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Sustainable Energy

A Better Way to Keep Milk Fresh

Founded by Americans, Promethean Power brings a thermal battery system to small dairy farmers in India and Bangladesh.

Millions of dairy farmers in India rely on dirty, expensive diesel fuel to keep their product fresh.

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 Like India, Bangladesh has an outdated power grid that supplies electricity sporadically—often as little as a few hours per day.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.

“We’re not delivering new forms of energy; we’re simply storing the intermittent power that they do get and parceling it out over time,” says White. Promethean Power has sold around 150 systems in India to date. The dairy collection center in Chetawala, in the state of Rajasthan, estimates that it saves around 40,000 rupees ($628) a month on diesel fuel and reduced milk spoilage since installing a Rapid Milk Chiller from Promethean Power. The center has increased its average daily milk production from 500 liters a day to around 800.

That’s a huge improvement in a country where more than 300 million people live without access to electricity and even villages that are nominally electrified often have spotty service at best. Prime minister Narendra Modi, who took office last year, has pledged to bring reliable electricity to the full population by 2022.

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