14th September 2016


Thermal Stores Wales

Thermal stores are an ever growing trend in domestic renewables heating technologies as a means of storing excess heat generated.  Thermal stores are proved to work particularly well with heat pumps, wood-fuelled biomass boilers, solar and wind energy water heating systems.

Thermal stores can be used as a renewable technology with a conventional boiler or immersion heater. They can also be used with an individual renewable heating technology or to combine different renewable heating technologies.

What is a thermal store?

Thermal stores are a way of managing and storing renewable heat until it is required.  In domestic settings, heated water is normally stored in a well-insulated cylinder (a buffer or accumulator tank).  Thermal stores may contain one or more heat exchangers, often 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, as long as it has been designed and sized to work with them all. It may store heat from a wood-fuelled boiler, solar water heating or a heat pump.

What 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.

A thermal store’s full potential can be fully seen 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.

What are the benefits of a thermal store?

  • They enable a renewable heating system to work more efficiently.
  • Thermal stores allow management of the difference in time between when heat is available and when it is needed.
  • They reduce the need to buy expensive fossil fuels to meet on-demand hot water or space heating.
  • They enable warm water to be heated up by a secondary heating source such as a electric immersion heater or conventional boiler.
  • They let you use a wood burning boiler stove or a stove with a back boiler at maximum efficiency without overheating the room.

Types of thermal store

There are many different thermal stores to chose from, each designed with different technologies in mind.  Here are some of the most common ones:

Combining technologies

A thermal store allows you to link up different heating systems.  This is an especially 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:

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

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 payments.  This is because solar panels that are part of a system that also provides heat for a purpose other than domestic hot water are not eligible for the scheme.

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 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.

Because thermal stores used with wood-fuelled heating systems are often designed to provide hot water for space heating as well as domestic hot water, they tend to be rather large.  Thermal stores linked to wood fuel heating systems are most commonly known as accumulators or buffer tanks. Typically they hold between 500 – 5000 litres of water and can store hot water for days if properly insulated.

Smaller thermal stores also work well with boiler stoves and stoves with back boilers. These tend to be in living spaces and are fuelled 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% of their output into water, in comparison to stoves with back boilers that may only put 20% into water.

The sizing of a thermal store for a wood-fuelled heating system is dependent on many factors, in particular the type of wood fuel being used.  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.  Whereas a pellet boiler will need a small thermal store as it can cope quite quickly with changes in heat demand.

A thermal store working in conjunction with a log burning batch boiler will need to be fairly 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 is something for the installer to determine as part of the total system design.

Thermal stores for solar water heating

Thermal stores perform very well along with solar water heating systems as they allow solar thermal heat to be used for space heating as well as heating water.  A solar thermal array may harvest far more heat than would be needed for hot tap water alone.

With a simple design and control strategy a thermal store can also be designed to prioritise solar thermal heat above other sources. This will mean that if solar heat is available, no other heat source will be used.

Thermal stores for heat pumps

An air source or ground source heat pump works more efficiently with less wear on the pump and compressor when it does not have to continually cycle on and off 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 adjust its output.

A way to avoid the short cycling of a heat pump is for it to be linked to a thermal store.  However, there are other ways, for example;  leaving a part of the heating system permanently open.