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Solar oven designed by Steen Carlsen

Clean Power

Solar Oven — Simple & Crucial

Last year, in the the spring, I met Steen Carlsen. He has travelled extensively in Asia and East Africa, including Zambia, which is why I came to hear of him. Steen has a background in electronics and electrical engineering and we have had many interesting conversations already. Unfortunately, Steen is terminally ill, and is in a hurry to realize a few projects before it is too late. When he showed me his solar oven project, I suggested we tell about it here, in the hope that his detailed description of this simple and crucial technology could reach more people who would benefit greatly.

It is our hope that this report will serve as a technical source for anyone who wants to make instructions for building and using solar ovens in regions of the world where any of the more modern approaches are not yet possible. Steen mentions refugee camps as places where no commodity, let alone energy, is a given.

The report contains rare and very detailed technical details on the principles of thermal conductivity. It’s a must read for anyone who needs to manage temperatures in any system really. It’s an exercise in capturing, confining, and not wasting thermal energy.

The following are excerpts from Steen’s report. Download the full report here.

Considerations, design calculations, construction, test and evaluation of a cavity walled solar oven

The use of fossil fuels presents many problems, not least in refugee camps. Single-walled solar ovens have proved to work in Ladakh (in the Himalayas), although, a higher operating temperature is desired. This report investigates the feasibility of designing a dual (i.e., cavity) wall and double-glazed solar oven, with the aim to raise the operating temperature as near to 100 degrees Celsius as possible.

A prototype has been built and tested in Aarhus, Denmark (Latitude 56o 16’N; Longitude 10o 20’E), where it reached a maximum temperature of 83.0 degrees Celsius on August 13, 2022. The test of the oven complied well with the theoretical calculations.

A crucial design process is to add up the three heat flows — the one from the conduction, the one from the convection, and the one from the radiation. Knowing the sum of each of these three heat flow contributions (denoted ΣP) and knowing the difference in temperature between the desired temperatures TA and TB, I can now calculate the resulting thermal resistance RTh = |TB – TA| / ΣP.

In a thermal system, a thermal resistor compares to an electric resistor in an electric system.

In a thermal system, a thermal capacitance compares to an electric capacitance in an electric system.

In a thermal system, a heat flow compares to an electric current in an electric system.

In a thermal system, a temperature compares to a voltage in an electric system.

Based on calculations, test results, and the lessons learned, I believe that the temperature of a subsequent prototype can be increased by another 5–15 degrees Celsius. The design, the technology, the choice of materials and required tools have been aimed to be adapted to the conditions in refugee camps. I have tried to use either recycled or cheap lightweight materials. I have strived to make a design that can be made by a skilled handyman.

From the early 20th century, hay boxes have been used extensively to reduce the consumption of fuel. It seems obvious to merge the functions of solar ovens and of hay boxes, which would imply fuel/firewood savings even during the rainy seasons. Another additional purpose for the solar ovens is to sterilise contaminated water to reduce the high mortality rate caused by muddy drinking water.

Photo by Steen Carlsen

This report is aimed at refugees, refugee camp managers, NGOs and other organisations, do it yourself people — and whoever might have an interest. In any design — in any activity — a feedback from the end user is crucial. Please help me improve by providing me with feedback, may it be suggestions, questions, experiences, or whatever.

There is, however, a deadline for the feedback, as I suffer from a number of incurable diseases. It is my hope with this report to reach out to organisations and individuals who have an interest to use the report and hopefully to take my work forward.

It is my wish that this report shall be accessible free of charge to everyone on the internet. I have no economic interests in this project.

Aarhus, Denmark, December 16th 2022.

Steen Carlsen MSc. E.E.
Regenburgsgade 11, 8000 Aarhus C. Denmark.
Mobile: +45 23636968

As a reference and comparison, please see page 90 in USAID’s report Fuel-Efficient Stove Programs In Humanitarian Settings.

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Written By

Jesper had his perspective on the world expanded vastly after having attended primary school in rural Africa in the early 1980s. And while educated a computer programmer and laboratory technician, working with computers and lab-robots at the institute of forensic medicine in Aarhus, Denmark, he never forgets what life is like having nothing. Thus it became obvious for him that technological advancement is necessary for the prosperity of all humankind, sharing this one vessel we call planet earth. However, technology has to be smart, clean, sustainable, widely accessible, and democratic in order to change the world for the better. Writing about clean energy, electric transportation, energy poverty, and related issues, he gets the message through to anyone who wants to know better. Jesper is founder of and a long-term investor in Tesla, Ørsted, and Vestas.


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