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The Renewable Mix
Jamais Cascio, 12 May 05

It's not hard to find people arguing against the greater use of renewable energy sources by making the following kind of argument:

(Renewable Technology X) can't replace our current energy use because of (Insert Well-Known Reason), so it's pointless to pursue its development.

The Well-Known Reasons often boil down to what the industry calls "intermittency" -- the inability of the given energy technology to provide reliable amounts of power. And, constructed as that straw man, it's true: photovoltaics are ineffective at night; wind turbines are non-productive in still air. But as we've said here time and again, nobody is talking about replacing the entire energy infrastructure with a single production source. What will allow renewable energy to succeed is a distributed mix of a variety of sources connected via a smart power grid.

Now researchers at the Environmental Change Institute at Oxford University are arguing the same thing.

[Dr. Graham] Sinden initially looked at just three generation technologies: wind, solar and dCHP [domestic combined heat and power] — in effect, hi-tech domestic boilers, which produce electricity as they heat water. He ran computer models of power output based on weather records going back up to 35 years, and found that electricity production could be optimised by creating a mixture of 65% wind, 25% dCHP, and 10% solar cells. The high proportion of wind is because the wind blows hardest in the winter, and in the evening — when demand is highest. The dCHP also produces more at peak times, when demand for hot water and heating is also strongest. Solar makes a smaller contribution, and produces nothing at night. But it is still important to have it in the mix as it kicks in when wind and dCHP production is lowest.
It is also essential to disperse the generators, whether wind turbines or rooftop solar cells, as widely as possible. By increasing the separation between sites, you can be sure that power is always being generated somewhere and so smooth out the supply curve.[...]

Putting these figures together with estimates of Britain's available renewable resources, wind (onshore and offshore) could realistically provide some 35% of the UK's electricity, marine and dCHP each 10-15%, and solar cells 5-10%. In other words, more than half the UK's electricity could ultimately derive from intermittent renewables.

This contrasts to current plans, which call for renewables to amount to about 20% of the UK's generation needs.

DCHP systems are particularly interesting -- increasingly common in Europe, much less so in the US. In a separate article about Sinden's argument, dCHPs are described thusly:

The dCHP takes the form of gas-fired domestic boilers which generate electricity as they warm water using a Stirling engine, producing 1 kilowatt of electricity for every 8 kilowatts of heat. So dCHP produces most electricity when demand for hot water and central heating is greatest, neatly coinciding with peak demand for electricity.

(We posted about a commercially-available Stirling engine dCHP back in December of 2003.)

This, of course, only accounts for the production side of the equation. There are myriad examples of ways to improve the efficiency of buildings (phase-changing wax insulation, anyone?), and increasingly sophisticated tools for monitoring and reducing power consumption (I'll be writing about a new one in the coming days). Amory Lovins makes the point again and again -- efficient use is even more important than efficient production.

Mixed/Distributed Renewables + High Efficiency Living = The Bright Green Future.

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Comments

I would encourage you to take a good look at http://www.generadoreolicowm.com which is a vertical axis wind generator that's apparently capable of working at slow wind speeds as well as at large wind speeds (slow and large relative to horizontal axis) - all while using standard building material as none of the pieces are large or require special strength.

There's a couple of animations (3D and from above), and I feel both are needed to show the main concept: don't fight the wind when you come back to the starting position. Just the concept is worth a visit!

The inventor is currently looking for funding to start production. Can anyone here contact him if interested?


Posted by: Lucas Gonzalez on 12 May 05

Thanks for this Jamais. Was there anything in the report about the potential for really serious effciency improvements, or was it focused on shifting supply?


Posted by: David Foley on 12 May 05

The power in a flow of a fluid (such as air) moving at speed v is 1/2Aρv^3.  Theoretically, you can extract about 59% of this energy with an ideal turbine.  It doesn't matter if the axis is vertical or horizontal, the v^3 factor means that there just isn't that much energy in low-speed flows of air; you can't capture what isn't there.

The GEWM device is going to be considerably worse; it appears to rely on drag rather than lift, and lots of air is going to leak through those partially-open vanes.

The inventor is looking for investors, because the smart money is elsewhere.


Posted by: Engineer-Poet on 12 May 05

Not an expert myself, and never tried it. He claims it could be used with high speeds too, without having to stop it when the wind is too fast. I don't know how difficult it is to actually try these things in real life.

On an aside, being an ignorant myself, I wonder about all the "efficiency" issue. If, hypothetically, I could extract a low percentage of a slow wind, but the machine cost were next to zero and it were easy to build with fablabs - wouldn't we want it? I really don't know - that's why I ask. :-?


Posted by: Lucas Gonzalez on 13 May 05

David, I couldn't find a link to the core research paper, only reports about it -- but it does appear that the focus was on the generation side of the equation.


Posted by: Jamais Cascio on 13 May 05

Just for a lark, suppose that you've got a 2 meter by 2 meter zone for a wind generator and the wind is blowing gently at 10 km/hr (2.78 m/sec).  If the air density is 1.29 kg/m^3, you'd have .5 * 4 m^2 * 1.29 kg/m^3 * (2.78 m/sec)^3 = 55.3 watts of power in the stream.  If you can recover half of that, you'd get 27.6 watts out.

Unless your investment in materials and maintenance is very low, harvesting power from slow winds just isn't worth it.


Posted by: Engineer-Poet on 13 May 05

Thanks! So I guess what's needed is a "wind collector" that's good at "medium" speeds and also at "high" speeds. A bit less than "medium" and then it has to be quite unexpensive. A lot less than "medium" and it has to be almost zero-cost.

I wonder if living systems can use very slow winds for something useful. Perhaps with really big "wings"? Perhaps the slow wind would be useful not because it has energy but because it ... I don't know ... provides information about something? Ok, probably there's nothing there - or is it?


Posted by: Lucas Gonzalez on 13 May 05

The best use for slow wind is in a ram scoop fed ac unit for your home. You force the air underground where it cools then up through your home and out through roof vents. Even fairly slow wind can be used to cool in such a setup. You can also use such a system to heat if you pass the air through a solar hotbox setup.


Posted by: wintermane on 13 May 05



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