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.

Sunday, 19 January 2014

Understanding the Weather and making Use of the Knowledge

Understanding the Weather and Making use of the Knowledge

Back in April 2013 I bought a weather station, only a cheap one, but it allows samples of wind, rain and temperature to be taken automatically and stored on a computer.

As of now, I have around 400,00 samples of the weather that have been taken every 10 minutes for the last 9 months.

The software that came with it only works on Windows and allows you to download the weather and look at the data and also plot graphs, how ever the software was so awful that the graphs are worthless. The software was little better than rubbish.

Being a computer programmer and all round nerd I decided to export the data to a Linux computer and write a program so that I could analyse it. Although the program isn't finished it does work and gives most of the information I want.

I was interested in knowing a few things such as is it worth having a wind turbine, how much water falls on our little field and what weather and when provides good growing conditions and whether these conditions would allow Soya and other plants to grow well. Also how much wood for my heating do I need to heat the house each month.

I've now been able to answer some of my questions and have the data and a method that will enable me to gleam far more information in the future.

One of the obvious things that I wanted to know was the temperature of our site.

Monthly Temperature (click for larger image)

And also the rainfall...

Monthly Rain Fall (Click for larger image)
The rainfall was interesting but also raised another question. How much water falls on the field because the field floods and I am trying to make a drainage system and although my efforts have mostly worked the field is now saturated and my drainage channels are full. My drainage system can't take much more water. If I knew how many gallons it takes to saturate the field I then can also know when it will flood and therefore how many gallons on top of this I need to shift.

The amount of water that falls on our plot was quite staggering and caused me to recheck the answer many times. In the end I know I am correct and the amount of water that falls is huge.

(These next lines are taken directly from my program. The overall plot is around 0.7 acres)

159.71 cubic meters  (159705 litres / 35130.3 Gallons)  of rain fell on the plot of land in December

30.54 cubic meters  (30531.9 litres / 6716.08 Gallons)  of rain fell on the house area (House stables yard garden) in December

129.18 cubic meters  (129174 litres / 28414.2 Gallons)  of rain fell on the field in December

35,000 Gallons of water! one month for the plot! 28,000 Gallons for the field. All this falling on a compacted clay soil, no wonder it floods. I haven't gone back through the figures to see how much it takes to saturate the ground before a flood but that is one of the next jobs. I only know roughly when the ground saturated but that will be good enough.

The next question was how windy is the site and where does the wind come from. The met office says that it comes from the South West where we live but the results show differently. What ever the met office say the fact is that they don't do their measurements from my exact location and hills houses and trees have an effect on strength and direction.

The data I had was for each 10 minutes and I had to produce a wind rose which involved grouping the data from 16 directions into 8 and working out what was the predominant direction for each day, then working out how many days of each direction we had.

(Below is a direct output from the program)

Wind Rose for December 2013
 N (average:  0 mph) - 0 days
 NE (average:  0 mph) - 0 days
 E (average:  0 mph) - 0 days
 SE (average:  9.18 mph) - 29 days
 S (average:  0 mph) - 0 days
 SW (average:  4.24 mph) - 2 days
 W (average:  0 mph) - 0 days
 NW (average:  0 mph) - 0 days

 It's important to know that an average of 0 mph doesn't mean that no wind came from that direction only that on no day was the predominant wind in that direction. What this shows is that my location has my house and stables on the North and North West side, a small hill (I mean small) in the West and East and a town (centre 1 mile away) in the South West. These landmarks help to funnel the wind from a mainly South or South West direction to a South East direction. The met office wind is measured 10 metres up where as mine is measured around 3.5 meters up. The lower to the ground you measure the lower the wind strength will be.

The difference of wind from 10 metres up to about 3.5 metres is around 18% which explains why my measured wind is less than what the met reported but also the land marks reduced it a bit. I don't want to measure the wind 10 metres up as my plants aren't up there. I measure at 3.5 metres because that is above my green house but also closer to where the plants are so the measurements are accurate although the plants will be hit with another 10 or 20% less wind.

The highest gusts were interesting. We had a storm where winds of 90 to 100 mph were seen around the South East but I measured:

December 2013 Maximum gust of wind was day: 5 at 42.56 mph (samples taken every 45 seconds)

The met office measures the average wind speed and my highest wind average was:

Windiest day was: 27 with an average speed of: 18.91 mph predominantly from a South Easterly direction.

On that day the met office were predicting an average wind speed of 30+ mph so we can see how sheltered my site is. Even adding 18% to take into account the height difference we only saw 22 mph winds..

All these figures effect, massively, how well or not a wind turbine would operate.

Wind Turbines
How much potential power there is in wind is fairly easy to work out. You need to take into account pressure, air density, size of a propeller and wind speed. The air density and pressure aspect can be simplified which helps a lot.

Most wind turbines have a cut in speed of  around 6 mph and stop producing when the speed reaches 25 mph so I calculated how much wind there was between 6 and 25 mph and fed this into the general formula for calculating wind power. Next I had to take into account that this is only theoretical and the real world designs of turbines only capture and convert 10 to 40% of this power. I used 20% as a realistic figure because the cheaper a turbine is the less efficient it will be, and if I were to buy one it would be on the cheaper side.

Calculating how much power I would have generated with a 2 metre Wind Turbine situated on a tower 10 metres above ground at a rate of 13 pence per Kilo Watt unit I came to:

May: £10.34
June: £4.27
July: £1.60
August: £2.53
September: £2.43
October: £11.49
November: £5.54
December: £39.63

You can see that each month varies a huge amount. My own monthly spend on electricity is £70 per month and therefore I would be well short with a turbine of this size.

I will leave to another post many more details and all thoughts regarding whether a wind turbine is worth having and the costs involved plus the actual calculations but for now what I have achieved is a proper data set, a few months short at present, but one that is capable of answering my questions.