ASK THE EXPERTS: Wind is Hard

Intermediate
Destroyed wind system
Wind is a dynamic, and potentially destructive, energy source. Frequent maintenance is required to prevent system failure.
Destroyed wind system

The leading statement in the “2014 Wind Turbine Buyer’s Guide” (HP161) is right on—“Without question, wind is a tough renewable energy resource to tap.”

I love the Home Power cover shots showing people strapped to a 100-foot tower while a crane, probably another 20 feet over their heads, tries to avoid dropping several hundred pounds of metal on them. Why anyone thinks this is glamorous is completely beyond me. Home-scale wind is dangerous, expensive, takes a lot of real estate, and is so much more involved than PV that—for all but the very few with unlimited time and resources—wind is a no-go. That should tell people capable of looking past the glamour to pass by wind energy.

Yes, I know the arguments about how wind complements PV, but at what cost? At a wind velocity of 11 meters per second, the small Kestrel puts out 1 kW, according to your comparison sheet. That’s about three PV modules—which have a fraction of the cost, little or no maintenance, and 30-plus years of output with a 25-year warranty—not a five-year wind machine warranty.

And why are manufacturers publishing an 11 m/s wind speed’s output—who has that kind of wind? It’s unrealistic and a sad commentary on an industry that can’t compete in the renewable energy business outside of large-scale commercial turbines. How is anyone realistically justifying wind?

Sorry, but this confirms my belief that wind has very little place in small-scale energy production.

Robert Dee • via homepower.com

Small wind is not for the faint of heart. I talk most of my clients out of it, especially as the cost of PV modules continues to drop. For a wind-electric system to make sense, it requires a great wind resource; a dark and windy season (in the case of justifying an off-grid system); or a strong desire to just do it. It is a blast (if you like that sort of thing) to install and keep a system running, but it’s not cheap, easy, or reliable.

One of the presenters at the recent Small Wind Conference gave a presentation titled “Go Big or Go Home,” and I think there’s a lot of logic to that. The economics and the equipment quality both improve as machine size increases.

“Wind complements PV” is a reasonable off-grid approach. On-grid, it’s generally wiser to examine your resources and sink your money into generating energy with the most reliable and abundant resource—be that sun, wind, or falling water. With net metering, there’s little need to have your generating source producing evenly all year. PV can make most of your energy in your sunny season (your utility credits the surplus to your account), and then you can draw on the credit during times of lower production.

I agree that the 11 m/s value is a bit high for a rating—but that is an instantaneous wind speed, not an average. And any instantaneous rating is pretty useless for comparison with PV or with other machines, and for energy predictions. What’s really helpful is an energy rating at various average wind speeds, as shown in the article’s table. Then you can (with luck) find the average wind speed at tower-top height at your site and get a prediction of the kilowatt-hours a given machine may provide each year.

A “1 kW” machine and 1 kW of PV are not comparable. A 1 kW PV system rarely produces at full power, but has a fairly predictable energy (kWh) output if you know the peak sun-hours at the location. A wind turbine rated at 1 kW peak is not similarly predictable, since wind is a cubic resource. For example, cutting the wind speed in half yields about one-eighth the potential power. You’ll need to know the actual tower-top average wind speed to make a reasonable energy prediction.

You are wise to point out that the turbine cost is just one part of the system’s cost. Typically, it’s a small portion—in most cases, the tower and balance-of-system components each cost more than the turbine. Potential wind energy users need good pricing on the installed cost of all of the components before deciding to invest in a wind-electric system. Some will go for it regardless of the economics. In all cases, it’s wise to know the costs and the benefits.

Ian WoofendenHome Power senior editor

Comments (8)

James Knapp's picture

We have an 20kW PV off grid system with 72 DEKA Unigy II batteries in our incredibly energy efficient home located in the Cayman Islands. We've been running successfully now for 7 years with very little problems. We wanted to add a small wind system to provide charge for my batteries at night or on windy days. We were thinking that 2 1kW units would be perfect for us. I'm always happy to see 1kW being put into my batteries from the solar but I'm reading that I shouldn't bother for small amounts of energy produced from wind. If they really are that much trouble we won't bother. If anyone has had success with them please let me know. Thanks!

James Knapp's picture

Thanks to everyone that replied! I've decided that I'll do exactly as instructed - add more PV. In Cayman we don't have enough wind to make it cost justifiable as a dependable resource but when it is windy (maybe 4 months a year) there's plenty of it. I just thought I'd tap it when it's available. I had no idea about the trouble it could be nor the amount of maintenance required. At 65 years old I don't need the grief. Our solar system has powered everything for years with virtually no issues at all and I still have about a third of our roof space available for more PV and it's so cheap compared to using a generator or the grid for topping up the batteries. Diesel is over USD $5.60 per gallon and utility costs are $0.42 per kWh here. Again, thanks for the help. Much appreciated.
Jim

Ian Woofenden's picture
Hi James, An important thing to know is that the rated power of a PV array and the rated power of a wind generator cannot be compared directly. You probably have noticed that it's fairly rare to get rated output from your array. That's partly because of the variability of the resource (sunshine), and also because the modules are rated in "perfect" laboratory conditions that the modules rarely see. But with PV, we know how to derate the rated capacity and make a sensible prediction of the actual energy output. With wind, it's different. Since wind energy is cubic in nature, that "1 kW" turbine may only produce 1/8 of its rated power if the wind speed is half of the rated wind speed. And since wind is more variable than sunshine, you'll see the output wander all over the place. And you'll rarely see 1 kW out of a "1 kW" wind turbine, since we only get those peak winds a few percent of the time at best. I wish it were as simple as buying a couple of "1 kW" turbines and getting that output all the time. The reality is that in order to predict your wind energy output, you need to know your average wind speed at tower top, and then get good data on what the machines you are considering will deliver in that wind regime. Michael's advice about tall towers is spot on, and I also encourage you to see our next issue and the Wind Turbine Buyer's Guide for more advice and perspective. Wind energy might be an excellent complement to your PV system, but it's easy to be disappointed if you're not realistic about the resource, equipment, and costs. Regards, Ian Woofenden Home Power senior editor
Michael Welch's picture
1 kW wind generators can be helpful as you say, but unless you are willing to be sure of your wind source and put them on a tall tower, you are probably better off investing the money in solar to get your batteries filled up earlier in the day.
summervilleb's picture

Comparing wind turbine models using the average power output at a hub height wind speed of 11 m/s is an OK way to compare apples to apples. It probably makes more sense for a consumer to compare estimated annual energy output which is typically done assuming an annual average wind speed at hub height of 5 m/s, not an unrealistic wind resource.

Some say wind is hard and expensive and PV is cheap and easy but others think wind is dynamic and interesting, definitely not boring ;-)

Ian Woofenden's picture
Thanks for your comment, Brent, and I'll concede that your "OK way" might be closer to truth at times than my "pretty useless", considering our real-world options, with typically limited data. My preference for a single comparative measure would be at a lower wind speed, closer to the reality for most users, and more conservative, so we'd get more pleasant surprises and less disappointment. The ideal, of course, is to have excellent data from the actual tower top site, and accurate predictions of many machines' output. But that often is not available, so we do the best we can. _Attitude_ about wind energy is a big factor. Some see the fickle nature of the resource and the varied results of technology application as a downer, while others see it as the spice of life. One of my goals is to help people avoid the personal and physical crashes that come when unrealistic expectations meet reality. Regards, Ian Woofenden Home Power senior editor PS -- Your comments are straight up, so why is the picture of you sideways? ;-)
summervilleb's picture

Thanks for straightening me out, Ian.
Brent

GUY T MARSDEN's picture

When I got the small wind bug back in 2010, I decided to do my due diligence and research my site very carefully. I installed a weather station at the intended location about 10 feet above the ridgeline of my building. After studying wind data for a year and looking at the average wind speeds I realized it would be cheaper to add solar panels. I have detailed what I learned in detail on my website: http://www.arttec.net/WindPower/ind...

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