The initial brief for the National Energy Centre (Phase 1) was to create a building that would:
- Use 40% less energy for space heating/cooling than CIBSE standards.
- Be constructed at no extra cost than more conventional buildings.
- Have a long design life and provide a stress-free working atmosphere.
- Allow flexible use of space with an area for meetings and displays.
- Maximise winter solar gain without over-heating in summer.
Architects, Weston Williamson, won a competition to design the centre, with a team including Ove Arup who were responsible for the energy strategy.
National Energy Centre Phase 1 building fabric
The strategy was to provide a high thermal inertia (mass) using:
- Exposed concrete ceiling on the ground floor.
- Multiple (dual) layer dense cement fibre boards on the first floor ceiling.
- Blockwork walls.
- The ability to vent overnight in summer. This was achieved through:
|Roof||Zinc skin above 50mm air gap
80mm thermal board
|Walls||50mm air gap
|Floor||315mm concrete slab
National Energy Centre Phase 1 windows and ventilation
The window design is:
- Softwood frames by Rationel.
- Low-e glazing (centre U-value = 1.9).
- Four narrow external light shelves.
- One wide internal light shelf.
Vents with external louvres and lockable and insulated interior doors are situated next to each window, providing fresh air for occupants and allowing passive cross-ventilation of the working space.
National Energy Centre Phase 1 heating system
As the building has such a low heat demand, it was possible to install a domestic scale heating system based around an MHS Strata 1 condensing boiler. The boiler has a fully modulating heat output sliding between 20% and 100%. Its stainless steel heat exchanger is designed to return operating efficiencies between 88% and 96% (gross). This ensures that there are extremely low NOx and CO emissions. The unit also has a variable speed primary pump which allows the boiler to maintain the widest possible temperature difference across the heat exchanger (resulting in a seasonal efficiency gain of 7% to 10%). Control is through an electronic 7-day integrated timer and room thermostat, with a set back temperature for night and weekends. Local temperature control is mainly by thermostatic radiator control valves (TRVs). To avoid running the boiler at part loads during the summer, instantaneous electric hot water units were fitted at each point of use.
National Energy Centre Phase 1 monitoring and performance
The building was monitored for several years using an Eco-Warrior PC-based monitoring system was installed to log energy use and other relevant data, such as internal and external temperatures. Monitored data was verified against actual meter readings and utility bills. The strategy of providing a high thermal mass for the building worked very well, with occupants enjoying a very even climate inside the building with low energy use. In winter, this was generally achieved with the heating running more or less continuously from about 7.30 to 9.00 am, then requiring only short top-up bursts later in the day. For example, the small screen image shows internal and external temperatures on Thursday 15th February 2001. The external temperature rose from -3.5°C at night to a maximum of just 4.5°C by day, but the internal temperature at the monitoring point did not fluctuate by more than 1°C. And this was achieved with the central heating system only firing measurably in the first 1½ hours. Later in the day, most of the heating requirement came from solar gain or incidental gains from the occupants and office equipment.
Summer energy use was very low, as the building is naturally ventilated. The internal temperature can rise as high as 24 or 25°C during prolonged periods of warm weather, but again fluctuates very little between day and night. Working conditions are more than acceptable, as staff can maintain a good cross-ventilation through opening vents beside the main windows, and the external overshading prevents the direct solar gains that are so useful in winter, when the lower sun angle allows warmth into the building.
The Eco-Warrior system also shows quite clearly how the lighting strategy is working. On bright days, throughout the year, there is often no lighting circuit consumption at all downstairs, with only limited use upstairs. In the example shown, the peak demand on the graph is just 1,600W serving an area of 400m2. It is noticeable that the greatest lighting demand occurs when the cleaners come in at night and switch on all possible lights, proving that staff motivation is also an important part of minimising energy use.
Analysis of bills revealed that The National Energy Centre achieved total energy use considerably below the Building Research Establishment's best practice guidelines for a modern, naturally ventilated office. However, over time, consumption crept up for a number of possible reasons:
- The original occupants, who were highly motivated to keep energy use low and so actively managed the use of lights, left the building.
- The design of the ground floor originally provided for naturally lit, open-plan office space but this was later replaced by cellular meeting rooms, requiring much more lighting.
- The new occupants were involved in software development and database management, which required a more intensive use of computing, including an air-conditioned server room.
Nevertheless, the overall design continued to show good energy use, especially as a result of the high levels of insulation, which were well above UK building regulations at the time the building was constructed.