Wal-Mart's work in greening its fleet of cars and trucks netted savings of more than $50 million during the last year, and a first in its class diesel hybrid tractor trailer could provide even more savings.

Truck company Arvin Meritor has developed a diesel electric truck for Wal-Mart that can run on electric power only at speeds up to 48 miles per hour. The truck uses regenerative braking to recharge the batteries, and the diesel engine will recharge the batteries when they run low, says Wal-Mart's Chris Sultemeier.

The truck's lithium ion batteries and electric motor can power all of the electrical system, including keeping the cab heated or cooled as necessary. This can dramatically reduce the need for idling and cuts down on emissions. The truck can drive for up to 20 minutes on battery power alone, and will be tested around the Detroit area.

Wal-Mart is also testing five parallel hybrid diesel trucks manufactured by Peterbilt. Rather than providing all of the power to the truck at any time, the electric motors supplement the diesel engine when going up hills or during acceleration, and also can power the electrical system.

Sultemeier says Wal-Mart will explore both types of hybrid trucks and may at some point determine which avenue, if any, is the best path to proceed down.

Wal-Mart surpassed its internal goal of making its transportation 25 percent more efficient by 2008 (relative to 2005), and the company wants to double fleet efficiency by 2015. Much of the efficiency gain thus far has come from smarter logistics by filling the trucks at closer to their capacity and optimizing delivery routes, Sultemeier says. During the last 12 months the company moved 150 million more cases of products but drove 90 million fewer miles. "If you can take miles off the road, that's huge," he says.

Other efforts to green their fleet include a trial in Arizona that will use a combination of waste grease produced by the company with diesel, and a test of trucks in California that will burn liquefied natural gas.

The hybrid trials will be a good test of the capabilities of lithium ion batteries and the power management unit since braking a big rig produces a high density of energy.

One thing the company hasn't explored to date -- I suggested that those batteries, when they no longer become useful in vehicles, could be used to store solar energy. Wal-Mart has a goal of switching to 100 percent renewable energy across the entire company, and solar will be a considerable contributor, so it could be a natural fit.

If copied by its suppliers, Wal-Mart's advances in transportation efficiency could have a significant impact on emissions in the U.S.