Small Scale Pumped Thermal Energy Storage Modeling and Optimization

Abstract

Variable green energy sources were sought to supply the growing demand for energy without impacting the environment. Wind and solar energy are at the forefront of the energy transition. However, their intermittent nature poses a challenge that necessitates the development of energy storage technologies.
Several technologies were studied over the years including pumped hydro energy storage, compressed air energy storage, electrochemical energy storage, and pumped thermal energy storage.
Pumped thermal energy storage provides a mean to store excess electrical energy in the form of heat by employing a heat pump cycle during the charging process and a heat engine cycle during the discharging process. There are multiple variations of pumped thermal energy storage cycles of which Rankine and transcritical cycles. Unlike Brayton cycles, Rankine and transcritical cycles typically operate at low-temperature levels < 200o𝐶 which facilitate their
integration with low-temperature waste heat in order to boost their round trip efficiencies even beyond 100%. Therefore, the focus of this thesis was to model and optimize Rankine and transcritical thermally integrated small scale pumped thermal energy storage.
Critical to the storage system’s efficacy are the performances of its key components: the compressor and expander. Geometric models were developed and validated for these components, facilitating the determination of their isentropic efficiencies and their variation with pressure ratio and rotation speed.
Additionally, a small-scale pumped thermal energy storage system model was developed to study the system performance. The choice of working fluid and operating parameters were guided by both a simplified optimization scheme and a multi-objective approach utilizing the Non-Dominated Sorting Genetic Algorithm-II (NSGA-II). This yielded 𝑅13𝐼1 as the optimum working fluid with source temperature 𝑇𝑠𝑜𝑢𝑟𝑐𝑒 = 80o𝐶 and storage temperature 𝑇𝑠𝑡𝑜𝑟𝑎𝑔𝑒 =140o𝐶. This configuration led to a transcritical heat pump charging cycle and a subcritical heat engine discharging cycle employing pressurized water as the storage medium.
Consequently, the compressor and expander geometric models were integrated into the pumped thermal energy storage system model working with 𝑅13𝐼1 at the source and storage
temperature mentioned earlier which yielded a round trip efficiency of 𝜂𝑟𝑡 = 93%, energy density 𝜌𝑒𝑛 = 12𝑘𝑊ℎ𝑟/𝑚3, and exergy efficiency 𝜓𝑒𝑥 = 29%.
Finally, a case study was demonstrated with the integrated cycle employed in a Dutch household solar system where dual electric and thermal panels are employed for the electrical and heat energy input in the pumped thermal energy storage. The system was developed for a 1𝑘𝑊 electrical input from the solar system to be stored for further use during off peak hours.