Zone, spot HVAC systems could save fuel or extend EV range

  • 18-May-2012 02:30 EDT
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Zoning of HVAC concentrates cooling (shown) and heating on driver when rest of cabin is unoccupied. (Hyundai)

The concept of zoning the passenger compartment’s climate control system isn’t new. The control system that permits the front passenger to adjust air temperature to a different level from the driver dates back many years. And revisions to the HVAC case that enable a third duct with independent controls for the rear has been a popular addition to many cars. But new approaches to zoning are coming, as part of systems that can reduce cooling and heating output, as well as airflow volume, and direct airflow only to the occupants, specifically the driver if there are no other passengers.

The objective is obvious: improve the fuel economy, as A/C-on operation is now included in one of the U.S. EPA test cycles. But there is an equally important objective: to extend the range of electric vehicles (EVs), which use an electric-drive compressor for A/C and PTC (positive temperature coefficient) heaters, which may be an even greater current draw on EVs. These heaters are also used to supplement conventional heating on hybrids.

Several systems have been proposed. One available for application was introduced by Denso Corp. Two others, an early prototype from Hyundai and an experimental system from General Motors and Delphi, were described at the recent SAE World Congress in Detroit. The Delphi-GM project is focused on passenger thermal comfort with a spot cooling approach to improve efficiency, rather than just climate control of a cabin zone.

All HVAC systems can be “dialed” up and down to precisely match calculated heating and cooling requirements, such as cooling adjustment by increasing or reducing displacement of a variable displacement compressor or changing the on-off strategy of a cycling compressor clutch. There are various ways to modify the HVAC case to direct the airflow to specific parts of the cabin.

Denso, Hyundai systems

The Denso approach, which the company claims can save up to 20% of A/C-use energy in the driver-only comfort mode, uses an HVAC case with five separate sections covering three zones: driver only, front passenger, and rear seat. The case is sized to match existing cases for packaging. There are two side-by-side A/C-vent upper sections: one for the driver’s side and front side window and one for the front passenger’s side and its side window. There are three side-by-side lower sections: driver’s side footwell, floor duct to the rear seat, and passenger’s side footwell. The control system uses outside, lower-humidity air only for upper-dash (defrost-vent, to prevent window fogging) when either or both of the front zones are active in heat mode, which conserves interior heat for the footwell outlets. The Denso data does not include any detail on the heating and cooling strategies used.

The Hyundai system, still in development phase for the company's EV, is similar in concept but with an approach to execution that could be translated into driving range improvement for the EV. Hyundai studies showed that the nominal EV driving range of 160 km (100 mi) was reduced by 20-30% in A/C mode and by 30-50% in heating mode. The developmental system reduces power consumption by 17% in A/C mode at 35°C (95°F) and 20% in PTC heating mode at 0°C (32°F). The EV range with driver-only climate control increased in A/C mode by 9% and in heating mode by 4%, said Chunkyu Kwon of the company’s R&D Center, in a presentation at the SAE World Congress.

The case has the temperature blend-air door adjacent to and above the heater core toward the front. At the rear is a partition between the front passenger and driver’s side, plus a duct at one side for airflow to the rear seat. Control doors with actuators for the front passenger’s side and rear-seat duct can shut off airflow to those zones. Mode doors with actuators in the case direct airflow up or down for the front zones.

Hyundai developed a zone temperature control algorithm for the system, in which thermal load for each zone was estimated and tuned with CFD and then verified by vehicle testing.

However, the R&D program still has a long way to go, Kwon indicated. Despite the fact that only the driver’s zone was activated for power-saving evaluation, “the 17-20% seems small considering that 75% of the cabin volume was unoccupied.” Theoretically, he said, 75% of the energy should be saved because unoccupied zones require no HVAC, although a gap can be expected by the fact that “conditioned air streams in occupied zones inevitably flow through unoccupied zones, resulting in significant heat losses. The control logic will be refined further to reduce such losses by optimizing airflow rate, direction, and discharge temperatures.”

He said that the focus also will shift to passenger thermal comfort, “which is known to be more effective in reducing the power consumption because it targets passengers themselves, rather than the entire air volume of occupied zones.”

Delphi-GM project

Validity of that approach is what the Delphi project is intended to demonstrate, in the spot cooling system developed with GM under a grant from the U.S. Department of Energy (DOE). In its presentation at the SAE World Congress, the engineering team noted that unlike buildings, automobiles face a greater challenge in maintaining passenger comfort, in part because of major variations in thermal mass of materials on the interior and a heterogeneous thermal/airflow environment, including solar loads that can change instantly. Zone cooling alone still results in parasitic heat losses to interior materials, and airflow from the registers diffuses, so it may not reach the occupant at the velocities needed for comfort.

The spot cooling approach that Delphi-GM took was applied to 10-90 percentile occupants of the front seats of a GM five-seat crossover. Its bottom-line number in steady-state testing was just 30-50% of the HVAC airflow per passenger vs. a conventional HVAC system, using three 20-W thermoelectric modules with airflow nozzles for spot cooling. Additionally, spot cooling provides a shortened time to occupant comfort.

First, a CAD was generated with Siemens PLM Software’s NX Unigraphics, ANSYS Inc. design modeling software, and its FLUENT flow modeling simulation package. The model not only had to be capable of “understanding” all the aspects of the vehicle model but also the surfaces of an occupant relative to heat transfer, including clothed and bare skin. The passenger compartment itself would get a minimum amount of climate control—29°C (84°F) in A/C mode. So the greater the difference between that temperature and the spot cooling airstreams through the cabin ambient, the more complex the effects that had to be captured by CFD. The model for cabin temperature turned out to be 2.5°C (4.5°F) lower than actual measurement, which was attributed to modeling simplifications.

Perhaps the most difficult issue with spot cooling is finding out what actually makes most people comfortable, and for this project the Delphi-GM team used the University of California, Berkeley thermal comfort model. Inputs from a thermal manikin contributed to readings of EHT (equivalent homogenous temperature) and skin surface heat flux that were used by the UC Berkeley software. Delphi itself has considerable experience with driver’s seat spot heating and cooling control strategies (http://www.sae.org/mags/aei/3283), so it understands the complexities, and it tried several spot cooling configurations and airflows. The model was tunnel-tested for human comfort validation. ThermoAnalytics RADTHERM was used for analysis of the comfort and physiological modeling of the manikin, using inputs from CFD analysis.

People do not like left-to-right temperature and airflow velocity differentials or large temperature gradients on the body, the researchers said. Further, small skin temperature changes can make a big difference in comfort perception, they added, so the climate control and airflow nozzles must be carefully tuned to avoid overcooling or overheating any particular body part.

The researchers used cooling nozzles for the face (lower part of face, with nozzles in the rear of the seat headrest or alternatively the headliner), chest (symmetrically from headliner and overhead console or asymmetrically from A-pillar or cabin frame), and lap (from dashboard primarily to thighs, arms, and pelvis), plus a climate control seat. Although cooling to all four locations was the most robust system for all cabins and people, a trio of chest, face, and seat was very good too, they added.

Seats have a lot of thermal mass, so isolating occupants with seat cooling is valuable, and although not modeled, provided up to 25% of the total heat transfer in actual use in the tunnel tests.

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