Last updated: 12 June 2018
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. One end of the tube is left open to allow a slender cooking tray to be inserted. The tray has a handle that seals with a gasket against the glass tube. The opposite end is fitted with a fixed stopper or has had the tube sealed during the manufacturing process. Improvements in glass technology is allowing for larger diameter tubes to be fabricated, which will allow larger cookware to be inserted inside.
- June 2018: Prospects for a metal-lined Rand tube - One way of achieving this would be to make the inner tube from stainless steel while preserving the other elements of the design – the glass outer tube, the evacuated space between the tubes, and the selective coating on the outside of the inner tube. Such a design already exists, though not for cooking. Utility scale concentrated solar power plants deploy linear parabolic reflector troughs with evacuated collector tubes at their focus. These tubes usually contain heat transfer fluid at temperatures of up to 400 °C (752 °F). They are said to be very durable (Gordy Bishop, personal communication). They are similar in construction to evacuated tubes made entirely of borosilicate glass, except that the inner tube is constructed of stainless steel. Thermal expansion and contraction are accommodated by flexible bellows. Read Dave Oxford and Stewart Maclachlan's paper on Rand tube safety.
- 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 disadvantagesEdit
- 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 breakageEdit
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"
- June 2018: The Durability of Evacuated Solar Cooking Tubes - Some Preliminary Field Trials - Dave Oxford & Stewart Maclachlan
- January 2016: The Durabilitity of Evacuated Solar Cooking Tubes - Somae Preliminary Field Trials - Dave Oxford and Stewart Maclachlan
Audio and videoEdit
- November 2017:
- January 2017:
All construction plansEdit
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