Yes you need a temperature differential. In the reaction chamber you generate steam in a semi-confined space which raises the pressure there, hence the liquid would stop boiling if the temperature is not increased further, so a temperature difference is needed to maintain a pressure difference or "work up a head of steam".
People have used and are using lower boiling point liquids in "steam engines". A historical example is the "Naptha engine" or "Naptha launch" from the 1890s. More recently in general terms such an approach is termed an organic Rankine cycle (ORC) engine. Power can be generated at lower temperatures though there are also some potential drawbacks depending on the pressure volume characteristics of the working fluid. This approach is being used by large companies (Ormat technolgies, Turboden/Pratt and Whitney) for industrial scale waste heat electricity generation. Often some type of freon is used as the working fluid. Two interesting examples of groups looking at smaller scale applications are 1) Infinity Turbine which is building ORC engines as small as 1-12 kW. They have an interesting website which I haven't gone though entirely yet but would just give an excerpt from their write up on "magnetic coupling"
"Another interesting effect of the spinning magnet is that you can put a solid piece of copper on the outside, and it will heat up via inductive heating. So much so, that the copper will get red hot, and can melt solder.
The uses of a waste heat to energy turbine, such as the ITmini are vast.
In recent tests, we were able to run the ITmini in a ORC cycle, with only 115 F (40 C) of input hot water. ..."
Matteran Technology Matteran: Technology has also made what I suspect is a very significant advance in this area. By way of background, ORC engines and indeed steam electricity engines are closed cycle, the same working fluid is shuttled back and forth from vapor to liquid. An inefficiency in the Rankine cycle approach is that you need to return the liquid back to the reaction chamber which is by neccessity at high pressure. So the "boiler pump" has to fight against this high pressure when returning the liquid. Jeffery Sterling of Matteran Technology has recently patented a new system which replaces the boiler pump. You would need to look at the website but basically he uses a timed airlock approach to do away with the boiler pump. This approach should both make conventional steam generators somewhat more effecient and also allows for electricity production from temperatures as low as 150F. He also uses the Venturi effect to provide refrigeration at no additional input cost. Pretty cool!
To my mind another closed cycle engine that doesn't have a boiler pump is again the Stirling engine, though this has always been used as a hot air engine. So I still have the question of could you run a Stirling engine as something akin to an ORC or steam engine and would this be an improvement? It also seems to me that the work done in a steam engine or ORC engine is from the differences in pressure throughout the system. If you lowered the pressure of the whole system either closed loop steam engine or Stirling engine you should still (I am guessing) have the same relative pressure differences throughout the system and hence same work capacity, while introducing a partial vacuum would allow you to have water boiling at near room temperature, decreasing the input energy needed and preserving the advantageous environmental, safety and thermodynamic properties of using water as the working fluid. There have also been improvements to the standard steam engine approach recently especially in how to distribute heat to the working fluid in the reaction chamber. I guess I should shut up now and try and see if I can't build a Striling engine and/or closed loop steam engine. Thx Paul
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