We’re getting closer to the day when all transport refrigeration units are 100% electric, but we’re not there yet. Lighter Class 2 through Class 4 trucks already have fully electric refrigeration units driven off the truck’s engine. Larger trucks, however, demand more cooling capacity and energy than engine-mounted compressors and alternators can provide. For those trucks, fully electric operation is still a few years off.

Silent and emissions-free TRU operation is highly sought after in light- and medium-duty delivery trucks. They work in urban environments where customers and their neighbors would rather drivers didn’t leave their engines running while making deliveries. The same is true, and even more so, for larger trucks with diesel-driven refrigeration units.

Paul Kroes, power solutions business development manager for Thermo King, says switching to all-electric refrigeration units has multiple advantages over diesel.

“Getting rid of engine-mounted compressors will really help with reliability, and overall performance could actually be improved simply because you can run [an electric TRU] at more steady state,” he says.

If the vehicle is idling, the compressor isn’t running very fast, which significantly reduces its cooling capacity. On urban routes, there isn’t a lot of opportunity to get the engine running at higher speed to optimize cooling. So being able to drive the compressor at a steady speed with a battery electric motor offers a lot of performance advantages.

“You’d also see a lower total cost of ownership through reduced wear and tear on a secondary engine,” Kroes adds. 

With today’s light-duty internal-combustion-engine chassis, switching to a battery-electric TRU can be expensive. But opportunities begin to open up as more light- to medium-duty battery-electric trucks enter the market. With the batteries already built into the vehicle, adding a little more battery capacity to run an electric TRU is more viable. Many smaller systems, such as Carrier Transicold’s S and X series and Thermo King’s V series units, already offer electric standby options.

Over the next few years, as the industry moves toward more mainstream usage of those Class 1-4 delivery vehicles, the adoption of electric TRUs will follow, says Steve Hubbard, Thermo King’s lead electrification engineer.

“We are mostly in step with the rest of the trucking industry in terms of [rollout] timing relative to vehicle size,” he says. “Electric TRUs will mostly follow in the same rough timeline, but maybe a little bit more delayed because we are an add-on to most of those vehicles. We’ll have to wait for the industry to figure out what they want to do and then how we can integrate into that.”

Electrical Integration

Even as the smaller BEVs proliferate, they are coming up against a couple of now-familiar barriers: charging infrastructure and the cost relative to the return on investment. The technology is expensive, and payback is largely unknown, especially in terms of the residual values.

Getting the charging infrastructure into place for tiny fleets will be relatively easy, but they may not be able to absorb the cost of the trucks. For larger fleets, 50- to 100-truck food service fleets for example, charging infrastructure will be a bigger concern.

We already have plug-in infrastructure in place at many locations for electric standby service or precooling of trailers, but it’s not broadly used, says Kroes.

“That technology has existed for a long time and it is often referred to as electrification. But in our world, that’s more like electric standby,” he says. “It’s very underutilized, partly because of the misunderstanding of the benefits and partly because of a lack of infrastructure. Overall, it’s vastly more simple than what you’d need for a fully electric TRU, and the power requirements are much lower.”

Mobile Energy Sources

The migration toward fully electric refrigeration units will be hampered by another familiar problem: range anxiety, complicated by myriad TRU duty cycles. TRUs require a lot of energy, and that means large, heavy battery packs, or some means of recharging on the move.

ConMet’s recently announced PreSet Plus eHub can help supply some of that energy. It’s a motor/generator built into the hub that captures wasted braking energy, which can be used to power TRUs in some instances.

“When considering routes and opportunities for running all electric with ConMet’s eHub, it really all comes down to the energy capacity of the system,” says Caleb Lander, product manager for electrification. “To define this capacity, we need to evaluate the length of time the trailer will remain stationary while still running the TRU, and the duty cycle of the TRU during these idle periods.”

Lander suggests, for example, in an urban food service application with a multi-temp TRU conditioned in the yard with shore power and using a smaller battery, eHub could support eight to 12 stops per 10-hour day, averaging 15 to 45 minutes per stop.

“In this scenario, we would size a small battery allowing for three to four hours of runtime while stationary, even during peak load from the TRU,” Lander says. “And it’s scalable. Fleets can specify a version that meets their needs without buying more capacity than necessary.”

In Europe, Carrier Transicold just launched an all-electric trailer refrigeration system, Vector eCool, that uses a battery pack recharged by an axle-mounted generator that converts kinetic energy into electricity. The eCool system can be plugged into the electrical grid when the trailer is parked and will fully charge in under four hours, the company says. On the road, the battery pack is recharged using power generated by the axles and the kinetic energy created under braking.

The announcement did not detail the weight of the unit, but the release said, “Even with the system’s axle generator and battery pack fitted, it is still significantly lighter than a standard diesel unit with a full tank of fuel.”

“We believe the Vector eCool represents the future of refrigerated trailer technology,” said Victor Calvo, president, Carrier Transicold International Truck & Trailer. The company said it is looking at potentially bringing this technology to North America.

A hybrid Approach

Meanwhile, Carrier Transicold’s currently available Vector system uses a diesel engine to drive a high-performance generator to run the unit’s E-Drive all-electric refrigeration system. With no mechanical connection between the diesel engine and the refrigeration system components, much of the old hardware, including belts, pulleys, and refrigerant valves, are eliminated – along with their associated maintenance costs, says Bill Maddox, senior manager for product management at Carrier Transicold, Truck/Trailer/Rail Americas.

“The Vector system activates components only when needed, reducing engine runtime and associated fuel consumption,” he says. The Vector unit also generates heat, when needed, electrically.

The E-Drive can also be plugged into an electric power source when the unit is parked, enabling the refrigeration unit to run at full capacity without the noise, emissions, and fuel consumption associated with a diesel engine.

While it’s not a zero-emission alternative, there are still fuel savings (and CO₂ reduction) benefits.