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Evacuated tube solar cooker designs

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Last updated: 10 January 2017      
Evacuated tube grid

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GoSun Stove- Welcome to the Fuel-Free Frontier00:53

GoSun Stove- Welcome to the Fuel-Free Frontier

SLiCK SM70 photo, 8-19-15

The SLiCK SM70 is another example of an evacuated tube style solar cooker.

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.

News

Advantages and disadvantages

Advantages

  • Usually compact, and can cook quite efficiently with relatively small reflectors
  • Contemporary designs have aesthetic appeal

Disadvantages

  • 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

NEW:  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 what is meant by "the reality of vertical gradients when racking a vacuum tube vertically"

Resources

All Evacuated tube solar cooker designs

All Evacuated tube solar cooker construction plans

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