Performance study of heat-pipe solar photovoltaic/thermal heat pump system

doi:10.1016/j.apenergy.2016.12.145

Applied Energy

Volume 190, 15 March 2017, Pages 960-980

https://doi.org/10.1016/j.apenergy.2016.12.145 Get rights and content

Highlights

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The testing device of HPS PV/T heat pump system was established by a finished product of PV panel.
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A detailed mathematical model of heat pump was established to investigate the performance of each component.
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The dynamic and static method was combined to solve the mathematical model of HPS PV/T heat pump system.
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The HPS PV/T heat pump system was optimized by the mathematical model.
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The influence of six factors on the performance of HPS PV/T heat pump system was analyzed.

Abstract

A heat-pipe solar (HPS) photovoltaic/thermal (PV/T) heat pump system, combining HPS PV/T collector with heat pump, is proposed in this paper. The HPS PV/T collector integrates heat pipes with PV panel, which can simultaneously generate electricity and thermal energy. The extracted heat from HPS PV/T collector can be used by heat pump, and then the photoelectric conversion efficiency is substantially improved because of the low temperature of PV cells. A mathematical model of the system is established in this paper. The model consists of a dynamic distributed parameter model of the HPS PV/T collection system and a quasi-steady state distributed parameter model of the heat pump. The mathematical model is validated by testing data, and the dynamic performance of the HPS PV/T heat pump system is discussed based on the validated model. Using the mathematical model, a reasonable accuracy in predicting the system’s dynamic performance with a relative error within ±15.0% can be obtained. The capacity of heat pump and the number of HPS collectors are optimized to improve the system performance based on the mathematical model. Six working modes are proposed and discussed to investigate the effect of solar radiation, ambient temperature, supply water temperature in condenser, PV packing factor, heat pipe pitch and PV backboard absorptivity on system performance by the validated model. It is found that the increase of solar radiation, ambient temperature and PV backboard absorptivity leads to the increase of the coefficient of performance based on thermal (COP_th) of HPS PV/T heat pump system, while the increase of supply water temperature in condenser, PV packing factor and heat pipe pitch leads to the decrease of COP_th. Furthermore, the increase of solar radiation and packing factor leads to the increase of the advanced coefficient of performance based on both thermal and electrical performances (COP_PV_/_T), while the COP_PV_/_T decreases as the ambient temperature, supply water temperature in condenser and heat pipe pitch increase. The PV backboard absorptivity has little influence on the COP_PV_/_T of HPS PV/T heat pump system.

Introduction

Shortwave radiation can be converted to electricity using photovoltaic (PV) technology. However, the photoelectric conversion efficiency is only 5–20% [1]. It is found that the photoelectric conversion efficiency is dependant on the temperature of PV cells [2], [3], [4]. When the PV cells’ temperature is higher than 25 °C, the temperature increase of every 1 °C results in the reduction of power generation efficiency by 0.5% [5], [6]. It is also observed that the extracted heat from PV panel could be employed for use by photovoltaic/thermal (PV/T) technology [7]. Moreover, the photoelectric conversion efficiency could be improved due to the reduction of PV cells temperature. But it’s difficult to make full use of the extracted heat because of the intermittency of solar energy and low collector efficiency of PV panel. Therefore, various PV/T panels have been put forward in previous studies, including air-cooled PV/T panel [8], [9], [10], [11], [12], [13], [14], [15], [16], water-cooled PV/T panel [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], direct-expansion PV/T panel [27], [28], [29], [30], [31], [32], heat-pipe solar (HPS) PV/T panel [33], [34], [35], [36], [37], [38] and HPS PV/T heat pump system [39], [40], [41], [42], [43].

The simplest method to cool down PV cells is air-cooled PV/T panel by natural or mechanical ventilation [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, the natural ventilation method is restricted by weather conditions, and the mechanical ventilation method has a high cost. In addition, the increase of photoelectric conversion efficiency for this structure is poor due to the limited cooling effect. Alternatively, a circulating water channel could also be used to extract heat from PV panel [17]. The cooling effect of this method is better than that of air-cooled panel, but the inner structure of PV cells could be destroyed in cold weather conditions due to the freeze of water pipe on the backboard of PV panel. A more advanced PV/T structure is direct-expansion PV/T panel. Evaporation coils are placed beneath PV panel so that the refrigerant can pass through it [27]. The PV cells in this structure can be cooled down to a very low temperature, leading to higher photoelectric conversion efficiency and better utilization of the extracted heat. But this structure has some inherent problems, such as a mass use of copper coils and gas tightness.

As for the heat-pipe solar (HPS) PV/T structure, heat pipes are integrated with PV panel so that PV cells can be cooled down to a relatively low temperature. In addition, the freeze of water pipes can be avoided by using heat pipes. Pei et al. [34] designed a novel HPS PV/T system and developed a dynamic model to predict the performance of this system. The results indicated that the thermal efficiency was 41.9% and the electrical efficiency was 9.4%. Zhang et al. [35] established a simulation model of the HPS PV/T system using TRNSYS. The tank volume of the HPS PV/T system was optimized, the electric power generation was also calculated and the tilt angle of the HPS PV/T collector was optimized based on the model. When the HPS PV/T panel is combined with heat pump, the extracted heat can be fully utilized just like direct-expansion PV/T panel. Moreover, the HPS PV/T heat pump system not only has advantages of the above structures, but also avoids disadvantages that the above structures may occur. Zhang et al. [39] studied the dynamic performance of a HPS PV/T heat pump system. They established a mathematic model integrated the transient processes of solar transmission, heat transfer, fluid flow and photovoltaic power generation appropriately. It was concluded that the HPS PV/T heat pump system could harvest significant amount of solar heat and electricity, thus improving the solar thermal and electrical efficiencies. Fu [40] constructed a testing rig of a HPS PV/T heat pump system. Energy and exergy analyses were conducted to investigate the overall system performance and the optimum operation mode. A dynamic model of the HPS PV/T heat pump system was presented. Using this model, the dynamic parameters were predicted and analyzed under different intensities of solar irradiation, and the instantaneous system performance was also simulated and studied.

Previous researches on HPS PV/T heat pump system have achieved prominent results. Many mathematical models and/or testing rigs were established to predict and investigate the system performance. Some performance evaluation methods were also presented in the previous researches. However, for those studies, the dynamic mathematical models of heat pump are quite simple and difficult to provide more accurate prediction of system performance, and the stability is poor as well. Meanwhile, they provide little investigation and discussion on the optimization of the HPS PV/T heat pump system and the influence of affecting factors such as solar radiation, ambient temperature, supply water temperature in condenser, PV packing factor, PV backboard absorptivity and heat pipe pitch etc on system performance. In this paper, a relatively more detailed mathematical model of the HPS PV/T heat pump is established, which makes it possible to provide a more accurate prediction of the state parameters of refrigerant and water at the inlet and outlet of each component of heat pump, compressor power and system performance as well. The optimization of HPS PV/T heat pump system and the discussion on the influence of affecting factors on system performance are also presented in this paper. In addition, an advanced method that combines the quasi-steady state method and the dynamic method is used to solve the mathematical model of HPS PV/T heat pump system in this paper.

Section snippets

System descriptions

The HPS PV/T heat pump system consists of a HPS PV/T collection system and a heat pump system. The HPS PV/T collection system includes water-storage tank, circulating pump and HPS PV/T collector. The experimental set-up of HPS PV/T collector and heat pump is shown in Fig. 1. The HPS PV/T collector is mainly made up of PV panel, heat pipes, aluminum sheet and manifold. Fig. 2 shows the cross-section view of HPS PV/T collector. A piece of YL200P-23b PV panel (including PV cells and backboard)

The mathematical model of HPS PV/T collection system

The mathematical model of HPS PV/T collection system consists of six main equations: energy balance equation of glass cover, energy balance equation of PV panel, energy balance equation of aluminum sheet, energy balance equation of heat pipes, energy balance equation of manifold, energy balance equation of water-storage tank. In this study, some assumptions are made as follows:

(1)
The heat loss of water pipe is neglected.
(2)
The pressure drop of water circulation is neglected.
(3)
A mean temperature is

Program algorithm

The dynamic and quasi-steady state methods are combined to solve the model of HPS PV/T heat pump system. The dynamic method is used to solve the model of HPS PV/T collection system, while the quasi-steady state method is used to solve the model of heat pump system during the time step. When solving the mathematical model of HPS PV/T collection system, Newton’s backward interpolation formula is used to discretize the energy balance equations, then solve the simultaneous equations to obtain the

Results and discussion

The experiment is carried out in Beijing University of Civil Engineering and Architecture, a university in the north of China (116.3°E, 39.9°N). The solar collector is installed to be south-facing with a tilt angle of 30°. The experiment commences at 8:30 a.m. and concludes at 16:30 p.m. (August 12, 2015) with the data collection interval of 10 min. The mean solar radiation intensity and ambient temperature are 656 W/m² and 37 °C, respectively. The initial ambient temperature is 30.5 °C. The initial

Conclusions

A numerical and experimental study on the performance of a HPS PV/T heat pump system is carried out in this paper. A testing rig of the HPS PV/T heat pump system is constructed. A mathematical model, including a dynamic distributed parameter model of the HPS PV/T collection system and a quasi-steady state distributed parameter model of the heat pump system, is presented to assess the performance of HPS PV/T heat pump system. The mathematical model is validated with the testing data. The

Acknowledgements

The work of this paper is fully supported by Projects of National Key Research and Development Program (2016YFC0700104), National Natural Science Foundation of China (51206004) and Beijing Municipal Key Lab of HVAC (KF201004).

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Performance study of heat-pipe solar photovoltaic/thermal heat pump system

Highlights

Abstract

Introduction

Section snippets

System descriptions

The mathematical model of HPS PV/T collection system

Program algorithm

Results and discussion

Conclusions

Acknowledgements

Appl Energy

Renew Energy

Sol Energy

Appl Therm Eng

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Energy

Appl Therm Eng

Sol Energy

Appl Energy

Appl Energy

Appl Therm Eng

Appl Therm Eng

Appl Energy

Appl Energy

Appl Energy

Appl Therm Eng

Sol Energy

Appl Energy