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Written by Paul Weissler
Thermal storage, now coming into use as a low-cost method to maintain A/C cooling during idlestops in warm weather, has been identified bymodeling 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 RidgeNational 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 heatrejected by EV power electronics would be difficult to collect efficiently. To now, the vehicle"s traction batteries have been theonly power source for conventional heating, with some addedefficiency from reverse-cycling the A/C into heat pump operation. Heat needed for 46-min commute<br /> For EV drivers, the winter-heatingdemand can make a routine commute uncomfortable and even dreadful over longer distances. Adding enough extra battery capacity for acommute-length source of conventional warmth from PTC (positive temperature coefficient) or resistance heaters hasbeen deemed too costly. Preheatingthe cabin provides a short period of warmth, and features such as heated seatsand steering wheel can improve occupants" perception of warmth. However, the length of the average U.S. round-trip commute is46 minutes, according to a Gallup survey. Engineers recognize that more has to be done to make thewinter commute free of range anxiety or the need to recharge while the car isparked during the workday. Sufficient thermalstorage 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 latentheat at a relatively constant temperature. The use of PCM tomaintain A/C cooling during an idle stop, already available for some internal-combustion vehicles, is a relatively low-cost, easily packaged answer, asthe 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 intothe evaporator, chilling the airflowwhen the compressor has been stopped. For heating, the PCM is stored in chambers in an underhood heat exchanger that is in aloop 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"sprocessed differently and as a result would react with air and moisture. Therefore, the material must be stored carefully andprior to installation in the PCM Heat Exchanger (Hx) chamber, the chamber mustbe evacuated to remove any air and moisture. Using special equipment, the Hx then is charged with PCM and thechamber is sealed. The first-choice PCM, designated DPT 83, has amelting point (phase change temperature) of 83 C (181 F), sufficiently close tothe 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 heatcapacity is 348 joules/gram, highest of eight PCMs evaluated and well abovethe 200 J/g of older PCMs, which helps minimize package size. Slightly lower in performance but alsopotentially suitable is DPT 68, which has a phase change temperature of 68 C (154 F) and a latent heat rating of 342 J/g. With A/C idlestop, it is important for the PCM to recharge quickly from the A/C-enginerestart 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 heatexchanger 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 PCMnotwithstanding, 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 of33 kg (73 lb), including 21 kg (46 lb) of PCM plus 12 kg/ 26 lb for the PCM heatexchanger (charged to 120 C/248 F) to provide at least 20% range extension. ThePCM system also assumes a pre-heated cabin, which reduces thevehicle-in-operation demand to steady-state heating. For A/C operation, cold airpassing through the under-dashboard evaporator freezes the liquid wax-type PCMto form a cold solid. During idle stop, thecompressor stops and warm air passes over the solidified PCM to provide cooled cabin airflow. As the air gives up heat to thePCM, 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, itfunctions opposite of the idle-stop process. This configuration wasnamed ePATHS electric PCM-AssistedThermal Heating System. It consists oftwo heat exchangers: the PCM Hx and the under-dash passenger compartmentheater, with tubing and control valves to connect them. In addition, there is an electrical loop thatadds the conventional PTC heater. The PCM is a solid at ambient temperature. Thepre-charge starts with the EV plug-insystem that uses electricity to charge the EV battery pack and, via electricheating elements inside the PCM Hx, also heats the PCM to a liquid (while also operating the PTC heaters to pre-warmthe cabin). Electric pump circuit In vehicleoperation, an electric pump circuit (operating continuously and/or inpulse-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. Thisis somewhat similar to the IC-engine setup that flows engine-heated coolant to the cabin heater. The proposedsystem 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"smore heat available: when most ofthe 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 worthusing, and a dual-heater core is projected to take advantage. The PCM Hx continues to circulate the heatedglycol/water mixture through the forwardsection of the core to preheat the incoming airflow, adding an additional 10minutes to PCM heating, for a total of 50 min. The PTC heater is activated to complete theheating of the glycol/water solution asit flows through the rear section of the core. <br />
Date written: 20-Apr-2016 01:31 EDT
More of this article on the SAE International Website
ID: 3056
Thermal storage, now coming into use as a low-cost method to maintain A/C cooling during idlestops in warm weather, has been identified bymodeling 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 RidgeNational 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 heatrejected by EV power electronics would be difficult to collect efficiently. To now, the vehicle"s traction batteries have been theonly power source for conventional heating, with some addedefficiency from reverse-cycling the A/C into heat pump operation. Heat needed for 46-min commute<br /> For EV drivers, the winter-heatingdemand can make a routine commute uncomfortable and even dreadful over longer distances. Adding enough extra battery capacity for acommute-length source of conventional warmth from PTC (positive temperature coefficient) or resistance heaters hasbeen deemed too costly. Preheatingthe cabin provides a short period of warmth, and features such as heated seatsand steering wheel can improve occupants" perception of warmth. However, the length of the average U.S. round-trip commute is46 minutes, according to a Gallup survey. Engineers recognize that more has to be done to make thewinter commute free of range anxiety or the need to recharge while the car isparked during the workday. Sufficient thermalstorage 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 latentheat at a relatively constant temperature. The use of PCM tomaintain A/C cooling during an idle stop, already available for some internal-combustion vehicles, is a relatively low-cost, easily packaged answer, asthe 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 intothe evaporator, chilling the airflowwhen the compressor has been stopped. For heating, the PCM is stored in chambers in an underhood heat exchanger that is in aloop 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"sprocessed differently and as a result would react with air and moisture. Therefore, the material must be stored carefully andprior to installation in the PCM Heat Exchanger (Hx) chamber, the chamber mustbe evacuated to remove any air and moisture. Using special equipment, the Hx then is charged with PCM and thechamber is sealed. The first-choice PCM, designated DPT 83, has amelting point (phase change temperature) of 83 C (181 F), sufficiently close tothe 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 heatcapacity is 348 joules/gram, highest of eight PCMs evaluated and well abovethe 200 J/g of older PCMs, which helps minimize package size. Slightly lower in performance but alsopotentially suitable is DPT 68, which has a phase change temperature of 68 C (154 F) and a latent heat rating of 342 J/g. With A/C idlestop, it is important for the PCM to recharge quickly from the A/C-enginerestart 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 heatexchanger 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 PCMnotwithstanding, 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 of33 kg (73 lb), including 21 kg (46 lb) of PCM plus 12 kg/ 26 lb for the PCM heatexchanger (charged to 120 C/248 F) to provide at least 20% range extension. ThePCM system also assumes a pre-heated cabin, which reduces thevehicle-in-operation demand to steady-state heating. For A/C operation, cold airpassing through the under-dashboard evaporator freezes the liquid wax-type PCMto form a cold solid. During idle stop, thecompressor stops and warm air passes over the solidified PCM to provide cooled cabin airflow. As the air gives up heat to thePCM, 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, itfunctions opposite of the idle-stop process. This configuration wasnamed ePATHS electric PCM-AssistedThermal Heating System. It consists oftwo heat exchangers: the PCM Hx and the under-dash passenger compartmentheater, with tubing and control valves to connect them. In addition, there is an electrical loop thatadds the conventional PTC heater. The PCM is a solid at ambient temperature. Thepre-charge starts with the EV plug-insystem that uses electricity to charge the EV battery pack and, via electricheating elements inside the PCM Hx, also heats the PCM to a liquid (while also operating the PTC heaters to pre-warmthe cabin). Electric pump circuit In vehicleoperation, an electric pump circuit (operating continuously and/or inpulse-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. Thisis somewhat similar to the IC-engine setup that flows engine-heated coolant to the cabin heater. The proposedsystem 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"smore heat available: when most ofthe 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 worthusing, and a dual-heater core is projected to take advantage. The PCM Hx continues to circulate the heatedglycol/water mixture through the forwardsection of the core to preheat the incoming airflow, adding an additional 10minutes to PCM heating, for a total of 50 min. The PTC heater is activated to complete theheating of the glycol/water solution asit flows through the rear section of the core. <br />
Date written: 20-Apr-2016 01:31 EDT
More of this article on the SAE International Website
ID: 3056
