Thin-film Cascades An important technology for the production of high-efficiency thin film arrays is the ability of thin films to be produced in multi-bandgap “cascade” structures. In the cascade structure, short wavelength (high energy) photons are absorbed in a high bandgap material on the top of the solar cell. The high bandgap material is transparent to longer wavelength (low energy) photons, which pass through and are absorbed by a second layer consisting of a photovoltaic material with lower bandgap. In a current-matched two-element cascade, the efficiency can be approximated, as equal to the top cell efficiency plus half the bottom cell efficiency. The best currently demonstrated thin-film cascade, reported by Siemens Solar, uses an amorphous silicon top cell on a CuInSe2 bottom cell. The achieved efficiency is 12.5% (AMO). Solar Array Paddle For the photovoltaic system, parameters of structures to support solar cell array are as important as for solar cells. Generally the structure is called solar array paddle or solar array wing and classified by three types, that are rigid paddle, semi-rigid paddle, and flexible paddle. The rigid paddle is the cheapest type and has the highest-reliability. Usually it consists of aluminum-honeycomb sandwiched by sheet of aluminum (Al) or Carbon Fiber Reinforced Plastic (CFRP). Lightweight Lattice Panel (LLP) which has lattice structure made by CFRP is the lightest paddle in the rigid paddle. When the power level is lower than about 5 kW, rigid paddle is lighter than the other types. The power to weight ratio of this type is about 30 W/kg in the case of 5kW class with using Si cells. This paddle was employed by various satellites till now and is recently employed by INTELSAT-VIL The semi-rigid paddle consists of CFRP lattice and CFRP sheet tensioned in the lattice. The weight is almost same as that of LLP and is employed by Japanese Earth Resources Satellite - 1 (JERS-1), Engineering Test Satellite - VI (ETS-VI), etc. The flexible paddle is suitable for large-power use in space. It is easy to fold in the fairing of the carrier. The weight of this paddle is lighter than the rigid paddle in the case of large power (more than 5 kW). This paddle consists of a thin blanket containing wire harness and usually is extended by a mast mechanism on orbit. When the power level is below 10 kW, the weight of the mechanism for holding and expansion is constant and is about 100 kg to 150 kg. Therefore, the higher is power, the bigger is the ratio of power to weight. There are two ways to expand this paddle. One is roll-type method and the other is fold-type method. The typical example of the roll-type method is the paddle of the Hubble Space Telescope which was launched in 1990. The power generation of this paddle is 4.8 kW(BOL), and the weight is 270 kg. The ratio of power to weight is 17.7 W/kg. The size of this paddle in expansion is 11.82 m x 2.83 m (without its mechanism), the ratio of power to area is 143 W/m2. The typical example of the fold-type method is the paddle of Space Station Freedom (SSF). 8 cm x 8 cm Si cells are mounted on this paddle and the generation power is 27.8 kW. The ratio of power to weight is 43 W/kg (BOL) and ratio power to area is 95 W/m2 (BOL). [Bailey,1992] Problems of Large Scale Solar Array Wing When the size of the solar array is large and its generation power is high, we are faced with a lot of problems with which we have not had experience until now. In this case, if we use current technology for the solar array paddle, we must use a high-voltage solar array to reduce transmission loss between solar cells and power transmission area in the spacecraft. It is ensured that the limit of operating voltage of solar array is determined by plasma induced discharge. The voltage threshold for breakdown is dependent on the plasma density and it may be about several handreds volts in Low Earth Orbit (LEO). The reason for this phenomenon is that the exposed interconnectors of high voltage biased solar cells make complicated electrical fields near the boundary between interconnectors and coverglasses, which are insulators. Therefore this may not be serious in GEO. Another way to reduce the transmission loss is to employ an Alternating Current (AC) system. In this case, solar array must contain some inverters and this may reduce the power density of solar array. In GEO, there is a problem of charge-up to several kilovolts on the surface of the spacecraft caused by high-energy particles. It is considered that this phenomenon may induce arc discharge and the electrical function will suffer from the discharge. In the case of the current satellites, it is
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