Thermo-economic analysis of steam accumulation and solid thermal energy storage in direct steam generation concentrated solar power plants

In direct steam generation (DSG) concentrated solar power (CSP) plants, a common thermal energy storage (TES) option relies on steam accumulation. This conventional option is constrained by temperature and pressure limits, and delivers saturated or slightly superheated steam at reduced pressure during discharge, which is undesirable for part-load turbine operation. However, steam accumulation can be integrated with sensible-heat storage in concrete to provide higher-temperature superheated steam at higher pressure. In this paper, this conventional steam accumulation option (existing) and an integrated concrete-steam TES option (extended) are described and analysed, and their thermo-economic performance are compared taking the 50-MW Khi Solar One DSG CSP plant in South Africa as a case study. The results show that the extended option with five 10-m long, square cross-section concrete blocks, each with 3600 equally spaced tubes, provides an additional TES capacity of 177 MWh compared to the existing configuration as a result of utilising most of the available thermal power in the solar receivers. Moreover, the extended option delivers 58 % more electricity with a 13 % enhancement in thermal efficiency during TES discharging mode. With an estimated additional investment of $4.2M, the levelised costs of storage and electricity for Khi Solar One with the extended TES option are, respectively, 29 % and 6 % lower than those obtained with the existing TES option. With the extended TES option, the projected net present value of Khi Solar One increases by 73 %, from $41M to $71M, at an average electricity price of 280 $/MWh.