Fig. A 6.5 Example of large scale solar based energy supply system Electricity Heat / cool Fuel cell cars Water Hydrogen Fuel cells Hydrogen Heat driven cooling local cooling Main grid Electric cars Power plant Desalinati on Mirrors Heat storage Piped water Irrigation 6.3 Environmental impact on PV cell efficiency The efficiency of solar PV modules are measured under standard conditions that are supplied by the manufacturers. These conditions are: • A solar insolation of 1000 W/m2 on the module. • A surrounding air temperature of 25 C. • No dust on the module. Under operation these conditions are not met, the insulation is on average smaller and the temperature higher and there will be some amount of dust on the module. Calculation programmes for calculation of output from PV system always take into account the variation in insolation and the angle of radiation. The influence of the temperature and the dust is in most programmes counted for as a derating factor. In the programme HOMER, used in this analysis, a derating factor of 90% (10 % losses) has been used. This includes reductionfor higher temperature and for dust and other minor losses. For Oman the influence of the temperature is of particular interest as the temperature is very high. Therefore this has been analysed further. On the basis of laboratory tests and mathematical analysis it has been established that the drop in efficiency for solar modules amount to about 0.4 percent per increase in module temperature. The module temperature depends on the surrounding temperature, the insulation as well as on the way the module is mounted. A module mounted freely will be cooled by the air and the efficiency higher. The analysis has been made with a linear temperature model. This model determines module temperature through linear dependence, from solar irradiation (G) as follows: T Module = Ta + k * ( G / GSTC ).
The value of the constant “k” is dependent on the type of module installation and a measurement for the maximum possible temperature at maximum insolation (GSTC = 1000 W/m2) compared with the external temperature “Ta”: • Free-standing: k = 20°C. • Roof-mounted, with ventilation space: k = 30°C. • Roof or façade-integrated, without ventilation space: k = 45°C. With this model and on the basis of hourly values for insolation and external temperature from SEEB in Oman the following table has been prepared. Table A 6.1 Influence on the efficiency of solar PV modules by the cell temperature for different types of mounting Average outside air temperature during PV operational hours Degree C Average Module temperature during operation Degree C Average drop in efficiency, Percentage 1) Free-standing 31 44 7.9 Roof-mounted, with 31 51 10.6 ventilation space Roof or facade-integrated, without ventilation space 31 60 14.5 1) Compared to standard test conditions for PV modules. Insolation of 1000 W/m2 a and surrounding air temperature of 25 C. Table A 6.1 shows that the reduction in efficiency and output is about 8, 11 and 15 percent for the three different mounting. The corresponding maximum temperature (on the hottest day) is about 70 C, 80 C and 100 C for the three different mounting systems. The influence of dust depends naturally on the environmental conditions as well as on the cleaning frequency. In climates with frequent rain there will after some time be a steady state where dust and dirt is removed by the rain in the same speed as it is coming. The percentage reduction will then depend on the slope and is believed to be about 10% for a slope of 15 degree and smallest with a slope of 45 degree, where the rain is still effective. In Oman the slope should not be higher than 15 degree (for freely mounted modules) as this will reduce the output considerably. With the low and unpredictable rainfall it will be necessary to clean the PV panels from time to time in order not to reduce the output too much. For larger systems the efficiency will be continuously monitored and will indicate when a cleaning is necessary. Methods with use of water as well as without water exist. Also new materials and coatings for the glass have been developed that should reduce the problem. For the purpose of the analysis in this report the reduction as a result of temperature has been set to 8 percent and the reduction caused by dust to 2 percent.