March 1, 2011
Challenge: Make a solar cooker that works at night
contributor: Rob Goodier
Solar cookers solve real problems in health and conservation—they promote better air quality in the kitchen and healthy forests around a community. The problem is that they don’t work at night.
The aid organization Climate Healers stumbled on the issue last year after handing out solar cookers to women in two rural Indian villages. The broad, reflective parabolas that work so well to cook rotis, rice and dhal by day were useless when the sun went down, just when the women began to prepare their families’ dinner. And they were equally useless for cooking breakfast in the early morning.
Rather than scrap the solar idea, Climate Healers is designing a new cooker that stores the sun’s energy by day to release it at night. Students at the University of Iowa, who are working with Climate Healers, surveyed the women who would use such a cooker, if it is made, and the potential users were enthusiastic about the idea.
Climate Healers tested a design (pdf) that could serve as a baseline, but it needs improvement. For that, the organization has turned to the E4C community. You or someone you know might be able to help. Here are the details.
The Blazing Tube
The starting point is a modified Blazing Tube solar cooker. The original design places a three-gallon glass cylinder in the crook of a compound parabolic reflector—it’s basically a tube on the bottom of a half pipe. The tube is filled with one gallon of oil and an air bubble. It warms throughout the day and releases heat at night or the next morning through a heat pipe that the chef controls.
Climate Healers replaced the oil and air with three gallons of table salt. The salt heated to 626F (330C) at the center of the tube, sufficient to yield 1KW hour of energy. That’s enough heat to cook flat bread and lentils for about an hour.
But the design doesn’t quite work in rural India. “We don’t consider it suitable for the final design because it requires transporting large, fragile glass tubes to remote locations,” says Sailesh Rao, head of Climate Healers. Also, “While it stores energy adequately, it also leaks it. We expect that a solar cookstove engineered for storage from the outset will be designed differently, and this is why we are approaching the E4C community for help.”
The design, Rao says, should have three aspects.
1. Energy capture
Users should be able to set up the cooker and leave it alone while it gathers heat throughout the day. To do that, it will need the right kind of reflector. Climate Healers tested the cylindrical reflector, but Rao suggests that another design, such as the Scheffler, could work also.
2. Energy storage
The cooker should store 3-4KW hours of energy, according to the University of Iowa students’ surveys. It should deliver about half of the energy at night and the rest the next morning.
Rao would like to use locally available materials, rather than glass and salt. For example, he proposes sand-filled tubes that masons can build.
3. Energy delivery
The cooker should heat a pot to at least 300F (150C). “It is best if the energy delivery can be controlled by the cook so that she can use a low flame to simmer the lentils and vegetables and a high flame to roast the rotis,” Rao says.
Climate Healers posted the stove design as a challenge in an E4C workspace. Experts, innovators and anyone with an idea are encouraged to contribute. Can you help? If so, consider signing on. Your design could contribute to better health and healthier forests in India, and maybe worldwide.