Do not attempt to build this engine.
It will not work.
Think conceptually: The boiler is high pressure, the condenser is low pressure. The resultant differential pressure between the boiler and condenser “forces” the stem through the engine. Unlike compressed air, the high heat capacity of steam maintains elevated pressures during work extraction.
Without a differential pressure, you simply cannot extract mechanical power from steam. Consider a traditional boiler design, where a 150 psig boiler feeds a vented condenser. The boiler creates a differential pressure across the steam engine of 150 psig. Again, the pressure is the driving force of work extraction. The high heat capacity of steam slows the drop in pressure during work extraction – which is why water is the medium of choice for power plants, and not air. In the proposed design, you don’t begin to make steam until you draw vacuum on the boiler, which means that the boiler and condenser are operating at nearly the same pressure. The differential pressure across the steam engine will be practically nonexistent, and so will the power extraction.
There is another critical design flaw, which Tin Falcon pointed out – specifically:
Saturated water (212 degrees F, atmospheric pressure) contains 180.12 BTU of energy per pound mass. If you port the saturated water into an evaporation chamber, and draw a vacuum on it equal to 5 psia, here’s what will happen: Approximately 27.7% of each pound of water that enters the evaporation chamber will flash into steam. The steam will have a temperature of 162.18 degrees F, and a specific volume of 73.525 cubic feet per pound-mass.
The remaining 72.3% of each pound of liquid water that enters the evaporation chamber will reduce in temperature to 162.18 degrees F, and will remain inside the chamber. This water must go somewhere, or you’ll fill up the chamber. As designed, the liquid water will be drawn through the steam engine, with no work output.
If you port the saturated water into an evaporation chamber, and draw a vacuum on it equal to 1 psia, here’s what will happen: Approximately 61.3% of each pound of water that enters the evaporation chamber will flash into steam. The steam will have a temperature of 101.69 degrees F, and aspecific volume of 333.49 cubic feet per pound-mass.
The remaining 38.7% of each pound of liquid water that enters the evaporation chamber will reduce in temperature to 101.69 degrees F, and will remain inside the chamber. Again, this water must go somewhere, or you’ll fill up the chamber. As designed, the liquid water will be drawn through the steam engine, with no work output.
Smithdoor, whoever told you that this engine “has been in use for over 100 years” told you a lie.
Additional design consideration: the available solar flux from the sun is about 1-kw per square meter (this is for a bright sunny day). If you assume a reasonable thermodynamic efficiency for the solar panel of 40%, and a reasonable 5% thermodynamic efficiency for a piston-steam engine operating at low temperatures, you’ll require just over 37 square meters of solar panels per one mechanical horsepower out (again, that's assuming a bright sunny day). Point being - there is a reason the solar industry is failing in a disastrous fashion. :fan: