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Generating renewable electricity

Thermal energy stores

Benefits

  • reduce the need to buy fossil fuels
  • help renewable heating systems work more efficiently
  • combine with a secondary heating source

Thermal energy storage or thermal stores are vessels used to store excess heat generated from a domestic renewable heating system.

What is a thermal store?

A thermal store is a way of storing and managing renewable heat until it is needed.

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 several different technologies, provided it has been designed and sized to work with them all. It might store heat from a biomass boiler, solar water heating system, 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 several 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.

Benefits of a thermal store

Types of thermal store

Thermal stores work with an individual renewable heating technology or in combination with different renewable heating technologies. Thermal stores can also be used as a renewable technology with a conventional boiler or immersion heater.

Thermal stores have proved to work particularly well with wood-fuelled biomass boilers, heat pumps, wind energy and solar water heating systems.

There are several different thermal stores on the market designed with different technologies in mind. We’ve highlighted some of the most common below.

Thermal stores for wood-fuelled heating

Thermal stores are very important for the efficiency of biomass heating systems, particularly log boilers, which are designed to burn batches of logs at high levels of efficiency, rather than in small quantities throughout the day. A log boiler linked to a large thermal store can be used in this way. A thermal store can also reduce the time lag (which could be at least an hour) between lighting the boiler or stove and the demand for hot water, by storing water from the last time the stove or boiler was last lit.

As thermal stores used with wood-fuelled heating systems are usually designed to provide hot water for space heating as well as domestic hot water, they tend to be large. Thermal stores linked to wood fuel heating systems are commonly referred to as accumulators or buffer tanks. Typically, they will hold between 500 to 5,000 litres of water and can store hot water for days if properly insulated.

Smaller thermal stores (around 300 litres) can also work well with pellet or log stoves and stoves with back boilers. These stoves tend to be located in living spaces and are fed with fuel throughout the day. Pellet or log 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% of their output into water, whereas stoves with back boilers may only put 20% into water.

The sizing of a thermal store for a wood-fuelled heating system will depend on many factors, in particular the type of wood fuel being used. A pellet boiler can be paired with a relatively small thermal store, as it can cope quite quickly with changes in heat demand. On the other hand, a log boiler designed to burn logs in batches will need a large thermal store to take all the heat from the batch of logs in one go.

A thermal store working in conjunction with a log burning batch boiler will need to be quite large, probably no less than 25 litres/kWth and preferably 50 litres/kWth. The sizing of the thermal store connected to a wood fuel boiler should be decided by the installer as part of the total system design.

Thermal stores for solar water heating

Thermal stores work very well with solar water heating systems as they allow solar thermal heat to be used for space heating, as well as heating water. On a sunny day, a solar thermal array may harvest far more heat than would be needed for hot tap water alone. Combined with a thermal store also supplying space heating, this collected heat can be put to good work.

A thermal store can also be designed to prioritise solar thermal heat above all other sources. This will mean that if solar heat is available, no other heat source will come on.

Thermal stores for heat pumps

An air source or ground source heat pump will work more efficiently with less wear on the pump and compressor if it does not have to continually cycle on and off (known as short cycling) when the demand for heat is low. This is more likely to happen if your heat pump is relatively large, and less likely if it is relatively small and running continually to meet demand. It is also less likely if you have an air source heat pump with a motor that can modulate its output.

One of the ways to avoid the short cycling of a heat pump is for it to be linked to a thermal store (usually referred in this instance as a buffer tank). However, there are other ways, such as leaving a part of the heating system permanently open. Your installer, together with the recommendations of the manufacturer, will decide whether a buffer tank linked to your heat pump is appropriate, as well as what size buffer tank you should use.

Combining technologies

A thermal store allows you to link up several different heating systems, for example, a wood burning pellet or log stove and a solar water heating system. This is a particularly beneficial combination, as it provides you with hot water in the summer without having to light the stove.

You can also connect the following to a thermal store:

Costs

Thermal stores such as hot water cylinders are found in millions of homes, but the cost of connecting it to renewable sources can range from a few hundred pounds to over a thousand, depending on how many inputs you’re looking for, and how they will be controlled.

Please note when a solar water heating system is combined with a thermal store the system will not be eligible for domestic Renewable Heat Incentive (RHI) payments. This is because solar panels that form part of a system that also provides heat for a purpose other than domestic hot water are not eligible for the scheme.

This content has been developed by Energy Saving Trust in partnership with the OCTES project, with funding from the Northern Periphery Programme (NPP) and the Scottish Government.

Last updated: 10 March 2021