These allow a wind turbine or photovoltaic array to be used in synchronised connection with a grid supply. No battery store is required. Power generated is first used by the house loads and any surplus exported to the grid. In times of low wind, at night or when power consumption exceeds power generated, power is imported from the grid as normal. All power flows are ‘seamless’ from the point of view of the user.
Over a period it is possible for a property to achieve numerical electricity autonomy if energy generated equals that which is consumed. In this situation the grid effectively acts as an ‘energy store’ instead of a site battery system, avoiding the problems and expense of managing and replacing batteries. A downside is that the renewables system does not act as a back-up power supply in the event of power cuts. In many cases this may not be considered a big issue.
The photovoltaic array or wind turbine are connected to the grid supply by means of inverters which automatically adjust PV array or wind turbine loading to provide maximum efficiency and energy production.
Your Own Power Station
Instead of simply being consumers of electricity, ordinary domestic properties, offices and workshops can now generate their own power on-site to reduce both their electricity bills and their impact on the environment.
The sun is the source of all renewable energy and can be used directly as heat or electricity; or indirectly as biomass, hydro-electricity or wind energy.
By harnessing renewable energy sources other resources can be conserved. The combustion of fossil fuels with it’s release of carbon dioxide and danger of climate change can be reduced; and the risks of nuclear energy avoided.
Sufficient renewable resources must be available at a given site to make the use of wind or solar systems feasible. Once installed a wind or solar system will supply electricity cleanly and quietly; year in; year out; - providing an investment in the future.
Operation is fully automatic - when the sun comes out or wind blows they produce power. When the sun goes down, wind stops or there is a power cut the systems shut off.
System operation can be kept track of by periodically reading energy output meters. Various options exist for these including displays which show instantaneous output and record daily production.
Turning the Meter Backwards
Power generated is first used within the property to reduce electricity consumption and any surplus is exported to the grid.
In addition to the normal Import kWh meter which measures electricity consumed an Export kWh meter may be required to measure the surplus and a Generation kWh meter enables electricity generated to be measured.
Often the lower price paid for electricity sold compared to electricity bought makes it advantageous to make best use of all power generated, for example by using electricity during periods of high generation.
Connection of a Wind/PV electrical system to the grid must have local electricity company approval and approved equipment, installation methods and protection must be used.
Note that if the grid supply fails, to protect linesmen working on the grid the system will shut down, so these cannot be regarded as a standby source of electricity.
Solar systems generate electricity in proportion to the amount of sunlight received, so unshaded, preferably south facing areas are best the roofs of many houses are ideal! As a guide, 1kWp of well sited grid connected PV array in the UK will produce approx. 700-800 kWh of electricity per year and can be achieved using an array area of about 8 square metres.
A 3.5kWp system covering 20 sq. m can supply approximately 2800 kWh of electricity - more than half of the average annual electricity consumption for a household of four.
All generation is during hours of daylight and most in summer, least in winter. Best use is obtained when electricity consumption runs in parallel to electricity generation. Eg. Appliances such as washing machines or dishwashers, which do not have to operate at a certain time can be operated when the solar system is supplying plenty of power.
For properties with good wind sites (See: Siting A Wind Turbine) these systems are the ideal way to reduce electricity demand. Maximum generation is usually over winter months when demand is highest and power can be produced 24hrs per day. For example, a 2.5kW Wind turbine can produce between 2,500 - 5,000 kWh of electricity on an ideal site with 5 m/s average annual wind speed.
In contrast to solar electric systems, heating can be an appropriate load for the larger grid-connected wind systems.