Organic rankine cycle with propane gas as HTF.

[Salah 1]

Hi guys:
as I understand organic rankine cycle usually uses fluid with boiling temp less than water (below 100 celsius) with different heat sources such as waste heat, solar heat, geothermal heat...
Now I read sime articals about using propane gas which needs minus 42 degrees to start boiling...
In most articals it said that the evaporator temp is about 77 celsius and after that the high pressure gas ( in some cases about 30 bar)goes through expansion in the turbine resulting pressure drop (and making power) and after that the low pressure propane gas goes through condensor and goes out as liquid to be directed to the evaporator using pump to repeat the process.
My Q is how come the propane gas after turbine expansion goes through condensor with cooling water at 25 celsuis and goes out as liquid??
The propane needs either minus 42 degrees to be converted to liquid
Or the turbine outlet pressure is not atmosperic but much higher to match the condensor temp of 25 celsius for the propane gas to get converted to liquid state....
Thank you very much...

 

[georgeverghese]

Yes, turbine exit pressure will have to be much higher to enable C3 condensation.

For this reason, propane is not used as the fluid for ORC. Use butane or pentane or some other fluid with higher boiling point.

 

[goutam_freelance]

I have seen reference of use of Propane as ORC working fluid.

"An innovative solar-based Rankine-Brayton system for electricity generation was suggested by Mũnoz et al. [108]. Propane and R125 were found to be the most suitable fluids"

Reference:
A Critical Overview of Working Fluids in Organic Rankine, Supercritical Rankine, and Supercritical Brayton Cycles Under Various Heat Grade Sources:Abrar Sobhan Chowdhury, M Monjurul Ehsan



.https://pdf.sciencedirectassets.com...f085b040652525b5553&rr=8da0196578289135&cc=in

 

[LittleInch]

You need to determine what temperature and pressure you have at the turbine exit.

If it is more than 25C then it will cool down and become liquid, but you might need a compressor and cooling system ( which then reduces your power output) and it id difficult to get to a point where it is liquid at -42.

Ultimately its an energy equation, can you transfer more heat from your heat source than you release by the condensation process.

The higher the pressure you have at the base of the condenser, the higher the temperature and the less heat transfer you will get when you then heat it up and create more pressure from the now gaseous Propane.

 

[BrianPetersen]

Don't think of fluids as having fixed boiling temperatures. It has a variable boiling (or condensing) temperature which is a function of the pressure. So, the condensing temperature can be (let's say) 30 C, but the pressure is going to be a lot higher than atmospheric. https://www.engineeringtoolbox.com/propane-d_1423.html

The working fluids for Rankine cycles - a heat pump or air conditioner is a Rankine cycle machine! - have to be chosen based upon, among other things, what the high-side and low-side temperatures are going to be, and what the pressures are going to be at those temperatures. This is why there are so many refrigerants to choose from.

Propane is not too bad for an automotive air conditioner application (but it isn't used for that, because it's highly flammable/explosive in case it leaks out - which it could, if the vehicle is involved in a collision). Low side temperature about 0 C and 5-ish bar (= atmospheres) pressure, high side temperature about 60 - 70 C and 25-ish bar pressure, and the high side operation is below the critical point, though not by much. For a waste-heat-recovery application, you could for example select a high-side temperature and pressure in that range, or maybe as high as the critical point (96 C and 42 bar), and a low-side of let's say 35 C and 10 bar. Or lower temperature and pressure if you can get it, but that's going to depend upon local conditions at the time and place of operation.

It will not be very good in, let's say, the main stage of a utility power generation machine, where the high-side temperature ideally is going to be much, much higher than that. Water works pretty well for those.

 

[Salah 1]

Don't think of fluids as having fixed boiling temperatures. It has a variable boiling (or condensing) temperature which is a function of the pressure. So, the condensing temperature can be (let's say) 30 C, but the pressure is going to be a lot higher than atmospheric. https://www.engineeringtoolbox.com/propane-d_1423.html

The working fluids for Rankine cycles - a heat pump or air conditioner is a Rankine cycle machine! - have to be chosen based upon, among other things, what the high-side and low-side temperatures are going to be, and what the pressures are going to be at those temperatures. This is why there are so many refrigerants to choose from.

Propane is not too bad for an automotive air conditioner application (but it isn't used for that, because it's highly flammable/explosive in case it leaks out - which it could, if the vehicle is involved in a collision). Low side temperature about 0 C and 5-ish bar (= atmospheres) pressure, high side temperature about 60 - 70 C and 25-ish bar pressure, and the high side operation is below the critical point, though not by much. For a waste-heat-recovery application, you could for example select a high-side temperature and pressure in that range, or maybe as high as the critical point (96 C and 42 bar), and a low-side of let's say 35 C and 10 bar. Or lower temperature and pressure if you can get it, but that's going to depend upon local conditions at the time and place of operation.

It will not be very good in, let's say, the main stage of a utility power generation machine, where the high-side temperature ideally is going to be much, much higher than that. Water works pretty well for those.

Thank you very much guys!!
this information is what I was looking for about propane organic Rankine cycle.

 

[georgeverghese]

Note that design pressure for the high temp side of a propane based ORC is much higher than for many other fluids you could use for the same temp. See V-L tables for saturated propane. The higher the operating temp of the evaporator, the less is the waste heat recovered.