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 the Massachusetts Institute of Technology (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 MIT 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.

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