EPFL creates a solar cooker with solid potential in Switzerland

Solar cookers – or solar-powered ovens – can be used to cook foods at low temperatures (60–120°C) for anywhere from 30 minutes up to four hours. This makes them perfect for a range of dishes, such as potatoes, vegetables, rice, stews and even cupcakes. EPFL scientists have been working on an enhanced version of the conventional solar cooker since 2018. In research that appeared in Solar Energy, the scientists show how their design can operate in Switzerland’s sunniest areas, such as Valais Canton, for 240 days per year – or two thirds of the year. And in the country’s cloudiest areas (such as Zurich), it can operate for 155 days per year, a figure that surprised even the researchers.

The scientists in question work at EPFL’s Solar Energy and Building Physics Laboratory (LESO-PB). But how did building experts come to be interested in cookers? “It all started with a chocolate cake – and it’ll all end with a chocolate cake,” says Andreas Schüler, a research associate at LESO-PB. One day, Olivia Bouvard – a scientist at the lab – arrived at work with a cake she’d baked in a solar cooker. That got her colleagues interested in how these devices work. They ordered one from Solemyo, an association based in Geneva, and began to reverse-engineer its design.

Like a miniature building
“A solar cooker is actually like a tiny building with glazed facades,” says Schüler. Which is right up his alley, since he has been working on advanced window-glazing technology for the past few years. Schüler and his colleagues at LESO-PB made improvements to the size of the cooker they’d ordered and added more glazing – using, of course, their innovative type of glass. That boosted the amount of sunlight entering the cooker, but it did not increase the amount of heat loss, thanks to their glass’s superior insulating properties. The scientists worked with the Swiss Design Center to create a prototype of their model, and they filed for a patent in 2018. Then, using an EPFL Enable grant and an ENAC InnoSeed grant awarded the same year, they teamed up with TZ Menuiserie SA – a carpentry firm based in Valais – to manufacture ten prototypes.

The EPFL model is designed specifically for Switzerland. In addition to glass, it uses pine wood and is surrounded with aluminum to reflect and guide the sun’s rays. The cooker’s shape and glazing are engineered to create a “sunlight trap” so that it doesn’t have to be moved during cooking to follow the sun’s rays. With dimensions of 33 cm x 33 cm, it is large enough to hold a standard casserole dish. “We also looked into the idea of creating a collapsible, portable cooker, along with a fold-up casserole dish, that people could use when they go hiking or to the beach,” says Schüler, who himself is a proud owner of one of the prototypes.

“We believe our cooker could help reduce the load on Switzerland’s power grid during the peak lunch and evening hours. Cooking is the third-biggest use of household power in Switzerland, behind heating systems and water boilers. With our cooker, you could put all the ingredients in before going to work, for example, and have a meal ready for you when you get home – without using gas or electricity,” says Schüler.

Designs for other countries, too
The method for calculating how many days a year the solar cooker could work was developed by Dasaraden Mauree, an LESO-PB scientist specialized in modelling techniques. “We combined climatic data for Switzerland with our cooker’s operating data to develop a mathematical model that can subsequently be used for any other region in the world,” says Mauree. The scientists tested their model by collecting data from their cooker over one month, on the roof of their lab, and comparing the data with the model’s predictions. In addition to validating the model, these tests showed that their solar cooker, when equipped with aluminum reflectors, can deliver 55% more cooking days per year than the same cooker without reflectors.

Today the scientists are working on further increasing their cooker’s efficiency and on making it a “smart cooker” by incorporating an app that would let users monitor the oven when they’re not at home and that would send an alert when a meal is ready. They are also thinking about adjusting their design so it can be used in other countries through a technology-transfer approach. “We’ve had Master’s students work with us on this project, and their ideas and enthusiasm really helped us move it forward. We owe them a lot!” says Schüler. Now they can apply the lessons learned while developing the cooker to their research on making buildings tighter. So after shifting their focus from buildings to cookers, the researchers can now apply what they learned on cookers to buildings.

Jérémy Fleury, Dasaraden Mauree, Andreas Schüler and Olivia Bouvard. © Alain Herzog / EPFL