National Energy Centre Phase 2 building - an exemplary low-energy office
The second phase of the National Energy Centre in Milton Keynes opened in 2004, with a gross floor area of 430m2 able to accommodate up to 45 people, including storage and meeting space. This increased the total occupancy capacity of the National Energy Centre to over 100 people.
The Foundation monitors its energy use and has had a Display Energy Certificate for the building since they were first launched. Its most recent certificate is a 'B' rating (48) with gross annual energy use of 72kWh per usable square metre of floor area. Approximately 14.5% of this energy use came from the PV array, and the overall renewable fraction is higher due to the use of a ground source heat pump.
The west elevation of the building has a mixture of horizontal and vertical glazed areas including high level natural ventilation and vertical/horizontal shade panels. This has been constructed using a timber frame, with brick walls below the windows and on the north and south aspects, and low maintenance 'Thermowood' cladding produced from sustainable (Scandinavian) timber elsewhere.
National Energy Centre Phase 2 construction details
The floor level of the building was set at a similar level to the Phase I building to minimise the disposal of subsoil from the site. The east elevation was partially buried to help shield the Phase II building from solar gain and to use the stabilising effect of the temperature within the mass of the adjacent sub-soil. The ground is sloped towards the building to provide shielding up to a height of approximately 1 metre.
The mono-pitch roof has an outer skin of Rheinzink, supplied through Boss Metals, to ensure a long design life and ease of workability in fitting the solar PV panels and sunpipes. The ridged design assists with the generation of natural cross-flow ventilation of the building through the eaves level vents in the east and west elevations. All construction elements (roof, floor, walls and glazing) were built to higher standards than mandated under Part L2 of the UK Building Regulations.
The internal skin of the building is a load bearing timber frame construction, with a high level of yellow glass wool insulation. This was internally faced with plasterboard, and painted in light colours (to reflect natural daylight) with a water-based low-emission paint. The south elevation brickwork is extended beyond the sides of the building to provide shielding from the high summer sun.
Windows and ventilation
The windows are gas-filled low-emissivity doubled glazed units with frames from sustainably sourced softwood. The roof projects over the high level opening windows to shade them in summer. The high level vents on the west elevation can be seen here above the windows. This natural ventilation should reduce perceived temperatures by between 2-3°C in summer months.
The floor consists of several layers to maximise energy performance. From the ground up, it starts with 250mm suspended concrete beams, which offers limited summer cooling from its thermal mass.
The beams are insulated with two layers of insulation: 100mm of expanded polystyrene (shown stacked on the photo) beneath 25mm of polyurethane foam, with a heat reflecting coating on each side. Underfloor heating pipes were laid above this in a screed of heat-transmitting Gyvlon.
The south wall of the building is of straightforward brick construction with no glazed areas to minimise the chance of overheating in summer. As an additional shading feature, a trellis has been installed on which deciduous plants grow. The flue for the pellet stove can also be seen in this photograph, inside the red O.
The heating system is built around a ground source heat pump feeding a low-temperature underfloor heating system.
Soil temperature remains almost constant year round deeper than around 1m below the ground surface, as the heat of the sun percolates through the layer of topsoil. Ground source heat pumps capture solar energy by exploiting this effect, using a long water-filled coil to extract heat from the ground.
The Foundation needed to calculate the heat demand from the building very carefully, as on both cost and performance grounds it was important to avoid over-sizing the system. Calculations suggested a total fabric heat loss from the building of around 7kW (as a design temperature difference of 27K), with a similar maximum loss from ventilation. It was decided to install a 13kW Viessmann Vitocal 300 heat pump unit (left), which although theoretically very slightly under-sized should operate at a high load for much of the time.
The heat pump collects heat from ground loops containing a water/antifreeze mixture laid into three 50m trenches. Each trench contains approximately 250m of polyethylene pipe coiled into a "slinky" of around 1 metre in diameter, laid horizontally at a depth of 1.4m.
The external loops are pumped by a Wilo Salmson TOP S30/7 high efficiency pump.
The heat pump feeds a 200-litre buffer tank, from which a secondary circuit supplies the underfloor heating. To deal with exceptionally cold weather, a manually operated 3kW immersion heater is installed in the top of the buffer tank, but in the first winter of operation it did not prove necessary to use it. The building also has a biomass pellet stove as back-up heating.
Internally the heat pump drives an underfloor central heating system consisting of Rehau Universal pipe laid in a Gyvlon floor screed on the ground floor. The self-levelling screed was a nominal 35mm thick, allowing greater responsiveness from the system and enhanced insulation - this arrangement is shown schematically below. On the mezzanine, the heating pipes needed to be set above timber joists, so were set into Rehau's high conductivity aluminium plates (above right). In all, there are three underfloor heating circuits, individually controlled through zone thermostats, fed from the top of the buffer tank.
The building also includes a demonstration pellet stove, to provide top-up heating on very cold days. It is located quite close to the main entrance to the building, where the air is likely to be coolest, and well away from the thermostats controlling the main heating system. The stove is an Enviro Evolution, capable of delivering 7.2kW heat at 82% efficiency. Sustainably produced wood pellets are fed from an integral hopper into the stove, and the output can be adjusted by a variable speed auger changing the volume of pellets delivered. A convection fan helps distribute heat into the building.
As well as the windows on east and west elevations (there are no south windows to avoid overheating in summer), additional light is available in the centre of the building through eight monodraught sunpipes (light tubes). As can be seen, these protrude minimally above the Rheinzink roof, but (inset) provide a significant level of internal illumination - of up to 500W equivalent on a sunny day.
Solar hot water
The hot water system (for kitchen, toilets and a shower for cyclists) is electric, pre-heated by water from by a 2m2 evacuated tube solar panel, generously donated by Thermomax (now part of Kingspan Solar), shown here being fixed to the roof. Installation of this unit, along with the cost of the ground source heat pump system, has also been supported by a UK Government Clear-Skies grant.
The roof has 6.47kWp of photovoltaic panelsmounted on it to generate electricity, supported financially by a DTI PV Solar Grant. 20 Schüco S340K modules are arranged into four strings spread across three south-facing planes. Direct current from the modules is converted into 240V AC for grid connection; the system is designed to generate enough electricity to power the heat pump and PCs in the building on sunny winter afternoons.
The floors are carpeted throughout with Interface Entropy carpet tiles, which incorporate fibres from recycled materials. An additional benefit is that they can be laid in a random pattern, permitting easy replacement of any areas that become worn.
Rain water recovery
The building also incorporates a rainwater recovery and storage system for use in toilet flushing. This photo shows the 3,300-litre underground storage tank waiting to be installed..