How Solar Cooking Operates

In the mid-1700s, European naturalist Horace de Saussure created a small device designed to capture the sun's energy. During this period, scientists were extensively experimenting with sunlight-to-heat conversion, primarily using mirrors to focus the light to a single intense point. However, Saussure was intrigued by the heat-retaining properties of glass.

What Saussure invented was the earliest solar cooker, strikingly similar to what we now refer to as a solar box cooker. He arranged five glass boxes within each other and placed them on a black wooden slab. After several hours of sunlight exposure, the temperature inside the boxes reached nearly 190 degrees Fahrenheit (88 degrees Celsius). He used this box to bake fruit.

What started in the 18th century as a scientific experiment to understand why and how glass traps sunlight to generate heat, has now evolved into a vital tool in addressing hunger and disease in the world's poorest regions. (It also serves as an eco-friendly and innovative alternative to the conventional backyard barbecue.) The modern idea of solar cooking emerged in the 1950s but struggled to gain traction, possibly due to ineffective marketing by the United Nations [source: Radabaugh]. With the right equipment, the thermal energy from the sun can be harnessed to create temperatures high enough to cook food.

A solar cooker can perform almost any task that a stove or oven can, but it operates on a clean, renewable, and free source of energy. In this article, we'll explore how sunlight transforms into heat, review the various types of solar cookers available and their functions, discuss how solar cooking can be a potential lifesaver in many parts of the world, and analyze some of its limitations.

Let’s start by understanding the heart of the process: How does sunlight, or electromagnetic radiation, turn into heat?

Cooking With Light

Using stoves and ovens, we can prepare a variety of dishes, such as meat, vegetables, beans, rice, bread, and fruit in almost any style. Methods like baking, stewing, steaming, frying, and braising are all possible. A solar cooker can also achieve the same results, but with sunlight as the heat source instead of gas or electricity.

Sunlight itself isn't hot. It’s simply radiation, a form of light waves — essentially energy created by shifting electric and magnetic fields. While it may feel warm when it touches your skin, that’s due to the interaction between the light waves and the molecules in your skin. This process is akin to how one type of solar cooker, the box cooker, uses sunlight to generate high temperatures.

In basic terms, sunlight turns into heat when photons (light particles) moving within the light waves collide with molecules in a substance. The sun's electromagnetic rays are packed with energy. When they hit matter, whether solid or liquid, this energy makes the molecules within the material vibrate. This movement excites the molecules, causing them to jump around and produce heat. Solar cookers use various techniques to capture this heat.

The box cooker is a straightforward type of solar cooker. Roughly 3 to 5 feet (1 to 1.5 meters) in size, it functions as a sun-powered oven — a sealed box that warms up and traps heat. In its simplest form, the box cooker consists of an open-topped black interior box, with a piece of glass or transparent plastic placed on top. Often, reflectors (flat metallic or mirrored surfaces) are positioned around the box to help direct extra sunlight toward the glass.

To cook, place the box in direct sunlight with a pot of food inside, positioned on the black bottom of the box. As sunlight enters through the glass top, it hits the bottom, which heats up quickly. Dark colors are effective at absorbing heat, which is why the inside is black. The molecules of the box get energized and produce more heat. This trapped heat causes the oven to heat up progressively, much like a conventional oven. The food begins to cook as a result.

Box cookers can reach temperatures as high as 300 degrees F (150 degrees C) [source: SHEI]. This level of heat is sufficient for cooking meat safely.

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A parabolic cooker can achieve even higher temperatures, reaching up to 400 degrees F (204 degrees C), which is enough to fry food or bake bread. This slightly more intricate design employs curved, reflective surfaces that concentrate a large amount of sunlight into a small focal area. It operates similarly to a stove, and it tends to be large, sometimes several feet in diameter.

The pot sits on an arm that holds it at the center of the curved reflectors, suspended just above the point where all the light converges. This focal point becomes so hot, and the molecules vibrate so intensely, that the heat waves rise in a constant flow to hit the bottom of the pot.

Both parabolic and box cookers are quite large, making them challenging to transport. Additionally, box cookers tend to be heavy due to the glass used. A panel cooker, which incorporates parabolic reflectors placed above a box-style oven, is typically smaller and lighter. In this design, the cooking pot is placed inside a plastic bag during cooking, which acts as a heat trap, similar to the transparent top of a box cooker. These cookers are often used in camping situations.

While camping is a secondary use for solar cookers, their primary applications are related to addressing poverty, hunger, and disease.

How can cooking with sunlight make a difference?

The Benefits of Solar Cooking

Across the world, hundreds of millions of people face challenges in accessing cooking fuels [source: SCI]. In many cases, electricity and gas are out of reach, leaving only charcoal and firewood as options — with charcoal often being too costly. As a result, families rely on wood. The issue is that in many impoverished, rural areas, such as in Africa and India, trees are in short supply. Cooking meals for an entire family every day requires a significant amount of wood, and available resources are rapidly depleting. Families must walk for hours to gather firewood and spend most of their limited money on fuel, leaving less for food.

This situation leads not only to hunger but also to widespread disease.

With 1 billion people lacking access to clean water, the ability to easily pasteurize (heat to a point where microbes are killed) becomes essential [source: SCI]. Every year, 2 million people die from bacterial diseases that could have been prevented by pasteurizing drinking water [source: SCI]. It's also critical to heat meat and vegetables grown with contaminated water to ensure their safety. These foods can harbor harmful bacteria, worms, and viruses, causing deadly diseases such as hepatitis A, giardia, and E. coli infections.

Pasteurization doesn't require extreme heat. Water can be pasteurized at just 150 degrees F (65 degrees C), while food requires a slightly higher temperature of 180 degrees F (82 degrees C) [source: SCI]. Solar cookers, which can be as affordable as $10 for basic models, easily reach these temperatures and cook meals in just a few hours, depending on the type of cooker and food. The best part is that people don't need to travel long distances to gather fuel; they can simply step outside and harness the sun — at no cost. In fact, using a solar cooker for just one year can eliminate the need for 1 ton (0.9 metric tons) of firewood [source: Justus].

Another issue that solar cooking addresses is the pollution generated by burning wood. Fires release harmful pollutants into the air, and the smoke is not only bad for the environment but also dangerous to those who breathe it. When cooking indoors over open flames, people inhale fine particulates that can cause serious health problems, including both lung and heart diseases. An estimated 1.5 million people die each year from this type of air pollution [source: Madrigal]. Solar cookers eliminate the need for open flames, providing cleaner air for everyone.

While solar cookers offer a promising solution for many developing nations, they come with certain limitations. The very advantage that makes them effective — using sunlight as a fuel source — also poses a challenge. Solar cooking is most feasible in regions that experience dry, sunny weather for at least half of the year. Countries like India, Brazil, Kenya, and Ethiopia are some of the ideal places for this cooking method [source: SCI].

A more significant issue is that even in places like India, the sun doesn't always shine. Solar cookers are ineffective during the night or on cloudy days. This means that firewood is still required during the hours when the sun isn't available. Nevertheless, cooking most meals with sunlight instead of firewood helps alleviate some of the challenges faced by those with limited access to fuel.