Carnot battery with steam accumulator and pebble bed thermal energy storage

Carnot batteries can store excess electricity from intermittent renewable solar or wind sources and generate power in periods of peak consumption. A novel design of the Carnot battery is proposed based on thermal energy storage by the combination of a steam accumulator and a pebble bed in a series configuration. During the Carnot battery charging, steam is generated by high temperature heat pumps with CO2 compression cycles, pressurized and superheated by electrically driven steam compressors. The obtained superheated steam flows through the pebble bed porous structure and transfers heat to alumina balls in direct contact. Steam from the pebble bed inflows the steam accumulator. During the Carnot battery discharging phase, the steam outflows the steam accumulator, superheats in the pebble bed and expands through the steam turbine, which is connected to the electric generator for power production. The advantages of the studied Carnot battery are a simple design and operation of the steam accumulator as the thermal energy storage unit, the accumulated steam directly serves as working fluid in the steam turbine and the coupling of the pebble bed with the steam accumulator enables steam superheating that is required for an increased steam turbine efficiency in power generation. Water is low-cost and more convenient in respect to safety, plant engineering and operational aspects in comparison with other working fluids. In addition, there is no need for additional heat exchangers for the heat transfer between the working fluid and the storage medium. Maximum temperature and pressure of the thermal storage medium are 303 °C and 0.7 MPa, respectively. The temperature and low-pressure values enable application of mature technology and lower investment costs, where the capacity specific cost of 471 €/kWhe is reached. The presented Carnot battery design is supported with numerical simulations of thermal energy charging and discharging transients in the pebble bed and the steam accumulator with own original and validated modelling approaches. The obtained Carnot battery charging electric power is 9.5 MWe for 6.9 h, while discharging power is 2.3 MWe for 9.4 h, which is suitable for the electric grid power control on a daily period.