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Auto-AC has big energy role to play in reducing emissions

A report by the International Energy Agency (IEA) is highlights both direct and indirect emissions caused by automotive air-conditioning, recommending governments take a holistic approach to addressing these issues that includes the adoption of low-GWP refrigerants, improved system efficiency and increased vehicle electrification.

Few reports of this kind cover both direct and indirect emissions, usually focussing on refrigerants escaping to atmosphere, the energy consumption impact of running air conditioners or simply assuming air-conditioning usage when calculating transport emissions.

Titled “Cooling on the Move: The Future of Air Conditioning in Vehicles”, the report estimates that improving auto-AC energy efficiency and using refrigerants with a GWP of less than 1 would eliminate 950 megatonnes of CO2 equivalent emissions.

“Further emission reductions could come from decarbonising vehicle fuel supply and through greater electrification of the vehicle stock, especially if coupled with low-carbon electricity,” says the report.

It recommends that air-conditioning energy consumption be included in fuel economy standards and that vehicle testing methods are expanded to include minimum performance standards for specific air-conditioning components.

Today, air-conditioners in passenger cars, vans, buses and trucks collectively consume the equivalent of almost two million barrels of oil per day, according to the report, which estimates this could almost triple to 5.7m barrels by 2050 without further policy action.

“At the same time, annual combined emissions from energy consumption and refrigerant leakage could more than triple to 1300 million tons of CO2 equivalent.”

Much of this is driven by an expected doubling of the global vehicle fleet and an increase in average the number of kilometres travelled by 2050, with much of this growth in warmer climates where auto-AC must work harder and more often, and increasing living standards mean more people expect to travel in air-conditioned comfort.

The report calculates that energy consumption causes 70 per cent of auto-AC greenhouse gas emissions, with the leakage of high-GWP refrigerants accounting for the remaining 30 per cent.

Energy consumption of auto-AC varies from just three per cent in colder climates to 20 per cent in hotter parts of the world, peaking at 40 per cent if summer conditions are combined with traffic congestion.

With all of its recommendations adopted, the report estimates that improvements in energy efficiency could limit energy consumption to the equivalent of 2.8m barrels of oil and that with both low-GWP refrigerants and partial electrification of the vehicle fleet, by 2050 total greenhouse gas emissions from auto-AC would be 20 per cent lower than today.

Improved system design

The report says auto-AC systems tend to be over-sized due to the need for quick pull-down of high cabin temperatures to a comfortable level and operate at a fraction of their capacity once this is achieved.

This could in part be addressed by encouraging behavioural change, such as parking in shade or pre-cooling of vehicles (which most electrified vehicles are capable of), which could reduce energy consumption by up to 30 per cent.

Improved insulation, window coatings and reflective paints could also contribute by reducing cooling loads.

In the report’s best-case scenario, these measures, combined with improved HVAC and drivetrain system efficiency, could reduce the energy impact of car and van air-conditioning by as much as 67 per cent.

From an HVAC system perspective,  improvements to blower efficiency and more efficient compressors such as variable displacement designs can yield improvements as high as 36 per cent, or up to 50 per cent in hot and humid conditions.

These figures apply to cars, with buses yielding a 50-60 per cent improvement and heavy trucks 40 per cent.

According to the report, the reason a lot of HVAC system efficiencies are left on the table include the fact that auto-AC energy consumption in not factored into fuel efficiency test procedures, meaning there is no perceived consumer benefit to help justify the investment required to engineer and install better equipment.

A key exception to this is full-electric vehicles where HVAC efficiency provides a clear increase in battery range, a benefit everyone understands.

The report says Brazil is the only country that requires the air-conditioning to be running while testing vehicles for energy consumption and emissions. No country currently mandates the direct measurement of, our places a limit on, the energy consumption of auto-AC systems.

Part of the reason is the lack of an appropriate, standardised and internationally accepted method of measuring the contribution of auto-AC to the overall energy consumption of a vehicle.

However, the report recognises the technical challenges involved in accurately replicating ambient temperature, humidity, solar load and airflow over the condenser, as well as reproducing the test cycle at various temperature settings.

Refrigerant choices

The report identifies the study by Mahle and Tata into the A2 flammable refrigerant R152a (GWP: 124) that demonstrated it to have a lower life-cycle carbon footprint than R134a and R1234yf in hot climates when used in a secondary-loop system that keeps all refrigerant outside the passenger compartment.

It also refers to ongoing studies in China that are evaluating the use of A3 flammable R290 (a hydrocarbon with GWP of 3) refrigerant in a secondary loop setup.

Regardless of technology, the report recommends the broader worldwide adoption of regulations that limit the use of high-GWP refrigerants, as is already starting to happen in several developed nations under the Kigali Amendment to the Montreal Protocol.

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