Thursday, 30 January 2014

Wind Turbines and Green Energy

Wind Turbines and Green Energy

Much has been written about Green Energy, in fact it has become very political and we are all paying a Green Tax on our electricity bill. Bills keep rising despite the fact we have energy saving light bulbs and many of us are trying to use less power. To me, the reason our bills don't fall, is because the less we use the more the big energy companies have to put up their prices. They simply don't want to make less profit. The bills also rise because we pay a new Green Tax. Add onto that the fact that all big businesses seem to turn in bigger profits during a lean time for consumers it seems only sensible if there is an alternative.

Is it worth jumping onto the band wagon and generating your own electricity?

With the number of Wind Turbines on the market it may seem like a no brainer but delve a bit deeper and the picture looks totally different - at least financially. 

Ideally I'd like to make the decision to generate green power but reality has to come into effect and that reality is money.

How much power is in the wind?

With regard to a rotary blade turbine working out how much power is in the wind is actually very easy:

Power in Watts in the wind = air density * Swept area of blades * Wind Speed in Metres per second cubed

This can be simplified by using a constant to replace air density which relates to pressure, altitude and temperature. The figure that is used for this constant is 0.625. The swept area of the blades is a simple calculation using PI and the radius of the blades which gives us PI X radius squared.

The formula used is therefore:

Watts = 0.625 * (PI * (R * R)) * (Speed * Speed * Speed) where radius is in metres and speed in metres per second.

If we had a 2 metre wind turbine (the blades spam 2 metres) and the wind was blowing at 10 metres per second the maximum power we could in theory extract from the wind would be:

Watts = 0.625 * (3.141 * (1 * 1)) * (10 * 10 * 10)
Watts = 0.625 * (3.141) * (1000)
Watts = 1963.125

That's almost 2KW and if the wind blew like this for 1 hour we would generate 2KWH and at an electricity price of around 13p per Kilo Watt Hour (KWH) we would have generated 26p over that hour.

That's the theory.

In practice a Mr Betz worked out that you can't extract all of this power. In fact he deduced that the maximum power that can be extracted is 59% of this theoretical power if a perfect turbine blade was made. Obviously nothing is perfect and in reality when you take losses from cables, inverters, magnets, power curves etc etc the normal amount of power that is extracted by a wind turbine is reduced to between 10 and 40%, with 40% ones being more one off turbines built in the lab or very very expensive ones. A good figure to expect to achieve for standard wind turbines would be closer to 20% and this is the value I will use from now on.

20% of the 1963 watts is 392 watts (0.392KW). If you generated this for an hour you could save yourself  5.2 pence.

Theory and reality are quite a bit different.

It helps to get a handle on things

It would help to understand how much power this really is and what it can do. A typical Microwave oven is between 650 and 850 Watts. So a wind speed of 10 m/s hitting a wind turbine that is 2m in diameter running for 1 hour would not be enough to run your Microwave.

It would also help to understand what effect different wind speeds make on the possible power you can generate. If you halve the wind speed to 5m/s the power drops...

Watts = 0.625 * (3.141 * (1 * 1)) * (5 * 5 * 5)
Watts = 0.625 * (3.141) * (125)
Watts = 245.39

20% of 245.39 = 49 watts

By halving the wind speed your power has dropped to an eighth. That is because the formula uses the cube of the speed. The cube of 10m/s is 1000, the cube of 5 is 125.

You can see that changing the wind speed by just a little will drastically effect the amount of power you can generate. 49 watts wouldn't even be enough to run an old style light bulb in your living room (which was normally a 60 watt bulb). Modern energy saving bulbs will be between 8 and 12 watts so you could possibly run 5 new style bulbs with this power.

Even if you doubled the size of your turbine to 4 metres you would quadruple the amount of power. Double the wind speed and the power goes up by 8 times.

When deciding on whether you should get a wind turbine the most important thing to know is have you got enough wind. There is no point thinking wind doesn't matter because I'll just put up a bigger turbine. If you thought like that you'd end up with a huge turbine, spending huge amounts of money which simply wouldn't be worth the cost.

Wind Speeds

It's one thing to find out that the Met Office say your area has an average speed of 5 mph but your actual site will be different as my wind data in my previous post (Understanding weather and making use of the Knowledge) demonstrated to me. If your own site wind speed is only slightly different you'll generate either a lot less power, or possibly a lot more. Sod's law say a lot less.

Doing a real site survey is a must because trees, hills, buildings and height make a big difference. 

In the past people have put up wind turbines on their house roof based upon the area's average wind speed expecting to save £250 per year on their electricity bill. The install costs came in at around £2,000 and they thought they'd have an 8 year pay back on their investment. The reality turned out that often they saved £27 per year giving a pay back of 74 years. That's a big difference and a big outlay for almost no gain.

It's important to understand where they went wrong. Firstly you can't rely on average wind speed figures, secondly the Met Office figures are normally measured from 10 metres up which is more than many homes and they measure the wind on a site that is often flat with no obstructions any where near. 

Wind speed reduces as you go down in height, it also gets greatly effected by buildings, even if the turbine is above your roof. Your roof will produce turbulence.

Power Curves

Wind turbines have what is called power curves. What this means is that when they state that they produce x number of watts at a certain wind speed, thereby giving their efficiency, it doesn't follow that it will be as efficient at different speeds. In fact they will definitely be less efficient at different speeds so you need to choose a turbine that has a power curve that suits your wind speed spreads. Ideally you would pick a turbine that is at it's most efficient at your known average speed (taken from your site survey) and one that can handle your less often high wind speeds.

Cut Ins and Cut Outs

All turbines have what is called a wind cut in speed. Below this speed the turbine will not generate any power. If your average wind speed doesn't reach this cut in speed then there is no point getting the turbine. Each turbine can have a different cut in speed.

Many wind turbines have a cut in speed of around 3 metres per second. This is often accompanied by words such as "low cut in speed" to describe the turbine. 3 as a number does sound low but if you convert it to miles per hour (multiply by 2.24) you get 6.72 mph.

Cut out speeds are where a wind turbine will cease to work. It may even put the breaks on. Most cut out speeds are in the range of 18 to 25 metres per second but since our average wind is way below these cut out speeds they can be largely ignored unless you're in a particular windy part of the country or on top of a mountain.


The reality is that the faster the wind and the bigger the turbine the more power you can produce but to get the most out of the wind means installing a big heavy lump of a turbine as high as possible. The turbine wants to avoid obstacles since they get in the way of the wind and stand on a tower. The problem with putting heavy objects on top of a tower or pole is that the wind wants to blow them over and providing strength to the tower means big and heavy duty or guide lines both of which cost a lot of money and take up a lot of room.

A wind turbine is mechanical, mechanical things wear out and go wrong from time to time and need maintenance. The turbine is connected via cables to an inverter (an electronic gadget that makes the wind power usable). We all know that electronic things go wrong, how many of your other electronic gadgets have lasted 20 years with out failing.

I have calculated what different size turbines could produce at my location if they were installed at a height of 10 Metres with electricity prices at 13p per KWH and overall system efficiency of 20%.

Based upon my windiest month, December, power for the whole month was:

1 metre turbine = 76.232 KWHs of power (£9.92)
2 metre turbine = 304.928 KWHs of power (£39.65)
3 metre turbine  = 686.088 KWHs of power (£89.2)
4 metre turbine = 1219.71 KWHs of power (£158.57)
5 metre turbine = 1905.8 KWHs of power (£247.76)

Based upon my least windy month, July, power for the whole month was:

1 metre turbine = 3.0695 KWHs of power (£0.4)
2 metre turbine = 12.278 KWHs of power (£1.6)
3 metre turbine = 27.6255 KWHs of power (£3.6)
4 metre turbine = 49.112 KWHs of power (£6.39)
5 metre turbine = 76.7375 KWHs of power (£9.98)

The difference between a windy month and a hot summer month with little wind is around x24. You can generate 24 times the amount of power from a poor month compared to a good month.

Many wind turbines are at their most efficient when the wind speed is at 12 metres per second (26 mph). That is when they produce their rated output (ie, their headline figure). My location, over the last 10 months, has not had a single day where the average wind has reached 26mph. The best seen was approx 22mph.

The average speed of the windiest month, taken as an average for all hours of that month, came in at 9.2 mph.

Any turbine that I installed would hardly ever (apart from a few minutes, the odd hour) deliver it's stated power and in reality I would only ever achieve 20% or so of what the turbine could possible deliver.

Ignore the sales hype, if you want to install a wind turbine get a proper site survey done, find out your average wind speed and get a realistic idea of what you can achieve.

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