I am interested in ORC-system. If I look to the hp-diagrams of the main working fluid they do not need a superheating stage, but in reality I saw that in many ORC the fluid is also superheated. Could you explain me the reason of that?
Hi Serena:
there are not any real differences between an Organic and conventional steam Rankine cycle, except for the type of working fluid selected, the shape of the liquid-vapor saturation dome and the temperature of interest. Superheating of vapor in the cycle has the main purpose of increasing the average temperature of the cycle heat input process, thereby increasing the overall cycle efficiency (per Carnot). Of course, the turbine's maximum inlet temperature of your cycle is limited by the heat source temperature from your heat recovery application.
Because the formation of liquid droplets in the turbine due to partial condensation of the working fluid is detrimental to the blades (causing pitting and errosion), superheating can also be used to prevent condensation in the turbine during expansion, as done in the steam Rankine cycle for example. Depending on your working fluid, this may not be an issue; it depends on the shape of the liquid-vapor saturation line.
You can learn much about this subject from studying the well documented steam Rankine cycle and its many variations.
Hope this helps. Good luck in your research!
Jean-Michel
Hi Jean-Michel,
thank you for your answer. I am very familiar with the steam rankine cycle, not with organic fluid.
In ORC one of the advantages is that you do not need superheating beacause of the shape of saturation curve, you do not have droplet formation. Do you know if the superheating in ORC is an exception or is the normal design?
Thank you again.
Serena
a slight superhated state is advisable in ORC for the following reasons:
- as Jean-Michel said, the higher is the upper temperature of the cycle, the higher would be the cycle efficiency. anyway, if your ORC is for waste heat recovery, you should aim to maximize the power output rather than the efficiency
- even if you prefer an expansion that starts at the saturated vapor state, a slight superheating would compensate the thermal losses that you might have along the passage from evaporator to turbine.
-superheating allows an expansion process always in the superheated regime for lots of working fluids. this is a fundamental requirement for dynamic machines (axial or radial turbines) but also for displacement expanders. these devices are more suitable than turbines for small scale applications. anyway, also with these kind of machines, a superhaeted expansion in needed to maximize the power output. indeed, if you have a wet expansion, the condensation process would reduce the mass of the working fluid trapped in the expansion cell. this fact would lead to a pressure drop, thus to a lower expansion work
hope this helps
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