Figure 7.14 Coaxial geometric TPV converter [Angrist, 1982] The radiative collection efficiency, for an emissivity ratio of 0.1 or less, can be as high a 80%. In addition the solar cell efficiency of spectrally tuned cell is presented in the following graph. We can observe that the efficiency drops considerably. Then, for the operating temperature of 1500 K, the cell and emitter combination can only provide an efficiency of 30%. Nonetheless, the total conversion efficiency from radiative power to electrical power is greater than 20%. If we consider a 50% loss for the emitter conversion of heat to radiation we obtain 12% for the total system efficiency. With higher operating temperature such as 1800 K, this overall efficiency can be substantially increased up to 16 %. Additional information about the selective emitter thermal- to-electric conversion efficiency could be found in [Chubb, 1992]. Two main types of TPV systems exist. One uses a selective emitter material to radiate in the energy band where the PV cells are the most efficient. The other alternative is to heat a black body which serves as a thermal emitter. In this case it is preferable to use a filter to allow only the bandgap energy of a specific PV cell and reflect all other photons back to the emitter. This filter protects the solar cell from additional heat to prevent degradation of the PV material. In addition, suitable geometry such a coaxial system can minimize reflection losses by directing the energy that is not absorbed on initial impact to another converter surface. However, TPV systems require cooling for the PV cells. This subsystem handicaps the TPV concept because of the additional weight needed. In this regard, TPV, thermoelectric and thermodynamic conversion systems are very similar. New Concept of Thermal Engine Called “Gyroreactors” The principle behind this technology can be compared to the air flow in the atmosphere. On the ground the air gas is heated and then rises in the higher atmosphere where it is cooled. The same phenomenon is applied here. A rotating engine generates a gravity field that attracts cold gas on the outside. This gas is heated by passing through copper fine plates. Energy is collected on the receiving area and convected to these plates. The hot gas is then moving against the gravity field and through a fixed turbine which enables this engine to rotate. At the exit of the turbine a water radiator cools the gas. The rotating components such as the radiator and the heat exchanger need advanced industrial production methods which cause high manufacturing cost. This concept provides a few advantages very useful for space power systems. Less surface is needed for concentrators and radiators than the usual cycle engine. The power flux needed is 40 W/cm2 which is easily provided by the actual concentrators. However, the main advantage is the high efficiency achievable with this system. 45% efficiency is predicted for its thermodynamic cycle that operates over a temperature range from 850 K to 330 K. This efficiency is three times higher than what is usually measured at these temperatures for the classical gas turbines. These numbers correspond to an electric power of 400 W/kg based on the engine mass. These achievements have considerate impact on the modern space application.
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