How to control turbine in Organic Rankine Cycle system

[Lei]

I am currently working on turbine design for small scale ORC system to produce power from low-enthalpy heat source. In ORC, turbine inlet pressure and flow rate are controlled by pump, and turbine inlet temperature is determined by evaporator when the flow rate of refrigerant and enthalpy of heat source are constant.
During turbine operation, I am curious about how to control turbine outlet pressure (condenser inlet pressure) and rotational speed. Is the outlet pressure controlled by turbine, condenser or buffer tank? How to change turbine speed?

 

[Nidum]

If everything else is truly constant then turbine speed is determined by the load . Reducing load means speed increases . Increasing load means speed decreases .

I would be very surprised though if everything else did really remain constant . Generally in turbine systems any change results in everything else changing .

Can you post diagrams showing the practical arrangement of components and the thermodynamic cycle for this system ?

 

[anorlunda]

@Nidum is correct. You can't begin to talk about speed without first describing the load.

Exit pressure is not "controlled" it is minimized. The lower you can get the exit pressure, the better. For example in a steam turbine cycle like in power plants, a condenser operating at a vacuum pressure follows the turbine.

 

[Lei]

Hi Nidum and Anorlunda,

Thank you so much for your reply.

The thermal cycle of ORC is shown in Figure below. The refrigerant liquid captures energy from waste heat at evaporator and is vaporized to vapour. Then high pressure and high temperature vapour goes into turbine and expands across turbine passage to produce mechanical energy. After existing turbine, the vapour is condensed into liquid in condenser and the liquid is pumped back to evaporator, initiating a cycle.

The minimum pressure of turbine outlet is determined by the property of the refrigerant and the saturation temperature in condenser.
Nidum,as you mentioned, if the working condition of turbine is stable, the turbine speed is determined by load. Does it mean that the turbine speed is controlled by adjusting generator load?
I will appreciate it if you have any further thoughts on this.
Regards