facilitated efficient waste heat recovery and holds potential for applications in flexible electronics, wearable devices, and personalized thermal management. The experimental model is illustrated in Figure 17.

Sok et al. [22] presented a novel model for predicting the thermal and electrical performance of TEGs and HEs in waste heat recovery systems. By focusing on heat transfer coefficients and pressure drops in a louvered corrugated fin HE, the study introduced two methods using user-defined functions (UDFs); the first offers high accuracy but requires significant calibration for each fin pitch, while method 2 simplifies the UDFs, achieving reasonable prediction errors without extensive tuning. The model was validated with experimental data under various operating conditions, demonstrating its ability to predict TEG performance. The results indicated that the model can effectively simulate thermal behaviors, including heat transfer and pressure drop, and predict electrical power with less than 20% deviation. In addition, Asaduzzaman et al. [68] investigated the performance of TEGs for exhaust heat recovery from ICEs, focusing on converting exhaust heat into electrical power. Two types of setups were constructed using copper and steel materials, both incorporating triangular channels to facilitate heat exchange (Figure 18a–c). The copper-based TEG setup achieved a maximum power output of 2.96 W at an exhaust temperature of 297 °C, while the steel-based TEG produced 2.0 W at 305 °C. The copper setup demonstrated a 48% higher power output and slightly better conversion efficiency. The results suggest that copper-based TEGs more efficiently convert exhaust heat, leading to lower expelled exhaust temperatures and reduced entropy loss.

Luo et al. [69] focused on improving the performance of annular thermoelectric generators (ATEGs) in WHR based on the performance loss caused by temperature drops along the exhaust pipe. A novel annular thermoelectric module (ATEM) is proposed, with the cross-sectional area of thermoelectric elements increasing along the heat flow direction. The authors evaluated its performance through a three-dimensional multi-physics numerical model and optimized its parameters. The study showed that the optimal length difference between thermoelectric elements is 0.06 mm, which maximizes output power and efficiency. The ATEG achieved an output power of 76.66 W and efficiency of 1.45%, with a significant improvement of 8.97% and 8.93%, respectively, over traditional models.

Quan et al. [70] focused on enhancing the fuel economy of a mild hybrid electric vehicle (HEV) with an integrated starter and generator (ISG) by incorporating an automobile exhaust thermoelectric generator (AETEG). The primary goal was to optimize energy distribution using a model predictive control (MPC) strategy combined with a dynamic programming (DP) algorithm for torque distribution between the ICE and the ISG. The AETEG’s role is to charge the battery and support the hybrid power system through a maximum power point tracking method. Results showed that the MPC-DP strategy with the AETEG reduces fuel consumption by 5.63%, improves fuel economy by up to 7.53%, and ensures better charge stability than traditional strategies (such as fuzzy logic control and rule-based energy management). Moreover, it reduces emissions and smoke opacity, contributing to improved environmental performance in HEVs. The same authors also investigated the performance and fuel-saving potential of an automotive thermoelectric generator (AUTEG) system integrated into a sport utility vehicle (SUV) (Figure 19), evaluating how parasitic power losses, such as backpressure, AUTEG weight, and coolant pumping power, affect the system’s net output power and fuel economy [71]. A multi-physics field coupling model was tested to estimate the heat transfer and power generation performance of the AUTEG, finding a maximum net output power of about 50 W, with a conversion efficiency of 0.1% and a fuel economy improvement of 0.21%. The study showed that vehicle speed, backpressure, and AUTEG weight significantly optimize fuel efficiency, finding the best results between 108 km/h and 125 km/h. This work provides a framework for optimizing AUTEG systems and assessing their impact on fuel economy.

Luo et al. [72] focused on enhancing the output performance of automotive thermoelectric generators (AUTEGs) by introducing a new heat pipe configuration. The heat pipes are arranged in a staggered up-and-down pattern, directly interacting with the exhaust gas to improve heat absorption. A hybrid numerical model combining computational fluid dynamics (CFD) and thermal–electric models was used to predict the system’s performance under varying parameters. The study examines the effects of staggering distance, fin thickness, and fin spacing on the AUTEG’s efficiency. The findings indicate that the staggered configuration significantly boosts the heat transfer efficiency, increasing the AUTEG’s output power. The optimal staggered distance is identified as 1.5 mm, and the ideal fin thickness and spacing are 0.5 mm and 1.0 mm, respectively, resulting in a 15.07% improvement in net power output compared to the original structure.

A heat exchanger downstream of the exhaust pipes risks reducing engine performance. In this regard, Quan et al. [73] investigated the performance interaction between the AETEG and the ICE, focusing on how heat exchangers impact ICE performance due to backpressure. Figure 20 shows the experimental setup of the model proposed by the authors. A comprehensive numerical model integrating the ICE with different heat exchangers in the AETEG setup was developed and validated with experimental data. The findings revealed that the heat exchanger’s inner topology significantly influences backpressure, with the chaos-shaped heat exchanger causing the highest pressure increase. Increased backpressure raises the hot side temperature of the thermoelectric modules (TEMs), improving the AUTEG’s output power and reducing ICE efficiency, brake power, and fuel economy.

Luo et al. [74] presented a fluid–thermal–electric multi-physics numerical model to predict the performance of a TEG system for a car’s waste heat recovery. The model considers the full geometry, temperature-dependent material properties, the topological connection of thermoelectric modules, and impedance matching, providing a realistic simulation of a TEG system under actual operating conditions. The model uses exhaust temperature and mass flow rate data at different vehicle speeds to evaluate the system’s output performance, including power, efficiency, and losses. The model is experimentally verified using the test bench shown in Figure 21. The study showed that the position of the thermoelectric modules on the heat exchanger significantly affects the uniformity of the output power, and that system performance improves at high speeds.

Wen Du et al. [75] introduced the tubular thermoelectric generator (TTEG), a novel approach to converting waste heat into electrical energy specifically designed for fluid-circulating circular channels. The primary objective of the research was to demonstrate the advantages of the TTEG over traditional flat-plate thermoelectric generators (FTEGs) by offering a more compact, efficient, and scalable solution for waste heat recovery. The TTEG, with its full-ring thermoelectric elements, provides a larger heat source contact area and a more effective configuration, leading to significantly better thermal efficiency and higher output power. The study revealed that the TTEG outperforms the traditional FTEGs by 62.5% in power generation under identical test conditions, highlighting its suitability for high-temperature differential applications. Gürbüz et al. [76] investigated and improved the performance of TEGs for exhaust waste heat recovery in propane-fueled spark-ignition engines. The study presented a novel TEG design that incorporates two engine coolant exchangers (Ec_hex) with copper serpentine pipes to allow propane to pass through, enhancing the cold side efficiency and increasing the temperature difference (ΔT) between the hot and cold sides of the TEG. The paper aims to evaluate the impact of this propane input on the TEG’s power generation and energy conversion efficiency across a range of engine speeds (1500–5000 rpm). The results showed that the TEG with propane increases the output power by 11.5–12.1% compared to the standard design, with a maximum DC output of 90.2 W and energy efficiency of 3.02% at 4500 rpm.

Quan et al. [77] analyzed solutions for optimizing the structural parameters of polygonal heat exchangers (HEXs) in automotive exhaust thermoelectric generators (AETEGs) to enhance energy conversion performance while minimizing backpressure. A comprehensive fluid–thermal–electric coupling model was developed to simulate and analyze the influence of HEX width and length on flow dynamics, thermal behavior, and electrical output. The objective is to identify optimal HEX dimensions using an in-vehicle compatibility index that considers power output, conversion efficiency, temperature uniformity, and backpressure. Simulation results show that wider HEXs reduce efficiency and power, while longer HEXs increase power and pressure drop but lower efficiency. The optimal design parameters were determined to be H = 130 mm, Nw = 1 row, and NL = 6–8 columns. At 600 K and an exhaust flow of 40 m/s, the system achieved 0.97% efficiency and an output of 118.24 W. This work provides a practical method to enhance AETEG design and integration in vehicles with minimal impact on engine performance.

Quan et al. [78] also present a study that aims to optimize the design and integration of automobile exhaust thermoelectric generators (AETEGs) to enhance fuel economy while maximizing net power output. A multi-physical field model was developed to simulate thermal and electrical behavior under varying TEG unit configurations. Using GT-SUITE software, the impact of different TEG numbers on power loss, output, and vehicle performance was analyzed. The main objective is to balance power generation with backpressure and system weight to improve overall fuel efficiency. Results show that five TEG units yield a maximum output power of 721.76 W and improve fuel economy by up to 3.33%. However, backpressure and weight cause major power losses, highlighting the need for a lightweight, efficient design. The study provides insights for selecting optimal TEG configurations to minimize energy loss. Future work will explore lightweight materials and assess carbon emission reductions. This research supports smarter AETEG deployment in next-generation fuel-saving vehicles.

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Electric and hydrogen vehicles

Wang et al. [79] investigated the integration of an ORC with HEVs to improve energy efficiency by recovering waste heat from the ICE. Given the complexity of managing multiple energy sources in an HEV–ORC system, the study proposed the use of deep reinforcement learning (DRL) for energy management. A dynamic simulation model of the HEV–ORC system was established, and the DRL-based energy management system (EMS) was compared with a traditional rule-based EMS. The results demonstrated that the DRL-based EMS can achieve 2% greater fuel energy savings by ensuring higher average engine and motor efficiencies and a more stable ORC power generation and battery state. Lan et al. [80] presented a method for optimizing a TEG for use in extended-range electric vehicles (EEVs) by considering two key criteria: TEG protection and high net power density. Unlike conventional vehicles, where engine power varies across different driving cycles, EEV engines operate at a fixed point, making them ideal for TEG optimization (Figure 22). The study evaluated trade-offs in TEG performance, including weight, electric pump power consumption, and exhaust backpressure. The optimization process increased the net power density of the TEG by 11.6%, and fuel economy testing showed that the EEV–TEG system results in a 1.7% reduction in equivalent fuel consumption compared to a conventional EEV. This work offers theoretical and practical insights for designing efficient TEGs in EEV applications, enhancing fuel efficiency and reducing emissions.

Lee et al. [81] proposed an optimization method for waste heat recovery systems (WHRS) in electric vehicles (EVs) to enhance heating performance in winter. Unlike conventional systems, which recover heat at a fixed temperature, the proposed method dynamically adjusts the temperature based on operating conditions. Heat pump and electric device models were developed to evaluate the performance under different driving conditions, including speed, duration, and ambient temperature. The results showed that the optimal WHRS changes with conditions, allowing up to a 13% reduction in power consumption by recovering heat at the ideal temperature. The model can predict the optimal WHRS before driving, improving energy efficiency and extending the driving range.

Wilberforce et al. [82] focused on potential waste heat recovery from proton exchange membrane fuel cells (PEMFCs) in transportation and portable applications. PEMFCs generate significant heat (45–60% of the hydrogen energy input), necessitating an efficient cooling system to prolong their lifespan and maintain performance. The work emphasizes the importance of thermal management and the potential for heat recovery to enhance efficiency and reduce energy consumption. Various heat recovery strategies are discussed, including coupling PEMFCs with organic Rankine cycles, thermoelectric generators, and adsorption cooling systems. Additionally, the study highlighted the need for experimental research on these systems’ environmental performance and economic evaluations. It also suggests exploring the integration of PEMFCs with hydrogen storage units for improved performance, especially in cold climates. The aim is to optimize energy use, reduce greenhouse gas emissions, and improve system robustness through innovative heat recovery solutions. Based on the same propulsion system, Wang et al. [83] also examined the contribution of the waste heat recovery (WHR) system to hydrogen power technologies for land transportation, focusing on hydrogen internal combustion engines (HICEs), proton exchange membrane fuel cells (PEMFCs), and solid oxide fuel cells (SOFCs). WHR improves HICE efficiency from 43.92% to 51.05%, reaching 63.7% with advanced technologies. In PEMFCs, WHR has increased efficiency by up to 3.96%, potentially exceeding 69.2% in future designs. SOFCs combined with gas turbines achieve 60–70% efficiency, reaching 78% with WHR. SOFC–WHR offers the highest electrical efficiency, while HICE–WHR excels in mechanical efficiency and durability. PEMFC–WHR has better power density, making it ideal for urban vehicles, while HICE–WHR is more suited for trucks on long routes. SOFC–WHR remains too large for land transport. WHR enhances all three technologies, each with specific advantages. Future advancements will further optimize these solutions for efficiency and sustainability.

3.2. Energy Recovery from Braking Systems

Saif et al. [84] explored the use of a TEG to enhance the operating range of HEVs by converting waste heat into electrical energy. The primary aim was to propose a TEG model that utilizes gas velocity pressure and turbulence kinetic energy at peak temperatures to improve battery efficiency. The concept is based on regenerative braking, where kinetic energy that would normally be lost as heat is captured and converted into electricity. This process, driven by the heat from the brake disk, reduces energy loss and extends the battery range of HEVs. The study investigated the impact of temperature and fluid dynamics on the performance of the TEG model through computational fluid analysis (CFD). Ultimately, the goal has been to increase the efficiency of HEVs by capturing more energy from waste heat, contributing to reducing environmental pollution and enhancing the overall performance of electric vehicles. Coulibaly [85] explored a novel approach to improving vehicle energy efficiency by utilizing TEGs for energy recovery during braking. The primary aim is to assess the potential for converting thermal energy from friction in braking systems into electrical power. The research involved the thermal analysis of brake pads and discs, using finite element simulations to evaluate the energy available under various temperature conditions. Despite the relatively low efficiency of TEGs (around 0.3%), the results showed that up to 4 W of electrical energy can be recovered in typical braking systems, which is sufficient to power onboard vehicle devices. The study compared two types of brake discs—full and ventilated—and found that cooler ambient temperatures and the ventilation of the disc reduce energy recovery efficiency. However, even under these conditions, TEGs can still recover a meaningful amount of energy. Furthermore, Saif et al. [86] addressed the growing importance of regenerative braking in electric and hybrid vehicles as a means to improve energy efficiency. The focus of the work was to present the design of a TEG that captures waste heat generated from the braking process and converts it into electrical energy. The primary goal is to store this electrical energy in the vehicle’s battery, thus extending the driving range of HEVs. The manuscript explores the relationship between temperature and current within the TEG system, analyzing how heat can be efficiently harvested from the brake pads and transformed into power. Advancements in thermoelectric materials and heat exchangers are considered to optimize energy recovery by minimizing thermal resistance and improving heat transfer.

3.3. Comparative Analysis and Limitations Related to Energy Harvesting Applications Using Thermal Energy

Recovering waste heat is an essential strategy for improving vehicle efficiency, lowering fuel consumption, and reducing environmental impact. Various heat recovery solutions are being explored to optimize energy use across different powertrain technologies—internal combustion engines (ICEs), electric vehicles (EVs), and hydrogen-powered vehicles. Regarding ICEs, two primary technologies stand out: TEGs and the ORC. TEGs convert heat directly into electricity, offering a compact and maintenance-free solution, but their efficiency remains relatively low, typically between 1% and 3% (Zhao et al. [87], Burnete et al. [67]). ORC systems, by contrast, can achieve much higher recovery rates—often between 3% and 10%—making them more suitable for heavy-duty applications (Rijpkema et al. [59], Di Battista et al. [62]). Some studies suggest that advanced high-temperature cycles, such as the Kalina cycle, could push efficiency up to 40% under optimal conditions (Roeinfard et al. [66]). Additionally, exhaust gas heat recovery (EGHR) systems are being explored to improve fuel efficiency in gasoline engines, leading to modest but meaningful gains of 0.5–0.8% (Kumar et al. [61]). For electric vehicles, the main challenge is managing thermal losses effectively, particularly in extending the battery range. One of the most promising solutions is optimized heat pump technology, which dynamically regulates energy use and can reduce power consumption by up to 13% in cold conditions (Lee et al. [81]). Another emerging area is thermoelectric energy recovery from braking systems, which can convert waste heat from braking into electricity, recovering between 0.3% and 4% of lost energy (Saif et al. [84], Coulibaly et al. [85]). While regenerative braking is already common in EVs and hybrids, the additional use of TEGs in braking remains limited by the efficiency of thermoelectric materials.

Waste heat recovery is particularly relevant in hydrogen-powered vehicles for proton exchange membrane fuel cells (PEMFCs) and hydrogen internal combustion engines (HICEs). PEMFCs generate significant amounts of heat, which can be recovered through ORC systems, TEGs, or adsorption cooling, improving efficiency by up to 3.96% (Wilberforce et al. [82]). Similarly, HICEs—comparable to traditional combustion engines—can integrate WHR systems to raise efficiency from 43.92% to 51.05% (Wang et al. [83]). Meanwhile, when combined with gas turbines, solid oxide fuel cells (SOFCs) achieve the highest recorded WHR efficiencies, reaching up to 78%. However, their size makes them impractical for road vehicles (Wang et al. [83]).

Comparing these technologies highlights several trade-offs. ICE-based WHR systems can recover more energy but require additional components that add complexity and cost. Electric vehicles focus on improving internal energy efficiency but remain constrained by battery capacity and thermoelectric material limitations. Hydrogen vehicles offer a balance between traditional and renewable technologies, but integrating WHR systems into fuel cells presents challenges in terms of cooling and system design. Advancements in scientific materials, system integration, and energy management strategies will determine the viability of these technologies. The best solution will depend on the specific use case, whether it is maximizing energy efficiency in long-haul trucking, extending the driving range of EVs, or making hydrogen fuel cells more competitive for mainstream adoption.

Table 2. Main comparison features for the thermal harvesting systems; the specified temperature values refer to the operating temperatures (e.g., related to exhaust gases) at which the system works.

References	Propulsion Category	Harvesting Technology	Conversion Efficiency	Recovered Power	Operating Temp.	Design Features	Advantages	Challenges
Y. Zhao et al. [87]	Internal Combustion Engine	Thermoelectric Generators (TEGs)	1.80%	Max output value 236.02 W	200–650 °C (1)	Direct heat-to-electricity conversion	Exergy loss analysis, optimizing exhaust parameters	Low power output, integration complexity
R. Sok et al. [60]			-	1.1 kW	250 °C (1)		EG integration for diesel engines, boosting BTE by 1.1%
G. Sharma et al. [65]			3.5%	37 W	-		TEGs on diesel engine exhaust, max output 37 W
R. Sok et al. [22]			-	3 W	200–300 °C (1)		TEG–heat exchanger modeling, accurate power prediction
Md. Asaduzzaman et al. [68]			4.65% steel TEG, 4.63% copper TEG	2.0 W steel TEG, 2.9 W copper TEG	297–300 °C (1)		Copper vs. steel TEGs, copper has 48% higher power
D. Luo et al. [74]			1.53%	38.07 W	363.15 K (1)		Numerical model for TEG placement impact
Quan et al. [77]			0.97%	118.24 W	623 K		Structure optimization of AETEGs
Quant et al. [78]			5 TEG units reach 1.06% at 125 km/h	721.76 W	663 K		Solution for AETEG system optimization and assessment of fuel efficiency
J. Rijpkema et al. [59]		Organic Rankine Cycle (ORC)	1.6% (water) (2); 2.5% (ethanol) (2); 3.4% (cyclopentane) (2)	0.5–5.7 kW 1.8–9.6 kW 1.0–7.8 kW	260–320 °C (1)	High energy recovery, suitable for trucks	ORC for heavy-duty diesel engines, 3.4% recovery with cyclopentane	Requires additional components, space constraints
D. Di Battista et al. [62]			10%	3 kW (I stage), 3.5 kW (ORC stage)	300–400 °C (1)		ORC + turbo-compounding, up to 10% brake power recovery
N. Roeinfard et al. [66]			25–40% (HTKC), 8–13% (ORC)	10–25 kW (HTKC), 2–7 kW (ORC)	700–900 K (3)		ORC vs. Kalina cycle, HTKC achieves 25–40% efficiency
S. Lee et al. [81]	Electric Vehicles	Heat Pump Optimization	Up to 13% (energy saved in EV)	-	-	Optimal WHRS (waste heat recovery strategy)	Multi-level heat recovery strategy reduces power consumption by 13%	Dependent on driving conditions
Md.Z.U. Saif et al. [84]	Braking Systems	Thermoelectric Generators (TEGs) in Braking	0.3–4%	41.98 W	326 °C	Heat recovery during braking for battery charging	TEG model using brake heat for HEVs, CFD analysis	Limited power output, efficiency depends on braking conditions
A. Coulibaly, et al. [85]			0.3%	4.25 W (4) 3.25 W (5)	25–200 °C (6), 25–175 °C (6)		Brake disc TEGs, recovery up to 4 W
Md.Z.U. Saif et al. [86]			-	71.136 A 97.001 A	250 °C (7) 326 °C (7)		Impact of temperature on TEG efficiency for regenerative braking

⁽¹⁾ Exhaust gas temperature; ⁽²⁾ percentage of recovered energy; ⁽³⁾ temperature range for turbine inlet; ⁽⁴⁾ thermal analysis of a full brake disc; ⁽⁵⁾ thermal analysis of the ventilated brake disc; ⁽⁶⁾ temperature range between the ambient and hot sides of the disk; ⁽⁷⁾ temperature generated from the brake pad of the HEV.

shows the main characteristics of each study analyzed in this section.

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Discussion of limitations related to thermal-energy-based harvesting applications

Waste heat recovery technologies face several key challenges to overcoming the limitations of their development and application. For example, a lamination in the integration of TEGs and ORC systems remains complex due to space constraints; as in the case of ORC-based systems, additional components are required for operation. Improvements are necessary to exceed the efficiency limits, particularly in low-power applications where output remains limited, such as optimizing the TEG’s geometry to maximize contact with heat sources and improve heat flux, combining TEGs with other energy recovery technologies, such as heat recovery systems, to maximize the recovered energy, developing and using thermoelectric materials with higher efficiency in converting heat into electricity (such as bismuth telluride, silicon germanium, or new metal alloys that can improve performance), and balancing the costs. The dependency on specific operating conditions, such as temperature variations and driving cycles, affects overall performance, so numerical modeling and optimization strategies require further refinement for performance prediction. Maintaining consistent efficiency across varying braking conditions makes braking system heat recovery challenging in the context of braking heat harvesting. Another significant hurdle is ensuring the long-term durability and reliability of these systems. Finally, future advancements in this field should focus on hybrid approaches integrating multiple recovery technologies for enhanced energy efficiency.

4. Hybrid, Solar, and Wind Energy Harvesting Systems

A great solution to increase the amount of energy that can be recovered from a vehicle is the combined use of mechanical and thermal harvesting systems. Combining multiple harvesting technologies allows hybrid systems to exploit energy from different sources, thus increasing the overall electrical power output. In this way, efficiency can be significantly improved. The works reviewed in this section cover various energy harvesting sources, including wind, solar, thermoelectric, and piezoelectric technologies. Each of these approaches offers distinct advantages in specific vehicular applications, contributing to the broader goal of sustainable transportation. With the growing demand for eco-friendly mobility solutions, extensive research has been dedicated to developing innovative energy-harvesting technologies for electric vehicles (EVs) and connected vehicle systems. These advancements enhance energy efficiency, reduce dependence on external charging infrastructure, and improve vehicle autonomy. Researchers integrate renewable energy sources directly into vehicle designs to create self-sustaining systems that optimize power generation and consumption. This analysis explores various harvesting methodologies, assessing their efficiency, implementation challenges, and practical applications.

4.1. Hybrid Harvesting Systems

In developing hybrid systems, Park et al. explored computer-proximal energy harvesting as an alternative to traditional wiring to power automotive sensors and electronic accessories [88]. The authors’ aim was to reduce installation costs, inconvenience, and failure risks by harvesting energy locally from wind, light, vibration, and heat. The study investigates various energy sources around vehicles, presenting two prototypes: a thermoelectric parking assistant attached to the exhaust pipe and a wind-powered pedestrian display mounted on the front bumper. Unlike previous work on recharging car batteries, this research examines energy harvesting for retrofitting vehicles with intelligent devices. The study demonstrated how off-the-shelf and custom energy harvesters can power embedded computing and sensing systems, offering a foundation for incrementally integrating computation into cars. Kim et al. introduced an innovative hybrid energy harvesting system that combines thermoelectricity and piezoelectricity to enhance energy generation efficiency (Figure 23) [89]. The primary goal was to overcome the limitations of single-mode harvesters by leveraging the synergistic effects of oscillation-induced heat dissipation. Unlike conventional hybrid harvesters that merely sum individual energy outputs, this study demonstrates amplified power generation through dynamic thermal flow regulation. The research explored optimized cantilever beam geometries, showing that a trapezoidal design maximizes displacement and heat dissipation. Experimental results demonstrated that lowering the cold-side temperature (T_c) of TEGs increases the power output significantly. The system achieves a 26.8% boost in energy efficiency, certifying the potential for sustainable and self-powered applications in the automotive field.

Yadav et al. addressed the role of heavy-duty vehicles (HDVs) in contributing to pollution and identified areas of improvement for enhancing their fuel efficiency [90]. The research examined the potential of thermo- (TEG) and piezoelectric generators, as well as a regenerative braking strategy to capture and utilize waste energy. It suggests developing an integrated solution combining all three technologies to maximize energy harvesting and supply significant energy back to the HDVs. The aim is to optimize costs, reduce losses, and lower maintenance expenses by increasing the lifespan of vehicle components. The study highlighted that these technologies are environmentally friendly, reliable, and can power a hybrid powertrain, further improving fuel efficiency. However, challenges such as the low conversion efficiency of TEGs and the high cost of these modules remain.

4.2. Solar Harvesting Systems

Park et al. evaluate and validate the power output potential of photovoltaic (PV)-powered electric vehicles (EVs) by analyzing travel time and driving routes [91]. The research examines a vehicle equipped with PV modules on four sides (roof, rear window, left door, and right door) driving across seventeen road sections with varying inclinations and azimuths. The study incorporates the impact of terrain and building shadows to assess PV potential accurately. Results indicate that the roof module generates the highest power, followed by the rear window and door modules, with the rear window producing approximately 42% and the doors 27% of the roof’s output. A single 10 min drive generates 0.0158 kWh, amounting to 221 kWh annually under specific conditions.

Jin et al. [92] proposed a portable, auxiliary photovoltaic power system for electric vehicles (EVs) to address driving range and charging limitations. The system features a foldable scissors mechanism for the photovoltaic power generation module, enhancing portability. The electricity transfer module uses wireless power transfer to store energy in supercapacitors. The experiments show that the system can deliver a maximum output power of 1.736 W and 57.7% wireless power transfer efficiency. A case study conducted by the authors in Chengdu showed that the system can power the MINIEV vehicle for an annual distance of 423,625 km at an average consumption of 0.08 kWh/km. Future work will focus on improving efficiency through maximum power point tracking and solar tracking technologies. Mytafides et al. developed a novel approach for the integration of TEGs and dye-sensitized solar cells (DSSCs) into carbon fiber-reinforced polymer (CFRP) composites, resulting in a hybrid composite [93]. Carbon fibers offer high conductivity and play a crucial role as counter electrodes in DSSCs due to their suitability for the redox reactions of the electrolyte. Tests performed under simulated sunlight showed a significant power density of 6.4 mW/cm². This hybrid TEG- and DEEC-based energy harvester also opens up potential automotive applications for powering sensors and low-power electronic devices, contributing to energy efficiency. Furthermore, using lightweight composite materials such as CFRP, combined with innovative energy harvesting systems, can improve the sustainability and autonomy of vehicles, paving the way for greener and more integrated solutions in automotive design.

4.3. Wind Harvesting Systems

Shamselding [94] presented a novel combination of model reference adaptive control (MRAC) with several types of PID controllers, including PID, fractional order PID (FOPID), and nonlinear PID (NPID), optimized using a new COVID-19 algorithm. The study’s primary objective is to improve an electric–wind vehicle’s performance and dynamic response (Figure 24). This vehicle design includes a wind turbine installed at the front to capture wind energy, convert it into electricity, and store it in a backup battery, allowing the vehicle to continue moving while the primary battery charges. The paper compares different control algorithms applied to this new vehicle model, focusing on enhancing vehicle dynamics, including minimizing overshoot, rise time, and settling time. MRAC with the NPID compensator offered superior performance among the various controllers, compensating for nonlinearity factors such as air resistance and wheel friction. The study demonstrated the effectiveness of this approach in optimizing the vehicle’s energy efficiency and dynamic response.

Khan et al. [95] presented an approach for harnessing energy from a small-scale wind turbine mounted on EVs to support Internet of Vehicles (IoV) applications. Figure 25 shows the electric circuit of the harvester system. The aim is to reduce EVs’ reliance on the national grid by using renewable energy, specifically wind energy, for power generation. A wind turbine is integrated into the vehicle’s design to capture energy during vehicle mobility. The harvested energy is processed through a regulation circuit to supply the vehicle’s battery or supercapacitor. Aerodynamic analysis using ANSYS workspace 16.0 software is employed to determine the optimal orientation for maximum energy conversion efficiency. The study evaluates the feasibility of charging EVs at speeds ranging from 40 to 90 km/h, focusing on developing a DC voltage regulator to optimize input voltage ranges.

Zhao et al. proposed an adjustable Savonius vertical-axis wind turbine (SVAWT) to harvest wind energy for electric vehicles [96]. The SVAWT consists of three parts: an energy absorption module, an energy recovery module, and an energy conversion module (Figure 26). The energy absorption module has four blades with a staggered distribution, whose overlap ratio can be adjusted based on wind speed for higher energy efficiency. The energy recovery module adjusts the blade overlap without interruption using self-rotation and orbital revolution. The energy conversion module transforms mechanical energy into electricity, which is then supplied to the vehicle’s battery through a voltage regulator. The study shows that the power absorbed by the blades increases with wind speed, with RMS values ranging from 3.9 W to 7.1 W under different operating conditions. The proposed SVAWT demonstrates high energy recovery potential, stability, and adaptability, providing a viable solution for wind energy harvesting to support electric vehicles.

4.4. Comparative Analysis and Limitations Related to Energy Harvesting Applications from Solar, Wind, and Hybrid Solutions

The literature reveals considerable diversity in energy harvesting approaches and system designs and their demonstrated effectiveness for practical applications. For example, Park et al. [88] focus on self-sustaining systems in automobiles using computer-proximal energy harvesting, whereas Kim et al. [89] examine the synergy between piezoelectric energy harvesting and thermoelectric power for enhanced energy conversion. Some studies, such as Yadav et al. [90], aim to improve the fuel efficiency of HDVs by integrating several energy recovery technologies. The growing demand for sustainable transportation has driven research into innovative vehicle energy recovery systems. Technologies such as thermoelectric, piezoelectric, and photovoltaic generators offer promising solutions by converting waste energy into usable power. Recent studies have explored various configurations, from mounting generators on exhaust pipes and car roofs to embedding them in vehicle suspensions and pavements. Each approach presents unique advantages but faces constraints related to energy conversion rates, material performance, and environmental conditions. The combination of multiple energy harvesting methods has been investigated to enhance overall efficiency.

Table 3. The main comparison features for hybrid, solar, and wind harvesting systems.

Reference	Developed System	Harvesting Technology	Application Point	Specific Design Features	Performance
J.W. Park et al. [88]	Energy recovery for powering sensors and electronic accessories	Electric generator	Thermoelectric generator attached to the exhaust pipe and the wind fan on the front bumper	Electronic devices: parking assistant (RearSense) powered by a thermo-electric unit, pedestrian display powered by wind fan	RearSense: 3900 μWh for 20 °C heat difference; PedDisplay: 1700 μWh for 15 mph wind speed
S.-B. Kim et al. [89]	Hybrid generator with a TEG and piezoelectric system	Electric generator	Right-side engine head	Piezoelectric cantilever beam mounted on a TEG for amplifying power generation	7.619 mW and 0.5 g vibrational source; increment > 50% compared to no vibration
D. Yadav et al. [90]	Energy recovery from TEGs, regenerative braking, and piezoelectric solution	Electric generator	TEG on turbocharger exit and exhaust pipe; regenerative braking on braking system; piezoelectric on suspensions	Integration of multiple energy recovery methods	TEG: 1068 W, piezoelectric system: 332 W; breaking system: increment of generated current: 157% (low braking intensity), 238.72% (high intensity)
Park et al. [91]	Energy recovery from solar PV panels	Electric generator	Four sides of the vehicle: roof, rear window, left door, and right door	PV photovoltaic power generation modules attached on the car and tested with various inclinations and azimuths	0.0158 kWh for a single drive (approximately 10 min) and 221 kWh for one year (considering six hours a day)
Z. Jin et al. [92]	Energy recovery from solar PV panels	Electric generator	Car roof	Foldable scissors mechanism for efficient energy conversion (PVPGM, photovoltaic power generation module)	Max output power: 1.736 W, max. electricity transfer efficiency: 57.7%
M.A. Shamseldin [94]	Energy recovery from wind fan	Electric generator	Car front bumper	Combination of model reference adaptive control (MRAC) with different types of PID for catching the wind in the opposite direction of a moving vehicle	Recovery of 59.26% of the total wind power under ideal conditions
Z.A. Khan et al. [95]	Energy recovery from wind fan	Electric generator	Car front bumper	Wind fan integration for power recovery, reducing dependence on charging	Power: 100 W at a speed of 90 mph
Z. Zhao et al. [96]	Energy recovery from wind turbine	Electric generator	Vehicle’s front grille	Savonius vertical-axis wind turbine (SVAWT)	Blades’ absorbing power: 3.9 W ÷ 7.1 W (varies with wind speed)
C.K. Mytafides et al. [93]	Hybrid TEG- and DEEC-based energy harvester	Electric generator	Possible application on the external surface of the vehicle body	TEG and DSSCs within CFRP composites	Power density of 6.4 mW/cm2

reports a comparison of the main features of hybrid, solar, and wind harvesting systems.

Future challenges in vehicle energy recovery focus on improving efficiency, feasibility, and large-scale adoption, requiring advancements in energy conversion technologies. Optimizing thermoelectric, piezoelectric, and photovoltaic systems is essential, demanding innovative designs and high-performance materials. Developing lightweight, high-conductivity, and durable materials is key to enhancing long-term reliability while ensuring seamless integration within limited vehicle spaces. This integration, however, presents engineering constraints, necessitating smart design solutions to avoid compromising vehicle performance. Environmental factors such as temperature fluctuations and sunlight variability also impact energy consistency, making efficient storage systems crucial. A stable power supply can be ensured even under suboptimal conditions by mitigating these issues. Overcoming these challenges will drive the advancement of energy recovery technologies, fostering more sustainable and efficient transportation systems.

5. Harvesting Methodologies Implemented by Car Manufacturers in Vehicles

In recent years, energy harvesting in commercial vehicles has become an area of strong interest to improve energy efficiency and reduce fuel consumption. Several energy harvesting solutions have already been implemented, exploiting the motion, heat, and vibrations generated during vehicle operation. The main harvesting technologies implemented on vehicles on the market are as follows:

• Regenerative braking, widespread in electric and hybrid vehicles, allows the recovery of kinetic energy during deceleration, reducing fuel consumption and improving battery autonomy;
• Suspension harvesting systems use electromagnetic generators to transform suspension vibrations into electrical energy to power onboard electronic components;
• Exhaust heat recovery is mainly used in ICE vehicles. These models use TEG systems made of thermoelectric materials to convert waste heat into electricity;
• Integrated solar panels on the roof to power auxiliary systems such as air conditioning and lighting, reducing the load on the main battery.

Harvesting solutions have brought interesting benefits, such as greater energy efficiency by reducing energy waste and emissions and improving vehicles’ ecological impact. These technologies extend battery life by reducing the number of charge cycles and operating costs, as lower consumption and greater autonomy mean lower management costs for corporate fleets and heavy transport. The future evolution of commercial vehicles will focus on increasing the efficiency of recovery systems, integration with electric vehicles, and optimizing innovative materials to reduce cost and weight. The main harvesting solutions introduced by car manufacturers are described below.

5.1. Regenerative Braking and Mechanical Energy Harvesting in Commercial Vehicles

Mechanical energy harvesting is an innovative solution adopted by car manufacturers to improve energy efficiency in fully electric or hybrid models while reducing environmental impact by cutting emissions. The main technologies used are regenerative braking and energy recovery from vibrations. Energy recovery through regenerative braking is the first recovery method adopted by car manufacturers and is now widely used in electric and hybrid vehicles. With this methodology, the electric motor acts as an electric generator, exploiting inertia during deceleration without pressing the brake pedal. In this way, the energy produced is stored in the battery, as shown in Figure 27. Furthermore, thanks to advanced speed control algorithms, the traction motor controllers generate specific three-phase voltage and current waveforms, allowing the motors to slow down the vehicle gradually. This process reduces wear on traditional brakes and uses the counter-electromotive force generated during braking to produce electrical energy [97].

Each car manufacturer implements regenerative braking in a slightly different way, varying the energy recovery mode, the driving feel, and the integration with the traditional braking system:

• Tesla uses one-pedal driving technology, moving towards driving with just one pedal. This technology allows for high regeneration when the accelerator is released, reducing the need for mechanical brake use. In the latest models, it is no longer possible to manually adjust the intensity of the regeneration, but the system adapts the braking based on the battery and driving conditions.
• Toyota (hybrid and electric) focuses on a balance between regeneration and mechanical braking. Hybrid models such as the Toyota Prius combine regenerative and mechanical braking for a smooth and natural ride. In these models, regeneration is less aggressive than in pure EVs, which ensures a smoother transition between the two systems (progressive regenerative braking).
• BMW (i3, i4, iX, etc.) has created a system that can select the level of regeneration; in fact, many BMW models offer different regeneration settings, allowing the driver to choose between more aggressive braking or one more similar to a traditional car. In addition, the adaptive range automatically adjusts the intensity of the regenerative braking based on traffic conditions and the road.
• Volkswagen (ID.3, ID.4, etc.) has implemented the B (brake) mode in its vehicles, which increases regeneration and allows for driving like one-pedal driving. The D mode provides lighter regeneration, and the car decelerates more due to inertia.
• Hyundai and Kia (IONIQ 5, EV6, etc.) have implemented the i-Pedal system, which allows driving using only the accelerator pedal, with strong regeneration when released. These vehicles offer multiple regeneration levels, selectable via the paddles on the steering wheel. Finally, the intelligent regenerative braking of these models automatically adapts energy recovery based on the distance from other vehicles and the route type.
• Mercedes-Benz (EQE, EQS, etc.) offers different regeneration modes. The EQ range allows you to select various levels of regenerative braking, up to ‘D Auto’ mode, which automatically manages recovery depending on the traffic. The Mercedes recovery system integrates with advanced driver assistance systems (ADAS) to optimize braking and energy recovery.
• Porsche (Taycan) uses a more discreet regenerative braking system to provide a driving experience similar to a traditional sports car; the regeneration is activated only by operating the brake pedal.

The differences in regenerative braking systems depend on the engineering choices of each car manufacturer. Some brands (Tesla, Hyundai, and BMW) push towards a one-pedal driving experience, while others (Toyota and Porsche) aim for a smoother transition between regenerative and mechanical braking. The ability to customize the level of regeneration and integration with ADAS systems are key factors that vary between models.

Regarding energy recovery from vibrations, BMW has filed a patent describing a system that recovers energy from the suspension movement that the shock absorbers would otherwise dissipate [98]. The system consists of an electric generator mounted on a frame with a flywheel and a one-way clutch operated by a disc connected to the suspension control arm via an actuator similar to a stabilizer bar, as shown in Figure 28. The system transfers motion to the electric generator during the descending phase of the suspension by exploiting the energy stored by the spring and thanks to the one-way clutch. Furthermore, the one-way clutch during the motion transfer activates a small gear mechanism that accelerates the generator’s flywheel. It is preferable to use the expansion stroke of the spring since the compression phase, in most cases, is sudden, and the system must absorb as much energy as possible in the shortest possible time. This energy can then be stored in the car’s normal 12 V electric battery or in the high-voltage traction batteries of an electric vehicle [99]. This recovery system will be installed soon in top models such as the BMW i7.

5.2. Thermal Harvesting Systems in Commercial Vehicles

Car manufacturers are still investigating thermal harvesting, and the European Union has spearheaded this by funding the “Power-Driver” project [100], which aims to enable large-scale energy recovery from waste heat in Europe. The project targets the transport sector, which is responsible for a quarter of the continent’s total greenhouse gas emissions. It aims to recover waste heat from hot gases generated by ICEs. A TEG is a device that converts heat directly into electrical energy. For over four decades, BMW Group engineers have perfected a technology that NASA initially used to power space probes. This technology is based on the Seebeck effect, which generates an electrical voltage between two thermoelectric semiconductors at different temperatures. The BMW i3 uses advanced thermoelectric generators to convert waste heat from the exhaust system into electrical energy [10]. This technology improves the vehicle’s energy efficiency by 5–10%, extending its range and reducing energy consumption. In addition to making the i3 more economical and environmentally friendly, the innovation represents a step forward in energy management in electric vehicles, highlighting BMW’s commitment to sustainable and efficient solutions. In the past, the efficiency of TEGs was too low to make them suitable for automotive applications. However, recent advances in materials research have significantly improved the performance of TEG modules, making them more competitive and applicable to the automotive sector. Several car manufacturers, including Volkswagen, Volvo, Ford, and BMW (in collaboration with NASA), have developed waste heat recovery systems based on this technology. These thermoelectric systems promise improvements in vehicle efficiency, with estimated fuel savings of between 3% and 5%. In addition, the power generated by such devices could reach up to 1200 W, representing a significant step forward in the energy optimization of engines [101].

5.3. Solar Harvesting in Commercial Vehicles

With the growing interest in renewable and sustainable energy systems, the energy harvesting paradigm has extended to solar energy systems, leading some automotive companies to produce mass-produced vehicles that integrate photovoltaic cells. The term solar panel vehicle generally describes electric vehicles that incorporate photovoltaic cells into their design to convert solar energy into electrical energy and store it in the battery.

Regarding solar harvesting, Toyota and Hyundai have launched two vehicle models with a solar panel in their bodies since 2017. The Prius Plug-in (2017 Toyota Hybrid) was among the first commercial vehicles to include a solar roof (Figure 29a). The photovoltaic panels generate electricity during exposure to the sun, storing it in the auxiliary battery. The panels provide up to 180 W in optimal conditions, enough to travel about 5 km per day (equivalent to 1800 km per year) [102]. With the new Prius Plug-in model proposed in 2023 (prototype shown in Figure 29b), autonomy has been further extended, reaching approximately 9 km per day (approximately 3200 km per year) [103]. The recovered power is used to power the electronic systems or improve the main battery’s autonomy. Finally, using panels with the latest-generation photovoltaic cells (with an efficiency of 34% compared to 22.5% for the panels installed as standard) and a larger exposure surface, a power of 860 W and an electric range of 44.5 km are achieved [104]. Toyota’s BZ4X electric SUV (sport utility vehicle) also offers a solar roof as an option, enough to add around 1800 km of extra range per year [105]. This system contributes to the vehicle’s autonomy and to the powering of secondary systems (air conditioning and infotainment).

Hyundai entered the market by offering two models, the Sonata Hybrid and Ioniq 5, that integrate solar roofs capable of storing solar energy into the bodywork. The Sonata Hybrid (2019), in optimal exposure conditions, generates 205 W of energy, adding up to 1300 km per year (4 km/day) depending on use and climatic conditions (Figure 30) [106]. The solar roof can power the main and secondary batteries, contributing to the reduction of consumption. The Ioniq 5 model (2022) also includes an optional solar roof to support the main battery. Over a year, the panels can add a range in optimal conditions of 1200 to 1500 km, taking advantage of extended exposure to sunlight [107].

Squad Mobility BV (Breda, The Netherlands) launched the Squad Solar City Car onto the market in 2023 (Figure 31), a fully electric compact vehicle designed to comply with the UE (L6 and L7) and US LSV regulations [108]. The city car mounts a solar panel on the roof capable of providing a range of 22 km to 31 km, enough to cover the 12 km usually traveled in the city. The Lightyear 0 vehicle, produced by the company Lightyear (Venray, The Netherlands), integrates a total surface area of 5 m² of double-curved solar panels on the body (Figure 32), capable of guaranteeing approximately 70 km of autonomy per day (approximately 11000 km/year) with a power of 1.05 kW [109].

Another automotive company, Edison Future (Anaheim, CA, USA), has brought to market two heavy-duty vehicles that integrate a solar harvesting system: the EF1-T and the EF1-V [110]. The EF1-T Pickup Truck (Figure 33a) is a pickup truck with a retractable solar roof that extends to cover the entire pickup box (Figure 33b), providing up to 15–25 miles (24–40 km/day) of additional range on sunny days. [111]. The EF1-V Delivery Van is a multipurpose electric vehicle designed to adapt to different needs (Figure 33c), based on the same chassis and platform as the EF1-T pickup.

Table 4. Additional autonomy of vehicles with solar roofs from major car manufacturers; the reported autonomy values depend on exposure times to the sun and geographical locations.

Car Manufacturer	Model	Year	Extended Autonomy
Toyota (Toyota, Japan)	Prius Plug-in Hybrid	2017	5 km/day (~1800 km/year)
	Prius PHEV	2023	9 km/day (~3200 km/year)
	BZ4X	2022	1800 km/year
Hyundai (Seoul, Republic of Korea)	Sonata Hybrid	2019	~1460 km/year
	Ioniq 5	2022	1200–1500 km/year
Squad Mobility BV (Breda, The Netherland)	Squad Solar City Car	2023	22–31 km/day
Lightyear (Venray, The Netherland)	Lightyear 0	2022	70 km/day
Edison Future	EF1-T Pickup Truck	2025	24–40 km/day
(Anaheim, CA, USA)	EF1-V Delivery Van	2025	N.A.

reports vehicles equipped with solar harvesting systems, highlighting the manufacturer, models, year of release on the market, and extension of autonomy.

These systems are particularly effective in sunny regions, but their contribution to range remains limited compared to the capacity of the main batteries. However, they represent a step towards greater sustainability and energy optimization in modern vehicles.

Another solution for solar harvesting is to use external accessories. In this regard, the company GoSun (Cincinnati, OH, USA) has introduced the EV Solar Charger Deposit to the market, a portable charger for electric vehicles that allows you to recharge your car using solar energy (Figure 34). The device consists of a removable box containing foldable solar panels that, once laid out on the vehicle, can provide electrical power of up to 1100 W, contributing to an increase in autonomy of about 48 km per day. The device weighs about 32 kg and can be mounted on the vehicle’s roof rack. [112].

Mercedes-Benz has unveiled new research and future technologies, including a solar paint designed to generate power for vehicles, as shown in Figure 35. This photovoltaic (PV) coating consists of ultra-thin (5 μm) solar modules that are seamlessly bonded to the car body. The nanoparticle-based protective layer allows 94 percent of the sun’s energy to pass through. The solar paint has an efficiency rate of 20%, and a vehicle with a solar panel area of 11 m² could generate enough energy to travel up to 12,000 km per year under ideal conditions. In Stuttgart, it could power 62% of a 52 km commute, while in Los Angeles, it could cover 100%. The paint contains no rare earths or silicon, is made from non-toxic and recyclable materials, and is cheaper than traditional solar modules [113].

6. Thermal, Mechanical, and Environmental (Solar/Wind) Harvesting Technologies in a Horizontal Comparison

Mechanical and thermal energy are two forms of energy that differ in their characteristics and ways of manifestation. The first is the energy associated with the movement and position of an object; the other is the internal energy of a system due to the movement of the particles that compose it and the temperature and agitation of the molecules in a material. The comparison just described highlights the intrinsic difference between the two energy sources. Furthermore, exploiting environmental sources such as solar and wind energy requires different architecture and management. In this section, a horizontal comparison is performed between the various architectures analyzed.

The harvesting technologies described in this manuscript can be compared based on some characteristic parameters such as operating principle, energy efficiency, costs, scalability, and environmental impact, as highlighted in

Table 5. Horizontal comparison between mechanical, thermal, and environmental (solar/wind) harvesting technologies.

Key Parameters	Mechanical Harvesting	Thermal Harvesting	Environmental (Solar/Wind) Harvesting
Operating principle	Recovery of kinetic energy dissipated by the vehicle.	Recovery of waste heat produced by the vehicle.	Production of electrical energy from sunlight and the air flow produced by the moving vehicle.
Energetic efficiency	Mechanical harvesting architectures have the highest efficiency among harvesting technologies, recovering between 50–70% of the energy dissipated by the vehicle.	TEGs’ efficiency ranges from 5–10% and is limited by the materials, the physics of the conversion process, and operating conditions. Instead, ORC systems reach an efficiency range of 10–25% depending on the circuit’s thermodynamic losses, the working fluid, and the system design.	Commercial PV panels have a conversion efficiency that varies from 15 to 22%, even reaching 34% with the latest generation, and depends on the solar incidence and the environmental conditions. Wind harvesting reaches 30–45% efficiency but depends greatly on size, airflow, and installation location.
Costs	Low installation cost. High maintenance costs.	High installation cost. Low maintenance costs.	Low installation and maintenance costs for solar harvesting. High installation cost and low maintenance costs for wind harvesting.

.

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Operating principle

Thermal harvesting exploits the temperature difference to generate energy through TEG or ORC systems, which is ideal for applications with constant heat sources. The first convert the heat dissipated by the engine, exhaust system, or brakes to generate electrical energy, while ORC systems recover thermal energy through appropriate heat exchangers, using a fluid with a low boiling point, high thermal stability, and good heat exchange capabilities (such as cyclopentane, toluene, ethanol, R245fa, etc.). Mechanical harvesting is based on kinetic energy generated by vibrations or mechanical movements to produce energy by piezoelectric generators, electric generators, or electromagnetic systems integrated into suspension. This is suitable for significant vibrations, such as on heavy vehicles and uneven roads. Solar harvesting is based on PV panels installed on cars to convert sunlight into electrical energy, while automotive wind harvesting involves using small wind turbines installed on the vehicle at strategic points to capture the maximum available airflow.

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Energy efficiency

The efficiency of thermal harvesting architectures depends, by their nature, on the temperature difference and the quality of the materials used for energy recovery. In fact, the conversion efficiency of TEGs is between 5–10%, while ORC-based harvesting methods can reach an efficiency between 10–25%. In both architectures, the temperatures of the heat sources are in the range of 90–400 °C [114]. In the case of mechanical harvesting, efficiency exceeds 65% but varies with vibration intensity (amplitude) and frequency; piezo materials, for example, can convert a good part of the mechanical energy but are limited by the need for constant vibrations. In solar harvesting, the efficiency of commercial PV panels generally ranges between 15% and 22% and depends on external factors such as the incidence of sunlight, weather conditions, and the vehicle surface area available. The efficiency of wind turbines depends on the design and wind speed and have an efficiency ranging from 30% to 45%; however, the speed and direction of the wind can limit their effectiveness in the automotive field.

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Cost, scalability, and environmental impact

From a cost perspective, the elements used in thermal harvesting architectures are more expensive to produce and integrate. Still, once installed, they require less maintenance and are potentially more sustainable because they exploit the heat that would otherwise be lost into the air. However, the production of TEGs may involve the use of rare materials. Electromechanical harvesting systems are less expensive but require more maintenance due to the wear of mechanical components. The scalability of these systems depends on the ability to integrate harvesting systems into the vehicle. For solar collection, the cost of photovoltaic panels has decreased in recent years; however, integrating energy management systems and batteries on the vehicle’s surface can increase the final cost. Solar systems can be easily scaled, as more panels can be added based on available space and energy needs without compromising the aesthetics or aerodynamics of the vehicle. Although materials and industrial processes used in constructing solar panels may influence the environment, solar energy has a lower environmental impact than fossil fuels.

The costs of integrating wind turbines into vehicles can vary significantly. Automotive wind turbine technology is still in its infancy, and costs can be high relative to the energy benefits. Wind turbines are more complex than solar panels for vehicle integration because of space constraints and the vehicle’s aerodynamic design. Wind energy is a renewable energy source with a low environmental impact during operation.

7. Conclusions

Energy harvesting in the automotive sector represents a promising strategy for enhancing vehicle efficiency and sustainability by capturing and converting wasted energy from mechanical, thermal, and environmental sources. This paper is structured into sections, each dedicated to a specific type of energy harvesting, including mechanical systems (such as regenerative suspensions and piezoelectric technologies), thermal systems (such as TEGs and ORC systems), and hybrid solutions that integrate solar, wind, and mechanical–thermal recovery. Each section includes a detailed review of the technical and scientific state of the art, followed by a comparative analysis supported by specific tables containing criteria for comparison. These comparative analyses allow for a clear mapping of the strengths of each solution and highlight the challenges and future developments required to improve harvesting technologies. Furthermore, a dedicated section on commercial automotive solutions illustrates how current scientific research meets the needs of the automotive market, showing how research findings are translated into implemented and implementable solutions. The valuable contribution of this study lies in its structured approach to mapping the state of the art, identifying not only the current capabilities of energy harvesting technologies but also the future challenges that must be addressed to enhance their efficiency and integration further. By addressing these challenges, energy harvesting technologies can play a pivotal role in transforming the automotive industry, reducing dependence on traditional energy sources, and contributing to developing more sustainable and energy-efficient transportation systems, ultimately paving the way for a greener automotive future. In recent years, the adoption of EVs has accelerated significantly, helping to reduce the environmental impact of the transportation sector. According to recent data, global sales of electric vehicles exceeded 10 million in 2022, representing approximately 14% of the global automotive market. This shift has reduced CO₂ emissions; it is estimated that if 30% of new sales were electric by 2030, approximately 1.5 billion tons of greenhouse gas emissions could be avoided. Furthermore, using EVs helps improve air quality in urban areas, with an estimated 30% reduction in emissions of harmful air pollutants. However, it is important to consider that the overall environmental impact also depends on the energy source used to charge these cars: electricity produced from renewable sources can further reduce the carbon footprint of electric vehicles.