Economic and Climate Advantages: Secondary-Loop Motor Vehicle Air Conditioners (MACs)

This paper and presentation compare the thermal, economic and climate performance of existing direct expansion motor vehicle air conditioners (DX-MACs) using hydrofluorocarbon (HFC)-134a (global warming potential (GWP) =1300) with secondary-loop MACs (SL-MACs) using hydrofluoroolefin (HFO)-1234yf (GWP < 1) and HFC-152a (GWP = 138), both of which satisfy the European Union (EU) and Japan F-gas regulations and are listed as acceptable by the US Environmental Protection Agency (US EPA). In addition to a technical review of the SL-MAC system, the paper includes a part-by-part system manufacturing cost comparison and itemized ownership cost comparison taking into account fuel savings and reduced maintenance. The paper is timely because the Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer now requires both developed and developing countries to phase down the production and consumption of HFCs and at the same time encourages increases in energy efficiency. The developed country transition to HFO-1234yf is well underway, with the EU already requiring refrigerants with GWP less than 150, Japan requiring GWP < 150 after 2023, and the US EPA offering credits toward fuel efficiency standards for low GWP refrigerants. SL-MAC technology uses a refrigerant contained in the engine compartment to chill coolant that is circulated in the passenger compartment. The design uses a smaller refrigerant charge with fewer connections, which reduces refrigerant leak rates and lowers the frequency and cost of service. The thermal coolant ballast saves energy by allowing the compressor to turn off during acceleration and to engage during deceleration, with inertia converted to cold without carbon emissions. The thermal ballast prolongs passenger comfort during idle stop. A timeline of MAC history is also included.