Tham khảo tài liệu 'nonimaging optics winston episode 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Solar Thermal Versus Photovoltaic Concentrator Specifications 319 case of the CPC at least a small gap must be provided. Moreover these thermal aspects are responsible for the difference between thermal and photovoltaics with respect to the use of optically dense transparent media n 1 surrounding the receiver. This is difficult to apply for thermal concentrators and if excluded the concentration limit is reduced by a factor n2 for solar thermal systems which is n2 for typical materials . Another aspect to consider is the electric nature of the photovoltaic cell. The necessity of extracting the photogenerated current and of interconnecting the different cells must be considered at the optical design stage. Also the use of metallic mirrors in contact with the cell should be avoided to prevent short circuits. Obviously these problems do not matter for solar thermal systems. Also the spectral sensitivity of the receivers is different in both fields. For example the quantum efficiency of the silicon solar cells is very low over 1 200 nm. This means that the further infrared radiation of the sun which usually is nearly 25 of the sun power is not useful contrary to the solar thermal case. These spectral differences may make it so that a good optical material for photo-voltaics may be unsuitable for solar thermal applications due to its poor infrared response. Finally the last aspect to be considered here is the receiver geometry. This geometry affects the design importantly as we have seen in Chapter 5. In the case of medium to high concentration in photovotaics the active surface of the solar cell is always flat typically with a round or squared contour of the active area. Sometimes the cells tessellate to build a strip receiver. The inactive face of the cell is used to evacuate the heat by fixing a heat-sink to this face. Only low-concentration systems typically C 5 can use bifacial solar cells that is cells that can collect the sunlight on their two .
