Wind Power

Wind Power

Key Facts

On a global scale, it is estimated that the total recoverable wind resource is 8 times greater current World energy consumption.
In the UK, up to 25% of energy could be supplied by wind; beyond this point the national grid would become unstable.
Average wind speed is very important as the power from wind is the cube of the speed.
Small scale wind is excellent for remote area supplies. Intermittent supplies require batteries for electricity storage. Easily combined with small scale solar PV for reliable standalone hybrid systems. (1)
Use the Tip Speed Ratio (TSR) for comparing performance, and solidity for matching to chosen application. See below for explanations!
Best indicator for showing whether turbine will work in a specific area is to match the turbine's power curve (expected output at varying wind speeds) against anticipated wind speed at sight.
Benefits include reduced CO2, NOx and SOx. Energy payback can be in as little as 3-6 months.

Wind farms, both onshore and offshore (up to 10km from shoreline) have been developed. In the UK, around 1400MW is currently installed, with another 500MW of output planned, or in development (2). Growth is restricted by planning risks and a world wide shortage of manufacturing facilities.

The European capacity is 48,000MW with planned developments doubling that amount. (3)

A single wind turbines has a very small physical footprint. The largest of which can produce 5MW.

Power Law gives theoretical maximum for output from wind turbine.

A = area, i.e. area swept by the blades, which is p ∗ r²
v = velocity, i.e. wind speed
ρ = air density, which is generally constant.
P (watts) = 1/2 A ∗ ρ ∗ v³

Therefore, larger blades result in more usable energy, and higher wind speeds result in much more usable energy.

Nearly all modern wind turbines turn on a horizontal axis (HAWT). Experiments have taken place with vertical axis wind turbines, but these have failed to scale to useful sizes. (4)

Tip Speed Ratio (l) = Blade Tip Speed / Windspeed. Although fewer blades are better, two blades tend to self-destruct the machine, so a minimum of three is now the norm.

Solidity = Total Blade Area / Swept Area. High solidity machines have a low TSR and high torque. Applications such as water pumps benefit from this arrangement. Low solidity machines have a high TSR and low torque and are suited to electricity generation. The shape of the blades is critical.

Site assessments are vital for correct installation. Anemometers provide spot measurements, but are of little use. Isovent maps show general average speed rates (5), so really require a simple wind logger which takes a measurement every 10 seconds. Ideally, require speeds for a whole year. The DTI has made available a wind speed database for the UK (6). The results should show a skewed, normal distribution. It is also critical to identify any local obstructions, as turbulence caused by houses, trees, etc. can have a major impact on the viability of a site. The turbulence zone is 2 times height of object, the same in front and side, and twenty times height downstream. Other impacts to consider are noise, visual and birds (avoid migratory routes).

Wind has good DIY potential for small-scale (7)(8), or medium-scale for community projects (9). They can also be a key component of standalone hybrid systems (10). The well-named Proven (11) range of small wind turbines have an impressive track record.

Further Information