How Do You Cook With Sunlight?

It’s a hot sunny day. You park your car outside and go grocery shopping. After an hour, you return, only to find out that your car’s interior is scorching as compared to its exterior.

Your car, sort of, acts as a Solar Oven. Some creative minds have even successfully tried to bake cookies on their cars’ dashboards. They are calling it the “Dashboard Cooking”, by the way. What a time to be alive!

A solar oven is a simple yet fascinating device. It can come in very handy in certain situations, especially when you’re outside. You can prepare some appetizing dishes without lighting a fire or using electricity. Let’s dive in and find out how does a solar oven work!

What is Solar Cooking?

Solar cooking is the utilization of solar energy to cook food.

It is not as simple as just putting a pot full of necessary ingredients in bright daylight, then waiting a couple of hours to let it get baked. To cook something, a certain heating level needs to be maintained.

Solely the rays coming from the sun won’t be able to achieve a temperature high enough, because of heat leakage.

How Does a Solar Oven Work?

Sun’s rays bring heat energy with them. The heat energy can be absorbed by the food material, thus, heating it. However, as soon as the temperature rises above the ambient, the heat starts leaking, preventing a further rise.

Let’s try to understand it with an analogy. You have a water tank with a puncture in it. You turn on the pump to fill it up. As the water level in the tank hikes, the leakage also rises. When the water crosses a certain level, the leakage exactly matches the pumping rate, and the water level cannot ascend further.

The ‘water level’ here represents the temperature. The ‘water pumping rate’ is solar heating. The ‘water leakage’ is the heat lost to the surroundings.

If you want the water level to climb further, you can do it by elevating the pump rate, and by reducing the leakage. This is exactly how you can maintain a higher pot temperature i.e., by increasing the sunlight energy absorption and by prohibiting heat leakage.

Improve these two parameters enough, and you just made yourself a simple solar oven!

3 Working Principles of a Solar Cooker

Do you know that a solar oven works like a greenhouse? It is the same phenomenon that maintains life-sustaining conditions on our blue planet. In a greenhouse, solar energy can enter it to heat the internals, but the re-emitted longer wavelength radiation cannot escape.

A solar cooker operates on C.A.R: Concentration, Absorption, and Retention.

Concentration

The first principle is the concentration of solar energy. To reach blazing temperatures, concentrated solar power is needed. Reflective surfaces are used that can direct the energy from a larger surface area towards the cooking pot.

On a clear day, the solar intensity on earth is around 1,000 watts per square meter or about 90 watts per square foot. Compare this to your electric oven that operates on 2,000 watts!

So, to rival the electric stove lying in your kitchen, our solar cooker needs sun’s rays that are incident on an area of about 2 square meters or 21.5 square feet. Reflective panels of similar size can be used to concentrate the solar power towards the container.

Reflectors can be made of any shiny material such as aluminum foil, acrylic mirror, etc.

Absorption

So, the concentrators are doing their job, but the food is still not cooking. You find out that the cooking pot is made of high-grade steel. As it turns out, steel has a polished surface, hence, it acts as a reflector too.

You get the point. Concentration is of no worth if the food in the oven does not absorb the light falling over it.

The dark color absorbs more light. Therefore, color the outside of the pot with non-toxic dark paint. However, the coating should be thin so that the heat of absorbed light can be transferred to the inner conductive material easily.

Some pots are already black which is a plus point.

Retention

We are close because the pot is getting hot now! Only one stone is left unturned – the retention.

Remember the analogy? As the temperature rises so is the amount of heat leakage. It needs to be stopped so that a maximum temperature can be achieved and maintained. In other words, the heat needs to be retained.

Easier said than done, – but how can the heat be retained? If the pot is insulated then how will the absorption take place in the first place?

The answer is a plastic or a glass window. We place the pot inside the oven and cover its opening with plastic or glass. The opening lets the light in but stops the heat radiation from going outside.

Solar ovens can easily go over 300 degrees Fahrenheit in optimal sunlight.

Types of Solar Cooker

Three fundamental types of solar stoves are known. There are many offshoot designs based on these three types.

Panel Cooker

It has an intuitive design. Big reflecting panels are arranged around the “baking area”. The cooking pot is often enclosed in a transparent plastic wrapper. This helps to retain heat.

Panel oven can attain a baking temperature of around 300^oF (or 150^oC). They are like do-it-yourself (DIY) kind of stuff.

Parabolic Cooker

This design utilizes pre-built concave mirrors. The advantage of using a curved mirror is that it has a fixed focal point. This helps better concentrate all the incident solar light on food.

They have the same structure as that of a satellite dish.

Due to enhanced concentration, the parabolic cookers can achieve temperatures up to 400^oF (~204^oC).

A standard 1800 watts parabolic cooker has a diameter of 5 feet. They are usually not foldable, so it can be troublesome to move them around easily. Manufacturers claim that focal point temperature can get as hot as 1300^oF (700^oC) to 1800^oF (1000^oC). This can be a safety concern if not handled properly.

Box Cooker

It is a commonly used type of solar cooking vessel. It comes in a manageable form factor. Rectangular panels are installed at the opening faces of a box. The panels act as concentrators, while the side and bottom face of the box act as retainers.

The top opening, from where the concentrated rays enter the box, is sealed with a plastic or glass cover that reflects back inside heat.

Solar box cookers can also maintain temperatures up to 400^oF (~204^oC). The pizza box solar oven is famous among pizza lovers. 

Other than these types, Evacuated tube cookers are trending these days. The container is a double glass cylinder with a vacuum in between the gap for better retention. Claims are that it can reach above 660^oF (~350^oC) cooking temperature.

Evacuated tube cooker is advertised to be portable and easy to set up. Their price ranges from $200 – $250.

Advantages of Solar Oven

The benefits are obvious.

Solar stoves don’t require fuel of any type to run, a bright sunny day is all that is required. The evacuated tube ovens are portable. You can take them to a picnic spot, hiking, or on a camping adventure! Solar ovens can save you some bucks on your bill ($0.1-$0.25 per cooking hour) and provide a green alternative for traditional cooking.

Most of them require no maintenance at all. Only the reflectors need to get cleaned. If properly cleaned and not damaged, their performance does not degrade even a bit! They can last as long as the owner wants.

As no combustible materials are involved, solar ovens have low chances of causing any type of fire. Solar ovens provide a sustainable solar cooking solution for people living in rural areas or those living off-grid.

They can even cook in freezing winters.

Disadvantages of Solar Oven

Though the tech looks cool, these ovens are not a good option when you can’t sacrifice time. Solar cookers take 2-3 hours to properly cook or bake.

One of the major disadvantages is that a solar oven can not work at night. It won’t even perform well while preparing an early breakfast.

As sunlight is required to power them, they might be a bad idea for people living in cloudy cities. Similarly, the folks living in apartments might also face a shortage of sunlight.

Solar cookers such as parabolic ones can take up a lot of space and they may not be easy to carry around. As the concentrators’ size is large, they can be significantly perturbed at moderate wind speeds.

While these ovens come in very handy for picnics, do not pack them as your only cooking equipment – the weather may get unfavorable.

FAQs

Get ready for exciting answers to some of the frequently asked questions about Solar Ovens.

Conclusion

Solar cooking is indeed a charming idea. It is simple enough to be built as a fun project, at home, at a trivial cost. Commercial alternatives are also available that provide enhanced solar cooking experience and portability.

The compact evacuated tube cooker can act as a cool picnic gadget. For people living off-grid or in rural areas, a solar oven might be the best cooking device. They can last longer and require only solar energy to work.

For the areas that receive low light, the whole concept of “a stove powered by the sunlight” simply does not work.

Moreover, solar cooking is not a replacement for traditional cooking. They don’t work at night. A medium-sized solar cooker cannot prepare food for gatherings; it would simply consume a lot of time. Naive solar cookers don’t offer precise temperature control, – the ones that do, cost about the same as a regular oven.

Updated on 01-30-23