Off-design performance evaluation of thermally integrated pumped thermal electricity storage systems with solar energy

Abstract

Pumped thermal electricity storage that integrates with low-grade heat source (thermally integrated PTES, TI-PTES) have gained widespread attention due to the superior performance compared to traditional PTES systems. In this work, a TI-PTES system integrated with solar energy is constructed. R1233zd(E) is employed as the working fluid in the system. Select solar data from four typical days (Vernal Equinox, Summer Solstice, Autumn Equinox, Winter Solstice) throughout the year in Chuxiong City, Yunnan Province, which has abundant solar resources. Design conditions for the system are determined by a multi-objective optimization approach, and based on this, the off-design performance of both the charge and discharge processes are studied. The off-design models for the main components of the system are established. In the charge process, the weather conditions from 10:00 to 15:00 on a typical day are selected for a 6-hour charge period, and a parameter analysis of the system is conducted under different weather conditions. In the discharge process, the off-design performance of the system under different load levels is analyzed for two operating modes: constant pressure mode and variable pressure mode. The results indicate that in the charge process, the values of the compressor power under off-design conditions (Wcomp,off), the mass flow rates of the heat pump system (mRHP), and thermal energy storage system (mts) of Winter Solstice are the highest from 11:00 to 15:00 compared to the other 3 days. In the discharge process, the power-to-power efficiency (ηPTP) and Levelized Cost of Storage (LCOS) of the Vernal Equinox and Autumn Equinox reach their maximum (74.47 %) and minimum (0.247 $·kWh−1) values at an 80 % load level under variable pressure mode. Under constant pressure mode, the power-to-power efficiency and LCOS of the Vernal Equinox and Autumn Equinox reach their maximum (74.34 %) and minimum (0.232 $·kWh−1) values at 70 % and 40 % load levels. Comparing the off-design performance between the variable pressure mode and constant pressure mode in the discharge process, the variable pressure operation mode is recommended for load levels between 78 % and 85 %, while the constant pressure operation mode is recommended for load levels between 40 % and 78 %, as well as between 85 % and 120 %. The obtained results can serve as a reference for the off-design performance analysis of TI-PTES.