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Cooking up Solar Solutions

Solar power need not be complicated. Research into complex materials that convert the energy from sunlight into electricity is well underway, but offers only low efficiencies.

In contrast direct heating of water sidesteps the intermediary of converting sunlight into electricity and then using that to power a heating element in a water tank. All you need is some kind of pipework on a southerly facing roof in a hot climate. The pipework, painted matt black, carries a cold water supply and being held beneath glass heats up very quickly during the day, transferring the sun’s energy into the flowing cold water and producing piping hot water on the outflow.

Similarly, as any sadistic school child with a magnifying glass and a trapped ant will tell you, focused sunlight can produce a lot of heat in a small volume very quickly. Perfect for cooking.

There is no need for this inefficient conversion. Instead a reflective sunlight collector with a parabolic profile can be used to focus sunlight up and on to a hotplate. Such solar cookers are already well known and many a green barbecue enthusiast will have one in their garden near their greenhouses for cooking burgers, veggie and beef, and anything else they care to eat. The potential for summer savings in the developed world are immense if one removes the requirements for bottled gas and charcoal from the BBQ equation. You could spend more time and finances on creating a luxurious back patio. One complete with rattan garden furniture and colourful decorations.

In the developing world, however, solar cookers might be seen as an essential domestic tool.

  • In Ghana, for instance, Zouzugu villagers are using solar cookers to pasteurize water and so kill off waterborne disease, without the need to light a fire.
  • In Lesotho, small groups of women now have access to communal solar ovens they use for baking bread.
  • In the silk-producing village of Bysanivaripalle, India, an entire village uses solar cooking.
  • In Gaza, solar cookers are becoming increasingly common as fuel is in short supply.

The list goes on.

There are dozens of designs for solar cookers and solar kettles, some commercial, some created by aid agencies, others improvised based on published schematics. Fundamentally, they all work by concentrating sunlight. A curved sheet of polished metal or a mirror concentrates sunlight into a small cooking area. Black-bottomed pots and pans are the best receptacles for absorbing heat effectively. Some kind of casing can augment the effect by trapping the heat within the cooking area. This makes it possible to reach similar temperatures on cold and windy days as on hot days.

There is always room for improvement, and if solar cooking is to be adopted more widely and not simply seen as a novelty barbecue tool in the developed world or a rare convenience in the developing world, then inexpensive design improvements are needed.

With this in mind, Nitesh Rathore and Shailendra Shukla of the Department of Mechanical Engineering, at Banaras Hindu University, in Varanasi, India, have investigated the efficiencies of different approaches to solar cookers. Specifically, they have looked at two major designs: the flat plate box type solar cooker made of steel and the solar parabolic cooker which uses an acrylic mirror.

Shukla points out that there are two classifications of solar cooker: domestic and communal. Domestic type cookers may be either box-type or parabolic, but community solar cookers are usually of the parabolic concentrator type. There are also cookers that track the sun. “A tracking type parabolic concentrator is most suitable for community cooking including restaurants and hotels,” Shukla explains. “One can cook not only food, but also roast nuts, dry vegetables and fruits, pasteurize water and even bake bread, idly on a clear day at noon.”

The solar cooker is a very versatile machine. The domestic type kept in the sun is like having an oven kept to temperature provided you have clear skies. “You can place anything in it at any time and take it out when it’s done,” Shukla told Sciencebase, “What is important for solar cooking is not how hot the sun is but how clear the sunlit sky is. In most places in India one can cook for 70-80% of the days in a year. One can thus solar-cook in all seasons, with unmatched cost-efficiency.”

The teams tests confirm that solar cookers are just as “inefficient” in absolute terms as photovoltaic electrical generation systems. In other words, very little of the energy from the incident sunlight is converted into usable energy. However, with a solar cooker as opposed to a PV system, this matters little as long as the water gets boiled and the food cooked relatively quickly. Different cooker geometries and setup can affect cooking times significantly and they suggest various design modifications that could make solar cooking a viable alternative to a wood-burning fire for millions of people.

The team points out that the main problem with solar cookers in India is not efficiency or design but simply awareness. “There is less knowledge of solar energy utilisation equipment among the people of the country,” Shukla explains, “The use of SC could help in the conservation of conventional fuels, such as firewood and agricultural waste in rural areas of India.” Promoting a shift to solar cooking would preserve ecosystems, and reduce soil erosion and desertification, and unused animal waste could be employed as agricultural fertiliser rather than as fuel.

India has the right climate for solar cooking – with around 3000 sunshine hours a year – has thousands of villages and a population of well over 1 billion many of whom have very little money and limited access to fuels and who could well benefit from solar cooking.

Of course, if you can concentrate sunlight well, then you can sidestep the problem of low efficiency photovoltaics by simply using the heat produced at the focal point to run a generator produce steam for producing electricity.

Stirling Energy Systems and Tessera Solar are developing solar power collection dishes at Sandia National Laboratories called SunCatchers, the dishes are designed to pipe energy from the sun uses precision mirrors attached to a parabolic dish to a Stirling engine.

The Sterling engine (invented in the early nineteenth century) is a sealed system filled with hydrogen and as the gas heats up and cools down, its pressure rises and falls, this changing pressure drives the engine’s pistons. These pistons can then do mechanical work or drive an electric generator. So direct solar heating might not only be useful for cooking and boiling water, but could also be used to provide electric light and power for mobile phones, laptop computers, and other equipment, all without resorting to fossil fuels, or burning valuable wood and animal waste resources.

Research Blogging IconNitesh Rathore, & S.K. Shukla (2009). Experimental investigations and comparison of energy and exergy efficiencies of the box type and Solar Parabolic Cooker Int. J. Energy Technology and Policy, 7 (2), 201-212

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