Modern Wind Turbines, come in many heights and shapes. The smallest will work in wind speeds of 1.5m/s and the largest off-shore machines can work in gale force winds before they feather their blades and stop to prevent damaging themselves. Their efficiencies range from 15% for some of the early designs to 60% for some of the most recent. Whatever you need, you’ll be sure to find something for you.
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The first consideration to be made when considering wind power is how much space you have and how visible will it be to members of the public? There are two basic types of Turbine, the HAWT (Horizontal Axis Wind Turbine) that one normally associates with all wind farms and the normal farmstead wind turbine; and the VAWT (Vertical Axis Wind Turbine) that slightly resembles an upside-down mixer with slightly curved blades held in a vertical position. The latter type is usually lower and therefore less visible. VAWT turbines work better in ‘confused’ winds, but are only about 50% as efficient as the HAWT in comparable winds, mainly due to the greatly increased drag of the blades and the need to use power to get them started. They require either higher winds, an ancillary motor, or the use of a small Darrieus Rotor to overcome the inertia of the system and set the blades in motion. The link above gives a good account of each type of wind turbine. See also below - Variations on the Theme which describes the new wind lens - an additional bit of designing that doubles or even triples the output.
By using a modern Accumulator with your wind turbine system, this could provide all your own electricity. Or you could opt to have your Wind Turbine generator plumbed into the Grid, in which case your supply is taken directly from the turbine while it is generating and the extra is passed directly to the Grid. With this system an electricity supply company purchases your excess and the Grid supplies you with electricity in the normal way or at a reduced cost whenever your Wind Turbine is not generating, depending on your contract. Either way, in this country, you should be selling more than you can use.
For an HAWT with two or three blades radiating from the top of the tower/pole, you will need a piece of land that gets an uninterrupted wind flow. If you intend to rely on the prevailing wind it is likely to blow for only 2/3 of the available time, so the ability to auto-adjust to the direction of the wind is crucial. Wind that is ‘confused’ (blowing up or down on the blades) is a problem as it acts as a brake. This unfavourable wind is produced when a clear wind hits buildings and surrounding roof tops and trees, or even nearby hills and valleys. Horizontal axis wind turbines should never be sited closer than 10 times the height of the nearest building, hedge or woodland edge e.g. if the building is 20ft high, then the turbine should be at least 200ft away from the base of the building. Wide open, flat or gently undulating countryside with the prevailing wind blowing along a ridge is best.
For the VAWT, the wind can turn it from any direction - see references to Darrieus Rotors. The bottoms of the vertical blades are close to the ground, and it is normal to enclose the bottom with a stock fence for safety. This means that it can be sited in a garden/orchard etc. where the winds it uses will be at a lower speed than those utilized by the HAWTs. However the blades, require more wind to get them turning even with the smaller machines.
For the present, the best way of getting a good deal is to decide on the things you would like to do, know how many kWh that you use currently each year, and then approach the companies advertised to assess the best deal for your situation.
Be aware that wind turbines are rated according to the peak wattage that can be produced. This may be for winds of 8m/s, but it can be for 11m/s or even greater. If the latter, and your average windspeeds are only 8m/s, then you will never get the promised number of kW/MW over the year. This in turn means that your returns on income invested will be reduced and the time to payback will be similarly extended.Top of page
The initial costs of Wind Turbines being very high, the FiTs (Feed in Tariffs) are also quite high. They are all payable for 20 years and work out at 34.5p/kWh for a turbine rated at up to 1.5kW and 4.5p/KWh for the huge 1.5 - 5 MW rated machines. The lower rate for the most powerful machines reflects the fact that these are producing electricity on a commercial scale and as such their operation is a business in its own right and not simply an asset to a farm business or an home.
The electricity you produce will also be paid for by the importing company, and if you are a charity or community then there may be grants available to help you start-up.
The smallest HAWTs are ca 5kW; and an average domestic turbine rated at 11kW will stand ca 88ft. (26.8m) in height with 3 x ca. 2m long blades, and can supply 1,000 times its rating in kWh. The largest off-shore turbines currently stand over 100 m above ground level, have blades of 75m in length and ratings of 10MW, meaning that they can supply approx. 10,000MWh/yr (10GW - giga-Watts). It must be noted that this is the result of siting in ideal locations only.
In 2007 Germany’s 19,460 HAWTs were capable of generating 22.25GW and it can supply 7% of its electricity from Wind alone. Each year it is installing further Wind Turbine Capacity, and the knock-on effect of this is to reduce the cost of turbines, parts and installation for the homesteader, since the country as a whole imports in bulk. Currently most of the UKs imports are from Germany; and only if our demand increases sufficiently to import our own bulk supplies, will the costs come down.
Initial costs are changing all the time, but the following can be taken as a rough guide. Payback times with FiTs and payments from the Grid are in the region of 5 years. Overall Wind Turbines capture about 60% of the wind hitting the blade and due to the variability of the winds the final generation efficiency is 30% of the 60%. Therefore:-
- A Wind Turbine capable of generating 5kW/h will cost approx £25,000 +VAT and would generate approx. 7.884MW/year.
- The maximum microgeneration Wind Turbine capable of generating at 15kW/h will cost approx. £43,000 and would generate approx. 23.652MWh/year.
- The wind can blow for 24 hours, 365 days per year, unlike solar which is limited to daylight hours.
- The electricity from your WT enters your house by a single cable just like electricity from the Grid
- Small farm-based supply systems for the home only, may be suitable for servicing by the owner.
- VAWTs will work in locations where the wind is ’confused’ by nearby trees, or buildings. It could even be a feature of your garden.
- Depending on their height and visibility to the rest of the community, VAWTs may not require Planning Permission, meaning reduced expenses and time for completion of the installation.
- Winds tend to blow most in the winter, when electricity demand is highest.
- Both types of WT will require an annual Service and it is normal to set up a Servicing Contract on installation.
- The surrounding space must be large enough to lay the machine down for servicing; or the tower and nacelle need to be so big that a climber can service it from the top.
- The high torque generated by the VAWT means that it requires just as sturdy a base and is unsuitable for roofs; although there are a variety very small modified machines that are designed to produce small amounts of electricity from a city roof top.
- The initial outlay is high.
The variations on ways of abstracting energy from the wind are increasing constantly.
The newest variation seems to be the wind lens. A development that is well described in www.cleantechnica.com/2011/09/02/wind-lens-triples-turbine-output/ This could well not only increase the output per turbine, but also remove risks to birds and bats by making the top of the turbine more visible and preventing the formation of vortices at the tips of the blades.
The earliest design was the HAWT, being a long pole with an axis on the top that is parallel to the ground. This means that the blades, turn vertically to the ground. This design has stood the test of time, and is the one most often seen. The tips of the blades were, and sometimes still are pointed - a design that tends to create a vacuum that is known to kill bats as well as causing a drag that reduces the efficiency of the electrical generation. However, by bending the tip or adding a rod to it at right-angles to the line of the blade, the formation of a vacuum can be avoided, and the blade becomes Bat-Safe unless a bat actually flies into it.
The Slavonius Rotor is great for producing small amounts of electricity e.g. for opening an automatic gate etc. There are 3 basic designs with a minimum efficiency of 15%. The Slavonius Rotor is well described in www.angelfire.com/ak5/energy21/microsavonius.htm
The Darrieus Rotor could be considered to be the fore-runner of the VAWT and is well described in en.wikipedia.org/wiki/Darrieus_wind_turbine. They can and were used in stand-alone situations, but are now mainly famous as a small roof-top design or as ‘starters’ for the large VAWTs. As a ‘starter’ they are mounted on the top of the VAWTs shaft. They can be turned by a wind of only 1.5m/s and thus overcome the inertia of the VAWT allowing it to start to turn at low wind speeds. The VAWT is like a straight multi-bladed Darrieus Rotor and has become the turbine of choice for inland situations: where the winds are ‘confused’; where they are needed for domestic use rather than to supply the Grid; or where their low height allows them to blend in more easily with the landscape.Top of page
The highly visible, multi-sail Wind-mills of past centuries were used to capture the Potential Energy in the Wind and convert it into the Mechanical Energy needed to turn the heavy grind stones to mill grain into flour, or to turn gears and belts for small industry. However, these uses of the wind were limited to large flat eternally windy locations e.g. much of Holland where their picturesque presence on the land became an icon for the country - at least in the eyes of foreigners.
It was not until the discovery of Electricity and manufacture of Batteries, that some 'bright spark' realized that the same sort of structure could be used to generate Electrical Energy. This proved invaluable to all those out of reach of the nascent Mains Supply. In its simplest form, these very basic combinations of propellers on poles with a bit of wire etc. are still in use to power the now essential TVs for nomadic peoples. However, in the 1970s the need was seen to utilize the moving components of our Natural Environment in order to develop a Sustainable Energy Supply that did not involve polluting the surface of our planet; yet would be able to provide the huge amounts of electricity needed to power our modern lifestyles.
Wind Turbines currently come in an huge range of sizes and even essentially different shapes. However, all utilize the flow of air over a blade to create a turning effect; and all the modern ones, involve several levels of gearing in the Nacelle, before the generation of electricity begins. We have all seen very large Turbines in Wind Farms that seem to be scarcely turning, but inside they are rather like my lawn mower, with me pushing it slowly along, yet the blades turning so fast that I can not differentiate one blade from the next. The secret is in the gearing. The blades - usually 2 or 3 in the domestic sized machines and 3 or 4 in commercial ones to even out the turning forces - are attached to a large gear wheel with many teeth. This in turn makes contact with, and turns, a smaller wheel with teeth at the same spacing, with the result that the smaller wheel turns a lot faster than the big one. If the turning power available is great enough - and the bigger and heavier the blades are the more power they will input to the system - then the main shaft can turn a whole series of smaller graded gear wheels, spinning the smallest very fast indeed. The overall result is that these huge turbines, can turn very slowly and yet keep the coil spinning very fast within the magnet and thus generating lots of power even at low wind speeds. For the sailors amongst you a 2 on the Beaufort scale (leaves rustle) is equivalent to 2.5m/sec. and at 3 (a gentle breeze with leaves and small twigs moving) the average wind speed is 4.5m/sec. Most of the large Wind Farm sized Turbines start to generate electricity at a mere 3m/sec, though the smaller domestic ones may require up to 6m/sec. to start generation and need 8m/sec. to generate at their stated capacity. The wind speeds, necessary to provide the stated kW rating of your machine - MW (mega-Watts) are only applicable to wind farm sized turbines - will be given by the company supplying your installation. To do this they will measure the range of wind speeds available on your site over the course of a year. It is necessary to have this information on your wind speeds to get a good idea which size of turbine will be best for your site.Top of page
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- An article in the Summer 2011 issue of Practical Boat Owner magazine is also of interest for all off-grid systems. Whether they be on a boat or for a house, it is essential for you to have a Regulator. The regulator's job is to avoid the power output from the wind turbine from damaging the batteries by over charging, when the panels are charging under peak conditions.
- The following is a useful site for getting a quick and accurate idea of the average windspeeds in your area. it provides essential initial weather data for those thinking of going into either wind or solar energy. The author of the article (above) explains how to use the NASA website, but the url given does not seem to work reliably. After playing around with Google I have found that the following Link does work. science.larc.nasa.gov/research-AppliedSciences-SSE It opens on a page with a link to the SSE Website at the beginning of the second paragraph. This takes you to a page with "Meterology and Solar Energy" at the top of the left hand side. Click on this to take you to a page offering a search on either your desired map location or your Latitude & Longitude e.g. 50.6 and 3.3. You will be asked to register with your name and telephone number, but this is free. This takes you to a page with all the data you could possibly want with monthly averages over 22 years and the data for SSE HRZ is probably the best place to start. For those of you without a Practical Boat Owner, I will summarize some of the most relevant information in the Solar PV section.
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