Thermal Energy Storage in Plain Terms

What does thermal energy storage actually do?

It separates the time energy is collected from the time it is used. A solar collector produces heat at midday, but a house wants it at 7 p.m. A heat pump can run when power is cheap and hold the result. In every case, a material is warmed or cooled, insulated from its surroundings, and tapped later.

The usefulness of a store depends on three things: how much energy it holds, how fast heat can be put in and taken out, and how much leaks away while it waits. A big store with leaky insulation is a poor store.

Examples are everywhere once you look. An ice rink or arena that freezes water overnight, a school with a chilled-water tank for air conditioning, a solar hot-water cylinder in a basement, and a borehole field under a neighbourhood park are all thermal stores, at very different sizes.

How does sensible heat storage work?

Sensible storage raises the temperature of a material without changing its state. Water is the usual choice because it holds a lot of heat for its cost: about 4.18 kJ per kg for each degree. For a worked example, assume one cubic metre of water, roughly 1,000 kg, heated by 40 C. That stores about 1,000 x 4.18 x 40, or 167,000 kJ, which is around 46 kWh. The assumptions are idealised and ignore losses.

Rock, sand, soil and concrete also work, though they hold roughly half as much heat per unit volume as water, so the store must be larger. The ground is the cheapest material of all, and the basis of the borehole approach covered in borehole thermal energy storage.

What is latent heat storage?

Latent storage uses the energy absorbed or released when a material changes phase, such as ice melting into water. Melting one kilogram of ice takes roughly 334 kJ, equal to warming the same mass of liquid water by about 80 C. The advantage is a large amount of energy at a nearly constant temperature, which is handy for cooling and for compact stores. The disadvantages are cost, complexity and the narrow temperature at which the effect happens. Materials engineered for this purpose are sold in some building products, but claims should be checked against measured data.

Latent stores also show up in ordinary life. A frozen gel pack in a lunch bag does the same job as a much larger ice tank used to cool a building: it absorbs heat while it melts, then needs freezing again before the next use.

What about thermochemical storage?

A third route stores energy in reversible chemical or sorption reactions. In principle, the energy can be held with almost no standing loss, which would suit long periods. In practice, most of these systems are still at the research or early demonstration stage, and a homeowner cannot yet buy one as a routine product. Mention of them is useful mainly to show where the field might move.

How is heat stored for whole seasons?

Long-term storage depends on volume. As a store gets larger, the ratio of surface area to volume falls, so a smaller share of heat escapes. That favours big tanks, pits, aquifers and boreholes, compared in seasonal thermal energy storage, pit thermal energy storage and aquifer thermal energy storage.

Drake Landing Solar Community used both ends of the scale. Two 120 m3 insulated tanks handled day-to-day swings, and 144 boreholes about 35 m deep held heat across seasons. As an illustration only, one 120 m3 tank cycled through a 30 C swing holds roughly 120,000 x 4.18 x 30 kJ, about 4,200 kWh. The tanks fed the borehole field that is described on the borehole storage page, and the electric side is compared in types of home energy storage.

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