Generation Solar Renewable Energy Systems



	1kW wind turbine at an off grid home.

What's in a wind power system?

Learn about the major components of a wind power system.

How big a wind turbine will I need?

Tough question; but we can help! Here's an easy step by step guide to sizing a wind power system.

Feature article

When is wind power economical?

Many people feel the answer to their energy prayers is blowing in the wind. For some folks, this may be true. For many, it is not. To help decide if wind power is for you, a little background on wind power and its economics is in order.

Power in the wind is simply the energy of moving mass. Just like a ball rolling down a hill has energy of movement, the wind, which is a gas and has mass, has energy of movement. A wind turbine is a device that converts the wind’s energy of movement into another kind of energy, usually electrical energy. The amount of energy available to be captured from the wind at any given moment is based on several factors including the speed of the wind and the area over which you’re going to try to catch it. Intuitively, one can imagine that higher wind speed means more wind energy and larger catchment area means more energy. But how much? The single most important factor is wind speed because the maximum energy available in the wind is proportional to the cube of the wind speed. This means that if the wind speed increases by a factor of 2, the energy in the wind increases by a factor of 8! More than any other factor, we want a wind turbine to see the highest wind speeds possible. The next important factor is area – a large wind turbine blade span (the diameter of the circle created by rotating turbine blades) will capture more wind energy than a small blade span. Here the available energy varies with the square of blade span. So a turbine having a blade span two times another will capture four times the energy.

For any spot on your property, there will be more wind higher up off the ground than down close to the ground. This effect is called ‘wind shear’ and is important in wind site planning. Since we want to maximize the wind speed seen by a wind turbine, we should put it as high as possible. If you’ve got a project in mind and have some money left over you may be considering spending that extra money on a larger turbine. But wait!! Remember, wind speed increases with height, and power increases with the cube of wind speed. You’ll be better off putting the same turbine on a taller tower so as to capture more wind speed than putting a larger turbine on a shorter tower. Towers for wind turbines come in many different styles and price tags and everyone has their own preference. When it comes to power production, though, there is only one factor and that’s height. Economically, your incremental return on investment (power output vs cost) for a taller tower will be higher than that of the system as a whole.

So how much wind is enough? Is a wind turbine even worth considering? The answer to that depends on why you’re considering a wind turbine in the first place. If your objectives are purely economic, then you’ll need to verify that your site has a sufficiently high average annual wind speed. (Of course there are other factors, but wind speed is the first to look at.) Very roughly speaking, look for an annual average wind speed of at least 16km/h at the height where a wind turbine would be installed. That’s actually quite a bit of wind… 16km/h including all the storms and all the doldrums. Commercial wind farms require higher average wind speeds than 16km/h to be viable. If your objective is to combine wind and solar for an off-grid home, the rules are very different. Wind and solar tend to complement each other. This means that when the sun is shining the wind tends to be still and when the wind is howling it tends to be cloudy. Additionally, it tends to be windier in the winter, when there’s less sunlight and there’s more power in the wind when the temperature is lower. In this situation it may be sensible to install a wind turbine even if your average annual wind speed is not very high because the net result, combined with the solar system, will be more reliable energy overall.

In this article I’ve only just scratched the surface on wind system selection. However, it should give you a starting point for evaluating priorities. There are many excellent resources on wind turbines and wind systems. We recommend Paul Gipe’s book ‘Wind Power for Home and Business’ as well as Home Power Magazine, in general, and their periodic ‘Apples and Oranges’ wind turbine comparison.

Printable version (this function opens a new window)