If you are a homeowner considering using a wind turbine to power your home,
there are a number of considerations. Fortunately, there are also a number of
information sources to help you. The following will help you decide if a wind
system is practical for you.
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By investing in a small wind system, you can reduce pollution and reduce your
exposure to future fuel shortages and price increases. Deciding whether to
purchase a wind system, however, is complicated; there are many factors to
consider. But if you have the right set of circumstances, a well-designed wind
energy system can provide you with many years of cost-effective, clean, and
reliable electricity.
Wind energy systems provide a cushion against electricity price increases.
Wind energy systems reduce U.S. dependence on fossil fuels, and they don't emit
greenhouse gases. If you are building a home in a remote location, a small wind
energy system can help you avoid the high costs of extending utility power lines
to your site.
Although wind energy systems involve a significant initial investment, they
can be competitive with conventional energy sources when you account for a
lifetime of reduced or altogether avoided utility costs. They length of the
payback period — the time before the savings resulting from your system equal
the system cost — depends on the system you choose, the wind resource in your
site, electric utility rates in you're area, and how you use your wind system.
Small wind energy systems can be used in connection with an electricity
transmission and distribution system (called grid-connected systems), or in
stand-alone applications that are not connected to the utility grid. A
grid-connected wind turbine can reduce your consumption of utility-supplied
electricity for lighting, appliances, and electric heat. If the turbine cannot
deliver the amount of energy you need, the utility makes up the difference. When
the wind system produces more electricity than the household requires, the
excess can be sold to the utility. With the interconnections available today,
switching takes place automatically. Stand-alone wind energy systems can be
appropriate for homes, farms, or even entire communities (a co-housing project,
for example) that are far from the nearest utility lines. Either type of system
can be practical if the following conditions exist.
Conditions for stand-alone systems
- You live in an area with average annual wind speeds of at least 4.0 meters
per second (9 miles per hour)
- A grid connection is not available or can only be made through an expensive
extension. The cost of running a power line to a remote site to connect with the
utility grid can be prohibitive, ranging from $15,000 to more than $50,000 per
mile, depending on terrain.
- You have an interest in gaining energy independence from the utility
- You would like to reduce the environmental impact of electricity production
- You acknowledge the intermittent nature of wind power and have a strategy
for using intermittent resources to meet your power needs
Conditions for grid-connected systems
- You live in an area with average annual wind speeds of at least 4.5 meters
per second (10 miles per hour).
- Utility-supplied electricity is expensive in your area (about 10 to 15 cents
per kilowatt-hour).
- The utility's requirements for connecting your system to its grid are not
prohibitively expensive.
- Local building codes or covenants allow you to legally erect a wind turbine
on your property.
- You are comfortable with long-term investments.
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To get a general idea if your region has good wind resources, look at the
Wind Powering America Wind Resources page, which has state wind maps. The
maps will show you if wind speeds in your area are strong enough to further
investigate the wind resource. Of course, the maps are just a starting point —
the actual wind resource on your site will vary depending on topography and
structure interference. And a localized site with good winds, such as a
ridgetop, may not show up on the maps.
Another source for wind data is the
National Climatic Data Center, which collects data for selected sites and
makes area wind data summaries available for purchase.
You will need site-specific data to determine the wind resource at your exact
location. If you do not have on-site data and want to obtain a clearer, more
predictable picture of your wind resource, you may wish to measure wind speeds
at your location for a year. You can do this with a recording anemometer, which
generally costs $500 to $1500. The most accurate readings are taken at "hub
height" (i.e., the elevation at the top of the wind turbine tower). This
requires placing the anemometer high enough to avoid turbulence created by
trees, buildings, and other obstructions. The standard wind sensor height used
to obtain data for the DOE maps is 10 meters (33 feet).
You can have varied wind resources within the same property. If you live in
complex terrain, take care in selecting the installation site. If you site your
wind turbine on the top or on the windy side of a hill, for example, you will
have more access to prevailing wind than in a gully or on the leeward
(sheltered) side of a hill on the same property. Consider existing obstacles and
plan for future obstructions, including trees and building, which could block
the wind. Also realize the power in the wind is proportional to its speed
(velocity) cubed (v�). This means that the amount of power you get from your
generator goes up exponentially as the wind speed increases. For example, if
your site has an annual average wind speed of about 5.6 meters per second (12.6
miles per hour), it has twice the energy available as a site with a 4.5 meter
per second (10 mile per hour) average (12.6/103).
In addition to reviewing your site and particular situation and goals, you
should also
- research potential legal and environmental obstacles
- obtain cost and performance information from manufacturers
- perform a complete economic analysis that accounts for a multitude of
factors
- understand the basics of a small wind system, and
- review possibilities for combining your system with other energy sources,
backups, and energy efficiency improvements.
Establish an energy budget to help define the size of turbine that will be
needed. Since energy efficiency is usually less expensive than energy
production, making your house more energy efficient first will likely result in
being able to spend less money since you may need a smaller wind turbine to meet
your needs.
Potential Legal and Environmental Obstacles
Before you invest any time and money, research potential legal and
environmental obstacles to installing a wind system. Some jurisdictions, for
example, restrict the height of the structures permitted in residentially zoned
areas, although variances are often obtainable. Your neighbors might object to a
wind machine that blocks their view, or they might be concerned about noise.
Consider obstacles that might block the wind in the future (large planned
developments or saplings, for example). If you plan to connect the wind
generator to your local utility company's grid, find out its requirements for
interconnections and buying electricity from small independent power producers.
Pricing a System
When you are confident that you can install a wind machine legally and
without alienating your neighbors, you can begin pricing systems and components.
Approach buying a wind system as you would any major purchase. Obtain and
review the product literature from several manufacturers. Lists of
manufacturers are available from the American Wind Energy Association;
however, not all small turbine manufacturers are members of AWEA. Manufacturer
information can also be found at times in the periodicals listed below. Once you
have narrowed the field, research a few companies to be sure they are recognized
wind energy businesses and that parts and service will be available when you
need them. Also, find out how long the warranty lasts and what it includes.
Ask for references of customers with installations similar to the one you are
considering. Ask system owners about performance, reliability, and maintenance
and repair requirements, and whether the system is meeting their expectations.
The Economics of Wind Power for Home Use
A residential wind energy system can be a good long-term investment. However,
because circumstances such as electricity rates and interest rates vary, you
need to decide whether purchasing a wind system is a smart financial move for
you. Be sure you or your financial adviser conduct a thorough analysis before
you buy a wind energy system.
Grid-connected-system owners may be eligible to receive a small tax credit
for the electricity they sell back to the utility. The National Energy Policy
Act of 1992 and the 1978 Public Utilities Regulatory Policy Act (PURPA) are two
programs that apply to small independent power producers. PURPA also requires
that the utility sell you power when you need it. Be sure you check with your
local utility or state energy office before you assume any buy-back rate. Some
Midwestern rates are very low (less than $.02/kWh), but some states have
state-supported buy-back rates that encourage renewable energy generation. In
addition, some states have "net billing," where utilities purchase excess
electricity for the same rate at which they sell it.
Also, some states offer tax credits and some utilities offer rebates or other
incentives that can offset the cost of purchasing and installing wind systems.
Visit the DSIRE web site, which contains
a database of financial incentives for wind energy. Check with your state's
department of revenue, your local utility, public utility commission, or your
local energy office for information.
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All wind systems consist of a wind turbine, a tower, wiring, and the "balance
of system" components: controllers, inverters, and/or batteries. Hybrid systems
use additional equipment, like photovoltaic panels and diesel generators to
ensure electricity is available at all times.
Wind Turbines
Home wind turbines consist of a rotor, a generator mounted on a frame, and
(usually) a tail. Through the spinning blades, the rotor captures the
kinetic energy of the wind and converts it into rotary motion to drive the
generator. Rotors can have two or three blades, with three being more common.
The best indication of how much energy a turbine will produce is the diameter of
the rotor, which determines its "swept area," or the quantity of wind
intercepted by the turbine. The frame is the strong central axis bar onto which
the rotor, generator, and tail are attached. The tail keeps the turbine facing
into the wind.
A 1.5-kilowatt (kW) wind turbine will meet the needs of a home requiring 300
kilowatt-hours (kWh) per month, for a location with a 6.26-meters-per-second
(14-mile-per-hour) annual average wind speed. The manufacturer will provide you
with the expected annual energy output of the turbine as a function of annual
average wind speed. The manufacturer will also provide information on the
maximum wind speed in which the turbine is designed to operate safely. Most
turbines have automatic speed-governing systems to keep the rotor from spinning
out of control in very high winds. This information, along with your local wind
speed distribution and your energy budget, is sufficient to allow you to specify
turbine size.
Towers
To paraphrase a noted author on wind energy, "the good winds are up high."
Because wind speeds increase with height in flat terrain, the turbine is mounted
on a tower. Generally speaking, the higher the tower, the more power the wind
system can produce. The tower also raises the turbine above the air turbulence
that can exist close to the ground. A general rule of thumb is to install a wind
turbine on a tower with the bottom of the rotor blades at least 9 meters (30
feet) above any obstacle that is within 90 meters (300 feet) of the tower.
Experiments have shown that relatively small investments in increased tower
height can yield very high rates of return in power production. For instance, to
raise a 10-kW generator from a 18-meter (60-foot) tower height to a 30-meter
(100-foot) tower involves a 10% increase in overall system cost, but it can
produce 25% more power.
There are two basic types of towers: self-supporting (free standing)
and guyed. Most home wind power systems use a guyed tower.
Guyed-lattice towers are the least expensive option. They consist of a simple,
inexpensive framework of metal strips supported by guy cables and earth anchors.
However, because the guy radius must be one-half to three-quarters of the
tower height, guyed-lattice towers require enough space to accommodate them.
Guyed towers can be hinged at the base so that they can be lowered to the ground
for maintenance, repairs, or during hazardous weather such as hurricanes.
Aluminum towers are prone to cracking and should be avoided.
Balance of System
Stand-alone systems require batteries to store excess power generated for use
when the wind is calm. They also need a charge controller to keep the batteries
from overcharging. Deep-cycle batteries, such as those used to power golf carts,
can discharge and recharge 80% of their capacity hundreds of times, which makes
them a good option for remote renewable energy systems. Automotive batteries are
shallow-cycle batteries and should not be used in renewable energy systems
because of their short life in deep cycling operations.
In very small systems, direct current (DC) appliances operate directly off
the batteries. If you want to use standard appliances that require conventional
household alternating current (AC), however, you must install an inverter to
convert DC electricity to AC. Although the inverter slightly lowers the overall
efficiency of the system, it allows the home to be wired for AC, a definite plus
with lenders, electrical code officials, and future homebuyers.
For safety, batteries should be isolated from living areas and electronics
because they contain corrosive and explosive substances. Lead-acid batteries
also require protection from temperature extremes.
In grid-connected systems, the only additional equipment is a
power-conditioning unit (inverter) that makes the turbine output electrically
compatible with the utility grid. No batteries are needed. Work with the
manufacturer and your local utility on this.
Hybrid Systems
According to many renewable energy experts, a stand-alone "hybrid" system
that combines wind with photovoltaic (PV) technologies and/or a diesel generator
offers several advantages.
In much of the United States, wind speeds are low in the summer when the sun
shines brightest and longest. The wind is strong in the winter when there is
less sunlight available. Because the peak operating times for wind and PV occur
at different times of the day and year, hybrid systems are more likely to
produce power when you need it.
For the times when neither the wind generator nor the PV modules are
producing electricity (for example, at night when the wind is not blowing), most
stand-alone systems provide power through batteries and/or an engine-generator
powered by fossil fuels like diesel.
If the batteries run low, the engine-generator can be run at full power until
the batteries are charged. Adding a fossil-fuel-powered generator makes the
system more complex, but modern electronic controllers can operate these complex
systems automatically. Adding an engine-generator can also reduce the number of
PV modules and batteries in the system. Keep in mind that the storage capability
must be large enough to supply electrical needs during noncharging periods.
Battery banks are typically sized for one to three days of windless operation.
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