Hybrid storage solution steam-accumulator combined to concrete-block to save energy during startups of combined cycles

Abstract

This work presents a novel steam accumulator and concrete-block storage system (SACSS) to recover part of the energy lost through the steam cycle side during startups of combined cycle power plants (CCPPs). The steam accumulators are integrated with sensible-heat concrete storage to provide superheated steam resulting then to a higher efficiency and safer steam turbine operation compared with systems based only on saturated steam. An economic analysis is performed considering two different scenarios: i) a CCPP able to execute fast startups using a Benson-type heat recovery steam generator (HRSG) and ii) a CCPP operated with conventional startups which employs a typical drum-type HRSG. It is worth mentioning that the second scenario is based on measured data. The economic optimization of the SACSS is carried out focusing in four design variables: number of steam accumulator units, storage pressure, concrete-block length and outer concrete diameter. The optimum solution presents a net present value of 4.45 M€ and a payback period of 3 years for the CCPP suitable for fast startups. For the CCPP operated with conventional startups, a net present value of 2.53 M€ and a payback period of 3.4 years are obtained. The net present value grows around 60 % in both cases if the benefits from carbon credits are considered. In addition to the efficiency improvement, the SACSS could be used to preheat critical sections of the heat recovery steam generators, reducing the thermal stress and the fatigue damage during fast startups. Finally, the emissions avoided thanks to SACSS are estimated to be around 3 640 and 2 175 tons of CO2 per year, for fast and conventional startup cases, respectively.