Very interesting project, hope it works out.   I would suggest though that instead of keeping less serious contributors out by permission you instead do things by allowing everyone, but then having another section that is write only to the serious contributors.  More welcoming for people who may become serious contributors.  Or more welcoming yet allow anyone to edit anything and then filter the additions into the right places, as wikipedia does.

Anyway, I am skeptical about the idea of a "scalable" design.  Of course most such designs can be scaled a bit, but I think a better approach, which may also be easier since it breaks the problem down some, would be to produce a set of designs.  Because scaling or customizing such a machine will ultimately require some of what is, in fact, redesign.  If anyone, even people who have no expertise are to make the machines that are safe and will work as well as they followed the build instructions, then they have to be able to use the design as-is.  We need a library of designs, not these hypothetical "scalable" designs.

If this sort of thing caught on, sets of approved designs would probably be what evolved anyway, as the tested designs were copied by people who were hesitant to try to scale things.

Anyway, I have a suggestion to drop on an expander which, after having looked at a lot of different expander types (claw compressor, screw compressor, roots blower and gear pumps, gearrotor, globoid expander, gear pump, there was one very clever one from a pump of the type which is for pumping sewage and concrete but I forget what it is called, it uses a universal joint and a sort of helical shaped mass moving within another mass that is almost the negative shape but not quite, the various turbine systems, diaphragms, piston engines of various types like the normal crankshaft and nutating disk types flexible shaft kind ("the green steam engine") and totally linear kind, wankel/rotary, and various cockamamie schemes that I thought up from scratch, etc.),  I think there are two expanders that really stand out:

It you want electricity only, the entirely linear one combined with a linear alternator looks interesting.  Easy to make, very few moving parts, can have a very high expansion ratio, etc.

The other one is the modified claw compressor kind.  For clarification see the wikipedia article for claw compressor.  Anyway modifying it for use as an expander for high temperature steam:  Instead of a claw, it can be a circular mass at the end of the male rotor tips.  So it is more of a club than a claw.  A cylindrical prism basically attached at the end of a rod of square or rectangular cross section.  The only purpose of this is to increase the expansion ratio - the claw shape implies a dead space at top dead center otherwise.

  Have external gears that synchronize the two rotors so they never touch.  This is done on oilfree compressors and gearrotor expanders already.   The gears are lubricated with normal lubricant because they are cold.  The hot expander requires no lube because there is not metal to metal contact.  This eliminates the oil. (BTW the return pump could operate in a similar way but a rearrotor pump might be more appropriate.)

Okay, so you see the point is that the club part (rather than claw) is acting as the piston head, with the cylinder replaced by the other rotor.  Then you need a way to inject and exhaust the steam, obviously.  It could be injected/exhausted through a hole in the face of the casing, as with a compressor.   The interesting things about this sort of expander being the smaller number of parts, the inertia required to reverse the piston motion is not a problem so it can operate at higher RPMs, potentially much lower wear and therefore maintenance because the cold synchronizing gears and the rotor bearings are the only part where there is any contact, and they can be at low temperatures last a very long time when appropriately lubricated (and consume a lot less lubricant).  There is only 2 surfaces that bear high loads, the rotor bearings.  No piston rings needed since there is no wear in that area.

It looks a lot simpler to make than a reciprocating piston type, less alignment and little fiddly bits to make maybe. 

Hi Mark and folks,

Hi Mark,