Solar-Powered Refrigerated Trailers Could Eliminate Millions of Tons of CO2 Emissions

Reefers cooled by diesel emit a lot of CO2. Why not put solar panels on their roof?

solar powered transport trailer
Solar powered refrigerated trailer.

XL Fleet

Reefers, or refrigerated trailers, are cooled by diesel fuel, with electricity from either the tractor's generator or an Auxilary Power Unit (APU) mounted on the trailer. An idling diesel tractor burns a gallon of fuel per hour; an APU, 4/10ths of a gallon. According to XL Fleet, which makes "electrification solutions for commercial and municipal fleets," about 50,000 diesel-powered reefers are sold every year in the U.S.

XL Fleet recently announced it is working with eNow, which makes solar and battery systems for electric Transport Refrigeration Units (eTRUs).

"XL Fleet and eNow are collaborating on the design and development of the system that will power eTRUs, as a replacement for conventional diesel-powered systems. XL Fleet is developing the high-capacity integrated lithium-ion battery and power electronics technology that will be installed underfloor on the Class 8 trailer, providing approximately 12 hours or more of run time between charges. eNow will integrate this system into its architecture, including solar panels mounted on the roof of the trailer to maintain the battery charge and extend operation." 

According to the press release, "each conventional diesel power refrigerated trailer can use as much diesel as a delivery truck uses in a day, so there are large opportunities for diesel and emissions savings with electrified refrigerated trailers."

This got our attention since the question of the carbon footprint of imported versus local food has long been a controversial issue on Treehugger. We asked for the data behind the statement. Tod Hynes, Founder & President of XL Fleet, tells Treehugger:

"Refrigerated trailer fuel consumption is highly impacted by internal and external temperatures and operating conditions. Based on customer data, trailers can consume approximately one gallon of diesel fuel per hour, and run for 24 hours (including sitting in a yard/parking lot), which totals 24 gallons of diesel fuel per day."

It should be noted the APUs on trailers are more efficient: According to manufacturer Thermoking, they burn 0.4 gallons per hour or 9.6 gallons per day. But let's use the XL numbers for now.

Burning diesel emits 22.4 pounds of carbon dioxide per gallon, so a trailer full of lettuce is emitting 538 pounds of carbon dioxide per day. In her research into the cold chain for my class at Ryerson University, my student Xin Shi found a head of lettuce spent an average of 55 hours in a refrigerated truck, so just cooling a trailer full of lettuce emits 1,232 pounds of carbon dioxide. (Did we mention that lettuce is stupid?)

There are more than half a million reefers in operation in the U.S., so electrifying them would mean a dramatic reduction in emissions. Given that a Class 8 tractor-trailer rig gets about 6 miles to the gallon, electrifying the tractor would make an even bigger difference, but even just electrifying the refrigeration would reduce carbon dioxide emissions by 15%.

All of this confirms my thesis that the carbon footprint of food transportation is grossly underestimated, and is why eating local makes a difference in your carbon footprint. Because this is such a contentious issue, let's do the math on lettuce.

There are 24 heads in a case and 600 cases in a transport trailer, or 14,400 heads in a transport trailer. The 55 hours the lettuce travels is in a truck probably moving 2/3 of the time at an average of 55 mph and 6 miles per gallon, burning 332 gallons, pumping out 7,453 pounds of carbon dioxide. Add the cooling and it totals 8,685 pounds of carbon dioxide, over four tons per trailer load. divide that by the heads of lettuce and you get 0.6 pounds of carbon dioxide per head of lettuce, just moving it.

It's not much, but given that lettuce is 97% water, it is what Tamar Haspel described in The Washington Post as "a vehicle to transport refrigerated water from farm to table." Until every trailer and every tractor pulling it is electrified, we should think twice about where our food comes from, and we should recognize that eating local matters.