Thermal energy storage comprises multiple pathways where the input and output energy is either heat or electricity. Conventional thermal storage uses concentrating solar power (CSP) to heat the storage media, which predominantly is a molten nitrate salt with composition 60 wt.% NaNO3-40 wt.% KNO3, also known as solar salt. Efforts are underway to use electrical resistive heating to replace CSP, with additional storage media considered such as crushed rock, sand, concrete, brick, or cast iron. Liquid air energy storage (LAES) involves liquefaction of air using a standard refrigeration cycle, followed by extracting stored energy by heating the liquid air, resulting in orders of magnitude higher volume, to generate electricity by driving a gas turbine.

The different types of thermal energy systems are:

• Pumped heat energy storage (PHES) (AC in, AC out)
• Sensible heat-based thermal energy storage such as heat storage media such as molten salt, sand, concrete, thermal oil (AC in, AC out)
• LAES (AC in, AC out)
• Latent heat energy storage, which is in the applied research stage and is not covered in this report
• Thermochemical energy storage, which is in the applied research stage and is not covered in this report.

All systems considered had electricity input and output. Charging is done by electricity input (heater for sensible heat, power for compressor for pumped heat storage, and power for refrigeration cycle for LAES) with an exception for some hybrid systems where fuel is also used. These hybrid systems were included to assess the impact of their additional complexity on cost and performance of thermal energy storage. A brief review of thermal energy storage technologies is presented and is by no means an exhaustive list of technology designs that have been proposed in the literature.

The results presented below represent the consolidated cost and performance estimates of the above. For information on each of the thermal storage system types, see the 2022 report.