Large-scale thermochemical energy storage using high-temperature solid cycles – impact of electricity system composition

The replacement of fossil fuels with electricity from renewables is a main option in climate change mitigation. Thus, future electricity systems are expected to consist of a large share of variable renewable electricity (VRE) and these will require flexibility measures to be applied in order to increase the value of VRE, i.e., to reduce VRE curtailment.

Energy storage can offer flexibility since it can disconnect electricity generation from the demand, and thermochemical energy storage (TCES) using high-temperature solid cycles appears as a technology with the potential to store energy at different time scales (from hours to seasons). The use of fluidized bed (FB) reactors allows for implementation of TCES at a large scale.

This work investigates how large-scale TCES influences the cost-optimal investment and operation of electricity and district heating (DH) systems in the electricity price area of southern Sweden. TCES offers the possibility to absorb non-dispatchable electricity and store it as chemical energy (in the form of reduced solid material), which can subsequently be discharged as dispatchable heat. Based on previous studies at process-level, this work assumes iron oxide as the redox material and green hydrogen as the reducing agent for the chemical reduction.

A previously developed energy system cost optimization model is further developed to also account for heat production flexibility including TCES with high-temperature solid cycles. The model minimizes the investment and operating costs of electricity generation and heat production units while meeting the demands for electricity and heat without adding carbon dioxide to the atmosphere.

The results show that the extent to which the cost of electricity for heat production can be reduced depends on the operational flexibility of the heat production units and the extents of hydrogen and solids storage. To achieve a low electricity cost for heat production, investments in overcapacity are required.