Dish/Stirling hybrid-heat-pipe-receiver design and test results

A 75-kW/sub t/ hybrid receiver, intended for dish/Stirling application, has been designed, fabricated, and tested. The receiver is a 6-x scale-up of our earlier successful bench-scale hybrid concept. It is a major extension of the bench-scale concept to a compact package comprising a fully-integrated solar absorber, gas-fired surface, heat pipe, combustor, and recuperator. The device is built around a sodium heat pipe having a spherical-dome solar absorber and a pin-fin-studded, cylindrical-sidewall, gas-fired surface. The combustion system uses a metal-matrix burner, with premixed air and natural gas. The recuperator is a folded-membrane design. The receiver is designed for simultaneous solar and gas-fired heating, with a nominal throughput of 75 kW/sub t/. The nominal operating (sodium vapor) temperature is 750 C. The receiver has been ground tested (gas only) at throughput power levels from 18 to 75 kW/sub t/and output temperatures up to 750 C. It was tested in four different orientations, corresponding to sun elevations of 12, 22, 45 and 80 degrees. The tests have established several landmarks at the 75 kW/sub t/ power level, including: (1) preheat of fuel/air mixtures above 600 C without preignition, (2) internal wall temperatures over 800 C with minimal warping, particularly at critical internal seals, and (3) 68% thermal efficiency including parasitics. We believe the efficiency could be boosted to 75% by the addition of an external insulation package. Our tests also verified smooth ignition, as well as the absence thermocouples, differential pressure gauges on all major flow elements, and calorimetry. Some nonfatal problems occurred during the tests, including occasional transient leakage at an internal seal, and warping of the burner matrix. Late in the scheduled tests, a hot spot developed on the heat-pipe gas-fired surface. This behavior is believed to be the result of a wick flaw; it has been seen in other heat pipes, and has been the subject of an ongoing separate effort. Design details and rationale will be presented, along with test data illustrating the behavior of the receiver, and demonstrating its efficiency.