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Heat pump technology a game changer for widespread electric vehicle adoption

By Craig Van Batenburg (Article reproduced by permission of SAE International)

Kia Soul EV_Heat pump technology

The Automotive Career Development Center (ACDC) that I founded over 20 years ago provides training and tech support to the world’s independent electric-vehicle (EV and hybrid) technicians, who will be asked to fix and maintain the systems as they age.

Our facility in central Massachusetts includes a variety of hybrid, plug-in hybrid and battery-electric vehicles (BEV) that we use for both teaching and research.

It has also supported my own investigations into cabin-heater technology and its annoying “your range may vary” impact on BEV use in northern climates.

This curiosity began when I bought a new 2011 Chevrolet Volt to add to the ACDC fleet. The Volt, a series-type hybrid, has an electric-only range of about 56km in ideal thermal conditions.

If I drive back roads at 56km/h, the Volt may deliver 64km of pure-electric use before the combustion engine kicks in. During winter, however, that EV range plummets to less than 32km.

A couple of years after buying the Volt, a used 2011 Nissan Leaf came our way. This early EV, with its 24kWh air-cooled battery, could go about 129km per charge in the summer, but only make 80km in winter.

Then in 2017, ACDC leased a Chevrolet Bolt EV. It offered a big jump in summertime range – 370km. But when the mercury dropped so did the Bolt’s driving range, to 257km.

Life with an electrified vehicle in New England was shaping up to be problematic. What did they all have in common? The Leaf and Bolt all generate cabin heat by using their high-voltage battery pack to heat antifreeze for the heater core – an “old-school” solution! Volt uses its ICE for heat in certain conditions, but not always.

In 2020 when the Bolt’s lease was up, it was time to look for a new EV for our training work as well as for longer-distance driving.

The Hyundai Kona EV was on my list but its cousin, the Kia Niro EV, won out. Why? The Niro features a heat pump, rather than hot water, to heat the cabin.

Heat pumps are not new in EVs: Nissan offered one in 2013 on upper-trim models of the Leaf, and Kia’s Soul EV brought a slightly improved heat pump. The Niro EV takes the technology one step further.

Coolant vs. heat pump

When affordable modern electric cars began entering the global market in 2010, their ability to keep driver and passengers comfortable in cold weather – without compromising driving range – became a concern for customers and EV development engineers.

To provide their cabin heat, approximately 3.8L of coolant was heated using the high-voltage battery for power. The hot coolant was then pumped into the heater core. 

This approach has been around since 1893, when the pioneering mechanical engineer Margaret Wilcox patented a method for warming the passengers by directing air from over the engine to the vehicle interior. Soon after, engine coolant became the medium. Using waste energy is smart; wasting energy is not.

Wilcox’s invention was still in use in the 2011 Leaf. Unfortunately, it decreased the EV’s driving range by 30 to 40 per cent in cold weather, as the heat was generated from the primary energy source on board – the car’s battery pack.

Nissan’s adoption of heat-pump technology was a revelation. It reduced the load on the high-voltage battery from approximately 7kW to 2kW, for the same amount of heat generated. This allowed the improved Leaf to go further on a charge while keeping the occupants comfortable.

Conceptually, heat pumps have been around since the 1850s. They have been used in buildings since the 1940s. How do they work? In simple terms, a heat-pump system heats the cabin using the temperature delta between a refrigerant and the outside air, creating a heating effect while consuming less power than a conventional vehicle heater.

Essentially in a heat pump the condenser and evaporator trade places by adding another TX valve before the condenser.

The HVAC controller, depending on a request for heat or air-conditioning, calls upon one expansion valve or the other. A single refrigerant circuit can be used for both cooling and heating.

A more efficient air-conditioning system is achieved through optimisation of the heat exchanger design that implements low power consumption, along with heating and cooling according to set temperatures, and temperature control when switching between cooler air and heater.

In this way the heat is then produced in the cabin as you try to cool the air outside the vehicle.

Heat-pump technology saw rapid iterations to improve performance, from Nissan’s first automotive use in the early Leaf, to Kia’s tweaks, and then to Denso’s further improved heat pump used by Toyota. These changes came because EV customers wanted more range in cold weather.

Inside the heat pump

You may find value in examining the generic heat pump illustration (below) we use in the ACDC classroom to help repair technicians understand the concept. 

The system has two heat exchanger/evaporators (11 and 2). One of them, number 2 in the schematic, is used for heating the cabin. The condenser (5) has two jobs as we will explain. The compressor (1) is a typical high-voltage scroll type and uses an accumulator (9). The accumulator (9) prevents any liquid from entering the compressor.

Two 12-volt solenoid 3-way valves (3 and 6) are fitted before each TX valve (4 and 10) with a bypass line to direct the refrigerant either to the expansion valve or around it. The “3-way valves,” as they are referred to by the OEMs, have only two directions.

The switching of the valves will allow the refrigerant to keep going straight through the same pipe or be redirected to another pipe. It has only two choices, and only one expansion valve is open at any one time.

There are also subsystems that are needed to support a heat pump in damp and very cold weather, or a subsystem to make it perform better. Expect more improvements in the future.

Chiller

If there was ever a poor choice of descriptors, calling something that gets hot a “chiller” would be it (8).

This component cools the PE (7; power electronics) but its purpose in the HVAC system is to warm the refrigerant on its way back to the compressor. That makes it easier for the compressor to do its job.

Less effort equals less energy from the high-voltage battery and more range. A lot more range? No, but every little bit helps.

The PE coolant had not been used on the Nissan Leaf before, so now we are recycling energy (a Kia idea) we otherwise would lose as waste heat. Clever, those engineers, applying Reduce, Reuse and Recycle to transportation.   

Condenser icing

When the heat pump is on and the climate outside the vehicle is wet and above freezing (1°C to 7°C), the exterior of the condenser (5) may freeze as the air moving through it lowers the air pressure and the temperature drops.

If the condenser has iced up, the heat-pump system stops working. What now? 

The HVAC computer will turn the A/C back on; this will now melt the ice as the condenser gets hot. Air flaps in the evaporator will move the cold air from the A/C outside as the high-voltage air grid heater (12) is turned on to heat the cabin.

At this point the main battery pack (13) will be asked to provide more energy than when the heat pump was the sole provider of cabin heat.

As the outside (ambient) temperature drops below freezing and/or the air dries out, the front condenser will stop icing up. This is all done without the driver knowing what is going on.

To stay warm is a human desire. In the beginning of the modern electric-vehicle age, many early EV adopters had to resort to their cars’ heated seats and heated steering wheel – while wearing a jacket, hat, gloves and heavy socks – to stay warm while optimising vehicle range.

Opting for cabin heat or reaching your destination is not a choice that EV drivers should have to make.

About the Author

Craig founded ACDC after 25 years as a workshop owner. He is engaged full time in understanding and teaching the technology used in hybrids and EVs.

As well as independent repairers and technicians, Craig’s audience now includes factory techs and engineers from OEMs and suppliers including Honda, John Deere, Midtronics, Delphi, Autoliv and others.

He describes his training style as “down-to-earth, knowledgeable and dynamic.”

During the 2020 COVID lockdown, he and the ACDC staff compiled a college-level book on electrified-vehicle technology, including fuel cell vehicles, which was recently published.

A number of people have travelled from Australia and New Zealand to train at ACDC and many more have taken online courses.

Craig is among the international speakers confirmed for Wire & Gas 2022.