We’re all familiar with the concept of batteries to store electrical energy, but thermal batteries are somewhat less common and the benefits and uses less understood. What’s more, thermal batteries can be used to store heat and cold.
Water based thermal storage
This is the most common form of thermal storage. Called ‘Accumulators’ or ‘Buffers’, these are distinct from the sort of Domestic Hot Water tank we’re all used to.
Accumulators are large volumes of hot water placed in-line in a water based heating system. The benefits arise particularly where a heat load is not consistent, or where a boiler has a high thermal inertia, such as a biomass boiler. Having a permanent store of heat available reduces the need for a boiler to short-cycle (i.e. turn on and off regularly), thus improving its efficiency. It can also mean the boiler can be down-sized, as it need not be sized for the peak load.
Phase-change materials
These are materials that release or store energy when they change phase eg solid to liquid, and they are used for thermal storage when a wider range of operating temperatures are required (when compared to water). They also have the advantage of having a higher thermal density, so lower volumes are required.
Unlike water based systems, PCMs are often used to store at very cold temperatures, useful in cold stores.
PCM based thermal storage is particularly useful in decoupling demand for heat (or cold) from the generation of energy. For example, a Heat Pump run on low rate overnight tariff electricity.
There are a wide range of PCM such as inorganic salts, or organic materials such as oils.
Thermal Batteries
Thermal batteries typically use a solid material such as sand or crushed rock to store heat and are suited to having an electrical energy input. Very high recovery temperatures are possible, so via heat exchangers, process steam is available.
The main benefits of thermal batteries are the ability to take electricity that may be available on a low night rate, or generated from on-site renewables, and store the potential energy for later use in a different form.
Conclusions
This has been a very basic oversight into the main principles of energy storage. There are many technologies and techniques available. The key benefits are:
- Improved operational efficiency of boiler or refrigeration plant
- The use of low cost electricity
- The ability to store ‘cold’
- The potential to down-size boiler or chiller plant