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Thermal Storage: How much could you save?

Posted by Alex Barrett on 10 April 2017 at 2:34 pm

Storing the energy produced by solar panels is a hot topic. Developments in battery storage for solar photovoltaic systems are constantly improving, but when it comes to storing energy, thermal systems have a lot of advantages.

Solar thermal systems work by pumping a mix of water and antifreeze through a panel of pipes. During the day this fluid is heated by the light of the sun and is transferred to the hot water system. Solar thermal systems don’t usually produce enough heat to completely replace a house’s boiler. Rather, they ensure that the water has a warmer base temperature, so that the boiler uses less energy to heat it up for use in taps and appliances.

Hot water can be produced throughout the day while the sun is shining, and stored in a well-insulated tank so that it is available at night when the solar panel is no longer directly producing heat. The Centre for Sustainable Energy claim that “A well-installed and properly used solar hot water system can save a household £55 a year when replacing gas heating or £80 a year when replacing electric immersion heating.”  [1].

Similar principles can be applied to households without a solar thermal installation. An immersion heater can be run during periods when electricity is cheap, or when an intermittent generator, such as a wind turbine, is producing a lot of power. This heat is then stored for use later on. 

Another option is the night storage heater. This device also makes use of the cheap electricity at night to heat up a thermal “bank”. This bank is a mass of clay or ceramic bricks which have good thermal storage properties [2]. The high cost of electricity, both for the consumer and the environment, means that these are only really advisable if you are already heating your house with electricity. Boilers are generally more efficient [3], Even if they do rely on local burning of fossil fuels. If you plan to generate all of your electricity through renewable sources then thermal bank technology might be a good one to consider, but for most an efficient boiler is a better option.

This doesn’t mean that the thermal bank is a dead end. The principles of storing heat in solid material can be applied on a larger scale to increase the use of ground source heat pumps in larger, commercial buildings [4].

Ground source heat pumps work in a similar way to solar thermal systems, but rather than drawing heat from a solar panel it is instead provided by a series of pipes embedded in the ground [5]. The temperature just below the ground is far more stable than that at the surface, since soil retains a lot of heat. A ground source heat pump draws this heat out of the ground, but if it does so too quickly then it can deplete the resource. Ground heat is replenished when the sun warms the ground. Use too much of it during a long winter and you could freeze the ground around your pipes preventing the system from working.

Thermal banks can solve this problem. Pipes buried beneath tarmac surfaces capture heat during sunny periods. This is stored in a well-insulated bank of clay, usually buried beneath the foundations of the building. This thermal store is thus recharged during the summer months and can be tapped throughout the winter, when the ground source heat pump might otherwise be less effective.


  1. Centre for Sustainable Energy: Solar Hot Water
  2. Centre for Sustainable Energy: Night Storage Heaters
  3. The Energy Saving Trust: Electric Heating Systems
  4. ICAX Interseasonal Heat Transfer systems
  5. The Energy Saving Trust: Ground Source Heat Pumps

Image credit: Boatbuilder via Wikipedia (CC BY-SA 3.0)


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1 comments - read them below or add one

Andy in Hawick

Andy in HawickComment left on: 29 April 2017 at 12:32 pm

There are some nice developments on the thermal storage front with phase-change materials offering more heat storage in a smaller volume. See, for example,

It is also worth bearing in mind that the heat available in the ground comes not only from the heating of the surface by the sun but also thermal flux from below. Typically, these are roughly the same size.

As David Mackay pointed out ( you cannot heat all of suburbia with ground-source heat pumps without significantly reducing the heating load of the houses and/or deliberately pumping heat back underground during the summer.

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