(392c) Pumped Thermal Electricity Storage: Modelling of a Reversible Recuperative sCO2 Pumped Brayton Cycle for Variable Power Operation

Introduction

Decarbonization of the energy grid involves increasing use of renewable sources such as solar and wind. However, these energy sources have inherent intermittency issues. Thus, it is vital to develop highly scalable energy storage technologies to complement this increase in renewable energy generation. The focus of this work is one such technology, Pumped Thermal Electrical Energy Storage (PTES), utilizing heat pumps and heat engines to store electricity in the form of thermal energy [1,2] and more specifically, a reversible recuperated closed Brayton cycle employing supercritical CO­­₂ as the working fluid. In times of surplus electricity production, the process operates as a heat pump, storing incoming electrical energy in a heat storage medium as thermal energy. Conversely, during times of increased electricity demand the process is reversed, operating as a heat engine, and producing electricity. The most flexible and effective solution for the controllability of the power grid is to have such electricity storage units to operate with variable power charging and discharging rates. The current study uses Aspen HYSYS for detailed modeling of part-load operating conditions and MATLAB Simulink is used for dynamic modeling and control studies. Previous studies for modeling of part-load and off-design operation of similar systems used simplified models for compressor, turbine, and heat exchanger modeling [3,4]. In our work, these units are modeled more rigorously, with turbomachinery maps sourced from machine builders and considering temperature distributions in the heat exchangers.

The cycles operate a set of compressor-expander pairs. These have set design points at which they operate at their highest efficiency. When modelling the process, this design point is taken into consideration and the turbomachines are chosen accordingly. However, during operation the charging or discharging power may fluctuate and cause the process to deviate from this design point. It is important to understand the behavior of such systems under off-design conditions and develop control strategies to deal with these deviations. This study uses turbomachinery maps to investigate part-load operation and control strategies of such a system.

Part-load Performance Analysis

Studying the part-load characteristics is crucial to achieve high efficiencies when the power is varying [4,5]. In this study, the PTES power consumption, mass flow rate and thermal efficiency were studied. Fig. 1 shows the layout of the charge and discharge cycles of the PTES model and Fig. 2 shows the performance maps used to model the main compressor.

In summary, this study seeks to provide a detailed investigation for the full-load and part-load performance of supercritical CO­­₂ based recuperated Brayton cycles for Pumped Thermal Electricity Storage. Based on those results, a dynamic model is established as well as plantwide control strategies for coordinated operation with the power grid. The overall goal of this research is to achieve flexible operation capabilities with PTES systems while maintaining near-optimal performance. The steady-state and dynamic simulation results we present will provide novel insights for utilizing this new energy storage technology to its full potential.

References

[1] Mercangöz, M., Hemrle, J., Kaufmann, L., Z’Graggen, A. and Ohler, C., 2012. Electrothermal energy storage with transcritical CO2 cycles. Energy, 45(1), pp.407-415.

[2] Morandin, M., Maréchal, F., Mercangöz, M. and Buchter, F., 2012. Conceptual design of a thermo-electrical energy storage system based on heat integration of thermodynamic cycles–Part A: Methodology and base case. Energy, 45(1), pp.375-385.

[3] Frate, G.F., Paternostro, L., Ferrari, L. and Desideri, U., 2021, June. Off-Design of a Pumped Thermal Energy Storage Based on Closed Brayton Cycles. In Turbo Expo: Power for Land, Sea, and Air (Vol. 84966, p. V004T07A004). American Society of Mechanical Engineers.

[4] Yang, J., Yang, Z. and Duan, Y., 2020. Part-load performance analysis and comparison of supercritical CO2 Brayton cycles. Energy Conversion and Management, 214, p.112832.

[5] Xingyan, B., Wang, X., Wang, R., Cai, J., Tian, H. and Shu, G., 2022. Optimal selection of supercritical CO2 Brayton cycle layouts based on part-load performance. Energy, 256, p.124691.