The generation of electricity (and income) from wind power is a confusing topic, laced with technical terms, esoteric units, preconceptions and the occasional outrageous claim. This section aims to explain the main sources of confusion and to provide a basic introduction to the topic, which we feel is essential to anyone planning to make a significant investment.
Wind speed
All measurements use standard SI units, so wind-speed is measured in metres per second (m/s) rather than knots or using the Beaufort scale. Conversion utilities are widely and freely available online.
For any given site, the wind will blow at different speeds at different times. Taken over a representative period (such as a year), the frequency with which the wind blows at different speeds can be used to derive an average. Sites are often referred to on this basis, so a 5m/s site is not very windy, whereas an 8m/s site is excellent. At the same site (or indeed at any site) the wind blows faster (in all conditions) as you go higher; this is due to the effect of surface roughness (from obstructions such as trees) slowing down the wind. This means that a site will have a range of average wind speeds, depending on height, so the taller the mast you put your wind turbine on, the better (at least as far as physics is concerned; planning may disagree).
To confuse matters a little further, the rate at which the average wind speed increases with height changes, depending on surface roughness, so for a nice smooth prairie, it’s not so important to have the highest tower possible but if your local topography is very confused, you definitely should.
NOABL
For the whole of the UK, a database exists which predicts (quite roughly) the average wind speed of each kilometre grid square (the NOABL database). This is a useful tool for making an initial assessment of the wind potential of a site but it is based on a computer model, not actual observations and with a resolution of 1km, it has to be treated with a degree of care.
The image below shows an extract from the NOABL database showing wind conditions at different heights for the kilometre grid square covering Ecodyn’s offices in Fife. The database gives average wind speed figures of 7.9, 8.5 and 8.9m/s at 10, 25 and 45m above ground level.
However, the grid square including our offices also includes Norman’s Law, which is the biggest hill for miles around, so, while the average wind speed for the square may be correct, the average wind speed for the office, which is sheltered in the lee of the hill, will be significantly less.
Weibull
The frequency distribution exhibited by wind speeds on any site follows a pattern similar to the “normal” distribution, or bell curve but skewed towards lower and away from higher wind speeds. This is known as the “Weibull” distribution (and an example is shown below for an 8.5m/s site):
If you know the average wind speed, you can use the Weibull distribution to derive the frequency distribution. Put another way, you can calculate how often the wind will blow at different speeds. Again, to add another layer of complication, the appropriate Weibull distribution also depends on surface roughness, so it varies from one site to another.
Power (response) curves
Every turbine generates more electricity as it gets windier but the shape of this power curve varies considerably from one turbine to the next. Some are designed to operate best in low winds and they may be too delicate to operate at all at higher speeds; some protect themselves by shutting down when it gets too windy, others modify the blade angle in various ways so that they can continue to generate in any conditions. The example below is from a Northern Power 100kW turbine:
People often get very hung up on “cut-in” speed and “cut-out” speed. These are the lower and upper speed limits at which any turbine will operate. Some turbine manufacturers make great play of the fact that their models never shut down, others insist that theirs will turn in the lightest of airs. It is worth considering such claims in the light of a couple of facts: (1) the energy available from the wind varies as the cube of the speed, so compared to a good blow, there is really nothing worth having at lower wind speeds, (2) if you look at the Weibull curve, you can quickly see that for most sites (and this is the curve for a very windy site) the wind practically never blows faster than 20 or 25m/s, so even though your turbine may be generating its maximum rated output in these conditions, they don’t happen very often.
Efficiency
Another frequent source of confusion is “efficiency,” which is often described as the percentage output that a turbine actually achieves, compared to what it could achieve if it generated at its peak rated output round the clock, 365 days a year. This is in no sense what efficiency means and, in the context of a wind turbine, is doubly meaningless.
The maximum rated output of a turbine (measured in kW) is a simple but misleading way of describing its “size.” All the rated output really tells you is the most that a particular turbine is capable of generating; it does not tell you how much it will generate. You could put a huge turbine in a poor site and it would generate less than a small turbine in a great site simply because there is not enough wind, or you could put a low wind turbine in a high wind site and it would not perform as you would hope, as it would be shut down to protect itself a lot of the time.
This use of the term efficiency is often used deliberately to mislead by opponents of wind turbines, who complain that the wind power industry makes inflated claims. A 2MW turbine is no such thing, they claim, as it will only generate 30% as much energy as a 2MW coal-fired power station. This is clearly and wilfully to miss the point. The true, engineering definition of efficiency is the ratio of energy out compared to energy in. Since the wind is free and has no detrimental environmental impacts, our only responsibility as far as efficiency is concerned is to harvest as much of it as possible.
Using the correct definition of the word, different models of turbine do vary in their efficiency; that is to say, the effectiveness with which they capture the wind and convert it into useful electricity. However, this is as much dependent on the site as the turbine, which is why it is so important to select the right turbine for your site.
Power Output
While the term efficiency, at least as commonly used, may be nonsense when used in the context of wind turbines, we still need a useful and valid way of comparing different turbines at different sites. The only robust way of doing this is to take the Weibull wind distribution and multiply it by the power curve; that way, you know (a) how often the wind will blow at each speed at your site and (b) how much power your chosen model of turbine will generate at a given wind speed; multiply them together (and take into account availability, see below) and you get the total output for that turbine at that site. An example of this approach (again for the Northwind N100 turbine) is shown below:
This illustrates what we mean by a good wind site (e.g. 8m/s) and a poor wind site (e.g. 5m/s). Although the difference may appear small, the annual energy output at the good site will be around 2.5 times as much.
Availability
The best wind turbine on the best site is no use if it isn’t working. Availability is the term used to describe, on average, what percentage of the time a turbine is operational. It doesn’t relate to the site at all, so if it isn’t windy enough to make the turbine work half the time, this is not included in the availability figure. Availability is the proportion of the time when a turbine could be working, that it actually does.
Most serious turbine designs have a documented availability across the fleet of somewhere in the region of 95%. Some manufacturers guarantee this, with various forms of compensation should your own turbine fail to live up to their claims.
Free Energy
Although there are variations in mechanical and electrical design, the basics of how a wind turbine works are the same for every model from every manufacturer. The basic principle of a turbine is to extract energy from the movement of the air; a certain mass of air travelling at a certain speed has a certain amount of energy and your job as a wind turbine designer is to capture as much of it as you can, so that it can be converted into useful electricity. The maximum amount that can be captured depends on the swept area of the turbine; this is the area described by the arc of the turbine blades. If you imagine a cylinder with its base described by the swept area and its length described by the distance the air moves in a certain time (say a second) then the total amount of energy available depends on the volume of this cylinder. The volume of the cylinder increases with an increase in the area of the swept area (or as the square of the blade length) and or an increase in the length (which is the distance the air travels in a second, so it varies with wind speed). Confusingly, the energy contained by moving air also increases with the square of the speed so overall, combined with the “increase in length” of the cylinder, the energy available from the wind increases as the cube of the wind speed.
Turbine designers can’t do a lot about wind speed (apart from using taller towers), so the best way to increase the performance of a turbine is to increase the swept area. When choosing your turbine, swept area is arguably more important than peak output. However, as the wind speed increases, the energy captured from the air by the blades soon approaches the maximum that the generator can handle, so you need a system that can reduce it. This can be done by various means, such as:
changing the pitch of the blades (e.g. Enercon, Vestas, C&F)
yawing the turbine head out of the wind (e.g. Hannevind, Vergnet, Fortis)
coning the disc (e.g. Proven)
deploying tip brakes (e.g. Gaia, AOC)
or by a combination of these methods. For most turbine designs, there comes a point when the risk of damaging the machine outweighs the benefit of generating electricity, at which point most designs put the brakes on and wait for the storm to pass.
For the real enthusiast, the maximum theoretical limit for generation from a wind turbine is described by Betz’ Law, which you’ll find on Wikipedia. Turbine nerds can compare how well their favourite designs fare against this ultimate benchmark. And if you are still with me at this point, the actual output of your turbine compared to the theoretical maximum determined by Betz’ Law is the best definition of efficiency.
Electricity
All of this only gets you as far as making the shaft of the turbine rotate; you still need to convert that rotational energy into electricity. There are two main ways of doing this (with several variations): permanent magnet (synchronous) and induction (asynchronous) generators.
Synchronous generators are arguably more efficient and tend to come on at lower wind speeds, however the electricity that they generate varies in voltage and frequency, so it cannot be connected to a domestic supply or exported to the grid until it has been put through a rectifier (which converts it to direct current, which has no frequency but still has variable voltage), then through an inverter (which converts the direct current to alternating current with a frequency that is matched and synchronised with the local grid, while also adjusting the voltage similarly to suit local conditions).
Asynchronous generators (which almost invariably operate on three-phase, see below) use power supplied from the mains to generate an electromagnetic field. They automatically generate voltage, phase and frequency-synchronised electricity, so do not need inverters but they must match the rotation of the turbine to the frequency of the grid. With a couple of exceptions, turbines with asynchronous generators therefore have a constant rotation speed, whereas those with synchronous generators can vary. (Those exceptions are the Windflow, which uses a variable slip gearbox and the Harbon, which uses a variable speed drive generator.)
Phases, frequency and voltage
The electricity which you use in your house is alternating current (AC) with a frequency of about 50Hz (Hertz). This means that the potential difference between the live and neutral wires varies from plus to minus (i.e. it reaches a maximum potential difference from live to neutral, then from neutral to live) fifty times a second. The term voltage is used to describe the scale of this potential difference. In plumbing terms, voltage is equivalent to the height, or head of water, whereas current, measured in amps would be equivalent to the flow. Pressure times flow gives us the energy in a water-based system; voltage times current in an electrical one: a volt times an amp equals a watt. Measuring the voltage in an AC system is a bit meaningless, as it shifts completely fifty times a second, however increasing the voltage does increase the power available; the calculation required to work this out is a bit too esoteric for our purposes here but if you are determined, it is the root mean squared (RMS) of the voltage.(http://en.wikipedia.org/wiki/Alternating_current#Mathematics_of_AC_voltages).)
This system is a single “phase.” That is to say, there is a single frequency, or sine wave of potential difference, between live and neutral. To get more power through the system, the next step up is “three phase” where there are three wires, with the same 230V RMS 50Hz voltage between each of them and neutral. Applying the same esoteric calculation that we glossed over before, the voltage of a three-phase system does not come to three times the voltage of a single phase but √3 times the phase to neutral voltage, giving a nominal three-phase voltage of about 415V.
As a rule of thumb, the District Network Operator (DNO) will let you put about 15kW (or sometimes up to 20) into a domestic single phase supply, so having a three-phase supply gives you the advantage of being able to have a 50kW turbine (or thereabouts) while your single-phase neighbour can only have 15kW. This can mean that it is cost-effective to pay the DNO to upgrade your connection from single-phase to three-phase, if you want a bigger turbine. Of course you can connect far bigger turbines than this but these require a special connection all of their own.
Transformers
As the peak power output of your dream turbine increases, the next bottleneck will likely be your transformer. This is the gray box, usually on two telegraph poles, which converts the local grid power (which is at 11,000 volts, or 11kV, also known as high voltage, or HV) to a more manageable 230 (or 415V, low voltage or LV). As your turbine generates LV power, the transformer also steps it up to HV as it is exported to the grid.
Your transformer has a maximum rated capacity, which may be something like 25, 50, 100 or even 200kVA. As you already know, a volt times an amp equals a watt so, ignoring reactive power for now (which again, you can find on Wikipedia), if you have a 100kVA transformer, there should be no reason why you can’t have a 100kW turbine feeding into it. The transformer (and the line connecting to it) are capable of delivering 100kW, so broadly speaking they are also capable of accepting it.
In practice, your DNO would be unlikely to allow a 100kW connection into a 100kVA transformer, as you have to have some slack (again, see reactive power in Wikipedia), so in this case, they would charge you to upgrade the transformer to the next size up.
Local (LV) voltage rise
If you connect a sizeable turbine into your mains distribution board and your house is connected to your transformer by hundreds of meters of tatty old cable, you will get an unacceptable voltage rise on your side of the transformer, as the resistance between the turbine and the transformer is too great. This would have the effect of blowing up your new plasma TV when the wind is blowing, which would be unfortunate and potentially dangerous, so in this case your DNO would insist on upgrading the line from your house to the transformer, even if the transformer itself doesn’t require upgrading.
DNO
Even if your LV cable is fine and you have a transformer with plenty of capacity, there is always the chance that your neighbour has already applied to the DNO to connect a wind turbine and they have secured all the spare capacity in the line. This is why it is crucial, once you have established what your transformer can accept and what size of turbine you want, that you put in an “application for embedded generation” immediately.
Depending on the size of the turbine, your DNO will then undertake a grid survey of sufficient detail to enable them to assess whether your turbine will cause them any difficulties. If so, they will usually offer you the option to upgrade a section of the local network. If you are putting up several hundred kilowatts of wind turbine, this may be cost-effective but for the usual medium-scale turbine, usually not.
As a very rough rule of thumb, you can estimate what capacity should be available to you (ignoring what your neighbours have already bagged) by taking the capacity of your primary station, halving it for the first kilometre you are away from it, halving it again for the next two, then again for the next four and so on. So if your primary station has a capacity of 5MW and you are 8km away, then in principle you should be able to get around 300kW in without having to pay for a major upgrade.
Units
Finally, perhaps the main source of confusion in wind turbines: the units. We said some pages back that everything would be in standard SI units (metres, kilograms etc.) however here we have to digress. The watt (W) is the standard unit of power, so the kilowatt (kW) is a thousand watts. In physics, terms like power, energy and work all have very specific meanings, so we have to be careful here. What we are after is energy, which is the application of power over time (hence kilowatt-hour, which is the quantity of energy you get if you apply a kilowatt for an hour). For reference, a kilowatt is roughly the power output of a single element of an old-fashioned electric bar fire; a modern kettle draws about three kilowatts, so if your kettle takes four minutes to boil, you just used about 0.2kWh. This is no longer an SI unit but it is very convenient: it’s a meaningful size and it is universally used as the “unit” of electricity on your bill.
When you start considering wind turbines, particularly the bigger ones, it can be easier to consider energy production in megawatt-hours (MWh, each of which is 1,000 kilowatt-hours) and you will usually consider it in the context of a common time baseline, like a year, so we end up with a measure of annual energy production in MWh/yr. The next step is to factor in how much it costs to install your turbine and how much you get for the energy you produce, which is where we get to the crucial consideration of £/MWh/yr and then £/£.
Feed-in Tariffs
The UK renewable energy market was transformed on 15th July 2009 with the implementation of the Feed-In Tariff (FIT) regime. This requires electricity suppliers to buy renewable energy at a fixed price, which is set for 20-25 years, depending on the technology involved. Once your installation has been commissioned, the price you receive for each unit of electricity you generate is fixed and index-linked.
These changes mean that it is easier to calculate with confidence the return on investment for a wind turbine installation project, using the known FIT values, the performance of each turbine model, and the altitude and average wind speed of a given site, all of which are readily available.
When FITs were first introduced, the payment levels set resulted in an explosion in the installation of solar photovoltaic panels; three times as many as had been predicted. FITs are not paid from Treasury funds, they come direct from the electricity bills of every consumer in the country. Nonetheless, the UK government was concerned that the explosion in PV installations would exhaust the budget that had been allocated, placing too high a burden on consumers and not achieving the planned reduction in greenhouse gas emissions. Emergency measures were therefore taken to reduce the FITs level for PV. These were not handled particularly well, resulting in a series of legal challenges and appeals, and further uncertainty but whatever the final outcome, from 3rd March 2012 the tariff for PV will be reduced to 21p, bringing payback more into line with other technologies.
On 9th March 2012, a consultation document was issued by DECC, setting out proposals for changes to the FITs regime for everything else. This document can be found at http://bit.ly/w1rayy but the main points are summarised below:
What’s in it
| kW capacity | Current tariff (p/kWh) | Proposed tariff (p/kWh) |
| >1.5-15 | 28.0 | 21.0 |
| >15-100 | 25.3 | 21.0 |
| >100-500 | 19.7 | 17.5 |
| >500-1500 | 9.9 | 9.5 |
| >1500-5000 | 4.7 | 4.5 |
Implementation date for new rates 1/10/2012.
Automatic annual price reduction (of at least 5% from April 2014)
Tariffs for 1.5kW to 1.5MW wind installations set to provide 8% return @ 6m/s
Automatic reduction in rates triggered when each technology achieves a certain total installed capacity.
Proposed capacity triggers for wind are 111MW for 2014 and 137MW for 2015. (Total wind capacity installed pre-FITs was 5MW, FITs Yr1 14.2MW, FITs Yr2 to Q3 15.7MW.)
Preliminary accreditation process for wind projects over 50kW to provide comfort to investors (although for the purposes of automatic capacity-based degression, these installations count as deployed on accreditation).
Preferential rates and treatment for community projects.
What’s not
No mention of any change to capacity bands.
No change to the ability to restrict turbine output (e.g. install a 900kW Enercon E-44, restricted to 500kW).
The structure of the FITs system generates some perverse incentives. For example, the banding means that there is no point financially in developing an installation with a total capacity of, say, 600kW. Assuming similar (pro-rata) performance for two installations, one with a total capacity of 450kW and one 600kW, you would expect the larger installation to generate 1/3 more electricity; however, because it would fall into the 500-1500kW tariff band, you would receive only half as much for each kWh generated, so income would be around (1/2 X 4/3 =) 2/3 as much as the smaller installation.
Ecodyn has developed a system which uses these data to calculate the return on investment for any of the turbines in our portfolio and for any location in the UK. Demand from some of our larger commercial and community clients has extended this portfolio to include the Enercon turbine range which extends up to 2.3MW, so that we now have detailed performance and cost data for effectively every turbine available. This facility allows us to provide very accurate and comprehensive economic data for prospective turbine sites.
To qualify for FITs, any installation must be installed by a Microgeneration Certification Scheme registered installer and the turbine itself must be certified either under MCS or, for turbines over 50kW, under ROO-FIT.
There are a lot of cowboy operators out there who are happy to recommend the wrong turbine in the wrong place if they think they are going to make a sale. We have been around for twelve years and still have repeat business from some of our first customers. We take great pride in our integrity and we will always give you the best advice we can, even if it means losing business. We would rather not have a customer than have an unsatisfied customer.
And if your site is not suitable for a turbine, you can still benefit from investing in renewable energy elsewhere; just ask us how.
