cavity design reduces the heat leaks even further. The heat leak along the path from the receiver cavity to the Brayton engine at the counterweight position is considered to be about 5% of the incident energy. This can be achieved by a well insulated gas line between the focal point and the Brayton engine. A flexible link is required between the two-axis tracking dish if the engine is positioned on the ground. An alternate approach would be the use of a secondary reflecting surface at the focal point to the receiver placed behind the dish surface. This Cassegrain approach would place the cavity receiver much closer to the Brayton engine and minimize line heat leaks, but introduce additional reflector losses. A fixed receiver concept was introduced by Honeywell (Ref. 1) using a probe holding the receiver through a slot in the dish surface. These various approaches have to be evaluated in detail to trade off initial capital cost versus heat leak. Many questions of performance and reliability are raised when a flexible fluid line is used to carry the heat from the two-axis tracking dish to a fixed position on the ground. The probe through the slot approach avoids a flexible joint, as does an approach of using a Cassegrain geometry to reflect the energy back through the dish center to a fixed receiver on the ground. For the case of the small heat engine located at each dish, a Cassegrain reflector could be used to deliver the heat to the heat engine hung at the counterweight position of the dish. A flexible electric connection would be used to transport the power from the two-axis tracking dish to the ground. Although several approaches have been identified, a detailed study is required to find the most appropriate concept. The receiver cavity can be open to air at temperatures up to 900°C if nickel alloys such as Inconel are used as the heat exchanger tubing material (Ref. 6). Inconel has a stable oxide coating in air up to 900°C. For a long life, design stresses should be limited to 1000 psi. This is possible with low pressure closed
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