Thermal Energy Storage Systems
Storage types and sizing
Thermal energy storage decouples production from demand. Chilled water storage uses sensible heat: 1 ton-hour = 12,000 Btu, and with a 10 F usable delta-T one cubic foot of water holds about 624 Btu, so plan roughly 19 cubic feet per ton-hour plus unusable stratified volume. Ice storage uses latent heat and needs only about 2 to 3 cubic feet per ton-hour, but requires a chiller that charges at low evaporating temperature, costing efficiency and raising kWh. Hot water storage serves boiler plants and solar thermal the same way.
Strategies and trade-offs
Full storage shifts the entire on-peak load, maximizing demand reduction and requiring the largest tank. Partial storage runs the chiller alongside storage during the day and is usually more capital-efficient. Control modes include chiller-priority, storage-priority, and constant proportion. The economics rest on demand charges and time-of-use differentials, not total kWh; storage typically increases site energy slightly because charging still requires compressor work, and ice charging occurs at a lower COP.
Traps
Confusing tons of capacity with ton-hours of capacity; ignoring standby and tank heat gain (storage efficiency typically 85 to 95%); assuming storage saves energy when it mainly saves demand dollars; and overlooking that ice-capable chillers have different selections, flows, and efficiencies.
Worked example
A 500-ton peak with a 10-hour on-peak window requires 5,000 ton-hours for full storage. A 2,000 ton-hour ice tank at 2.5 cubic feet per ton-hour needs 5,000 cubic feet. Under partial storage, a 300-ton chiller plus a 2,000 ton-hour tank covers the same day: 300 x 10 + 2,000 = 5,000 ton-hours.