The choice of the working fluid for any given application is a key issue and should be done based on specific applications to achieve maximal efficiency. For working fluids in ORC, a green energy alternative, there are some requirements to keep in mind:
Low pump consumption and high critical point
•Positive or isentropic saturation vapor curve
Avoid wetness in flow path, i.e. avoid damages of flow path elements
•High vapor density
Decrease sizes of equipment (expander and condenser)
High pressures usually lead to higher investment cost and increasing complexity
•High stability temperature
Prevent from chemical deterioration and decomposition at high temperatures
•Low environmental impact and safety level
•Good availability and low cost
Dry, Isentropic and Wet Fluids
The slope of the saturation curve of a working fluid in a T-s diagram can be positive (e.g. isopentane), negative (e.g. R22) or vertical (e.g. R11), and the fluids are accordingly called ‘‘wet’’, ‘‘dry’’ and ‘‘isentropic’’ fluids. Wet fluids usually need to be superheated to avoid liquid droplet impingent in the turbine blades during the expansion. Dry or isentropic fluids do not need superheating.
Superheating in ORC
Operation between two isobaric curves is
not recommended because fluids superheat, which contributes negatively to the cycle efficiency for dry fluids. For wet fluids, superheat is mostly necessary
for turbine expansion safety and improvement of the cycle efficiency.
High latent heat, high density and low liquid specific heat are preferable.
Fluid with high latent heat and density absorb more energy from the source in the evaporator therefore giving higher turbine work output compared to a low latent heat and density fluids. This helps to reduce the required flow rate, size of the facility/equipment, and pump consumption.
Fluid Compatibility with Materials
Organic fluids usually suffer chemical deterioration
and decomposition at high temperatures. The maximum operating temperature is thus limited by the chemical stability of the working fluid. Additionally, the working fluid should be noncorrosive and compatible with engine materials and lubricating oil.
And in terms of environmental and safety aspects, as mentioned above, fluids with high latent heat, high density and low liquid specific heat are preferable because they absorb more energy. And that’s in the end what we want, isn’t it?