Testing zero carbon

Greenwatt Way, in Slough, Berkshire, is more than a zero carbon Code for Sustainable Homes Level 6 development. It’s a live testbed for, amongst other things, five different types of energy generation. This means it has an energy centre on a scale very different from what I had expected of a ten home scheme made up of two and three bedroom houses and one bedroom flats.

For a start there’s the 8,000 litre thermal store weighing in at 11 tonnes when empty, 19 when full. This is connected to a low temperature district heating system – designed to reduce heat losses and maximise heat source performance – which pipes hot water through heat exchangers in each of the homes to feed the hot water supply. The bathroom towel rails and single radiator which, due to the high levels of insulation within the homes, are all that’s required for space heating are fed directly by the district heating system.

Energy generation currently consists of air and ground-source heat pumps, a biomass boiler, solar thermal panels and solar photovoltaic tiles – one bay within the energy centre sits empty awaiting a fuel cell. Each of these technologies has been sized to meet the full heating requirements of the site and the heat pumps and biomass boiler will be run independently to demonstrate their ability to meet the energy needs of the homes.

Behind Greenwatt Way is SSE (Scottish and Southern Energy) which is investing over £3.5m. The scheme was designed by PRP Architects, built by Bramall Construction and engineered by AECOM. The homes are being rented out to SSE and Slough Borough staff, along with local residents and, for the next two years, will be monitored to improve understanding of energy usage and requirements and the individual occupant’s interaction with the homes. The findings will contribute to studies which SSE is carrying out in collaboration with the University of Reading, NHBC and BRE.

As PRP Architects explained to me, Greenwatt Way is primarily designed as a place where people will want to live. The aim has been to replicate a larger, ‘healthy’ community where good neighbourhood interaction helps encourage a more sustainable lifestyle. For this reason there are community as well as private spaces and the residents have to walk past one another across the development to reach the communal bins, bike store and car parking.

See cutaway of house here

Key features:

  • To reflect the different construction methods used across the UK, four of the homes are built from timber frame and the remainder in masonry block.
  • The homes were deliberately oriented to have east and west facing façades as this is the most challenging orientation to deal with both in terms of passive solar design and in maximising south facing roof areas.
  • A high efficiency of heat recovery was necessary on the ventilation system, with summer bypass and a natural ventilation strategy to mitigate overheating in summer.
  • A north facing rooflight above the stairs in the homes allows natural daylight penetration and, in summer, provides an opening to draw warm air out of the house.
  • Low water use fittings are used and the homes include a grey water system for recycling bath and shower water to flush toilets and recover waste heat. A centralised rainwater harvesting system collects rainwater which is stored and used to flush toilets and provide water for irrigation and car washing.
  • The roofs of the homes are covered with solar PV tiles (63 kWp in total) which provide enough renewable electricity to achieve net zero carbon emissions in each of the homes irrespective of heat source.
  • The homes include low energy lighting, the latest energy efficient kitchen appliances, VPhase voltage optimisation and smart meters which SSE will use to monitor the homes’ energy and water usage.
  • Outside, the houses have their own private patio as well as a communal bike shed and garden with space to grow vegetables. Charging points have been installed for electric vehicles.

Image credits: PRP Architects