You see them at the side of roads, in parking lots and pull-off areas of highways—Class 8 over-the-road trucks with their diesel engines idling through the night to run the heating or air conditioning that assure comfort in the resting drivers' sleeper compartments. A lot of fuel might be saved—and CO₂ emissions reduced—with energy-efficient alternatives to extended idling.

The National Renewal Energy Laboratory, a federal facility based in Golden, CO, took on the challenge. The idle fuel-usage studies it found were from 2005; the outdated figures estimated idling is 7% of total diesel fuel consumption, so NREL focused on individual Class 8 trucks, proposing a package of technologies that can annually save 774 gallons on A/C cooling load alone compared with an idling truck; at $3 per gallon for diesel fuel, full payback of an investment in sleeper cabin climate-control equipment would come in about three years. 

Only about half the states in the U.S. currently restrict idling for over-the-road trucks and laws often are observed in the breach, as some answers raise cost and reliability issues.

Driver comfort important 

The trucking industry and regulators know there's a clear opportunity to save fuel and improve air quality. But resting drivers can't be confined to cocoon-like compartments with an occasional wisp of cool air in summer and/or wrapped in heavy blankets in winter.  

Today's sleeper compartments make driver comfort and convenience high priorities.  Volvo VNL Class 8 series single-bed units are 118 ft³/3.34 m³ with a flat roof,  and two-driver models with high roofs (77 in/1955 mm) have bunk beds in a 371 ft³/10.5 m³ living space—considerable area to be climate-controlled.

NREL's name points to its research in such areas as photovoltaics, wind and biosciences, but much of its work is on cost-effective solutions to real-world "lower-tech" problems. It has provided considerable study on climate-control related electric-vehicle range issues and at the 2016 SAE World Congress explained its extensive research into reducing Class 8 truck idling fuel consumption.

There are alternative approaches for climate control in sleeper cabs to avoid extensive idling, including battery-powered auxiliary A/C, fuel-fired  or electric heaters and small diesel engines that serve as auxiliary power units (APUs). The popularity of diesel APUs peaked some eight years ago and the adoption rate today represents just 9%, explained Jason Lustbader, NREL senior research engineer.

NREL looked at many aspects of what it proposes as "complete cab solutions," with emphasis on low-cost additions or substitutions. "The project goal was to reduce cab thermal loads to enable smaller, lighter and more cost-effective idle-off climate-control equipment," explained Lustbader. The goal: a 30% reduction in big-rig sleeper climate-control loads with no-idle solutions that pay back within three years or less.

The NREL project evaluated four aspects in an integrated strategy: the solar envelope (as overall A/C energy use is in the equation), heat and cold conductive pathways into the sleeper, efficient equipment and the volume of the sleeper. In this effort, NREL works with industry partners, particularly Volvo Group NA, PPG and Aearo Technologies, which makes energy/sound absorbing materials.

35.7% reduction in A/C load 

NREL testing showed its complete cab solution exceeded the goal, with its most advanced approach producing a 35.7% reduction in A/C load and 43% decline in heating load, with an even greater reduction of 53% with a more advanced approach to insulation. Translating those savings into dollars, NREL found that with the complete cab solution to reduce loads and adding a battery-powered A/C system for the sleeper, 774 gallons of fuel per year could be saved. Savings and cost analysis for heating the cabin is still underway, although some preliminary data was announced.

Prior testing by NREL indicated the best bang for the buck was in specific insulation, interior curtains, window shades and paints. Although all the actual testing was done at NREL headquarters, the data was plugged into NREL's own load estimating software, CoolCalc, to give the results a nationwide scope. The process provides fuel-use estimation by combining thermal loads with an A/C performance model to calculate an electrical demand. The load was combined with modeling to be able to determine fuel use from recharging the battery pack. Because long-haul trucks operate across the country, the model used the 200 most representative weather stations nationwide to calculate total fuel saved.

Details that led to NREL's quantified results:

Thinsulate in "plus" package 

Baseline insulation consists of foam attached to the door and body trim panels of the sleeper. The advanced package instead uses one- and two-inch insulation blankets with a thermal conductivity rating of 0.03-0.05 W/m-K.  In addition to that advanced package is a "plus" addition of 0.25-in (6.35-mm) layer of 3M's Thinsulate, a synthetic fiber thermal insulation material which has a reflective radiant barrier. It is installed between the interior trim and the structure of the sleeper.

The three levels of insulation for the sleeper were combined with three different packages of privacy shades for the cab glass and sleeper curtains between the cab and sleeper:  standard insulation with advanced shades and curtains, advanced insulation with standard shades and curtains and a package of fully advanced insulation, shades and curtains.  The first two packages produced almost the same reduction in UA (heating load): 20.6% and 20.7%. The fully advanced setup resulted in a 43% reduction. 

"Advanced plus" insulation also was evaluated against standard shades and curtains, advanced shades and open curtains and advanced shades and curtains. This "Complete Cab Plus" configuration yielded the greatest reduction in UA at 53%.   

However, even with Plus insulation and advanced shades, leaving the sleeper curtains open to avoid a claustrophobic-feeling sleeper caused the heating load reduction to drop markedly to just 21.6%. 

No-idle equipment  

A Dometic no-idle 7000-BTU A/C system was installed. This truck sleeper system is battery-operated, has a three-speed blower and will run for more than 11 hours, the company claims. The system was powered by the laboratory's 120-v A/C for testing to obtain the most accurate data. However, the analysis assumed the use of 104 A-h AGM (absorbent glass mat) batteries that would recharge while the truck is in operation.

For heating, NREL chose a forced-air heater with the diffuser oriented to avoid air stratification. The heater was operated at 90°F (32°C) to provide a sufficient gap compared with ambient temperate for accurate measurement of the heat transfer coefficient (clearly much higher than would be used solely for sleeper comfort). A fuel-fired heater will be evaluated in future testing and it is expected to enhance the overall results.

Paint-color effect

Exterior paint color affects sleeper thermal loads only when the sun is out for a long time and intensity is high, typically in summer. However, because it does affect the total vehicle A/C load, the chosen white color was found to deliver a 20.8% load reduction compared with black. As an alternative, solar-reflective blue was compared with conventional blue and showed a 7.3% saving. 

The white color combined with the NREL Complete Cab Plus package to deliver the lab's 35.7% cut in overall A/C energy consumption. 

NREL's complete solution delivered results that equate to operating the trucks in year-round moderate temperatures. Modeling with CoolCalc showed major reductions in the number of both heating and cooling days in all parts of the country. This seems to provide an opportunity to save by downsizing the battery pack and other components in a fully optimized Class 8 climate-control system.