IIR Rankine Conference 2020: Which alternative working fluids for ORC systems?

The IIR International Rankine Conference 2020 – Advances in Heating, Cooling and Power Generation – which took place on July 26-29 July in Glasgow, UK, was the opportunity to highlight the Organic Rankine Cycle (ORC) as a proven way to convert into electricity energy from low-temperature sources such as geothermal heat, industrial waste heat, geothermal or solar heat ^{(1) (2)}.

 

In 2017, the number of operational ORC power plants was estimated to be 1754, generating around 2700 MW of electricity. ORC cycles operate in a similar manner as the conventional steam power plants, but instead utilize fluids with lower boiling temperatures. Furthermore, ORC systems can be optimized to reject heat via air-cooled condensers, instead of water-cooled condensers. This can help reduce the dependence on water for power generation ⁽²⁾.

 

An important consideration regarding the design and optimization of ORC systems is the selection of the working fluid. HFC245fa, is commonly used as working fluid in ORC applications. However, its relatively high GWP (880) justifies the current research work on alternative fluids having a lower climate impact while exhibiting thermodynamic properties at least as good. HFOs (hydrofluoroolefins), HCFOs (hydrochlorofluoroolefins) and hydrocarbons were the main options presented during the IIR Rankine Conference.

 

In their study ⁽²⁾, T. A. Jacob and B. M. Fronk experimentally investigated in-tube condensation heat transfer coefficients and pressure drop of HFO1233zd(E) at representative ORC conditions, and compared the results with R245fa data and correlations from the literature. Experiments were conducted in a smooth round tube with inner diameter of 4.7 mm, mass fluxes from 100 to 400 kg.m^-2.s^-1, and saturation temperatures of 45 °C to 60 °C. The results were then used to design and compare the size and pressure drop of an air-cooled condenser for a 1 MW ORC plant using the two refrigerants. Replacing R245fa with R1233zd(E) has led to 2.7% reduction in size but an 18.9% increase in the refrigerant pressure drop for equivalent power production.

 

G. Righetti et al ⁽³⁾ have compared the thermal and hydraulic performance of HFC245fa with those of HFO1233zd(E), HFO1234ze(Z) and HCFO1224yd(Z) during flow condensation and boiling under working conditions suitable for ORC. A general approach was adopted, basing on the second law of thermodynamics by means of suitable Performance Evaluation Criteria implemented on semi-empirical heat transfer coefficient and pressure drop models, previously validated on experimental measurements for different type of tubes. The results show that the alternative refrigerants to R245fa present comparable and in most cases better performance in both condensation and boiling.

 

M. Amat-Albuixhech et al ⁽⁴⁾ have experimentally tested the suitability of using HCFO1224yd(Z) in an existing low-temperature and small-scale HFC-254fa designed ORC. The drop-in replacement tests show similar behaviour between the two refrigerants and although R-245fa provides higher expander power output, net cycle efficiencies are similar, and several adjustments can be introduced to compensate the lower electrical output.

 

M. T. White and A. I. Sayma have presented a parametric thermodynamic optimisation study to identify optimal working fluids among 20 potential fluids and optimal cycle architectures for small-scale Rankine cycle power systems for a range of different heat-source and heat-sink conditions. The results indicate that the top five working fluids, when considering performance over all of the case studies considered, are isobutane, isopentane, n-propane, HFO1233zd and n-pentane. When using isobutane and n-propane, supercritical cycles are generally optimal, whilst subcritical cycles are generally preferred for isopentane and n-pentane. R1233zd can be used in both types of cycle. Moreover, it is found that fluid selection is not significantly affected by heat-sink availability, whilst subcritical cycles are preferred at lower heat-source temperatures, and low heat-source temperature drops, whilst supercritical cycles are better for higher heat-source temperatures and are most suitable when trying to maximise power output.

 

Sources:

⁽¹⁾ B. G. Meyer et al. Potentials of Zeotropic Mixtures with Large Temperature Glides as Working Fluids for Organic Rankine Cycles (ORC) and Heat Pumps. https://iifiir.org/fr/fridoc/142594

⁽²⁾ T. A. Jacob et al. Experimental Investigation of Low GWP Alternative R1233zd(E) for Use in Organic Rankine Cycle Condensers. https://iifiir.org/fr/fridoc/142573

⁽³⁾ G. Righetti et al. Heat-Transfer Assessment of the Low GWP Substitutes for R245fa in ORC. https://iifiir.org/fr/fridoc/142714

⁽⁴⁾ M. Amat-Albuixhech et al. Experimental Drop-In Replacement of HFC-245fa by HCFO-1224yd(Z), A Low-GWP Working Fluid Candidate for Low-Temperature Organic Rankine Cycles. https://iifiir.org/fr/fridoc/142635

⁽⁵⁾ M. T. White et al. Fluid Selection for Small-Scale Rankine Cycle Plants: Can You Draw Some Lines in the Sand? https://iifiir.org/fr/fridoc/142633