Besides increasing the power and reducing the size or number of cells used, concentrators have the additional advantage that cell efficiency increases under concentrated light. The increase in efficiency depends largely on the cell design and the cell material used. Another advantage of the concentrator is that it can use small individual cells because it is harder to produce large-area, high-efficiency cells than to produce smaller – area cells. Due to the very low temperature coefficient of the III-V multi-junction concentrator solar cell, the performance of CPV systems is much less affected by temperature than any other PV technology, i. e., the loss of efficiency is approximately one third that of c-silicon modules.
This characteristic is extremely important for the best solar sites in the world, which are generally located in the equatorial regions where ‘high temperatures’ and ‘direct solar radiation’ with virtually perfect incidence angles with values up to 2800kWh/(m2a) are available. Because of the low temperature coefficient, the efficiency and the electricity production of CPV systems are only slightly affected by high ambient temperatures in comparison to other PV technologies. The drop in efficiency with rise in temperature is by far the smallest for CPV systems as can be seen in Figure 5.8; the efficiency drop for a temperature difference of 40°K (e. g., from 25°C at standard testing conditions to typical operating cell temperatures of 65°C) is by far the smallest for CPV systems. The CPV technology, therefore, guarantees energy production during the entire day as well as maximum energy yield.
The other benefits that make CPV technology most cost-effective are: lower environmental effects, high potential for recycling, short energy payback time, requirement of less water for cooling compared to nuclear power plants and concentrating solar thermal systems, and most impressive potential for cost reduction.
There are, on the other hand, several drawbacks in using concentrators. The concentrating optics, for example, are significantly more expensive than the simple covers needed for flat-plate modules, and most concentrators must track the sun throughout the day and year to be effective. Thus, higher concentration ratios mean using not only expensive tracking mechanisms but also more precise controls than flat-plate systems with stationary structures. High concentration ratios are a particular problem, because the operating temperature of cells increases when excess radiation is
Efficiency loss Loss of efficiency (%/40K)
-2 -4 -6 -8 -10 -12 -14 -16 -18 -20
concentrated. This, in turn, generates heat that disturbs the long-term stability of PV cells. Hence, the cells have to be cooled.