Saguaro Land is a series from The Show looking at the Sonoran Desert — the lushest, hottest desert in the world that happens to be our home.
This time, The Show is talking food. But, not anything cooked in a fancy kitchen — or in a kitchen at all.
It's a solar oven throw down. Last month, more than 400 engineering students at the University of Arizona took part in the obviously outdoors competition using no gas and no electricity, just the Sonoran Desert sun to heat up their ovens.
Byron Hempel, a professor of practice in the Chemical and Environmental Engineering Department at UA, says he’s taken part in the solar over throw down for the past few years, and in fact, this was the first year he wasn’t an active participant. Hempel spoke with The Show about the event.
Full conversation
CAROLINE WICKERS: Hello, we are Team AAMC Theaters at the University of Arizona Engineering Solar Oven Throwdown. Uh, my name's Caroline Wickers. We're testing our—
MARK BRODIE: That's right, a Solar Oven Throwdown. Last month, more than 400 engineering students at the U of A took part in the obviously outdoors competition using no gas and no electricity, just the Sonoran Desert sun to heat up their ovens. We asked a few of them to record what they were doing and how it was going.
CAROLINE WICKERS: Currently, it's quite sunny but with a few clouds. So, the clouds are actually causing us to have dips in our temperatures, so we were starting to rise to closer to 150, but now we had to drop back down and we're getting closer again to 140 due to that. Our predicted temperature is 160° now uh due to the cloud coverage. Um, we have—
MARK BRODIE: After the event, I spoke more about it with Byron Hempel, a professor of practice in the chemical and environmental engineering department at the University of Arizona. He says he's taken part in the Solar Oven Throwdown for the past few years. In fact, this was the first year he wasn't an active participant. And I asked him what the event is all about.
BYRON HEMPEL: So, the Solar Oven Throwdown is a culminated event, or a culminate event, between uh all the different instructors and the students that have built for about four or five weeks, depending on their curriculum, uh this solar oven. And so it's a challenge that we give to students to try to have the highest temperature, as well as do really well in what's called a performance index. We actually have two performance indexes that they try to do really well on.
MARK BRODIE: So, what makes a good solar oven? Like, what do you have to have, what are some of the design elements that make it particularly successful?
BYRON HEMPEL: So, there's many things that go into a solar oven. The first and foremost is a great team spirit. Uh, these are all very collaborative builds. And so, each team of four students is trying to get the highest internal temperature of an oven. So, we actually have a really cool equation. It's uh our temperature IO, or inside oven, and there's quite a few different variables there. So, we have to think about reflectors, uh reflecting sunlight into our oven. We're thinking about uh any kind of colors that might help absorb light. We're thinking about uh different insulation around what would be a box inside of a box. So, our inside box is our oven, and then our outside box is what's going to contain all that insulation. So, we have all kinds of different design parameters, geometric shapes, um so many different considerations to make uh for our solar ovens to be as hot as possible, while also trying to spend as little money as possible, too.
MARK BRODIE: So, how hot did some of these ovens get?
BYRON HEMPEL: Oh, man. Some of them got really hot. We actually had a record temperature this year, even though it was cloudy and there was some fluctuations in our incident sunlight. Uh, so our temperature to— this year was 252.2°C, which is well over 450°F.
MARK BRODIE: Wow. Like, you could cook real food in that.
BYRON HEMPEL: You could really cook real food. I wouldn't suggest it, though. Um, these solar ovens are not FDA approved um and so they're uh there's a lot of glue and other things involved in there. You probably wouldn't want to eat what's inside these ovens.
MARK BRODIE: But in theory, I mean, if you used this kind of technology, you know, some of what your students were doing and maybe, you know, just had some tighter construction methods, it sounds like it's the kind of thing you could, you know, cook meals in.
BYRON HEMPEL: Oh, absolutely. You totally could, especially in Arizona where you have a lot of incident sunlight coming in. Uh, so ideally, if you were going to build a solar oven at home, you would want to make sure that everything is food safe. Uh, you might even change your design to being a parabolic shape of reflectors. Uh, our teams, it— they actually have more of um geometric, flat shapes of reflectors. If we did the parabolic shape, we would actually light all of our cardboard on fire for our teams to design really well. So, we we actually purposefully restrained our designs so they wouldn't get much hotter.
MARK BRODIE: I mean, I guess that's one way to increase the temperature, though.
BYRON HEMPEL: It definitely is. So, if you really want to uh set some high record temperatures at home, you would probably do a parabolic shape.
MARK BRODIE: Are you somebody who cooks with a solar oven, like outside of the school setting, or like, have you eaten food that has been cooked in a solar oven?
BYRON HEMPEL: Uh, I don't actively use a solar oven. It does take a little bit of time to heat up. Um, over the summertime, it probably wouldn't take that long at all. Um, I'm purp— I'm more of a natural gas fan uh inside Tucson whenever I cook. But I have eaten some cookies, biscuits, and other things that have been cooked by solar ovens. They are quite tasty, and it works really just as well as an oven.
MARK BRODIE: So, in your mind, like, is the Sonoran Desert— is the Phoenix area, the Tucson area, like, are these maybe some of the better places around if you are somebody who wants to use a solar oven to do that?
BYRON HEMPEL: Oh, 100%. So, if you um kind of have a feeling for it, any place that's great for solar panels uh for producing your own electricity is also going to be a great place for solar thermal uh objects like your solar oven, or maybe you're even using like a solar water heater.
MARK BRODIE: So, you mentioned that it can take a little while for the solar oven to heat up if you want to use it. Are there particular foods that that are maybe better suited to be cooked in a solar oven than others?
BYRON HEMPEL: Uh, that would not be my expertise, but I could assume that, as with anything, especially if you're trying to bake, you want to try to reach your optimal temperature first before placing your food in. Um, and if you build your solar ovens really well, have a lot of insulation, have very little uh ways for convection to pull out the hot air, if you have really good resistances to any type of heat sink, as we would say, uh then you could actually get pretty hot temperatures fairly fast. Uh, these students got their ovens at max temperature in about 20 or 30 minutes, and then it fluctuated or maybe slowly rose after that. So, if you're cooking at home, that's really not too much longer than your in— than your oven inside your house, although this is a much bigger size to heat up.
MARK BRODIE: Right. Well, and I wonder, is there a way to control the temperature? Like, if you wanted to cook something at, say, 350, do you just have to hope that that's as hot as your particular oven will go, or can you control that?
BYRON HEMPEL: Oh, that is a great question. Actually, I'm wondering if we should try to change our uh solar oven throwdown to see if you can get closest to 350°C. But uh you very much easily could. Um, the if you imagine most of your heat sink is from convection, or the movement of air, kind of think about radiators on your car, for instance. Those cool off your engine quite a bit just by having air pass over those hot plates.
MARK BRODIE: Yeah.
BYRON HEMPEL: So, if you have a little flap on your solar oven, you could probably very easily control the internal temperature. Uh, it might be hard to get uh calibrated perfectly, but I think with time and experience, people could get pretty good with controlling their solar ovens.
MARK BRODIE: All right, that is Byron Hempel, a professor of practice in the Department of Chemical and Environmental Engineering at the University of Arizona. Byron, thanks for your time. I really appreciate it.
BYRON HEMPEL: It was a pleasure, Mark.
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