Saturday, January 24, 2009

How much power do wind turbines make? and other questions

With a new wind turbine siting process looming for Waldo and Penobscot Counties (the same proposed wind farm straddling the county line), I'm getting numerous technical questions about wind power. The following is my response to one such question. A previous response, also germane, is at this post here.

(I often get asked the same question or much the same question several times by members of the public so I like to post the answers.)

Another question I got asked recently is, "why can't the State/Federal government do more comprehensive land planning for wind turbines so we can minimize the clashes with other important land uses such as tourism? Why do we always have to do it piecemeal?"

The answer to that one is, you don't have comprehensive state or federal-level planning regime in this country, in the same way you do in, say, most European countries. Since a key court case, Lucas vs, South Carolina Coastal Commission, and other confirming judgements, the only planning system with teeth is municipal or county zoning. If the zoning regime permits a turbine, the landowner or developer with a lease is going to be able to build it. So the impetus is on developers to propose sites. You can see a paper relating how we got in this particular fix in my chapter in Matthias Ruth's edited anthology Smart Growth and Climate Change, if you're serious about getting a full, reasoned answer to the question.

Dear Mick:

Several of us Thorndikeans [Thorndike: local town] have decided to form a citizens committee to research potential risks -vs- benefits of wind turbines. I thought since you are involved in the Mount View project you would be interested to know this is happening as the decisions made could impact that project and second, to ask you for some sources of information. I am seeking information about the economics of wind power. I know that wind is the most cost effective per watt, but I am looking for the larger picture. How much power can a wind project of this scale (say 30 turbines) provide? Would it be a drop in the bucket compared to what our community is currently using power wise or could they make a significant difference?

My goal is to gather enough sound informationto make an understandable presentation and then let people choose for them selves wether or not wind power is a good- albeit sometimes noisy, sometimes flickery- way to generate energy.

Dear XXXX:

The question you ask is fairly straightforward, but the details get technical fast.

First up, the costs and benefits. I'm going to direct you to a letter I sent XXXX, a former student of mine who was working with your neighbor, XXXX, to form some kind of wind power group. I got the impression, although I could be wrong, that XXXX were mostly worried about the negative impacts, so I wrote this letter to point out that there are benefits as well as costs. Very rational, I'm afraid, but that's what environmental scientists do.

You can get to it at

Let me know if you have trouble getting to it.

The main thing I would add is, since then, I studied up on some of the negative impacts of [another Maine wind power installation], and particularly questioning why their local noise level was higher than advertised. I think I know why, and how it can be avoided. It's most likely to do with the increase in noise when one turbine's noise is added to anothers, as well as the neighbor's buildings being downwind perhaps more often than was thought.

With better planning you'd likely be able to avoid these problems.

I would be happy to explain the noise characteristics of turbines and the aspect of site planning related to adding one turbines noise to anothers in abstract terms should you or your neighbors be interested.

As to benefits, wind power can be profitable, which is why private financiers are interested. Most essentially, the economics of wind power are not that different from the economics of any other industrial plant installation, where the primary factor is the interest on borrowed capital. Towns and municipalities can issue bonds with good interest rates, and so community-owned wind can be profitable.

The second factor is the site assessment work, which we are learning to do at Unity College, having just completed our first for Mt. View. The data you need is the frequency distribution of wind speed, not just the mean or average, and the power curve of a given manufacturer's make and model of turbine. There's a major increase in power produced per dollar of installation costs as the size of the turbines increases. The "square-cube law" applies. The power of a turbine increases relative to the square of the diameter of the swept area, and the cube of the windspeed, up to the cut-out speed of the turbine. This provides for a major economy of scale. Put simply, as turbine height and blade area increases, so KWH produced increases much, much more.

So, for instance, on the Mt. View site, using a medium-sized Northwind 100 KWH-rated turbine, the power produced according to our assessment would be 120-130 megawatt hours/year. Your house probably uses 6-10 MWH per year, so that's enough power for 10-14 houses. The turbine has a hub height of 32 meters and a 21 meter rotor diameter.

The GE 1.5 MWH turbines at XXXX have a hub height of 60-70 meters and a 77 meter rotor diameter. I would guess that they will produce each enough power for several hundred houses. Two orders of magnitude greater.

As you can see, the benefits increase greatly with larger turbines. Assuming the owners make 2 cents a KWH for their power, the 3 turbines at XXXX might bring in a million dollars gross per year. Likely they make more than 2 cents some of the time, with some kind of green rate. So, for instance, I pay 12.5/KWH cents to my power production company for hydropower because I want to use renewable power at home. That would be a lot more money.

A thirty turbine project would produce ten times the energy of a three turbine project, assuming it employed the 1.5 MWH models or similar.

So, in general, taking into account the effects on birds and bats, which have to be compared against negative effects from coal and oil and nuclear power, wind power is quite effective. There is, however, what's called the "base-load problem," which is just that the wind doesn't blow all the time in all the places where there are wind turbines. Generally, a good site is active 70% of the time, and up to 90%.

This means we'll still need to have up to 80% of our power from other sources. Hydropower is, however, a good base load supply in Maine.

The theoretical maximum load of regional or national electrical supply that could be met by wind turbines is usually given as 20%. We're nowhere near that yet in the US. The Danes, however, are set to exceed that this year for the first time, and plan to get up to 30% or more, so we'll see if they can do it. By the time they get to 30%, we'll be at 10% or less, so we don't need to try that particular experiment when they will do it for us.

I hope this helps. If you'd like more information, or you want me to explain any of this in person, I'd be happy to do so.

I hope you don't mind if I publish this reply on my blog too. It saves me writing it all out again when someone asks next time, as with the letter to XXXX above. I will make sure your name isn't on there.



Mick Womersley, PhD
Associate Professor of Human Ecology
Unity College

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