Storing heat instead of electricity
Much of the energy people use is not electricity at all but heat: warming buildings, heating water, cooking, and running industrial processes. Cooling is another large use of energy, especially in hot climates. Thermal energy storage keeps heat or cold in a medium so it can be used later, at a different time from when it was produced.
The attraction is practical. If the end use is heat, storing heat directly can avoid converting energy to electricity and back, a process that always loses some energy. Heat storage materials, such as water, are often cheap and widely available.
The three main approaches
Sensible heat storage. This is the simplest form: raise the temperature of a material, then use the heat later as it cools. A familiar example is a hot water tank. Water is a popular choice because it is cheap, safe and holds a lot of heat for its cost. Other materials, such as rock, concrete, bricks or molten salts, are used where higher temperatures are needed.
Latent heat storage. Some materials absorb or release a large amount of energy when they change phase, for instance from solid to liquid, while staying at nearly constant temperature. Ice is the common example: melting a given mass of ice absorbs far more energy than warming the same mass of water by a few degrees. Materials designed for this purpose are called phase change materials, and they can hold more heat in a smaller volume than a simple water tank across a narrow temperature range.
Thermochemical storage. Here, heat drives a reversible chemical reaction, and the energy is released when the reaction runs backward. This can store energy for long periods with limited loss, but it is less mature and is still mostly at the research and demonstration stage.
A simple calculation
The energy in sensible storage depends on mass, the material’s specific heat, and the temperature change. Water’s specific heat is about 4.2 kilojoules per kilogram per degree Celsius. Heating 100 kilograms of water, roughly 100 liters, by 40 degrees stores about 100 times 4.2 times 40, or 16,800 kilojoules. Dividing by 3,600 kilojoules per kilowatt-hour gives about 4.7 kilowatt-hours. This is a small amount compared with a daily household heating need, which shows why storage tanks are sized in the hundreds of liters or larger.
In homes and buildings
Hot water cylinders. Many homes already have a tank that stores heated water. If it is heated when electricity is cheap or when rooftop solar is producing, it effectively acts as a battery that costs far less than a chemical one. This can interact with time-of-use tariffs, since heating can be shifted to cheaper hours. The topic of water heating itself is covered separately.
Buffer tanks with heat pumps. Heat pump systems often include a buffer tank to smooth operation and allow the heat pump to run at times that suit the grid or the owner.
Storage heaters. Electric storage heaters warm up bricks or similar material overnight and release heat during the day. They have been used for decades in some places to take advantage of off-peak power.
Ice storage for cooling. Some large buildings make ice at night, when electricity demand is lower, and use it to cool the building during the day, reducing the need for air conditioners to run at the peak. This helps lower peak demand, which is useful for the electricity grid.
Building mass. The walls, floors and structure of a building also store heat. Good home insulation and air sealing help it hold on to what it has gathered, and design that uses this thermal mass can moderate temperature swings.
In district systems and industry
District heating. Some towns and cities heat many buildings from a central source through a network of insulated pipes. Large insulated tanks or pits can store hot water for hours, days or even seasons, letting the system capture surplus heat and deliver it when demand rises.
Concentrated solar plants. Some solar thermal power stations store heat in molten salt, which allows them to keep generating electricity after sunset. This is a well-known example, though such plants are a small part of global generation and require sunny sites.
Industrial heat. Many factories need heat for processes such as drying, food production or chemical manufacturing. Thermal storage can capture waste heat or use cheap renewable electricity when it is plentiful, then supply heat on demand. Higher temperatures are harder to store and use efficiently, and the right approach depends on the process.
Limits
Heat loss. Stored heat gradually leaks away. Insulation reduces the loss, but longer storage generally means more of the energy is lost, and effective insulation adds cost.
Temperature. The usefulness of stored heat depends on its temperature. Low-temperature heat is fine for space heating but cannot drive many industrial processes.
Getting electricity back. Converting stored heat to electricity is possible but inefficient compared with using the heat directly, so thermal storage is best matched to heat demand.
Space and cost. Large tanks need room, and cost-effectiveness depends on how often the system is cycled.
Safety. Hot water and molten materials present burn, pressure and scald risks, and legionella can grow in poorly managed stored water. Installation and servicing of pressurized or high-temperature systems should be left to qualified, authorized people following local rules.
How it fits alongside other storage
Thermal storage complements batteries rather than competing across the board. Batteries are flexible and suited to electricity needs, while thermal storage is often the cheapest way to shift heating and cooling. Together with options like pumped hydro, it contributes to the wider picture of energy storage, and which mix works best depends on local demand, climate and costs.