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Development of advenced hybrid heat pipe receivers in dish/stirling systems for decentralized power production

Exploitable results

The objective of the project was to contribute to a commercialization of the dish/Stirling technology that can provide the decentralized power demand of small communities in Mediterranean countries utilizing concentrated solar radiation. A new low-cost method producing heat pipe capillary structures has been developed in order to reduce one of the main costs in hybrid heat pipe receivers. Controlled open porous layers for new-type heat pipe capillary structures have been produced with radio-frequency plasma spraying (RF-PS). The RF-PS capillary structures show the possibility of low-cost production of capillary structures, not only for heat pipe applications. Refractory metals as well as ceramics have been sprayed using typical heat pipe materials, e.g. nickel based alloys. Porosities of up to 80% have been reached with molybdenum and of up to 50% with nickel based alloys. Two prototype sodium heat pipes have been built and tested successfully. Up to a capillary height of 400 mm, heat fluxes of more than 100 W/cm2 could be obtained. With RF-PS capillary structures, costs of hybrid heat pipe receivers could be decreased at least in the range of factor 5 to 10. A hybrid heat pipe receiver for dish/Stirling systems has been developed, which allows reliable power generation independent on solar radiation. Basic element is a sodium heat pipe, using conventional spot welded screen wick structures with arterial webs. This heat pipe can transfer any power combination from gas and solar input up to 45 kW thermal power with hardly any temperature drop. The power is introduced either through the solar absorber surface and / or over the gas heat exchanger and is transported to the Stirling heat exchanger tubes by the working fluid of the heat pipe (sodium). At the Stirling heat exchanger there is an equal heat flux distribution, improving efficiency and lifetime of the system. A low emission, high efficiency and high density combustion system for high temperature applications has been developed, adapted to the hybrid heat pipe receiver. It is a lean premix pre-vaporize (LPP) combustor using combustion gas recirculation (CGR) to lower the combustion temperature. To recirculate combustion gases and to mix air, fuel and combustion gases, the system is equipped with an internal ejector driven by the inlet air. For high efficiency an air preheater is used. An automatic control system has been developed and implemented. Then the hybrid heat pipe receiver has been installed in a dish/Stirling system at Plataforma Solar de Almeria, Spain and tested successfully in all operation modes, solar-only, gas-only and hybrid mode. A total of 360 Stirling engine operating hours have been accumulated. A study of the market potential for hybridized dish/Stirling units and of adequate sites concerning solar radiation for Morocco, has been performed, showing a huge potential for decentralized power production with hybrid dish/Stirling systems in Morocco.

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