Performance analysis of a 1.5 MW organic Rankine cycle in a Carnot battery system for grid balancing services

Abstract

Due to the increasing share of variable renewable energies in the energy mix, large-scale (>1 MW), long duration (>4 h) energy storage systems are becoming essential to ensure secure and stable energy supply. Carnot batteries, a combination of a power-to-heat, a thermal storage and a heat-to-power system, could provide a possible solution. Organic Rankine cycles have been considered as power-toheat technology for Carnot batteries. However, it is not clear which grid services these organic Rankine cycles could deliver. To answer this question, a 1.5 MW organic Rankine cycle, suitable for integration in a Carnot battery, was modelled dynamically in Modelica. Dynamic finite volume heat exchanger models were used, while the operation of the pump and turbine was considered quasi-steady state due to their low time constant compared to the heat exchangers. The storage tanks were assumed perfectly mixed and modelled by time-varying boundary conditions. Furthermore, a control strategy driven by the requirements of the electrical grid was proposed. The dynamic model simulates and evaluates the organic Rankine cycles’ response to the qualification test profiles for grid balancing services. It was found the organic Rankine cycle can deliver a 1 MW capacity for downward and upward secondary and tertiary reserve. Delivery of a 0.5 MW symmetric capacity for primary reserve is currently not possible due to the required full activation time. Nevertheless, the results indicate that the delivery of secondary and tertiary grid balancing services with the organic Rankine cycle could be an additional revenue stream to increase the financial feasibility of Carnot battery systems and it is thus worthwhile to investigate its potential financial benefits.