[Gasification] solar collector

[Daniel Chisholm]

On Mon, 2007-05-02 at 20:34 -0500, GFWHELL at aol.com wrote:
> I believe I could build an ORC system using this form of heat  collection.
> Using say a water cooled refrigeration condenser to capture the heat and  
> using R 22 as the fluid to push a scroll compressor, what are my  chances?

I assume you want to use an off-the-shelf refrigeration compressor,
using its built-in induction motor as an induction generator?

Here are my thoughts:

* You won't want to run a head pressure more than what you would get
when using it as an air conditioner compressor - say 250-300psi.

* Without bothering to do further engineering, it would be OK to
superheat the vapour to the same level that it would be when operating
as a compressor.  Off the top of my 250F or so pop into mind.

* You would be running the R22 flow in the opposite direction that it
was designed to do.  For a scroll compressor, that should be OK.  The
three issues I can think of are motor rotation, cooling, and
lubrication.

* You would be reversing the direction of rotation of the motor.  For a
3-phase motor this is no problem, just swap two of the wires.  But for
the more likely case of a single phase motor, you will have to see if it
is possible to reverse its direction of rotation.  If you have a junk
unit to cut apart and look at the motor, it might be a lot easier to
determine this.

* Usually the motor is positioned in the incoming (cool) refrigerant gas
side.  It still will be, however the temperature of the cool side gas
will be somewhat higher than when it is operating in compressor mode.
This may or may not be a problem.

* Refrigeration compressors are designed to have lube oil in their sump,
and have it carried throughout the system by the refrigerant.  In
reversing the direction of the refrigerant flow, you want to make sure
that the important parts (the motor bearings) still will receive lube
flow.  Also, you want to make sure you don't expose the oil to more high
heat than it would see in compressor service.

I think it sounds like an interesting project to try.  You have to
understand in advance the sort of efficiency you can expect since you'll
be operating with a fairly low hot side temperature.  Assuming your cold
side (condenser) is 25C, the Carnot limit efficiency is 24.3% - which is
the theoretically-limited maximum possible efficiency.  You might
realistically expect to get somewhere between 10% (easy) and 15% (really
tough) thermal efficiency.

FWIW, instead of having to add a feedwater pump, you can use a much
simpler device to return the condensed refrigerant to the boiler.  WHile
it is an utterly brilliant idea, I can't claim any credit for it at all,
since it is what the old steam locomotives used.  You simply gravity
feed your condensate from your condenser into a pipe connected to your
boiler, with two isolating valves (to configure it like an airlock).
You open the top one to let it fill with liquid, close the top one, then
open the bottom one to let the liquid drain (via gravity) into the
boiler.  No reason that these valves can't be solenoid-operated, and on
a automatic timer.




-- 
- Daniel
Fredericton, NB  Canada