Hydrocarbon refrigerants

Thermodynamic properties of hydrocarbons

 

Hydrocarbon refrigerants are chemically stable over a wide temperature range, non-toxic and environmentally friendly with extremely low GWP and zero ODP. Hydrocarbons also have excellent thermodynamic properties along with physical and chemical properties that allow them to be particularly energy efficient. ^[1] In vapour compression refrigeration and air conditioning systems, the most used hydrocarbon refrigerants include: R290 (propane), R600a (isobutane), R1270 (propene/propylene). ^[2], ^[3]

 

Main thermodynamic and environmental properties of hydrocarbon refrigerants. ^{[1], [4]}

Refrigerant	No.	Chemical formula	Molecular mass (kg/kmol)	Normal boiling point (°C)	GWP (100 year)	ODP
Methane	R50	CH4	16.0	-161	30
Propane	R290	C3H8	44.1	-42	<1	0
Butane	R600	C4H10	58.12	0	<1	0
Isobutane	R600a	C4H10	58.12	-12	<1	0
Ethene/Ethylene	R1150	CH2=CH2	28.1	-104	3.7	0
Ethane	R170	C2H6	30.07	-89	1.4	0
Propene/Propylene	R1270	C3H6	42.08	-48	<1	0
Dimethyl ether	RE170	C2H6O	46.07	-25	1	0

 

 

Safety standards and charge of hydrocarbon refrigerants

 

According to ISO 817 standard, hydrocarbons are classified as A3 refrigerants, low toxic and highly flammable refrigerants. ^[2] Nevertheless, hydrocarbon refrigerants are safe to use when handled according to safety procedures and installed in compliance with manufacturer's instructions.

According to an IIR Informatory Note, one of the safety measures required to prevent potential fires or explosions with flammable refrigerants is to minimise refrigerant charge and leakage. ^[5] Since most of the charge is stored in heat exchangers, refrigerant charge can be lowered by reducing the internal volume of the heat exchanger, for instance by using microchannel heat exchangers. ^{[4], [6]} 

The charge limits for flammable refrigerants can be found in the following safety standards for refrigeration, air conditioning and heat pumps (RACHP): ISO 5149, IEC 60,335-2-40 and IEC 60,335-2–89 at international level and EN 378 in Europe. For instance, the upper charge limit for domestic refrigeration is 150g, while the upper charge limit for commercial refrigeration ranges from 500g to 1.5 kg depending on standards. ^[2] The occupancy and size of the room are factors to be taken into consideration in the use of hydrocarbon refrigerants. For systems with a charge size over 150g, the room size must be such that a sudden loss of refrigerant does not increase the average gas concentration in the room beyond a given practical limit. ^[1]

Aside from flammability concerns, cost is one more impediment to the adoption of hydrocarbon refrigerants. Depending on the application, the investment required for the use of hydrocarbons may be expensive due to the additional costs related to safety requirements.

 

Applications of hydrocarbon refrigerants in refrigeration and air conditioning systems

 

Hydrocarbons can be used either in a system designed specifically for their use or as a replacement to other refrigerants along with some modifications in the equipment. In retrofitted equipment, compatibility with lubricant and safety measures are paramount. ^[1]

 

Hydrocarbons in domestic refrigeration and air conditioning

 

Hydrocarbons, especially R600a (isobutane), are considered the main energy-efficient and cost-competitive alternative for domestic refrigerators. ^{[4], [7], [8]} In a study on a household refrigerator, a ternary mixture of R290/R600/R600a increased the coefficient of performance (COP) by 14% and the exergy efficiency by 3.43%, compared to HFC-134a.  ^[9] To mitigate safety risks, recent studies have focused on optimising configurations in order to minimise both the refrigerant charge and the energy consumption in domestic refrigerators. ^[8] 

In many countries, the use of HFCs in residential air conditioners is still widespread. Nevertheless, conversion of production lines to R290 (propane) in China, South East Asia and South America is underway. Restrictive safety standard requirements limit market introduction, except for small and portable units. ^[10] Hydrocarbon-based air conditioning systems are commercially available for low-charge indoor applications such as mini split, window and portable air conditioners, and more recently in split and rooftop ducted systems. ^[4]

Flammable refrigerants such as propane can be safely used in window air conditioners, due to their compact size and low refrigerant charge volume. ^[11] In one study, R290 was used as a drop-in replacement in a HCFC-22 room air conditioning system with the addition of a liquid-suction heat exchanger. The new system achieved a 38% increase in COP. ^[12] In another study, a prototype high efficiency window air conditioner using R290 achieved the same cooling capacity as a baseline R410A model and achieved an EER 17% higher than that of units promoted by the U.S. Environmental Protection Agency “Energy Star program”. ^[11]

 

Hydrocarbons in commercial refrigeration

 

According to a recent review article, HCFC-22, HFC-404A, and HFC-134a still dominate small-sized refrigerated retail equipment. Nevertheless, the adoption of hydrocarbons such as R290 and R600a in display cabinets and freezers is gradually advancing thanks to charge reduction technologies. ^[7] For instance, a 28% reduced energy consumption has been reported when using R290 in glass-door bottle coolers compared with older HFC-134a bottle coolers. ^[4] In China, results from a performance study have shown a significant reduction in energy consumption when using R290 in a vertical display cabinet under both freezing and refrigeration conditions. ^[7]

 

Hydrocarbons in automotive air conditioning ^{[4], [13-14]}

 

Regional safety standards and charge requirements for the use of flammable refrigerants are not standardised in automotive air conditioning. For instance, according to UNEP’s RTOC 2018 assessment report, retrofits with hydrocarbons were legal in some Australian states but illegal in others as well as in the USA. Hydrocarbon refrigerants could be used in new systems, provided that safety issues have been mitigated, for instance by introducing a secondary loop circuit that allows to separate flammable refrigerants from occupied spaces. ^{[4], [13]}

 

Hydrocarbons in ultra-low temperature applications ^[3]

 

R170 (ethane) and R1150 (ethene) are the only hydrocarbon refrigerants with normal boiling points suitable for ultra-low temperature refrigeration (below -50°C). R170 has been reported to be suitable for low temperature commercial equipment as well as large commercial and industrial equipment. In a theoretical study, R170 and R1150 were used in the lower stage of a cascade system. Compared to other selected refrigerants, R170 had superior performance in terms of COP but not in terms of volumetric cooling capacity. Other studies investigating the use of an ejector in a R290/R170 cascade refrigeration system have found superior performance in terms of COP, cooling capacity and reduction in electricity consumption.

 

Hydrocarbons in heat pump applications ^[15]

 

In large capacity heat pumps, the use of hydrocarbons is limited by the maximum charge of 500g required by the IEC standard and HFOs are therefore more suitable. For small and medium capacity systems, however, compression heat pumps using hydrocarbon refrigerants have great potential for development in commercial applications.

According to UNEP’s RTOC 2018 assessment report, a limited number of low-charge heat pump water heater installations applying R290 had been sold in Europe. ^[4] Nevertheless, a more recent UNEP report indicates that R290 is fast growing in monobloc equipment for water heating appliances, where the safety restrictions allow its use. ^[10]

According to a recent review article, when the temperature of the heat source ranges from 40°C to 80°C and the output temperature ranges from 80°C to 120°C, R600 and R600a as well as HFO-1234ze(Z) are the most suitable refrigerants for these medium and high temperature heat pump applications. R600, R600a and R601 can provide higher output temperatures up to 135°C with COPs between 1.9 and 7.0. For output temperatures below 70°C, R290 is the most popular option, with COP values in the range of 1.3–6.0.

 

Energy efficiency and environmental impact of hydrocarbon refrigerants ^{[1], [4]}

 

Hydrocarbon refrigerants are an energy-efficient alternative to HFCs. When using hydrocarbons, various studies have reported energy savings ranging from 4.4 to 18.7% compared to HFC-134a and from 2.65 to 13.5% compared to HCFC-22. ^[1] Various studies have also reported reductions in refrigerant charges with hydrocarbons by 40 – 56% compared to HFC-134a and a reduction by 12.9 – 58% compared to HCFC-22. ^[1] This is due to the lower density of hydrocarbons compared to HFCs. A lower refrigerant charge helps reduce refrigerant consumption and, consequently, reduce possible CO₂-eq emissions during leakage and at end of life. ^[4]  In a recent study, the total equivalent warming impact (TEWI) metric was used to determine that using hydrocarbons instead of an HFC along with optimising the design of a refrigeration system jointly reduced the CO₂-eq emissions. ^[17]

 

Useful links for further information

 

 

 

References

[1]          K. Harby, « Hydrocarbons and their mixtures as alternatives to environmental unfriendly halogenated refrigerants: An updated overview », Renew. Sustain. Energy Rev., vol. 73, p. 1247‑1264, juin 2017, doi: 10.1016/j.rser.2017.02.039.

[2]          D. Colbourne, K. O. Suen, T.-X. Li, I. Vince, et A. Vonsild, « General framework for revising class A3 refrigerant charge limits – a discussion », Int. J. Refrig., vol. 117, p. 209‑217, sept. 2020, doi: 10.1016/j.ijrefrig.2020.04.024.

[3]          A. Mota-Babiloni et al., « Ultralow-temperature refrigeration systems: Configurations and refrigerants to reduce the environmental impact », Int. J. Refrig., vol. 111, p. 147‑158, mars 2020, doi: 10.1016/j.ijrefrig.2019.11.016.

[4]          UNEP, « Refrigeration, Air Conditioning and Heat Pumps Technical Options Committee (RTOC) 2018 Assessment Report », 2018. [En ligne]. Disponible sur: https://ozone.unep.org/sites/default/files/2019-04/RTOC-assessment-report-2018_0.pdf

[5]          IIF-IIR et D. Colbourne, « Flammable refrigerants, 36th Informatory Note on refrigeration technologies. », 2017. https://iifiir.org/en/fridoc/flammable-refrigerants-36-lt-sup-gt-th-lt-sup-gt-informatory-note-on-141136 (consulté le juill. 27, 2021).

[6]          E. Allymehr et al., « Numerical Study of Hydrocarbon Charge Reduction Methods in HVAC Heat Exchangers », Energies, vol. 14, no 15, Art. no 15, janv. 2021, doi: 10.3390/en14154480.

[7]          E. Gao, Q. Cui, H. Jing, Z. Zhang, et X. Zhang, « A review of application status and replacement progress of refrigerants in the Chinese cold chain industry », Int. J. Refrig., vol. 128, p. 104‑117, août 2021, doi: 10.1016/j.ijrefrig.2021.03.025.

[8]          W. Cho, D. S. Jang, S. H. Lee, S. Yun, et Y. Kim, « Refrigerant charge reduction in R600a domestic refrigerator-freezer by optimizing hot-wall condenser geometry », Int. J. Refrig., vol. 117, p. 295‑306, sept. 2020, doi: 10.1016/j.ijrefrig.2020.05.012.

[9]          M. Fatouh et H. Abou-Ziyan, « Energy and exergy analysis of a household refrigerator using a ternary hydrocarbon mixture in tropical environment – Effects of refrigerant charge and capillary length », Appl. Therm. Eng., vol. 145, p. 14‑26, déc. 2018, doi: 10.1016/j.applthermaleng.2018.09.008.

[10]       UNEP, « Technology and Economic Assessment Panel (TEAP) 2021. Progress Report (Volume 1) - advanced copy », 2021. [En ligne]. Disponible sur: https://ozone.unep.org/node/11983

[11]       B. Shen et B. Fricke, « Development of high efficiency window air conditioner using propane under limited charge », Appl. Therm. Eng., vol. 166, p. 114662, févr. 2020, doi: 10.1016/j.applthermaleng.2019.114662.

[12]       D. M. Nasution, M. Idris, N. A. Pambudi, et Weriono, « Room air conditioning performance using liquid-suction heat exchanger retrofitted with R290 », Case Stud. Therm. Eng., vol. 13, p. 100350, mars 2019, doi: 10.1016/j.csite.2018.11.001.

[13]       M. Mohanraj et J. D. A. P. Abraham, « Environment friendly refrigerant options for automobile air conditioners: a review », J. Therm. Anal. Calorim., oct. 2020, doi: 10.1007/s10973-020-10286-w.

[14]       S. Wongwises, A. Kamboon, et B. Orachon, « Experimental investigation of hydrocarbon mixtures to replace HFC-134a in an automotive air conditioning system », Energy Convers. Manag., vol. 47, no 11, p. 1644‑1659, juill. 2006, doi: 10.1016/j.enconman.2005.04.013.

[15]       D. Wu, B. Hu, et R. Z. Wang, « Vapor compression heat pumps with pure Low-GWP refrigerants », Renew. Sustain. Energy Rev., vol. 138, p. 110571, mars 2021, doi: 10.1016/j.rser.2020.110571.

[16]       M. Ghanbarpour, A. Mota-Babiloni, B. E. Badran, et R. Khodabandeh, « Energy, Exergy, and Environmental (3E) Analysis of Hydrocarbons as Low GWP Alternatives to R134a in Vapor Compression Refrigeration Configurations », Appl. Sci., vol. 11, no 13, Art. no 13, janv. 2021, doi: 10.3390/app11136226.