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We wanted to build a solar cooker as high-tech as the house, so we decided on a paraboloid shape that uses mirrors to concentrate the sunlight onto a cast iron cauldron. The mirror assembly rotates smoothly on azimuth and altitude axes. Azimuth position is held by friction, altitude by an adjustable metal strap. In operation, the aim needs to be tweaked every 5-10 minutes to keep things cooking. Auto tracking would be nice, but way too complicated. It works fine as is, and can cook a hot dog in slightly more time than a microwave oven (once the cauldron is heated up). Here’s the specs: Maximum Temperature = 100°C = 212°F |
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By using a cast iron cauldron we take advantage of thermal mass. The iron holds enough heat to maintain the temperature while the lid is removed and replaced. Cooking in water increases this effect, and that’s what we usually use. If you like your hot dogs charred and crispy, this cooker is not for you. But if you like your hot dogs steamed and juicy, it works great. We’ve cooked meatloaf and pot pies without water, and warmed many a muffin for breakfast. The acrylic lid helps hold the heat in and allows easy monitoring of the contents. Note the condensation on the lid. The H2O is boiling. |
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Maximum temperature at noon on a clear summer day will just hit 100°C (212°F). That’s more than enough to safely prepare most food for human consumption. A lab grade thermometer verifies this claim. I rounded up the reading, since the insertion of the thermometer required the lid to be unsealed during measurement. That cools the system rapidly. And I’ve seen this cooker boil water. As Heisenberg noted, the act of observing a system will change its state. |
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I later added an 8 AWG copper support to get the classic appearance of grilled hot dogs. I was hoping the thermal conductivity of the copper would help concentrate heat. The support feet were filed flat to increase thermal contact area with the cast iron. |
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As you can see, it’s still not quite what you get on a real grill, and the hot dogs weren’t truly crispy or charred, but at least they no longer looked boiled. 🙂 |
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This spectacular image shows the precise concentrating action of the paraboloid. I reflected sunlight off a large plane mirror into my darkened shop. One of my assistants and I fired up a couple of fine cigars to fill the room with smoke. The reflected rays of sunlight were thereby made visible by the scattered photons. Note that, with individual plane mirrors approximating a paraboloid, we can’t get a perfect focus. However, the focal volume pretty well matches that of the cast iron cauldron, so the cauldron does intercept most of the reflected energy. |
The solar cooker was used less and less over time, but cranked out several hundred hot dogs for visitors during our annual Open House. After I retired from teaching, the cooker started gathering dust. I decided to donate it to any educational institution that could use it. Note that it is not weatherproof, and needs to be stored inside. It has wheels on the base so it’s easy to move. As of 2023, the new owner of this cooker is yet to be determined. If you want it, email me and explain why, but preference will be given to museums, schools, or other educational institutions. And you’ll have to pick it up. It’s too large to ship.