Green Tech

Promethean Power Systems is an MIT spin-off that's swinging for the fences: using cutting-edge thermoelectronic technology, it wants to build a solar-powered refrigerator with no moving parts.

It plans to show off a prototype unit, designed specifically for rural areas in India, at the Emerging Technologies conference at the Massachusetts Institute of Technology next month.

If successful, the company's end product will be a small building to store milk, medicines, or other perishable goods in India and other places that don't have reliable electricity.

An artist's rendering of solar-powered cold storage unit.

(Credit: Promethean Power)

On the roof will be a small array of solar panels. Inside will be electronic components which use thermooelectric modules--essentially a semiconductor material sandwiched between ceramic tiles. These devices will convert electricity from the solar panels into cold air.

Thermoelectric modules, typically made out of bismuth telluride, are already used in low-energy applications like cooling car seats or computer chips. Promethean Power and a handful of other companies are trying to marry the technology to renewable energy and apply it far more broadly.

Because current thermoelectric cooling isn't very efficient, the key technical challenge is designing a system that's energy-efficient enough to be commercially viable.

On the business side, selling high-tech gear to poor countries sounds like a difficult proposition. Some people have suggested that Promethean Power adapt the technology to high-end products like picnic bags that can cool drinks with mini-solar panels.

But co-founder Sorin Grama says the company's market research has identified a business customer with money to spend, namely food distributors and processors in India looking for an alternative to costly diesel generators.

"That's where the money is. That's the kind of billion-dollar market that can sustain this kind of business," Grama said. "Anything else is just a niche."

With a storage unit that runs in part on solar power, these food or dairy companies can rely less on costly--and heavily polluting--backup diesel generators. Also, they will be able to collect produce or milk from farmers once, rather than many times, a day and save on transportation costs.

If the technology is cost-effective, it can be adapted to many uses, he said.

Plotting a new technology path
Solar panels or another renewable energy resource could already power a traditional compressor-based cooling unit. But the energy needs are very high and would require a lot of expensive solar panels.

Promethean Power itself started out trying to commercialize a process developed by MIT students that uses the sun's heat and an organic Rankine cycle to make electricity for cooling. Another approach is adapting an evaporative cooler for refrigeration.

A thermoelectric module connected to a small solar panel. With electricity, one side gets hot and the other cools.

(Credit: Martin LaMonica/CNET News)

But after they couldn't get an exclusive license for the original technology approach, Grama and co-founder Sam White chose to take a new path to the same market. The solar-thermelectronics combination gives the company a shot at breaking new ground technically, using solid state electronics.

"The old technology doesn't have as much room to improve," Grama said. "We're riding the wave of cheaper PV (photovoltaics) and thermoelectronics getting better."

In a phenomenon known as the Peltier effect, when current passes through a thermoelectric material, one side gets hot and the other cools. So the cold storage unit will also have to wick away a substantial amount of heat and isolate the cooling.

The plan is to make ice during the day using the solar panels. That stored ice will provide cooling at night or when the sun isn't shining.

It's an arrangement that won't eliminate the need for a diesel generator but a distributor or food processing company can use a smaller portable generator and save on fuel, said White.

Disruptive?
Right now, Grama and a handful of collaborators are writing the software for the embedded electronic controllers to make the cooling mechanism energy-efficient enough to work with a small number of solar panels.

Other companies and researchers are trying to find ways of tapping the thermoelectric effect in different ways, such as taking waste heat from car exhaust systems and turning it into electricity.

Promethean Power Systems co-founders Sorin Grama (left) and Sam White across the street from their Harvard Square, Cambridge office.

(Credit: Martin LaMonica/CNET News)

GMZ Energy earlier this year received seed funding from Kleiner, Perkins, Caufield & Byers to develop and commercialize thermoelectrics using nanotechnology.

The Boston area company, which is building off of research from Boston College and MIT, intends to make low-cost materials for use in refrigeration, air conditioning, waste heat recovery, and solar thermoelectrics.

After finishing its prototype, Promethean Power intends to look for outside funding. After deciding to take a very different tack to solar-powered refrigeration, it decided to keep thing small and low-budget.

It has been funded by angel investors, the founders, and received some money from an MIT business idea grant. It's building its prototype with equipment donated from suppliers, including National Instruments.

"It's amazing what not having a lot of money will do. You can't afford to make a lot of mistakes," said White.