Solar Cooking

One of the highlights of my summer at Ampersand Sustainable Learning Center was learning to cook with the sun. My fellow intern and I bonded right away over our love of vegetables and passion for the kitchen. We were eager to learn more sustainable ways to prepare our favorite dishes and to experiment with new recipes and cooking methods. Over the next two months we prepared nearly all of our meals (except for a few overcast days) with parabolic and box oven solar cookers. I was pleasantly surprised at how simple and fun it turned out to be to adapt recipes and make just about any dish using only the sun!

Solar Cooking Basics

Passive solar principles and thermodynamics can be used to capture and direct solar energy to cook food. While there are a few different methods and models of solar cookers, this is generally accomplished through some combination of reflective panels that concentrate the light and heat of solar rays towards the pot, and trapping and absorbing heat with glass and dark colored metal, which is then conducted (direct contact) or radiated (transmitted through air) into the food.

Dark pots and pans are critical for solar cooking, so that they absorb the energy directed towards them rather than reflecting it away again. If your pots don’t have dark enamel, they can be painted with high-temperature barbecue paint. Don’t use just any old paint, which will off-gas toxic fumes into your food!

When possible, prepare food when the sun is high in the sky, late morning to early afternoon. If you have a fairly predictable storm pattern (like afternoon thunderstorms during monsoon season in the southwest), plan ahead to cook your meal before the clouds roll in. Obviously a region with many sunny days, like the southwest, is more amenable to solar cooking than a foggy coastline.

The two types of solar cookers we used to prepare daily meals at Ampersand were the parabolic “stovetop” and the solar box oven.

The parabolic is made of carefully positioned metallic plates that reflect the sun’s rays towards the center rack, where you place your pot. In full midday sun, the parabolic can reach up to 500°F, making it perfect for boiling, simmering, sautéing, or almost anything you would use a stovetop for, usually requiring similar cooking times — and the same vigilance to keep things from sticking or burning, especially when uncovered! To work effectively, it also needs to be rotated every 30-60 minutes to follow the sun. When the sun is still low in the sky in the morning, or on overcast days, it will produce significantly less heat, which means it will cook much slower and may not work well for some dishes — frying or brown rice, for instance. It is still surprisingly efficient at capturing solar rays however, allowing you to still prepare quick cooking grains like oatmeal, quinoa, or millet. Bringing the food to cooking temperature with another fuel source first and then letting it simmer on the parabolic can be another technique for more energy efficient cooking for partly cloudy days less suited for exclusively solar cooking, or for speeding things along.

Thick cast iron pots, with or without enamel, work well on the parabolic mid-day helping reduce the risk of burning. The parabolic can also be used to bring food more quickly up to cooking temperature before transferring it to the box oven “slow cooker.”

Box Oven
The solar box oven is perfect for hands-off cooking. It captures heat from the sun through the top glass panel, which is held in by insulation, reflected toward the center by the metallic sides, and absorbed and then conducted and radiated back to the pot from the black bottom. The design is simple enough to build yourself, and can either be placed in the south wall of a structure or on top of a swivel chair so that it can be oriented towards the sun throughout the day.

The solar box oven maxes out at about 350°F, on a cloudless day when the sun is high in the sky and after the box oven has been facing the sun long enough to warm up. It has all the advantages of a low-heat slow-cooker — you can walk away from your food for hours, letting it marinate in its own juices and deepen in flavor, without worrying about the food burning or having the nutrition blazed out of it. It’s perfect for casseroles, and also can work well for many baked goods, especially quick breads like cornbread and muffins. The timing can be quite flexible, often within the range of an entire hour, making it easier to fit around your schedule.

We often assume that when we are busy, the only solution is to rush everything along as fast as possible. But actually, a slower but more flexible and low-maintenance option might sometimes meet our needs at least as well as the quickest shortcut. All it requires is putting some effort into advance planning. The solar box oven is a great example of this. Away from home all day? Do your meal prep the night before or first thing in the morning, stick it in the box oven before you leave, orient it towards the south or slightly southwest, and come back in the evening to enjoy your slow-cooked dinner. A few caveats: If you don’t get home until an hour or two after dark, your food may have cooled as the temperature dropped, especially if it’s chilly out, and need to be reheated with another energy source. A larger amount of food in a thick pot will retain heat longer. If the sky clouded over for most of the day, it may need a little time on the stovetop to finish cooking. Nonetheless, this can often be a fun, simple, convenient, ecological way to let the afternoon sun cook dinner for you.

There are also methods for speeding up the process, and for cooking a wide variety of foods in the solar box oven. For instance:

  • Food can be placed in dark mason jars (tinted or covered with barbecue paint) — screw on the lid for a mini “pressure-cooker,” great for steaming veggies.
  • Food can be brought to cooking temperature on the parabolic or another stovetop.
  • Turning the oven towards the sun before you start meal prep will preheat the oven. When not needed to cook a meal, black jars or bottles of water can be heated in the oven — the water can then be used for cooking, or the jars can be left in the oven to radiate heat out to the cooking food.
  • Small, thin, short and wide pots provide more surface area, particularly in contact with the bottom of the oven, maximizing heat transfer. Dividing food into a couple smaller pots and cooking smaller amounts of food, cut into smaller pieces, also shortens cooking time. By the same token, the more pots and jars and so on are jammed into the oven at the same time, the longer it’s going to take for any of them to cook. If you find solar cooking fits well into your life and you are serious about using it as a primary cooking method, you’ll probably want more than one of these ovens.
  • Intrigued? Check out the Ampersand site for photos and more information on various solar cookers, including where to acquire your own — or how to build one yourself!

    Adapting Recipes
    Many stovetop recipes can be transferred directly to the parabolic, with only a possible slight increase in cooking time midday. Light colored foods without a lid — fried rice or tofu, for instance — do tend to a little longer than darker foods, but the difference is minor except in marginal weather conditions. Pan-frying seems rather tricky (but I find pan-frying tricky in general), and I admittedly have not attempted it yet. Deep-frying doesn’t really seem feasible, but stir-fries, sautéed and braised veggies, stews, soups, and so on work just as wonderfully. Beans do seem to take longer in my experience. Usually we’d start them after breakfast and plan to eat them for dinner — lunch could be a stretch, especially if a few clouds passed through. Garbanzos seem beyond the capacities of solar cooking, unless you can find some much fresher than the hard-as-rocks offered by most stores.

    In the solar box oven, it seems to generally take between two to three times as long for things to bake as in a conventional oven. I found 3-4 hours to usually be about right for most casseroles and quick breads. You may be able to pull it out sooner if its the hottest part of the day, and it could be a little longer if it’s partly cloudy or you are starting in the early morning or late afternoon. Keep covered throughout. For baked goods that demand a crispness and can’t really be covered — like cookies, muffins, or veggie burgers — it may be necessary to wedge the oven door slightly open or wipe condensation off the glass from time to time.

    Baked goods with a crust, like quiche or pie, are more challenging for solar baking. Pre-bake the pie crust before adding the filling. This information comes second-hand, as I haven’t tried this yet myself, so I can’t speak to how long this generally takes.

    Stew, soup, and legume recipes can also be adapted to a solar box oven. I don’t have much experience with this, since a parabolic cooker is so much more convenient, but if you only have a box oven to work with, plan to make a small batch and give it much of the day to cook, and/or cook some of the ingredients ahead of time and then mix it all together and set it to simmer and meld a few hours before you want to eat. A small pot or a few “pressure cooking” jars of a quick cooking legume like lentils should be able to cook in half a day of sun, but larger beans might present more of a challenge. I’d be dubious to try unless I had a full day with ideal weather conditions. Sprouting the beans first would shorten the cooking time some, along with enhancing nutrition.

    Recipes requiring precise temperature control or really high heat aren’t good candidates for solar cooking. (Many yeasted/sourdough breads spring to mind, though with some experimentation they might be workable on the parabolic). Pancakes or homemade tortillas also sound daunting. Heck, we never even managed to find a satisfactory way to rewarm and soften up tortillas with the sun. But otherwise, in my experience most recipes can be adapted quite readily to solar cooking.

    Perhaps the biggest adjustment is learning to prepare further ahead. I sometimes fell into old habits that didn’t fit with solar cooking, such as planning to whip together a quick stir-fry after work, only to be reminded that while everything might sizzled wonderfully in the pan for a quick lunch, it sits cold in the pan for ages as the evening sun dangles over the horizon, quite possibly swallowed up in clouds. Generally, starting lunch right after breakfast, and dinner right after lunch (or eating leftovers) seemed the most reliable strategy.

    Some Tried-and-True Favorites
    Here are some of the recipes I have successfully cooked with the sun. I’ve done my best to explain how for most of these. Certainly many more of the recipes on this site are well-suited for solar cooking… I encourage you to explore the possibilities for yourself!