By Dave Elliott
Heat pumps are seen as a clever way to get an energy upgrade, with the input energy driving a compression cycle, pumping heat collected from outside a building into radiators inside, like a fridge working in reverse. Most systems use heat from the air or from the ground, but there are also some water-source systems. For example there are large water-source heat pump schemes in Scandinavia, feeding heat to district heating networks. About 60% of the total energy input for Stockholm’s Central Network is provided by a district heating plant with six large heat pumps using the sea as a heat source. Warm surface water is taken during summer, while in winter, the water inlet is in 15m depth where the temperature is at constant +3°C. Helsinki in Finland also has large heat pump plant producing district heating with capacity of 90 MW, as well as cooling, with capacity of 60 MW, using heat from the sea and from wastewater led into the sea from a central wastewater treatment plant.
These are large projects, but a medium-scale system is being developed in the UK, using Mitsubishi’s Ecodan pump, which was voted the best new product or technology at the 2014 Climate Week Awards. It’s the first application of a system of its kind in the UK, and is backed Mike Spenser-Morris, a local developer and director of the Zero Carbon Partnership. The heat pump will use the Thames to provide hot water for radiators, showers and taps in nearly 150 homes and a 140-room hotel and conference centre at Kingston Heights in Richmond Park, cutting heating bills, it’s claimed, by up to 20%. It’s based on using water drawn from two metres below the surface of the Thames, where the ambient temperature, sustained by ambient heat from the sun, stays at around 8C to 10C all year round. A system of heat exchangers, pumps and condensers boost that to 45C. The electricity used to power the system is supplied by Ecotricity, which makes it zero carbon. According to a report in the Independent on Sunday, the system is thought to have cost about £2.5m, though this is for a ‘first of a kind’ project. The cost of future systems should be lower, and the Renewable Heat Incentive can offset supply costs.
Energy Secretary Ed Davey told the Independent on Sunday: ‘This is at a really early stage, but it is showing what is possible. You never have to buy any gas- there are upfront costs but relatively low running costs. I think this exemplifies that there are technological answers which will mean our reliance on gas in future decades can be reduced. Here you have over 100 homes, you have a hotel with nearly 200 bedrooms and a conference centre that won’t be using gas. It will be using renewable heat from the nearby River Thames. This is a fantastic development. My department is exploring the potential for this sort of water-source heat pump across the UK, so we’re going to map the whole of the UK for the potential’: www.independent.co.uk/environment/climate-change/exclusive-renewable-energy-from-rivers-and-lakes-could-replace-gas-in-homes-9210277.html
As the Independent noted, in theory, any body of water, including tidal rivers as well as standing water such as reservoirs and lakes, can be used as long as they are in the open and heated by the sun. The Government has a target of 4.5 million heat pumps across the UK, though most will be using heat from air or ground and will be small domestic units. Prof. David MacKay, until recently DECC’s chief scientific adviser, has described a combination of heat pumps and low carbon electricity as the future of building heating. However, as I’ve noted before, there are limits to the viability of small domestic systems: they make most sense in off gas-grid areas. Larger units, feeding district heating networks, are more efficient, and make more sense in urban areas, where there are large heat loads. Operation at the larger scale also make it easier to provide an effective maintenance regime, important for heat pumps, which need careful adjustment and servicing to maintain optimal performance. Otherwise the coefficient of performance (CoP), usually expected to be around 3, can fall dramatically. For example, in winter in damp cold countries like the UK, the external heat absorption pipes of air source heat pumps can develop a film of frost, reducing the heat flow. Without regular de-icing, the pump then has to work harder, potentially, in the extreme, reducing the CoP to perhaps 1 or less- making it less efficient than a simple one bar electric fire.
Moreover, large or small, the current type of heat pump run on electricity, and it’s been argued that the idea of shifting to heat pumps instead of gas for home heating on a national scale may be suboptimal, since using heat pumps run on mains electricity generated in large gas fired-plants, may be no more efficient than using gas direct in a domestic scale condensing boiler. It’s also argued that the wide-scale use of electric heat pumps is impractical, since the electricity network could not supply the large amount of power needed – the gas grid carries 4 time more energy than the power grid. It’s perhaps worth noting in this context that in the 1950’s, Southbank’s Festival Hall was heated by a large 7.5MW gas fired heat pump using the Thames as a heat source, although it seems it was taken out mainly as it produced too much heat: it was oversized www.architectsjournal.co.uk/home/rolls-royce-performance/181204.article#
There is now renewed interest in gas-fired absorption cycle heat pumps. They are less efficient than the electric motor driven compression-cycle variant, but gas is cheaper/kWh than electricity, much of which, after all, is made inefficiently by burning gas (and coal), so a 50% net fuel saving is claimed. At the World Renewable Energy Congress in London in August, Prof. Bob Critoph from the University of Warwick noted that there were now three domestic gas-fired systems on or very near to market (Robur, Vaillant, and Viessmann) with others under development. He proposed a mixed heating solution with both gas-fired and electric heat pumps, and also the use of hybrid electric heat pump-gas boiler systems, e.g. for older properties. He felt that the proposed mix, whilst not being the minimal emission route, was an affordable and pragmatic solution to domestic heating. There are of course other novel ideas, for example solar thermal fired absorption cycle heat pumps, which may have relevance even in the UK, with the combined air source/solar Solaris system claimed to be 25% more efficient than standard air-source electricity-powered units depending on location: www.uk-isri.org/case-studies/solaris and http://cordis.europa.eu/publication/rcn/16280_en.html
Whatever the heat and power source, are heat pumps the way ahead? Some say that large community scaled gas-fired combined heat and power (CHP) plants, with CoP equivalents of up to 20, are better in energy efficiency and carbon emission terms than heat pumps of any scale or type. That may be true at present, but, longer term, if electric heat pumps use green electricity, or gas fired heat pumps use green gas (biogas or stored gas produced using surplus wind/solar-derived power), then net emissions would be near zero. Although the same would be true for green gas fired CHP.
In the final analysis, given its high CoP, CHP seems to have the edge for the moment, but, in economic terms, the optimal systems choice may depend on the location and the size of the load. One of the largest gas-fired heat pump systems so far is the 140kW unit at Open University: http://www.modern-building-services.co.uk/news/archivestory.php/aid/9841/__65279;Ener-G_teams_up_boreholes_with_absorption_heat_pumps_.html
In some locations, large water sourced units may make sense, but large gas-fired units might have even wider applications. But then so may CHP, linked to district heating networks. However, to complicate matters further, it may not be a straight choice between CHP and heat pumps: e.g. a heat pump can be run using electricity from a CHP plant, while using the heat from the CHP plant as its heat source, thereby upgrading the heat output. Plenty of room for innovation! http://setis.ec.europa.eu/system/files/JRCDistrictheatingandcooling.pdf