2016 SAE Congress: Thermal storage is solution to EV winter range loss

  • 20-Apr-2016 01:31 EDT

Illustration of ePATHS.Main section at left has PCM heat exchanger.Two-stage heater core is at right, with PTC heater at back side. 

Thermal storage, now coming into use as a low-cost method to maintain A/C cooling during idle stops in warm weather, has been identified by modeling studies and laboratory tests as also offering an inexpensive and effective solution for heating the cabin of electric vehicles. The study results were presented by researchers from Mahle and the Oak Ridge National Laboratory during the 2016 SAE World Congress.

Electric vehicles typically lose up to 60% of their operating range during cold weather operation due to the power required to heat the passenger compartment, while the small amount of heat rejected by EV power electronics would be difficult to collect efficiently. To now, the vehicle's traction batteries have been the only power source for conventional heating, with some added efficiency from reverse-cycling the A/C into heat pump operation.

Heat needed for 46-min commute

For EV drivers, the winter-heating demand can make a routine commute uncomfortable and even dreadful over longer distances.

Adding enough extra battery capacity for a commute-length source of conventional warmth from PTC (positive temperature coefficient) or resistance heaters has been deemed too costly. Preheating the cabin provides a short period of warmth, and features such as heated seats and steering wheel can improve occupants' perception of warmth. However, the length of the average U.S. round-trip commute is 46 minutes, according to a Gallup survey. Engineers recognize that more has to be done to make the winter commute free of range anxiety – or the need to recharge while the car is parked during the workday.

Sufficient thermal storage in Phase Change Material (PCM) for a complete commute could improve the EV's prospects as year-round transportation in cold-winter markets, according to the Mahle and ORNL experts. PCM is a class of materials that go through a phase change (solid-to-liquid or vice versa) while absorbing or releasing a large amount of latent heat at a relatively constant temperature.

The use of PCM to maintain A/C cooling during an idle stop, already available for some internal-combustion vehicles, is a relatively low-cost, easily packaged answer, as the total required cooling interval typically is less than one minute. And if necessary, the engine and A/C system can be restarted to maintain comfort. For IC vehicles, the PCM is held in storage areas built into the evaporator, chilling the airflow when the  compressor has been stopped.

For heating, the PCM is stored in chambers in an underhood heat exchanger that is in a loop with coolant flow to the under-dash heater core. The material is in the paraffin wax family - similar to that used for supplemental cooling, but it's processed differently and as a result would react with air and moisture. Therefore, the material must be stored carefully and prior to installation in the PCM Heat Exchanger (Hx) chamber, the chamber must be evacuated to remove any air and moisture. Using special equipment, the Hx then is charged with PCM and the chamber is sealed.  

The first-choice PCM, designated DPT 83, has a melting point (phase change temperature) of 83°C (181°F), sufficiently close to the EV coolant temperature specification of 85°C (185°F). This should enable it to provide heater coolant temperature approximately equivalent to what a PTC (Positive Temperature Coefficient) heater system would deliver. Latent heat capacity is 348 joules/gram, highest of eight PCMs evaluated and well above the 200 J/g of older PCMs, which helps minimize package size.

Slightly lower in performance but also potentially suitable is DPT 68, which has a phase change temperature of 68ºC (154ºF) and a latent heat rating of 342 J/g.

With A/C idle stop, it is important for the PCM to recharge quickly from the A/C-engine restart and be ready for the next idle stop. But for cabin heating, the more critical demand is ensuring remaining PCM heat storage isn't lost through the workday in cold weather (-10°C/14°F) while the EV is parked outdoors. Through an eight-hour workday, the PCM heat exchanger is projected to retain 80% of the remaining latent heat by employing a lightweight insulation package made with vacuum-insulated panels.

However, the engineering target is 90% heat retention and the Mahle and ORNL researchers believe that target can be met with improved construction.

Weight penalty

The improved PCM notwithstanding, the PCM system does add weight. But  (depending on the lithium-ion battery pack used) the overall mass increase is no more, or even less, than increasing the size of the battery pack. The researchers decided on a total package of 33 kg (73 lb), including 21 kg (46 lb) of PCM plus 12 kg/ 26 lb for the PCM heat exchanger (charged to 120°C/248°F) to provide at least 20% range extension.

The PCM system also assumes a pre-heated cabin, which reduces the vehicle-in-operation demand to steady-state heating. 

For A/C operation, cold air passing through the under-dashboard evaporator freezes the liquid wax-type PCM to form a cold solid. During idle stop, the compressor stops and warm air passes over the solidified PCM to provide cooled cabin airflow. As the air gives up heat to the PCM, the PCM changes from solid to liquid until the A/C restarts or the cold storage is exhausted.

When the PCM is used for heating the cabin, it functions opposite of the idle-stop process. This configuration was named  ePATHS—electric PCM-Assisted Thermal Heating System. It consists of two heat exchangers: the PCM Hx and the under-dash passenger compartment heater, with tubing and control valves to connect them. In addition, there is an electrical loop that adds the conventional PTC heater. The PCM is a solid at ambient temperature.

The pre-charge starts with the EV plug-in system that uses electricity to charge the EV battery pack and, via electric heating elements inside the PCM Hx, also heats the PCM to a liquid (while also operating the PTC heaters to pre-warm the cabin).

Electric pump circuit

In vehicle operation, an electric pump circuit (operating continuously and/or in pulse-width-modulated mode) runs a water-glycol mixture through the PCM Hx, absorbing heat from the molten PCM and flowing to the under-dash heater. This is somewhat similar to the IC-engine setup that flows engine-heated coolant to the cabin heater.

The proposed system is projected to comfortably exceed the targeted 20% increase in EV range. Using 70% outside air at -10°C/14°F, it would operate in PCM-only mode for 40 min.

Total PCM heating is equivalent to 3.3 kW·h. However, there's more heat available: when most of the PCM latent heat is exhausted, there still is some residual heat, even if at about 60°C (140°F) it is at too low a temperature for full PCM heating function.

That's energy worth using, and a dual-heater core is projected to take advantage. The PCM Hx continues to circulate the heated glycol/water mixture through the forward section of the core to preheat the incoming airflow, adding an additional 10 minutes to PCM heating, for a total of 50 min. The PTC heater is activated to complete the heating of the glycol/water solution as it flows through the rear section of the core.

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