Secondary loop and heat pump climate control under evaluation once more

  • 09-Dec-2015 02:28 EST
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Direct expansion A/C (top) vs. secondary loop. (Click arrow at top right of this image to view additional images.)

Is secondary loop climate control about to make an engineering splash? It got a close look for A/C when the industry was considering low-global-warming alternatives to R-134a, because it permitted confining a system using a refrigerant with safety/environmental issues to a safe underhood area. But once R-1234yf was seen as a safe albeit mildly-flammable choice, usable with under-dash circulation in a direct expansion cycle with existing technology, secondary loop went into mothballs. However, at the recent SAE Thermal Management Systems Symposium (TMSS) it was once again identified as a promising alternative.

Direct expansion vaporizes the liquid refrigerant in the under-dash heat exchanger—the evaporator—a single heat exchange loop. A carefully packaged secondary loop allows safe use of R-152a, a low-cost, low-global-warming, modestly more flammable refrigerant.

However, secondary loop requires an additional heat exchange circuit—from the refrigerant to an underhood coolant chiller circulating water and antifreeze mixture into an under-dash heat exchanger for cabin climate control. The second circuit reduces overall system efficiency somewhat, of course. So why would it be worth a new look?

Heat pump, thermal storage

Essentially reversing the refrigeration cycle (using control valves) allows the compressor to operate as a heat pump, both for electric vehicles (EVs) and fuel-powered cars. If the system—A/C only or with heat pump circuitry—is a secondary loop, there are potential fuel-economy benefits with an idle stop system. The secondary loop is effectively “engine-off” liquid thermal storage and can provide long periods of cabin comfort. 

According to a study presented at TMSS by the Department of Mechanical Engineering of the University of Maryland, the engine-off A/C cooling storage from secondary loop can be sized to exceed 10 minutes. Developing a system for thermal storage of heat for an EV is an obvious area for study, and research in that area reportedly will begin when funding, perhaps from a UN agency, becomes available.

The use of a water-antifreeze secondary loop also simplifies the plumbing in vehicles with rear HVAC. Instead of high-pressure refrigerant lines, the system can use lower-cost heater-like lines and hoses.

Because the greatest gains are in A/C, a team of researchers looked at large population areas where the climate includes a long cooling season. India, the example chosen, has winter temperatures of 20-25°C (68-77°F) in the mainland southeast, and long hot and humid warm seasons. By taking this geographic example, it could be seen if an optimized secondary loop A/C system could match or exceed the efficiency of a direct expansion system. 

The question and accompanying research was presented at TMSS by Sangeet Kapoor and Prasanna Nagarhalli of Tata Motors, and Timothy Craig and Mark Zima of Mahle's Delphi Thermal. They worked with refrigerant scientist James Baker and Dr. Stephen Andersen, retired EPA climate issues executive and now research director of IGSD (Institute for Governance and Sustainable Development), a non-governmental organization.

Refrigerant savings

Secondary loop is tough to package, adding a refrigerant-to-coolant heat exchanger/chiller, the pump to circulate the coolant, and extra piping. However, there is a 20% reduction in refrigerant, which results in a significant cost saving if it’s R-1234yf. Using R-152a, because it is very low in cost, saves even more. The coolant mixture used was 30% antifreeze, 70% purified water, chosen for heat transfer efficiency.

A dynamometer test program looked at COP (Coefficient of Performance) for the secondary loop, running it through a complete Federal Test Procedure (FTP) at 28°C (82°F), the New European Drive Cycle (NEDC) at 28°C and an SCO3 cycle, the supplemental EPA cycle for A/C operation, which is run at 33°C (91°F).

The tests were run with (1) no A/C, (2) continuous A/C with the cabin soaked to ambient temperature, (3) with the A/C compressor disengaged at idle, and (4) on acceleration.  Although there was a modest improvement with the compressor stopped at idle, the combination with the disconnect-on-acceleration, taking greater advantage of the inherent thermal storage, produced dramatic fuel-economy improvements on all three cycles. The numbers: 6.5% on the FTP, 11.5% on the NEDC, and 12.5% on the SCO3.

R-152a has a global warming number below 150, so it is lower than international regulatory limits. Although this is higher than R-1234yf’s under 10, unlike R-1234yf it does not produce TFA (trifluoroacetic acid, a plant growth inhibitor) as a degradation product, although currently not regulated.

The compressor with a circuit reversal to operate as a heat pump, providing heat for the passenger compartment, is superior in a secondary loop because it also provides engine/EV-off heat from the thermal storage. Even a half-way step—heat pump in a direct expansion system—has demonstrated value for an electric drive vehicle, where conventional electric heating (resistance or Positive Temperature Coefficient) can sharply reduce winter driving range. The heat pump is much more efficient than conventional heating.

Nissan, Kia EVs

Both the Nissan Leaf and Kia Soul EV offer direct expansion heat pump systems, presently using R-134a systems like the companies’ gasoline-engine cars, but with R-1234yf on the horizon. There is a baby step into secondary loop: a water-antifreeze chiller circuit for cooling EV electronics, and Kia estimates a 27% improvement from the heat pump in electrical heating efficiency over the winter temperature range. 

Conventional (resistance and Positive Temperature Coefficient) heating has a major effect on EV range in winter. A study with various sizes of cars, reported at TMSS by Dr. Gregor Homann of Volkswagen AG and Prof. Jurgen Kohler of the University of Brunswick, showed a loss of 20-50% with conventional electric heating. The greatest drop, not surprisingly, was suffered by the smallest vehicle, which has the smallest battery pack, in testing at -7°C (+19°F).

Heat pumps have a positive COP in ambient temperatures starting from above -15 to -18°C (0 to +5°F). They approach 2.8-3.5 at 10°C (50°F), the COP depending on refrigerant used and pump design. So although the heat pump makes a significantly positive contribution at moderately cold temperatures, in really cold weather conventional electric heating still is needed. 

The heat pump might draw heat from warmer sources than ambient air, such as EV electronics, motor, and the battery pack. But these would require packaging a sophisticated ducting system and can make only minor contributions, if any, at low ambient temperatures. In many areas, however, the ambient temperatures during heating seasons are sufficiently high to make the heat pump a valuable addition for both EVs and even many fuel-powered vehicles.

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