A 'thermal battery' for villages in India

Startup Promethean Power had to ditch its dream of solar-powered milk chillers in rural India, but the company hit on a novel energy storage system that's cheaper and overcomes the unreliable grid.

SOUTH BOSTON--From this grungy warehouse converted into a startup incubator, Sam White and Sorin Grama plot how to chill milk for poor Indian villagers.

The four-year journey of their startup, Promethean Power, has included several trips to India and dramatic engineering detours. Now finally, their rapid milk chillers, which feature a novel energy-storage technology, will be installed at three milk processing centers in India in the coming months.

The two company co-founders had originally intended to build a solar-powered milk chiller that would improve the lives of Indian farmers and advance renewable energy. But despite engineering some nifty power electronics, the company last year made the painful decision to abandon solar.

In the end, the technology proved too expensive, too complicated, and too large for use in Indian milk processing centers, said White. "A lot of the challenge has been the cost, and falling in love with the original dream of solar," White said from Promethean Power's headquarters in the Greentown Labs clean-tech incubator , a hardware hacking space it shares with other fledgling companies.

Instead, its $8,000 chillers will use a large "thermal battery" that can rapidly chill milk. The main advantage of the system, developed with Olin College through a National Science Foundation grant, is that it can chill milk even when the grid is down, which it frequently is in Indian villages.

That means local milk collection stations in villages could operate for longer hours, rather than have to rush milk off to centralized processing centers before it spoils. Indian farmers, some of whom may have only one or two cows, can then earn more money.

"There's always an impact when you put technology at the local level of a village, but they just didn't have it before," White said.

Cold energy storage
The thermal battery is a large container that stores a specially designed liquid that doesn't freeze, even below the freezing point. People pour milk, which has just been milked from cows, onto a cylinder-shaped tank, which is cooled by the thermal battery's liquid. As the milk flows over the cylinder, it's rapidly chilled to an appropriate temperature for storing.

And when the grid is not available, the thermal battery can run for several hours on a car battery, and so it avoids dirty diesel generators. The first machines Promethean Power will deploy are roughly the size of a large refrigerator. To remove heat from the cooling liquid, it circulates through a traditional compressor loop, the same used in heat pumps, air conditioners, and refrigerators, White explained.

Over a period of three intense weeks last year, the team in India worked around the clock with engineers back in Boston and Olin College, optimizing the system while keeping the design simple and inexpensive enough for this application. The composition of the cooling liquid, which includes phase-change materials, is a key part of the company's technology, but it is made with abundant and cheap materials, White indicated.

Although the cooling technology is designed specifically for their chosen application, it could ultimately be used to make air conditioners more efficient or be used for in-truck refrigeration in developing countries, according to the NSF.

The thermal battery will get its debut at processing centers that collect milk from thousands of villages, which will be a crucial test. Grama, White, and other employees got this far with a persistence bordering on obsession. But their deep knowledge of their chosen application--milk collection in rural India--has helped them tune their product over the years.

"(People we work with in India) totally think we're nuts. But the best part of the job is to experience such incredibly extreme things," said White.

 

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