In the face of growing need to reduce carbon emissions from fossil fuel the demand for alternative renewable and low carbon sources of electricity generation is always increasing. Advanced technologies for the conversion of low grade heat sources into power like Organic Rankine Cycle (ORC) generators have been proven and successfully implemented to recover different heat sources such as industrial waste heat, solar energy, geothermal, and biomass energy. ORC generators are known for their ability to capture low grade heat, from temperatures as low as 80°C up to 400°C. Their increasing efficiency through innovation, reliability and low cost of maintenance is advancing the number of installations as a cost effective solution for converting low grade waste heat to power.
Selection of working fluid is one of the most important factors determining the ORC efficiency, performance and safe operation. Material compatibility, fluid thermodynamics, toxicity, flammability and other properties must be also considered in accordance with the ORC generator requirement. Desirable characteristics of the working fluids include appropriate boiling point temperature, chemical stability, low latent heat, high critical temperature/pressure, low density, low surface tension, suitable specific volume, high thermal conductivity, non-toxic, non-corrosive and compatible with the ORC construction materials.
Working fluids usually can be categorised according to the slope of the saturation vapour line in a T-s diagram.
A dry fluid has positive slope of the saturated vapour curve and are usually of high molecular mass, e.g. HFE7100 m=250
A wet fluid has negative slope of the saturated vapour curve and are generally of low molecular mass, e.g. Water m=18
An isentropic fluid has infinitely large or nearly vertical on the saturated vapour curves and are generally of medium molecular mass, e.g. R245fa m=134
Dry and isentropic working fluids that do not require superheating are more suitable for the ORC generators as they eliminate concerns of impingement from liquid droplets on the expander blades (Bao & Zhao, 2013).
Figure 1: T-s diagram of working fluid- dry, isentropic and wet (Qui, 2012)
The organic working fluids with low boiling temperatures commonly refer to hydrochlorofluorocarbons (HCFCs), chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). Most of the CFCs and HCFCs are threat to the ozone layer depletions but not HFCs and HFEs. Hence, HFCs or better environmental friendly fluids will replace HCFCs ultimately due to zero value of ozone depletion potential.
Figure 2: T-s plots for various dry working fluid (Qui, 2012)
Although HFCs have no ozone depletion threat, certain HFCs have high global warming potential threat (GWP). For example HFC-134a has GWP of 1,300 and HFC-245fa has GWP of 950 (US EPA, 2014). Therefore research has been currently undergoing in this field to replace high GWP working fluids to low or zero GWP fluids. It has been identified that in certain industries HFC-134a is currently considered being replaced with HFO-1234yf which has low GWP less than CO2 (Nielsen, et al., 2015).
Fluid selection studies in the scientific literature cover a broad range of working fluids but only few working fluids are actually used in commercial ORC engines. Below are the lists of few commercially available working fluids:
HFC-134a: Mainly used in geothermal power plants or in very low temperature waste heat recovery
HFC-245fa: Low temperature working fluid, mainly used in waste heat recovery.
N-pentane: Used in medium temperature geothermal and some waste heat recovery.
Toluene: Waste heat recovery
Silicone Oil: Used in high temperature waste heat recovery
OMTS: CHP power plants
In order to improve the thermodynamic efficiency & economic performance of the system, some researchers have shown interest to mix certain working fluids. But it has been identified by Feng, et al., 2015 that working fluid mixtures does not always provide better thermodynamic performance and have worse economic performance than the pure working fluids. Further research and experimental analysis need to be conducted on more fluid mixtures to determine good operation parameters and mass fraction, which then might exhibit better thermodynamic and economic performances than the pure working fluids.
After the techno commercial research and consideration of environmental impacts it has been identified that currently R245-fa is the most suitable working fluid for low temperature waste heat recovery application (Wang, et al., 2011). It is also identified that when R245-fa is employed, thermal efficiency of ORC is higher comparing other fluids. Further research has been undergoing in order to establish a better alternative working fluid with lower GWP to replace R245-fa, for example R245-ca with GWP of 560 is still not in use today but might be a possible working fluid in the future (US EPA, 2014).
Bao, J. & Zhao, L., 2013. A review of working fluid and expander selections for organic Rankine cycle. Renewable and Sustainable Energy Reviews, 24(1), pp. 325-342.
Feng, Y. et al., 2015. Thermoeconomic comparison between pure and mixture working fluids of organic Rankine cycles (ORCs) for low temperature waste heat recovery. Energy Conversion and Management, 106(1), pp. 859-872.
Nielsen, O., Anderson, M. & Wallington, T., 2015. Atmospheric chemistry of short-chain haloolefins: Photochemical ozone creation potentials (POCPs), global warming potentials (GWPs), and ozone depletion potentials (ODPs). Chemosphere, I(129), pp. 135-141.
Qui, G., 2012. Selection of working fluids for micro-CHP systems with ORC. Renewable Energy, I(48), pp. 565-570.
US EPA, 2014. Global Warming Potentials of ODS Substitutes. [Online]
Available at: http://www3.epa.gov/ozone/geninfo/gwps.html
[Accessed 16 November 2015].
Wang, E. et al., 2011. Study of working fluid selection of organic Rankine cycle (ORC) for engine waste heat recovery. Energy, 36(1), pp. 3406-3418.