The Oxford Solar House

by the CADDET UK National Team

Click here to see a graph showing the electric balance

Click here to see a Schematic diagram of the PV installation

Introduction

Building-integrated photovoltaics (PV) could provide between 3% and 75% of the total energy demand in the UK, according to current research. As part of a project to evaluate the potential of PVs to contribute to the UK's domestic energy supply, a solar house was built in Oxford in 1994. This house is being used as a working example of PV power applied to the domestic situation and for practical instruction of architectural students at a local university.
 A 4 kW PV system is integrated into the roof structure and there is a 5 metres squared solar thermal domestic hot water pre-heating system.

The House

Built on a south facing site with good solar access, the house was designed to require a minimum of energy for heating, cooking and lighting, maximising the contribution of the solar electric supply without impairing the comfort of the occupants. The building was completed in March 1995 and a 2-3 year period of measurement and monitoring is now under way.
 Inside, the house is laid out with rooms arranged around a central core incorporating a service duct, stairs to the first floor and a hallway to the front entry porch. Bathrooms are positioned over the kitchen to reduce the pipe run, and hence minimise material use. The front and back doors are protected by buffer spaces, a porch to the north and a two-storey double glazed conservatory to the south, with a balcony on the first floor between two bedrooms.
 Warm air is taken in from the conservatory air space through ground and ceiling level vents and/or French doors and circulated through the house, by convection, to the kitchen and upstairs bathrooms, where it is expelled through windows. Low-energy, high-efficiency gas-fired appliances are used for cooking and (in winter) pre-heating water for the washing machine and dishwasher, and heating three radiators in the north facing rooms for two hours a day. At ground level, a wood burning stove is the main source of heating. The walls, windows, floor and roof are well insulated for low heat loss. Triple-glazed windows are used throughout the house except in the sun spaces.
 The use of gas appliances removes a potentially large electricity burden that would normally be connected to the utility supply and creates an opportunity for biogas to be demonstrated eventually in place of the mains gas supply.
 The PV system is regarded as the main power supply and was designed to export surpluses to the utility, importing power only when it is unavoidable such as at night and in overcast conditions. To spread the electricity requirements of the house, careful timing of the use of the low-consumption appliances is essential.
 In summer, the house receives about four peak sun hours daily and energy surpluses are predicted to be around 12 kWh per day. In winter the house receives about 0.6 peak sun hours per day, however the house is expected to have an overall positive energy balance as the summer surplus is greater than the winter deficit.

The PV System

The PV system is connected in parallel to the electricity supply. Energy from the solar array will be consumed by the ac loads in the house with any excess being exported into the utility supply. Any shortfall in output from the array will be made up by importing from the supply connection. This is a fully automatic process, completely invisible to the householder.
 The PV modules are mounted between the skylights of the roof. The skylights open from the second floor room and provide good access for maintenance if required. Edge frames have been used and modules are carried on an aluminium substructure mounted to the roof. These frames are designed to be fastened by simple means to a standard roof structure.
 The modules in the array are wired together in three groups, connected in parallel to the inverter. The PV system working voltage was chosen to be nominally 333 V dc for good inverter efficiency. This can present a hazard to installation and maintenance personnel and therefore training is required for installers and users.
 The location for the inverter is a closed area on the second floor at the same level as the PV modules. It has a fire resistant lining, a smoke detector and is vented. This location ensures that module output cables are kept relatively short with protection devices in place. Standard wiring and wiring installation methods were used for the ac circuits in the house.

 

Three areas of monitoring are currently being undertaken:

  • electrical generation and performance of the PV system;
  • thermal comfort (humidity and temperature) of the house;
  • solar hot water system performance (cost effectiveness of different solar collector technologies in the UK climate).

Several environmental parameters are also monitored such as horizontal and roof irradiance, as well as outdoor ambient temperature.

Performance

The solar house has been operating very well after an initial adjustment period. During the first 9 months, the balance of exported/imported electricity gave 130 kWh surplus of exported electricity.
 The graph above shows monthly performance data from June 1995 to February 1996, obtained by reading the export and import electricity meters.
 The first results monitored at the solar house show that the PV technology is very robust. These results also verified that a high level of insulation can be achieved in a residential building without technological barriers and that the strategy of cutting the energy demands to a minimum as a requirement for using renewable energy sources is appropriate.

For more information contact the CADDET UK National Team in Oxfordshire.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.