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A Primer on Small Wind Generators Michael Bergey, Bergey Windpower Co. History of Wind Energy
Books
Keeping Current
Wind
Energy
Wind Turbines-Design Considerations
Performance
Towers A wind turbine must have a clear shot at the wind to
perform efficiently. Turbulence, which both reduces performance and
"works" the turbine harder than smooth air, is highest close to the
ground and diminishes with height. Also, wind speed increases with
height above the ground. As a general rule of thumb, you should
install a wind turbine on a tower such that it is at least 30 ft
above any obstacles within 300 ft. Smaller turbines typically go on
shorter towers than larger turbines. A 250 watt turbine is often,
for example, installed on a 30-50 ft tower, while a 10 kW turbine
will usually need Remote Systems Equipment The balance-of-systems equipment used with a small wind turbine in a remote application is essentially the same as used with a PV system. Most wind turbines designed for battery charging come with a regulator to prevent overcharge. The regulator is specifically designed to work with that particular turbine. PV regulators are generally not suitable for use with a small wind turbine because they are not designed to handle the voltage and current variations found with turbines. The output from the regulator is typically tied into a DC source center, which also serves as the connection point for other DC sources, loads and the batteries. For a hybrid system the PV and wind systems are connected to the DC source center through separate regulators, but no special controls are generally required. For small wind turbines a general rule-of-thumb is that the AH capacity of the battery bank should be at least six times the maximum renewables charging current, including any PV elements. The wind industry has had good experience using battery banks that are smaller than those typically recommended for PV applications. Related Wind Energy System Components The balance-of-systems equipment used with a small wind turbine in a remote application is essentially the same as used with a PV system. Most wind turbines designed for battery charging come with a regulator to prevent overcharge. The regulator is specifically designed to work with that particular turbine. PV regulators are generally not suitable for use with a small wind turbine because they are not designed to handle the voltage and current variations found with turbines. The output from the regulator is typically tied into a DC source center, which also serves as the connection point for other DC sources, loads and the batteries. For a hybrid system the PV and wind systems are connected to the DC source center through separate regulators, but no special controls are generally required. For small wind turbines a general rule-of-thumb is that the AH capacity of the battery bank should be at least six times the maximum renewables charging current, including any PV elements. The wind industry has had good experience using battery banks that are smaller than those typically recommended for PV applications. Being Your Own Utility Company The federal PURPA regulations passed in 1978 allow you to interconnect a suitable renewable energy powered generator to your house or business to reduce your consumption of utility supplied electricity. This same law requires utilities to purchase any excess electricity production at a price (avoided cost) usually below the retail cost of electricity. In about a half-dozen states with "net energy billing options" small systems are allowed to run the meter backwards, so they get the full retail rate for excess production. Because of the high overhead costs to the utilities for keeping a few special hand-processed customer accounts, net energy billing is actually less expensive for them. These systems do not use batteries. The output of the wind turbine is made compatible with utility power using either a line-commutated inverter or an induction generator. The output is then connected to the household breaker panel on a dedicated breaker, just like a large appliance. When the wind turbine is not operating, or it is not putting out as much electricity as the house needs, the additional electricity needed is supplied by the utility. Likewise, if the turbine puts out more power than the house needs, the excess is instantaneously "sold" to the utility. In effect, the utility acts as a very big battery bank and the utility see's the wind turbine as a negative load. After over 200 million hours of interconnected operation we now know that small utility-interconnected wind turbines are safe, do not interfere with either utility or customer equipment, and do not need any special safety equipment to operate successfully. Hundreds of homeowners around the country who installed 4-12 kW wind turbines during the go-go tax credit days in the early 1980's now have everything paid for and enjoy monthly electrical bills of $8-30, while their neighbors have bills in the range of $100-200 per month. The problem, of course, is that these tax credits are long gone and without them most homeowners will find the cost of a suitable wind generator prohibitively expensive ( note that this is has changed in certain states, and countries where renewable energy buy-back programs and incentives are once again being made available). A 10 kW turbine (the most common size for homes), for example, will typically cost $22,000-29,000 installed. For those paying 12 cents/kilowatt-hour or more for electricity in an area with an average wind speed of 10 mph or more, and with an acre or more of property (the turbines are big), a residential wind turbine is certainly worth considering. Payback periods will generally fall in the range of 8-16 years and some wind turbines are designed to last thirty years or more.
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The wind has been an important source of energy in the
U.S. for a long time. The mechanical windmill was one of the two
"high-technology" inventions (the other was barbed wire) of the late
1800's that allowed us to develop much of our western frontier. Over
8 million mechanical windmills have been installed in the U.S. since
the 1860's and some of these units have been in operation for more
than a hundred years. Back in the 1920's and 1930's, before the REA
began subsidizing rural electric coops and electric lines, farm
families throughout the Midwest used 200-3,000 watt wind generators
to power lights, radios, and kitchen appliances. The modest wind
industry that had built up by the 1930's was literally driven out of
business by government policies favoring the construction of utility
lines and fossil fuel power plants. 
In
the late 1970's and early 1980's intense interest was once again
focused on wind energy as a possible solution to the energy crisis.
As homeowners and farmers looked to various electricity producing
renewable energy alternatives, small wind turbines emerged as the
most cost effective technology capable of reducing their utility
bills. Tax credits and favorable federal regulations (PURPA) made it
possible for over 4,500 small, 1-25 kW, utility-intertied wind
systems to be installed at individual homes between 1976-1985.
Another 1,000 systems were installed in various remote applications
during the same period. Small wind turbines were installed in all
fifty States. None of the small wind turbine companies, however,
were owned by large companies committed to long term market
development, so when the federal tax credits expired in late 1985,
and oil prices dropped to $10 a barrel two months later, most of the
small wind turbine industry once again disappeared. The companies
that survived this "market adjustment" and are producing small wind
turbines today are those whose machines were the most reliable and
whose reputations were the best.
By far the best source of general information on the
technology and application of small wind turbines is a book written
in 1993 by Paul Gipe. Mr. Gipe has more than 15 years experience
with small wind systems and is a world renowned author and lecturer
on the subject. This book, Wind Power for Home & Business, is
soft-bound and a little over 400 pages long. Gipe's book is easy to
read and is filled with examples, illustrations, and a lot of common
sense. We highly recommend it.
The best way to keep current with the progress of wind
energy development, both small and large scale, in the U.S. is to
join the American Wind Energy Association - AWEA
Wind energy is a form of solar energy produced
by uneven heating of the Earth's surface. Wind
resources are best along coastlines, on hills, and in the
northern states, but usable wind resources can be found in most
areas. As a power source wind energy is less predictable than solar
energy, but it is also typically available for more hours in a given
day. Wind resources are influenced by terrain and other factors that
make it much more site specific than solar energy. In hilly terrain,
for example, you and your neighbor are likely to have the exact same
solar resource. But you could have a much better wind resource than
your neighbor because your property is on top of the hill or it has
a better exposure to the prevailing wind direction.
Conversely, if your property is in a gully or on the leeward side of
the hill, your wind resource could be substantially lower. In this
regard, wind energy must be considered more carefully than solar
energy.
Most wind turbines are horizontal-axis propeller type
systems. Vertical-axis systems, such as the the egg-beater like
Darrieus and S-rotor type Savonius type systems, have proven to be
more expensive. A horizontal-axis wind turbine consists of a rotor,
a generator, a mainframe, and, usually, a tail. The rotor captures
the kinetic energy of the wind and converts it into rotary motion to
drive the generator. The rotor usually consists of two or three
blades. A three blade unit can be a little more
efficient and will run smoother than a two blade rotor, but they
also cost more. The blades are usually made from either wood or
fiberglass because these materials have the needed combination of
strength and flexibility (and they don't interfere with television
signals!). 
The rated power for a wind turbine is not a good
basis for comparing one product to the next. This is
because manufacturers are free to pick the wind speed at which they
rate their turbines. If the rated wind speeds are not the same then
comparing the two products is very misleading. Fortunately, the
American Wind Energy Association has adopted a standard method of
rating energy production performance. Manufacturers who follow the
AWEA standard will give information on the Annual Energy Output
(AEO) at various annual average wind speeds. These AEO figures are
like the EPA Estimated Gas Mileage for your car, they allow you to
compare products fairly, but they don't tell you just what your
actual performance will be ("Your Performance May
Vary").
Wind resource maps for the U.S. have
been compiled by the Department of Energy. These maps show the
resource by "Power Classes" that mean the average wind speed will
probably be within a certain band. The higher the Power Class the
better the resource. We say probably because of the terrain effects
mentioned earlier. On open terrain the DOE maps are quite good, but
in hilly or mountainous terrain they must be used with great
caution. The wind resource is defined for a standard wind sensor
height of 33 ft (10 m), so you must correct the average wind speed
for wind tower heights above this height before using the AEO
information supplied by the manufacturer. Wind turbine performance
is also usually derated for altitude, just like an airplane, and for
turbulence. Wind turbine manufacturers can usually provide
computer-aided performance predictions for their turbines at
virtually any site.
As a rule of thumb wind energy should be considered if
your average wind speed is above 8 mph (most, but
not all, Class 1 and all other Classes) for a remote application and
10 mph (Class 2 or better) for a utility-intertied application. If
you live in an area that is not too hilly then the DOE wind resource
map can be used to fairly accurately calculate the expected
performance of a wind turbine at your site. In complex terrain a
judgment on the site's exposure must be made to adjust the average
wind speed used for this calculation. In most situations it is not
necessary to monitor the wind speed with a recording anemometer
prior to installing a small wind turbine. But in some situations it
is worth spending $300-1,000 and waiting a year to perform a wind
survey. Manufacturers and equipment dealers can help sort
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