How Do You Store Heat Without Getting Hot? Melt Something

Heating a solid raises its temperature until it reaches its melting point, at which the temperature stops rising even though heat continues to enter. The energy is going into breaking the arrangement holding the molecules in place rather than into making them move faster, and only when the whole quantity has melted does the temperature begin to climb again. The amount involved is large compared with ordinary heating, with the energy needed to melt ice equalling that required to heat the same water through about eighty degrees. The process runs in reverse on cooling, releasing that same quantity at the same temperature, which is what makes the behaviour useful for storage.

Materials are chosen for where they change state and how they behave doing it:

• •Paraffin waxes, chemically stable and available across a range of melting points
• •Fatty acids, similar in behaviour and derived from plant or animal sources
• •Salt hydrates, storing more per unit volume and prone to separating over cycles
• •Water and ice, cheap and effective where the useful temperature is zero
• •Molten salts, used at high temperature in solar power installations
• •Metal alloys, for specialised high temperature applications

The applications share a need to hold a temperature rather than to reach one. Medical and vaccine shipping containers use packs that melt around the required temperature, which holds the contents within a narrow band for days regardless of outside conditions, and this is considerably better than ice, which is too cold for many products and offers no control. Building materials incorporate microscopic capsules of wax in plasterboard and plaster, which absorb heat during the day and release it at night, reducing temperature swings without any mechanical cooling. Electronics use them to absorb bursts of heat from components that run intermittently. Solar thermal power stations store heat in molten salt so that generation continues after sunset. Clothing and bedding use them for comfort.

Living systems and ordinary weather exploit this behaviour continuously, which is worth noticing because it makes the principle concrete. Sweating cools a body because evaporating water absorbs a large quantity of heat from the skin at constant temperature, which is the same mechanism running in the other direction. Frost protection in orchards works by spraying water onto trees before a freeze, since the water releases heat as it turns to ice and holds the buds at zero rather than letting them fall below it, which looks perverse and is sound. Large lakes moderate the climate around them partly through the heat absorbed and released as ice forms and melts. Sea ice formation releases enormous quantities of heat into polar air. The behaviour is not a laboratory curiosity but a major term in how the planet handles heat.

Several difficulties limit how widely these materials are used and each is being worked on. Thermal conductivity is poor in most organic options, so heat enters and leaves slowly, which limits how fast the store can be charged or discharged and is addressed by adding conductive structures or graphite. Some materials supercool, staying liquid well below their freezing point and failing to release their heat when required, which is addressed with additives that trigger the change. Salt hydrates separate into components over repeated cycles and lose capacity. Containment is necessary since the material is liquid half the time, and leakage into a building fabric is a serious matter. Cost per unit of stored energy remains higher than simply using a large mass of water where space allows.

Melting absorbs a large quantity of heat at a constant temperature, and freezing releases it again at that same temperature, which is the whole basis of the technique. Waxes, fatty acids, salt hydrates and molten salts cover different temperature ranges. Vaccine shipping, temperature-buffering building materials, electronics and solar thermal storage all use it, and poor conductivity, supercooling and containment are the limits.