The reliable route to renewable energy

Published:  09 April, 2007

collage
Heat pumps can exploit renewable energy from a variety of sources and upgrade it to useful temperatures for space heating and generating domestic hot water. Clockwise from lower left: boreholes at Sharrow School in Sheffield, the paddock at the Yorkshire Energy Centre and outdoor air using a fan-assisted coil.

Despite the powerful green image of solar energy and wind power, heat pumps are by far a more effective way of exploiting renewable energy, as we found out from PHIL MOORE.

The problem with most types of on-site renewable energy, as increasingly required to obtain planning permission, is that they are erratic and temperamental.

The output of solar thermal and solar photo-voltaic systems depends on how much sunlight is available.

Using wind power to generate electricity is also erratic — and there is the paradox that planning permission to erect a wind turbine may be refused so that, in turn, planning permission for the project may be refused on the basis of renewable energy not being generated on site.

‘In stark contrast to solar energy and wind energy,’ explains Phil Moore of Eco Heat Pumps, ‘Heat pumps can reliably exploit renewable energy 24 hours a day.’

He further explains, ‘Unlike solar thermal used to produce domestic hot water, there is no need to worry about there being too much solar energy to cope with. A heat pump can simply be turned off if there is no demand.’

While heat pumps require direct-acting electricity to convert low-grade heat to higher-grade heat, a COP of 4:1, for example, means that 75% of its energy output is derived from renewable energy. ‘That is why,’ says Phil Moore, ‘Heat pumps are an effective way of getting renewable energy in buildings, and they can go a long way to meeting the overall renewable-energy requirements of a building.’

He speaks from the experience of Eco Heat Pumps having been involved in a wide range of projects, mainly ground source using shallow pipework or boreholes.

The company is based in Sheffield and has developed a strong working relationship with Sheffield City Council. One project is South Yorkshire Energy Centre, which offers advice and services to improve energy efficiency in both domestic and commercial buildings. There are display boards for solar PV and wind energy — and a working installation of a ground-source heat pump delivering space heating and domestic hot water with a much shorter payback than solar thermal could achieve.

Phil Moore tells us that this refurbished building was formerly a bakery making pikelets (known in the rest of the country as crumpets). The heat pump replaced a gas-fired system and serves radiators. Renewable energy is drawn from pipework buried in the paddock outside. For a small office building, such pipework could be buried beneath a car park with a porous surface to allow water to percolate into the ground and help recharge it with thermal energy.

250 m of pipe is buried about a metre down and carries a mix of water and glycol to carry energy to the Thermia Diplomat unit installed inside the building, which contains the heat pump with an output of 10 kW and storage for domestic hot water. On a rather chill day in winter, the water/brine was arriving at the heat pump at 3°C and leaving at –1°C, with the temperature elevated by the heat pump to deliver heating and hot water.

While this installation was a replacement for a gas-fired system, Phil Moore explains that a different mind set is required to get the best from a heat pump. Because they cannot cost effectively deliver the high heat outputs that gas can, heat pumps are better suited to continuous heating — perhaps with night setback. On the plus side, the life of a Thermia heat pump is 20 to 25 years, and annual servicing is not required.

A few roads away, a much larger system is being installed to heat a new school. Because of the large heat requirement (200 kW) renewable energy will be extracted from the ground using 21 boreholes, each 90 m deep. At this depth, the average temperature is 10°C. Depending on the nature of the ground, higher COPs can be achieved than with shallow pipes, but at a higher capital cost. The overall life cost will, however, be lower.

The system will serve underfloor heating with an expected COP of 4:1 and provide hot water with a COP of 2.8:1. The underfloor circuits will also provide cooling in summer simply by circulating cool water from the ground; the ‘EER’ for cooling will be over 100:1.

Another project in a nearby house uses the same type of indoor unit as the South Yorkshire Energy Centre, but with its energy supplied by a fan-assisted coil outdoors to transfer heat from ambient air using a water/glycol mix. Inevitably the seasonal COP is lower than if energy were drawn from the ground, but the capital coast is lower. Even so, Phil Moore says that this approach is effective at getting renewable energy into a building.

‘Heat-pump technology no longer has to prove itself,’ says Phil Moore. ‘An increasing number of organisations are quite comfortable specifying it. The rest will follow.’



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