I brought up the ether thing because I’m interested in doing exactly what you’re talking about. Ether boils at 100F.
R-11 at about 75F, but using it is a no-no.
Some of the freons boil at well below zero:
R-12 at -22F.
R-13 at -115.
R-14 at -198.
Ammonia at -28F.
Machinery Handbook lists two alcohols: One at 150F, one at 173F. Don’t ask me; I dunno. But they don’t look like great candidates for mid-range solar heat. Acetone and lacquer thinner I have no dope on. Ether is the one that looks good to me.
One thing I keep harping on is that your system doesn’t have to be dazzlingly efficient if your heat-source is free. You only need to look at buckbang, which keeps the total problem manageable. You wanna chew this some more, come on back. I’m interested.
This is the idea I had. Up here in the great white north there are a lot of people with outdoor wood heaters to heat there home. They work great, keep the mess outside, produce an even heat in the home assuming the delivery system in the house is good, and so on. Only one problem. They don’t provide for any means to get to the heat if the grid goes down. So I was thinking of a way to tap some of that heat to produce some power, at least enough to power the pumps and fans of the unit itself. Then why not make a little more and size it so maybe you can cut your utility bill somewhat. So we have a supply of 180deg. water, constantly flowing from heat unit to house and back. Can we run that water through an engine on it’s way back out? Sure, but what kind. That is where I am right now. I started listening in on the stirling engine topic, as well as any other that might work. Any thoughts out there?
Well, fudge! If you were going on up into a steam system, you might be able to make everything work with steam pressure, but at 180F, the possibilities are more limited. Can’t very well mix these funny fluids with water, and tricky to keep them separated. Sleep on it.
There has been several messages on the subject of using low vapor point compounds as a means of generating power. While the concept of vaporizing a liquid to produce energy is not new, many on this forum seem to think that bringing the point of vaporization down so that low tech energy (solar heating) can make energy production practical. Of course – what is practical depends on individual circumstances. If you are a good back yard mechanic with free materials and time – almost any project can be practical. However, for the rest of us building such a machine would not.
You see – producing (mechanical) energy (to make electricity) efficiently isn’t a matter of a vaporization point, or of even the temperature per-si. What you need is to move as many BTU from the source to the sink as possible. The greater the differential, the greater the efficiency. And that’s the problem. Lowering the working temperature of the heat source while relying on ambient temperatures at the sink results in a low differential, a low BTU exchange, and low yield. You would have to compensate for this by construction a very large collection exchanger (your heat source) and an even larger heat sink. Like I have said – it isn’t that it can’t be done, it just how big of a power plant do you want to power your house?
Now, there are some ideas I have to increase the differential. Such ideas do make the machine more complicated, but would increase efficiency by combining cogeneration in the design. For instance – (for colder climates) on the heat source side one could preheat the fluid using solar then use a burner or stove to vaporize the fluid. Exchangers in the flue would increase efficiency. On the heat sink side the first heat yield would heat your water, then your home, then the rest would heat somewhere else. Vapor traps would increase efficiency further still. While such concepts are not off the shelf, they could be constructed and integrated into the structure.
Frankly – my plans are to build a power systems using an integrated, free-piston Stirling (linear) generator. It will likely be oil fired, but the heat byproduct will be mostly utilized in heating the water and the air space. The combustion gas heat gets recovered as does the sink side gets constructively used. IMHO, it is a more compact system of production.