Fun with solar cooking

It is early October and our mid-day temperatures are still consistently in the 90’s. This actually feels like a relief from a few weeks ago, when 90s were our ‘lows’, and our highs hit over 115F. Heat and UV radiation pose serious challenges in Phoenix, from the critical issues of heat vulnerability and heat mortality, to the marginalizing effects heat has on different socio-economic groups (e.g., stakeholders with limited access to cooling appliances or transportation). At the same time, heat also presents new opportunities for imagining what could be: we can creatively harvest heat or create broader systems that energize communities around the topic of climate change.

To examine both the challenges and the opportunities presented by extreme heat, we have been running a diary study over the summer, which asked people to document how heat effects their lives. Jennifer and I are now wrapping up the interviews, and at the most basic level, we are finding that heat really does impact people’s routines. Nutrition and exercise are two examples: who wants to go running or use the stove/oven when it’s over 100 degrees out?

So this past week, I have been experimenting with and thinking about different ways to use heat for good, especially in the domain of food preparation. Solar cookers are an obvious direction, but while there is no shortage of DIY solar cooker designs, information on how effective these are (and the tradeoffs between complexity and efficiency) is harder to come by. In the most basic terms, a solar cooker maximizes heat gain and minimizes heat loss. Most examples I’ve seen follow a similar pattern to do this: light is directed onto a cooking area and everything is insulated.

As a first-try attempt to both make a solar cooker and burn off some post-CHI steam (pun intended), last Friday, Jennifer, Piyum and I threw together a bunch of (really) DIY ideas using just what we had in the lab and a few things I picked up at home depot.

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Jennifer made a control box/tinfoil version (which reached 150F), and Piyum did some experimentation with layered lids to prevent convection. Fun fact: heat loss happens through radiation, conduction, and convection (see this tutorial), so it’s not surprising that insulation made a huge difference. A simple styrofoam cooler with a mirror inside gets you to 200F.

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I think if you actually sealed the lid properly (ours is just taped in one place), you can easily boil water in this, which is neat, right? Another fun fact, the best insulator is actually a vacuum, so this commercially-available $360 solar cooker users vacuum tubes.

Many baking recipes tend to call for 350F, and with that as my goal, this week, I set out to improve a few things. I used mirror acrylic to optimize on the heat gain, and built out a two-layer enclosure with bits of styrofoam in between to minimize the heat loss.

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I also added a black metal platform to simulate where the at-scale cooking container might go (most likely something cast iron). I used two layered acrylic lids and also, quite pessimistically, assumed this temperature sensor (max 125C) would help me log the temperature.

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The sensor maxed out in under 5 minutes, and reverting back to the old-fashioned oven thermometer, I saw upwards of 275F (outside temp was around 90F).

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I placed a magnifying sheet on the top, which I’m not actually sure made any difference because I never bothered with any controls since it’s Friday.

So 275F is actually pretty good: some slow-cooking recipes call for 250F to roast chicken or pork, along with many slow-baking bread and dessert recipes. I think with slightly better insulation (i.e., black enclosure and a lid that’s not half-taped because it’s Friday), as well as an actual cooking surface that retains heat (i.e., a dutch oven), this would easily get to 350F. A modified version of this would also work as a solar dehydrator or a hot composter. Looking forward to building out more of these ideas in the coming weeks, especially now that a few thermocouples are on order!

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About staceykasu

Stacey Kuznetsov is an Assistant Professor at the School of Arts, Media, and Engineering, with a joint appointment at the School of Computing, Informatics, and Decision Systems Engineering at ASU.

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