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Small Wind
Energy Systems for the Homeowner
In the 1920s and
'30s, farm families throughout the Midwest used wind to generate
enough electricity to power their lights and electric motors. The
use of wind power declined with the government-subsidized
construction of utility lines and fossil fuel power plants. However,
the energy crisis in the 1970s and a growing concern for the
environment generated an interest in alternative, environmentally
friendly energy resources. Today, homeowners in rural and remote
locations across the nation are once again examining the possibility
of using wind power to provide electricity for their domestic
needs.
This publication will
help you decide whether a wind system is practical for you. It will
explain the benefits, help you assess your wind resource and
possible sites, discuss legal and environmental obstacles, and
analyze economic considerations such as pricing.
Benefits of Wind Power
A wind energy
system can provide you with a cushion against electric power price
increases. Wind energy systems help reduce U.S. dependence on
fossil fuels, and they are nonpolluting. If you live in a remote
location, a small wind energy system can help you avoid the high
costs of having the utility power lines extended 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. The
length of the payback period-the time before the savings resulting
from your system equal the cost of the system itself-depends on
the system you choose, the wind resource on your site, electricity
costs in your area, and how you use your wind system.
Is
Wind Power Practical for You?
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 9
miles per hour (4.0 meters per second).
-
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 10
miles per hour (4.5 meters per second).
-
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.
Is
Your Site Right?
The U.S.
Department of Energy (DOE) has compiled wind resource maps that
are available from the American Wind Energy Association and the
National Technical Information Service (see Source
List). The DOE maps are good sources for regional
information and can show whether wind speeds in your area are
generally strong enough to justify investing in a wind
system.
Wind-turbine
manufacturers can use computer models to predict their machines'
performance at a specific location. They can also help you size a
system based on your electricity needs and the specifics of local
wind patterns. However, you will need site-specific data to
determine the wind resource of 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 site 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 tower where you will install the wind turbine-see the
section on "Wind System Basics" that
follows). 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 33 feet (10 meters).
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 winds
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 buildings, which could block the
wind. Also realize that the power available in the wind increases
proportionally to its speed (velocity) cubed (v3). 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 12.6 miles per hour
(5.6 meters per second), it has twice the energy available as a
site with a 10 mile per hour (4.5 meter per second)
average.
Additional Considerations
In addition to
the factors listed previously, 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 (see the case study),
-
understand
the basics of a small wind system,
and
-
review
possibilities for combining your system with other energy
sources, backups, and energy efficiency improvements.
You should
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 (see
"Wind System Basics,"
which follows). 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 (AWEA, see Source List); however,
not all small turbine manufacturers are members of AWEA.
Manufacturer information can also be found at times in the
periodicals listed in the Reading
List. 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. The case study that follows
illustrates the many factors and calculations you will need to
consider. 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. For 1996, it was 1.6 cents per kilowatt hour. 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. (The Energy Efficiency and Renewable
Energy Clearinghouse-see Source
List-has more information on net billing.)
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. Check with your state's department of revenue, your
local utility, public utility commission, or your local energy
office for information.
Wind System
Basics
All wind systems
consist of a wind turbine, a tower, wiring, and the "balance of
system" components: controllers, inverters, and/or
batteries.
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
14-mile-per-hour (6.26-meters-per-second) 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 30 feet (9 meters) above any obstacle that is within 300
feet (90 meters) 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 60-foot tower
height to a 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 home
buyers.
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
Wind Systems
According to many
renewable energy experts, a stand-alone "hybrid" system that
combines wind and photovoltaic (PV) technologies offers several
advantages over either single system. (For more information on
solar electric-or photovoltaic-systems, contact the Energy
Efficiency and Renewable Energy Clearinghouse-see Source
List.)
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.
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.
The
Future of Wind Power
By investing in a
small wind system, you can reduce your exposure to future fuel
shortages and price increases and reduce pollution. 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.
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Case
Study: Wind Power Economics of a Home System
Note: In this analysis,
we have assumed a certain set of conditions, such as wind
regime, maintenance costs, etc. Your analysis will differ for
your set of circumstances. This case study is for illustration
purposes only.
A New England homeowner is
considering taking out a 20-year loan to purchase a $10,000
wind system (turbine, tower, inverter, and battery storage)
for generating her own electricity, instead of paying her full
electricity bills for the next 20 years.
Assume that the wind turbine
she has chosen is rated at 3 kilowatts with the turbine 80
feet (24 meters) above the ground, and that she lives in a
Class 4 wind regime (average wind speed of 12.5 to 13.4 miles
per hour [5.6 to 6 meters per second] measured at 33 feet [10
meters] above the ground). Given these assumptions, the
turbine can produce an estimated 9000 kilowatt hours (kWh) per
year, or 750 kWh per month. Also assume, for the sake of
simplicity, that she will use all of the electricity herself
and will not sell any back to the utility. Therefore, the
value of the electricity to her is equal to the retail price
she pays the utility; in this case, 12 cents per kWh.
Continuing to Pay Electricity
Bills
If she continues to pay her
electricity bills without the wind turbine, the retail value
of the electricity is $1,080 the first year. In later years,
the price of electricity increases. For this analysis, we
assume that the cost of electricity increases at the same rate
as inflation-3% a year. Thus, the 9000 kWh will cost $1,112 in
the second year, $1,146 the third year, and so forth, until
the total inflation-adjusted cost of electricity for 20 years
is $29,020.
Purchasing a Wind
System
She can obtain the
least-expensive loan by taking out a second mortgage on her
home. She can borrow $10,000 at 8%, and make payments of
$1,019 for 20 years. But she can deduct the portion of her
payments that go toward interest at her 30% combined federal
and state tax rate. Thus, after taxes, her annual payment is
$779 for the first year, and increases to $996 as the interest
deduction decreases in later years.
However, there are other
costs to owning a wind turbine. Her property taxes will be
higher because the wind turbine increases the value of her
property. She will pay additional insurance since her standard
homeowner's policy does not cover liability from the wind
tower. And she will hire a local mechanic to climb the tower
and grease the bearings every year. Altogether, she figures
these operations and maintenance (O&M) costs will be about
1 cent/kWh or $100 per year in today's dollars. Let us assume
for this analysis that taxes, insurance, and labor rates
increase at the same rate as inflation. Thus, annual O&M
costs increase to $175 in the 20th year. So, over 20 years,
her total inflation-adjusted cost for buying a wind system is
$19,678.
Net Present Value of Both
Options
However, our example is still
not complete. Economists tell us that future dollars are worth
less than present dollars. It is better to have money now,
rather than in the future, so we can use it to invest and earn
more money. Even though inflation increases her annual
electricity payments after 20 years to $1,894, those are
future dollars, so they are worth less than today's
dollars. Economists refer to this devaluation as the net
present value factor, the rate at which future dollars are
discounted compared to present dollars. This discount rate is
equal to the rate of return that she could make on an
investment of equivalent risk and liquidity to a wind turbine.
In this evaluation, assume her opportunity for return on
investment with today's dollars (i.e., the discount rate for
her future dollars) is 10% a year.
Therefore, projecting her
electric utility payments into the future to, say, the end of
the first year, the dollars are worth 90% of what they were at
the beginning of the year. At the end of the second year, the
dollars are worth 90% of what they were at end of the previous
year. (Notice the value of her future dollars depreciates at a
compounded rate.) Considering these adjustments, her annual
electricity payment in the 20th year is actually worth only
$156 in today's dollars. Thus, her total cost of buying
electricity for 20 years, adjusted for inflation and present
value factors, is only $8,927 in today's dollars.
Another way to think of it
is that her payment in the 20th year is really a
deferred payment. She does not have to pay $29,020
today. Since the utility company allows her to pay her bills
as she uses the electricity, she does not have to make
any large capital expenditures. So she has more of her money
to invest for 20 years. This would not be true if she had to
pay for 20 years of electricity up front.
But net present value
factors also apply to purchasing a wind system, because she is
making deferred payments on her loan. Her payments of $1,154
in year 20 are really worth only $95 in today's dollars, for
instance. Therefore, her total cost for buying a wind system,
adjusted for inflation and net present value, is only $6,426
in today's dollars.
The Final Analysis
So
in real terms, she saves $2,501 over 20 years by purchasing a
wind system, as opposed to continuing to pay her electricity
bills. An added benefit is that she would avoid the release of
40 tons (40 metric tons) of carbon dioxide, 800 pounds (363
kilograms) of nitrogen oxide, and 280 pounds (127 kilograms)
of sulfur dioxide into the atmosphere-the amount of pollution
that a utility company in the Northeast would emit to supply
her electric load for 20 years, on
average. |
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