Performance analysis of a hybrid photovoltaic-thermoelectric generator system using heat pipe as heat sink for synergistic production of electricity
Introduction
The urgent need to replace conventional fossil fuel energy sources promotes the exploitation of green and sustainable energy sources. Solar energy with eco-friendly, pollution-free, and inexhaustible features plays a vital role in the reform of alternative energy systems [1]. In general, one of the most pervasive ways of harvesting solar energy is to convert it directly into electricity through the photovoltaic effect [2], [3]. Photovoltaic (PV) cells are semiconductor devices that can convert incident sunlight into electrical energy. However, restricted by the bandgap of semiconductor materials, only a fraction of the solar energy can be utilized by solar cells, whereas the rest of the energy is transformed into thermal energy [4]. The undesired heat results in the reduction of conversion efficiency and life span of the PV system. This situation is graver for concentrated photovoltaic (CPV) systems. The dramatic uprising temperature of the solar cells due to higher heat flux input causes cell degradation and even permanent damage [5]. These issues weaken the performance of the PV system, one of the potential ways to improve the behavior of the PV system is to utilize the residual heat to produce extra electricity [6].
Similar with PV module, thermoelectric generator (TEG) is also a kind of semiconductor component that directly converts thermal energy into electricity via thermoelectric effect (also known as the Seebeck effect) [7]. Very recently, incorporating TEG unit with PV system to harvest excess heat has gained great attentions. The unique advantages of noise-free, zero hazardous emission, the absence of moving parts, modular design, compact structure and high stability make it more competitive compared with other innovative technologies to combine with PV system [8], [9]. Furthermore, it is a promising way to enhance the comprehensive performance of the new hybrid photovoltaic- thermoelectric generator (PV-TEG) system. PV can only utilize a portion of the solar spectrum (including ultraviolent and visible regions), while TEG unit extracts the thermal energy from the infrared region. Therefore, the combined PV-TEG system is beneficial to use a wider range of solar spectrum [10]. Particular attention is needed when designing a PV-TEG coupling system, because PV module and TEG unit have completely different requirements for operating temperature. The PV module requires lower operating temperature to ensure the efficiency of power generation, while larger temperature gradient between the hot side and cold side of the TEG unit is longing [11]. Consequently, the coupling relationship between the PV and TEG module is quite complex, and a continued in-depth investigation is needed [12].
A series of the theoretical and numerical studies on PV- TEG system have been extensively explored in the last few years. Babu et al. [13] conducted a theoretical analysis of hybrid PV-TEG system in MATLAB/SIMULINK environment. The proposed layout reported a production of 5% additional energy with an increase in overall efficiency of 6%. To predict the maximum theoretical performance of the tandem unconcentrated PV-TEG configuration, Bjørk et al. [14] established an analytical model. It is revealed that the maximum increase of 4.5 percentage points was achieved for combined case compared with the standalone case in efficiency. Kraemer et al. [15] presented a methodology to optimize the performance of the hybrid spectrum beam splitting PV- TEG system with a solar cell operating at ambient temperature. The results proved that the maximum efficiency of the system was dominated by the spectral efficiency of the solar cells and the TEG efficiency. Mahmoudinezhad et al. [16] suggested that the power generation of the coupling system increased with the incident solar irradiation, and the overall performance could be enhanced by optimizing the materials properties and geometries of the TEG unit. A transient numerical study of nanofluid cooled PV/T-TEG system was considered by Rejeb et al. [17], the overall electricity generated by the PV/T-TEG with 0.5% graphene/water nanofluid and PV/T-TEG compared to PV/T collector is respectively 11.15% and 9.77% for the summer day.
In addition to the above mentioned theoretical and numerical researches, the field experimental tests have also exerted a tremendous fascination. A tandem design of a PV- TEG system fabricated with optimized thermal management was developed by Zhu et al. [18]. The results showed that a high peak efficiency of 23% in the outdoor test was achieved by introducing the TEG unit to the PV system, which was also 25% higher than that of the reference PV system. Yin et al. [19] claimed that the performance of the concentrated PV-TEG system was superior to the conventional CPV system under specific coupling conditions. Zhou et al. [20] conducted a series of the field tests to explore the newly fabricated photovoltaic-thermoelectric/thermal (PV-TE/T) system (so-called full solar spectrum utilization system). The experiments were performed on sunny and cloudy days with different solar irradiations. The yielded electrical power of the PV-TE/T system was 11.2% higher than the PV-TE system on sunny days. While, in part cloudy days, this value was increased to 35.6% owing to the existence of a thermal collector.
Heat pipe devices, as passive thermal management equipment, have been amply discussed to improve the overall performance of the hybrid PV-TEG system. Shittu et al. [21] presented a comparative analysis of a concentrated PV- TEG system with and without a flat plate heat pipe. Under the concentration ratio of 6, the gained results demonstrated that the efficiency of the PV-TEG system with heat pipe is 1.47% and 61.01% higher than that of the reference PV-TE and PV systems. A novel structure of PV/T system combined with TEG and gravity-driven heat pipe (GHP) using water as working fluid was proposed by Wen et al. [22]. The results showed that the overall performance of the system could be enhanced by joining the TEG module with the system. Li et al. [23] investigated the influence of different environmental conditions (such as solar irradiation, ambient temperature, and wind speed) on the performance of the microchannel heat pipe-based PV-TEG system. The numerical results of the one-dimensional model were in good agreement with the experimental results. A novel heat pump-PV-TEG system combined with micro-channel heat pipe was introduced by Song et al. [24]. The results indicated that the addition of TEG in condenser could elevate the electrical and overall performance of the system.
In spite of the above mentioned studies, the optimal system structures and other influencing parameters have also been examined. For example, Abdo et al. [25] recently proposed a novel sandwich type PV-TEG system using a microchannel as the heat sink. The author suggested that the new system was superior to conventional tandem PV-TEG system, especially for the concentrated PV system. However, the additional electricity consumed by the power device may affect the economic feasibility of the system.
From the above literature survey, it can be concluded that the majority of the discussed structure in hybrid PV-TEG system is in tandem. However, in this tandem construction, there exists a game relationship between PV cell and TEG unit working temperature. From one hand, PV cell needs lower working temperature, while TEG unit desires higher temperature difference (higher hot side temperature and lower cold side temperature). On the other hand, the existence of TEG unit hinders the heat conduction from PV to heat sink, and also the existence of PV cell impedes TEG unit to obtain higher heat flux.
Furthermore, there also remain some gaps that need to be further addressed. Firstly, a great number of numerical studies have been done using one-dimensional heat transfer model and constant material properties rather than three-dimensional model and temperature dependent material properties to predict the performance of the system, which is not the real reflection of the operating conditions. Secondly, the possibility of taking FPHP as a heat sink for sandwich (or bifacial) type of PV-TEG system to eliminate parasitic energy consumption is also not mentioned. Thirdly, as reported by Chen et al. [26], the performance of the heat pipe might vary with different working fluids, while this point is also rarely considered in the FPHP based PV-TEG system in open literature.
Therefore, to settle the competition relationship between PV module and TEG unit and test the feasibility of the hybrid bifacial FPHP-PV-TEG system, a three-dimensional numerical investigation is presented in this study. The remaining part of this paper is arranged as follows: Section 2 describes the different system components used in this study, Section 3 presents the numerical model prepared for this research, and the obtained results are reported and discussed in Section 4. Finally, Section 5 summarizes the main conclusions and potential challenges derived from this study.
Section snippets
System description
In this study, two different layouts of hybrid concentrated PV-TEG systems are developed. As shown in Fig. 1(a), the conventional concentrated FPHP-PV-TEG system comprises an optical concentrator system, PV module, TEG unit, and flat plate heat pipe equipment. The composition of each component in conventional FPHP-PV-TEG system is displayed in Fig. 1(b). In this tandem design, the PV module is directly attached to the top surface of TEG unit. The sunlight illuminates on the PV module through
Mathematic model description
The following steady-state numerical models are presented to describe the operating principles of the PV module, TEG unit, and FPHP device. Each component has been established separately before they joined together to form the conventional FPHP-PV-TEG system and the bifacial FPHP-PV-TEG system.
Influence of solar concentration ratio
To investigate the impact of solar radiation on the performance of the two different systems when water is selected as working fluid and other conditions (i.e, convective heat transfer coefficient is regarded as 1200 W/m2/k) are kept constant, various solar concentration ratios are considered. The initial solar radiation is chosen as 1000 W/m2, and the obtained results of the two systems are listed in Fig. 7. For the sake of identifying the different systems, the abbreviations of Ta and Bi in
Conclusion
In this study, for the sake of handling the competitive relationship between PV module and TEG unit in tandem design and enhancing the overall performance of the conventional PV-TEG hybrid system. A newly designed bifacial PV-TEG layout using FPHP as heat sink has been developed. The performance of the proposed system is compared with the conventional tandem case. COMSOL Multiphysics software is used to perform the 3D numerical simulations, and the different models prepared for this examination
CRediT authorship contribution statement
Yuanzhi Gao: Conceptualization, Methodology, Formal analysis, Writing – original draft. Dongxu Wu: Methodology, Formal analysis, Writing – original draft. Zhaofeng Dai: . Changling Wang: Investigation. Bo Chen: Investigation. Xiaosong Zhang: Conceptualization, Supervision, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research described in this paper is supported by the Key R & D Program of Jiangsu province (No. BE2018118), the National Natural Science Foundation of China (No. 51876034). Acknowledgments are also given to Chao Yang, for his contribution to language editing and meaningful discussion in this paper.
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