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Page 2 of 14 Li et al. Carbon Footprints 2024;3:6 https://dx.doi.org/10.20517/cf.2023.54

INTRODUCTION
The electrification of vehicles, in principle, stands out as one of the most effective strategies for achieving
carbon neutrality in urban transportation, which can significantly reduce reliance on fossil fuels depending
on how electricity is generated, mitigate the emissions of both Green House Gases and public health
relevant environmental pollutants from vehicles, and thereby lower the costs associated with environmental
management and public health. Currently, almost all globally manufactured vehicles are equipped with
[1]
MAC (Mobile Air Conditioning) . As the largest energy-consuming facility on electric vehicles, apart from
the motor, the energy conversion efficiency of the air conditioning has a profound impact on the electric
vehicle’s driving range. Heat pump technology has emerged as the predominant technical solution for air
conditionings in electric vehicles. However, the traditional automotive air conditioning refrigerant, R134a,
exhibits a severe decline in heating performance when utilized in heat pumps as ambient temperatures
decrease. In instances where the temperature falls below -5 °C, R134a automotive heat pumps typically
require supplementary electric heating, resulting in a notable reduction in COP (Coefficient of
Performance), and consequently generating substantial indirect carbon emissions. Furthermore, the
exceedingly high GWP (Global Warming Potential) of R134a, which reaches 1,430, also points to its
greenhouse effect through significant direct carbon emissions. Considering the entire life cycle of
automotive air conditionings, including operation, maintenance, and disposal, refrigerant leakage is
anticipated at various stages. Given China’s extensive vehicle fleet, with an estimated 0.7 kg of R134a
[2]
charged in the air conditioning of a vehicle , the equivalent carbon content of China’s vehicles is
approximately 280 million tons CO -eq. Therefore, the adoption of low-GWP refrigerants such as HFOs,
2
R290, or R744 in electric vehicles as replacements for R134a holds paramount significance for the urban
transportation sector’s low-carbon transition.

Following China’s official announcement on September 15, 2021, of its formal accession to the Kigali
Amendment to the Montreal Protocol , China has embarked on a historic chapter in addressing climate
[3]
change and strengthening the control of HFCs and other non-carbon dioxide greenhouse gases. In order to
identify environment-friendly alternatives for the widely used R134a refrigerant in automotive air
conditioning, a series of attempts have been made by both the automotive industry and academia. However,
due to the complex operational conditions of automotive air conditioning systems, the selection of
alternative refrigerants to R134a requires careful consideration. The chosen refrigerant not only needs to
have a low GWP but also must balance factors such as efficient cooling/heating capabilities, low
flammability, and reasonable cost. Currently, elevated research attention has been directed towards
refrigerants such as R1234yf, R744, and R290 , and the GWP and Adp.GWP values of these refrigerants are
[4]
presented in Table 1.

The HFO-based refrigerant R1234yf, jointly developed by Honeywell and DuPont, exhibits thermal-physical
[5]
properties remarkably similar to R134a , requiring only minor adjustments to existing systems. However,
similar to R134a, R1234yf demonstrates lower heating efficiency at low temperatures. Therefore, the use of
R1234yf in heat pump air conditioning requires additional supplementation with PTC (Positive
Temperature Coefficient) electric heaters to meet heating demands in low-temperature conditions.
Additionally, patent barriers and the higher cost of R1234yf have limited its widespread adoption . R744
[6]
refrigerant, also known as CO , is considered a promising natural low-GWP refrigerant for electric vehicle
2
air conditioning. Wang et al. conducted experimental studies on the heating performance of a R744
automotive heat pump . Their results indicated that the R744 automotive heat pump system can provide
[7]
3,600 W of heat supply at -20 °C, with a system COP as high as 3.1. However, due to the low critical
temperature of CO , the R744 automotive heat pump system must operate in a transcritical cycle, with
2
operating pressures approximately 6-8 times higher than that of R134a systems. This requirement for high-