Capturing heat


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Five Earth-Friendly
Cooking Technologies
and How to Build Them

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Capturing heat

  1. 1. Capturing HEAT Five Earth-Friendly Cooking Technologiesand How to Build Them By Dean Still and Jim Kness Aprovecho Research Center Illustrations by Loki Quinnangeles, Cathy White, Shon Lenzo, Lynn Forbes, Amelia Ettinger
  2. 2. Aprovecho is a center for research, experimentation, andeducation on alternative technologies that are ecologicallysustainable and culturally responsive. Our fields of study includeorganic gardening, sustainable forestry, indigenous skills, andappropriate technology. The center is located on a beautiful 40acre land trust near Eugene, Oregon.Since 1976, Aprovecho Research Center has been involved withdeveloping energy efficient and non-polluting inventions thatreflect current research but which are designed to be made inmost any country. The tools are designed to be self built and selfrepaired. The technologies are used at the research center.Students and staff are constantly working to improve designs forefficiency, ease of use, and general utility.The center is largely supported by its internship program. Threeten week semesters are offered per year. Classes begin in March,June, and September. Classes are both lecture and hands-on,providing the college aged or older student a chance to live inand learn with a community of teachers dedicated to sustainableliving and voluntary simplicity. Please contact us for furtherinformation.Additional copies of this book are available for $7 postpaid from: Aprovecho ResearchCenter, 80574 Hazelton Road, Cottage Grove, Oregon 97424, (541) 942-8198. Copiesare available for the cost of postage to those working to benefit the poor. 2
  3. 3. Capturing HEAT 2nd Edition—Newly revised and updated. By Dean Still and Jim Kness Contents:4 INTRODUCTION 4 Saving Biomass Resources Worldwide7 MARIA TELKES SOLAR COOKER 8 How to Design a Powerful Solar Cooker 10 How to Build the Plywood Telkes Cooker16 CONICAL COOKER 16 Reflections on Solar Cooking19 THE WINIARSKI ROCKET STOVE (ESTUFA ROCKY) 20 How to Build the Rocket Stove 22 Insulation and High Mass in Stoves (and Houses) 26 Concerning Stove Efficiency 26 How to Make a Rocket Stove with Tin Cans29 HAYBOXES (INSULATED COOKERS)31 THE ROCKET BREAD OVEN 33 How to Construct the Winiarski Rocket Oven35 VISIT THE RESEARCH CENTER 3
  4. 4. Introduction This booklet is an introduction to more more efficient ways of preparing food, raisingefficient ways of cooking food using renew- crops, building composting toilets, cisterns,able resources. The five stoves and ovens etc. Today, our focus has widened a bit topresented here are like well-proven recipes: include research into solar technologies:each has been built and tested and used at the desalinators, dryers, refrigerators, waterResearch Center. They all use solar energy to heaters and cookers. The wood burning stovecook food; some use it directly and others are designs have become a great deal more effi-designed to burn biomass, like wood, that cient and they fit into an integrated "system"stores sunlight in the form of chemical energy. of cooking that has evolved over the years. Aprovecho uses each new design beforeteaching people about it, to make sure that adesign is worthwhile. The staff at Aprovechoconcentrate primarily on household technolo-gies, figuring that cooking is the most impor-tant job that is done involving technology. Ifpeople dont cook, they dont usually get toeat! So, here are five "recipes" for ecologicallysensitive ways of making the foods that youlove. Many of these designs have been built invarious countries around the world. Eachculture has produced a stove slightly differentfrom antecedent designs. In many ways thisprocess is very much like a recipe where eachculture produces exotic and wonderful varia-tions of a theme. We invite you to make yourvery own stove and then make somethingdelicious with it! Why not share successes,both technological and culinary, with neigh-bors and friends who might appreciate both? Saving Biomass Resources Worldwide Aprovecho has been involved with stove The Winiarski Rocket and cooking systems for more thanfifteen years. The Research Center near Cot-tage Grove, Oregon was created by consult- Deforestation due in part to fuel needsants to the Peace Corps and other Aid Agen- represents a major threat to ecosystems incies who worked overseas helping to create many developing countries. Obviously, the 4
  5. 5. answer to threatened forests is to grow treesat a faster rate than forest products are con-sumed. Wood can then be used at a sustain-able rate, where less is taken than produced.Taking a greater amount insures a diminishedresource and, if the trend continues, an even-tual loss. Many countries in the world willrun out of wood, long before they run out ofgasoline and oil! No cooking tool will be universally adapt-able in different situations and cultures. Onedesign will not be applicable to all settings,because resources, climates, acceptance,patterns of use, etc., vary from place to place.The designs presented here are single at-tempts at melding working principles withavailable resources, which includes the hu-man component. All designs are expected tobe modified to fit into a given situation. Thatswhy Aprovecho believes very strongly inteaching people to be designers, not in justteaching designs. No single unchangingdesign can be considered an "appropriatetechnology"; local people must help in thedesign process to create a useful appliance. The five cooking devices make up an inte- savings. For example, a fuel efficient stovegrated system of cooking that allows the user coupled with an insulated cooker can saveoptions and choices. Any design will help in tremendous amounts of firewood. A fuel efficient woodstove, by itself, saves no morecooking more efficiently, but using devices in than 30 to 40 percent of the fuel used in cook-combination can result in more dramatic ing over an open fire. The use of an insulated cooker is essential. This system involves designs that are easy to learn to make. When its sunny, its possible to cook using a very powerful solar oven, with many reflectors. You can boil water using sunshine by building a conical concentrator. When the sun isnt shining, which has been known to happen here in the Pacific North- west, the staff at Aprovecho can cook using a "Rocket" wood fired stove which is very efficient and almost smoke free. Baking is done in 5
  6. 6. a "Rocket" style bread oven, which is veryinexpensive and can bake about 70 pounds ofbread at a go. (Both the stove and oven weredesigned by Dr. Larry Winiarski.) Aprovechovery rarely does any simmering of food.Instead food ready to simmer is placed insidea well insulated box, where it finishes cookingwithout worries over scorching, boiling overand the like. Its important for the appropriate technolo-gist to introduce the element most likely tomake a positive difference. For example, aRocket stove can be less helpful than a Rocketbread oven in places where baking consumesa lot of wood. Building a bread oven may bethe first priority. Insulated cookers can savemore fuel than is gained by replacing an openfire with an efficient wood burning stove. In a sunny climate, direct solar cooking maybe the wisest choice. However, solar cooking Conical Solar usually slow to be accepted because it is youre concerned with conserving naturalnew and slower and a bit limiting. Starting resources, whether its oil or wood or what-out with a haybox, which is easier to use, ever. The appropriate technologist with amight make more sense. In any event, know- hundred designs in the back pocket seemsing a range of options allows choices when more likely to produce an acceptable device, with local input, than the tech- nologist with only a few designs back there! A good A.T. designer is a combination of engineer, anthro- pologist and political scientist. Following are general descrip- tions for constructing each of these devices. Each is designed to be inexpensive and simple to build. All are in use at Aprovecho and you are sincerely invited to visit and experience these cookers in operation. Please feel free to adapt these ideas and principles to your unique situation and needs. The extent to which your needs end up being satisfied by your cooker, will determine how successfully we have combined to produce an appropriate technology. 6
  7. 7. Maria Telkes Solar Cooker The solar cooker that is recommended was glass cover 30" x 30". This particular design isdesigned and tested by Dr. Maria Telkes useful in higher latitudes. Directions are givenduring the 1950s. (Dr. Telkes invented many showing how to adapt the design to all lati-solar devices. She could very easily qualify as tudes.)the mother of Appropriate Technology.) Thisoven easily reaches 300 degrees R and will This solar cooker works well because ittouch 400 R on hot summer days. It isnt combines some important design criteria:necessary to constantly reorient it, because thestove is powerful enough that it works even •The box is surrounded by reflectors, whichwhen its not aimed exactly at the sun. We direct visible light down into a box through arecommend building fairly large scale cookers glass cover. (On a sunny day, each square footbecause solar energy is diffuse. It takes a of earth in sunshine receives about 200-300pretty big reflector and glass top to build up btus of energy per hour. One btu is thethe heat and temperatures necessary for easy amount of energy it takes to raise one poundcooking. (The cooker in these plans has a of water one degree R)glass cover 24" x 26"; we also use one with a •The box is well insulated and relatively 7
  8. 8. airtight. has about 2,000 btus per hour entering the box when used near Aprovecho. The cooker is •The glass is at a 60 degree angle, minimiz- only about 30% efficient so about 660 btusing shading. Double panes of glass help a will actually cook the food per hour. 660 btusgreat deal as the air space in between the divided by 150 equals 4.4. Therefore, about 4.4panes helps to insulate the box. pounds of an easily-cooked food (including •The box is metal lined and painted com- the weight of the water) can be made per hourpletely black so that visible light is efficiently in this cooker. Hard-to-cook items will requireabsorbed and changed into infrared or heat more time. To accomplish this type of power-energy. The infrared energy is absorbed by the ful cooking, make sure that:glass and its escape is slowed by the insula- A.) The glass is as perpendicular to thetion inside the walls, so that heat leaves the average position of the sun as possible. Withbox slowly enough that internal temperatures the Telkes cooker, aiming the glass directly atcan rise. the sun will optimize the effectiveness of the •The pot sits on a black metal floor so that reflectors. The maximum amount of sunlightheat enters the pot through conduction (metal will be reflected down into the box. Take intoto metal contact) as well as through convec- account whether more cooking is done duringtion (air to metal contact.) the summer or winter. To figure out the angle of the sun above the horizon, subtract the local latitude from 90. How to Design a Powerful Solar Cooker Then add 23.5 degrees to the result. This will give you the highest sun position in the sum- Here is one simple way to design a solar mer. Subtract 23.5 degrees from the result tocooker that will work well: find the suns lowest position in winter. For example, the latitude at Aprovecho is 44 Figure out what you will normally want to degrees north. 90 minus 44 equals 46.46 pluscook. 23.5 is 69.5 degrees. This is the highest point Figure out how quickly you need to cook reached by the sun in summer.the food. 46 minus 23.5 is 22.5. The sun only reaches Figure out how many btus you need to 22.5 degrees above the horizon in the dead ofcook the food. (As a very rough rule of winter. Since Aprovecho does most of its solarthumb, figure on needing about 150 btus per cooking in the middle of the summer, wehour per pound of food to cook food in "nor- place the glass in the Telkes cooker at 60mal" time periods.) degrees. Aim the glass in your cooker at the most advantageous angle. Depending on your latitude, the cooker willreceive between 200-300 btus per square foot B.) The reflectors are at 120 degrees to theper hour. Its about 300 btus near the Equator glass. Another way to say the same thing isand closer to 200 btus near 45 degrees North that the reflectors are positioned at 30 degreesand South Latitude. Only approximately 30% to the incoming rays of light. The reflectorsof this energy will actually cook the food. can be about as long as the glass surface they face. Longer reflectors will only aim sunlight Size your solar cooker accordingly. For at the opposite reflector. (See diagram A.)example: A solar cooker with a total of 10square feet of sunlight directed into the box, C.) Make the box well insulated, as airtight 8
  9. 9. How to Construct a Telkes Solar Cooker Make the box first. Begin by cutting out all of the pieces. In this case we used 1/2 inch plywood. 9
  10. 10. as possible and, if economically feasible, use btus per hour depending on latitude. It isdouble panes of glass with a small air gap easy to figure out in advance how muchbetween the two panes to increase the energy is available in any design for cooking.insulative value of the glass cover. This isespecially important in cold, windy climates. The accompanying diagrams show how toInsulation made up of cardboard and alumi- build a medium to large Telkes cooker. Thisnum foil works well. Good insulators also cooker is made out of plywood or sheet metal.include wood ash, charcoal, rigid foam, etc. In the U.S., its easiest to make the stove from(See notes on insulation in the Rocket Stove these materials. It is important to remembersection, pp. 20-23.) that a high mass stove body will absorb a lot of heat that could have initially gone into the D.) In a Telkes type cooker, paint the inside cooking process. (A heavy pot will do theflat black. Have the black pot rest on a black same. See "Insulation and High Mass inmetal floor so that we take advantage of Stoves (and Houses)", pp. 22-23.) A lowerconduction and convection. (Any part of the mass oven will heat up much more quickly.interior of the box can be shiny instead of The higher mass stove will only assist in theblack, if sunlight will hit the surface and be cooking process when the pot is cooler thandirected at the black pot. Then the pot is most the stove body. Its important to insulate theefficiently absorbing the solar energy. In some stove body from the stove interior by usingsituations, however, shiny interior walls low mass, highly insulative or reflectivereflect sunlight out of the box through the materials. A solar cooker made from bricks orglass.) earth without interior insulation will waste energy in heating up the stove instead of the E.) Since hot air rises, the best theoretical food. A solar cooker is also relatively lowplace for the door is on the bottom of the powered so its usually better to use thecooker. However, this may be difficult. The energy directly without storing heat for laterTelkes cooker has a back entrance door. use. F.) Remember that heat and temperature are It is also possible to make the body of thetwo different things. Heat is the amount of stove out of scrap metal, such as metal con-btus available for cooking. The internal tainers. We are confident that it is quite pos-temperature of the box is only the point in sible to make both the stove body and reflec-degrees where the amount of heat gained and tor from any sort of wired together cans orlost equalizes. Its important to have a high found metal.enough temperature and enough heat energyfor cooking to occur. A large Telkes model willbe a powerful solar cooker. Solar cookers withless square footage of reflector area will cook How to Build the Plywood Telkes Cookerat lower temperatures, over a longer period of Begin by constructing the box first. In thistime. case we used one-half inch thick plywood. G.) The square footage that is multiplied by Cut out all of the pieces shown in the dia-200 to 300 btus per hour is not the square gram. Bevel the edges that meet the glass atfootage of the reflectors! It is, instead, the 30 degrees. Cut the door into the back or sidessquare footage of the intercepted sunlight. of the box. Each joint is glued and nailed(See diagram A.) The Telkes cooker is only of many. But all designs work in the samemanner. Each receives the same 200 to 300 The glass cover fits over the plywood and is supported by the insulation as well, which 10
  11. 11. 11
  12. 12. completely covers all interior wooden sur- The interior of the reflector needs to be cov-faces. You can nail or glue rigid foam insula- ered with something very shiny like alumi-tion to the walls or build up the one inch thick num foil or mylar. Contact cement worksinsulation out of alternating layers of card- well.board and aluminum foil. If you choose thecardboard option, contact cement works The box works better covered with twobetter than white glue. It resists moisture panes of glass. One pane works too. Glue thebetter. Big washers, with nails or screws, are glass to the insulation and plywood, using ahelpful to hold the cardboard to the walls. sealant, preferably silicone. The silicone canRemember that the front and back insulation also be used as a spacer between the twoneeds to be beveled. Surround the cardboard sheets of glass. Leave a small air hole so thatinsulation with aluminum foil, shiny side the heated air can escape and not build upfacing in towards the food. Make this "pack- pressure between the panes of glass.age" as tightly and strongly wrapped aspossible to keep moisture out of the card- Remember that the black pot (hopefully,board. Paint the whole interior black with low mass) should sit on a black cookie sheetwater based flat paint. Leave the cooker in the or painted piece of metal. Make sure that thesun for a couple of days to burn off the gases door fits tightly, try to minimize air leaks! Thefrom the paint before using the cooker. oven cant get hot with a lot of air infiltration. Following these criteria will result in a solar The reflector wings, which in this case are cooker that should hover around 300 degreesmade out of 1/2" plywood, need to be sup- F. even when full of food. On hot days, beansported by some type of brace. We make the will cook quickly, bread will bake and brown,braces out of wood. The reflectors meet the chicken will get crispy and french fries willglass at an angle of 120 degrees. Hinges hold disappear in foaming oil as they fry. Thethe reflectors together. Its possible to make results will impress you!the hinges from leather or old bicycle tubing. 12
  13. 13. Cut the door out of the back panel. Remember to bevel the front and back panels at 30 degrees.The back and front are nailed onto the bottom The sides are nailed onto the bottom, back, andfront. 13
  14. 14. The glass is glued to the top of the boxusing silicone sealant. If two panes of glass Wooden reflector supports that meet the are used, another bead of silicone forms a glass at an angle of 120 degrees are screwed gasket between the two. or nailed to the box. 14
  15. 15. Diagram A: A comparison of varying reflector angles for solar cookers, showing the amount of admitted sunlight and the advantages of a 30 degree angle. 15
  16. 16. Conical Cooker Telkes solar cookers are great for baking of inches to one side of the wedge for a flapfood. However, the sunlight isnt concen- that will join the cone together. Cut out thetrated enough to quickly accomplish more piece and fasten the edges together. Thisdifficult tasks, like boiling water for tea or method quickly makes a 45 degree To quickly boil water, its necessary touse a solar cooker with a different shape. A After forming the cone, you can screw twoparabolic dish will concentrate all of the plywood plates a couple of inches apart to thesunlight that hits its surface to a point. If the sheet metal. Pieces of 2 by 4 fill in the gaptea pot were at that point, the water would between the plywood plates. The stand sur-boil much more quickly than in a Telkes rounds the cone and two large bolts form ancooker where the sunlight is less concen- axle for rotating the apparatus to the propertrated. sun angles. There is another shape which concentrates A quart sized can with a hole punched intosunlight very well and its much easier to the top holds the water. Its held in the middlemake than a parabola. This is a cone at 45 of the cooker by a cylinder, open at the top,degrees. Cones have been used for more than made of sheet metal. The can full of water100 years to concentrate sunlight.They are simple to make and canaccomplish many of the same tasks asthe more difficult-to-make parabola. The French mathematics professor,Augustin Mouchot, invented theconical cooker in the 1870s. Thecooker that he used in Africa was atruncated cone with a glass-enclosedmetal pot in the middle. The cookerconcentrates light into a line thatforms the central axis of the cone.Telkes cookers concentrate sunlight toa plane, 45 degree cones concentrateto a line, and parabolics concentrate toa point. Although parabolics have thegreatest amount of concentration, acone focusing to a line is quite suffi-cient for boiling up the tea, or evenrunning a steam engine. Easy construction is a major advan-tage of conical cookers. To build oneyou need to draw a wedge of 105.5degrees out of a circle. Add a couple 16
  17. 17. slides about one quarter itslength into the cylinder whereits held in place. A one gallonglass jar is inverted over the canand cylinder, providing glazingand insulation. The interior of the lid from theone gallon jar is cut away, leav-ing the threaded part. The cut-away lid slides down over aquart sized can and is fitted intoa circular hole cut in a piece oflumber attached to the plywood.The one gallon jar screws intothe lid and can be taken off andbe replaced, using pot holders, a towel, etc. usually boil in less than 12 minutes.The jar completely encloses the water in itscan. (Please see the drawing.) When much more hot water is wanted, its probably easier to fill a whole kettle and wait The conical boiler is aimed by the "nail for an hour or two until it boils in the Telkes.method". A long nail or dowel is nailed into The problem is that the sun moves enoughthe plywood base and is fitted so that its that for good results, the conical must be re-parallel to the central axis of the collector. aimed at the sun about every 15 minutes.When the nail throws no shadow, the collector Many people might find this more troubleis aimed right at the sun! than planning ahead and using the slower Telkes. But, for "tea for two", in a hurry, The sheet metal for our 54" conical concen- theres nothing like a Mouchot conical!trator cost about 17 dollars. It was made frompart of a 4 by 8 panel of sheet metal. Wecovered ours with mylar but aluminum foilwould also work. We used contact cement as Reflections on Solar Cookingthe glue. Weve also made conical cookers outof tin cans wired together. Bands of bamboo I worked in Mexico for ten years untilheld the cone together. moving full time to Aprovecho. This experi- The intent in trying the ence pointed out a lot ofconical was to see if we realities, one of which iscould make a solar cooker that no matter how wellthat would boil water in a device works inan easier way than, say, a theory, if people dontwood fired stove. And it use it, it doesnt reallyactually is easier to boil work at all! Solar cook-small amounts of water in ers are a very goodthis device, since you just example of this prin-fill the can and aim. ciple, because evenTheres no fire to start, though solar cookingetc. A pint of water will "works" it has been 17
  18. 18. almost universally rejected worldwide. All of a full array of reflectors (a Telkes cooker)the cookers that I first built in Mexico were raises the temperatures and amount of btusfound unused a year later. to the point where faster cooking can be done. A conical or parabolic can do concentrated When asked why the initial cookers werent tasks, like boiling or frying, as quickly as anyhelpful people listed the following reasons: other stove. But, as the cooker becomes more concentrated, it needs to be aimed more 1.) Cooking outside in the sun is uncomfort- frequently and can have trouble with and unusual. Both the cone and parabola concentrate sun- 2.) The cooker worked too slowly and light by focusing it in front of the reflector.wouldnt cook beans. The Telkes reflects sunlight downwards into a box, below the reflector. 3.) The solar cooker was unreliable andmore trouble than cooking with wood or gas. The Telkes cooker is easy to recommend. It convinced me, a solar sceptic, that solar cook- I hoped that we could find a solar cooker ing is a viable possibility. A Telkes cookerthat would be more attractive, since its very with a 24 by 26 inch glass cover will cooksunny in Mexico and solar cooking is the one food easily and quickly. Weve used one nowcooking method that doesnt diminish an for years. I believe that powerful solar cookersearthly resource. One of the ways that are easier to accept than slower cookingAprovecho addressed this problem was to try models. But, a large solar cooker is not mo-to make sure that the solar cooker was more bile, it can be expensive to build, and stillpowerful, to work almost as fast as other requires a change in cooking routines. Allstoves. We tried to find designs that were solar designs should be considered becausepleasant, and even fascinating, to use. Both no one design will be universally "best."the Telkes and conical cookers meet thesecriteria! Of course, the solar cooking option that is almost universally accepted is to burn bio- Both can perform rapidly and reliably. The mass. Wood and other plants have storedconical can make boiling water for coffee or direct sunlight in a chemical form through thetea very rapidly. But, it can be irritating to use mysterious process of photosynthesis. Directbecause of the glare. The Telkes has no prob- solar energy is diffuse; wood has a built-inlem with glare because the light is directed concentrated advantage.down into the box. But since it is less concen-trating it takes a longtime to boil water. Different shapes seembest suited to differentsolar tasks. A box with aglass top (a flat platecollector) is great atheating water for bath-ing. With one addedreflector (a "Solar BoxCooker") temperatureswill rise high enough todo slow cooking. Adding 18
  19. 19. The Winiarski Rocket Stove (Estufa Rocky) One pound of dry wood releases about available at a fixed rate and can disappear8,600 btus of energy when burned. Gasoline behind clouds. If youve got it, wood is ais only about twice as concentrated. It took a powerful, convenient source of fuel.long time for the tree to grow. The dailyamount of sunlight was captured and By using wood efficiently, people can cookchanged into chemical energy. In fact, trees food using branches and twigs instead of splitare very much like batteries, storing energy. logs. Gathering fallen branches can bringWood is like a battery that has been storing people firewood without killing the trees, ifenergy for decades. The energy is concen- the rate of use matches the resource. Thetrated and ready for use at any rate, depend- branches are already a handy size so peopleing on need. Also, this "battery" does not lose can be spared the labor of splitting logs asits charge while sitting, cannot spill acid on well. The trees continue to grow while peopleyour pants, and usually smells good! cook with wood. Burning wood or any biomass also has agreat advantage over burning petroleumproducts. Trees absorb carbon dioxide whilegrowing and release the same amount whenburned. The tree reduces the amount of C02in the atmosphere and then replaces it. Burn-ing fossil fuels, made from plants millions ofyears old, can only increase the amount ofatmospheric C02, since absorption done bythe plants happened so long ago. Burningbiomass does not increase the amount of C02in the atmosphere in the same way that burn-ing petroleum can. The 54" in diameter conical concentratorfocuses about 10 square feet of sunlight at thepot. Each square foot of earth receives be-tween 200 to 300 btus per hour on a sunnyday. So, the conical cooker delivers about twoto three thousand btus per hour to the pot.Thats approximately equal to the amount ofenergy thats held in a small bit of 2 by 4lumber only four inches long! The wood isvery concentrated and we can burn it asquickly as we want. Direct solar energy is The low mass rocket stove. 19
  20. 20. Dr. Larry Winiarskis low mass Rocket stove The "Guatamalan" Rocket stove is made uphas proven itself to be the most efficient of six parts. Two of them are cut out of anycooking stove that we have used so far. It is five gallon metal container. This makes thefar more efficient than high mass cooking external body of the stove. Weve used soystoves like the Lorena. We use Rocket type sauce containers, paint buckets, etc. A shortstoves now to cook, bake, heat water, warm chimney (10"-12" is optimal) is made, in thishouses, etc. example, from stove pipe. Its also possible to make the chimney from tin cans or more The Rocket stoves are based on an ingen- sturdy scrap metal. (A longer chimney willious combination of principles: smoke less and may be preferable for that reason.) The combustion chamber is insulated inorder to keep the fire hot (above 1,100 F.) toburn the wood more completely, reducingsmoke which is uncombusted fuel. How to Make the Stove The cooking stoves are low mass, robbing With a can opener or hatchet, etc. take offless heat from the pot. the lid of the can. An insulated chimney creates a very strong g Then cut a 4" round hole in the middle ofdraft which helps the wood to burn fiercely. It the lid and a 4" round hole in the lower frontalso makes the stove easier to light and to use. side of the can, about 1" up from the bottom ;e.An insulated chimney has significantly more ; of the 5 gallon can.draft than an uninsulated chimney. A 4" stove pipe elbow at 90 degrees is The wood is burned at the tips and is placed inside the can with the larger endshoved in towards the fire which controls the e protruding out the hole cut in front of the can.rate of burn, reducing smoke. Two parallel cuts, 1/2" apart, are made in The air/fuel mixture is regulated. A small this protruding section of elbow, on the low-opening is provided for incoming air. Too est side of the pipe. This section is foldedmuch air just cools the fire. down, creating a lip so the elbow cant fall back into the container. The incoming air is preheated, especially in na downdraft stove, which helps to keep the A straight section of 4" chimney pipe is thenfire above 1,100 F. for complete combustion. fitted to the other end of the elbow as a chim- ney. It is cut off 1" below the top of the can. A skirt surrounds the pot on all sides. A (Remember that a short chimney is moresmall gap between the skirt and the pot al- efficient. A long chimney will keep flamelows hot flue gases to rise up near the pot, from hitting the pot. Conversely, a tallergreatly increasing heat transfer. The flame hits chimney will produce less smoke.) The lid,the sides of the pot as well as the bottom. with a hole in the middle, is fitted over the straight section of chimney pipe, after the An insulated skirt reduces heat loss. space between the chimney and stove body is Since the stove operates at very high tem- filled with lightweight insulation.peratures, there is almost no smoke and it is Insulation is small trapped pockets of air. Itpossible to cook directly on top of the chim- is this air that slows down heat transfer. Sand,ney. The flame, in contact with the pot, as- brick, earth, cement, etc. do not contain manysures efficient heat transfer. 20
  21. 21. 21
  22. 22. trapped pockets of air and are poor insulators almost immediately if its empty and aimed at the sun. But, full of beans and rice, it takes a Examples of good insulation include: pum- while to reach higher temperatures. The massice rock, vermiculite, perlite, wood ash, dead of the food and water absorbs the heat ini-coral and aluminum foil, if it traps air. tially. The old Lorena stove, which Aprovecho helped to design, is so heavy that a lot of the Usually, we use wood ash. As long as it heat goes wastefully into the stove, instead ofstays dry, wood ash is a great insulator and it into the pot of food! Sand and clay and earthis found near fires. are not good insulators. To finish the stove make a pot support, An earthen or cement Rocket stove takes asimilar to a grill, out of heavy gauge fencing long time to heat a pot of water, when firstor wire and place it on top of the container. started. All of the mass in the stove is "rob-Alternatively, its possible to rest the pot on bing" heat from the pot, where you reallytop of the container itself. In this case, its want the energy to go. Thats why we use lownecessary to cut large holes in the top of the mass insulation around the fire in our currentcontainer so the air can flow unimpeded past designs.the bottom of the pot. These holes shouldhave the same cross sectional area as the With low mass insulation the heat can do itschimney and firebox, in this case 4" in diam- job with less waste. The small isolated pocketseter. of air in the insulation do not absorb much heat, and heat passes very slowly through Then, make the flat shelf that fits inside the insulation. Sand, clay, earth, etc. do not insu-fire chamber and helps to separate the sticks. late well because they do not contain manyThe shelf can be made out of a flattened tin isolated pockets of air.can and slides into the combustion chamber.The stove will be much more efficient if used Remember its the air in a material matwith a skirt around the pot. The skirt is sim- creates the insulation. The function of theply a sheet of metal that is as high as the pot. material around the air is only to separate theThe skirt surrounds the pot, leaving an even air pockets. Usually the more the materialgap of about 1/4" between the skirt and the weighs, the more it absorbs and diverts thepot, so the hot flue gases are directed to con- heat. A Rocket heating stove uses high mass,tact the pot sides. The skirt gap is equal to the heavy materials to capture heat that wouldcross sectional area of the chimney. An insu- otherwise flow up out of the chimney and outlated skirt is double walled with insulation of the room. The high mass sucks up heat andbetween the walls. stores it. In a heating stove, mass can play an important role. But, in the case of a cooking We use the Rocket stove at Aprovecho and stove, mass just takes heat from the pot andwe recommend it. Its much easier to use than lowers efficiency, especially when starting thean open fire and uses less firewood. Please let cooking know what you think of it. Help us tocontinue its evolution towards perfection! Mass is necessary in solar buildings to absorb and store heat from the sun. An empty room, full of air, wont store very much heat. Insulation and High Mass in Stoves (and But a cement wall will get hot enough to keepHouses) the room toasty all night long. The Maria Telkes solar cooker gets very hot This room will stay warm for a much longer 22
  23. 23. time, however, if it is insulated. In a good comfort. An uninsulated, earthen-walledsolar house, the mass is contained inside the house in that type of climate ends up refriger-building and is surrounded by insulation in ating its occupants.the ceiling and walls. A high mass house, inwhich the walls are built out of earth or ce- High mass houses have also been buriedment, will have troubles if it doesnt have underground. The benefit is that the house, ininsulation on the outside. The high mass in contact with the earth, will stay at the tem-the walls will suck up a tremendous amount perature of the earth, which is about 58 de-of heat and lose it to the outside. grees F. However, this house too, if it doesnt have exterior insulation, will require heroic Youd need to make an earthen wall four efforts to raise the interior temperature. With-feet thick to equal the insulative value of 3.5 out insulation, the heat will, in effect, beinches of fiberglass insulation (R-ll). Heat trying to raise the temperature of the entirewill easily pass through earthen or cement Earth.walls, and it will take a lot of energy to heatthe house. For this reason, the bestadobe houses had doublewalls. The air space betweenthe walls acted as insulationwhich blocked the flow ofthe heat from the inside tothe outside. If a high masshouse has external insula-tion, the heat that has beentrapped in the interior masswill be contained. But with-out insulation, not only willit take a very long time toheat up the mass, but theheat will only be wastefullydissipated to the outside. Of course, adobe houses inhot climates did not reallyneed double walls. Themajor need in the desert wasfor cooling, and the masshelped in this regard.Uninsulated, shaded, highmass walls will stay at thedays average temperature.In the desert, the averagetemperature can be quitepleasant. But, in a coldcloudy climate, the averagetemperature is too low for 23
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  26. 26. Mass and insulation, when combined, make compared to open fires outside in the wind.for a perfectly lovely, easily heated solar They can play a very important part in de-home, or an effective stove. However, when creasing the need for large amounts of fire-mass is wrongly used, or is thought to be like wood for cooking.insulation, mistakes in construction are made.An understanding of these two factors can However, an indoor, open fire is not neces-help us all in our search for a bountiful, not sarily an inefficient cooking method. It takes asparse, simplicity. low mass stove with a skirt to improve upon it. A high mass stove can easily use more fuel than a simple indoor fire, especially when the stove isnt in constant use, which is frequently Concerning Stove Efficiency the case. It really all depends on how carefully people make fires. When Dr. Winiarski and I first walkedaround San Nicolas, the ranch in Mexicowhere I have a house, he surprised me bysaying that the Rocket stove wouldnt save a How to Make a Rocket Stove with Tintremendous amount of firewood. He saw how Canswell the ranchers made fires and thought thata fuel efficient stove would save only 30% to In many countries around the world it is40% more firewood. advantageous to use tin cans to make a chim- ney. Tin cans are often available when there is When people are careful and expert, the no other chimney material. The cans will burnindoor open fire (or three stone fire, as its out in about a month. However, its possiblesometimes called) is a relatively efficient to replace the cans rather easily. (Of course,cooking method. its always possible to make a fired clay chim- ney and firebox, that will last much longer.) Traditional methods can work very well. Infact, the indoor, open fire can be more efficient Youll notice from the diagram that it isthan higher mass, supposedly fuel efficient possible to cover the chimney with a mixturestoves. Of course, this probably plays a part in of clay and sand (60% sand mixed with 40%the cultural resistance to new methods of clay works well). When the tin can chimneycooking. The introduced methods are not burns out, the sand /clay chimney takes itsnecessarily superior in every way. place. The ash insulation, of course, is still used between the outside of the clay/sand God made fire, people made pots. The trick chimney and the metal stove improving efficiency is not so much toimprove the efficiency of combustion, but to In Zaire, many Rocket stoves were built forimprove the transfer of heat into the pot. The and by Rwandan refugees using the contain-skirt around the pot is very important. The ers that had held relief supplies. The containerstove body keeps wind away from the fire, was cut up to make the chimney and firebox.keeping the fire hot enough for more com- Pumice rock or wood ash filled the interior ofplete combustion to occur. The stove "forces" the stove between the external body of theusers to feed wood more slowly, to make stove and the chimney. It is even possible tobetter fires. It can be tempting to build an make the external stove body out of sand andoverly large open fire, since a large fire wont clay as long as insulation surrounds thego out as easily. internal chimney. A chimney really helps to burn up smoke, to keep the fire going, and to Fuel efficient stoves can save a lot of fuel direct flame at the pot. 26
  27. 27. The stove body, shelf, and skirt support.The shelf is insertedinto the firebox. The skirt support rests on top of the stove body. The skirt is also shown. 27
  28. 28. Three pot supportsan inch long sit onthe skirt support. The pot rests on the pot supports.The skirt directs theheat around the pot. 28
  29. 29. Hayboxes (Insulated Cookers) When the sun isnt shining and people are shavings, sawdust, newspaper, fiberglass,cooking with wood, a lot of energy can be feathers, cotton, sponge, fur, wood ash, char-saved by using a haybox. (Hayboxes are coal, etc. If you use one of these, allow 4" ofsometimes called fireless cookers.) After wall thickness. If you use cork, cardboard,boiling the food for a few minutes, the hot pot aluminum foil, styrofoam, rigid foam, etc. youis placed inside an insulated box, where it can reduce the thickness to two inches.finishes cooking. The retained heat is enoughto simmer the food to completion. A haybox One of the most efficient hayboxes is madedoes all of the simmering for us, saving not by filling the air space around the pot withonly energy but effort as well. Saving the cushions, held in place inside a box.firewood that wastefully went into simmeringthe food results in tremendous savings! A haybox also makes cooking much easier.No more dealing with watched pots to guardagainst boiling over or scorching. AtAprovecho, we usually make lunch whilegetting breakfast. All we do is quickly boil upthe beans and rice and then pop them intohayboxes where theyll emerge hot and readyfor lunch. It saves a lot of time and effort. Hayboxes have been used for centuries in Fill the space around the pot withdifferent countries. cushions. They work by keeping food hot enough Another simple method is made from two(over 180 F.) for cooking to continue. The boxes, one inside the other. The space be-design criteria are simple: tween the two boxes is filled with insulation. Make sure that there is plenty of insulation An insulated lid or cushion fits over the top.around the pot, especially the bottom and top. The pot should fit into the box as snugly aspossible. The lid on the haybox should make a good,airtight seal. A wide variety of materials, besides hay,can be used for insulation. Remember thatany material that traps isolated pockets of airwill insulate well. Examples of insulation are:straw, rushes, chaff, popped grain, wood Two boxes with insulation between them. 29
  30. 30. Or we can invert the box and have it sit A haybox can be something very simple,upon an insulated base on which the pot also like a hole in the ground that is filled withrests. Raising the pot up onto another higher hay, straw, etc. The pot is placed inside theplatform, like a piece of plywood, makes a hole and covered by thick layers of insulation.double seal to keep air from escaping. The hole could also be made in a block of sand/clay mixture above ground so as to be more easily reached. Simmering food in a haybox will take about one and one-half to three times longer to cook than on a cooking stove. A large mass of food works better than smaller amounts. A tight lid on the pot helps as well. Using one quarter less water with grains is better since less water is lost to evaporation. Its possible to wrap the pot in a towel before putting it in the haybox. Bacterial poisoning can be avoided by two simple steps. Make sure that the food is ini- tially boiled for at least 5 minutes. Then keep the lid closed from that point on and reboil meat dishes before serving. We cant imagine cooking now without An insulated box resting on a platform. hayboxes. Theyre truly one of those rare devices that save energy and make life sim- pler at the same time! Weve used them for Another type of haybox is made from a more than ten years and highly recommenddouble bag of material lined with insulation. them. Hayboxes are so fuel efficient, becauseA draw string closes the top. The neck of the the haybox does all of the simmering "forbag should open wide so that the pot can free", without using energy that would haveenter and be withdrawn easily. mostly gone into making steam, anyway. The Rocket stove/haybox combination is capable of cooking with very small amounts of bio- mass. And, this method is easier, safer, and more convenient! A double bag lined with insulation. 30
  31. 31. The Rocket Bread Oven Dr. Winiarski and I built this very efficient cient oven:oven in 1990. It is used about once a week atthe Research Center for baking bread, etc. It The hot air is in contact with the bottom, thecan hold more than 20 loaves. Sixty-six sides, and the top of the inner drum whichpounds of bread can be completely cooked contains the bread.using 11 pounds of dry wood. The hot air is forced through a tight gap The oven is built out of 55 gallon drums and that forces the heat to touch the inner drum,can be made inexpensively. heating it more efficiently than would happen if the gap were wider. The oven can be built in different ways. Onemethod uses two drums. The inner drum The inner drum is low mass and is insu-holds the bread. It is completely closed except lated by the aluminum foil that creates airfor three doors cut into the front of the barrel. gaps between each layer. (Aluminum foil alsoSix layers of alumi- reflects infrarednum foil, each radiation back intoseparated by a the inner drum, atsmall air space, the bread.)insulate betweenthe two barrels. The insulated(Weve also built firebox keeps thebread ovens with- fire hot, assistingout aluminum foil. complete combus-Instead a third tion and reducingbarrel surrounds smoke.the second. Insula- The insulatedtion fills a 4" gap chimney creates abetween the 2nd great deal of draftand 3rd barrels.) A and air is pre-gap of about an heated before it caninch, between the cool the fire.inner barrel and thefoil, allows the hot The fuel is eitherflue gases to rise up fed vertically orbetween the two horizontally. Verti-barrels. In effect cal feed allows forthis gap becomes a more preheating ofpart of the chimney. air and the wood is more self-feeding. Many elements Horizontal feed isact together to more what peoplemake this an effi- The downfeed-downdraft bread oven. are used to, and 31
  32. 32. doesnt suffer from backdrafting as much. The doors were cut as cleanly as possible. AHorizontal feed is only slightly less efficient. large door, covering all three holes, swings on hinges attached to the outer barrel. It closes The fire chamber and fuel magazine can be against two pieces of angle iron that fill theformed in various ways. (Please refer to the gap between the two barrels.drawings.) The oven at the Research Centeruses a 14 gallon drum as the feed box. It has a This oven saves a great deal of fuel whenfired clay liner inside it, surrounded by pum- compared to a beehive earthen oven. Peopleice rock as the insulation. This liner creates cannot believe that a few sticks can be heatingthe fire chamber and tunnel that directs the the entire interior of the drum to cookingfire at the bottom of the inner drum. (Its also temperatures.possible to make a horizontal fire box pat-terned after the smaller Rocket cooking In Mexico, its been the design most univer-stove.) sally praised. Its easy, with a bit of practice, to regulate the temperature, and large A metal cap, also insulated, covers both amounts of food can be made at one time. Webarrels and sits 1" above the inner drum, recommend it very highly!allowing the flue gases to reach the chimneyunimpeded. A six inch in diameter section of Earthen stoves can use so much wood, tochimney pipe exits vertically from the cap. bake so little bread! At the very least, earthen stoves can be covered by insulation (wood Three shelves fit into the rings of the barrel. ash, etc.) so that the heat isnt as easily lost.We cut circular shelves out of metal fencing to The low mass oven heats to 350 degrees F. inbe a bit bigger than the diameter of the drum. twenty minutes and keeps at that temperatureThese shelves were forced down into the with small branches added to the fire.barrel, to spring flat and find support in therings.The split drum slides over the inner barrel. How flue gases pass between the drums. 32
  33. 33. How to Construct a Winiarski Rocket Oven Cut three door openings in a 55 gallon drum. Leave the top and bottom on the drum.Gut the top and bottom out of a 55 gallon drum. Split the drum longitudinally. Slide the split barrel over the barrel with the door openings. The oven can be placed on top of a ring of bricks.A gap of one inch is created between thefirst and second drum. Bolts hold the drums in this position. A sacrificial plate can be bolted to the The gap between the inner and outerbottom of the inner drum, protecting it drum is not blocked by this ring. The outer-from prolonged exposure to the flame. most drum rests on the inner edge of the brick wall. 33
  34. 34. Split the third drum longitudinally. Cut an 8 inch in diameter hole in the bottom of the drum. An elbow of stovepipe fits into the 8 inch hole. The stovepipe ends 2 inches below the oven. The space between the stovepipe and Its also possible to make the firebox for the inner wall of the third barrel is filledthe oven out of a third 55 gallon drum. with insulation, such as wood ash.A covering of sheet metal is screwed over the openings in the bottom barrel. This diagram shows construction details of the door which covers all three openings in the bread oven. 34
  35. 35. Visit the Research CenterPlease visit Aprovecho whenever youre in Oregon! We have a self guidedtour through both the campus, garden, and forest. Just call for directions...Aprovecho is a non-profit, membership supported organization. Membershipdues are $30 per year which includes a newsletter, News From Aprovecho,detailing results of current research.The cost of the ten week internship is $1,830 per semester which includesroom and board in the straw bale dormitory. Please make arrangements earlysince the internship tends to fill a semester in advance.Capturing Heat: An Appropriate Technology Design Manual will be availablein September of 1999. It is a more complete version of this booklet andincludes: how to heat water and houses with both direct and indirect (stored)solar energy, simple refrigeration, composting toilets that work!, desalinationand drying designs, etc. Please contact us for details.The following designs are included in the larger Capturing Heat:
  36. 36. Aprovecho is a non-profit, membership sup-ported organization. Dues are $30 a year andinclude a newsletter detailing results of currentresearch. We also offer a 3 month internship programfor people who want to learn about organic gar- dening, sustainable forestry and appropriate tech- nology. The cost is $500 per month which in- cludes instruction, room and board. Sessions be-gin March 1, June 1, and September 1 in Oregon. We also offer a one-month January session inMexico. Classes include lectures, discussion,practical work, and field trips, and run 8-5 Mon- day through Friday. Please contact us for com-plete information. Our phone number is (541) 942-8198; fax (541)942-0302. Please call in advance of your visit. Ifyou would like written information, please send aSASE to Aprovecho Research Center, 80574Hazelton Road, Cottage Grove, Oregon97424.