Working fluid selection

Most thermodynamic cycles make use of the latent heat (advantages of phase change) of the working fluid.

There are also technologies in heat pump and refrigeration, where working fluid does not change phase, such as reverse Brayton or Stirling cycle.

Finding the optimal working fluid for a given purpose – which is essential to achieve higher energy efficiency in the energy conversion systems – has great impact on the technology, namely it does not just influence operational variables of the cycle but also alters the layout and modifies the design of the equipment.

A suitable fluid must exhibit favorable physical, chemical, environmental, safety and economic properties such as low specific volume (high density), viscosity, toxicity, flammability, ozone depletion potential (ODP), global warming potential (GWP) and cost, as well as favorable process characteristics such as high thermal and exergetic efficiency.

Existing research is largely focused on the selection of pure working fluids, with vast number of published reports currently available.

[2] Many authors like for example O. Badr et al.[3] have suggested the following thermodynamic and physical criteria which a working fluid should meet for heat engines like Rankine cycles.

There are some differences in the criteria concerning the working fluids used in heat engines and refrigeration cycles or heat pumps, which are listed below accordingly: Traditional and presently most widespread categorisation of pure working fluids was first used by H. Tabor et al.[4] and O. Badr et al.[3] dating back to the 60s.

The third category is called isentropic, which means constant entropy and refers to those fluids that have a vertical saturation vapour curve (regardless of a short negative slope somewhat below the critical point) in temperature-entropy diagram.

If the fluid is of dry-type, the isentropic expansion necessarily ends in the superheated (also called dry) steam zone.

Another problem is the extent of how dry or isentropic the fluid behaves, which has significant practical importance when designing for example an Organic Rankine Cycle layout and choosing proper expander.

The new classification is also based on the shape of the saturation vapour curve of the fluid in temperature-entropy diagram similarly to the traditional one.

Some scientific papers deal with the application of multicomponent working fluids in Organic Rankine cycles as well.

Traditional classification of pure working fluids. 1→2 shows isentropic expansions from saturated vapour states.
Novel classification of pure working fluids. [ 5 ]
Compatibility of traditional and novel classification of pure working fluids. The shape of the saturated vapour curve of the fluid depends on the specific isochoric (molar) heat capacity (c v ) of that state via degrees of freedom (f) of the molecules. [ 6 ] [ 7 ]