Uniform cooling of photovoltaic panels: A review

Abstract

Cooling of PV panels is a critical issue in the design and operation of concentrated photovoltaic (CPV) technology. Due to high cell temperature and non-uniform temperature distribution, current mismatching problem and hot spot occurs on the cell resulting in either reduction of efficiency or permanent structural damage due to thermal stresses. Temperature non-uniformity on the surface of PV panel has a major impact on the performance of CPV systems and directly increases cell temperature and series resistance. This review paper highlights the importance of uniform PV cooling by exploring the possible causes and effects of non-uniformity. Cooling techniques with low average cell temperature and uniform temperature distribution are analyzed. Economic and environmental impact on the importance of cooling of PV systems are discussed and an experimental case study is presented for comparison between uniform and non-uniform cooling methods. Immersion cooling is a promising solution for uniform cooling and has been reported to reduce the cell temperature to 20–45 °C for CPV systems. Heat pipes reduced the temperature down to 32 °C with the best case temperature non-uniformity of 3 °C. Passive cooling by heat sinks was found to reduce the cell temperature as low as 37 °C for high concentrations but with an expense of large heat sink area. Active cooling by microchannels, impingement cooling and hybrid microchannel-impingement cooling were found to be most effective in dissipating high heat flux from PV surface. Cell temperature was reported to decrease to 30 °C for 200× CPV using impingement cooling. For hybrid cooling, deviation of 0.46 °C surface temperature was obtained. Using PCM materials temperature of panel was controlled within 28–65 °C whereas optimization of heat exchanger designs also showed low and uniform temperature across surface. The impact of non-uniformity was found to be significant for all PV systems however the effect is more pronounced in CPV systems.

Introduction

Thermal management of photovoltaic (PV) systems plays a key role for low and high concentration systems. The overall performance of solar panels is strongly dependant by the affect of PV cells׳ operating temperature. Typically the efficiency of solar cells is around 15%. The remaining component of radiation absorbed in the cell transforms into thermal energy which causes the temperature of the cell to be increased. This further causes the panel efficiency to decrease as the open circuit voltage and fill factor is reduced with rising temperature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The drop in efficiency due to these reasons decreases the global system performance and leads to an increase in cost per unit power. To achieve optimum and better solar cell performance, innovative cooling techniques for maintaining the solar cell temperature at operating conditions are required.

In addition to cell temperature, non-uniform temperature and non-uniform illumination across the cells have been found to affect the cell efficiency and overall system performance in a negative way [12], [13], [14], [15], [16], [17]. Series connected PV cells faces greater damage as current directly varies with light, so the current in an array of series of identical PV cells will be restricted by the solar cell with the minimum irradiation. This failure is called as the ‘current matching problem’. This issue can be solved by using bypass diodes or by keeping a uniform temperature across each series connection [13], [18]. The uniform temperature across PV can be achieved by using conventional and non-conventional cooling mechanisms with novel designs for high heat dissipation. Low solar cell temperature and high temperature uniformity are one of the most important characteristics affecting the overall performance of PV systems [13]. Non-uniformity in temperature distribution affects the PV system performance in two ways: (1) cells may experience efficiency loss due to loss in power output; (2) temperature variation induces thermal fatigue because of large amount of thermal cycles and stresses. This further cause the irreversible damage to solar cell due to excess localized heating across the region and reduces the reliability of the system [13]. The cooling method should be such that it keeps the average cell temperature at its minimum with a uniform distribution. This will help in the appropriate design of CPV systems and for proper calculation of the global PV performance. Fig. 1 shows typical behavior of solar cell with increasing temperature. As the cell temperature increases, electrical efficiency of the solar cell decreases [19].

The aim of this paper is to highlight the need for uniform cooling of PV panels for low and high concentrated systems by exploring the possible causes and effects of non-uniformity in PV systems. The review on uniform cooling mechanism for PV panels is currently not available in the literature. To address the above issue, cooling methods which can cater the requirement for high heat flux and uniform cooling has been reviewed from literature and discussed. Economic and environmental impact of cooling systems for PV panels is also studied. An experimental case study is presented for the validation of uniform and non-uniform cooling techniques. Finally potential cooling techniques that could be utilized for decreasing temperature non-uniformity has been compared and analyzed for improvement in the overall performance of the PV systems.