Text and charts from The Wind Power Book by Jack Park (copyright © 1981) are reprinted by permission of the author and Cheshire Books.
Harnessing the wind is a fantastically appealing idea. Anyone who's stepped out into a howling autumn blow knows instinctively that there's tremendous power in the movement of air, but the would-be windplant owner must decide whether his or her area has enough wind year round to justify the expense of such a system.
Windplant designer and builder Jack Park—owner of Helion Inc., president of the American Wind Energy Association, and author—has addressed that subject in a new text: The Wind Power Book. In the opinion of MOTHER EARTH NEWS' technical staff, it is the clearest, most understandable explanation yet of the factors that anyone must consider before either purchasing or constructing a wind energy system.
In fact, we were sufficiently impressed by the book to secure the right to reprint a portion of the volume's introduction ...which we hope will help to give everyone who reads it a sound understanding of the whys and wherefores of wind power.
Today, the wind can be harnessed to provide some or all of the power for many useful tasks such as pumping water, generating electricity, and heating a house or barn. Let's examine a few of these more closely.
Pumping water is a primary use of wind power. Daniel Halliday and others began manufacturing multi-bladed windmills for this purpose in the mid-nineteenth century. Halliday's work coincided with advancements in the iron water-pump industry. Soon the combination of wind machines and iron water-pumps made it possible to pump deep wells and provide the water for steam locomotives chugging across the North American plains. The demand for wind-powered deep-well pumps created a booming wind power industry at the turn of the century. Sears sold those machines for about $15, or $25 for a tower.
The wind has also been harnessed to provide mechanical power for grain grinding, sawmill operation, and even driving a washing machine. While I don't envision next year's Kenmore washing machine to come complete with a tower, blades, and drive shaft in lieu of an electric cord and plug, mechanical power from a wind machine can prove useful.
Electricity can power just about anything, and its generation from wind power seems to grab the lion's share of attention. You can pump water, run washing machines, grind grain, heat houses, and read books if you have electric power. As soon as electric generators from old cars became available, farmers started building "light plants," or homemade wind generators. Such early mechanics magazines as Popular Science and Modern Mechanix demonstrated how to convert water-pumping windmills into wind chargers by using junked generators, bicycle chains, and the family wind machine.
Many Midwest farmers already had gasoline or kerosene generators to charge their batteries, and the addition of wind power helped reduce fuel costs and wear-and-tear on generators. Out of all of this backyard activity grew the pre-REA windcharger industry. Some half-million wind systems once existed in the United States alone, but it's not clear from historical records whether this number includes the water pumpers along with the wind chargers.
Farmers used wind-generated electricity to power a radio, one or two lights for reading, eventually an electric refrigerator or a wringer washing machine, and not much else. Electric irons for pressing clothes, electric shavers, and other gadgets built to run on direct current appeared, but most of these proved unrealistic uses for wind-generated electric power. In fact, they may have contributed to the demise of wind electricity when rural electrification began. Electric appliances performed much better on an REA line, which wasn't subject to dead batteries. "Let's go over to the Joneses, Pa. They got one of them new powerlines. Maybell says her refrigerator don't defrost no more!"
Rural electrification put numerous wind chargers out of business. In the Midwest you can drive for miles on an empty dirt road, following a long electric powerline to only one, or perhaps two, homes at the end of the road. Leave one road and follow the next. It's the same story. REA lines were installed and wind generators came down. Sears catalogs touted all the marvelous gadgets one could buy and plug into the newly installed powerline.
Electric stoves, hot curlers, electric air conditioners, two or more televisions ...these aren't very realistic loads to place on a wind-charged battery. However, wind power can contribute to the operation of these devices, especially if grid power is already doing part of the job. With such cogeneration (wind power used together with grid power), the more wind power available, the less grid power needed.
In another application, wind power can provide heat for warming our households, dairy-barn hot water, or just about anything else for which heat is used as long as the heat is not needed in a carefully controlled amount. This wind heating concept is called the wind furnace, and it's one of our most useful applications of wind power. Wind furnaces can use wind-generated electricity to produce the heat, or they can convert mechanical power into heat directly.
Wind machine design must begin with a realistic assessment of energy needs and available wind resources. When confronted by inexperienced people observing my wind machine, I'm asked most often, "Will it power my house?" Taking this question to its most outrageous extreme, I'm often tempted to reply, "Just how fast would you like your house to go?" But usually I just ask, "How much power do you need at your house?" Blank stares, mumbled confusion, sometimes ignorant silence follow. Then, "Well, will it power the average house?"
Apparently many people would like to install a $1,000 wind machine and "switch off" good old Edison. This is a fantasy. It might be reasonable to use the wind to power your house if old Edison is a $30,000 powerline away from your new country home, but most end uses for wind power will be somewhat less extravagant.
A successful wind power system begins with a good understanding of the intended application. For example, should you decide that water pumping is the planned use, you must determine how high the water must be lifted and how fast the water must flow to suit your needs. The force of the wind flowing through the blades of a windmill acts on a water pump to lift water. The weight of the water being lifted and the speed at which the water flows determine the power that must be delivered to the pump system. A deeper well means a heavier load of water. Speeding up the flow means more water to be lifted per second. They both mean more power required to do the job, or a larger load.
This concept of load is crucial to the understanding of wind power. Imagine that instead of using a windmill you are tugging on a rope to lift a bucket of water from the well. This lifting creates a load on your body. Your metabolic processes must convert stored chemical energy to mechanical energy. The rate at which your body expends this mechanical energy is the power you are producing. The weight (in pounds) of the bucket and the rate (in feet per second) at which you are lifting it combine to define the power (in foot-pounds per second) produced. How long you continue to produce that power determines the total amount of mechanical energy you have produced.
The kind of application you have in mind pretty clearly defines the load that you'll place on your windpower system. Knowing something about that load will allow you to plan an energy budget. "What the #$°/a&," you ask, "is an energy budget?" Let's explain that with an analogy.
When you collect your paycheck, you have a fixed amount of money to spend. You probably have a budget that allocates portions of your money to each of the several bills you need to pay. With any luck, something is left over for savings, a few beers, or whatever else you fancy. Energy should be managed the same way, and if you live with wind power for very long, you'll soon set up an energy budget.
Setting up an energy budget involves estimating, calculating, or actually measuring the energy you need for the specific tasks you have in mind. If you plan to run some electric lights, you must estimate how many, how long, and at what wattage. If you plan to run a radio for three hours each evening, you'll have to add that amount of electrical energy to your budget. If you want to pump water, you should start with estimates of how much water you need per day and calculate how much energy is required to pump that much water from your well into the storage tank.
Wind furnaces require that you calculate the amount of heat needed. In some cases, you need to calculate only the heat required to replace the heat lost from your house when it's windy. Such a system works only when it's needed. Your energy budget will now be in heat units, probably British Thermal Units (BTU) ...which can easily be converted to horsepower-hours (HPH) or to kilowatt-hours (KWH): energy units more familiar to wind machine designers.
Your utility bills and the equipment you already own will help define your energy needs. For example, average electrical energy consumption in U.S. residences is around 750 KWH per month, or about one kilowatt-hour per hour. Or, more specifically, most residential well pumps are rated at one to three horsepower. You can easily determine how long your pump runs and arrive at the total energy required per day, per week, or per month. In short, you really need to get a handle on your energy needs before you can proceed to the design of a wind power system.
You will also need to determine your energy paycheck. There are two possible approaches:  Go to the site where you intend to install the wind machine and analyze the wind resource, or  go searching for the best wind site you can find. The former approach is more direct. You own some property, there is only one clear spot, and that's where the tower will be planted ... along with your hopes for a successful project. The latter approach offers more avenues for refinements and better chances for success. In any event, the larger the paycheck, the less strain on your energy budget ...and the smaller and less efficient a windmill needs to be.
In either approach you need to measure, estimate, or predict how much wind you can expect at your chosen site. I've talked with folks who claimed to be wind witchers, possessing the ability to use a wet index finger and predict the wind speed with great accuracy. I've talked with people who installed wind generators back in the days before rural electrification. Most of these machines were installed in areas now known to be quite windy, with average wind speeds of 14 to 16 miles per hour (MPH). When asked, these folks almost always guessed that the wind averaged 30 MPH or more. Those old wind chargers were installed haphazardly. "Heck, anywhere you stick one, it will work just fine."
Once you have established an energy budget, you have effectively established a standard for the performance of your wind power system. That puts you in a different league from the pre-REA folks. Your system will be good if it meets the energy budget. Theirs was good because it was all they had. Your site analysis should be careful and conservative. If it is, the wind system you plan will probably serve its purpose. If not, you'll have to be happy with what you get, just like the folks before the REA. As you gain familiarity with your system, you might learn how to save a little power for later. Or maybe you'll build a larger wind machine because you like wind power so much.
From an engineering standpoint, the available wind power is proportional to the cube of the wind speed. Put another way, if the wind speed doubles, you can get eight times the wind power from it unless the tower collapses. If you are off by a factor of two on your wind speed estimate, you'll be off by a factor of eight on the power you think is coming to you. Remember those farmers who guessed their average wind speed to be 30 MPH, although it's been measured at around 15 MPH. A factor of two.
Folks who have lived in an area for a long time can usually tell you what seasons are windy and the direction of the wind during those seasons. But they're not very good at estimating wind speeds. You'll have to measure the wind speed yourself, or use some accurate methods to estimate it.
Once you have established your energy budget and collected adequate wind resource data, you can begin the task of system design. Whether you intend to design and build the windmill yourself, assemble one from a kit, or buy one off the shelf, these are necessary preliminaries. Too frequently, designers disregard energy budget and site analysis. Instead, they make arm-waving assumptions on the use of the wind machine and where it might be installed, and use other criteria like cost as the design goal. Some of the best wind machines on today's market have been designed this way, but it's very important to recognize that most manufacturers of mass-produced wind machines have left the tasks of energy budget and site analysis up to the buyer. The information provided in later chapters of The Wind Power Book will help you make these analyses before you select a factory-built wind machine suited to your needs or set about designing and building your own system.
A thorough energy budget should tell you something about the time of day, week, or month when you need certain amounts of energy and power. For example, if everybody in your house rises at the same time each morning, your electric lights, hot curlers, coffeepot, television set, toaster, stove, water heater, and room heaters probably become active all at the same time. An enormous surge in electricity use occurs, the same problem Edison has. Their generators idle along all night doing virtually nothing until everybody wakes up at once. If you had a wind electric system, it would be really nice if the wind at your site were strongest when the loads on the system were greatest. Chances are, though, that your consumption doesn't coincide with the wind. So, you can either try to synchronize your loads with the wind or store wind-generated energy until you need it.
The methods of energy storage are legion, but only a few are practical. If wind is being used to pump water, your energy storage might be a familiar old redwood water tank. Electrical storage has traditionally taken the form of batteries, which is still the most reasonable means of storage in many installations. Cogeneration allows you to send wind-generated electricity out to utility lines (running the meter backward) when you don't need it all. In effect, old Edison becomes the energy storage for the wind system.
There are a number of exotic ways one might choose to store excess wind energy. You might dynamite an enormous mine under your house and pump it up with air from a wind-powered compressor. This compressed air can then power a small generator, sized for your loads, as well as provide aeration for the tropical fish tank. If you own enough land, you can bulldoze a large lake and pump water up to it with wind power. A small hydroelectric turbine will produce electricity as you need it. In fact, you might sink two telephone poles out in the yard and use a wind-powered motor to power a hoist, lifting a '56 Oldsmobile up to 100 feet. As it descends, the motor that lifted it becomes a generator. Such a mechanism could provide you with 500 watts of electricity for about 15 minutes ... maybe enough to burn the toast! There are as many possibilities for energy storage as there are crackpot inventors around, and some of these possibilities are just as crazy.
Some systems provide energy storage as an inherent part of the design. The wind furnace, where wind power is being used exclusively for heat production, is a good example. Heat storage is energy storage: You may store energy by heating water, rocks, or a large building with the excess heat. But probably you'll be heating with wind power when you need heat the most. Wind chill can draw heat from a house much more quickly than heat loss occurs under no-wind conditions. So little, if any, storage would be necessary.
But for most applications, some energy storage is mandatory.
Wind system design is a process of balancing energy needs against wind energy availability. Besides picking a good site and buying or building the right wind machine, you have to select a suitable storage system, plan all wiring or plumbing, build a tower, support it with guy wires, and get building permits and neighbor approval. The Wind Power Book is organized to help lead you through the design process. Whether you intend to design and build the entire system or just to assemble it from factory-built parts, a systematic approach will help you achieve a wind power system worthy of your efforts.
A clear distinction must be made between energy and power, two different but closely related quantities. Briefly, power is the rate at which energy is extracted, harnessed, converted, or consumed. It equals the amount of energy per unit time, or
Power = --------
An equivalent relation between these entities is
Energy = Power X Time
The amount of energy extracted or consumed is therefore proportional to the elapsed time. For example, a typical light bulb draws 100 watts of electrical power. One watt (1 W) is the basic unit of power in the metric system, Leave the light bulb on for two hours, and it will consume 200 watt-hours (100 watts times 2 hours equals 200 watt- hours, or 200 WH). Leave it on for ten hours, and it consumes 1,000 watt-hours, or one kilowatt-hour (1 KWH), the more familiar metric unit.
In the English system, energy is measured in foot-pounds, British Thermal Units, and a host of other units that don't concern us here. One foot-pound (1 ft.-lb.) is the amount of mechanical energy needed to raise one pound one foot high. One British Thermal Unit (1 BTU) is the amount of thermal energy needed to heat one pound of water 1°F. Power is most often measured in horsepower and in BTU per hour. One horsepower (1 HP) is the power required to raise a 550-pound weight one foot in one second:
1 horsepower = 550 -----------
Note that the units of power are expressed in units of energy per time, as one would expect.
Conversions between metric and English units require that you know a few conversion factors. For example, one horsepower equals 746 watts, and one kilowatt-hour is equal to 3,413 BTU. Thus,
100 W = --- HP = 0.134 HP
10,000 BTU = ------ KWH = 2.93 KWH
Rarely is it a simple task to estimate the wind energy available at a particular site; the wind speed is constantly changing. During one minute, 300 watts of power may be generated by a windmill, or 300 watt-minutes of energy (which equals 5 WH). During the next minute the wind may die, and you get absolutely no energy or power from the machine. The power output is constantly changing with the wind speed, and the accumulated wind energy is increasing with time. The wind energy extracted by the machine is the summation or total of all the minute-by-minute (or whatever other time interval you care to use) energy contributions. For example, if there are 30 minutes during a particular hour when the windmill is generating 5 WH and another 30 minutes when there is no energy generated, then the machine generates 150 WH (5 X 30 = 150) of wind energy. If there are 24 such hours a day, then 3,600 WH or 3.6 KWH are generated that day.
Energy and power are derived from the wind by making use of the force it exerts on solid objects, pushing them along. Buildings designed to stand still against this force extract very little energy from the wind. But windmill blades are designed to move in response to this force, and wind machines can extract a substantial portion of the energy and power available.
The wind energy available in a unit volume (one cubic foot or one cubic meter) of air depends only upon the air density p (Greek "rho") and the instantaneous wind speed V. This "kinetic energy" of the air in motion is given by the formula
-------------- = 1/2pV2
To find the kinetic energy in a particular volume of air, you just multiply by that volume. The volume of air that passes through an imaginary surface—say the disk swept out by a horizontal-axis windmill oriented at right angles to the wind direction—is equal to
Volume = AVt
where t is the elapsed time (in seconds) and A is the area (in square feet or square meters) of the surface in question. Thus, the wind energy that flows through the surface during time t is just
Available Energy = 1/2pV3At
Wind power is the amount of energy which flows through the surface per unit time, and is calculated by dividing the wind energy by the elapsed time t. Thus, the wind power available under the same conditions as above that is given by the formula
Available Power = 1/2pV3A
Both energy and power are proportional to the cube of the wind speed.
If all the available wind power working against a windmill rotor could be harvested by the moving blades, this formula could be used directly to calculate the power extracted. But getting such an output would require that you stop the wind dead in its tracks and extract every last erg of its kinetic energy. This is an impossible task. Some non-zero windspeed must occur downstream of the blades to carry away the incoming air, which would otherwise pile up. Under ideal conditions, the maximum power that can be extracted from the wind is only 59.3% of the power available, or
Maximum power = -----pV3A
In practice, a wind machine extracts substantially less power than this maximum. For example, the windmill rotor itself may capture only 70% of the maximum power. Bearings will lose another few percent to friction ...generators, gears, and other rotating machinery can lose half of whatever power remains. Pushrods, wires, batteries, and monitoring devices will lose still more. The overall "system" efficiency of the entire wind machine is the fraction of the wind power available that is actually delivered to a load or to a storage device:
Efficiency = ---------------
Thus, the power extracted by a particular wind machine with system efficiency E is given by the formula
Extracted Power = 1/2pV3AE
The final output of a wind machine is greatly reduced from the power that is really available in the wind. In practice, values of E commonly range from 0.10 to 0.50, although higher and lower values are possible.
One more factor is needed before the formula above can be used in your calculations: a conversion factor, K, that makes the answer come out in the appropriate units, whether metric or English.
The final formula combines everything so far presented:
This is a very important formula, perhaps the most important in The Wind Power Book.
EDITOR'S NOTE: Another book that should definitely be a part of any prospective wind power user's library is a new treatise by Donald Marier (editor of Alternative Sources of Energy magazine) called Wind Power for the Homeowner: A Guide to Selecting, Siting, and Installing an Electricity-Generating Wind Power System.
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