Richard Martin | MIT Technology Review
July 21, 2015


Sometime this week a large milk refrigerator will arrive in Dhaka, the capital of Bangladesh. Since Bangladesh produces nearly four million tons of milk per year, that hardly seems remarkable; but this is a special kind of refrigerator.

Made by Promethean Power Systems, a company based in Pune, India, and Boston, the system keeps milk chilled with a thermal battery that stores energy and releases it, as cooling power, over the course of a day. Like India, Bangladesh has an outdated power grid that supplies electricity sporadically—often as little as a few hours per day. Rural dairy farmers on the subcontinent bring their milk to village collection centers that typically rely on diesel generators, a costly, dirty way of providing electricity.

Two Americans, Sam White and Sorin Grama, founded Promethean Power in 2007 to address a simple but widespread and pressing problem: how to keep milk cold without burning diesel fuel. They’ve been selling refrigerators in India for two years; this week marks their first export to neighboring Bangladesh.

“We’ve been at this for eight years,” says White, and “we’ve gone through all sorts of different technologies, attempts, and failures to figure out a solution.”

At first, he says, they were determined to craft a technology that relied on solar power—a noble attempt that ultimately failed because solar power, like grid power in India and Bangladesh, is by its nature intermittent, and refrigerators need constant power. Eventually they settled on a thermal energy storage system that uses a phase-change material to store energy in the form of ice. When the grid is operating, a portion of the material freezes, and the battery circulates that thermal energy into a heat exchanger to keep milk chilled over the course of the day. The thermal battery can store up to 28 kilowatt-hours of energy.

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