Solar cooking, when decarbonization becomes a pleasure

With a solar cooker, you can make ratatouille, brioche, cookies or even professional cooking, as is the case at Le Présage restaurant or during the solar cooking festival in Marseille.

Solar cooking also addresses major issues. Today, 2.6 billion human beings do not have access to a “clean” cooking system and use wood or charcoal for one of their essential needs, with disastrous consequences: deforestation, emission of greenhouse gases and pollutants leading to the premature death of approximately 2.5 million people per year, not to mention the obligation of populations to devote a large part of their day to the search for fuel, putting them in danger and limiting their available time to go to school or university.

The solar cooker could therefore seem to be a technological, ecological, economic and social solution. Solar cookers do their job well, they use free energy that is widely available in many parts of the world to meet an essential need.

So why aren’t solar cookers more widespread in France and the rest of the world?

How it works ?

The principle of a solar cooker is quite simple: we try to bring in as much solar energy as possible using mirrors and surfaces that absorb very good sunlight (black), and we try to have a minimum of heat loss through insulation such as rock wool or glass.

There are several solar cooker technologies, such as box cookers, parabolic cookers, vacuum tube cookers and Scheffler dishes. Each technology has its own temperature, ease of construction or use, advantages and disadvantages.

Three solar cookers under the snow.
Fringe2013, Wikimedia, CC BY-SA

A first explanation for their very limited adoption could be their inefficiency, or their inability to reach the temperatures necessary for the various cooking methods. It is true that “box” type cookers, generally made of cardboard or wood, which are the simplest and cheapest, have difficulty reaching temperatures above 150°C and take a long time to heat up.

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To speed up the heating and/or increase the temperature reached by a solar cooker, it is necessary to increase the concentration of the sun’s rays using additional mirrors and to reduce heat losses from the cooking dish by improving the insulation of the cooker.

A solar oven. There are several types, each with its advantages and disadvantages – here, the “box” type.
Erik Burton, Flickr, CC BY

Vacuum tube cookers are an extremely promising compromise. They consist of two glass tubes fitted one inside the other: the outer tube is transparent while the inner tube is painted black. They are separated by a small layer of vacuum: the best insulator we know! To cook food, just insert it into the inner tube. This type of cooker has so little heat loss that the internal temperature of a single tube left in the sun can reach over 110°C. With a simple system of mirrors to concentrate a few extra rays on their surface, one can easily reach 300°C in one hour.

The two disadvantages of these systems are the sensitivity to thermal shocks of the glass (uncovered sauce dishes should be avoided for projections) and the fact that it is compulsory to cook outdoors. Scheffler’s parabolas are so-called indirect cookers and make it possible to overcome this last problem, but for greater complexity.

A vacuum tube and its parabolas to redirect the sun's rays
An evacuated tube solar oven, reaching 260°C thanks to its concentrator.
Thomas Fasquelle, University of Aix Marseille, Provided by the author

A second valid reason could be the price: solar cookers require a potentially large but not prohibitive initial investment – ​​a few tens of euros for a box cooker, around 200 € for a vacuum tube cooker, and several hundreds or thousands of euros for parabolic cookers.

Solar energy, too complex to manage?

A third well-known reason is the intermittency of solar energy. When cooking, it is very unpleasant not to be master of time, in both senses of the latter. It is therefore the lack of a simple and inexpensive storage system that is the main defect of solar technology.

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Many research efforts are being made in this direction, such as the use of phase change materials, i.e. solid materials which melt at the temperature of use of the cooker (for example 200°C), absorbing a very large amount of heat, then resolidifying and releasing the heat when the sun is gone. The problem of storage is then to find the right compromise between efficiency and storage capacity on the one hand, and price and complexity on the other hand.

Solar cookers, not “cool” enough?

Finally, the slow spread of solar cookers can be explained by the image they convey and sociology.

Indeed, solar cookers today seem to be restrictive to use compared to the alternatives: most of us want to cook at any time, in any weather and indoors.

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But wouldn’t there be at least one cooking activity practiced outdoors, only when the weather is nice, and consuming energy that is not always carbon-free? The famous BBQ! The solar cooker has its place to replace or supplement it, provided it is efficient, easy to use and has high added social value.

The association Les Festins Photoniques, based in Marseille, thinks that several solar cooker technologies, in particular tubes, can fulfill this service. Optimization of insulation and optics, in particular using so-called non-imaging technology, could also lead to solar planchas.

The restaurant Le Présage, based in Marseille, thinks that we can go further in the use of these technologies. It uses an easy-to-build and easy-to-use technology, the Scheffler parabola, to make professional and quality collective cooking. Although costing several thousand euros, this parabola has the advantage of allowing professional solar cooking indoors and at high temperatures (400°C for the cooking plate at Le Présage restaurant). Thus, perhaps with the democratization of high-performance, user-friendly tools, allowing you to cook refined dishes, solar cookers will go from being a restrictive solution to a tool of social utility!

As far as developing countries are concerned, the social question relates more to poor communication between the promoters of solar cooking and the populations to whom it could benefit: failure to take into account local cooking needs and methods in the solutions proposed (for example the size of family-type cookers, or the desired temperature for the traditional local dish), lack of training and demonstration of the performance of the cookers, lack of exemplarity from developed countries that make massive use of fossil fuels or electricity for the kitchen, etc.

So what should be done to speed things up?

To accelerate this process, research and development institutions are working on four aspects: energy storage (not yet mature because it is too complex and too expensive), the development of cheap cookers that are easy to build by and for individuals ( under rapid development, often with plans/manuals offered in open source in different languages, as in France, the improvement of the most promising technologies such as Scheffler’s parabolas (in Marseilles the restaurant Le Présage is associated with the IUSTI laboratory to improve their solar cooker based on this technology), and above all taking into account the needs of populations upstream of a solar cooking development project.

At the same time, the players in this field are trying, at least in France, to transform the “constraint” linked to the use of solar cookers into a pleasure for the eyes and the taste buds, but also a game. we hope to decarbonize uses while creating pleasure.

Finally, there is a need for more funding for solar cooker development projects that are carried out by humanitarian associations and certain specific programs.

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