ReviewA review of PV–T systems: Thermal management and efficiency with single phase cooling
Introduction
With the growing environmental concerns and the depletion of fossil fuel, the world is paying more attention to the use of renewable energy. One source of renewable energy is solar energy, harvested through photovoltaic (PV) cells. The photovoltaic cells convert the solar energy into electrical energy. The solar-to-electrical conversion efficiency of PV cells is relatively low, which means most of the absorbed thermal energy is unused. This unused thermal energy could lead to tremendous temperature increase in the PV cells. It is known that PV efficiency decreases by 0.45% for every degree rise in temperature [1]. This shows that the efficiency can drastically drop since the PV temperatures can reach as high as 80 °C particularly in the summer. For this reason, various PV cooling methods have been investigated. Effective cooling of PV modules not only increases efficiency but also leads to an increased longevity in the life of the PV cells as the thermal stress are decreased.
A newer and alternative to the classical and more efficient version of the PV module is the photovoltaic–thermal (PV–T) module. PV–T not only makes use of the solar energy by converting it to electrical energy, but also makes use of the excess thermal energy. This is done most often by using a working fluid or coolant, such as air or water, to remove the excess thermal energy that would otherwise raise the PV cell temperatures. This extracted thermal energy can then be used for heating purposes. Therefore, the cooling fluids not only increase the electrical efficiency but also the thermal efficiency of the PV–T system.
Fig. 1 shows the cross-sectional view of a typical PV–T module cooled by water, which have been used by researchers, including Tiwari and Sodha [35] and Zevallos et al. [36]. The module is composed of several “layers” made of different materials, such as glass, silicon (solar cell), EVA, tedlar, copper (flow channel), fluids, and insulating materials to function as a hybrid electrical/thermal collector. Obviously, conjugate heat transfer is the energy transport mechanism across the layers with different thermal resistances. Of the most notable are the convection–radiation heat transfer on the PV panel surface and the convective heat transfer inside the cooling channels, which can be controlled to improve the efficiency of the system.
Several reviews on the subject of PV–T have previously been carried out by researchers in the past years. For example, Tyagi et al. [2] reported the advances in PV–T system and collector technology. Their work covered solar cells material, concentrating collectors and PV–T collectors. Chow et al. [3] reviewed the advances over the past decade in material, collector designs and geographical implementation. Moradi et al. [4] reported the effects of various control parameters on the efficiency of PV–T systems. Their work includes the identification of the parameters and their ranges of the existing research. Riffat et al. [5] carried out a through review of the PV–T system with emphasis on various types of PV–T systems, parameters affecting the performance of the PV system, qualitative evaluation of the electrical and thermal outputs, and various analysis models used. Their work however is limited up to the year 2011.
In this paper, we will provide a review of the recent advances in thermal management systems for the PV–T technologies over the last five years. The thermal and electrical performances under various thermal management systems are summarized. Up to date, various methods of PV–T cooling exist with different benefits and a varying degree of effectiveness. The most prominent methods of cooling include both single phase based active and passive cooling, with either air or water. This study primarily identifies and discusses the prominent methods of thermal management in PV–T analyzed by various authors using either experimental or computational methods. It is well known that experimentally testing of certain cooling configurations is very expensive. For this reason, various mathematical models are also discussed in this paper.
It should be mentioned that research has also been carried out on the use of two-phase cooling techniques. These methods work on the basis of latent heat transfer from the PV modules to the working fluid, leading to phase change. For example, such method has recently been investigated by Pellicone et al. [6] who applied the scheme using microchannels. Reeser et al. [7] also investigated the effects of two-phase cooling using R134 refrigerant, on concentrated photovoltaic cells. Although numerous authors have investigated the two-phase cooling methods, this study focuses on the review of single-phase flow cooling techniques for PV–T systems. We would also like to acknowledge that due to limited space, not all of the papers on this subject, some of which are excellent, are included in this paper.
Section snippets
Air cooling
One of the most common and easily accessible methods of cooling is through air-cooling. This is where air is used as the working fluid to remove heat from the PV–T system either through natural convection or forced convection [31], [32], [33], [34]. Force convection, where air is forced through the system usually by a fan, can further be broken down into continuous cooling or intermittent cooling. Unlike in the case of natural convections, efficiency calculations in forced convections must also
Water cooling
Water cooling methods provide improved performance over the air cooling methods due to the increase in heat carrying capacity of water over air. These methods make use of water, chilled or unchilled, as the working fluid. Unlike in air cooling methods that are limited to traditional natural and forced convections, water cooling has a broader range. Some methods that implement water cooling are natural and force convections, front water cooling, heat pipe and immersion techniques. This section
Unconventional cooling methods
In addition to air and water based cooling techniques, which are popular and relatively cost-effective. Researchers have also developed other unconventional cooling method for applications in PV–T systems. For example, Huang et al. [28] have applied phase change materials (PCM) to cool the PV system, and investigated the effect of convection and crystalline segregation on the heat transfer efficiency. Chen and Wei [29] utilized a glass vacuum tube (GVT), shown in Fig. 19, coupled with a heat
Efficiency by different cooling methods
Fig. 20 shows the difference in electrical efficiency from before to after cooling that various authors have achieved with various cooling methods. It can be seen that many methods are able to increase the efficiency by over 10%. There is a great potential to achieve a very high efficiency improvement through the application of cooling technologies.
Although air or water is readily available, to achieve more significant improvements in the efficiency, chilled water must be used as a coolant
Conclusion
In the review, we summarized various cooling methods and reported results over the last five year, with emphasis on thermal management and efficiency using single phase flow based cooling techniques. Various air-cooled and water-cooled, including natural and forced convection and immersion techniques, have been discussed. All of the reported methods were compared on their ability to reduce module temperature and increase efficiency. The reviewed results shown that PV system’s efficiency and
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