Parametric study of photovoltaic/thermal wickless heat pipe solar collector

https://doi.org/10.1016/j.enconman.2021.114236Get rights and content

Highlights

  • Describing the concept and parametric conditions of novel acetone PVT/WHP.

  • Developing a numerical model to evaluate the performance of the system.

  • Investigating the impact of the operating conditions on the system’s performance.

Abstract

Wickless heat pipes are an effective passive heat transfer device that, due to their improved heat transfer capabilities, can enhance photovoltaic system efficiency. A new photovoltaic-thermal acetone wickless heat pipe solar panel (PVT/WHP) is described in this study. The main parameters affecting the thermal and electrical efficiency of the solar panel, such as wind velocity, incident radiation, water inlet temperature, heat pipe number, and collector surface area, as well as inner heat pipe behavior and operating limits, are studied using a mathematical model. According to the reduced temperature simulation results, the theoretical PVT/WHP module's average electrical, thermal, and overall efficiency were approximately 12.52%, 43.75%, and 56.27%, respectively. The study is being used to assess the thermal efficiency of PVT/WHP under Tunisian climatic conditions, and the simulation results show that PVT/WHP outperforms conventional PVT water-based systems with a maximum thermal and electrical efficiency gain of approximately 21.9% and 14.2% respectively. The current parametric study provided a framework for assessing such a system's behavior and providing useful flexibility to achieve the best possible system performance.

Introduction

The solar photovoltaic thermal collector has the advantage of being able to produce both heat and electricity at the same time [1]. As a PVT system can extract heat from the back of a photovoltaic cell, its electrical efficiency is claimed to be higher than that of a conventional photovoltaic system. Several researchers design and test many PVT systems as follows: (i) PVT air collectors, (ii) PVT water collectors, (iii) PVT greenhouse dryers, (iv) integrated PVT roof, and PVT facades [2]. PVT using air as a cooling fluid can be integrated into the building to reduce PV systems temperature and provide space heating, and they have been more effectively applied in European real-building project applications.

PVT/water systems are extensively studied and can also be used for heating water [3]. However, due to a variety of factors, including technological and economic assessments, as well as public awareness and standardization, the PVT/water system's higher performance has been limited. However, increasing the system's energy efficiency to ensure efficient investments and decisions remains the biggest challenge. Technology that needs further research and development work on emerging innovations, including the design and manufacture of thermal absorbers, choice of materials and coatings, energy conversion and efficiency, performance monitoring, optimization of systems, control, and reliability, may also be planned [4]. There are good commercial prospects for BIPVT (Integrated Photovoltaic-Thermal Building) systems, especially for integrated roofing, insulation properties, and material savings, and heat pump support systems [5].

Hybrid PVT systems can be classified based on working fluid or into flat-plate, flexible, and concentrated configurations, with an electrical, thermal, and combined efficiency range of 6.7–15%, 22–79%, and 40–87% respectively [6], [7]. Several studies on the electrical performance of PVT devices using various solar cells and operating fluids have been conducted [8], [9]. Current findings have shown that various types of thermal absorbers are well suited to different PVT modules such as sheet-and-tube configuration, rectangular tunnel with or without fins/grooves, micro-channel heat pipe array, an extruded heat exchanger (PV layer back sheet attached), and roll-bond heat exchanger [10], [11].

Heat pipes with their different types could passively transport a large amount of thermal energy in a small unit size. Wickless heat pipes, also known as closed thermosyphon or gravitationally assisted heat pipes, have the advantages of the basic structure, low cost, lightweight configuration, high reliability, and long service life. Furthermore, the heat pipe working medium adopts phase change heat transfer, the overall heat pipe temperature gradient is minimal, and so the solar panel has good temperature uniformity.

In China, the use and testing of heat pipes in a PV solar collector has been undertaken. Based upon the idea that wicked heat pipes and a PVT flat-plate collector are integrated into a single unit, the heat pipe PVT (HP-PVT) collector system experimental rig was designed and developed by Pei et al. [12], [13]. They found that HP-PVT collectors could be used in cold environments without freezing and can also minimize corrosion. The results indicated that the HP-PVT system can provide 45 °C hot water for 172 days/year in Hong Kong without axillary heating and the daily thermal and electrical efficiencies could reach 41.9% and 9.4%, respectively. Tang et al. [14] have developed a new method for cooling photovoltaic panels using a micro heat pipe system. As compared to traditional PV panels, using air as a cooling medium results in a reduction of around 4.7 °C in PV cell temperature and an improvement of 8.4% in electrical power output. Furthermore, using water cooling, the cell temperature is reduced by around 8 °C and electrical power output increased by 13.9%.

Shittu et al. [15] studied a micro-channel heat pipe-based photovoltaic-thermoelectric system experimentally. In this design, the thermal energy extracted from the photovoltaic cells and transmitted through a flat heat pipe is reused to create more electricity by using a thermoelectric generator. Results show that the hybrid system provides enhanced performance compared to the photovoltaic system and the absence of insulation behind the micro-channel heat pipe enhances the electrical performance of the hybrid system.

Recently, González et al [16] designed a new (PV/T) wicked heat pipe solar collector panel with lauric acid as a PCM tank added to the backside of the photovoltaic panel. Experimental was tested under two different operating configurations and resulted in an overall daily efficiency of 50% for the first operating configuration and 30% for the second configuration. They noted, however, that the ambient temperature has a big impact on the heat that is generated for the proposed design. However, future studies may be carried out focusing on the insulation effect.

Gang et al. [17], [18] proposed a novel photovoltaic/thermal (PVT) water wicked heat pipe system tested under various experimental conditions. The results revealed that water mass flow rate has a significant effect on PVT overall performance, which improves as the water mass flow rate rises. Besides, any decrease in the heat pipe tube's pitch distance leads to an increase in the PVT heat and electrical efficiencies. Zhu et al. [19] also investigated the impact of heat pipe pitch spacing on the energy efficiency of the heat pipe PVT collector and found that smaller heat pipe spaces had better PVT electrical energy output. Despite the important results, many parameters including heat pipe diameter, panel surface, and optimal heat pipe pitch spacing have not been investigated.

Zhang et al. [20] propose a transitional model for a wicked HP-PVT system. An experimental study was also conducted on a sunny day and a cloudy day. Sensitivity analysis was conducted investigating the effect of heat pipe geometrical parameters such as evaporator and condenser lengths, heat pipe width, and working fluids type. Results have shown that thermal and electrical efficiencies increase with an increase in heat pipe diameter, condenser and evaporator section dimensions as well as heat pipe number.

Zhang et al. [21] have introduced a novel solar photovoltaic/loop-heat-pipe (PV/LHP) concept for heat pump operation. Test findings showed that the electrical, thermal and overall performance of the PV/LHP module was approximately 10%, 40%, and 50% respectively under the specified test conditions. The effects of operating parameters on the device’s operation, such as solar radiation, ambient air temperature, wind speed, glazing cover, and the number of heat-absorbing pipes, were also investigated. Diallo et al. [22] simulated a solar PVT collector employing a loop heat pipe employing a microchannel heat pipe evaporator and a PCM triple heat exchanger which presented system improvement of 28% and could theoretically achieve an electric efficiency of 12.2% and a thermal efficiency of 55.6%.

Qian et al. [23] have developed a new concept for the design of an integrated PVT system using oscillating heat pipes. The concept is intended to convey heat as a facade-assembled component from the hidden PV cell layer. The primary components included are oscillating heat pipes, risers and headers, finned tubes, graphite conductive plates, metal frames, PV laminate modules, and back insulations.

Behrooz et al. [24] simulated the PVT type of the closed two-phase loop mini tube thermosyphon (CLTPT) solar water heater using EES software and results show that optimal five loop tubes can be used. The highest water tank temperature near 72 °C is achieved in the evening and the maximum thermal and overall panel efficiencies are found to be 70% and 80% at noontime, respectively.

Souliotis et al. [24] conducted an experimental analysis of the thermosyphonic type of PV/T system under the climatic conditions of Patras. The results show that PV cell temperature is reduced, and that the device can meet both hot water and energy requirements for domestic applications with a reasonable degree of performance.

Hughes et al. [25] conducted a numerical analysis to increase the efficiency of photovoltaic panels, based on the output of the finned heat pipe connected to the back of the photovoltaic panel studied using CFD. They proposed and studied to calculate the enhanced heat dissipation and thus to increase the efficiency of the photovoltaic panel.

It is inferred from the literature review that PVT/HP solar panels can be a feasible way to enhance PVT system PVT performance. Methods have not yet been investigated and reported for wickless heat pipe PVT panels (PVT/WHP). Moreover, most PVT wicked heat pipe systems used water as working fluid while both acetone and methanol showed better performance than water as heat pipe fluid working medium [26], [27] and several studies have used acetone as a working fluid in heat pipe solar collectors to achieve better performance than other working fluids [28], [29]. Shafieian et al [30], [31] carried out strategies to improve the thermal performance of heat pipe solar collectors and revealed that the highest exergy efficiency occurs in the heat pipe thermosyphon acetone for air velocity of 2 m/s.

Wickless Heat pipes are a two-phase, passive heat transfer system (no external power source) that can transport a large amount of heat and have a high thermal conductivity. In comparison to other PVT systems, they can preserve a relatively uniform temperature distribution across the panel and provide a fast-thermal response. As compared to wicked heat pipe systems, they have the advantage of being simple to build, maintain, and cost.

An assessment investigation for a photovoltaic/thermal acetone wickless heat pipe (PVT/WHP) solar system was conducted in this study, which fulfilled a gap in the literature. The main goal of this study is, therefore, to carry out a parametric study to evaluate parameters affecting the overall performance of the acetone PVT/WHP system. The PVT/WHP system was also analyzed under Tunisian climatic conditions and a mathematical model was developed to achieve the present goal. The model was verified using an energy balance analysis and experiment results on a PVT solar collector under the same conditions.

Section snippets

Thermal analysis

The main components of the flat-plate PVT modules are the glass cover (optional), flat-plate PV module, adhesive, aluminum thermal absorber, and insulation (Fig. 1). The adhesive often consists of Ethylene-Vinyl Acetate (EVA) and a layer of Tedlar-Polyester-Tedlar (TPT). The insulation layer prevents heat from escaping into the surrounding area [33]. For wickless heat pipe, acetone is used as a working fluid with a 50% full fit ratio. The thermal resistance network is shown in Fig. 2 where we

Result discussions

The key simulation parameters, such as ambient conditions, cooling system parameters, and heat pipe specifications, are summarized in Table 1. The parametric analysis's main goal is to investigate the effects of different factors on the solar collector, such as irradiation, the number of heat pipes, velocity, ambient, and water inlet temperature. The research is extended to assess the system in the city of Tunis (North Africa), which is located at 36° 49′ 08″ latitude and 10° 09′ 56″ longitude.

Conclusion

This paper described a new solar photovoltaic/wickless heat pipe water heating system that uses acetone as the working fluid in the heat pipes. A mathematical model was developed and solved based on the heat balance mechanism to simulate the performance of the PVT/WHP system. A parametric analysis was performed, as well as simulations under Tunisian climatic conditions. It was found that:

  • 1.

    Lower incident radiation, higher wind speed, and lower ambient temperatures result in higher electrical

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.

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