Hybridization

Because dish/engine systems use heat engines, they have an inherent ability to operate on fossil fuels. The use of the same power conversion equipment, including the engine, generator, wiring, switch gear, etc., means that only th e addition of a fossil fuel combustor is required to enable a hybrid capability. For dish/Brayton systems, addition of a hybrid capabil ity is straightforward. A fossil-fuel combustor capable of providing continuous full-power operation can be provided with minimal expense or complication. The hybrid combustor is downstream of the solar receiver, Figure 5, and has virtually no adverse impact on performance. In fact, because the gas turbine engine can operate continuously at its design point, where efficiency is optimum, overall system efficiency is enhanced. System efficiency, based o n the higher heating value, is expected to be about 30% for a dish/Brayton system operating in the hybrid mode.

For dish/Stirling systems, on the other hand, addition of a hybrid capability is a challenge. The external, high -temperature, isothermal heat addition required for Stirling engines is in many ways easier to integrate with solar hea t than it is with the heat of combustion. Geometrical constraints makes simultaneous integration even more difficult . As a result, costs for Stirling hybrid capability are expected to be on the order of an additional $250/kW e in large scale production. These costs are less than the addition of a separate diesel generator set, for a small village application, or a gas turbine for a large utility application. To simplify the integration of the two heat input sources, the firs t SAIC/STM hybrid dish/Stirling systems will operate on solar or gas, but not both at the same time. Although, the cost of these systems is expected to be much less than a continuously variable hybrid receiver, their operational flexibilit y will be substantially reduced. System efficiency, based on higher heating value, is expected to be about 33% for a dish/Stirling system operating in the hybrid mode.

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