Integration of a steam accumulator with a biomass power-generation system for flexible energy storage and discharge: Effect of the initial steam pressure on the steam discharge profile and levelized cost of storage

Abstract

Carnot batteries (pumped thermal energy-storage systems) are promising systems to reduce the cost of electricity storage and balance intermittent variable renewable energy. In this study, a steam accumulator (SA), which is a sensible heat-storage unit for the Carnot-battery system, was integrated with the existing steam Rankine cycle of a biomass power plant (2000 kWe, inlet steam temperature and pressure of 480 °C and 6.3 MPa, respectively). The surplus electric power of the renewable energy was converted into steam with electric heaters and stored in the SA as saturated steam. The dynamic profile of the steam discharge from the SA was modeled with Excel Visual Basic for Applications, and the energetic performance was calculated with Aspen Plus V12. The effect of the initial steam pressure in the SA (7.0–10.0 MPa) on the steam-discharge profile and levelized cost of storage (LCOS) was investigated for the case of a power increment of 500 kWe (25 %) and 4-h discharge per day. In addition, the influence of the capacity on the LCOS was investigated from 5 % to 30 %. The estimated required volume of the SA was 154.1 m3 (7.0 MPa) to 51.6 m3 (10.0 MPa). The capital cost for installing the SA system (the SA, electric heaters, superheaters, high-pressure pumps, and valves) was higher for lower initial steam pressure (0.586 MUSD for 7.0 MPa and 0.341 MUSD for 10.0 MPa) because a larger SA volume was required. The capital cost of the electric heaters was 0.294 MUSD irrespective of the initial steam pressure in the SA. The LCOS was 0.317 ± 0.035 to 0.347 ± 0.043 USD/kWhe at initial steam pressures of 10.0 to 7.0 MPa when the plant lifetime was 20 years. The sum of the capital and maintenance costs in the LCOS can be reduced to <0.10 USD/kWhe, excluding the cost of the charging electricity and labor cost. These results indicate that it is feasible and promising to integrate a SA system with an existing biomass power plant to add flexibility of power generation at relatively low cost, especially, in the case that the cost of surplus electricity is zero or negative.