What is a Thermal Store?

A thermal store is becoming increasingly common when used in conjunction with renewable technologies such as wind energy, solar thermal heating, air & ground source heat pumps and wood-fuelled heating. A store enables the energy created to be stored and used at a more suitable time. For example, solar thermal panels will generate hot water during the day when the sun is shining and then that can be held using a thermal store until the evening when it is needed for hot baths and heating once the sun has gone down.

A thermal store differs from a conventional hot water cylinder in that the water does not come out of your taps directly from the store. Instead, it is heated up by passing through a heat exchanger that transfers heat from the thermal store water to the mains or tap water. Heat exchangers can transfer lots of heat in a short time so you can have a mains pressure shower or fill a bath very quickly.

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 needs to be planned in when you are considering using renewable technologies as it can make the energy generated available at a time when you need it.

Benefits of a Thermal Store

– Allows the 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.

– Allows 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.

Types of Thermal Store

There are a range of thermal stores on the market designed for different applications.

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 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.

As a thermal store used with a wood-fuelled heating system is usually designed to provide hot water for space heating as well as domestic hot water, they tend to be fairly 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 5000 litres of water and can store hot water for days if properly insulated.

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

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 spring or autumn day, a solar thermal array may harvest far more heat than would be needed for hot tap water alone. Combined with a thermal store can supply space heating which if the primary heating is through under floor heating in a highly insulated property may mean that no other source of heat is required.

With a simple design and control strategy 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 (short cycling) when the demand for heat is low. This is more likely to happen if your heat pump is relatively large 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). GOS Heating will help you decide whether a buffer tank linked to your heat pump is appropriate, as well as what size buffer tank you should use.