Making it hotter

One of the technical breakthroughs that has supported the growth of human beings into a population that dominates the planet was the invention of the cooking pot. This took place some 18,000 years ago, in China or Japan. But wherever it was invented, the importance was that it turned food into something safe to eat by ensuring the destruction of all possible pathogens. This is still true today. For food to be safe, it needs to be cooked to a high temperature and right through. Even plain water is not truly safe to drink in a hot country unless it has been boiled for at least 20 minutes. However, because of the large number of humans on the planet, using fuel for cooking causes problems. Not only does burning fossil fuels add to global warming, but cutting trees down for cooking fuel is even worse, not only removing CO2 sinks, but also leading to increased possibilities for soil erosion and flooding – especially in hot countries. One solution is to use the sun. There is a plethora of designs, mostly based on boxes with reflective foil aiming sunlight into the interior, which is painted black. While these are a great idea, they suffer from two drawbacks. One is that, unless the sun is very strong, they don't tend to get hot enough to kill all the nasty bugs. Secondly, they tend to produce their cooked food in the early afternoon, when people are least inclined to eat it. The Challenge Our challenge this month is therefore to come up with a better solar cooker. Cost has to be minimal, because the target markets include refugee camps and tropical villages. The cooker should also be capable of being used on days when the sun is not at maximum strength and at latitudes that are not entirely inside the tropics. It also has to be easy to use: anything that requires precision setting up and careful maintenance is not going to last long. Electronically controlled systems or those requiring photovoltaic cells are also non starters. The construction needs to be simple and rugged and should use locally available materials where possible. While labour is available to undertake adjustments, human attention should be minimal. Above all, it should be able to deliver hot food towards the end of the day. The solution, offered below would appear to solve all the problem elegantly and at remarkably low cost without recourse to any kind of electronics. Once you see it, you may consider it obvious, except that it is innovative enough to be protected by patent. See if you can come up with anything better. Solution The solution to our July challenge comes from Peter Smith, who lives in Cumbria. And his design works there, at least in Summer. His two breakthroughs are to use two Fresnel lenses to concentrate the sun's rays, and a lot of sand to give it enough thermal inertia to continue to cook, or at least to keep the food hot, even when the sun is going down. A Fresnel lens is a flat piece of transparent plastic with a series of prismatic rings moulded into it so that it behaves like a convex lens. They are commercially available in order to build TV projection systems with screens much larger than the actual display. That used for a solar cooker need only be fairly crudely made compared with those used for TV displays. In the Peter Smith design, the cooker is made as an insulated vessel filled with sand, plus some copper pipes to improve heat distribution. The lens is mounted on a frame and moved about once every half hour to keep the sun's rays directed into the cooker. In the prototype system, one lens is used to heat up the sand, before the cooking pot is put inside. On a summer's day in Cumbria, the arrangement reaches 80 deg C in three to four hours. Heating time is likely to be greatly reduced at more southerly latitudes. Smith says of it that the design is largely finalised, "It's just a matter of fine tuning the dimensions now."