Going underground

Why have some of the cleverest ideas been abandoned down the centuries? Tapping into the thermal performance of the ground to heat or cool a building is a brilliant technology that never made its way into our modern world — until now.

In Hatfield, Hertfordshire, work is under way at an £8 million school, designed by Capita Ruddle Wilkinson, that will feature a 21st-century interpretation of this technology. When completed in September, Howe Dell Primary School will be the first building in the UK to incorporate an interseasonal heat store. The store has been developed by Mark Hewitt and Andy Ford, directors of the London-based Interseasonal Collection & Exchange (Icax) and funded by the Carbon Trust to the tune of £250,000.

The principle of using the ground to moderate temperature is ancient. The Persians created elaborate underground labyrinths to cool their homes 5,000 years ago, while in northern India during the 15th and 16th centuries the Moguls were using the modifying influence of the earth to create ideal living conditions in their palaces.

Only now, with our focus fixed firmly on creating passive solutions, coupled with the technical ability to make use of this information, can we understand and learn from these ancient technologies, and adapt them to suit modern construction.

Both Hewitt and Ford argue that interseasonal heat transfer is a simple concept. It works by capturing summertime solar heat energy and storing it for use in winter. But the challenge is to create a 21st century working solution, making it as user-friendly as possible and finding a way for this technology to be installed using everyday construction materials.

Heat builds gradually

The beauty of Icax’s interseasonal heat store at Howe Dell is that it’s invisible. To the south of the main building, underneath the tarmac playground — tarmac’s high emissivity makes it an excellent material for transferring incoming solar radiation — an array of pipes has been laid, which collect heat. The sun warms the fluid —water plus an anti-ice agent — in the pipes, which is then transferred to an array of storage pipes laid beneath the school buildings. That heat is then conducted into the ground. Over the summer a body of heat builds up gradually underneath the school, and in winter it is transferred to the buildings.

“The smart part of this is understanding how to control the process so that the capacity can be sized correctly,” says Hewitt. “There’s no point in making something too big, because it’s expensive and a waste of money. The skill is making the installation the right size. We are providing a constant amount of heat — 60% of overall heat demand — that is sized according to that demand.”

Better building physics

The beauty of Icax’s inter-seasonal heat store is that it’s invisible

Getting the technology to a stage where it can heat a real building has taken the duo 10 years of hard graft. Hewitt is an architect at D-squared, where he is a partner; while Ford is a services engineer and founding director of Fulcrum Consulting. They came together because of a shared interest in finding more creative ways to bring physics into buildings.

Jointly they ran a three-year postgraduate architecture unit at the University of North London, now London Metropolitan University, on the subject. The course was instrumental in forming the basis of what Hewitt and Ford would later achieve.

“The course was an amazing exercise in trying to understand how architects’ minds work,” says Ford. “Mark and I had separate but parallel thought processes, and all these things were pulling us towards a solution.”

Before filing their patent and setting up Icax in 1999 — with the addition of third director Edward Thompson — Ford and Hewitt were separately collecting information relevant to the technology. Working with Brian Ford, now professor of architecture at Nottingham University, Hewitt was using computer data loggers on Mogul buildings in northern India to examine thermal performance — with fascinating results.

Meanwhile, Andy Ford had been analysing work carried out by the Rocky Mountain Research Centre, Arizona, on making underground buildings passively warm. Ford also monitored concrete buildings buried in the earth in a series of experiments in east London’s Mile End Park, rearranging the insulation so that thermally, the earth became part of the structure. He was also asking how heat could be transferred from the surrounding earth to buildings.

“You’ve got the temperature you want, but it’s not in the building. How do you get from A to B?” says Ford. “We looked at the earth tube, as well as the filtration of air through the ground, and continued to do experiments on thermal mass within the building fabric.”

Measuring energy use

During his experiments with thermal mass, Ford created a tightly sealed, well insulated and shaded structure. This happened to be the Elizabeth Fry Building at the University of East Anglia, which had insulating Termodeck extruded structural concrete slabs integrated into its fabric. Ford then measured in great detail where the energy used actually went. The purpose was to see whether buildings could be developed that relied on underground heat stores.

As well as conducting individual research, together Hewitt and Ford gathered a vast amount of data during years spent computer modelling thermal movements in the ground. All of this information helped to develop Icax.

The package can be installed easily: the whole aim is to move towards off-the-shelf technology

“We had been looking at how heat migrates in the ground, and were developing ways of calculating precisely where heat will be if you need to use it, what happens when you use it, and how much is left,” says Hewitt. “It’s only been possible since the arrival of computational fluid dynamics software that you can look at heat migration over a 10-year period. By having that capability, one can have a degree of certainty to put it responsibly into someone’s building.”

Computer modelling has enabled the firm to see that the technology works, but to familiarise people with IHT and to allow any problems to be ironed out, a building still needed to be constructed. Howe Dell, where Fulcrum was appointed as services engineer, became a test-bed for the technology after Ford and Icax persuaded Hertforshire County Council that the benefits far outweighed the minimal risks.

Hewitt and Ford say the heat produced for the school will be ample. The difficulty lies in predicting exactly what the school’s energy use will be. “Imagine this school was put to another use, or if one teacher is frenetic about closing doors while another isn’t,” says Hewitt. “The heat demand on the building could double because of these different approaches, and how the building is used. But on those days in February when it may be particularly cold, a conventional gas boiler is there as back-up.” And when heat is not required, for example when the building is empty, the interseasonal heat store is simply switched off.

Apart from the obvious benefits, the technology has no negative environmental or planning issues. It does not require a licence, as boreholes do, or an environmental impact study, as wind turbines do. It has no emissions, makes no noise, and is virtually invisible. The sensors plugged into the ground are the exception. These measure temperature at different locations, relaying the information to the plant room. Eventually, a small screen in the school’s foyer will provide data on the performance of the various elements so all those who visit can monitor the technology.

Ford says that combining the skills of an engineer with those of an architect has helped enormously in making sure the technology moves from a great idea to a workable solution. “We understand the building industry. We know what’s got to be done to make it acceptable and make it fit into the sector. It’s got to be affordable, predictable, and must not slow down the building process.”

He adds that this understanding has helped to produce a package that’s easy to install: “Part of the process of innovating is understanding how to move a product to market. We’re offering a package that can be installed easily. That’s been the whole aim — to move towards off-the-shelf technology.”

As well as being used at Howe Dell, the technology is about to be installed in a prison extension in Lancashire, as well as in a Highways Agency-funded project at Toddington, Bedfordshire. Heat collected in the summer will be stored beneath the road and used for de-icing it in winter. The repercussions of this could be huge, argues Hewitt. “The potential for the road to become a source of heat-collecting for surrounding buildings or neighbourhoods is vast if you scale it up.”

As confidence in the technology grows, Icax is being swamped with enquiries from the UK and abroad, especially Japan and the Gulf. But for the time being at least, demand is outstripping supply. Until Icax has the resources, the company will continue to devote its energies to ensuring that every project it commits to is completed and “delivered perfectly”.

If what Ford says is true — that “everybody is going to have to use these physics in the future to make their buildings work” — then interseasonal heat transfer could be the heating and cooling technology of the future.