Thermal storage evaporators offer idle-stop cost saving for A/C operation

  • 15-May-2012 01:32 EDT
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Illustration shows thermosyphon effect on refrigerant in tubes. Primary thermal wax storage is between tubes, but if more cooling is needed, wax chambers are extended (as shown) to surround refrigerant inlet and outlet lines in tank.

Idle stop-start is creeping onto the option lists at U.S. automakers, because in the real world it boosts fuel economy up to about 8% and there are regulatory credits. The situation is somewhat complex, requiring an understanding of what the regulations are and where they are believed to be going. Right now, idle-stop boosts city fuel economy 3.3 to 3.5%, which can add 1.0 mpg to the window sticker’s city number.

Further, the system provides a 2.9-g/mi credit to apply to the fleet carbon limits, which average about 250 g/mi. That credit translates to higher CAFE (corporate average fuel economy). Automakers would like idle-stop to provide an off-cycle credit, too, as there aren’t enough stops in the U.S. EPA fuel economy/emissions test cycles to reflect real value of the system. However, EPA feels any additional credits should be for changes not now given by the carbon credits.

Idle-stop creates a cooling comfort issue in hot weather as the A/C turns off. So its fuel efficiency value is reduced by the need to deactivate idle-stop in very hot weather and/or restart the engine when cabin temperature reaches a specified threshold. Idle-stop also poses other real-world problems, cost included.

The present alternative is an electric-drive compressor, but that adds about $150 or more to the manufacturer’s cost for the feature, already about a $400 option to cover the cost of a heavy-duty starter and AGM (absorbent glass mat) battery, electronic control modules, etc.

The lowest-cost alternative coming on stream is an A/C evaporator with phase change material compartments—cold storage to maintain A/C cooling during idle-stops. The storage medium is thermal wax and the greater the amount, the longer the system can maintain cooling during an idle-stop.

How long does the cooling have to be maintained to cover most idle-stops? There is no definitive answer because North American idling studies to date were aimed at measuring total idle times for effect on emissions. A 1993 Canadian government study in Ottawa indicated 71% were a minute or less. A 2003 study in Sacramento, CA, by the University of California, Riverside College of Engineering, said 67% of all stops under six minutes were a minute or less, establishing a good co-relation.

Thermal wax storage is intended to maintain comfortable airflow during an idle-stop, which may be measurably higher than the perhaps up to 7°C (45°F) airflow exiting the registers in maximum A/C, in fact as high as 15°C (59°F), particularly in high ambient. So the performance of a thermal storage unit is evaluated by itself, with subjective assessments of comfort by passengers. Merely shutting the engine and A/C compressor yields about 15 s of comfort from residual cooling capacity in the evaporator core and cool air in the HVAC case. But if most idle-stops are under a minute, a one-minute performance should be generally adequate and cabin temperature should rarely reach discomfort level and trigger an engine restart.

Behr has been selling to BMW for its European 7 Series a thermal wax storage system it rates for a peak of 40 s cooling with ambient at 30°C (86°F). Thermal wax is contained within an additional heat exchanger ahead of the evaporator. Denso has said it is developing a thermal storage evaporator but has released no details.

A working version of a Delphi system was exhibited at the recent SAE World Congress in Detroit, which the company said will be OE-installed in 2015. It unofficially estimated the OE incremental cost at “under $20.” Delphi has two different wax storage configurations. The evaporator is the popular plate-fin type, although a micro-channel tubular type evaporator also will become available.

For a “standard” application, wax is stored only in longitudinal chambered areas between the refrigerant tubes just under the top tank. When more compressor-off cooling is wanted, the thermal wax-filled chambers are extended into the tank area where refrigerant inlet and outlet lines connect to the core. The lower, between-tubes areas hold about 240 g (8.5 oz) of wax. The area in the tank adds space for another 60 g (2.1 oz), which provides a linear increase (about 25%) in cooling time.

When the A/C is running, the flowing refrigerant vapor solidifies the wax. During an idle-stop, warming refrigerant transfers heat to the wax in the upper part of the evaporator, and as the wax melts, the refrigerant chills. The warmer refrigerant in the lower parts of the tubing rises, starting a thermosyphon effect that continues as long as the wax has cold storage capacity. The blower fan continues to operate, pushing air through the evaporator to cool it and then out the A/C registers.

Peak times for continued A/C cooling are not necessarily meaningful, because wax recharge time varies according to the length of the previous idle-stops and ambient temperatures. So although recorded, Delphi prefers not to quote them. Assume the car goes through engine stops of 60 s or less with at least 45 s for wax storage recharge between idle-stops with the Delphi evaporator. The 300-g (10.6-oz) design with the compressor off will consistently maintain comfortable airflow with maximum vent temperatures of 15°C (59°F) or below, at 27°C (81°F) cabin temperature, Delphi said.

The Delphi evaluations for compressor-off were made with the blower fan lowered two speeds, a comfort-extending step. A/C was in recirculation mode in all cases except compressor running and ambient at 25°C (77°F) ambient, at which it was in outside air mode.

Thermal storage in the evaporator has important secondary effects on idle stop-start vs. a system with an electric-drive compressor. If the A/C is being maintained during the idle-stop by electric-drive compressor operation, the cabin airflow can be lower, enhancing comfort. However, the current draw on the battery means that in stop-and-go operation, the idle-stop strategy may have to become less aggressive for the vehicle to retain enough battery capacity for restarts. Yes, recharge time for the thermal wax has to be factored in, too, but that occurs with the engine running and the time is likely to be lower than for a battery powering an electric-drive compressor.

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