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Wind turbine selection - relating wind speed to power
Nova-wind's 6kw machine is one of a new breed of small wind turbines aimed at
the individual house, farm or small industrial/commercial application. Once you know your wind,
then you can compare wind turbines to see how individual machines stack up in your location.
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Feed in tariffs have made a lot of people, who are perhaps not being driven by
environmental motives to consider wind turbines. For those of us whose drive is sustainability,
then to generate your own power is one of the basic goals and FiTs has given us the opportunity
to make it affordable. For those who are looking at it purely as a financial investment, then it
is essential they understand how wind power is going to repay their investment. Get it
wrong and your money could end up being blown away with the wind.
So what do you need? First off, you need a windy site. I hear lots of individuals tell me they
have loads of wind where they live, but what can appear to be a windy site often turns out to be
disappointing when monitored. Wind turbines are not cheap and are a long term proposition - do
not even think about committing yourself without monitoring the wind at a height and position
where the turbine is intended to go, and you need to monitor the wind speed and direction for
preferably a year but at a very minimum for four to six months. There’s more information about
this topic on our website under the wind power heading but for this article I want to look at how to
analyse a wind turbine’s performance. Understanding performance is key to making the right choices.
I’ll try to keep this as simple as possible, after all, what we are trying to do is decide which
turbine will be the most cost effective for a particular site. Before we go on, I should say that
converting wind energy to electrical energy is a complicated business and you can easily get
bogged down with complex formulae and turbine efficiency ratings to try and achieve a realistic
estimate of annual power production. In this article I try to keep it simple and based on the
facts that are available. As mentioned in the previous paragraph, first off we need our wind
speed data. It is important to understand why we can’t just use the government’s average wind
speed data for the postcode or grid reference. The average wind speed for a location I am
currently monitoring is 6.5m/s (about 14mph) and that’s a pretty strong wind. Using the data for
the Evance 9000 wind turbine as an example, we can demonstrate just how inaccurate using a broad
average for the year can be. Consider the week’s wind data as shown in the table. From this we can
see that we had a quiet start to the week, then some strong winds then calm again at the end of
the week. With these figures the week’s average is 6.5m/s but using Evance’s power data and the
daily averages, we would have generated an average 2.18kw per hour during the week. If the wind
speed had been 6.5m/s consistently for the week, then the power generated would only have been
1.4kw per hour, that’s 40% less power yet both can claim a 6.5m/s average. This could be a
difference of 7,000 kw over the course of a year and in Fits’ terms, in the region of £2,000.
The table above shows the average daily wind speed over a week. The average speed
over the period is 6.5m/s but the power generated is around 40% higher than the manufacturers
figures for that wind speed. Knowing the wind at your location is essential.
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I hope that hasn’t been too confusing and serves to show just how inaccurate using an annual wind
speed average can be. That’s why it’s essential to monitor and use real data for your particular
site. That brings us to another important point. Often, monitoring will show the wind speed to be
quite different to the figure shown for a location on the government’s average wind speed website.
To use a real case, a man I know who had a wind turbine installed three years ago was told he
could anticipate generating between 2,500 and 5,000 kWh per year. The listed average wind speed
for the site is 5m/s. Reality has turned out to be a lot different with the turbine producing
1,100kWh during 2008 and only 1,000 during 2009. This is less than half the lower anticipated
figure and anyone looking for a return on their money with these figures would be disappointed.
This is money down the drain. I can’t stress too many times, if you fancy wind power, you need to
monitor your site at the height and location that the wind turbine will operate. Yes, it will add
a few hundred pounds to the bill but in the long run it could save you wasting thousands. Don’t
rely on the salesman’s ‘guaranteed’ returns.
Once the data is flowing from your anemometer, it’s time to start looking at the effectiveness of
various wind turbines that will suit your needs. To do this we need the turbine’s power data and
access to a spreadsheet. There are packages out there that can work the figures out for you if
you aren’t too good on a spreadsheet. I have my own Excel spreadsheet with the various calculations
in place. It’s very basic but does the job. With the spread sheet you can quickly relate your
wind speed to the power output for any particular wind turbine. I am currently monitoring a site
at 20 minute intervals which gives 72 average readings per day. By adding up the power figures
for those 72 intervals I get a pretty accurate figure of how much wind there is, and the amount
of power each wind generator would have produced on that particular day. The more days of data
you have and the shorter the time monitoring interval, the more accurately you can predict your
annual output. The site I am currently monitoring is showing an average wind speed of 5m/s from
40 days worth of data. This is around 3/4 of the 6.5m/s listed average for the site but it’s
early days yet. At 5m/s the Evance turbine would generate 576w every hour so over the 40 days at
that average it would have generated around 553kw. The actual wind speed data shows that just
over 1179kw would have been generated over the 40 days. That’s a big difference.
Now for the interesting bit. Seeing how different wind turbines stack up when it comes to
generating power. For this I am going to compare the performance curves of three similar sized
turbines. The graph shows wind speed against power output. By plotting the machines that you are
interested in against each other it is easy to see how each turbine performs at different speeds.
The smallest turbine on the graph is the 5kw Evance 9000, then there is the well known, 6kw Proven
11, and a new machine, the RLE nova-wind 6kw. The part of the graph that is interesting is the
range from 4 to 10 m/s. Under 4m/s, power is minimal and the bulk of winds we get in this country
are under 10m/s (22mph). The way wind turbines are designed, once they reach generation speed
they start to climb to their peak output with increased wind speed. The key to a turbine’s
effectiveness (in the winds generally found in the UK) is how quickly power generation ramps up).
You can see from the graph that the new nova-wind ramps up faster than the remaining two. What
this means is that it will generate more for a given speed than the others. The Evance is rated
at 5kw rather than 6kw so you would expect the nova-wind to produce more but if you look at the
curve for the Proven 6kw, the smaller machine is actually producing more power between 5 and 8m/s
and these are the more likely wind speeds to occur. When compared to the nova-wind, the graph
shows the Proven turbine lagging considerably until we get to around 10m/s. In fact at 7m/s you
will get over a kwh more from the nova-wind than the Proven according to these figures and that’s
significant when they are both rated at 6kw.
The graph shows the power curves for three comparable 5 - 6kw wind turbines. The speeds at
which a turbine provides it's power is critical if you want to maximise production.
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By looking at the data this way, you can easily see which machines offer the most promise. You can
then equate their power figures from your data and see exactly how much each machine would
generate and whether the sums add up. I must point out that both Evance and nova-wind’s data are
available from their websites but Proven do not give out the information. I have emailed to
request the information but to no avail. For their data I have estimated the figures from the
performance curve for the Proven 11 at
bettergeneration.com.
I cannot confirm the accuracy of the data but the site claims it is based on data supplied by the
manufacturer.
Naturally, this is only one aspect of the decision making process. Cost and reliability are also
key issues. After all, if a turbine breaks down then it will cost to get it repaired (once the
warranty period has expired) plus there will be no power generated during this period so downtime
is a critical issue. It is also worth remembering that there are significant differences in the
length of warranties and it always pays to read the small print and see what is covered or not as
the case may be. Some warranties are as short as a year, with a number at 3, and 5 now emerging
as the market norm. Some companies such C & F Green Energy even offer a 10 year warranty. With
this length of warranty you should see a payback before it runs out but don’t forget that the
design Life of 20-25 years for a wind turbine means there will be plenty of years when repairs
will cost. These costs along with servicing and maintenance can make a very large dent in your
return on investment and also extend the payback period considerably, even to the point of making
the whole project uneconomic.
Once you have narrowed your search down to a few machines that will (on paper) give you the
power you wish to generate for the price you wish to pay, then it is time to go an see the units
in action. Make sure you talk to existing customers as they are the best choice if you want a
genuine appraisal of the product. The topics of build quality, design, repairs and maintenance
costs will be investigated in future articles.
Contact point
For further information on the wind turbines mentioned in this article:
C & F Green Energy.
Evance Wind.
nova-wind.
Proven Energy.
Want to comment or have something to add regarding wind power?
Then email with your views. Shortly we shall be introducing a comment box directly under each article. Until then, please email and we shall add your comments.