1. Introduction
Indoor farming has been considered as an alternative measure to dealing with low and inconsistent food supplies from conventional agriculture. Drastic weather changes caused by global warming have pushed farmers and the agriculture industry to seek other forms of farming which provide a more dependable food source [
1]. Urban farming methods, which include vertical farming, rooftop greenhouses and indoor farms, have been highly adopted as the human population continues to increase [
2,
3]. Urban indoor farms have advantages when compared to conventional outdoor farms, including lower carbon footprints, more efficient use of farming inputs, and year-round crop output. Nonetheless, the strongest disadvantage of adopting indoor farms is the high amount of energy sources required to maintain an optimum growth environment [
4]. A large amount of electricity is needed to sustain indoor farms due to the usage of artificial lighting, automatic devices, heating, cooling, and ventilation. More than half of the share of electricity production comes from fossil fuel sources, which include coal, natural gas, and oils [
5]. The emission of greenhouses gases from the burning of fossil fuels has negatively impacted the environment, causing global warming, acid rains, and rising sea levels [
6]. Increasing the use of alternative energy sources is crucial in order to avoid causing further damage to the environment. The usage of renewable energy sources, such as wind, hydro, biomass, wave, solar and geothermal, have shown an increase in the recent years [
7]. This change is expected to continue and accelerate the conversion of primary energy sources from fossil fuels to renewable energy sources. Solar energy is the most easily obtainable form of energy, especially with the availability of solar photovoltaic (PV) panels and solar thermal collectors.
Solar energy has been implemented on agricultural land as a cost-saving measure by mitigating the power supply from the grid by using solar PV panels, as well as helping farmers gain additional income from selling electricity [
8]. Adopting solar PV for electricity generation while growing crops has shown its benefits, thus leading to a substantial amount of different research on the feasibility of implementing this approach [
9,
10,
11,
12]. Moreover, the crucial need for sustainable agriculture causes a shift in the farming approach to introducing solar energy technologies to support the electricity usage for agricultural activities [
13]. However, the reliability of a standalone PV is questionable since the power output is entirely dependent on the solar irradiation received and is sensitive to ambient temperature changes [
14,
15]. Thus, combining solar PV with other renewable energy technologies could help to improve its reliability and performance. Recently, solar photovoltaic-thermoelectric generator (PV-TEG) hybrid systems have been gaining the interest of many researchers as one of the more promising solar hybrids. A TEG is a solid-state device with no moving parts, which makes it easily integrated with other applications. Due to its principle of converting heat into electricity, it is usually used in waste heat recovery applications. This makes TEGs attractive to be coupled with solar PV, where excess heat from the PV panel can be used to produce additional electricity.
The unconcentrated PV-TEG is the most commonly studied system. The typical arrangement of the hybrid system is to attach the TEG behind the PV panel. Often a heat sink is introduced in order to maintain a constant cold side temperature [
16,
17,
18]. The arrangement of TEG with PV plays a vital role in producing the maximum overall power output by the hybrid system. In an unconcentrated PV-TEG system, PV and TEG would thermally benefit more with a series connection, as opposed to a separate tandem arrangement as reported by Bjørk and Nielsen, 2018 [
19]. The analytical investigation showed that a tandem arrangement using spectrum separation only increased the maximum efficiency by 1.8%, as compared to a combined system of 4.5%. Zhang and Xuan, 2019 [
20] found that a V-type groove arrangement for a PV-TEG achieved a slightly higher efficiency than the typical flatly combined arrangement, due to an improvement in the PV cell’s absorptivity.
Besides that, the types of solar PV used in the hybrid system also impact the overall efficiency of the system. Kossyvakis et al., 2016 [
21], carried out some lab experiments using different solar cells combined with a TEG. The hybrid system is able to provide a higher power output when combined with dye-sensitized solar cell than with a polysilicon solar cell at room temperature operation. This finding aligned with Bjørk and Nielsen, 2015 [
22], where cadmium telluride achieved the highest efficiency compared to polysilicon, amorphous, and copper indium gallium selenide (CIGS). However, both studies found that the overall efficiency of the system deteriorates with increment in temperature due to the loss of efficiency from the PV cells.
Other factors, such as a TEG’s figure of merit (zT) and weather conditions, play a crucial role in ensuring that the maximum power output is achievable for the hybrid system. Van Sark, 2011 [
16], investigated the feasibility of PV-TEG hybrid modules where a model of the hybrid system was created and simulated. The author found that the hybrid system produces an increase in efficiency between 8% and 23% according to the type of PV module integration at a constant figure of merit value. The author emphasized on possible further improvement in the overall efficiency through increment on the figure of merit value of the TEG. This was supported by Chávez-Urbiola et al., 2012 [
17], through theoretical analysis on the efficiency of a TEG, which was higher for zT = 4 than zT = 2.4. Other than that, the internal resistance of the combined system circuit must be minimized in order to avoid unnecessary power losses. Park et al., 2013 [
18], reported an increase of 3.8% through the addition of a TEG to a sole PV circuit, given that coupling losses were made between the PV cell and the TEG. The author mentioned that the potential of upscaling the hybrid system would result in a better overall performance.
Moreover, the PV-TEG system has also been modelled to investigate the effect of actual weather conditions on the hybrid system. Rezania et al., 2016 [
23], evaluated the potential of the hybrid system under the weather of several sample cities in Europe using a thermal equivalent model. The authors found that the summer weather condition produces a higher power output than in winter for the hybrid system. However, the opposite holds for the overall efficiency of the system where a slight drop in efficiency was observed in July as compared to in January. Yin et al., 2018 [
24], used one day weather data in Nanjing, China to analyze the one day performance of the PV-TEG system. The findings showed an improved performance of the hybrid system, especially during 11:00 to 13:00, where the highest TEG efficiency was obtained.
In this paper, a functional prototype of an unconcentrated PV-TEG hybrid system is designed and sized based on the power output supply required by the load used for indoor farming. An experimental investigation to test the performance of the hybrid system under Malaysia’s weather conditions was carried out. The PV-TEG hybrid system performance as a combined system and individually were obtained and are discussed. The possibility of night time power production from the TEGs are also examined. The effects of changing TEG circuit connections using a combination of series and parallel in fluctuating temperature differences are explored. The sufficiency of power output generated by the hybrid system with the load requirement is evaluated. Finally, the feasibility of the designed hybrid system is analyzed through comparison with a sole PV system in regard to the efficiency of systems.
4. Conclusions
This research focused on the design of a PV-TEG hybrid system for potential indoor farming applications. In striving for agricultural sustainability, usage of renewable energy for agriculture activities was placed as a priority, especially with the continually improved methods used in agriculture, such as the indoor farming approach. Solar energy technologies, particularly solar PV, were the most suitable choice for countries with a hot and humid climate like Malaysia. In order to combat the unreliability of solar PV systems, a hybrid PV-TEG system was designed to further improve the efficiency of a sole PV system. The hybrid system was designed to provide sufficient load to the LED used in indoor farming. The system was experimentally tested under actual Malaysian weather conditions in the months of December, February, and March. The system performance was analyzed based on the changing ambient and panel temperature, solar irradiance, and the effect of TEG connections to the overall power output. From the experiment conducted, the significant findings are summarized as follows:
The power outputs of PV for December, February, and March were slightly varied, ranging between 15 W and 20 W. The variation was caused by factors such as partial shading and solar module temperature while running the experiment. Whereas the TEGs power output was profoundly affected by the weather conditions, with the lowest power output obtained in December, followed by February and March. Different connections of the TEGs in the hybrid system were investigated in this study. The findings showed that the best connection for high and consistent power output from the TEGs throughout the day was a series connection. The matched optimal load for the series was considerably higher than a parallel and series combination, with a range between 50 Ω and 60 Ω, which affected the power output when used in the colder month. Although night time power production was possible with series connections, it also raised some uncertainties when applying for long term usage. Thus, for practical reasons, a combination of parallel and series connections with higher TEG numbers in series is preferred, such as that done for the month of March.
The hybrid system designed was able to successfully provide sufficient power output to the LED load for the months of December, February, and March. This study affirmed that the hybrid system main power output was from the PV panels, whereas additional power output produced from the TEGs was minor and incomparable to the PV output. Thus, the load was fully supported by the PV system instead of the hybrid system. The PV system was able to support not only the existing load, but also an additional load of a pump, despite it having a low power output of 15 W to 20 W in February and March. This suggests that a fully independent indoor farm relying on only renewable energy sources is possible with the hybrid system designed in Malaysia.
In the present result, the combination of PV-TEG has shown that the performance of the hybrid system was slightly improved when compared with a sole PV system. The hybrid system was found to have an increase in efficiency of 0.06%, as compared to stand alone PV. The hybrid system efficiency was also shown to be affected by TEG efficiency. A drop in efficiency by 1.68% was found when the hybrid system operated in cooler temperature, where the lowest TEG efficiency was obtained in the month of December. Regardless, this study showed the possibility of an improved hybrid system during favourable weather conditions, which were in hotter and less rainy months. Nonetheless, the hybrid system performance was better in a cooler month as compared to hotter months. Future work on improving the design of the PV-TEG system, such as by incorporating heat sinks, may provide a better outcome for the hybrid system.