Wind Turbines - Introduction
Alvesta Ltd manufactures and distributes probably the world’s most efficient design of wind turbine under licence. This is the vertical axis wind turbine (VAWT) which has many advantages over the more usual “propeller” type horizontal axis wind turbine (HAWT).
These vertical axis turbines are self starting, turning at low wind speeds, usually around 1.5 m/s and generating useable power at about 3.5 m/s. They take wind from any direction and work in confused air flows. This means for most sites they are able to cope with wind which has been affected by buildings or trees and still generate meaningful amounts of electricity. VAWTs go into a stall in winds of about 25 m/s but continue to rotate and produce power and they can withstand winds of up to 55 m/s. HAWTs have to furl their blades at high wind speeds and generate much less power at higher wind speeds.
The company is based in Devon, where we are fortunate to have high mean wind speeds and we provide a large amount of our own energy with a turbine – to see your mean wind speed and how much available wind energy you have, visit the Dti website or contact your local weather station. Generally speaking you will need to have a mean wind speed of above 4.3 m/s or at least 150 days per annum at 5.5 m/s or more to make a turbine a viable proposition.
The Dti wind database can be found at:
http://www.bwea.com/noabl/index.html
You can enter your post code and discover your grid reference which can then be entered into the website to show what mean wind speed the computer model would predict for that 1 km grid square.
It is important to note that the Dti figures are just an extrapolation of a computer model, apply to a 1km grid square and the actual wind speed on any site within the grid square can vary considerably and will be affected by the topography. Sites on open south west facing hills are likely to have a higher AMWS and sites on hills facing in a different direction or with buildings or tall trees to the southwest might have a lower AMWS. If you have any doubts we can run a data logging anemometer for a period to quantify it.
AMWS = Annual Mean Wind Speed
If the mean wind speed at your location is better than 5.5 m/s then either a 3 blade or 6 blade turbine wind turbine will almost certainly be the best option for you, depending upon whether you are seeking to sell power back to the grid. If the mean wind speed on your site is lower but above 4.3 m/s we would probably recommend a 6 blade turbine to make the most of the lower mean wind speeds.
Micro-generation of electricity through wind turbines is one of the most cost effective of the clean energies, although the cost effectiveness depends upon the windiness of your site and the method of connection.
Wind turbines produce variable amounts of electricity in terms of phase, amplitude and voltage. As such they can either be used for direct heating (resistive heating is very tolerant of voltage fluctuations); charging batteries or more commonly direct connection to the electricity grid via an inverter. The latter is a more expensive solution, the most preferable and the least tolerant of variances from the supplied grid. In this application the wind turbine is effectively using the grid as a battery – when the wind is blowing the wind turbine covers the electrical demand of the house and any excess is sold to the grid. Direct heating systems are also popular linked to storage heaters or UFH systems with an electric boiler or heating wires buried in the floor.
The table below shows some comparisons between the annual megawatt hours (mWh) produced by our VAWTs and some comparable HAWTs at various annual mean wind speeds (AMWS) in metres per second, this shows that at low wind speeds the HAWTs generate marginally more power but once the wind speeds pass 5 m/s the VAWT designs generate increasingly more power.
* kW (kiloWatt) = 1000 watts: One 100-watt light bulb on for an hour, is 0.1 kWh (100/1000)
AMWS m/s |
Alvesta |
Alvesta |
Alvesta |
SWWP |
Iskra |
Westwind 20kW |
|
|
|
|
|
|
|
4 |
1.6 |
2.6 |
7.9 |
3.2 |
5.0 |
17.0 |
5 |
5.3 |
8.7 |
26.2 |
5.2 |
8.7 |
30.0 |
6 |
13.2 |
21.9 |
65.7 |
8.0 |
13.1 |
46.0 |
7 |
22.9 |
37.9 |
114.9 |
10.8 |
17.1 |
60.0 |
8 |
33.6 |
55.5 |
168.1 |
|
20.2 |
71.0 |
9 |
43.5 |
71.8 |
217.5 |
|
22.2 |
78.0 |
We try to achieve a payback of 10 years or better for our customers in terms of the cost of the turbine against the value of the electricity generated and would not normally recommend installing a wind turbine on sites where the AMWS is less than 5 m/s.