Latent heat and cold storage: Science and technology
Why melting and freezing hold more than simple warming
What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.Latent stores hold energy in melting and freezing, which packs more energy into a narrow temperature band than simply heating a material. Ice for air conditioning is mature; paraffin and salt-hydrate PCMs are niche; molten silicon and aluminium remain early and risky.
Mechanism
FavourableA phase-change material absorbs a large amount of heat while it melts and gives it back as it freezes, at an almost constant temperature. Ice does this at 0 C and is used to store cooling; salt hydrates absorb 100 to 290 J per gram, roughly 28 to 81 kWh per tonne (calculated). At the hot end, 1414 Degrees uses silicon, which melts at 1,414 C and stores about 500 kWh per tonne. Chilled water and liquid nitrogen are cold stores that work mainly by temperature difference or boiling rather than melting.57
Latent heat, salt hydrates 100-290 J/g (about 28-81 kWh/t, calculated)5 Silicon PCM (1414 Degrees) about 500 kWh per tonne7 Volumetric storage, salt hydrates 50-130 kWh per m35Material scarcity and toxicity
FavourableWater for ice costs about USD 0.05 per kg as listed, and salt hydrates USD 0.09 to 2.53 per kg. None of the common PCMs relies on critical minerals. The drawbacks are chemical: salt hydrates are corrosive, and paraffins and fatty acids are flammable. Silicon and aluminium are abundant, though their production is energy-intensive. Ice systems often use glycol in the loop.56
Material cost, ice / salt hydrates USD 0.05 / 0.09-2.53 per kg56 Storage material cost, salt hydrates / paraffins USD 0.90-40 / 7-30 per kWh5Degradation and ageing
MixedIce and water do not wear out, so ice tanks age like plumbing. Salt hydrates are harder: they can supercool by more than 5 C (stay liquid below their freezing point), melt unevenly and corrode containers, and all PCMs need sealing to stop them degrading in air. High-temperature metal stores face container corrosion and thermal stress; no field cycle-life data were found.5
Supercooling, salt hydrates above 5 C5Variants compared25679111213
| Variant | Working temperature | Storage density or scale | Status |
|---|---|---|---|
| Ice storage for air conditioning | 0 C | CALMAC tanks average about 160 ton-hours; over 4,000 sites (company data) | Commercial |
| Salt-hydrate PCMs | 0-65 C (range reviewed) | 50-130 kWh per m3 | Niche commercial |
| Paraffin PCMs | typically 20-40 C (listed range) | USD 7-30 per kWh material | Niche commercial |
| Molten silicon | 1,414 C | about 500 kWh per tonne | Demonstration (1414 Degrees, company data) |
| Molten aluminium alloy | not stated in sources | 13 kWe pilots with 13 hours | Developer Azelio bankrupt 2023 |
| Chilled water | above 0 C, no phase change; about 7 C rise | about 7.6 kWh per m3 (calculated) | Commercial in district cooling |
| Liquid nitrogen | cryogenic | no figure found; see liquid air tile | Lorry refrigeration trials; developer Dearman entered administration in 2019-20 |
Sources
- Thermal Battery systems: CALMAC ice storage (Trane sales brochure IB-SLB001, company data) Trane Technologies, 2020
- Review of Low-Cost Organic and Inorganic Phase Change Materials with Phase Change Temperature between 0 C and 65 C (Hirschey et al., Purdue High Performance Buildings Conference) Oak Ridge National Laboratory, 2021
- Study on the hybrid energy storage for industrial park energy systems, Table 2: thermal storage materials (National Science Open 3(3)) National Science Open, 2024
- Review of Carnot Battery Technology Commercial Development (Novotny et al., Energies 15(2), 647) MDPI, 2022
- Is Azelio's abrupt bankruptcy a bad omen for long-duration energy storage? (news report) Canary Media, 2023
- Evaluating chilled-water storage for district cooling systems HPAC Engineering, 2016
- Leeds to trial liquid nitrogen engines (Dearman transport refrigeration, with Cenex and Leeds City Council) (developer claims as reported) The Engineer, 2018
- Dearman Engine Company enters administration Jones Day, 2020