NewTrans1.txt[9/15/2024 8:15:35 PM] of orbits whose useful portion will be very high, not to say geostationary. Let us assume that h = 10,000 km. At best, for D = 30 m, we find d = 2.1 m. Given the usual quality of beams from high-power lasers, a minimum ground spot diameter of between five and twenty meters seems more realistic. However, even for J = 5 m, and a 100 MW laser, the energy density will only be 0.5 kW/cm2, therefore below the threshold defined above. Conversely, we could look for what would be the optimum concentration of energy at the receivers, depending on the receiver adopted. 6. ENERGY RECEPTION A number of sensors are capable of transforming the laser beam into electrical or mechanical energy. Overall, these sensors are no different from those that could be used to exploit direct sunlight. However, the specific characteristics of laser radiation, namely its temporal coherence (or monochromaticity) and its spatial coherence (perfectly “ordered” radiation, with zero entropy) allow us to expect higher yields than what would be obtained from solar radiation. Since electrical energy is generally the most convenient form, we first think of photovoltaic, thermionic or thermoelectronic cells. 6.1 Photovoltaic Cells There are cells adapted to all wavelengths between 0.4 and 11-12 gm. Their working wavelength being defined by the laser, the semiconductor gap can be exactly adapted to the energy of the received photons, and under these conditions efficiencies of 30% to 40% seem to be expected. It should be noted, however, that cells intended to operate in the infrared will have to be cooled to maintain an acceptable efficiency, which will require on the one hand a poorly focused laser beam (to lower the energy density on the ground), and on the other hand cooling circuits that will penalize the overall efficiency of the system. 6.2 Thermionic and Thermo-Electronic Cells These elements are still at a preliminary stage. Their efficiency would be around 40%, their optimum use would require a high energy density. 6.3 Thermal Engines Overall, electrical conversions do not seem, even in an optimistic hypothesis, to allow high efficiencies to be achieved. The situation is a little more favourable with thermal machines, especially if we take into account that the coherence of the beam from a laser makes it possible to obtain, with appropriate optics, very high concentrations of energy. Under these conditions, the high temperatures accessible would make it possible to achieve efficiencies surely higher than 60% and perhaps beyond by combining several successive cycles (for example Brayton and Rankine). Various other principles have also been proposed to achieve efficiencies of 90%, but have not yet been the
RkJQdWJsaXNoZXIy MTU5NjU0Mg==