Last updated: 8 March 2017
With the advent of cheap evacuated glass tubes that are manufactured in China, is has become practical to use them to create Evacuated tube solar cooker designs. The first such use is apparently Alex Kee in Malaysia, who demonstrated such a cooker at the Solar Cookers and Food Processing International Conference (2006) in Granada, Spain. Such tubes were originally produced as a component of popular solar water heating systems. Evacuated tube literally means that the cooking chamber is constructed of two layers of blown glass in the shape of a sealed tube, where the air has been removed between the layers.
Heat loss happens primarily by conduction and convection through a medium. With no air between the layers of glass the chamber is nicely insulated, well suited for retaining cooking heat. The chamber is so effective it often does not require a large reflector to capture sunlight. The ends of the tube are open so a slender cooking tray can be inserted. Improvements in glass technology is allowing for larger diameter tubes to be fabricated, which will allow larger cook pots to be inserted inside.
- February 2016: Steam collection in evacuated tube cooking: Faustine Odaba from NAREWAMA, describes the behavior of steam inside a tilted vacuum evacuated tube.
- August 2015: Solarmate is a recent product developed by the company of the same name. It is compact and easily carried. It has been designed more for quickly heating water for hot drinks.
- August 2015: Evacuated tube observations and performance - Dave Oxford and Stewart MacLachlan
- August 2015: The SLiCK SM70 is an evacuated tube solar cooker. With relatively large reflectors it is well suited for solar cooking in high latitude locations such as the United Kingdom where it was developed. It will cook well at other locations as well.
Advantages and disadvantages
- Usually compact, and can cook quite efficiently with relatively small reflectors
- Contemporary designs have aesthetic appeal
- The cooking chamber requires careful handling to avoid thermal shock and breakage of the glass tube
- Glass technology somewhat limits the size of opening of the cooking chamber
Possible explanation for tube breakage
Dave Oxford writes: "We were puzzled by the 'vertical' breakages. They differed from the breakages sustained while the tubes were horizontal. In horizontal mode, the tube imploded and the glass dropped vertically, as you would expect, because of the vacuum. In vertical mode, heating water, they exploded - that is, the glass ended up a good distance from the cooker. Since writing the paper, we've revised our views about what accounts for this. We believe that there is a different mechanism at work - superheating. That is, in such a narrow tube, there is little opportunity for convection within the water column. Even large copper domestic water heating cylinders are known to stratify. So, we now think that some of the water near the bottom of the column reaches a temperature above 100 °C (212 °F), especially in full sun. When convection does occur, and as the pressure decreases as it rises through the column, it flashes abruptly to steam, causing an explosion. A similar thing happens when liquids are heated in microwave ovens, and localised heating occurs. I must stress that this is just our latest hypothesis, and we have no solid evidence to support this. It would be quite difficult to test. Meanwhile, we are wary of boiling water in a vertical tube, though heating to any temperature below boiling seems perfectly safe. Not sure what is meant by "the reality of vertical gradients when racking a vacuum tube vertically"
Audio and video
- January 2017:
- January 2017: Facebook video showing steaming loaves of bread being removed from the trough solar cooker
- January 2016: The Behavior of Steam Inside a Vacuum Evacuated Tube - Faustine Odaba, NAREWAMA
- January 2016: Durability of Solar Cooking Tubes - some field trials and observations (Abstract - Field and Destructive Testing of Solar Vacuum Tube Cookers) - Stewart MacLachlan/Dave Oxford CONSOLFOOD 2016, SLiCK Solar Stove, United Kingdom