Your choices so far:
1 Electricity; 2 Wind
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | 1 Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| 2 Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
The energy found in moving air is mainly exergy and since this is also the form of energy in electricity, wind shall be used to produce electricity and basically nothing else. The total wind power production in Europe is rapidly growing and forecasts say that by 2020 it will be about the same as the total hydropower production.
Wind power stations come in all sizes from only few kW up to 5 MW. The smallest units are suitable for installation in the single household. However, there are a number of restrictions that may apply and before a single-home installation is done, you are advised to take good care to see which restrictions and considerations apply in your own area.
Wind power stations come basically in two different models:
- Those with a vertical axis, like the egg-shaped Darreius wind turbine or the H-turbine and
- Those with a horizontal axis, mounted on a tower and with a one-to-four-bladed propeller
The latter type – horizontal axis turbines – is completely dominant today and the most common type will have three-bladed turbines since these show a good compromise between the total weight of the turbine and the total efficiency. The potential production increase when adding one more blade does usually not defend the weight increase and the subsequent expenditures for the tower and for the foundation.
The energy potential in the blowing wind is usually characterised in "W/m2 swept area", i.e. it is the power latent in the wind measured per square meter at a right angle to the direction of the wind. The energy potential is proportional to the wind speed raised to the power of three which means that the power produced in a wind-power mill is rapidly varying, which may cause problems for the receiving electricity grid. Data about the total wind power potential are supplied as "wind-maps" from the national meteorological offices.
All windpower installations suffer from the so-called "Betz limit" according to which the highest possible, theoretical efficiency is limited to about 60% (16/27 to be accurate). In modern installations the efficiency may reach more than 55%. There are a number of factors determining the total efficiency, among others the design of the blades, the control (pitch or stall) and the tip-speed ratio.
To achieve maximum annual production the wind speed at which production starts is one important factor and modern turbine blades tend to have a design where the inner part of the blade is wide so as to start the turbine already at relatively low winds. This is of course only applicable for larger turbines, 1-2 MW or bigger.
To achieve maximum efficiency modern turbines tend to have pitch control so that the blades can be rotated around their own axis. By this, the total force acting on each blade can be maximised and the rotational speed of the turbine adapted to the velocity of the approaching wind. Since a variable rotational speed would produce a varying electricity frequency, these turbines will typically have an AC-generator, a rectifier and an inverter installed. By this arrangement and by the use of multi-pole generators, the gearbox has been eliminated in many makes of wind turbines, prolonging life and reducing maintenance cost.
The combination of lower start-wind speeds and higher efficiencies has raised the capacity factor so that, today, one may assume the total, annual, production from a wind power generator on land to be about 30% of the annual potential as found on a wind-map. At sea, the capacity factor is even higher, exceeding 40 and sometimes approaching 50%.