CAMBRIDGE, Mass. -- It seems like a no-brainer: Remanufacturing products rather than making new ones from scratch -- widely done with everything from retread tires to refilled inkjet cartridges to remanufactured engines -- should save a lot of energy, right?

Not so fast, says a new study by researchers at MIT.

In some cases, the conventional wisdom is indeed correct. But out of 25 case studies on products in eight categories done by a team led by Professor of Mechanical Engineering Timothy Gutowski, there were just as many cases where remanufacturing actually cost more energy as cases where it saved energy. And for the majority of the items, the savings were negligible or the energy balance was too close to call.

Why are the new results so different from what might have been assumed? The MIT team looked at the total energy used over the lifetime of a product -- a life-cycle analysis -- rather than just the energy used in the manufacturing process itself. In virtually all cases, it costs less money and less energy to make a product from the recycled "core" -- the reusable part of the product -- than to start from scratch. But the catch is that many of these remanufactured products are less energy efficient, or newer versions are more energy efficient, so the extra energy used over their lifetime cancels out the savings from the manufacturing stage.

A simple and familiar example is retread tires. They do indeed require less energy to make than new tires, but their rolling resistance might turn out to be just a bit higher, which would mean their energy advantage is eaten up by the extra gas used while driving on them.

The study, published in the journal Environmental Science & Technology, is the latest from Gutowski and his students that, as he puts it, "takes what appears to be a simple, straightforward problem and shows that the world is a far more complicated place than people thought." The paper was co-authored by MIT materials science and engineering graduate student Sahil Sahni; Avid Boustani SM '10, a recent graduate from the Department of Mechanical Engineering; and Stephen Graves, the Abraham J. Siegel Professor of Management in the MIT Sloan School of Management. The work was supported by the MIT Energy Initiative and the Singapore-MIT Alliance.

What often turns out to be the case, Gutowski says, is that "new technology shows up that is so much more efficient, from an energy point of view, that you should get rid of the old device" rather than having it fixed or buying a remanufactured version. For example, the efficiency of many new appliances -- such as refrigerators and washing machines -- are so much improved over older models that, in terms of energy use, a new model is almost always the better choice.

Unfortunately, that is typically not the way people and businesses make their choices. "The decision is always on cost, not energy," Gutowski says. For example, a remanufactured electric motor, in which the metal core is reused but wound with new wires, is typically 0.5 to 1 percent less efficient than a newer motor. "There is still a cost advantage" to the remanufactured motors, he says, "but from an energy point of view, it's the opposite."

Other effects stem from remanufactured products going to different markets than their original destinations. For example, old cell phones can be remanufactured, but the remanufactured phones tend to be sold in developing countries. "If it goes to a different market, it leads to an expansion of the market," Gutowski explains, so the overall level of cell phone usage globally -- and the energy needed to power them -- ends up increasing.

For some kinds of products, the benefits of remanufacturing are unequivocal. It clearly makes sense to remanufacture anything that consumes an insignificant amount of energy when it is being used, he says -- for example, furniture.

Gutowski emphasizes that this research does not necessarily suggest a specific course of action. For any given product, there may be other reasons for preferring the remanufactured version even if it produces a net energy penalty. For example, remanufacturing may reduce the burden on landfills, reduce use and disposal of some toxic materials, or produce needed jobs in a particular area. And the expanded use of cell phones may have important social benefits, such as contributing to the recent wave of revolutions in North Africa and the Middle East. "We're not saying you shouldn't do it," he says -- just suggesting that it's worth understanding the decision's effects in their entirety.

"You think you're doing the right thing, it sounds so simple," Gutowski says. But when it comes to understanding the true impact of purchasing decisions on energy use, "things are far more complicated than we expect."

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