Green Automobile Air Conditioner Technology Demonstration Project

It’s nearly 40 degrees centigrade (˚C) (104 Fahrenheit) and your car is sandwiched in the middle lane of a stalling queue of cars and trucks crawling down the highway, headed back home from a stressful day at the office. You switch on the car radio and a weather forecast predicts five more weeks of a scorching heatwave. What could be better than knowing that, tucked under the engine hood, is an ingenious device that will not only keep you comfortable for each of those hideous commutes, but will save you enough money for some activities your family has been looking forward to?

The car is kitted out with a newly configured mobile air conditioning (MAC) system containing a coolant that stores energy. The compressor in the prototype engages when the vehicle is decelerating or going downhill and disengages when it is accelerating or climbing, maximising energy efficiency. Saved energy is stored in the anti-freeze coolant. And the new MAC system not only saves money but also cuts down the likelihood of more frequent and longer heat waves in the future – because it contains a refrigerant chemical known as HFC-152a that is better for the climate, more energy efficient than its alternatives, and more reliable, so you don’t have to service your AC as often.

With CCAC spport, Indian automobile producer TATA Motors Limited (TML) worked with German automotive supplier Mahle and the Institute for Governance & Sustainable Development (IGSD) to build and test the new mobile air conditioning system in a sport utility vehicle in 2017 and 2018. This technology demonstration project was funded by Climate and Clean Air Coalition. The vehicle has now gone through a series of extensive laboratory and road tests on city streets and highways. When it is launched on the market, it will be the first such system, known as secondary loop mobile air conditioning (SL-MAC), installed in a commercially available vehicle anywhere in the world.

It relies on a clever innovation – a secondary loop of coolant running between the refrigerant in the engine bay and the passenger compartment of the car. Most air conditioning in cars today relies on a refrigeration circuit partially located in the passenger cabin. But because HFC-152a is slightly flammable, the SL-MAC experts have taken a different approach. Engineers had to figure out how to eliminate fire risk to the passengers. So, they came up with a neat swap, positioning the refrigeration components within the engine compartment instead of the passenger cabin and introducing a secondary loop. This channels a separate coolant from the refrigerant unit back to the passenger cabin.

At first glance, using a flammable refrigerant sounds risky. But according to Dr. Stephen O. Andersen, research director at IGSD in Washington DC, which coordinated the project, the design is safe because the refrigerant charge is small, tightly contained, and isolated from the driver and passengers.

“The engine bay is a comfortable location for this refrigerant, and the refrigerant has been engineered to be safe in this system," he explains. “And petrol and other flammable liquids in the engine compartment, such as brake fluid, hydraulic oil, and lubricants, are safely used.”

Back to the matter of the driver’s wallet. The advantage of HFC-152a is not just that it’s one-tenth the global warming potential (GWP) of the common refrigerant – 138 times that of carbon dioxide compared to the status quo refrigerant HFC-134a, which has a GWP of 1300 – it is also that, in hot and humid climates, it reduces the fuel used by a car’s air conditioning, which can consume up to 20% of the fuel a driver puts into the car. Thus, SL-MACs with HFC-152a are more fuel-efficient than existing HFC-134a air conditioning systems, resulting in significant cost savings for drivers. The SL-MAC system installed in the Tata SUV, for example, resulted in overall vehicle fuel savings of 1.9% to 2.6%. That would save a typical SUV driver in India 23-31 litres per year based on a vehicle driven 15,000 km per year.

In addition, the new air conditioning system cut refrigerant leakage in half, reducing both emissions and service and maintenance costs for the vehicle’s owner. How does the SL-MAC achieve these impressive savings? According to the US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA), the secret to SL-MAC efficiency is its cold storage ability: “Secondary loop systems have added value in that they have the ability to store cooling within the loop, which in turn allows for ‘free’ cooling to occur during deceleration events, and then [be] delivered to the cabin during engine idle off condition (with engine start/stop feature) or while accelerating.”

In the TATA prototype, a computer programme determines when the MAC compressor engages. Adjustments by that programme as the car moves are key to improved MAC energy efficiency. It is an impressive feature of the TATA prototype. The compressor is responsible for most excess fuel consumption associated with vehicle air conditioning in current systems. But TATA has designed a system that, in terms of its concept, resembles regenerative braking – in which energy that would otherwise be wasted is stored and reused.

In a similar way, the compressor in the prototype engages when the vehicle is decelerating or going downhill and disengages when it is accelerating or climbing, maximising energy efficiency. Saved energy is stored in the anti-freeze coolant. That feature is possible in any vehicle equipped with a secondary loop or other cold storage device, but the specimen produced by the TATA team is the most finely tuned and highly engineered so far. As the test results demonstrate, the energy savings achieved by this feature more than compensate for the weight of extra parts needed under the car bonnet to accommodate the refrigerant and secondary loop.

The savings in service cost and fuel easily pay back the minor extra upfront cost of around $35 for the SL-MAC system. “Our automotive partners estimate that in India, you need to service older HFC-134a mobile air conditioning systems two or three times in the lifetime of a car. In the new SL-MAC, you service it as little as once,” points out Kristen N. Taddonio, Senior Climate and Energy Advisor to IGSD. That means fewer trips to the garage, and less inconvenience when booking an appointment and waiting. For some commercial fleets, it could cut costs significantly.

Then there is the comfort factor. In most vehicles, it takes 60 seconds for the air conditioning to work every time you start the car. In the SL-MAC, cooling starts immediately after short stops subject to the coolant being cold enough. With the reservoir of energy in the cold antifreeze coolant, the car occupants remain comfortable for longer even if they leave the engine switched off.

But let’s keep in mind the whole reason behind the invention in the first place – the climate. If automakers replaced all HFC-134a MACs with HFC-152a SL-MACs in cars worldwide starting in 2020, we would save billions of tonnes of greenhouse gas emissions by 2050.

The amount of refrigerant needed to cool an SL-MAC-equipped vehicle is less than that of conventional MACs. SL-MAC cars also have a lower leakage rate due to fewer joints in the primary refrigerant circuit. That is why the SL-MAC used with HFC-152a achieves extraordinarily low emissions from the refrigerant, as well as from fuel used to power the MAC.

That is a major consideration, given the massive contribution refrigerants currently make to global warming. According to climate scientists, HFCs would account for an increase in global warming amounting to 0.5˚C if no action is taken – a major portion of the maximum 1.5 or 2˚C of further warming that the planet can tolerate. Of that HFC contribution, 25% to 35% comes from automobiles.

The secondary loop does add complexity. With ab initio engineering of the SL-MAC, including architecture, integration with the engine management systems, higher quality heat exchangers and fittings, it is possible to overcome any disadvantage.

“Automotive manufacturers are looking for every competitive advantage in terms of improving fuel efficiency and reducing costs, so the SL-MAC represents an exciting opportunity,” says Dr. Nancy J. Sherman, Director of Technical Assessment, IGSD. Environmental considerations also weigh heavily on automotive manufacturers and branding due to regulations in place or on the horizon. Any major car producer is of course going to consider fitting similar systems to electric vehicles too. Nowadays, any brand that lays claim to advanced, cutting edge design has to include climate-friendly equipment.

How does SL-MAC techonology work?

A refrigerant loop in the engine bay containing HFC-152a chills an antifreeze-like coolant in a separate secondary loop that connects to the passenger cabin, cooling the occupants. Heat from the cabin is transferred through the coolant to a condenser in the engine compartment, where the heat is expelled to the air. During the heating and cooling cycle, components known as heat exchangers either evaporate liquid, converting it into cold gas, or condense gas, converting it back into a hot liquid. As the conversion takes place, the heat is transferred from gas to liquid or vice-versa.

The pilot testing and design was conducted with the support and backing of United Nations Environment. Automotive parts manufacturer MAHLE and Tata Motors, along with IGSD, received funding for developing the SL-MAC system from the Climate and Clean Air Coalition. 

Relevant papers from the study can be found below.