Thermo-Economic Optimization of a Carnot Battery Under Transient Conditions

Abstract

With the efforts to decarbonize the energy sector comes a growing demand of electricity, most of which is to be supplied by renewable generation in a carbon-neutral future. To balance the variability inherent to most renewable energy sources, some form of energy storage is required. In this work, a short review of current systems is made with a particular focus on Carnot Batteries, whose operating characteristics, long life and low environmental footprint make them competitive for daily energy storage. A transient model was developed to simulate the full operation of a Carnot Battery composed of a Vapour Compression Heat Pump and Organic Rankine Cycles in conjunction with sensible thermal storage. The key performance parameters were identified, and a Pareto optimization was carried out by balancing costs and performance across 25 configurations of storage temperature spread and heat exchanger pinch point. It was concluded that the wider storage spreads and higher pinch points lead to lower costs as they decrease the size of the water tank and the heat exchangers, and to lower efficiencies as unfavourable temperature gradients are created for the heat pump and heat engine. A Pareto front was identified, consisting of 10 configurations that were able to either optimize one criterion, or balance of two or more criteria, and conclusions were drawn as to the applicability of each configuration.