Researchers from the Technical University of Braunschweig and Volkswagen have found a CO2-based heat pump to be the most efficient of four options studied for mobile air conditioning (MAC).
The study analysed the performance of four different air-conditioning systems in motorised vehicles, with the aim of creating a generic system for mobile cooling and heating.
The study – authored by J. Koehler et al. and published last autumn – is entitled ‘Worldwide Electrical Energy Consumption of various HVAC systems in BEVs and their Thermal Management and Assessment – Control Architecture and Results’.
Two of the systems tested had a positive temperature coefficient (PTC), one using the HFO R1234yf and the other using CO2 (R744). The other two systems used a reversible heat pump, and the refrigerants tested in that applications were R134a and R744.
The simulation was complex, involving several departments and software tools, bridged together by TSC Center as middleware. Various vehicle parts were added to the simulation, adding to the complexity.
The simulation of the car’s passenger compartment used a mean energy balance for all components and an average temperature inside the vehicle.
TIL Suite Library simulated the thermal systems in the refrigeration cycle. Properties of liquids, gases and refrigerants were provided by TIL Suite Media Library.
The controller model has a generic design, allowing the same model to be used in real-world or simulated refrigerant systems. The sample size is 0.01 seconds.
The climatic conditions were retrieved from GREEN-MAC-LCCP, a tool for assessing the life-cycle climate performance of MAC systems – the LCCP 2013 version of which includes worldwide climate data. The tool references 211 cities with six temperature bins, representing the temperature all year. Each temperature bin shows climate data from 6am to 12pm.
For this study, the researchers also assumed that all vehicles were electric.
Taking this data as a baseline, the researchers found that the reversible system with a heat pump using CO2 is the most energy efficient worldwide. The data was measured in terawatt hours (TWh). The CO2 system is projected to consume 311 TWh per year, based on the assumption that all cars are electric. The second most efficient system uses R134a, consuming 320 TWh per year. The difference is most significant in colder climates.
The researchers gathered data about Europe, the United States and China. In Europe, the advantage of the CO2 heat pump system is the greatest. In the U.S. and China, differences between climate regions had more of an impact on the results. Energy savings from choosing CO2 were greater in those countries’ cooler northern climates than in warmer south.
The data for the specific regions was calculated in Watt-hours (Wh) per kilometre and year. In all regions the CO2 heat pump system is the most efficient. On average, in the United States the CO2 heat pump needs 22 Wh/(km year), compared to 23 for the R134a heat pump system. In Europe, the R744 system is projected to consume 18 Wh/(km year), while the R134a system needs 20 to run. In China 27 Wh/(km year) would be needed for the CO2 system in electric cars. The HFC system would average 28.
This data shows that the CO2 system is the most energy efficient in many different areas of the world. For electric cars, this translates to more mileage without charging the battery. The biggest advantages for the R744 systems are projected in cooler climates.
The systems tested without a reversible heat pump needed more energy on average in almost every setting.
Thus the energy saved by choosing CO2 as the refrigerant reduces indirect emissions from the air-conditioning system. CO2 also has the lowest global warming potential (GWP) and emits the least direct emissions. The CO2 system was thus found to be the most climate-friendly solution.