Rotary Screw Engines for ORC Systems | Jordan

[voltech444]

I've been reading a lot about rotary screw compressors and using them as expanders in ORC systems. There are about 3-4 companies making these for ORC systems and they seem to be very reliable and efficient. They are much more tough than turbines; they can take dry steam ,wet steam, saturated steam, even hot water, and of course many different types of refrigerants. The other good thing is they operate at about 1/10 the speed of a turbine; they can also operate at much slower speeds although they get less efficient. I think 10k-20k rpm is the perfect speed because you can get high power to weight ratio's but still be able to work with those rpm's with conventional gears. Once you get up and over 100k rpm it gets very difficult to work with those kinds of speeds.

So what I was thinking is a gas turbine built with rotary screw compressors and expanders. Have the rotary screw compressor push the air into a combustor where fuel is injected and burned, these gases go into the rotary screw expander and expand the gas. The compressor and expander could be connected with gears or belts or chains.

Another setup I thought of would be to have a turbocharger converted into a gas turbine, this would act as the gas producer. The gas that comes out of the exhaust turbine could go into the rotary screw expander. The power shaft from the expander could be geared down or directly coupled to a alternator.

In the other post i wrote about injecting water into the afterburner of a gas turbine I was thinking to due that in this setup; inject water after the exhaust turbine which would flash into steam and expand through the expander. It would create more mass flow and keep the temperatures down inside the expander. I was also thinking that maybe the steam created from the water injection could capture the waste heat from combustion and use it to turn the expander.

What do you all think, is something like this even possible?
Thank you,
Jordan

 

[Baluncore]

where fuel is injected and burned, these gases go into the rotary screw expander and expand the gas.

If you lubricate your screw turbine with oil, it will vaporise/burn in the hot exhaust gas.
Without lubrication, how will you seal against leakage of the hot gasses?
Will there be abrasion of the meshed screws?
Will there be corrosion of the screw turbine materials?
The screws in the turbine are thermally isolated, how will you cool them.

 

[voltech444]

There are two types of rotary screw compressors; oil-free and oil spray or bath. I would think the oil free version would be best, it could be toleranced so that when it heats up it would have the correct tolerance. The rotors could be made or coated with silicon nitride which has great thermal properties and great friction properties. I've read on some other sources about pumping oil or coolant through the passages in the rotors to keep them cool. With enough injected water it may be possible to keep the rotary inlet temp down to about 400-500* F . Rotary compressors can deliver up to 100 psi in a single stage. higher efficiency and pressures are possible with a two stage system.

 

[Baluncore]

With enough injected water it may be possible to keep the rotary inlet temp down to about 400-500* F

Does such a low input temperature not cripple your thermodynamic efficiency? See; http://en.wikipedia.org/wiki/Heat_engine#Efficiency

Rotary compressors can deliver up to 100 psi in a single stage.

To produce 100 psi they have oil bath or spray to seal the clearance between the screws. Without oil, but with clearances sufficient to allow for thermal expansion, the hot gasses would pass through the screw clearance.
Every new internal combustion engine ever designed has presented problems with sealing the hot products of combustion.