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Your Position: > Knowledge >> wind turbine >>> How to make a Small Wind Turbine

How to make a Small Wind Turbine

Cindy / 2014-08-23
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 The simple answer is yes it can but it depends on many factors  like how big is wellsee small turbine and how much power does your house consumeor even how windy is it on your property. They all affect the answer. This article is somewhat detailed but it discusses the most important issues around how to consider those factors. It's easier if you understand how small wind and solar is unlike just flipping a switch on a generator or connecting to the utility. You basically become your own utility and have the ability and the need to manage your own power. You can do that manually or get automated equipment that will look after the power management for you. As you might expect, simple manual systems are relatively cheap. Automated systems are more expensive but more reliable. Many people who who are learning about renewable energy wonder if a wellsee small wind turbine or some wellsee solar panels can totally replace the utility power. This is a common goal because utility power is getting more and more expensive and sometimes unreliable more importantly we all want more control over the price and availability of the power we use. I use barely 400kW hours each month, and live in a modern energy efficient home, with new electrical appliance fridge, stove, washer, dryer, dishwasher etc. The cost shown on my bill is about $20 (at 5 Cents/kWhr) and yet my whole Utility bill is over $65 so the real cost is more like 15-16 cents per kW and the note in my bill this month said they've applied to raise the delivery price another 6-20% starting in May 06. It seems logical to anyone, that if there is freely available power in the wind and sun on my property every day, why not use that instead of expensive and rising utility power. 

Quite simply the answer is YES and NO  the qualifier is "your house". Since every house is different every family's energy demands are different the answer is "it depends". Yes a 1-2kw small wind turbine with maybe a few hundred watts of solar will do the trick, if you own a very energy efficient home and appliances like most Europeans have. . . and most importantly you do not heat your home or your hot water with electricity. To do that you need a 7-10kW machine that is grid-tied . . and now you're talking $35-$50,000 Cdn just for the turbine installation. Most Europeans use 300-400kWhrs per month on average. But the answer is NO if you have a 3500 square foot Canadian home with a hot tub and electric base board heaters. Bear with me for a paragraph of technical stuff and you'll begin to see the essence of the small turbine sizing in a new light.

One of the most inefficient things you can do is turn electrons directly into heat and so any appliance that does that, like a water heater or stove or dryer is going to take a lot of electricity. Well, you would think then that all you need is a larger turbine . . yes? . . . YES that's true with a big enough turbine you can do it all . . but how big? Wind and solar are both "low density" energies. That means you need a large collector to capture a large amount of energy. There is a simple formula for calculating the wind power available in Watts. 

It's P (watts) = 1/2 X Air Density X Area (Cross Section) X Velocity (cubed) X the efficiency of the system (Cpe).

If you produce say 1000 watts continuously for a full hour then you have produced one kilowatt hour of Energy. Power is work in Watts. Energy is work done over a period of time (1 hour) Kilowatt hours. The density of the air is a minor variant so a number like 1.2 would be pretty close most of the time. Area is the cross sectional area of the turbine blades (the collector and the only thing you can control). What REALLY MATTERS is the density of the power available in the wind . . . that's what mass of air is passing through our turbine blades each hour. Think of it as a continuous column of air that we can weigh some section of . . (an "hours worth" of air for example). The bigger the mass in an hour the more energy we can extract from it in an hour. That mass can be slowed and therefore it can "give up energy" to the blades that are changing the direction of that mass of air particles by creating lift using an airfoil. As we redirect or change the direction slightly we transfer kinetic energy in the wind into lift and rotational energy in the turbine. But a German fellow named Betz back in the 1800s figured out that you can only extract 16/27ths (~59%) of that energy before the wind sees your turbine as a solid obstacle and simply goes around it. So about 59% is the most you can extract even if your turbine is 100% efficient.

Most small wind turbine systems have an overall efficiency of about 25%. some achieve 30-35%. . . so to start with, you can expect to get only 30% of 59% of the total energy available. If there was 1kW of power in the size column of air your turbine is collecting from then you can only expect to actually collect 177 watts (30% of 59% of 1000w). There are only so many "watts per square meter" cross section of wind. Let's say you have need 5 kw of power and the energy density is only 55 watts per square meter (12mph) then you need a turbine with almost 9 meter blades (that's over 18 meters across 250SqM cross section . . HUGE!) and it needs to be operating at near 45% efficiency. 

What really increases the power available dramatically is the speed (velocity) of the wind. That's a cubed function . . meaning each time the wind speed increases only 2-3 mph you have double the power available to capture. What you get at 20mph (32.2km/hr) is 8 times more than you have available at 10mph (16km/hr) . . That cube function is 2 X 2 X 2 or 8 times the power.... so you can actually get 5kw from a turbine with 1 meter blades (2 meters across) and about 3.5 sq Meter cross section . . but . . the velocity of the wind has to be over 45mph (~75km/hr). The wind at 45mph has a power density of nearly 2800watts per square meter. Can you imagine what it is during a hurricane like Katrina packing winds of over 120mph . . well I'll tell you . . it's over 50,000 watts per square meter. Let's look at a typical personal wind system cross section of 3.5 Sq Meter and see how the energy equation works.



The upper purple part is the energy that is missed or unable to be captured at a certain wind speed. The blue part of the column represents the potential power that can be captured by a VERY efficient machine (roughly 40% efficient) and the two stacked columns together represent the total energy in a 3.5 sq Meter cross-section of wind. Look at the first column (10mph) there is no energy there . . barely 32watts per sq meter and you can only expect to capture 30-35% of that if your 2 meter cross section system is 35% efficient overall. So don't get overly concerned about "low start-up" speeds that manufacturers quote. At a 6mph startup there is only 7 watts per square meter and the output of your small turbine can't be more than 9-10 watts total. If it's not blowing at least 10mph there's nothing there to capture unless you have a REALLY big turbine. .

So to achieve any particular power . . low wind speed - low power density - BIG turbine needed . . . . High speed wind - high power density - smaller turbine needed. Trouble is the wind velocity is changing all the time. One minute it's calm . . next day it's blowing like heck and you need a turbine that is not too big to get blown away, but big enough to capture some useful amount energy. That is, it needs to be as efficient as possible. Most of the time the wind may only be 10-12mph. If this is your "average" wind then it's only producing 10-20 watts per square meter cross section so most people would think it's hardly worth trying. Well since we've noticed the velocity is a cube function we can expect nearly twice the energy if the wind only goes to 15mph (24km/hr) we can expect 40-50watts/sqM and if we have a turbine with 3.5 SqM cross section then we can capture about 180W/hr of power. Just going to 19mph that increases to about 360W and whenever there's a good wind like 25-30mph (40-50km.hr) we can expect to get 800-1400 watts from your turbine with 1 meter blades. At 30 mph there is over 850 w/SqM power density and if I only get an hour of good wind like that it's as if I collected energy for over 16hrs at 10mph (16km/hr).

So a small wind turbine can produce a lot when the power density is high . . and that is only indirectly related to your "Average" wind speed. What matters is how often does the wind get above 15-20mph and how big and how efficient is my turbine. In general I'd say the average Canadian home could benefit from a horizontal axis turbine (HAWT) between 3-12 square meter cross section if it's at least 30% efficient including blades, turbine and drive system. More on horizontal vs vertical in the article on Vertical Axis Wind Turbines (VAWT) in the next FREE Wind News.

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