Dispatchable Solar Combined Cycle

Abstract

Successful deployment of large amounts of renewable solar and wind energy has created a pressing need for significant additions of grid connected energy storage. Excess renewable generation is increasingly necessitating curtailment or derating of renewable or conventional generators. The CAISO Duck Curve [8] illustrates the challenge caused by very large quantities of solar generation. Both large scale energy storage and flexible ramping are needed for renewable resources to be financially sustainable and to meet CO2 reduction goals. The Dispatchable Solar Combined Cycle (DSCC) integrates Concentrating Solar Power (CSP) with Thermal Energy Storage (TES) in a holistic combined cycle configuration to meet the challenges of the CAISO Duck Curve by delivering flexible capacity with dispatchable solar power. Energy cost from DSCC is comparable to that from a Combined Cycle Power Plant (CCPP), and substantially below the alternatives: Photovoltaic plus battery or Photovoltaic plus combustion turbine. DSCC also enable far higher integration of renewable power and far larger renewable capacity factors than the Integrated Solar Combined Cycle (ISCC), which typically has no storage. The innovative DSCC system: • uses energy storage to deliver power when it is most valuable, • increases the capacity factor to deliver more renewable energy, • improves the power plant Heat Rate to reduce fuel consumption, and • reduces the cost of power while addressing RPS and storage mandates. In DSCC, the CSP and TES are used primarily for latent heat: the evaporation of steam, and the Combustion Turbine (CT) exhaust gas is used primarily for sensible heating, especially superheating steam. This simplifies the integration of low-cost storage media, such as paraffinic oils or concrete, instead of molten salt, since high temperature storage is not needed. A single pressure, non-reheat steam cycle suitable, allowing for simplicity of design and operation, reducing costs and facilitating faster startup and ramping. With DSCC, the steam turbine generates about the same power as the CT, unlike a typical CCPP where about half the power comes from the steam cycle. The additional steam production reduces the Heat Rate about 25% compared to CCPP. The DSCC approach is ideally suited for repowering existing CSP plants, to provide firm capacity that can dispatch at valuable evening peak periods, increase the power output, and reduce fossil fuel use compared with conventional CCPP or peaking plants. This paper will outline the DSCC concept, and provide performance estimates for a reference plant.