The electronic devices and networks annex (EDNA) reports that by 2022, there will be around 50 billion devices connected to the internet [
1]. Some estimates even claim this number could exceed 100 billion [
2]. The wave of IoT is emerging very fast and becoming part of the mainstream electronic industry. Thus, people and society tend to adopt smart devices. These devices are equipped with a wireless terminal and effective sensors connected in a network that can gather data, features, statistics, and all sorts of information from the surrounding environment. Internet connections in embedded systems, controllers, transport systems, wearable devices, commercial security systems, and other objects are envisioned. IoT-based devices need an uninterrupted power supply to ensure the operation of their activities [
3]. Therefore, providing the necessary power to maintain all the devices operational for their projected lifetimes is challenging. The corresponding energy demand for IoT devices is very considerable due to their limited energy sources, replacement barriers, ecological obstacles, environmental risk, etc. [
4,
5]. Future projections for WSNs to allow the IoT indicates a doubling between 2018 and 2023, which will result in a substantially higher energy demand [
6]. Till now, the most significant energy sources for IoT sensors are batteries. An estimate shows that more than 23 billion batteries will be needed to power up the IoT devices in 2025 [
1], but the rising demand for the batteries needed to power up the IoT appliances is harmful because batteries contain harmful chemicals including lead, cadmium, zinc, lithium, and mercury. Using a battery is also challenging in remote areas because of limited charging facilities and accessibility for replacement. Therefore, energy harvesting (EH) from ambient energy sources, such as light, heat, radio frequency (RF), vibration, etc., is inevitable [
7,
8,
9,
10]. This would be an efficient solution to overcome the limitations and mitigate the energy demand for uninterrupted functioning by powering up the billions of IoT devices. In this context, numerous EH systems have been developed for outdoor and indoor applications. However, ambient resources provide low power and are dependent on time-varying environmental parameters, which is insufficient to power up IoT sensors sequentially. Based on the power generation capacity, a single energy source is often insufficient to power up all the sensor nodes; therefore, additional energy sources may be introduced as a secondary power supply. The world’s first multiple or hybrid power system comprising PV and diesel power was started up on 16 December 1978, in the Papago Indian Village (Schuchuli, AZ, USA) [
11]. Nowadays, hybrid EH systems are increasingly gaining recognition among researchers and industry. Tadesse et al. proposed an electromagnetic energy source (ES) paired with a piezoelectric ES. The fabricated prototype produced 0.25 W using the electromagnetic mechanism and 0.25 mW using the piezoelectric mechanism, at 35 g acceleration and 20 Hz frequency [
12]. Guilar et al. proposed an energy-saving photodiode array, which can generate 225 μW/mm
2 at 20,000 lux [
13]. Two energy sources—RF energy and TEG— with 78% efficiency were fabricated by Lhermet et al. [
14]. The harvested energy can run 30 integrated chips (ICs) and consumes 5 nW power. Based on the priority, only one source can provide the required power at a time. The main drawback of this proposed system is that it cannot generate power simultaneously. The authors in [
15] recommended a PV-TEG dependent HEH system for the indoor ambient environment. Average power of 621 mW is extracted in the integrated HEHS device at an irradiance of 1010 lux, nearly three times as much energy is obtained in single-source EH. Authors in [
16] proposed a modular design that pulls in its power from each linked harvesting device. Using a lithium-ion (Li-ion) or nickel-metal hydride (NiMH) battery extends the system’s dependability. With the inclusion of three energy sources, PV, piezoelectric (PZT), and RF, the authors in [
17] proposed a multi EH device that can provide up to 2.5 V and total power of 6.4 mW. A platform combining three distinct EH sources from PV, TEG, and PZT with the input voltage range of 20 mV–5 V is proposed in [
18]. A time-based power monitoring system is used to track the harvesters’ power, and a peak efficiency of 96% is achieved whereas, the inductor sharing for the PV boost performance is 78%, TEG boost is 86%, and PZT is 83%. In [
19], an MPPT EH device with an expandable control for charging and discharging a lithium polymer (LiPo) battery is proposed for PV and vibration energy. The device shows an overall efficiency of 75–85% for 24-h experiments in a WSN. A battery-free energy harvester based on thermal and the vibration energy is designed in [
20] for aircraft health monitoring. The use of a low bias current of only 10 nA per branch ensures low power consumption. Dini et al. [
21] designed an autonomous, self-starting, battery-less energy harvester for wearable devices and WSN combining PZT, PV, and TEG transducers. The total current consumption is 47.9 nA per source during all the energy sources are enabled. The test shows the peak single-source efficiency is 89.6%. G. Chowdary et al. [
22] presented a HEH device with available power levels of 25 nW–100 µW. The 180 nm chip has an output voltage of 1.5 V with the highest efficiency of 87%.
Table 1 summarises a comparison of different hybrid energy harvesting systems.
In this study, an ambient source-based hybrid energy harvester (HEH) is developed to power the IoT-enabled WSNs continuously. A small solar PV cell and thermoelectric generator (TEG) are used to develop the HEH device. Among the two sources, light sources are abundant in the environment. The PV cell can work well in an indoor or outdoor location. Thus, it will work as the primary source of the proposed system, and the TEG will work as the secondary ES. An ESP32 Wi-Fi module connects the complete system with the internet to monitor the sensor data. For energy backup, an SC is used. The proposed HEHS can overcome the limitations of a single-source energy harvester. It will mitigate the IoT sector’s energy demand, extend the sensor life span and the integrated system.
The rest of this paper is organised as follows: The EH methodology for WSNs is introduced in
Section 2, which is divided into three parts: ambient energy sources in
Section 2.1, solar energy harvesting system in
Section 2.2, and thermal energy harvesting system in
Section 2.3. After introducing the EH sources,
Section 3 presents the proposed HEHS.
Section 4 describes the experimental setup and methodology, including the simulation model, hardware components and the complete prototype. In
Section 5, the simulated and experimental results are presented and discussed. Finally, conclusions are drawn in
Section 6.