In the second article in my series on energy storage, I explore the area of thermal energy storage (TES), which involves the collection of excess thermal energy for later use. This and can be achieved through a range of technologies, at an individual building, district or even regional scale. The technologies fall into three broad categories:
- Sensible heat storage based on heating or cooling solid or liquid media such as water, sand, molten salts, or rock;
- Latent heat storage involving changing the phase of materials eg from a solid to liquid state; and
- Thermochemical storage which uses chemical reactions to store and release thermal energy.
With just under half of the UK’s energy consumption going on heating, the idea of thermal energy storage combined with renewable generation could be an important factor in the decarbonisation of the energy market.
Sensible heat storage
A “sensible” heat storage process is one that can be sensed by a change of the temperature. There are a large number of thermal storage installations globally based on the heating and cooling of various solid or liquid media. At the domestic level, ground-source heat pumps involve the circulation of water and anti-freeze through buried pipework to extract thermal energy from the surrounding soil and rock.
On a district level, projects such as the 52-home district heating system at Drake Landing in Canada involves seasonal storage of water heated through rooftop solar collectors being stored underground for use in winter. A similar project in Denmark covers 1400 buildings.
These schemes can require a large amount of physical space, which can affect their suitability, and their efficiency can be undermined if the buildings being served are not well insulated, as is the case with much of the existing UK housing stock.
Outside the UK, where district heating (or cooling) schemes have a longer history, local infrastructure has grown up with them, and schemes are starting to switch from fossil-fuels to renewable energy sources. Schemes are also seeking efficiency gains, for example by using buildings themselves for heat storage, or adopting advanced thermal insulation coatings such as those used at the innovative TES scheme at the Department of General Services in Sacramento, California which provides chilled water for cooling, steam for heating, and control air to 23 buildings (shown in the main image for this post).
At the grid-level, molten salts are used for storing heat at high temperatures and can be used in conjunction with concentrated solar power. Such schemes can delivery utility-scale energy solutions, such as the 110 MW facility at Crescent Dunes in Nevada, which has 10 hours of full load storage, enabling on-demand energy production day and night. There are similar projects at Postmasburg in South Africa (100 MW) and Copiapó in Chile (260 MW). Such schemes have greatest potential in scarcely populated, hot areas where solar capacity is greatest and space is not constrained.