I'm not a Hvac guy, but,

[eng1]

Typically, steam cycle (Rankine cycle) powerplants are used when you have a heat source that is hot enough to run a steam turbine. For example if you are burning coal or natural gas.

ORC's are typically used when the temperature source is too low to run a conventional steam cycle. They can also be used as a "bottoming" cycle where the discharge from a conventional steam turbine then cross exchanges with the inlet of the ORC. This is called a combined cycle power plant.

More commonly, you may hear of a combined cycle power plant when referring to a natural gas turbine that is "stacked" on a steam turbine (the discharge from the natural gas turbine heats water to create steam and run a steam turbine).

So they are meant for different applications. The use of ORC's is growing fairly rapidly since the most common geothermal resources in the world are relatively low temperature (not hot enough to run a steam cycle).

Fairly interesting stuff, although I would say that it really isn't a DIY project.

[supertek65]

i would not recommend that!

What you are describing is an ORC (Organic Rankine Cycle). It uses the same basic thermodynamic cycle as a steam powerplant (steam meaning water for the working fluid) although it uses a organic (meaning carbon in the molecule) working fluid. Some even use typical commercial refrigerants such as R134a. There have been special refrigerants that are designed specifically for ORC applications (R245FA for example).

The cycle efficiency is dependent on the temperature gradient that you have (how cold the cold sink, or condensor, is versus how hot the heat source, or evaporator is). In very basic terms it is a refrigeration cycle in reverse. If you have a "hot source" of about 200F and a cold source of 70F (typical outside air temp) then the cycle efficiency will be somewhere around 5-6%.

You read that right Five to Six percent efficient.

That means that in order to generate 20 HP, you need about twenty times that from your heat source. This also means that you have to reject about nineteen times that to the cold source (this means a fairly large condensor is required).

You could build one on your own, but it would be fairly costly and you really need to read up on it (and thermodynamics). I would suggest using a screw compressor, they have been used in some small applications as expanders. A centrifugal compressor would most likely need to be re-wheeled to be an expander (the blade angles are basically completely different, plus you need some kind of guide vane to guide the gas into the wheel). A screw compressor run backwards is much simpler.
If you really feel the need after reading this to learn more, I would suggest RefProp, which is a thermodynamic software that can be used to design the cycle.

Also, for those HVAC guys interested, Carrier built some small ORC's (~200 kW) using chiller parts. They are now built by UTC under the brand Pratt and Whitney (the power division). The marketing geniuses came up with the name "purecycle".

[eng1]

Screw expanders are not a new concept. Neither is an induction generator (or induction motor, they are the same thing).

Screw compressors built for refrigeration servicve have been used as expanders (so have re-wheeled centrifugals). See attached.

[freerider]

Kind of a neat one using a Carrier 19xr
http://www.yourownpower.com/Power/

[dave91169]

I would be curious as to what fuel is to be burned to operate this creature. You mentioned a 20 HP compressor; Onan's standard of 1 HP for every 500 watts of electricity means you're trying to come up with about 10 KW. If you're using propane, natural gas, gasoline, or diesel fuel as your source, just get a generator unit that runs on that type fuel and be done with it!
By the way, no form of "make-it-yourself" electricity can be done as economically as purchasing it from your local utility (one guy tried to run a major hotel on three natural-gas generators - worked well until the generators wore out LONG befure he broke even!). Solar is as close as it gets, except for the cost of the panels. Geothermal would be great if you were to own a geyser.
If you're doing this as an experiment/hobby/"see-if-I-can-do-it", good luck!
Dave E