Thread: Boiling a fluid to make an inverse water wheel?

Ok, how about this;

Submerge a waterwheel type structure with cups facing downwards on one side. Under the cups you have some kind of heating element. The fluid is methanol or some other low boiling fluid. A very small amount of heat from the element will produce bubbles, which rise and are caught in the cups, the wheel turns and can generate electricity or whatever.

I'm using this principle in another application, but thought it could be quite an efficient means of generating electricity from heat. If the meths is sitting at forty something degrees and boils around 50, it shouldn't take hardly any energy to tip it over. And since you're effectively generating force from gravity (the weight of the fluid on the other side of the wheel) you're kind of getting ten times your force back. ie producing one litre of bubbles will give you 9.8 Newtons of force (for water, 8ish for meths).
So (excuse my dodgy maths) 125 litres of bubbles in an hour would give you 1 kiloWatt hour?
One cola bottle per minute, doesn't sound very much and that much energy could easily be produced through concentrated solar. Slight risk of explosion...

How efficient do you think it would be, overall?

zreo, it won't work!

Why not?

I know it works in principle, I've tried it.
It's just a matter of how well...

Your math is indeed dodgy. If you generate 1 liter of gas and displace 1 liter of water, the buoyant force is 9.8 newtons. If that force is applied over a depth of 1 meter that's 9.8 newton-meters. 1 newton-meter is a watt second. So 125 liters of water displaced per hour gives you 125*9.8=1225 watt-seconds. There are 3600 watt seconds in a watt-hour, so you are at 0.34 watt-hours, not 1 kilowatt-hour.

Another problem is that the deeper you go, the more pressure you have to work against, so your boiling point will go up.

This principle is extremely common and used in almost any power plant.
The practical construction is different than what you present, but also a lot more efficient.
It is also used in (experimental, but promising) solar plants where mirrors are focussed on a tube with some kind of oil.

The volatility of the fluid is not really relevant: if the fluid can't absorb a lot of energy, it can't give a lot of energy to the mechanical conversion stuff. Besides, you give the impression that vaporising a fluid at low temperatures is somehow more energy efficient, but it's not. Vaporising fluids generally consumes quite a lot of energy.

Originally Posted by sugarandfat

Ok, how about this;

Submerge a waterwheel type structure with cups facing downwards on one side. Under the cups you have some kind of heating element. The fluid is methanol or some other low boiling fluid. A very small amount of heat from the element will produce bubbles, which rise and are caught in the cups, the wheel turns and can generate electricity or whatever.

I'm using this principle in another application, but thought it could be quite an efficient means of generating electricity from heat. If the meths is sitting at forty something degrees and boils around 50, it shouldn't take hardly any energy to tip it over.

Two words: Latent heat.

This is the energy it takes to make the phase transistion (in this case from liquid to gas). For methanol it is in the order of 1,100,000 Joules per kg.

The vapor density of methanol is .77 kg/m^3 or .00077 kg/liter.

Thus is will take 847 J to create 1 liter of gas or 105875 J to produce 125 liters.

A joule is a watt-sec so you will have to put in 29.4 watt-hours of energy to generate the .34 watt-hrs of energy you can get out of the system.

much more efficient to just boil the liquid and run a turbine. besides that alot of the vapor in your contraption will recondense on the way up.