Solar Cooking

Solar Cooking

Practical activities to investigate how the sun

can be used for food preservation and

cooking.

Renewable Energy School Workshops: Solar Cooking

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Acknowledgements Solar Cooking is one of a series of five publications collated by the Queensland Sustainable Energy Industry Development Group, a nongovernment alliance of organizations whose aim is to enhance the sustainability of Queensland’s energy supply. Each of the topics in the series contain a range of practical activity-based workshops for use in Queensland schools and aims to allow students and teachers to explore and discover the fundamental principles that underpin sustainable energy. Other activity sets in this series include • • • •

Global Warming and Climate Change Passive Solar Building Design Photovoltaics (Solar Electricity) Wind Power

The project “Expand community knowledge, understanding and uptake of renewable energy and energy efficiency technologies” was undertaken with the assistance and support of the Queensland Government, through the Sustainable Industries Division of the EPA and the Commonwealth Government, through the Australian Greenhouse Office. QSEIDG C/ Built Environment and Engineering, QUT PO Box 2434 BRISBANE QLD 4001 Ph 07 3964 9126 Fax 07 3864 1516 Email [email protected] www.qse.org.au

Special thanks to Peter Williams for assistance in collating these materials.

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Solar Cooking provides a range of activities, suitable for mid-primary

through to senior secondary school, allowing teachers to choose the level of activity most appropriate to their students’ needs. A little background information for teachers is also provided, as well as a list of additional resources that will provide teachers with more in depth background information, ideas for assignments and/or more practical activities. Teachers and students are encouraged to utilize these additional resources (predominantly from the Internet) to enhance their understanding of the topic and to keep up with the rapid developments in the area of sustainable energy.

These Solar Cooking workshop activities have been provided from the organisations acknowledged below, and we gratefully acknowledge their valuable contribution and their willingness to allow these materials to be more widely disseminated:

Workshops 1

Research Institute for Sustainable Energy, Murdoch University, South Street, Murdoch WA www.rise.org.au

Workshop 1 & 2

Energy Action Australia, c/ ecco2sol Global Energy Solutions, 36 Deniven Street, Corinda QLD 4075 www.esded.com.au

Workshops 4 & 5

ATA (Alternative Technology Association), PO Box 2919, Fitzroy VIC 3065 www.ata.org.au

Workshop 3

Taken from a Solar Education resource produced by the Victorian Solar Energy Association, 1992. Publication now out of print.

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Learning about Energy Renewable energy should not be studied as an isolated topic, without consideration of the context in which it operates. Whilst renewable energy technologies play a very important role in reducing the greenhouse emissions in our society, a holistic approach to energy demand and supply, one that first addresses energy services and energy efficiency, is needed if our society is to become sustainable. The following information about energy resources and energy services is taken from Chapters 1 and 2 of the book Introduction to Renewable Energy Technologies, published by the Renewable Energy Centre, Brisbane North Institute of TAFE, ISBN 1 876880384.

Energy sources The society we live in uses energy - lots of it - to run the systems and services that we depend on. Where does all this energy come from? All our energy sources come from the natural world, so where can energy be found, and in what form? Often the source of energy is not considered by the end user because the source is so remote and the energy is delivered by the relevant utility without us needing to know where it came from. It is important to be aware of the technology supplying our energy needs and to be aware of the impact that technology has on the environment. Energy can be generalised into two categories: renewable and nonrenewable. Renewable sources of energy are those which are continuously replenished by natural processes on the earth within relatively short periods e.g. 24 hours, a week, or a year. Examples include solar, wind and hydro energy. Geothermal energy is the only one for which the energy available may decline locally over time because of human use.

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The sun rises daily offering a fresh supply of energy every 24 hours regardless of how much it gave us the day before. The wind can blow day or night regardless of the season, and even if we empty our dams, the rain will come again to renew our hydro resources. We know that the sun will continue to rise daily and that wind and rain will continue to happen. Since these sources of energy are renewed within the course of a human life, they are classed as renewable. Non-renewable energy sources are those with finite reserves, and are not renewed within our lifetimes. They may take millions or billions of years to form, so in practical terms, once they are used they are gone forever. These are the fossil fuels such as oil, gas and coal; and nuclear fuel i.e. uranium. The major advantage of these fuels, and the reason why we have been able to use them so extensively, is that they are highly concentrated energy sources, and they can be easily stored.

Energy use Prior to the Industrial Revolution, human societies were largely dependent on renewable energy sources such as solar energy. Solar energy was used to heat, cool and light homes and to dry crops. Wind and water power ground wheat and pumped water. Wood was used for cooking, crop drying and space heating. Now in most countries, we use a combination of fossil fuels, nuclear fuels and renewable energy. In Australia, it is estimated that 94% of our energy demand is met from fossil fuels and only about 6% from renewable energy. However, the contribution of renewable energy to our total energy demand has been largely underestimated. For example, the contribution renewable energy makes to crop drying and production, or to space heating, cooling and lighting of buildings is not currently included in government estimates of energy consumption. The solar energy used to dry clothes on an outdoor clothes-line is never accounted for, yet when the same clothes are dried in an electric clothes dryer, the energy required is part of the official economy and is therefore counted.

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Energy services The whole purpose of our use of energy is that it provides us with services. Whether it is light to read by, refrigeration for food or just getting from place to place, it is the service that we are after, not the joules. Joules in themselves are of no use to us! Whenever we design an energy system then, we need to consider the system from the starting point of energy services. Your lifestyle determines what energy services you require, and this is where the whole story starts. In a domestic dwelling, these energy services are usually fairly clearly defined. They depend most of all on lifestyle. They also depend to some extent on location. A list of the major categories of energy services are as follows: Heating

Space heating, water heating and cooking

Cooling

Space cooling, refrigeration

Lighting

Visual (task oriented) & mood

Entertainment etc.

TVs, DVDs, music systems, Computers, electronic games

Communication Phones, computers, office equipment

Work

Cleaning and maintenance appliances and tools (e.g.

washing machines, dishwashers, vacuum cleaners, power tools, lawn mowers etc), office equipment

Other

Water pumping, waste disposal

Energy could be supplied for these services from different energy sources and in different combinations. Where energy is supplied by a utility, this will usually be electricity or gas. Where a dwelling is remotely situated, a generator and portable gas cylinders are often used to provide the same services. Your aim should be to select the source that is most appropriate for each service. This means taking into account the first and second laws of thermodynamics (i.e. efficiency and energy quality), other factors such as environmental impacts, as well as the usual economic constraints. This often means replacing fossil fuel energy sources with renewable energy sources, and it always means maximising the efficiency of energy use.

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The following table shows the proportions of energy use in the home in Queensland. Water heating is usually the largest consumer of energy, often accounting for more than one third of the energy use in the household. The energy required for space heating, cooling and refrigeration depend very much on climate. It is interesting to note the dramatic increase in air-conditioning in many parts of Australia in recent years. Is it because the climate is much hotter or because our lifestyle expectations have changed?

Energy Service

Australian average*

Brisbane average**

Proportion of Household Energy Use (End- Use)

Proportion of Household Energy Use (End- Use)

Water heating

27%

38%

Refrigeration

9%

16%

Cooking

4%

10%

Lighting

5%

11%

Space heating & cooling

39%

11%

Standby / ghost power

4%

6%

Other / appliances

12%

8%

*Data from AGO 1999 ** Data RMIT – Green Plumbers (data for 1998) NOTE 1: For Brisbane between 1994 and 2001 there has been an increase in the daily average energy use per person of 20.4% (Courier Mail. Oct 2001) NOTE 2: the typical standby power is now said to be around 11% of the daily energy use.

Composition of household energy use in Australia and a sample for Brisbane. Of course there is no such thing as an ‘average’ home, and your household energy use may vary from these figures considerably. Variations are largely dependent on climate and life style choices.

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Solar Cooking Background information Humans have used the sun for centuries to preserve food and in many parts of the world the sun is still used directly for food preservation, e.g. • Sultanas and other fruit are dried on racks in the sun in Mildura, Vic. • Rice is often dried in parts of Asia by spreading it on the road • Fish is dried on house roofs in Zambia • Salt is collected by pumping sea water into large, shallow lakes. The sun evaporates the water, leaving behind the salt. The first recorded purpose built solar cooker was made in 1767. Whilst not used for mainstream cooking in industrialized countries today, there are thousands of solar cookers being used in India, China and Africa for meal preparation. Today solar cookers are made in two broad categories, with some models being a mixture of the two types. Solar ovens are enclosed cookers that trap the sun’s energy to achieve moderate oven temperatures. They are extremely useful for slow cooking of relatively high large quantities of food, and can be used for steaming, boiling, stewing, roasting, simmering and baking. Concentrator solar cookers use mirrors or concave disks to concentrate the sun’s energy directly on to the food. They can achieve higher temperatures than ovens, cooking the foods quicker, but must be frequently refocused on the sun, and can cause burns and eye injury if not carefully used.

Key principles There are three main design features of solar ovens: a sealed container, a window to let the sunlight in, and various methods of keeping the heat in the container. The container can be a box (e.g. cardboard, polystyrene, wood, metal) or other container (e.g. a wheelbarrow, a hole in the ground . . .) Prepared by the Queensland Sustainable Energy Industry Development Group 2004

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The ‘window’ to let in the sunlight may be clear plastic, a sheet of glass or even an old window! It needs to be orientated so that it faces the sun (it will ‘collect’ the most sunlight if it is perpendicular to the sun.) The bigger the window, the more sunlight penetrates, hence the greater potential for heat. Keeping the heat in is the trick to achieving temperatures required for cooking. The three essentials are including materials that absorb the heat (e.g. black coloured), insulation materials (to stop the heat from escaping and to keep the temperature inside the container hotter than outside) and a tight air seal. The main principle of solar concentrator cookers is to use a variety of methods and materials to concentrate or focus the sun’s energy on one particular point. Any shiny, reflective material may be suitable. (See picture on front cover).

Additional resources 1. www.greenhouse.gov.au: website of the Australian Greenhouse Office: lots of information about renewable energy in Australia, government programs, policy issues etc. 2. www.epa.qld.gov.au/environmental_management/sustainability/energy: Qld government website with specific information about Queensland programs and resources. 3. www.rise.org.au: Research Institute for Sustainable Energy, Murdoch University: RE Files: a series of fact sheets on renewable energy technologies 4. www.ata.org.au: Alternative Technology Association, a not-for-profit environmental organization; provides information, advice and publications about renewable energy and other sustainability issues to the community. 5. http://solarcooking.org: loads of information, photographs and plans for many different models of solar cookers. 6. http://home.earthlink.net/~drduggee/solar.htm shows solar cookers that you can actual buy. 7. The Do-It-Yourself Solar Energy Project Book, Rick Swindell & Jim Richmond, Griffith University, 1994. This book suitable for senior secondary school students, contains stepby-step instructions to building four projects, including a solar concentrator barbeque and a solar still (which could be adapted to be a solar dryer fro fruit and vegetables.) 8. Solar Cooking Interest Group (Australia); contact Sunny Miller: [email protected]

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Workshop 1: Pizza Box Oven Context You have probably noticed that, even on cold days, the inside of a car which has been parked in the sun is a lot warmer than it is on the outside of the car. This is because clear materials, such as glass and plastic can trap the heat from the sun, just like the greenhouse gases do in our atmosphere. We can use this principle to cook a variety of foods in solar ovens.

Curriculum Links Science, Energy and Change, level 2-3

Equipment needed • • • • • •

Medium Sized Pizza Box Black Plastic or Cardboard Aluminium Foil Clear OHP Plastic Masking Tape Craft Knife

What to do Using a craft knife carefully cut a U shape on the lid of the Pizza box, by drawing a square 5cm in from all of the edges. Cut along line at the front and sides of the box. Cut a square of clear OHP plastic that is about 1cm in each direction larger than the flap you have just cut in the box. Tape the clear plastic on the inside of the box so that it is covering the flap you have just created. It must be air tight! Cut a square of black plastic about the same size as the bottom of the box and tape it to the bottom of the box (opposite the clear plastic). Tear a sheet of aluminium foil and fold it around the flap (the face that faces inside the closed box). Take the oven out into the sun and place the box with the window facing the sun. Use the flap to reflect more sunlight into the window. As Pizza Box solar ovens do not get very hot, try melting or heating up foods rather than cooking raw food. Chocolate covered marshmallow biscuits are perfect for testing your solar ovens. Prepared by the Queensland Sustainable Energy Industry Development Group 2004

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Workshop 2: Parabolic Dish Cooker Context By shaping a highly reflective surface into a parabolic dish, you can concentrate the sun’s energy on to one particular focal point. This activity shows how to make a very basic solar concentrator dish.

Curriculum Links Science, Energy and Change / Earth and Beyond: level 2+

Equipment needed • A cardboard box • Aluminium cooking foil • Aluminium pie plate • Metal spoon • Kebab skewer (or metal skewer) • Something to marshmallows)

cook

(e.g.

What to do Lay several thicknesses of a soft blanket over a table and put the pie plate on it. Smooth the edges of the plate by rubbing it with the back of the spoon. Then, using the back of the spoon, start from the centre and rub in a spiral, gradually working out to the edge. Repeat until you have bowl shape. Line the inside of the box with foil, then place your reflecting bowl at the bottom. Turn the box onto its side. Poke the skewer through one front edge of the box, through your marshmallow, and out through the opposite side of the box. Experiment in the sun to find the best focus position. marshmallow every minute so it gets energy on all sides. Think about other ways you can make other solar dish cookers . . .

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Turn the

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Workshop 3: Parabolic Trough Cooker Context The model to be built in this activity is an example of a parabolic trough or line focus type collector. The sun’s rays are focused along a line at the focal point of the parabolic curve. If a tube (painted back) carrying water is placed along the focal line it will heat the water. Alternatively a thin rod can be positioned along the focal line. If food (e.g., sausages, apples etc.) is placed on the rod the collector can be used as a cooker.

Curriculum Links Science, Energy and change: Level 5+

Equipment needed • • • • • • •

Plan for a half parabola A cardboard box Light weight poster board. Adhesive tape and adhesive spray 500 mm of strong wire (or a straightened wire coat hanger). Stanley knife or similar Aluminium foil used for cooking (300 mm width).

What to do Use the pattern provided on the following page. This has a focal length of 75mm. The shape provided is a ‘half parabola’. Use this to make a full size parabolic template as shown. Using your template mark the parabolic shape on the end of a cardboard box and cut it out with your knife. Cut the poster board to suit the length of the box and lay it into the curve of the box. Use spray adhesive to glue the aluminium foil onto the poster board. This will become your reflector. Cut the ends to suit the box and mark the focal point accurately. Mount the ends to the box, with holes punched at the focal points Place a cooking rod (e.g. length of straight strong wire) through the focal points. Try cooking sausage, apples, bananas etc.

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Workshop 4: Solar sausage sizzler Context This activity uses a number of mirrors to focus the sun’s energy.

Curriculum links Science, Energy and change: level 5+

Equipment needed • 11 bike spokes 30cm long • 1 long stainless steel skewer • 2 pieces of softwood 460 x 70 x 19mm • 1 piece of softwood 460 x 190 x 19mm • 9 pieces of mirror 180 x 45mm • 6 x 50mm nails • 2 x 10mm self tapping screws and washers • strong packing tape • Tools: pliers, a drill and small drill bits, hammer • a ruler

What to do: Making the base Using only two nails or some strong sticky tape, join the two pieces of 70cm wide wood together face-to-face. You will need to be able to separate the two pieces later, so only use two nails. Now mark a line, 10mm in from one edge, along the full length of one face of the wood. Put an ‘X’ on this line 25mm in from one end of the wood, and continue marking every 50mm. You should end up with nine marks across your line. Next drill holes right through both pieces of wood where the ‘X’s are. You need to make sure that these holes go through at right angles, and that they are about the same diameter as the bike spokes. Once you have drilled all nine holes, separate the two pieces of wood and nail one down each side of the larger piece of wood. The large piece of wood becomes the base, and the two rows of holes should be at the top.

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Making the mirror holders Cut off the small bend at each end of the spokes, and bend nine of them to the shape shown in figure 1. Make sure that the ends are in a straight line and that the bent spoke lies flat on the bench.

Carefully slide the long end of a spoke through one of the holes, then slide the short end through the matching hole in the other side. Repeat this with the other eight spokes. Now tape one mirror to each of the bent spokes, and bend the long ends that protrude from the side of your cooker at right angles so that each spoke now has a handle that you can use to aim the mirror with. Make sure that the mirrors do not touch each other when rotated.

Making the food holder Cut the small bent end off the two remaining spokes and bend one end of each one into a small circle just big enough for the screws to go through. At the other end of the spokes bend a small ‘U’ shape to hold the skewer. You can see the shapes in figure 2. Now screw one spoke onto each side of the cooker, near one end. You might want to make a small hole for the screw first to make it easier. You can see how the finished cooker looks in the photo. Your cooker is now ready to test. Get the food you want to cook, put the skewer through it, and rest the skewer on the ‘U’ shape at the end of each of the two spokes. We used a vegie sausage because they are precooked and only need to be heated, and don’t spit fats all over your nice clean mirrors like a meat one will. Take the cooker outside and face it so that the food end is pointed towards the sun. Now rotate each mirror so that the sunlight is reflected onto the food. As you adjust each mirror in turn, the spot of light on the food should look brighter and brighter, and the food will start to cook, although it will take a while. Tilt the cooker up at the back if the mirrors are casting shadows on the ones behind them. If you have used meat, make sure that it is properly cooked before eating it, or you could get food poisoning! If you are not sure, stick with vegies! You can also make kebabs, where you chop vegetables into pieces 3-4cm square and put them on the skewer.

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Workshop 5: Solar herb dryer Context A solar dryer, for food preservation, allows herbs, fruit and vegetables to be dried faster than sun drying, as well as protecting the food from contamination and damage from inclement weather, dust, insects, birds and animals. Contrary to solar ovens, a solar dryer needs to allow for air circulation to stop the food from spoiling.

Curriculum links Science, Energy and Change, level 4+

Equipment needed • • • •

Lengths of 25 x 50 mm plantation pine or similar Black wire fly screen Sheet of black painted aluminium Various nails, screws, glue etc

What to do Make two wooden frames using the lengths of timber (or use old two picture frames of the same size!) Attach the fly screen across the bottom of one of the frames, trimming edges. Attach 6mm plywood corner reinforcing on top of the fly screen to make it more sturdy and to provide an air gap between the frame and the ground or bench on which the frame will be placed. This becomes the base of the dryer. Attach a sheet of black painted aluminium on the top of the second frame. Drill 16mm holes in each of the four wooden sides. This will be the lid of the dryer. Spread herbs (e.g. parsley, sage, oregano, thyme, mint, lavender) or fruit (e.g. sliced banana, tomato) on the flywire and put the lid on. Place the dryer in the sun, directed at the best angle. Drying will take from several hours to several days, depending on your climate and the food.

How does it work? The metal lid absorbs heat from the sun which is then radiated onto the food below. Air is drawn through the gap at the bottom of the dryer, through the flywire and passes out through the holes in the sides of the lid. The air dried product is not exposed to direct sunlight, so retains a strong flavour and aroma. Prepared by the Queensland Sustainable Energy Industry Development Group 2004

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Other design ideas Use a bale of hay: cut into it to create space for your food container. Add heat absorbing materials, your cooking container and a glass lid! Try using polystyrene or cardboard fruit boxes, a wheelbarrow . . . .

Cooking hints and recipes • Try to keep the moisture content of the food as low as possible • To cook eggs: simply place a few eggs, still in the carton, in your solar oven. No need to add water. They may be cooked in about 75 minutes. • To cook corn on the cob: place the cob of corn in a black sock and place in your oven! • Fruit: try sun dried apple or banana chips! • Make a solar muffin pizza (use an English breakfast muffin, some tomato sauce, pineapple and cheese)!

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