Storing heat: the underestimated complement to wind and solar

The form of storage must match its eventual use

Thermal storage retains energy as heat or cold for later use. This can involve heated water, solid materials or substances whose phase changes absorb and release energy. The International Renewable Energy Agency describes these technologies as a way of separating energy supply and heat demand in time. The right temperature is just as important as the amount of energy stored. Lukewarm water can be useful for certain heating tasks, but does not automatically meet the requirements of an industrial high-temperature process. Storage for a few hours also differs from seasonal solutions. Anyone comparing technologies should therefore first establish the eventual application. A large stated capacity alone answers neither the question of the usable temperature nor that of the possible discharge power in the operation concerned.

Flexibility has physical limits

A thermal store can, for example, allow a heat pump to run at times when renewable electricity is available. The stored heat then supplies the building later. This shifts electricity consumption rather than automatically reducing it. Heat losses occur during storage; higher storage temperatures can also reduce the heat pump's efficiency. The benefit therefore depends on insulation, sizing and controls. Space requirements are also relevant, particularly in existing buildings. For buildings supplied by renewables, the International Energy Agency stresses the interaction between storage and flexible energy use. This does not, however, amount to a recommendation to equip every system with the largest possible tank. An appropriate solution is guided by the actual load profile and avoids unnecessary temperature margins and additional technical complexity in day-to-day operation.

From individual homes to heat networks

In larger supply systems, thermal storage can connect several producers and consumers. Possibilities include using intermittently available waste heat or shifting renewable heat generation in time. Larger installations can offer different technical possibilities from an individual home, but require suitable sites and coordinated infrastructure. With seasonal storage, long-term losses and geological or structural conditions also become important. Economic viability comes not from the store alone, but from avoided peaks in generation, usable energy and appropriate investment. A transparent assessment therefore also includes electricity for pumps, maintenance and service life. Thermal storage is particularly interesting when it fills a clearly identifiable gap. As a mere additional component without a clear operational purpose, however, it can tie up resources without delivering a corresponding system benefit.

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