1. Originally Posted by diarmaid
Wow, that's a very interesting idea. Im wondering was it a big windmill to drive the compressor efficiently, or how big was the tank?
Big! 5000 litres or something like that. The size you see driving around with the label Carbon Dioxide refrigerated liquid.

I did some Googling:

http://www.iop.org/EJ/abstract/0305-4624/12/3/I05

http://www.fieldlines.com/story/2006/3/28/7320/18629

This one also mentions using the compressed air for cooling;
http://www.worldchanging.com/archives/000486.html

I also found this at;

http://www.greencarcongress.com/2006...en_engine.html

I do not know how reliable the efficiency figures are, they may be a bit optimistic.

The energy efficiency in conversion from electricity to hydrogen is 70-80%, let's say 75%. Then, the energy efficiency of hydrogen-ICE is 45% at most, then the efficiency of the generator powered by the hydrogen-ICE is ~90%. Multiply all together: .75 x .45 x .90 = .30, or only 30%. So, the hydrogen-ICE-genset route is only 30% efficient.
However, if you would use compressed air at 5000 psi (~300 bars) as energy storage medium, then assuming 90% efficiency for the air compressor that is directly connected to the wind turbine, and 90% efficiency for the air motor running off the compressed air, and 90% efficiency for the generator running off the air motor, then you will have: .9 x .9 x .9 = .73, or 73% efficiency, much better than the hydrogen route. Of course, you will need a high-capacity heat exchanger during the compression process to take away the heat that is released from adiabatic compression (to achieve quasi isothermal compression) and to obtain the heat from the ambien during the adiabatic expansion process by the airmotor (to achieve near isothermal expansion). If you have good insulation for your compressed air tanks and do not wish to store energy for long, then you do not need too good a heat exchanger, just enough cooling to prevent thermo-structural damage to your air tanks. The "aircar" (theaircar.com) is rather ridiculous in mobile application due to the large bulk of the compressed air tank and the need for large-size heat exchanger to realize near-isothermal expansion to maximize the car's range, but for a stationary energy storage medium where bulk and weight would be more tolerable, I think that this would be a very viable idea. Unlike battery storage medium, which has about comparable efficiency at ~70-80% with comparable bulk and weight, the steel air storage tanks are cheaper to make with less environmental impact and will last practically forever, while the battery will wear out after several hundred charging cycles or after only a few year's time.
I wonder what would be the reason why compressed air storage means not adapted to store excess wind turbine production? Or for storing excess electrical output from your home PV panels?

2. A bit off track but there are a few compressed air concept cars about now.
http://www.theaircar.com/

3. Originally Posted by diarmaid
Thanks for answering my questions epineh, I hadn't thought about having disconnects in series but definately a must do with any system above 12V. Are there any specific reasons why you don't want to go with bty's apart from what you have already alluded to? Although I dont really have a choice, its always good to know the 'cons' of a system aswell.
First of all, sry for the late reply, been away with work. Didn't really have a huge issue with batteries, mainly the amount of space, safety (got 3 kids). There is the whole other side of the eco-friendly argument, introducing heavy metals and acids into the home and the manufacturing processes involved in making the batteries in the first place blah blah blah...

Unfortunately, they are a neccesary evil for the home power producer, I just would like to create or use a more elegant solution (its on my to do list).

Plus they generally only have a five year life span, the more you have, the more it costs at replacement time.

Beats the hell outta me what the alternatives could be though, more things to study...

Russell.

4. Originally Posted by epineh
mainly the amount of space, safety (got 3 kids)......

Plus they generally only have a five year life span, the more you have, the more it costs at replacement time.

Beats the hell outta me what the alternatives could be though, more things to study...
Russell.
Thanks for the reply, I am also concerned about kids ( Dont have any yet, but sure one will pop eventually! ), but I figured I'll build a wooden lockable enclosure.

On the OtherPower.com site they recommended using forklift bty's, which is where I got the idea from. One guy said he's been using the same forklift bty now for 28yrs and still going, while he feels sorry for his friends changing their car btys every few years!

• Originally Posted by epineh
.... There is the whole other side of the eco-friendly argument, introducing heavy metals and acids into the home and the manufacturing processes involved in making the batteries in the first place blah blah blah...

Unfortunately, they are a neccesary evil for the home power producer, I just would like to create or use a more elegant solution (its on my to do list).

Plus they generally only have a five year life span, the more you have, the more it costs at replacement time.

Beats the hell outta me what the alternatives could be though, more things to study...
Russell.
According to that comment I copied into my post a short distance above compressed air energy storage is comparable to batteries, however I do think the efficiencies mentioned are too optimistic. If you are lucky enough to be in a location where you have hydro based on storage rather than run-of-river then pumped storage is probably the most efficient.

Another energy storage method that is certainly better than batteries in terms of energy density is kinetic storage in an ultra high speed flywheel running in a vacuum. I haven't looked for any recent articles about applying this to storing excess wind energy but there are groups working on applying it to very high capacity uninterruptible power systems where the flywheel stores enough energy to maintain supply for several minutes while a big genset is started and brought up to load.

• Dear diarmaid,

Step number one is to get some idea of how much wind energy is available at your site. Try and find out what the mean annual windspeed is at a meteorological station near to you. They usually measure at a height of ten metres above ground over open country.

Once you have this information, look at this Danish website

http://www.windpower.org/composite-85.htm

It has all the information you will need to estimate how much electricity you may generate, and is an absolutely brilliant. Do not be put off by the maths.

Best wishes

Martin

• Great site martinw.

• This project is steadily moving itself up my 'to do' list, due to a lot of power out's. Must be due to winter coming in....power lines falling and all that sort of thing. A small temporary one would be a good way to learn the basics. I already have a 300w 12V to 220V inverter and can carve blades easy enough. All I need is a spare 12V bty, regulator, and an alternator.

Ok so....whats the best wood to carve the blades out of?

I think I read somewhere that mahogany is good, but anyone have any ideas. Im thinking light but strong. Which mahogany is not, but is!

• So....whats the best type of wood for turbine blades?

• How about fabricating a mahogany box, then carving ?

John

• Why not use high density polyurethane foam and then skin them with fiberglas and resin. These blades are not going to be rotating at high rpm and I would think you need is something light with low rotational inertia.

• Yeah foam and resin is certainly a good idea. It was mentioned in the cnc wind turbine thread but wasn't used in the end. I just think it'd be handier to use wood for a first small and temporary turbine since I have all the necessary tools. But as a small temporary one it'll only be about 30inch diameter so the weight is a major factor to allow it to turn, and I think mahogany would be fairly heavy. What about balsa wood, would it be strong enough?

• Page 4 of 6 First 123456 Last