How effective are wind turbines
The UK benefits from one of the most reliable electricity supplies in the world. This is founded on a diverse mix of different electricity generating technologies, of which onshore wind is a key component, and a sophisticated National Grid which controls both the amount of power generated and the distribution of that power.
In 2011 onshore wind generated 10,372 GWh of electricity, enough to power 2.4 million homes. This represented about 44% of UK renewable electricity production, and just under 4% of all electricity produced. By 2020 it is estimated that onshore wind could provide 24-32TWh electricity, enough to power up to 7.7 million homes.
Variability and Efficiency
Wind energy is a variable (or intermittent) source of energy but that does not mean it is inefficient. Wind turbines tend to generate electricity for around 80-85% of the time without any significant losses. In comparison thermal generation results in significant heat losses converting heat energy in the resource used into electricity output. In 2010 thermal efficiencies for coal and combined cycle gas turbine generation were 36.1% and 47.6% respectively.
However, whilst turbines are efficient, wind speeds vary which impacts how much power is produced and what is called the 'load factor' of a technology. The load factor is the accepted measure for the percentage of a theoretical maximum output of 24 hours a day, 365 days a year that a particular generating plant or technology achieves, with no technology achieving 100%. Over a 5 year period the average onshore wind load factor was 26.2% with offshore wind at 28.2%. For comparison in 2010 coal fired power stations averaged 40.9%, biomass 53.3%, nuclear 59.4% and Combine Cycle Gas Turbine 60.6%.
Impact on the Grid of Balancing Supply with Demand
Back-up capacity has always been an integral part of the national electricity supply. Energy demand during an ordinary day varies four-fold, and all power plants require down-time for maintenance and repair. To cope with changes in supply and demand the national system operates with spare capacity that can be used when needed, within which it is currently able to balance variation of output from different technologies including wind generation. In 2011,12, National Grid estimated an average of 4.9GW of reserve was needed in the UK, with only 0.3GW of that dedicated reserve for wind power. However this additional reserve displaced the output of existing generating stations in order to maintain the balance of supply and demand, so there was no net increase of power on the system at any one time (the exact level of reserve changes on a daily basis throughout the year).
Looking forward to 2020, Government is working with National Grid to manage the challenges of balancing as new generation such as wind power and nuclear comes on line. In June 2011 National Grid published a report: Operating the Electricity Transmission Network in 2020, that demonstrates how the system will be effectively balanced through: improved wind forecasting; demand side response measures; a step change in operational systems and a better understanding of generation embedded in the local network.
Beyond 2020 it is expected that demand side management, electricity storage and interconnection, as well as conventional generation, will play a greater role in maintaining the stability of the grid.
A higher share of intermittent renewable sources in the future is expected to require additional fossil fuel-fired generation capacity. The Committee on Climate Change: Renewable Energy Review 2011 shows that once the inefficiency of back-up capacity is taken into account, as well as other available measures to address intermittency, a system in which 30 to 64 per cent of electricity is generated by renewable sources can still emit less than 50g of CO2 per kWh in 2030. This is significantly less emissions-intensive than the most efficient gas combined-cycle turbines, which produce around 350g CO2 per KWh.
All information on this page is taken from the Department of Energy and Climate Change's website: http://www.decc.gov.uk/en/content/cms/meeting_energy/wind/onshore/questions/onshore_q2/onshore_q2.aspx