New type of solar/wind powered generator

[Jandell]

Carl Johnson at http://mb-soft.com/public3/electzzz.html seems to have an effective and simple way of generating electricity, his invention basically uses low-pressure air/water to push a large piston.  The forces involved can be large, for example if wind is used at 800Pa (80kg/m^2) you can get 400W if a 1m^2 piston takes 2 seconds to travel 1 metre.  The machine can work off wind, flowing water, heated air or any other low-pressure fluid or gas.

The most interesting possibility he talks about is that his machine can very effectively capture solar energy, which is about 5kw/m^2 on a sunny day.  He suggests building a greenhouse to capture the heat, which can then be pumped through his machine, and after all the losses are considered the efficiency is about 66%.  He says he has built and tested a small version and it worked as expected.

The only problem is that if you're using low-pressure air you need huge amounts of it to make a decent amount of electricity, he says about 3m^3/second for 2.4kW.  I'm having alot of trouble finding out how much energy you get from say 1m^3 of pressurised air expanding to 1.1m^3 at atmospheric pressure, hopefully someone who know their physics can help me here.  Im guessing it's approximately P x DeltaV / 2?  If so the machine seems to be weaker than Carl suggests, but he's a nuclear physicist and he obviously knows his maths better than I do.

He explains how his machine works but he hasn't built a full-scale version of his invention yet or explained exactly how you would pump so much air through the machine, I've drawn a plan of what I think might work.

Any comments would be welcome, e.g. how to make it more efficient or how to properly calculate the energy generated.

Operation (see drawing):

1) The 400kg piston (1m2) lifts 1m, drawing in 1m3 of air.
2) The 400kg piston drops, pumping the air through the greenhouse (H, say volume of 10m3), lifting the 395kg piston (with area 1m2) by 1m
3) the air is heated by the greenhouse (say by 10 C), expands (by say 0.07m3), and lifts the 395 kg piston another 7cm
4) The 395kg piston drops and the process repeats

Using P1V1/T1 = P2V2/T2 it seems that a temperature difference of 20 C will produce the 0.07 m3 increase in volume.
Total energy gain is -400x10x1 + 395x10x1.07 = 226J or 226W if it happens once/second.  Does anyone know if this is useful/accurate? 

It's simple to design/understand/build and produces the energy continuously, day and night, as the heat from the greenhouse can easily be stored in thermal mass.

[Jandell]

Looks like Carl doesn't know his maths better than I do after all :], I'm really jk.  He seems to have made a small mistake and overestimated how much power you can get from heated air. 

For 2.5kW the machine will have to use 40m3/s of air heated by 10C, which is impractical.  If however the air was heated 50C on a hot day/with insulation you could get about 1.6kW from it per m^3 which would be perfect for anyone who could afford to build a proper plant.  If megacorporations didn't mind going broke it could replace every other power plant.

Carl's invention also works very well using water, for example he says on his site that it runs at about 94% efficiency on a head of just 2-3 feet.  As the inventor explains, current hydropower schemes use turbines which need hundreds of feet of pressure to produce water at less than about 25% efficiency iirc, they could produce nothing at all from a 2 foot drop.  Water is about 1000 times heavier than air and so small quantities can produce very useful power.  A 2 foot drop at 1m^3/s would produce about 5.9kW of power, and even if losses amounted to say 30% thats still about 4.1kW of useable energy.  It might be something to think about if you have a steady river nearby.

Currently I'm building a small wind powered version of the machine, I'll post the results and plans here once it's finished.

Also I made a mistake with the first post, because the 395kg piston is squashing the heated air the volume will be decreased and it would have only produced less than 68J/cycle.

[TJBrooks]

Have been perusing C Johnson's solar generator and then found this conversation, and would like to know how large a greenhouse would have to be to produce 50 kWh?