SHENYI WU and CHENGUANG XIONG
The operating temperature is one of the important factors that can affect the efficiency of the PV panels. The effects of temperature on photovoltaic efficiency can attribute to the influences on the current and voltage of the PV panels. This can be easily found on the I-V curve of the panels. It results in a linear reduction in the efficiency of power generation as temperature increases . The efficiency of some types of PV cells is very much dependent on their operating temperature. For crystalline silicon solar cells, the reduction in conversion efficiency is 0.4-0.5% for every degree of temperature rise . Therefore, reducing the operating temperature of photovoltaic cells is important for the PV panel to work efficiently and protect cells from irreversible damage.
Passive Cooling Technology for Photovoltaic Panels for Domestic Houses. © Wu S and Xiong C. International Journal of Low-Carbon Technologies 0 (2014). doi:10.1093/ijlct/ctu013. Licensed under the Creative Commons Attribution 3.0 Unported License, http://creativecommons. org/licenses/by/3.0/.
A number of researchers have worked on cooling the PV panels with different approaches. Air circulation is probably the most simple and natural way for this purpose. In order to enhance convection heat transfer, fins were used to extend the heat transfer area. Edenburn  developed a device, made up of linear fins on all available heat sink surfaces, used for cooling single cells passively. Araki et al.  did a further research on passive cooling technologies and found that good thermal conduction between cells and heat spreading plate was important. Combining PV and solar thermal collectors (PV/T) is another way of cooling PV panels. Tonui and Tripanagnostopoulos  reported their experiment on modified PV/T collectors, and results showed the maximum temperature reduction achieves 10oC by natural ventilation and 308C by forced ventilation.
As a good cooling media, water has been widely used for PV cooling in various forms. It is very suitable for PV/T systems. Kalogirou  studied a water-based PV/T system consisting of four monocrystalline PV panels in the Cyprus and achieved an increase of average annual electrical efficiency from 2.8 to 7.7% with the payback periods of 4.6 years. Tripanagnostopoulos et al.  compared electrical efficiency of PV/WATER, PV/ AIR and PV/FREE and PV/INSUL under ambient air temperature of 29oC. They achieved the maximum increase by 3.2% with PV/ WATER.
Krauter  investigated the method of covering PV modules with a flowing water film above. With the additional evaporation heat transfer, it was claimed that they could decrease the cell temperature up to 22oC and obtained a net increase from 8 to 9%. Abdolzadeh and Ameri  used water spray to cool the PV panels and achieved increasing the efficiency of cells by 3.26 to 12.5%. Kordzadeh  studied that a thin continuous film of water running on the front of the surface of modules obtained better electrical efficiency because of reducing reflection loss and surface temperature.
To avoid additional energy consumption incurred for cooling the PV panels, Furushima and Nawata  reported a model with cooling water being supplied from a city water supply system by Siphonage and the cooling system did not require any additional energy input on the site. Wilson  studied the gravity-fed technology where water was transported from upstream sources like river to downstream sources by gravity. The results obtained from this work showed a 12.8% increase in electrical efficiency as a result of 32oC temperature reduction.
Other technologies were also used to enhance the heat transfer for cooling the panels. Akbarzadeh and Wadowski  reported an innovative gravity-assisted heat pipe system to optimize the cooling of concentrated photovoltaics. It was found that the temperature at the surface of solar cells did not exceed 46oC during a 4-h test, and the efficiency was increased by 50%. Huang et al.  initially integrated PCM into BIPV system and used fins for improvement. Biwole et al.  established a numerical model and used CFD to simulate heat and mass transfer of PCM at the back of photovoltaic panels. Their results showed that adding PCM at the back of panels can maintain the operating temperature below 40oC.
Active cooling is effective to cool PV panels. However, with the additional power consumption involved, the active cooling purely used to lower the operating temperature does not have obvious benefit in the net gain of efficiency. The technologies such as PV/T (photovoltaic thermal) system or the PV-SAHP (photovoltaic solar heat pump) system [16, 17] seem to address the issue stated earlier by combination of two systems. But the fact that PV/T has to at a higher operating temperature in order to supply useful heat means the gain by cooling is limited. What is more, the higher initial investment and the final benefit with PV/T technology is contributed to thermal energy rather than electricity . This renders the PV/T being not an effective technology for the original purpose. Therefore, finding a simple and feasible way to cool the PV panel without requiring further energy input is still much sought after.