Sunday, September 14, 2014

Solar Cookers Get Hot

Mechanical engineer Asfafaw Haileselassie Tesfay demonstrated an advanced new solar cooker at Mekelle University in Ethiopia in March, 2014. (Credit: Asfafaw Haileselassie Tesfay) Click to enlarge.
Of all the innovations that could markedly benefit humankind, a reliable solar cooker remains one of the most imperative.  Countless people across the developing world still cook their food by burning wood or even cow dung, causing respiratory problems and severe deforestation in some regions.

The quest for a practical solar cooker has gone on for decades and produced dozens of models ranging from $70 cheapies to $400 deluxe models.  But none of them have caught on significantly for use in the developing-world because they can’t store heat.  Without the ability to store heat, a cooker cannot be used, for example, on cloudy days.

But now a group based at the Norwegian University of Science and Technology (NTNU) in Trondheim is reporting a breakthrough in solar cookers.  They say it will lead, within a year and a half, to the production of practical solar cookers that can store enough heat during a sunny day to work for an entire additional day, even if it is cloudy.  The breakthrough was recognized with a US $8,000 award from the Norwegian Ministry of Petroleum and Energy this past August.

“Our vision is to solve problems in developing countries,” says Guro Grytli Seim, the CEO of a company, Morpho Solar, that was formed recently to capitalize on the Norwegian advance.  She notes that nine Ph.D. students and 20 Master’s degree candidates worked on the heat-storage technology, with funding from the university, the Norwegian Investment Fund For Developing Countries, and the electric utility Trønder Energi.  The system uses the principle of phase-change to store and release heat.  The key ingredient is a salt with a melting temperature of 220 °C, Seim says.  Sunlight concentrated by a parabolic reflector melts the salt; when heat is needed, the salt is allowed to solidify, which releases heat.

The system can have a thermal efficiency of 80 percent, Seim explains.  For comparison, an electrical system based on photovoltaic cells and batteries would be only about 15- to 20-percent efficient.

The team also developed two different techniques to transfer heat from the parabolic collector to the cooking surface.  These technologies, one based on steam and the other on oil, allow the sunlight-collecting parabolic dish to be outside a home and the cooking surface to be inside.

Solar Cookers Get Hot

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