Thermochemical storage: Engineering and performance
Why real reactors fall well short of the materials' promise
Why real reactors fall well short of the materials' promise What this tab covers Engineering and operating performance: round-trip efficiency, whether power and energy can be scaled separately, footprint and siting, and safety.Thermochemical storage separates power from energy and keeps heat without losses, but reactors, gas handling and heat transfer cut real efficiency well below the materials' promise.
Round-trip efficiency
MixedLosses arise from heating reactants, handling water vapour or CO2, and incomplete reactions. A modelling study of calcium looping for solar power found storage efficiency of 48% to 62% without separating spent sorbent and 64% to 76% with ideal separation; converting the heat back to electricity in a CO2 Brayton cycle then runs at 31% to 44%, according to the literature it cites. US oxide work set a 93% thermal efficiency target for storage alone. In storage itself the loss is close to zero, the reason for interest in seasonal use.45
Calcium looping storage efficiency (model only) 48-62%; 64-76% with ideal separation4 CO2 power cycle efficiency cited for CaL (literature) 31-44%4 Oxide storage efficiency target (DOE) 93%5Power and energy decoupling
FavourableYes. Reactor size sets power and the amount of stored reactant sets energy, and charged products can sit in silos at room temperature. The SaltX pilot pairs 0.5 MW with 10 MWh, a 20-hour ratio (calculated). The CREATE prototype stores about 182 kWh in three modules holding 400 litres of salt each.27
SaltX pilot 0.5 MW / 10 MWh, about 20 hours (calculated)2 CREATE prototype about 182 kWh in 1,200 litres of salt, about 150 kWh per m3 (calculated)7Footprint and siting flexibility
MixedA typical West European home needs about 7 to 12 GJ (about 2 to 3.3 MWh, calculated) of seasonal heat storage; at an assumed system density of 1 GJ per m3 that is roughly 10 m3 of reactor. The CREATE store was more than 20% smaller than a water tank of the same capacity, a modest gain once reactors and vacuum vessels are counted. A review concluded that one-cycle-a-year seasonal storage with pure salt hydrates is unrealistic at large scale.37
Seasonal store, West European home about 7-12 GJ; about 10 m3 at 1 GJ/m33 CREATE vs water tank over 20% smaller7Safety and thermal runaway risk
MixedHazards include hot reactors, pressurised or vacuum vessels, oxygen released by metal oxides, CO2 handling and corrosive or acid-forming salts such as magnesium chloride. Uncontrolled recombination would release heat, but no incidents were reported in the sources. Process-safety and pressure-equipment rules apply.3
Sources
- Review of Carnot Battery Technology Commercial Development (Novotny et al., Energies 15(2), 647) MDPI, 2022
- A review of salt hydrates for seasonal heat storage in domestic applications (Donkers et al., Applied Energy 199, 45-68) Eindhoven University of Technology repository, 2017
- Operation maps in calcium looping thermochemical energy storage for concentrating solar power plants (Pascual, Lisbona and Romeo, Journal of Energy Storage 55, 105771) University of Zaragoza repository, 2022
- Lessons Learned: Developing Thermochemical Cycles for Solar Heat Storage Applications (Wong, General Atomics; conference slides, DOE Workshop on TES for CSP, January 2013) US Department of Energy, 2013
- Compact thermochemical heat storage prototype (CREATE project, IEA SHC Task 58) IEA Solar Heating and Cooling Programme, 2020