Cheap, Off-Grid Cooling

A hybrid refrigerator will bring efficient, cheap cooling to India.

A startup based in Cambridge, MA, has developed a new solar-powered refrigeration system for food storage in Indian villages that are off the grid. Promethean Power Systems' design is a hybrid of conventional compressor-based refrigeration and thermoelectric materials--semiconductors that convert electricity into cooling and vice versa.

Chilling in the sun: A conceptual illustration of a solar-powered refrigeration system that could be used in off-grid villages in India. Promethean, based in Cambridge, MA, plans to make the system efficient by combining thermoelectric- and compressor-based cooling. Credit: Promethean Power Systems

The chilling units will be cheaper than what is currently used in Indian villages, most of which are off the grid. In such villages, food distributors and processors store raw food products in traditional compressor-based cooling units that run on diesel generators. These cost about $12,000, says the company's cofounder Sorin Grama. And that cost, says Grama, doesn't include the escalating cost of diesel needed to run the units. During a month spent in India a year ago, Grama and his cofounder, Sam White, identified a crucial niche. "Customers kept asking for a cooling system that has low maintenance and operation cost," White says.

Grama says that even including the expense of the photovoltaic (PV) panels, his design would cost about the same as or slightly less than the diesel-powered refrigeration units. More important, it would have no fuel costs, and almost no maintenance costs. According to the company's initial calculations, using a compressor combined with thermoelectric modules would use 20 percent less power to generate the same cooling as a compressor alone.

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The design uses off-the-shelf components: silicon PV panels, thermoelectric modules, and a compressor-based refrigeration unit. The company's control system directs the two cooling components to work together so that they squeeze as much juice out of the solar panels as possible, Grama explains. Early in the morning and late in the afternoon, when the amount of sunlight is low, the solar panels won't generate enough power to run the compressor. But there will be enough solar power to run the thermoelectric modules, which would generate cooling until the compressor kicks in. Around midday, when the solar panels are working full throttle, the thermoelectric modules will use the extra juice that the compressor doesn't need to provide additional cooling.

Since Promethean was founded in 2007, it has built a laboratory-scale 60-liter chiller. Last week, the company secured funding with which it plans to build a 500-liter prototype that it hopes to test in India in 2009.