Wind Energy
Harnessing wind as a renewable energy source involves converting the power within a moving air mass (wind) into rotating shaft power. Modern wind turbines contain rotors fitted with aerodynamic blades, which utilize lift forces caused by the wind on the blades to turn the shaft. The shaft rotates inside a generator (located behind the rotor and blades) which generates electricity.
How much energy can we get from small-scale wind?
Wind power is proportional to the cube of the wind’s speed, so relatively minor increases in speed result in large changes in potential output.
Wind speed also increases with height so it’s best to have the turbine high on a mast or tower. Generally speaking the ideal sitting is a smooth-top hill with a flat, clear exposure, free from excessive turbulence and obstructions such as large trees, houses or other buildings within a few hundred meters.
To assess the average wind speed at a particular site, a general indication can be established by using the wind map, available on the SEAI website
Wind speed can be measured by using an onsite anemometer, you can hire the mast and monitoring equipment, or consultants can do this for you. Several months of data at least are required to achieve an accurate figure – the longer the better.
Local vegetation can also provide an idea of the wind speed on your site. Below is the Griggs-Putman Index of Deformity which can be used as an indicator.
Griggs-Putman index of deformity
For every Kw installed , the average energy production on a good wind speed site with a good turbine is 1750KwH per annum so for 5 Kw turbine the production will be 8750KwH .
Factors that will change this production amount include
- Local tall vegetation
- Access to SW winds
- Height above sea levels
- Other obstacles
Size of turbines
Turbines vary in size. Typical sizes are 0.6kW – 8 kW and above, with a cut-in (starting) wind speed of around 2.5-4 m/s, and a rated (optimum) wind speed between 10 and 12 m/s.
Turbines are rated to a certain capacity. However, this output is only achieved for the short time that wind speed is at its optimum level. As a rough guide, a good wind site will produce an average output of 30% of the rated capacity of the turbine.
The size of the wind turbine determines the total amount of energy generated each year. For off-grid systems, the size of the battery bank determines the time appliances can be run if there is no wind. The size of the inverter installed determines the number of appliances that can be run at the same time from the stored electricity.
For example, a 6kW system can provide 12 000 – 15 000 units of electricity (kWh) per year depending on site and location. This system would reduce carbon dioxide emissions by between 5 and 6.6 tonnes per year.
Stand-alone or grid-connected system?
Small-scale wind power is particularly suitable for remote off-grid locations where conventional methods of supply are expensive or impractical. Most small wind turbines generate direct current (DC) electricity. Off-grid systems require battery storage and an inverter to convert DC electricity to AC (alternating current – mains electricity).
A controller is also necessary to divert power to another useful source (e.g. space and/or water heaters) when the battery is fully charged.
It’s common to combine this system with a diesel generator for use during periods of low wind speeds. A combined wind and diesel system gives greater efficiency and flexibility than a diesel only system. It allows the generator to be used at optimum load for short periods of time to charge batteries when there is little wind, rather than by constant use at varying loads.
Please note that currently it is not possible to sell electricity back to the Grid in Ireland.
Maintenance
Turbines can have a life of up to 20 years but require service checks every few years to ensure they work efficiently. For battery storage systems, typical battery life is around 6-10 years, depending on the type, so batteries may have to be replaced at some point in the system’s life.
