What is inter-seasonal heat storage?

One of the frustrations of solar power in the UK is that there’s a lot of seasonal variability. It’s cold at the moment and so this is when my household uses the most energy. It unfortunately coincides with the point in the year when the solar panels generate the least power, thanks to the short winter days and lots of cloud.

Come May and June the situation will be inverted. The solar will be producing generous quantities of power that I don’t need. It would be so useful to be able to store all that surplus energy from the summer and use it now.

In fact, you don’t even need the solar panels. The Holy Grail of renewable heating would be to capture the natural heat of summer and salt some it away to enjoy in the winter. Depending on where you are in the world, the bigger issue might be cooling and you’d want to transfer heat the other way. The cold on winter nights could be banked and used to cool buildings in the summer.

Both of those are possible, and it’s called inter-seasonal energy storage, or inter-seasonal heat transfer. The nearest example I’m aware of to me is Howe Dell primary school in Hatfield, which was built as an exemplar eco-school in 2007, and my wife reported on it for the BBC when it opened. They have a pioneering heat exchange system, the first of its kind in the world. It works by running water through pipes just beneath the surface of the playground, which gets hot in the summer. The water is channelled into an insulated thermal bank installed under the school. In the winter the warm water is piped back up and run through the under-floor heating system.

Another project not far from me is Toddington motorway services on the M1. Here the heat is captured just under the road, making use of hot tarmac temperatures in the summer. The heat is stored and then automatically released to melt snow and ice, as an alternative to salting the road. It was a trial by Highways England, also a few years ago now in 2008.

There are a handful of other examples around the UK. ICAX, a company that specialises in innovative heating, has a list of projects using it, including a prison and a supermarket. Beyond the the UK there’s a small island community in Denmark that has thermal storage in water-filled pits. Drake Landing Solar Community, Canada, has another clever solution. They collect heat using thermal hot water systems on garage roofs, and pipe the surplus into 37 metre deep boreholes in the rock. The system is able to meet 97% of the community’s heating needs in this way, one of the most sustainable forms of heating available anywhere.

Elsewhere there are a handful of private homes with seasonal heat storage, and it’s more common in district heating systems in places such as Germany, the Netherlands, and Belgium. I very rarely hear about the idea in Britain. It hasn’t hit the mainstream, despite its obvious appeal for heating and cooling. One problem is that it can’t be easily retrofitted. There are a variety of technologies, but generally you need to bury the heat battery underground, where temperatures are stable. As the idea is still at the margins, every application is niche and bespoke, so it’s expensive to design and install even though the resulting heat is essentially free.

There was a substantial government review of interseasonal heat storage a few years ago, and nothing more came of it. I suspect that the research and development that was underway a decade ago has been partly overtaken by the possibilities of hydrogen, at least as far as the government is concerned. There has been a lot of interest in using surplus renewable energy in the summer to create hydrogen that could be burned for heating in the winter. Recent hydrogen trials may have deflated that idea, and so maybe interseasonal heat storage will get a second look.

I certainly hope so. It seems like a very intuitive way to heat or cool a building. The technologies are relatively simple compared to the magical physics that’s going on inside a heat pump. And it’s ultimately free, just holding over the generously given heat of summer for when we need it most.