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Thursday, December 17, 2009

Wind powering your house at $1 per Watt

The cost of residential, small size, wind generators has been decreasing quite a lot. Try searching for wind generators on Amazon.com and you will get results like these:



Should we conclude that you can wind power your house at the cost of about $1 per Watt?

If this was true, then we could power a small house with something like $2000, and the economic advantage of that would be evident to everybody.

Unfortunately, this is not the case. Here's why.




1. True cost of the wind generator

What you are buying on Amazon (or everywhere else) is not a whole wind generation system that you can set and forget. You are just buying the wind mill alone, that is a set of blades, an electrical generator, a controller, and usually some wiring and a pole.

Together with that you will need:
  • batteries (deep cycle, long lasting ones)
  • battery charge controller (sometimes it is included)
  • additional wiring and hardware
Moreover, if you want to use the power from those batteries to use your regular appliances, you need an inverter. If you're just plugging small appliances into it, it can be quite cheap. If you want to connect your wind generation system to the grid, it will be more expensive.


2. Real power rating vs nominal power rating


There is some confusion about the power rating that is advertised by sellers. When they say that a wind generator is rated for 1000 W (for example), it means that:
  • the maximum power that it will generate is 1000 W
  • it is going to generate 1000 W when a strong, constant wind is blowing, usually about 30 mph (13.4 m/s).
If the wind speed is higher than this, the generator will still generate 1000 W, reducing its efficiency. If the wind is even faster, mechanical protection of the wind generator will stop power production, to avoid damages.
A wind with a constant speed of more that 30mph is not that common, so you should't worry too much about not producing energy when it happens.

Let's see what happens when wind speed is lower than 30mph.
Unfortunately, power production does not scale linearly with the wind speed. In other words, a wind of 15mph will NOT give you half of the rated power.

A good approximation is that power production goes as the wind speed to the power of 3. In other words, if wind is blowing at 15 mph (which is max speed divided by 2), then power generation will be the max rated power divided by 8 (2 times 2 times 2). That is, in our example, 125W.

Therefore, no surprise when one of the reviewers on Amazon says that his 400W wind generator is giving just 50W with a moderate breeze of 15mph. That's perfectly right.


3. Maintenance

Keep browsing for wind generators and you will see that one of them, rated 600W, costs something like $2000. Why is that? Well, the main reason is that it comes with a 5 years warranty.

This is a strong hint: if they ask you to pay $2000 instead of $600 to have a 5-year warranty instead of the regular 1-year one, then they estimate that more that $1000 might be needed in maintenance in those 4 years!

This is not always true, of course, but don't be surprised if you have to spend some money in the 20 years that are needed for your wind mill to repay itself. Wind generators require little maintenance, not no maintenance.


Conclusions

Residential wind generation can be an extremely interesting choice as an investment in renewable energy. For sure it's a winning choice when you want to power an isolated building, far from the grid.

If instead you are connected to the regular electrical grid, then you should spend some time in doing some budgeting, sizing your system properly, and see if that's the best way to invest your money. It's usually a great way, just don't act before thinking.