OPTIMUM COUPLING OF THERMAL ENERGY STORAGE AND POWER CYCLES FOR CARNOT BATTERIES

Abstract

To enable the widespread exploitation of intermittent, low-cost, and non-dispatchable renewable energy technologies, energy storage plays a key role in providing the required flexibility. In the spectrum of energy storage systems, one out of a few geographically independent possibilities is the storage of electricity into heat, so-called Carnot batteries. For thermal-to-electricity reconversion, depending on the operating energy storage temperatures, conventional or advanced power cycles can be integrated into the system, yielding different techno-economic performances. This work proposes a methodology that enables decision-making in selecting the adequate power cycle and Thermal Energy Storage (TES) type for a wide range of operating temperatures between 200 and 800 °C. To select the optimum coupling of TES and power block, a techno-economic optimization has been conducted aimed at minimizing the Levelized Cost of Storage (LCOS) for different plant capacities and charging costs. The study explores various power block configurations, including Organic Rankine Cycle (ORC), steam Rankine cycle, and supercritical CO2 (sCO2) Brayton cycle. Additionally, it evaluates different TES options such as molten salt, particle, and air packed bed TES. Results highlight that, for a charging cost of 50 EUR/MWh, the most cost-effective combination of TES and power block involves sCO2 power blocks with recompression and intercooling, along with particle-based TES operating at temperatures between 600 to 800 °C and a temperature difference of 200 °C. ORCs are suitable for low temperatures (up to 350 °C) and high temperature differences, while the steam Rankine cycle is considered optimal between the low-temperature and the sCO2 preferred regions. Air-packed bed TES is suggested as a viable option when TES represents a large share of the capital cost, with low charging costs, low hot temperatures, or low temperature differences. Molten salt TES is ideal when its design temperatures align with the operating limitations of the salts. Particle-based TES is the most cost-effective choice across a wide range of temperatures, at small (10 MW) and large scales (100 and 200 MW).