Showing the way towards zero carbon emissions
On a bright clear day the entire heating requirements of this building at the National Energy Centre in Milton Keynes can be met by a a heat pump powered by electricity from Solar photo-voltaic cells.
A ground-source heat pump at the National Energy Centre at Milton Keynes can heat a new building with no carbon emissions.The capability of heat pumps drawing heat from the ground to reduce carbon emissions is being demonstrated to its limits by a project at the National Centre for Energy at Milton Keynes. A heat pump with an output of 13 kW is combined with a solar photo-voltaic array that can generate up to 6.47 kW of electricity. Ian Byrne, deputy director of the National Energy Foundation explains that on a bright sunny afternoon in winter, the PV array generates enough electricity for the heat pump to heat the building and produce domestic hot water, so that these services are provided with no carbon emissions at all. There are also solar panels to produce domestic hot water. When PV electricity is not available, power is drawn from the grid, using a low-rate tariff when appropriate. Even when electricity is drawn from the grid, carbon emissions are reduced by up to 60% compared with using a gas-fired condensing boiler. During the summer, it will be possible to export electricity to the grid — when the necessary equipment has been installed. Phase 2 of the National Energy Centre is a lightweight building of timber-frame construction with high levels of rock-fibre insulation to standards higher than required by Part L2 of the Building Regulations. Additional thermal mass is provided by brick walls below the windows on the north and south aspects, which represent the length of the building. Heating zones
There are three zones of underfloor heating, one each on the north and south aspects of the ground floor and one serving the mezzanine floor. Total floor area is 325 m2 within a footprint of 250 m2. About 60% of the heat loss is due to ventilation and 40% to the fabric. The floor consists of several layers to maximise energy performance. From the ground up, it starts with 250 mm suspended concrete beams, the thermal mass of which offers some summer cooling. The beams have two layers of insulation. 100 mm of expanded polystyrene is topped by 25 mm of polyurethane foam with a heat-reflecting coating on each side. Rehau underfloor heating pipework is laid above the insulation in a 65 mm screed of heat-transmitting Lafarge Gyvlon. The thermal source for the heat pump is from three coils laid horizontally in the surrounding ground by Geoscience. Each trench is a metre wide and 1.4 m deep and accommodates 250 m of pipe in a flat, looping coil. The use of shallow trenches avoids the shoring that would be needed with deeper excavations. Water is supplied to the Viessmann Vitocal 300 heat pump at 8 to 12°C. Maximum flow temperature is 40°C, with a minimum below 30°C. The surface temperature of the floor does not exceed 27°C. The performance of the system is being monitored, and Ian Byrne tells us that COPs of 3.1 to 3.4 have been achieved so far. Those figures are not as good as he would have hoped for, as COPs greater than 4 have been achieved in other European installations. Accurate
In designing the heating system, Ian Byrne explains that it was essential that the heating requirements should be calculated very accurately or the heat pump would have been oversized, adding to its cost and also to the size and cost of the external ground coils. The buildng has been heated since last Christmas, and Ian Byrne says, ‘After reasonably cold periods in winter, we realise that we have got it right, and the building has been pleasantly at temperature.’ If the heat pump should fail, two immersion heaters in the water-storage tank provide for the heating requirements — effectively direct-acting electric heating. The low-energy lighting system is augmented by eight Monodraught SunPipes that bring light into the heart of the working spaces. The building is naturally ventilated by three methods. The opening windows have trickle ventilators installed vertically in the frames; the windows are double-glazed with low-emissivity glass filled with gas between the panes. There are also high-level ventilators to provide cross-flow ventilation to the building and overnight purging and cooling — with no security concerns. Protection against solar gain is provided by the roof projecting over the high-level opening windows to shade them in summer. Other conservation features include the harvesting of rainwater from the roof for flushing toilets. This water is stored in a 3000 litre tank. Stressing that this project is intended as one that could be replicated in a lot of other buildings, Ian Byrne explains that the National Energy Foundation considered using an adjacent lake as the heat source for the heat pumps, but rejected it in favour of ground coils. Value
This project was part funded by the Clear Skies programme, but even so it is expected to have a very long payback period. Its value lies in reflecting some 27 000 installations in Sweden last year, where ground-source heat pumps are replacing oil-fired heating systems. Only about a hundred systems were installed in the UK last year, so the market is in its infancy. In the UK, however, such heat pump systems are in competition with gas-fired condensing boilers. Compared with the cost of a building, however, the cost of such a heat-pump system is very small. With the growing pressures to reduce carbon emissions, their case looks strong indeed. The National Energy Foundation is at Davy Avenue, Knowlhill, Milton Keynes MK5 8NG.