The Skinny on Small Wind

Summary: Wind is volatile. The conditions for effective wind harvesting greatly favor rural areas. Turbulence can be a problem for building-mounted turbines. Height above the ground makes a big difference. Feed-in Tariffs keep small wind economically viable in the UK. A properly-placed turbine can pay for itself after a few years.

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You can see wind’s effects on the environment, sure, but have you ever looked for the environment’s effects on wind?
Consider a house that stands 5 meters tall. Wind doesn’t simply hop over this house and keep going. No, the wind throws a fit of turbulence. Turbulence will be found on the upwind side of the building, directly above it, on either side of it, and downwind of the building as well — as far as 100 meters downwind. Give this building a bunch of neighbors, and the best breezes bypass the whole lot. Wind may be powerful, but it’s extremely volatile as well.
For the building-mounted turbine, then, turbulence poses a fundamental dilemma: the house on which it’s mounted reduces its effectiveness.

 wind flow around buildings

The warmer the colour and longer the arrow, the greater the wind speed. The ‘isolated’ case is equivalent to a rural setting in context of this report, while the ‘Urban’ case depicts a building in an urban environment with other buildings nearby on either side (not shown). Source: Centre for Renewable Energy System Technology (CREST) at Loughborough University. As seen in Small Scale Wind Energy: Policy Insights and Practical Guidance. The Carbon Trust, 2008.

Under the right circumstances, it is feasible that a building-mounted turbine could be economical. Turbines mounted on high rises have performed well, for example. And, of course, it’s less expensive to mount a turbine onto something that’s already there. Some houses do have exceptionally good wind exposure. But if we’re going to generalize, the best place for a small turbine is usually right where you’d expect: pole-mounted, in a clearing, atop a small hill, with the sheep grazing around it.

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orography and the physics behind wind

To conceptualize, rid yourself of the notion that wind is a linear force that sweeps evenly across space. Remember that wind is essentially the equalization of the air’s heat across zones of variable temperature and pressure. Everything this air touches has a shape and a temperature of its own, and these properties are in turn transferred to the wind, affecting how it continues to flow.
Place your hand near the ground and you’ll feel that even the flattest surface turns a breeze into still air as the wind moves across it. This is called drag. In order to escape this dead air, you have to go up. As elevation increases, wind speeds increase drastically for the first few meters of elevation, then taper off. This logarithmic increase in wind speed is known as vertical wind-shear.
Herein lies the problem with turbines in urban settings. When wind encounters a cluster of houses, the shear of the wind is displaced upwards, because to the wind, the rooftops present a new ground level. In other words, a turbine mounted 2 meters above a rooftop in a subdivision will be about as effective as a turbine mounted 2 meters above the ground in a field—something you don’t see very much. Raising this turbine from 2 to 9 meters above the rooftop — where it can access strong, consistent winds — is likely to triple its yield at leasti, although it is likely to compromise the structure of the house as well. Again, building-mounted turbines prove tricky.



 
But is it really worth the trouble to hoist your turbine way up into the air for a few extra meters-per-second of wind speed? Yes, in fact: as wind speed increases, the energy within the wind accessible to the turbine increases cubically. Put crudely: double the wind speed means 8 times the power. This is the physical formula, anyway; in reality, other factors like cut-in_and_cut-out_speeds enter the calculus,  so a turbine’s power approximates this cubic relationship only across a practical range of wind speeds. This range will be evident in your model’s power_curve.
The point remains: the extra cost incurred to mount your turbine higher will prove to have been well worth it.

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finding a site

Okay, so you’ve got a potential site for a turbine. You know that the wind at your proposed hub height is turbulence-free because you put a ribbon on the end of a pole and observed a constant stream rather than a squiggly, erratic dance. The next step is to get a definitive read on the site’s wind speed.
The NOABL and NCIC databases might give you an idea, but these data can’t account for extremely local topography, not to mention obstructions. You’re much better off running an anemometer for at least 3 months, better yet for a full year. In addition to average wind speed, the anemometer reading should give you wind speed distribution data — how often the wind is blowing at what speeds — which will allow you to make a much more accurate prediction of your turbine’s yield. The math gets complicated, so suffice it to say that you’re looking for an average wind speed of at least 5 m/s. How high do you have to go to get the wind speed you want? If it’s over 11.1 meters, you’ll have to apply for planning permission [for stand-alone turbines; 15 meters for building-mounted turbines]. If not, and if your proposed turbine meets all the criteria, then the government considers it permitted development, and no permission is required before breaking ground. Notable criteria: the turbine and the installer are MCS-certified, the swept_area is less than 3.8 m², the site will have only one turbine….
Still standing? Then let’s get to it.

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go time

In addition to a turbine, you’ll need to budget for an inverter, cables and switches, the cost of installation, permitting if you need it, and a grid connection if you desire. [Nota bene: if you’re going off-grid, a turbine complements solar panels well, as the two usually prefer opposite weather conditions. Make sure to get a charge controller for your batteries.] When it’s all said and done, it’s realistic for the turbine to have cost 1/3 to ½ of the entire system.
One more thing: maintenance. It will behoove you to have your turbine checked out once a year. Costs can range from tens to hundreds of pounds—lots of factors determine wear. A properly-maintained turbine will last over twenty years… and pay for itself well beforehand.

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revenue

Personal energy savings should be the driving financial factor behind all microgeneration installations. To sweeten the deal, though, the UK maintains a robust, multi-tiered feed-in tariff (FiT) scheme.
One part of the FiT is the Generation Tariff, by which you’re paid a set rate for each kilowatt-hour your turbine generates, period — even if you’re off the grid. Your rate depends on the capacity of your turbine, but this tariff is designed to give you an annual rate of return of between 5 and 8% ii. Although the rates for new installations are falling each year, your rate is fixed once your turbine is commissioned, and adjusted for inflation to boot.
Now, if you’re grid-connected, you can also sell your unused energy to receive an additional Export Tariff. That is, if your turbine is generating more electricity than you’re using at the moment, the difference is exported automatically into the grid. Beginning December 1, 2012, the export tariff rate for all eligible installations is 4.5 p/kWh—up from 3.2 for the previous two years. [FYI: instead of opting for the government’s guaranteed export tariff rate, you can enter a “Power Purchase Agreement (a legal contract between you and the supplier),” and sell your kilowatt-hours directly to your supplier instead.ii It might be worth asking about.]
Your MCS-certified installer should take care of all the paperwork, including giving the DNO all the proper notifications. Note: if you’re on a private network—connected to the DNO’s network through a hospital or an airport or the like—then you’ll need to consult with the network owners about their connection policies. You’re still eligible for the Generation Tariff.

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a sample bottom line

Now for the good part. Note how much wind speed makes a difference!
Let’s say you use a robust 5,000 kWh each year. Your supplier provides electricity at 14p/kWh, so your electricity bills come out to £700 annually… until, that is, you felt the winds of change, you installed some anemometers, ran them for a few months, reviewed the data, and then decided to install a one-of-a-kind 10 kWh-rated Bergey Excel. The total installation sets you back £45k.
Fast forward a year to the end of 2013. Although you used your Bergey to generate most your electricity this year, you had to draw 1,100 kWh from the grid because when there’s no wind, you have to blow-dry your Pomeranian after her shampoo. Apart from that, you were energy-conscious, saving the washing for windy days. So the remaining 3,900 kWh you used in 2013 came from the turbine.
Scenario 1: You didn’t chose the best site for your turbine. It generated a modest 8,000 kWh in 2013. [This is about 60% of the manufacturer’s claimed annual energy output at an average wind speed of 5 m/s, and it is well below the best performance of the rural, pole-mounted turbines of the Carbon Trust’s 2008-published field study.] The unused 4,100 kWh your turbine generated was exported, earning £184.50. Add to this the Generation Tariff: 8,000 kWh @ 21p/kWh = £1,680. Combine these with your savings—3,900 kWh of power you’ve generated yourself rather than buying it—and you’ve netted £2410.50 in 2013. Your Bergey pays for itself by the 18th year.
Scenario 2: You pick a decent site, with an average wind speed of 5.3 m/s and a healthy wind speed distribution, so your Bergey generates 16,800 kWh. Your unused, exported 12,900 kWh brings in £580.50. The Generation Tariff brings in £3528. Add your savings to this, and you’ve netted £4654.50 in 2013. Your turbine pays for itself in the 9th year, still within the manufacturer’s warranty period.
Scenario 3: You pick a good site. Your turbine operates at a 33% capacity_factor, generating 23,390 kWh in 2013, netting £7742.04/year, and paying for itself in its 5th year.
 
 

pointers

Your turbine’s rated wind speed is the speed at which it generates its rated_power; most of the time, your turbine will generate much less than its rated power. The ratio of actual output to the output if the turbine were constantly generating at its rated power is known as the capacity_factor.
Some potential customers are concerned about noise. The industry has listened, and turbines continue to get quieter. One company notes that trees are generally noisier than turbines. Vibrations into a house from building-mounted turbines might be more of an issue—if you’re concerned, discuss this with your installer.
While windows and cats still pose a larger hazard to birds than wind turbines do, a recent study suggests that turbines should be built 20 meters away from “valuable bat habitat.”
A nerdy tidbit: the maximum efficiency with which a turbine can glean energy from the wind is 16/27, or about 59.25%, known as the Betz limit. Essentially, this limit exists because once wind has passed through the turbine, the air molecules must have some energy left within them to evacuate and make room for the air molecules behind them.
 
 

cited material

i.  Small-scale wind energy: Policy insights and practical guidance. The Carbon Trust, 2008

ii. Distributed Networks Generation Guide, published by the ENA

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