Yes, we have all seen these quotes most often made by a Politician or wind farm developer, but what is the truth? Theses are simply statistical models and if the wind does not blow, like the last three days, the wind farm won’t even boil one kettle. In fact the wind farm may well be drawing power from the Grid to power it’s systems. Ever wonder why a turbine is turning even when the wind is not blowing? A mechanical necessity to prevent flatting. Points on the spindle become flat and causing severe vibration when the blades turn. The power to facilitate that comes from the grid and perversely the wind farm does not pay for it.
So back to the statitsics. A good friend and engineer, John Curtis, has provided the equation:
Reduced to its simplest form the calculation of how many houses can be powered by a wind turbine goes as follows:-
X = Number of houses
P = Rated power of turbine in kilowatts
E = Assumed availability per annum of power
Y = 8,760 = hours per annum of consumption
A = Assumed average power consumption of the house in kilowatts.
Note :- P. For turbines rated in Megawatts, multiply by 1,000.
A. This is the average electrical power consumption per household and is based on DECC figures of 4,700 kwh
E. This is the assumed percentage of rated power that the turbine will produce over the year and includes maximum and minimum figures averaged out. DECC use a figure of 0.3.
X = P x E x Y / A
For a 2-megawatt turbine we have
X = 2000 x 0.3 x 8760 / 4700
= 1,118.3 homes.
For simplicity, we can say that this calculation suggests that for every 1 megawatt of installed power we can expect to power 550 homes using average electricity. This says nothing about how many homes can be powered when there is no wind and does not say what happens when the weather is very cold and we need more power.
The 4,700 kilowatt-hours per annum is based on average consumption of an average house and assumes an average constant consumption of 536 watts. Actual instantaneous consumption varies greatly during the day when, for example one uses a kettle or an electric cooker.
The key figure is E because this reflects the actual annual power output of the turbine. In low wind areas this will be less than the assumed 0.3, whilst in high wind areas it may be more. For example, if we assume E as 0.25, the number of homes would be reduced to 930 homes.
OK, so that explains the techniccy bits but what does it mean in real life?
An example: taking the Galawhistle site, Infinis, on their website, are quoting capacity of 26.9%. They do actually have a rider at the bottom but how many ordinary people are going to be aware of that – virtually zilch.
* This calculation is based on the UK average capacity factor of 26.9% (taken from Table 7.4 in the Digest of UK Energy Statistics 2010, DUKES, 2010, for data in 2009) and an average UK household electricity consumption in 2009 of 4,423kWh (Digest of UK Energy Statistics 2010) which leads to a calculation of the equivalent electricity consumption of 29,000 households (rounded down to the nearest 500 households). It should be noted that the equivalent household electricity consumption calculated will vary depending on site wind speeds and actual household electricity consumption. Download the Digest of UK Energy Statistics 2010 – PDF 6.8MB.
Conclusion (1) I see that they are being a bit sneaky with the household consumption at 4,423 kWh in place of the 4.700kWh used by DECC. This has the effect of increasing the number of houses supplied. (2) The 30% capacity factor used by DECC is a bit high and the above figure of 26.9% is closer but includes on and off shore winds. A better figure for on shore alone would be between 20% and 25%. (3) they state “It should be noted that the equivalent household electricity consumption calculated will vary depending on site wind speeds and …”. This is crap because a house will use what it needs, irrespective of wind speeds. Now, if we take the 4,423 kWh and a 20% capacity figure then for each 1 megawatt installed capacity we would have 491 house holds and with 4,700k`kWh we would have 463 households.
Another example: The Thanet wind farm shows just how sneaky they can be. If we take availability as 0.25 – which is actual and based on their figures – and we use power consumption at 4,700 kWh as per DECC, then the homes powered are a maximum of 140,000, when the wind blows. i.e, about 100,000 less than they state as a top figure
What this means in layman’s terms is that if you multiply the installed capacity in megawatts by 463 you may be somewhere near reality. The wind farm figure will be 550 or more. There are so many variables here. An area with new energy efficient small houses will use far less than the average as will an area with a great number of solar panels. Old houses with poor insulation and council housing, such as the Highland area, with predominately electrical heating, will use far more. However this is of little interest to anyone as the figures are based onm averages. What we can say though is that the 30% of assumed availability is out by as much as 10% and that creates a vaste difference. Replace E at 0.3 by 0.2. Hope this explains some of the mystery and I welcome any commentators who can add to these comments.