ELECTROTHERMAL ENERGY STORAGE CARNOT BATTERY DEMONSTRATOR WITH DISCHARGING VIA ORC

Abstract

Hand in hand with the growth of installed capacity of intermittent renewable energy sources, new energy systems capable of providing flexibility to balance the power grid are required. Electrothermal energy storage with direct heat-to-power conversion and high-temperature storage can deliver the desired service with lower CAPEX compared to systems utilizing a heat pump during the charging period, while maintaining a reasonable round-trip efficiency (RTE).

In the case of a full-size unit, utilization of a steam Rankine cycle during the discharging period is essential to achieve high RTE. However, for small-scale P2H2P technology demonstration units, the ORC system appears to be a more suitable option for several reasons — including simpler operation, fewer auxiliary components, and better feasibility of the expander for low power outputs.

The energy system introduced in this paper is based on a recently built experimental 1 m³ packed bed high-temperature thermal energy storage (HT TES) unit, using gravel as the storage material and air as the heat transfer fluid. A 15 kW resistance heater is used for charging, with a maximum output temperature of 900 °C. The nominal capacity of the TES is 240 kWhₜₕ at a design temperature of 600 °C. Air is supplied to the system by a radial blower equipped with a VFD.

For discharging of the existing TES assembly, an ORC system was designed utilizing a novel working fluid R1233zd(E) with an intermediate pressurized hot water loop. The design of the ORC is adopted from a previous heat pump–ORC demonstrator and consists of a gear pump, plate heat exchanger (HX), rotary vane expander, plate recuperator, and condenser. The intermediate loop also provides an alternative option for direct discharge of stored heat using a simple hot water–air heater operating in parallel with the ORC heat exchanger.