Thermal Stores

What is a thermal store?

A thermal store is a way of storing and managing renewable heat until it is needed. In a domestic setting, heated water is usually stored in a large well-insulated cylinder often called a buffer or accumulator tank. A thermal store may contain one or more heat exchangers, usually in the form of internal coiled pipes or external flat-plate heat exchangers. It may also include an electrical heating element, such as an immersion heater.

A purpose-built thermal store can take inputs from a number of different technologies, provided it has been designed and sized to work with them all. It might store heat from a wood-fuelled boiler, solar water heating or a heat pump.

A thermal store can provide:

  • Space heating and mains pressure hot water.
  • Space heating only (which may be the case with a heat pump system).
  • Hot water only (common in the case of a solar water heating system).

The full potential of a thermal store is realised when it is used to store and manage a number of different heat inputs and outputs. For example, it lets you use your excess solar heat for space heating or to act as a heat pump pre-heat when this is used to supply hot water.

Thermal Storage Tank

Thermal Storage System Diagram

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The benefits of a thermal store

  • Allows management of the difference in time between when heat is available and when it is needed. For example, hot water produced by a solar water heating system during the day can be stored for use when little or no solar energy is available.
  • Enables warm water to be heated up by a secondary heating source such as a conventional boiler or electric immersion heater.
  • Enables a renewable heating system to work more efficiently. This is  particularly relevant to wood-fuelled heating systems such as log boilers that operate much more efficiently if they are used at maximum output rather than kept ticking over.
  • Let’s you use a wood burning boiler stove or a stove with a back boiler at maximum efficiency without overheating the room.
  • Reduces the need to buy expensive fossil fuels to meet on-demand hot water or space heating.

stores linked to wood fuel heating systems are commonly referred to as accumulators or buffer tanks. Typically they will hold between 500 to 5000 litres of water and can store hot water for days if properly insulated.

Smaller thermal stores (300litres) can also work well with boiler stoves and stoves with back boilers. These stoves tend to be in living spaces and are fed with fuel throughout the day. Boiler stoves and stoves with back boilers differ in the proportion of heat they put into the room or water. Boiler stoves will put around 65 per cent of their output into water, whereas stoves with back boilers may only put 20 per cent into water.

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Combining technologies

A thermal store allows you to link up a number of different heating systems, for example a wood burning boiler stove and a solar water heating system. This is a particularly beneficial combination as it means that you can have hot water in the summer without having to light the stove.

You can also connect the following to a thermal store:

  • One or more renewable energy technologies (solar, biomass, heat pump).
  • A conventional boiler.
  • An Aga-type range with a boiler.
  • Electrical heating elements such as an immersion heater.

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