Two significant goals in solar plant operation are lower cost and higher efficiencies. This is both for general competitiveness of solar technology in the energy industry, and also to meet the US DOE Sunshot Initiative Concentrating Solar Power (CSP) cost goals [1]. We present here an investigation on the effects of adding a bottoming steam power cycle to a solar-fossil hybrid CSP plant based on a Small Particle Heat Exchange Receiver (SPHER) driving a gas turbine as the primary cycle. Due to the high operating temperature of the SPHER being considered (over 1000 Celsius), the exhaust air from the primary Brayton cycle still contains a tremendous amount of exergy. This exergy of the gas flow can be captured in a heat recovery steam generator (HRSG), to generate superheated steam and run a bottoming Rankine cycle, in a combined cycle gas turbine (CCGT) system. A wide range of cases were run to explore options for maximizing both power and efficiency from the proposed CSP CCGT plant. Due to the generalized nature of the bottoming cycle modeling, and the varying nature of solar power, special consideration had to be given to the behavior of the heat exchanger and Rankine cycle in off-design scenarios.

Variable guide vanes (VGVs), which can control the mass flow rate through the gas turbine system, have been found to be an effective tool in providing operational flexibility to address the variable nature of solar input. The effect VGVs and the operating range associated with them are presented. Strategies for meeting a minimum solar share are also explored. Trends with respect to the change in variable guide vane angle are discussed, as well as the response of the HRSG and bottoming Rankine cycle in response to changes in the gas mass flow rate and temperature. System efficiencies in the range of 50% were found to result from this plant configuration. However, a combustor inlet temperature (CIT) limit lower than a turbine inlet temperature (TIT) limit leads two distinct Modes of operation, with a sharp drop in both plant efficiency and power occurring when the air flow through the receiver exceeded the (CIT) limit, and as a result would have to bypass the combustor entirely and enter the turbine at a significantly lower temperature than nominal. Until that limit is completely eliminated through material or design improvements, this drawback can be addressed through strategic use of the variable guide vanes. Optimal operational strategy is ultimately decided by economics, plant objectives, or other market incentives.

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