NREL looks to reduce idle climate control loads with CoolCab

  • 10-May-2013 06:03 EDT
CoolCab IR.jpg

Infrared camera imaging shows the heat loss in truck cabs. (Ken Proc)

In the U.S., an intercity long-haul truck averages approximately 1800 h/year for idling, primarily for sleeper cab hotel loads, consuming 838 million gal (3.17 billion L) of diesel fuel across the entire long-haul fleet. Including workday idling, over 2 billion gal (7.6 billion L) of fuel are used annually for truck idling. The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) is working on solutions to reduce idling fuel use through the CoolCab project. The objective of CoolCab is to work closely with industry to design efficient thermal-management systems for long-haul trucks that minimize engine idling and fuel use while maintaining the cab occupant comfort. If the fuel savings from new technologies can provide a one- to three-year payback period, fleet owners will be economically motivated to incorporate them, the researchers believe.

NREL conducted an experimental test program at its Vehicle Testing and Integration Facility in collaboration with Volvo Trucks; Aearo Technologies LLC/E-A-R Thermal Acoustic Systems; 3M Corp.; and Dometic Environmental Corp. The impact of thermal load-reduction technologies on idle-reduction systems was characterized by conducting thermal soak tests, overall heat transfer tests, and 10-h rest period A/C tests. Technologies evaluated included advanced insulation packages, a solar reflective film applied to the vehicle’s opaque exterior surfaces, a truck featuring both film and insulation, and a battery-powered A/C system. The results demonstrated technologies that reduce heating and cooling loads for long-haul truck idling by 36% and 34%, respectively, which yielded a 23% reduction in battery pack capacity of the idle reduction system.

The researchers also collected data for the development and validation of a CoolCalc HVAC truck cab model. CoolCalc is a simplified, physics-based HVAC load estimation tool developed by NREL that requires no meshing, has flexible geometry, excludes unnecessary detail, and is less time-intensive than more detailed CAE modeling approaches.

Test procedures

NREL is closely collaborating with OEMs and suppliers to develop and implement a three-phased strategic approach that evaluates commercially available and advanced vehicle thermal management and idle reduction technologies. Phase I is baseline testing and model development, Phase II involves thermal load reduction, and Phase III is idle reduction, during which the most promising technologies are applied to vehicles and evaluated further. To experimentally characterize the impacts of the technologies being studied, thermal test procedures were conducted in each phase of the project.

Thermal soak tests were conducted to evaluate the impact of technologies in an engine-off solar loading condition. This test procedure was used to characterize technology impacts on interior air temperatures in a modified truck compared to those in the baseline truck. During summer operation with passive vehicle thermal load reduction technologies, the best possible steady-state performance is to reduce the interior temperature to the ambient temperature. The percent of maximum possible temperature reduction was developed to describe this maximum possible reduction in interior air temperature rise above ambient.

Overall heat transfer (UA) tests were conducted to quantify baseline heat loss and the impact of adding commercially available and advanced insulation. The test was conducted at night with a 1-kW heat source in each vehicle. The sleeper curtain was closed, and the average interior sleeper air temperature was calculated by averaging eight k-type thermocouples with six located in accordance with the American Trucking Association Technology Maintenance Council’s recommended practice RP422A. The addition of two thermocouples suspended 0.46 m (1.5 ft) from the sleeper headliner and oriented above the standard sleeper floor measurements improved the accuracy of the average air temperature of the sleeper compartment by better capturing the air temperature distribution.

The UA value (W/K) was calculated by measuring the heater power and the temperature difference between the interior air temperature and ambient temperature. If the area of heat transfer in the sleeper is known—i.e., the total area of the boundaries of the sleeper living space—this test can be used to determine a truck’s thermal resistance, R-value.

Infrared imaging of the test vehicle was conducted to identify heat transfer reduction opportunities in the cab/sleeper compartments by locating high heat loss paths in the cab construction. In this procedure, a 1-kW heat source was placed inside the sleeper compartment and operated overnight to ensure radiant heat from the surrounding ground is minimized. The heat source inside the truck heats the air, inner walls, glass, and interior roof areas. Nighttime images of the front, rear, sides, and roof of the vehicle exterior were collected with a FLIR infrared imaging camera. Hot exterior surfaces, in contrast to the colder surrounding areas, identify high heat loss paths that can be selected for the placement of thermal load reduction technologies.

Infiltration tests were conducted to characterize the natural air exchange rate, or infiltration rate, between the cab interior and local environment. Air exchange rates can impact experimental results and are inputs to CoolCalc model development and validation. The test procedure was conducted by introducing a known inert gas such as sulfur hexafluoride (SF6) in a quantity between 0-1 ppm into the cab/sleeper compartments. A Brüel & Kjær gas analyzer was used to monitor the concentration of SF6 over the 3-h test period. The decay rate of the gas concentration in the cab was then used to calculate the infiltration rate, expressed in air changer per hour (ACH) over the 3-h test period.

Rest period A/C tests were conducted to characterize thermal management technology impacts on an electric no-idle A/C system. A 2050-W (7000 [BTU/hr]) Dometic electric A/C system was installed in the sleeper compartment of each truck. Each A/C unit was controlled to a setpoint of 22.8°C (73°F) and connected to a data logger that recorded current, voltage, and power. Data was logged over 24-h periods and was reduced to represent a typical 10-h rest period. The A/C systems operated most consistently in the daytime between the hours of 9:00 a.m. and 7:00 p.m. This time period was used for the 10-h daytime A/C tests. Electrical energy consumed in watt-hours was determined by integrating the A/C load over the 10-h test period.

The test program was conducted at NREL’s Vehicle Testing and Integration Facility in Golden, CO, at an elevation of 5997 ft, during the months of May through September. The experimental setup included a test vehicle, representative of current production, provided by Volvo Trucks North America and an NREL-owned control vehicle. The vehicles were oriented facing south and separated by a distance of 25 ft (7.6 m) to maximize solar loading and minimize shadowing effects.

A National Instruments SCXI data-acquisition system was used to record measurements at a sampling frequency of 1.0 Hz, which was averaged over 1-min intervals. The instrumentation for each vehicle included 52 calibrated k-type thermocouples, comprised of 30 air and 22 surface locations. The thermocouples were calibrated using an isothermal bath and reference probe, achieving a U95 uncertainty of ±0.18°C. Air temperature sensors had a double concentric cylindrical radiation shield to prevent errors due to direct solar radiation.

Test results

Baseline testing was done to characterize the production performance of the trucks, calibrate the control vehicle to represent a baseline test vehicle, and collect data for the development of a CoolCalc truck model. This calibration was done using several days of unmodified vehicle baseline data. In the thermal soak test configuration, the calibrated average interior air temperature in the control vehicle was on average 0.32°C (0.58°F) warmer than the test vehicle on validation days.

With baseline UA testing, the control truck tested 20.8 W/K higher than the test truck, on average. Therefore, on modified test days, the measured control truck UA value was reduced by 20.8 W/K. Two calibration validation days were used to verify that the adjusted control truck UA matched the measured test truck data.

Baseline A/C system test results showed that the A/C systems did not operate overnight at the set point and weather conditions tested. Therefore, a 10-h daytime A/C-on test period, from 9 a.m. to 7 p.m., was specified. Both systems toggled on and off throughout the day as the systems tried to maintain a set point of 22.8°C (73°F). Each system peaked at approximately 700 W and had similar start and stop times. This data was used to calibrate the total energy consumed over the 10-h rest period by the control truck A/C system to that of the test truck A/C system.

The objective of Phase II was to identify high value opportunities to reduce the thermal loads of the test truck. CoolCalc analysis, infrared imaging, and thermal testing were used to identify promising load-reduction strategies. The CoolCalc tool characterized an opportunity to reduce rise over ambient by as much as 25%, using a generic truck model, through application of exterior materials such as reflective paints, films, or radiant barriers to reduce the exterior absorptivity.

To experimentally evaluate the opportunity for radiant barriers and to confirm the CoolCalc parametric analysis, a reflective radiant barrier was applied to a truck’s exterior opaque surfaces. A 5°C (9°F) temperature reduction was achieved with the reflective barrier. Considering the differences between the generic model built in the CoolCalc tool and the test vehicle, the trends as well as magnitude agreed well. The differences between the test vehicle and the generic cab modeled in CoolCalc include geometry, material properties, cab constructions, and window locations and glazings. Although significant differences were observed between the experiments and the parametric study, it was apparent that lowering the absorptivity of the truck exterior will improve the thermal performance of a truck in a solar-loading condition.

Nighttime infrared imaging identified opportunities to enhance heat transfer performance of the vehicle. Higher temperature exterior surface areas resulted from higher heat transfer through the walls. Sidewalls, structural members in the backwall, and the roofcap presented opportunities to reduce conduction by applying commercially available and advanced insulation packages.

To evaluate the potential benefit of improving insulation, a 16.5-cm (6.5-in) thick R-19 insulation was installed in a test truck. The insulation was attached to the interior cab upholstery to provide a noncommercial reference, or upper limit for reduction in overall heat transfer. For the UA test, with the sleeper curtain open, the insulation resulted in a 7°C (12.6°F) increase in the average interior sleeper air temperature. The increase in temperature of the test truck over the calibrated control truck equated to a 20% reduction in overall heat transfer.

Phase III involved close collaboration with OEMs and suppliers to design commercial and advanced insulation packages for further evaluation with an onboard electric A/C idle reduction system. NREL collaborated closely with engineers at Volvo Trucks, Aearo Technologies/E-A-R Thermal Acoustic Systems, 3M, and Dometic Environmental. Several configurations were selected for evaluation: solar reflective film, Insulation Package I, Insulation Package I plus solar reflective film, and Insulation Package II. The baseline test data for an unmodified vehicle was compared to the modified vehicle with these different thermal load reduction packages. Thermal soak, UA, and rest period A/C tests were conducted to determine temperature impacts, heat transfer improvements, and A/C load reductions.

The 3M solar reflective film applied to the cab and sleeper exterior opaque surfaces resulted in a truck average interior air temperature reduction of 1.5°C (2.7°F) from the baseline. This reduction was close to expectations based on modeling for a light-colored vehicle; the benefit is expected to have been significantly larger if the technology was applied to a darker, higher absorptivity exterior. UA tests to characterize heating reduction of the solar reflective film were not conducted because this thin film was not expected to add significantly to the wall thermal resistance.

Insulation Package I, provided by E-A-R Thermal Acoustic Systems, was installed in the sleeper walls, floor, and roof. Overall heat transfer tests for Insulation Package I demonstrated an increase in interior air temperature of 5.7°C (10.3°F) resulting in an overall heat transfer coefficient reduction of 13 W/K. This equates to a 26% savings on heating loads required to maintain the baseline interior air temperature. Integrating the hourly savings shows an overall A/C load reduction of 20% during the 10-h daytime rest period A/C test. Adding solar reflective film to the Insulation Package I truck improved the A/C load reduction to 22%. As with the “film only” case, this improvement is expected to be higher with a darker colored vehicle.

Insulation Package II also had insulation from E-A-R Thermal Acoustic Systems installed in the walls, floor, and roof. Insulation was also added to the structural channels. This package demonstrated the most significant load reductions. The insulated truck had an average interior air temperature increase of 7°C (12.6°F) for a constant heat load of 965 W. This 23-W/K reduction in overall heat transfer coefficient equates to a 36% savings on heating loads required to maintain the baseline interior air temperature.

The hourly daytime A/C energy consumption for the baseline vehicle compared to the vehicle modified with Insulation Package II showed a 34% reduction in cooling demand during the 10-h daytime A/C test.

The Dometic electric A/C system is an idle-free system that can be powered by shore power or with an onboard battery package. With a battery-powered configuration, the system operates on four lead-acid batteries with a capacity of approximately 1500 W·h each. The total battery weight of the system is approximately 132 kg (290 lb). A 23% reduction in battery capacity resulting from application of Insulation Package II could equate to a mass reduction of approximately 29.5 kg (65 lb) and would have an associated reduction in battery pack volume.

This article is based on SAE International technical paper 2012-01-2052 written by Jason A. Lustbader and Travis Venson of National Renewable Energy Laboratory; Steven Adelman, Chip Dehart, and Skip Yeakel of Volvo Group North America; and Manuel Sanchez Castillo of Aearo Technologies LLC/E-A-R Thermal Acoustic Systems.

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