Ok bear with me as this sounds a bit bizarre, especially for a first post. For a while I have been thinking of how to take advantage of the fact that a black passive solar collector can collect low temp BTU's at low costs, but due to the low temperature it can not be utilized in a steam cycle efficiently. My idea is to operate a steam cycle at a pressure low enough so that the passive solar collected heat is hot enough to boil the water. Since I believe I can produce around 140 deg F. passive solar heat I want to run the boiler system at around 3 psia and the condenser at around .5 psia. I recognize that at such low temperatures the system would be extremely inefficient, since ideal Carnot efficiencies are 1-Tc/Th. So the saturated steam produced by the passive collector would then be superheated using a conventional burner. Since I plan to run a small 1-5kW steam engine the superheat would be go to no more than 500 deg F. This should produce a dramatic increase in efficiency, while not requiring significant amounts of energy. Looking at enthalpies at 3psia for 140 deg and 500 deg steam I am thinking that roughly 85% of the energy can come from solar energy and 15% can come from some combustible fuel. The obvious difficulty is that this cycle would have inherently low energy densities at such low pressures. I did some calculations and concluded that for a 5kW steam engine at 100 rpm that I'd be looking at a piston larger than 12" in diameter. However since such low pressures are involved it need not be built nearly as rugged as a conventional engine. I was also thinking that the boiler danger would be greatly minimzed since the system is under vacuum and the amount of energy contained in the tube on a volumetric basis would be quite low, if the tube imploded it would probably not be deadly. So I am hoping for any feedback I can get on this idea? Is there something basic that I am missing that makes it impossible? Thanks in advance for the help.