One of the less conventional solar oven devices is a solar kettle. These kettles fall within the category of a solar cooker and although more renowned for heating liquids, in certain instances can be used to cook food as well. Wikipedia describes these devices as “solar thermal devices that can heat water to boiling point through the reliance on solar energy alone”.

There are a handful of these solar kettle devices available, although fundamentally the more well-known designs are as follows:

The solar kettle-thermos flask is a solar thermal design, developed by Alex Kee in 2006, that uses an evacuated solar glass tube (solar vacuum glass tube) constructed from borosilicate glass to capture and store energy from the sun. The tube consists of an inner glass layer characterized by a dark exterior that heats up in sunlight, whereas the outer glass layer is transparent allowing sunshine to penetrate. This sunshine transports the solar infra-red energy and penetrates through this outer layer and subsequently through the vacuum layer onto the inner layer where it is absorbed.

The air is evacuated between these two layers with a consequent insulating vacuum. The vacuum effect is disproportionately powerful due to minimal air or the absence of air. Without air molecules present (or with very little air molecules present), heat cannot move around and remains trapped, which ensures the environment gets hotter and retains the heat for longer, in much the same way that a vacuum flask keeps liquids hotter for longer periods of time.

The stagnating temperature of solar vacuum glass tubes is a high 425 ° F (220 ° C), a prevailing dynamic that enables the solar kettle to store energy and boil water or other liquids. In addition this energy storage technology, using vacuum insulating properties, allows the kettle to run after sunset as long as the kettle has been charged up during the daylight hours.

This kettle has a simple assembly of a tube stand angled at approximately 45 degrees, through which the solar kettle is inserted and then adjusted towards the sun. Thereafter the tube is filled with approximately 2 liters of water and sealed tightly with a cork. Usually in full sunlight pasteurization temperatures can be reached within 2 hours. Solar kettles also tend to get hotter than solar cookers and can in certain instances a modified design can be used for cooking as well.

The Navitron’s C-zero Solar Kettle/cooker is a kettle designed and developed by Navitron Ltd of Oakham. Described as one of the most efficient kettles in the world it similarly runs off evacuated solar tube technology, uses no electricity whatsoever by running exclusively off solar energy, has a capacity of 2 litres, takes about 3 hours to reach boiling point starting at ambient temperature and retains heat for a disproportionately long time, taking in excess of 15 hours to cool right back to ambient temperatures.

Another variant is the Solar Billy which is also a very similar design which also uses evacuated solar principles to harness the solar energy from the sun needed to heat the kettle. This kettle takes between 30 minutes to 3 hours to boil water and will still heat up, albeit slower on an overcast day because the embedded vacuum tubes absorb infra-red rays.