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‘At heart it’s the same technology’: the heat pump that uses water instead of air

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Scientists in Edinburgh have developed a home heating system that draws its energy from the world’s most abundant resource: water.

The equipment can use sea water, rivers, ponds and even mine water to heat radiators and water for baths and showers, using the same technology as in air source heat pumps.

It is being trialled by Edinburgh University in an affordable housing project close to the Firth of Forth near the Forth Bridge, at a gold-mining museum in south-west Scotland and in a commercial greenhouse in Fife.

Another system is due to be installed this summer at the Scottish Seabird Centre in North Berwick, also drawing its energy from the Firth of Forth. All of the systems use water from the sea or nearby rivers.

It is the latest way of exploiting the ambient warmth in the natural environment to heat buildings, using the same technologies in air and ground source heat pumps.

The warmth of the sea or river water is captured by glycol, the liquid used in anti-freeze, which is then compressed in the heat pump. That compression makes it hot enough to heat water for radiators or baths. As it travels through the heat pump, the liquid cools down again, and the process repeats.

Dr Andrew Fraser Harris and Professor Christopher McDermott with the SeaWarm system – layers of looped tubing carrying the glycol, which transfers the warmth of the water to the heat pump. Photograph: Murdo MacLeod/The Guardian

Similar technology is already used in large district heating networks: water from the Clyde is used at the Queen’s Quay housing development at Clydebank near Glasgow. Sewage is being used to power district heating systems in places such as Stirling, Borders college in Galashiels, and in Granton, Edinburgh.

Unlike large-scale plants, the prototypes built by hydrogeologists at Edinburgh University are designed to be compact, easily portable and used in homes and smaller buildings, particularly in rural and coastal areas.

They are intended to provide another type of the small-scale green energy systems needed in huge numbers to replace gas- and oil-powered heating, as the UK moves towards a zero-carbon energy supply. The UK has about 23m gas boilers, and about 1m oil-fired boilers.

The team behind the design said water was normally a more predictable source of energy than the outside air, as the sea, lakes and rivers generally remain at a consistent temperature.

It sits alongside the mini-hydro schemes used in hilly areas or the ground-source heat pumps householders with large gardens can install, by running pipes down deep boreholes into the ground or by laying the pipes over a large area below the surface of the ground.

While air source heat pumps need to work harder in very cold conditions, the Edinburgh University team say their designs, which they have called SeaWarm and RiverWarm, can also use frozen water.

“It’s about trying out a whole series of constellations, but at the heart of it is the same technology,” said Prof Chris McDermott, from Edinburgh’s school of geosciences and the lead designer.

Gus Fraser-Harris, a hydrogeologist involved with the design, said the system would be more expensive to buy and install than an air source heat pump but cheaper than a ground source heat pump when its final version goes on sale.

The SeaWarm system collects the water in a large circular tub, which holds 3.7 cubic metres of water, roughly the same volume as 12 bath tubs. Inside the tub are layers of looped tubing carrying the glycol, which transfers the warmth of the water to the heat pump. It can use a water body 500 metres away from the building.

The chief operating officer of Lar Housing Trust, Mikko Ramstedt, in what was the jail of the Naval Barracks where the heat pump is at work, heating radiators. Photograph: Murdo MacLeod/The Guardian

They have called the system HotTwist and say the tub can be buried in the ground, which also helps to keep it at a constant temperature. It delivers 350% to 400% more heat than the electricity it needs to operate, Fraser-Harris said, comparable to the most efficient air source heat pumps.

The pilot project near Edinburgh, part of a project by LAR Housing Trust to convert an old naval barracks and prison into affordable housing project, involves pumping water up from the sea.

The gold-mining museum at Wanlockhead, south of Glasgow, uses gravity to transfer water from the river used for gold-panning down to the heat pump, while the greenhouses in Fife take water from a nearby burn.

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