A Comprehensive Review of Physical Models and Performance Evaluations for Pavement Photovoltaic Modules

Abstract

Pavement photovoltaic (PV) is an innovative energy-harvesting technology that seamlessly integrates into road surfaces, merging established PV power generation methods with conventional roadway infrastructure. This fusion optimally utilizes the extensive spatial assets inherent in road networks. This paper offers an exhaustive examination of the literature concerning the physical models and performance evaluation of photovoltaic pavements. This study delineates the essential three-tier structure of pavement modules and juxtaposes the advantages and drawbacks of design models across these strata, thereby facilitating the development of more suitable solutions for varying application scenarios. The importance of accommodating fluctuations in shadows and countering the heat island effect (HIE) is emphasized. Nevertheless, the technology remains in its nascent research phase, characterized by challenges associated with limited long-term durability and efficacy. Building upon these findings, this study addresses the challenges confronting pavement PV from three perspectives and outlines future prospects and recommendations for its progression.

1. Introduction

The excessive use of fossil fuels has led to an escalating threat of energy shortages and environmental damage, presenting a significant challenge to human survival [1,2]. China has announced plans to increase investment in renewable energy, setting ambitious goals to reach the peak of carbon emissions by 2030 and achieve carbon neutrality by 2060 [3]. This vision has driven the rapid deployment of renewable energy, particularly photovoltaic (PV) technology [4,5,6].

As an infrastructure, the construction, development, and function of roads play a pivotal role in advancing more sustainable development [7,8,9,10]. However, further increasing PV capacity in urban areas poses challenges. Taking Hong Kong as an example, by 2050, solar energy applications are expected to account for 1–2% of energy production [11]. However, the space available for PV installation in metropolitan areas is limited. In addition to installing PV panels on the roofs and facades of urban buildings, the integration of PV technology into traffic roads has gradually attracted attention. Taking into account the relatively large area of the road and using China’s data for 2021 as an example, the road area per 100 square kilometers is about 55 square kilometers [12]. Due to the relatively low utilization of PV panels during the day, this provides a greater opportunity to install PV panels on the road.

In the realm of solar energy collection, the practice of harvesting solar energy from roads has emerged as a topic of considerable interest, receiving extensive study over the past five years. Traditional off-grid PV systems are often installed on roadside poles [13,14]; however, more and more studies are now directly integrating PV panels into the road, which not only provides electricity for road lighting but also provides basic sensors and controllers for intelligent transportation infrastructure [15,16]. Pavement PV can not only be used as renewable power suppliers but can also be integrated into distributed energy systems of smart grids, such as the wireless charging of electric vehicles [17], and provide application scenarios for next-generation PV cells using new coatings [18,19]. In addition to electricity gains, pavement PV can also mitigate the heat island effect (HIE), and its thermal application potential is similar to road snowmelt [7,20]. Similar to PV tiles, pavement PV modules also have additional advantages such as piezoelectric and thermoelectric energy harvesting when combined with traditional asphalt concrete roads [21,22,23].

As a part of the road structure, the improved PV module not only has the function of power generation but also provides support for vehicle traffic. The primary advantage of this technology is that it does not require additional land. It is anticipated that the development of pavement PV will yield significant benefits in both economic and environmental domains. Previous studies indicate that large-scale asphalt pavement is a significant contributor to the urban heat island effect [24,25], while pavement PV has been shown to effectively mitigate this phenomenon [26]. By combining pumped storage [27], batteries [28], supercapacitors [29], and other energy storage technologies [30], its overall performance can be further improved. Additionally, it is anticipated that it will also be combined with upcoming transportation technologies such as snowmelt [31], wireless charging [17,32], and driverless technology [33] with pavement PV in the future, aiming to create a more environmentally friendly and intelligent road system.

The continuous advancement of PV technology and decreasing costs have facilitated the development of pavement PV [34,35]; however, research related to this technology is still in its early stages. Most studies are still limited to the laboratory sample stage. At the same time, researchers pay more attention to the composition and arrangement of physical models, with less emphasis on discussing various aspects of evaluation methods and results. Hence, it is highly significant to review the early research in this field, introduce the latest progress, point out the existing obstacles, and propose prospects, and future development directions.

This study will furnish a synopsis of the contemporary research status of pavement photovoltaics. After a brief introduction to the current application of photovoltaic technology on roads, the Section “Influencing Factors on PV Pavement” analyses the impact of shadow on photovoltaic power generation, the impact of the heat island effect, and the impact of different materials. In the Section on “Performance Evaluation”, the evaluation of the technology is summarized, including modular characteristics, power generation, and application benefits, and the corresponding evaluation methods are given. On the basis of the above literature review, the “Challenges and Prospects” Section puts forward the future challenges and prospects of road PV from three aspects.

2. The Current Situation of PV Technology on the Road

2.1. PV Module

Figure 1 depicts the electrical equivalent circuit of a PV cell using a single-diode model [36]. This model serves as the basis for deriving the engineering representation of PV cells; its math model is demonstrated in Equation (1).

$$I_{\mathrm{PV}}=I_{\mathrm{ph}}-I_d\left[\exp \frac{q\left(U_{\mathrm{PV}}+I_{\mathrm{PV}}{R}_{\mathrm{s}}\right)}{Ak{T}_{\mathrm{c}}}-1\right]-I_{\mathrm{sh}}$$

(1)

where U represents the output voltage generated by the PV cell, I_ph is the photocurrent, I_d is the current flowing across the diode, I is the output current of the PV cell, and R_s and R_sh represent the equivalent series resistance and parallel resistance, respectively.

PV cells are characterized by parameters solely under standard test conditions (STC). Thus, it becomes essential to formulate an equivalent engineering model for PV cells that aligns with the needs of practical applications. This is demonstrated in Equation (2).

$$\left\{\begin{array}{l}I=I_{\mathrm{sc}}\{1-C_1[\exp (\frac{U}{C_2{U}_{\mathrm{oc}}})-1]\}\\ C_1=(1-\frac{I_{\mathrm{m}}}{I_{\mathrm{sc}}})\exp (-\frac{U_{\mathrm{m}}}{C_2{U}_{\mathrm{oc}}})\\ C_2=(\frac{U_{\mathrm{m}}}{U_{\mathrm{oc}}}-1)/[\ln (1-\frac{I_{\mathrm{m}}}{I_{\mathrm{sc}}})]\end{array}\right.$$

(2)

where U_oc, I_sc, U_m, and I_m represent the open-circuit voltage, short-circuit current, maximum power point (MPP) voltage, and MPP current under STC, respectively; meanwhile, I represents the output current, and U represents the output voltage. To enhance the accuracy of the model, relevant parameters are adjusted. The correction formula is as follows:

$$\left\{\begin{array}{l}{\tilde{I}}_{\mathrm{sc}}=I_{\mathrm{sc}}\frac{G}{G_{\mathrm{ref}}}(1+a\Delta T)\\ {\tilde{U}}_{\mathrm{oc}}=U_{\mathrm{oc}}\ln (\mathrm{e}+b\Delta G)(1-c\Delta T)\\ {\tilde{I}}_{\mathrm{m}}=I_{\mathrm{m}}\frac{G}{G_{\mathrm{ref}}}(1+a\Delta T)\\ {\tilde{U}}_{\mathrm{m}}=U_{\mathrm{m}}\ln (e+b\Delta G)(1-c\Delta T)\end{array}\right.$$

(3)

where a, b, and c are the compensation coefficient, and a = 0.0025/°C, b = 0.0005 m²/W, c = 0.0028/°C. Variables with a wavy superscript indicate values under non-STC. Here, G_ref represents the reference irradiance, and G represents the current irradiance. The symbol ΔT denotes the difference between the current temperature and the reference temperature.

2.2. Pavement PV

Pavement PV power generation technology is an innovative practice that intersects the fields of PV power generation and traffic engineering. Compared with traditional PV power generation projects, pavement PV technology fully utilizes pavement space resources without occupying additional land or emitting pollution [37]. Furthermore, it can be consumed locally, effectively addressing the energy structure optimization challenge of PV power generation and local consumption [38]. As a new trend in the comprehensive utilization of PV power generation, “PV + highway” is in the embryonic stage of development [39].

PV power generation technology is used in the ”four world firsts” on roads or pavements [37], as shown in Figure 1. In 2014, Scott and Julie Brusaw built the world’s first pavement PV parking lot (Figure 2a). The Netherlands Institute of Applied Sciences installed the world’s first 70-meter PV bicycle lane in 2014, generating approximately 3000 kWh of electricity in the first half-year after completion (Figure 2b). In 2016, the world’s first “solar highway” was officially put into use in the town of Opertsche in Touruvres, northwest France. This is the first “solar highway” in the world that can be used for motor vehicles. This 1-kilometer-long highway generates electricity at an average rate of nearly 800 kWh per day. It generates about 280,000 kWh of electricity, which is sufficient to power the daily public lighting of a small town with a population of 5000 (Figure 2c). On 28 December 2017, Shandong Guangshi Energy Co., Ltd. completed the world’s first 1080-meter-long PV highway test section located on the south line of the Jinan Ring Expressway, with an annual power generation of about 1 million kWh (Figure 2d).

2.3. Pavement PV Module

In 2012, Tighe et al. developed a three-layer solar panel with a numerical model, comprising a transparent layer, an optical layer, and a base layer [40,41,42]. In the study, the thickness of each layer was determined, and a suitable grid was designed to ensure that the 125 mm solar panel had adequate slotting space; however, the overall size of the solar panel has not been thoroughly discussed. In 2016, Zha et al. proposed a numerical hollow-plate element structure for PV pavement. The new structure consists of three layers, namely, a polymethyl methacrylate (PMMA) transparent protective layer, a solar cell layer, and a prefabricated concrete hollow base [43]. In this study, they used ANSYS to analyze the following four factors of solar panels: (i) length, (ii) width, (iii) sidewall thickness, and (iv) PMMA-layer thickness. By comprehensively considering the mechanical properties, safety, and economy, the recommended overall structural optimization scheme is obtained; however, these four factors are not prioritized in this study, which requires further research to provide guidance for future system optimization.

Two solar panel models were developed by Dezfooli et al. in their study [44], where the PV panel was embedded between rubber and plexiglass panels or between double-layer porous rubber, respectively. They tested their energy production efficiency, surface safety ratings, and structural stability.

Ma et al. [45] explored the viability of solar sidewalks in Hong Kong, and they designed square solar PV floor tiles suitable for pedestrian use. The floor tile’s structure comprises non-slip tempered glass, solar cells, and supporting elements made of tempered glass. The authors evaluated the electrical, thermal, and mechanical properties through laboratory and field tests. The results demonstrate the solar floor’s outstanding performance in energy conversion, skid resistance, heat resistance, and compressive strength.

Li et al. [46,47] developed translucent concrete by integrating recycled tempered glass and epoxy resin. The results indicate that the light transmittance of the material increases with the grain size of the blend. They indirectly determined the curing time of concrete and assessed the strength variation within the initial five days. The compressive strength increases with the duration of curing; there was a significant increase in compressive strength from day one to day two.

Vizzari et al. [48,49,50] put forward a composite road surface design that integrates solar roads with water-permeable surfacing. This system comprises a semi-transparent layer, solar cells, a permeable layer, and a dense-graded asphalt base. The top layer—composed of transparent polyurethane-bonded glass aggregates—is translucent, while the porous layer functions as a solar heat gatherer for the flow of heat-transfer media. The workability of solar pavement was assessed in terms of road safety under both dry and wet conditions. The study investigated the curing time and viscoelastic properties of four distinct types of thermosetting polyurethane. The results indicate that the properties of thermosetting polyurethane, particularly the glass transition temperature, significantly influence the curing temperature. Subsequently, Vizzari et al. [51] conducted an in-depth analysis of the translucent surface layer. They utilized the factorial design method to investigate how various variables (including thickness, glue concentration, and glass aggregate dispersal) affect the mechanical and optical properties of the material. The results show that increasing the thickness of the glass aggregate layer and reducing the particle diameter will adversely affect the optical properties. Raising the polyurethane content enhances both the light-related and structural properties of the superficial layer; however, this content should not surpass 20% as it may diminish its anti-skid performance.

3. Influencing Factors on PV Pavement

3.1. Shadow Occlusion

Occlusion has a pivotal role in influencing the power generation efficiency of solar pavements. Given the unique application settings of solar pavements, attention must be given not only to the occlusion caused by nearby buildings and trees but also to the consequences of driving-induced shadows created by vehicle movement on the power generation of solar roadways [52]. Selvaraju et al. [53] examined the impact and load-bearing capacity of shadows on solar pavements. They discovered that the effect of driving shadows on the power generation of PV pavements is dynamic and heavily influenced by vehicle speed and length. Additionally, the load-bearing capacity is associated with the depth of the surface layer of the panel.

Concerning the behavioral features of traffic-generated vehicle shadows, Wu et al. [54] contended that traditional evaluation methods failed to fully consider the spatial correlation and dynamic obscuring of vehicles. This oversight has adversely affected the accuracy of PV pavement output power assessments and potentially undermined the safety and coordination of the power system. Liu et al. [55] introduced a novel approach for computing the obtainable solar irradiance in urban roadway environments, which is based on models encompassing streetscape, solar radiation, and traffic volume. This method has been validated in Boston, USA, and the findings reveal that the roads in Boston are capable of supporting the energy requirements of 763,100 electric vehicles operating annually within urban areas, thereby underscoring the immense potential of road power generation. It is noteworthy that the decrease in traffic volume solely led to a 5% reduction in solar radiation on the right side.

3.2. Heat Island Effect

Another key issue for PV modules is to mitigate the urban heat island effect; however, numerous studies have highlighted the significant impact of operating temperature on the power generation efficiency of solar cells. As the temperature increases, the power generation efficiency of solar cells tends to decrease [56]. Since a significant portion of solar radiation is converted into heat energy upon reaching the surface of the solar cell, the operating temperature of the solar cell often exceeds 50 °C and can even reach as high as 80 °C [57,58]; therefore, it is imperative to investigate the heat dissipation mechanisms of solar cells within pavement PV systems. To address the heat dissipation challenge of solar cells, the integration of PV technology with thermal technology offers an effective solution, namely, the PV/thermal (PV/T) system [59,60]. Although the research on solar PV thermal building-integrated systems (BIPVTs) is more extensive [61,62,63], there is a relative scarcity of studies exploring the application of PV/T systems in pavement contexts. Xie and Wang’s study alleviated the urban heat island effect by reducing the road surface temperature by 3–5 °C in summer [22]. Similarly, in [26,64], the heat dissipation effect of PV panel modules has also been clearly demonstrated. However, these experiments were carried out without considering the heat transfer effect of thick ground, so the results are relatively rough. These studies usually choose the type of asphalt or fixed-depth asphalt concrete pavement. Another research work by Zhou et al. [65,66] focused on the structural performance of PV panel modules. The structural design of PV panel modules is also an important research focus, including new sealant module structures [67]; self-compacting concrete [68]; adding steam chambers, water tanks, and sun visors [21]; hollow structures with water pipes [64]; and methods for mixing fine concrete (such as cement and fine aggregates such as sand) with optical fibers [69]. Toughened glass with a metal frame and moisture-proof layer performs well in terms of load-carrying capacity and cost and is therefore recommended for use [45].

3.3. Materials for Pavement Energy Harvesting

A significant number of studies have been conducted to enhance existing pavement materials and new materials have been proposed. There have been many achievements in this research, and it is still ongoing; however, in the following chapters, the materials directly or indirectly related to pavement energy harvesting will be reviewed and classified. The structure of the photovoltaic road surface is shown in Figure 3 [70].

3.3.1. Pavement PV Module Materials

In 2010, Kang-Won et al. conducted a feasibility assessment of installing solar panels on roads for solar energy collection. Their research found that thin film solar cells are not suitable for roads due to the large load-bearing period of roads and harsh environmental conditions; therefore, they proposed to develop new solar cells suitable for road energy harvesting [71]. To tackle this issue, Northmore and Tighe employed interlayers, reinforcements, and textured glass in 2012 to enhance the performance of solar panels [40]. In addition, George Washington University used onyx glass to create an accessible solar pavement channel [72]; however, the translucent glass layer restricts the power output of this walkable solar panel channel, limiting its overall efficiency [45]. In 2017, Dezfooli et al. conducted an evaluation of the power generation efficiency, structural performance, and safety aspects of solar panels and solar pavements. By incorporating novel solar cell layers and employing rubber pavements, they enhanced both the structural performance and power generation capabilities [44].

The thickness of the glass is a key parameter in the design of the transparent layer. For example, when designing a glass floor, it is necessary to leave a clear gap between the glass to reduce the tension of pedestrians walking on it; however, when it comes to solar pavement, a larger gap will result in reduced transmittance of sunlight. Therefore, solar panels typically consist of multiple layers of glass, ensuring that if one layer fails, the remaining layers can still sustain the design load. Therefore, many researchers have conducted research on the pressure-bearing capacity of pavement PV [73,74,75].

3.3.2. Pavement PV Materials

From the perspective of energy harvesting, the combination of piezoelectric, pyroelectric, thermoelectric, and PV materials into ordinary concrete can create a material called “energy harvesting concrete”. This type of concrete is capable of capturing and storing energy wasted by transportation infrastructure for reuse. Although the design and research of the pavement PV model is still in the preliminary stage, there are some issues in its practical application. Taking into account the unique characteristics of the aforementioned pavement PV models, the design evaluation can be divided into three main levels. By summarizing and comparing the advantages and disadvantages of diverse designs—combined with the needs of different practical application scenarios—a more suitable pavement PV solution can be obtained.

(1)

The surface–transparent layer

The materials utilized for the surface–transparent layer can be categorized into three primary types: tempered glass, reinforced resin (e.g., polymethyl methacrylate, commonly known as PMMA), and resin-bonded glass aggregate (see

Table 1. The pros and cons of designs in the surface–transparent layer.

Surface Layer	Pros	Cons
Tempered glass	With excellent compressive strength, high light transmittance, and long lifecycle, it can be modified on the surface and has the functions of self-cleaning and anti-skid enhancement	High cost, heavy weight, the skid resistance is relatively poor
Reinforced resins (Such as PMMA)	Affordable, lightweight	Prone to yellowing and aging, resulting in degradation of mechanical and optical properties
Glass aggregates bonded by resins	Performs well in anti-skid, and the performance can be adjusted according to the requirements	Comparatively poor transmittance and brief lifespan

). In order to ensure the long-term stability of the PV pavement, it is advisable to utilize tempered glass in the surface–transparent layer rather than reinforced resin. Although the skid resistance of glass is relatively poor, it can be improved by applying texture to the surface. Although the cost of resin material is low, it is easily affected by the environment, so it needs more frequent maintenance and updates. Furthermore, a layer of resin-bonded glass aggregate film can be applied over the tempered glass to further enhance skid resistance and safeguard against surface scratches. According to Andrew’s research, any glass with a thickness of more than 9.53 mm can withstand a traffic load of 480 kPa [76].

To verify the compression resistance of PV pavements, Yang et al. [74] measured the compressive strength of tempered glass with different anti-slip patterns, and the results are shown in Figure 4. It can be seen that both PV pavements with anti-slip patterns can withstand compressive strength above 14 MPa. To verify the compression resistance, Yang also provided example ground pressure values, as shown in

Table 2. Example ground pressure values [74].

Type	Example Ground Pressure Values
Human	5–50 kPa
Road-racing bicycle	620 kPa
Mountain bicycle	245 kPa
Passenger car	205 kPa

. It can be observed that the compressive strength of the tempered glass is significantly greater than the example ground pressure values; therefore, it can be concluded that the existing materials can effectively handle the scenario of PV pavement.

(2)

The middle–functional layer

The structure of the intermediate–functional layer can be categorized into solid and hollow structures, depending on whether the solar cell is in direct contact with the surface layer (see

Table 3. The pros and cons of designs in the middle–functional layer.

Middle Layer	Pros	Cons
Solid structure	Uniform surface compression force to maintain the stability of the overall structure	Solar cells are directly responsible for transmitting the load
Hollow structure	Solar cells are free from stress, and the placement angle can be flexibly adjusted	The causes of surface damage include uneven compression, complex manufacturing processes, moisture penetration, and so on
Silicon cells	High operating efficiency, cost-reduction	Easily damaged
Film cells	flexibility, low temperature coefficient, and degradation rate (CdTe) [77], high efficiency (GaAs) [78], and can work in low-light environment	Most types of efficiency are relatively low

). The surface layer of the hollow structure is easily damaged due to uneven stress; however, according to the grid design described in [42], this phenomenon can be effectively avoided. In addition, the immature sealing method may lead to water penetration, which is the main hindrance to the advancement of this structure. This is because the diffusion of water vapor can expedite the decline in solar cell efficiency. Consequently, the solid structure remains the primary form currently in use.

When choosing solar cells, the monocrystalline silicon cell stands out for its superior photoelectric efficiency and relatively low sensitivity to temperature variations. On the other hand, the film cell boasts inherently better compression strength. Therefore, one can select the most suitable cell based on their specific requirements. In the solid structure, the former is an ideal choice for walkable tiles, whereas the latter is suitable for application on roads or highways, capable of bearing the load of heavy vehicles. The operational temperature of PV cells can vary significantly even under identical working conditions [79], necessitating careful consideration in practical projects. Furthermore, several emerging solar cell technologies, including dye-sensitized solar cells (DSSC) [80], organic solar cells (OSC) [81], and perovskite solar cells (PSC) [82], could potentially offer promising and cost-effective alternatives in the field of photovoltaic pavements in the future.

(3)

The bottom–protective layer

The bottom–protective layer in PV pavements commonly features designs such as tempered glass, concrete floor, and resin and polymer substrates, as outlined in

Table 4. The pros and cons of designs in the bottom–protective layer.

Bottom Layer	Pros	Cons
Tempered glass	Excellent stability	Higher cost
Concrete floor	Large heat capacity, excellent cooling effect and compressive strength	Heavy and expensive, and requires large equipment to assist in installation
Resin and polymer substrate	Affordable, and some of them use waste and recycled materials, which have ecological benefits	Vulnerable to aging
Porous layer with working fluid	Cooling effect and external thermal benefit	Due to the influence of porous structure, the bearing capacity is low

.

Out of the three designs, tempered glass appears to be the most suitable choice due to its long-term stability and comparatively lightweight nature. While the resin and polymer substrate, constructed from scrap and recoverable materials, offers ecological advantages, its practical performance remains to be fully evaluated. Some researchers suggest the implementation of a porous layer containing a working fluid for achieving a cooling effect and harnessing external heat benefits; however, this porous structure may potentially compromise the overall bearing capacity of the module. Additionally, addressing the persistent issue of moisture penetration remains a crucial objective for all future designs of the bottom layer.

3.4. Energy Harvesting

The excess solar radiation on the road is effectively utilized as a valuable energy source through solar–electric or solar–thermal conversion methods. Solar–electric conversion uses solar photovoltaic (PV) technology to generate electricity. Solar–thermal conversion, on the other hand, involves a combination of thermoelectric and pyroelectric power generation technologies [83] to generate heat through the circulation of water or air through pipes hidden under the road surface [64,84,85,86], also named pavement-integrated photovoltaic thermal (PIPVT). Solar photovoltaic technologies for roads encompass various innovations such as solar panel roads [44,76], acoustic photovoltaic barriers (APVB)—also referred to as photovoltaic sound barriers (PVSB) or photovoltaic noise barriers (PVNB) [87]—and solar arches [88].

Researchers such as Bobes-Jesus conducted a thorough review of asphalt solar collectors, emphasizing the analysis of factors that influence the efficiency of heat collection [89]. On the other hand, Wang et al. explored the utilization of energy-harvesting technology in roadway and bridge construction [90]. Ahmad et al. similarly assessed the implementation of energy-harvesting technologies for sidewalks and roads, with a focus on pertinent technologies and materials [91]. Gholikhani et al. have conducted a comprehensive and recent review of the literature on a variety of energy-harvesting technologies, including electromagnetic collectors, piezoelectric collectors, thermoelectric collectors, pyroelectric collectors, photovoltaic collectors, and solar collectors [92].

Furthermore, Sharma and Harinarayana [93] delved into the concept of solar arches by examining two representative highways in India. As shown in Figure 5a–d, they conducted a detailed analysis of the feasibility of generating electricity using this technology. Based on the solar irradiance potential on the studied highways and the available area for installing PV panels, the maximum power output for each highway was estimated to be approximately 86 and 81 gigawatt-hours per square kilometer per year, respectively.

At the same time, Firoz Khan and Jae Hyun Kim conducted an analysis on the performance degradation of photovoltaic modules mounted on road pavements under damp-heat (DH) conditions, as shown in Figure 6a,b [94]. It can be observed that in the damp-heat environment, the rate of performance degradation decreases when the photovoltaic components are fixed on a concrete slab, compared to the situation without concrete.

To validate the efficiency of various pavement systems, including conventional pavement (CP), pavement integrated with photovoltaic thermal (PIPVT), photovoltaic (PIPV), and solar thermal (PST) technologies, Ma et al. [45] conducted tests and sampling on these harvesting systems across different seasons. The outcomes of these tests are presented in Figure 7. It can be seen that PIPVT has the highest power generation efficiency because it can obtain energy through photovoltaic and thermal energy simultaneously.

4. Performance Evaluation

Pavement PV fulfills the demands for promoting clean and renewable energy sources while encompassing diverse energy conversion applications including transportation engineering technology, smart road equipment, and electric vehicle charging. This can achieve a sustainable equilibrium between limited resources and societal environmental necessities, thereby contributing towards the aspirations of “carbon peak” and “carbon neutrality”. Through an examination of the pertinent literature and case studies, the following key insights can be deduced:

(1)

Pavement PV are primarily composed of three layers: the top–transparent layer, the middle PV layer, and the bottom–protective layer. These three layers need to work in coordination to ensure the proper functioning of pavement PV. The commonly used structures for pavement PV are solid panel and hollow panel structures. The former boasts good load-bearing capacity and stability but its flat surface is not suitable for efficient monocrystalline silicon solar cells as they may break. The latter, however, does not face the risk of solar panel fracture and allows for adjustable placement angles. Yet, hollow panels demand higher packaging and waterproof drainage performance;

(2)

Pavement PV offers sustainable power solutions for smart roads, enabling the decentralized expansion of renewable energy sources. This approach minimizes the need for extensive power lines and associated energy losses, thereby promoting smart transportation systems and the development of electric vehicles. Furthermore, it effectively addresses issues such as road ice and snow melting, reduces greenhouse gas emissions, and mitigates urban heat island effects. Additionally, pavement PV creates job opportunities while simultaneously reducing fuel/energy consumption for projects or nearby buildings, thus contributing to a more sustainable and environmentally friendly urban infrastructure. However, various factors including traffic loads, material performance degradation, as well as environmental conditions such as temperature, humidity, and dust, have resulted in the failure or reduced durability of test projects, falling short of initial expectations. Presently, it appears that a suitable technology for constructing pavement PV that can withstand the impact of traffic loads while efficiently generating power remains elusive. The challenges persist in ensuring the durability and safety of the structure, materials, and collection circuits;

(3)

Research on the environmental impacts, costs, and efficiency indicates that another obstacle to the widespread application of pavement PV is their high cost; however, current costs cannot represent the installation cost at an industry scale. With the development of materials, technology, and economies of scale, future costs are expected to decrease. Hollow panel pavement PV have the lowest net present value (NPV) and levelized cost of energy (LCOE) among the other types of pavement PV, owing to lower material and production costs. In terms of current costs, the benefits of pavement PV are insufficient to cover the 20-year lifecycle costs. Nonetheless, if the LCOE drops below USD 0.2 per kWh, pavement PV would become economically attractive. This is not hard to achieve for hollow panel pavement PV.

5. Challenges and Prospects

(1)

The electricity generation capacity of pavement PV is not solely determined by geographical location and climatic conditions but is also influenced by unpredictable factors such as the environment and traffic flow, causing instability. Consequently, to adapt to the constantly changing operational conditions, further research is necessary to optimize the circuit design of pavement PV, taking into account the unique characteristics of the road environment. Furthermore, targeted waterproof measures need to be designed to address the issue of permeability in the bottom–protective layer of pavement photovoltaics. Simultaneously, the development of corresponding energy storage technologies is essential to ensure that the electricity generation of pavement PV can meet load demands, thereby enhancing load management flexibility;

(2)

Currently, pavement PV faces a dearth of suitable detection methods and standardized evaluation criteria. Therefore, it is imperative to devise innovative testing methodologies and establish uniform evaluation standards to assess the load-carrying capacity, durability, sustained performance, and reliability of electricity generation output for pavement PV. At the same time, as pavement photovoltaics is a relatively novel technology, it is necessary to regularly update its standards during the initial stage of standard development. The wide coverage and distinct working environment of pavement PV render conventional road management and maintenance models obsolete. Research and exploration are imperative for maintaining and repairing damaged pavement PV during operation, encompassing surface cleaning, load-bearing structures, and electricity generation systems;

(3)

While pavement PV present a promising road energy production approach, they are currently not applicable to all road types. Heavy traffic increases the likelihood of structural damage, elevating costs and reducing durability. Therefore, it is also necessary to strengthen the structural design of pavement modules to optimize long-term stability and effectiveness; in addition, this also adds to the shading area and duration of pavement PV. When replacing traditional road surfaces with photovoltaic pavement modules, it is also necessary to consider whether they will have a negative impact on the movement of vehicles. To prevent uncontrollable factors and mitigate significant economic losses, it is crucial to undertake a thorough feasibility assessment of the pavement site prior to embarking on test projects. This comprehensive evaluation encompasses various aspects, including but not limited to, an on site solar resource assessment, traffic environment screening, and a detailed cost–benefit analysis. Furthermore, to enhance the system’s electricity generation efficiency, relying solely on a single energy collection method for pavement PV might not suffice for their developmental needs. Consequently, future research should focus on integrating pavement PV with other forms of energy collection technologies.

6. Conclusions

This paper provides a comprehensive review of physical models and performance evaluations related to pavement PV while also offering insights into future prospects. The integration of PV technology imbues roadways with the inherent capability of generating green energy, thereby transforming them from energy consumers to energy producers. The overall structure of PV roadways comprises three layers: the transparent–surface layer, the intermediate–functional layer, and the bottom–protective layer. Building upon this three-layer framework, a summary and comparison of the advantages and disadvantages of various designs for each layer aids in selecting appropriate models for diverse application scenarios. Standards for mechanical performance and stability testing, simulation methods for electricity generation, and assessments of socio-economic benefits are all introduced.

Presently, research on PV roadways is still in its nascent stage, with poor long-term stability and limited effectiveness constituting the primary obstacles to their development. Accordingly, it is essential to further optimize the structural design of the road surface modules themselves. Additionally, quantifiable testing standards must be established and periodically revised to ensure their relevance and accuracy. In practical engineering applications, the installation locations for PV roadways should be carefully chosen, accompanied by more comprehensive economic and environmental evaluations.