what about small wind turbines? They are more expensive in terms of each unit of energy that they can produce. But they have a role to play, especially for electrification of remote areas.
Windpower can supply a house with electricity. I know this because my own home is powered by the wind, and has been for over twenty years. I live in a windy location on the coast in Scotland, and I use batteries to store electrical power until I need it. In the beginning, I was very
happy to have one or two little 12 volt lamps. Now our home has all modern conveniences, including TV, computer, etc… all powered by the wind. My neighbours use windpower too.
micro-hydro, and never need to use batteries again. But there is no stream of water – just plenty of wind.
Solar systems will work almost anywhere. Photovoltaic (PV) panels simply need to see the sun and they work. Windmills are not so easily satisfied. In most places there is not enough wind to make windmills worth having. Windmills need to be on hills, open plains or coasts where the wind is stronger and more regular. They need to be on tall towers to avoid the effects of trees and buildngs. Then windpower can be much
cheaper than solar power, and it also works at night! Here on the Scottish coast it is cloudy and windy, so we harness the wind.
Wind and solar work well together (hybrid system). When there is no sun, it will often be windy, and so windmills can make a good back-up for solar systems. When there is sun, PV charges the batteries the same amount every day. In windy weather there is often a big surplus of energy. Windy weather can be a good time to ‘equalise’ the batteries by deliberate overcharging. On the other hand, here in Scotland I have started to use PVs as a back-up for my windmill. We run short of wind power in the summer when the sun is strongest.
Hybrid wind and solar systems can function with smaller batteries than sun or wind alone. Other sources can be used in hybrid systems: hydro, biomass, even diesel power can be a good choice if necessary.
ones. But it has taken time to develop the simple, rugged design. In recent years I have had opportunities to transfer this technology to the developing world. Quality is important, but high technology is not necessary. Small windmill production could be a perfect example of Schumacher’s principles: a local solution to a local need, bringing power to the people.
Cost of small wind turbines is not proportional to size, as is the case with an array of PV panels.
Larger units are more cost-effective in terms of electricity production. But smaller units are more affordable, and much more popular. Where a single family is concerned, a small unit is appropriate. Where village electrification is concerned, larger units, combined with PV, can offer better economics. In all cases, the results will depend on choosing a windy site.
circumstances, the output will depend on two things: wind speed and rotor size. A windmill with larger rotor blades will produce more power on average than a windmill with a smaller rotor. The average power determines the monthly energy output.
For example, consider two windmills which cost the same to buy. In one case the rotor diameter is 1.2 metres, and the rated power is 400 watts. In the second case the rotor diameter is larger: 1.7 metres, but the maximum power is only 200 watts. Most customers will buy the first example, because they feel that it will give them twice as much power. But the rotor is so small that it takes a very strong wind (35 mph) to reach 400 watts. This will very seldom occur, in reality. Instead the second machine will produce more power, because the area of wind it sweeps is twice as large. (Area depends on the square of the diameter.) This windmill reaches its rated output of 200 watts at only 22 mph,
while the smaller, windmill is ony producing 100 watts. Most of the time, the larger windmill outperforms the smaller one. Nameplate rated power is not very important.
It is actually cheaper to erect a small windmill on a site and monitor its output over a year. The output can be recorded with an amphour meter or a data logger. Or we can connect a typical household to the system and see if it meets their needs. This is probably the best proof of success, and the family in question will be the best advertisement, when it comes to selling windmills to their neighbours.
As a rough guide we can say that a good site has average windspeed in the range 4-6 metres/second (9-13 mph). A windmill with a 2-metre diameter rotor will produce 40-120 watts average power output. There are 24 hours in a day, so this equates to 1-3kWhours/day on average. 1
kWh equates to charging up two car batteries, or providing 200 watts for lighting for 5 hours in the evening. A smaller windmill with 1.4 metre diameter would only give half this much power because its rotor area is half as large.
Windmills themselves are notoriously unreliable machines. Breakdowns have traditionally been the norm.
These problems can be overcome. Windpower can be very successful. The following factors will help:
- A windy site for the windmill is essential
- A reliable design with minimum of breakdowns
- Local know-how, for low cost manufacture and prompt repairs
- A backup supply for times when the wind is low (PV, diesel, or even kerosene lamps)
Small battery-charging windmills can be produced in village workshops, and used to produce electricity without environmental damage. That’s a job worth doing!