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,

NOTE: I had posted this elsewhere, but reposted it here as it might be relevant to some concerns here.

As I understand it, OSE is still working on a piston steam engine
design. I tend to agree that a piston expander is the way to go for its
relative ease in fabrication. Unfortunately (at least from my
perspective) the OSE guys seem to be emphasizing high thermal
efficiency, and things start to get really complicated when one tries to
get high thermal efficiency in a small steam system. So, I'm just
throwing out my concerns for anyone who might find interest.

If
biomass is the fuel source, and the system is used on a residential
scale or any setting where the waste heat can be put to use, then it
seems preferable to sacrifice high thermal efficiency for higher
condenser temperature in order to make the most of the heat available
from the system. There are so many ways to use this heat that can
displace electricity otherwise used. A few examples beyond the obvious
like space heating and water heating include water pasteurization, space
cooling, biomass fuel drying, air circulation in the home by
convection, clothes drying, and food drying. In particular, I think
there is promise in a desiccant evaporative cooling system. Here the
heat from a high temperature steam condenser can be used to boil the
water out of the desiccant for regeneration, and the steam generated
from this process can be used again in other heating applications. The
distilled water that results is used for evaporative cooling. There are
many other possibilities. Basically, I think an emphasis on high thermal
efficiency could be counterproductive for some settings. Actually, I
prefer a configuration where electricity use is minimized to the point
where a micro heat engine is really unnecessary. Let photovoltaics and
wind turbines do this. A micro heat engine might be useful as a backup
only. However, a biomass furnace can provide the bulk of energy required
for a home as a source of heat for the aforementioned applications.

With
respect to a remote solar heat engine where putting waste heat to full
use might be impractical, then I can understand a focus on higher
thermal efficiency. However, even here it can be impractical. After all,
since the solar "fuel" is free, then the efficiency of a solar power
system is a four letter word: cost. Sacrificing some thermal efficiency
is justified if it can get the overall costs down. I have yet to see a
mention of the following configuration on the OSE forums, so I will
mention it here. I wonder if using conventional double acting, two
cylinder compound, piston steam engines (with slide valves) will yield
good results by integrating a heat transfer fluid (such as a thermal
oil) and a bottoming cycling using an organic working fluid. In this
configuration steam could be generated with thermal oil at a temperature
that allows for circulating lubricating oil with the steam/condensate.
The exhaust from the first compound engine can be kept at a high
saturation temperature to transfer its heat to an organic working fluid
selected to keep the same pressures as the steam engine at these lower
temperatures (NOTE: see saturation conditions for butane - it might be a
good candidate). Sure, there are some disadvantages here, but the
advantages include not having to maintain a high vacuum in a steam
condenser, and being able to use conventional steam engine systems with
little modification. There would be additional pumping losses required
for an ORC, but perhaps the higher pressure and smaller cylinders can
reduce friction and thermal losses for this to be a wash. There would
also be losses involved in the transfer of heat from the steam
to the organic fluid, but my research suggests that surprisingly small
copper heat exchangers can see very high heat transfer rates when
saturated vapor/condensate compounds with high density are used, and with little temperature difference. Anyway, if the
OSE guys are having difficulties in developing a piston steam engine
with high thermal efficiency, then just toss this idea around.

NOTE: This configuration would also show minimal superheat on the steam
side, and the pressures could be limited to 500 psig or so. This would
be a lot safer than many other approaches... especially in a DIY
setting.