Pumped thermal energy storage: A review

The global energy requirements increase every year and a major portion of that demand is borne by the non-renewable energy sources, especially by fossil fuels. Even though the share of renewable energy sources has increased substantially, the challenge of low energy density and long duration storage is still a major concern. One of the most matured power generation and energy storage technology is the pumped hydro-energy storage or PHES but it is limited by the geographical restrictions due to large water body requirements. Various energy storage technologies have been studied and developed in recent decades such as compressed air energy storage, liquid air energy storage, and electrochemical batteries, but these too are restricted either by geography or high costs. In this review we study a storage option that has garnered extensive interest in the recent years: pumped thermal energy storage or PTES. It is a highly versatile storage option which is not restricted by any geographical constraints. This is a comprehensive review of the PTES system encompassing performance parameters, power cycles, thermal analysis, and different variations of the system that make it versatile and robust. Roundtrip efficiencies greater than 60% have been reported in literature which make it a competitive resource for energy storage and power generation. Another major advantage of the system is its ability to be integrated with low-grade thermal energy sources that further improves the roundtrip efficiency. This review studies various factors affecting the roundtrip efficiencies and different variations of the PTES cycles, such as utilization of transcritical CO2 cycles, aimed at enhancing the roundtrip efficiencies. Thermo-economic analyses show that PTES systems are quite cost-competitive to already mature technologies like PHES. With further investigations and performance enhancements, PTES will only improve its prospects of becoming a game changer in thermal energy storage.