addition to this, the lifetime of cells today is more than 10 years. Finally, the advancement of manufacturing techniques has, during the same period, enabled the cost to be reduced by a factor of more than 5. Most of the cells manufactured today use crystalline silicon technology. This technical line is expected to be continued for some time yet with the development of silicon tape technology. Other techniques have been studied for some years. Gallium arsenide cells, for instance, have an efficiency noticeably higher than that of silicon cells (panels currently in space 20% for GaAs compared to 14% for silicon), and, in their present design on thinned germanium substrates they are not generally heavier, but their price is still higher which makes them less attractive. However, gallium arsenide has the advantage of withstanding space radiation better than silicon. Research today is oriented towards thin layer photocells. Development of such cells would enable a reduction in material costs as well as the implementation of more automated manufacturing techniqueswhile still obtainingsatisfactory efficiency. Moreover, advanced concepts of multispectral photocells are being studied with an expected efficiency of more than 40% (50% under simple concentrators accepting an off-tracking of about 2 degrees). Gallium arsenide can also be used for the photovoltaic conversion of an incident laser beam (with over 50% efficiency demonstrated in two laboratories), which could be a useful technique for transmitting energy through space. Also, the future utilization of lunar silicon could constitute an interesting field worth some consideration. Thermal Conversion Parabolic concentrators orientate the high flux solar radiation toward an absorber with a cavity, where a working fluid is heated up to drive conventional turbomachines. Fhe electrical energy is generated by alternators coupled to these turbomachines. The thermodynamic cycles envisaged are well-known in terrestrial applications: Brayton, Rankine or Stirling. For example, two prototypes of 5 and WkW with Brayton cycle unites were developed in the Ukraine and Russia respectively. The efficiency attained was 36.7%. The use of a gyroreactor, proposed in France, enables the attainment of an efficiency greater than 40%. A "gyroreactor" is a heat engine which takes advantage of an intense artificial gravitational field to obtain quasi-reversible compression and expansion of the working fluid (Xe or Kr). A significant drawback of thermal conversion lies in the fact that the expected high concentration requires a very accurate orientation of the satellite with respect to the line of sight of the Sun (0.1°).
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