Design Principles For Wood Burning Cook Stoves

Design Principles for Wood Burning Cook StovesStove TheoryChapter 1Stove TheoryEven an open fire is often 90% efficient at thework of turning wood into energy. But only asmall proportion, from 10% to 40%, of thereleased energy makes it into the pot. Improvingcombustion efficiency does not appreciably helpthe stove to use less fuel. On the other hand,improving heat transfer efficiency to the pot makesa large difference.Improving the combustion efficiency is necessaryto reduce smoke and harmful emissions thatdamage health. Improving heat transfer efficiencycan significantly reduce fuel use. Fire is naturallygood at its job, but pots are not as good atcapturing heat because they are inefficient heatexchangers. In order to reduce emissions and fueluse, the stove designer’s job is to first clean up the fireand then force as much energy into the pot or griddleas possible. Both of these functions can beaccomplished in a well engineered cooking stove.It is always best practice to add a chimney to anywood burning cooking or heating stove.Additionally, it is preferable to use a cleanerburning stove to protect air quality in and outsideof the house. Chimneys that take smoke and otheremissions out of the living space protect the familyby reducing exposure to pollutants and health risks.Even cleaner burning stoves without a chimney cancreate unhealthy levels of indoor air pollution.Unvented stoves should be used outdoors or inopen areas. When chimneys are not affordable orpractical using a hood over the fire, or openingwindows, or making vents in the roof under theeaves are all ways to decrease the levels of harmfulpollution. The use of a cleaner burning stove canalso be helpful in this regard but, if possible, allwood burning stoves should always be fitted with afunctional chimney!How can we design a stove that improves upon theopen fire? First, let’s list the advantages of the threestone fire when compared to some stoves:fNo energy is absorbed into the mass of a stovebody. High-mass stoves can absorb energy thatcould have gone into the pot. The three stonefire can boil water fairly quickly.fFire hits the bottom and sometimes the sides ofthe pot, exposing a lot of the pot to the hotgases.fSticks can be fed in at the appropriate rate as thetips burn, assisting complete combustion.fA hot open fire can burn relatively cleanly.Every stove suffers because it has some mass thatabsorbs heat. But an improved stove can stillachieve better combustion and fuel efficiencythan an open fire.How to improve combustion(make less harmful pollution compared to an openfire)fMake sure there is good draft into the fire.fInsulate around the fire to help it burn hotter. Ahotter fire burns up more of the combustiblegases and produces less smoke.fAvoid using heavy, cold materials like earth andsand around the combustion chamber.fLift the burning sticks up off the ground so thatair can scrape under the sticks and through thecharcoal.fPlacing an insulated short chimney above thefire helps to increase draft and gives smoke, air,and fire a place to combine, reducing emissions.This is a popular strategy used in many stovessuch as the Z stove, the Vesto, the Wood GasCamp Stove, the Rocket stove, the Tso-Tsostove, etc. The Eindhoven group used achimney above the fire in their cleanest burningdowndraft stove. Micuta built stovesincorporating this idea as well (Modern Stoves forAll, 1981). Winiarski developed the concept inthe early 1980s creating a stove that cleaned up7

Design Principles for Wood Burning Cook StovesStove Theorycombustion and improved heat transferefficiency (Capturing Heat One, 1996).fMeter the sticks of wood into the combustionchamber to make a hot, fierce, jumpy lookingfire that does not make much charcoal. This typeof fire will make less dangerous emissions,chimney clogging soot, and creosote. Heat onlythe burning part of the wood. Do not encouragethe non-burning wood to make smoke.fLimit the cold air entering the fire by using assmall an opening as possible. Small openings intothe fire also force the cook to use less wood,which can be burnt more efficiently.fA certain amount of excess air is necessary forcomplete combustion. Preheating the air helps tomaintain clean combustion.How to improve fuelefficiency(get more heat into the pot)ffff8Increase the temperature of the gas/flamecontacting the pot, having the hot air scrapeagainst both the bottom and sides of the pot in anarrow channel, using a pot skirt.Increase the speed of the hot flue gases that scrapeagainst the pot. The fast gases punch through aboundary layer of still air that keeps slowermoving gases from scraping against the surface ofthe pot (orgriddle.) Air is apoor heat transfermedium. It takesa lot of hot air tobring heat to thepot.Use metal ratherthan clay potsbecause metalconducts heatbetter than clay.The size of thefire determinesthe size of theFigure 2 - Appropriate Useof Pot Skirtchannel gap in the pot skirt and the maximumefficiency of heat transfer. Smaller fires that canstill please cooks but are not too big will beconsiderably more fuel efficient.fUse wide pots with large diameters. Using a widepot creates more surface area to increase thetransfer of heat. Make sure that the top of thestove slopes up toward the outer perimeter ofthe pot, as shown in Figure 2.Sam Baldwin’s Biomass Stoves: EngineeringDesign, Development, and Dissemination (1987) is avery good summary of how to make improvedstoves. It is highly recommended. Dr. Baldwinfigured out how the channel size between pot andskirt, firepower and efficiency are related. Here area few examples using a family sized pot:1.) A 1.7 kW fire with a channel gap of 6 mm thatforces hot flue gases to scrape against the potfor 15 cm will be about 47% efficient.2.) 4 kW fire with a channel gap of 10 mm thatforces heat to scrape against the pot for 15 cmwill be about 35% efficient.3.) A 6 kW fire with a channel gap of 12 mm thatforces heat to scrape against the pot for 15 cmwill be about 30% efficient.4.) A 8 kW fire with a channel gap of 14 mm thatforces heat to scrape against the pot for 15 cmwill be about 26% efficient.As an approximate, general rule of thumb,Baldwin recommends that a family sized stove thatburns less than one kg of wood per hour can use achannel gap between pot skirt and pot of 11 mm.If the stove burns 1.5 kgs per hour the gap needs tobe 13 mm. If 2 kilos of wood are burnt per hourthen the gap should be 15mm. Please refer toBiomass Stoves for complete information.In wood burning stoves a lot of the heat istransferred to the pot or griddle by convection.The amount of wood burnt per hour and channelgap are related. If the pot skirt gap is made toonarrow, there is insufficient draft and smoke backsup into the room.

Design Principles for Wood Burning Cook StovesIncrease heat transfer to the pot by keeping thetemperature of the hot flue gases as high as possible.Insulate everywhere the heat goes except to the potor griddle. If there is enough surface area in thestove for the hot flue gases to scrape against, theflue gases will be much colder by the time they exitout of the chimney. If exit temperatures in thechimney are above 2000 C, add more surface areato make use of the heat. Secondary pots or griddlesplaced near the chimney may never boil water butthey can help preheat cooking water and warmfood and dishwashing or bathing water.Stove TheoryFigure 3 - Top DownView of HayboxUsing a pot skirt also forces more heat into the potby forcing the hot flue gases to continue scrapingagainst the pot all along its sides in addition to itsbottom.A haybox makes even more efficient use ofcaptured heat. Placing the boiling pot of food in anairtight box filled with insulation holds the heat inthe pot, and food cooks without using added fuel(See Figure 4).Once the food has boiled, the fire can beextinguished. Placing the pot of food in aninsulated cooking box most effectively uses the heatto accomplish the task of cooking. The hayboxdoes all simmering without using extra fuel. Thistechnique saves tremendous amounts of wood. Andusing a retained heat cooker saves time for the cookwho lets the haybox do the simmering!Figure 4 - PlacingBoiling Pot in theInsulationFigure 5 - PuttingInsulating Lid on theHayboxFigure 6 - FoodContinues to CookInside the InsulatedHaybox9

Design Principles for Wood Burning Cook StovesStove TheoryCommon Misconceptions4. Packed earth or stone acts like insulation.1. Retained energy in the stove body helps tocook food.FALSEFALSEExperiments by Baldwin have shown that retainedenergy is mostly lost. Leftover charcoal can heatfood after the fire has been extinguished butretained energy in the stove body is usually toocold to effectively heat pots.Note that retained energy in a stove can beadvantageous if the stove is used for space heating.Dense materials absorb energy rather quickly whileinsulation slows the passage of heat. Insulation ismade of pockets of air separated by a light weightless conductive material.Insulation is light and airy. Heavy materials arebetter examples of thermal mass. Insulation helps astove to boil water quickly; thermal mass robsenergy from the pot which makes water take longerto boil.5. Anything is better than an open fire.2. Keeping energy in the stove by decreasing thedraft will help to cook food. Lowering the exittemperatures in the chimney means that the cookstove is operating well.FALSEAs stated, slowing down the draft hurts bothcombustion and heat transfer. Hot flue gases needincreased velocity to achieve good heat transfer.3. Using a damper in the chimney helps to makea stove work better.FALSEAgain, slowing down the draft in a cooking stove isusually detrimental. Dampers should not be usedin a well designed cooking stove.10FALSEAn open fire can boil water faster than many heavystoves. The three stone fire can be clean burningand relatively fuel efficient. While the open fire canbe wasteful when used carelessly, the early estimatesthat any stove was better has been replaced with anew respect for this ancient technology. Engineershave learned how to design improved cookingstoves by learning what is great about the threestone fire.

Design Principles for Wood Burning Cook StovesStove TheoryTesting is essentialMake stoves safe!Dr. Baldwin includes a remarkably thoroughchapter on stove testing in Biomass Stoves. Hepoints out that the testing of prototypes is necessarywhile the stove is being developed. Testing stovesalso helps determine if the model is marketable,whether production costs are as low as possible, andif improvements are needed. Testing should happenduring the entire life of a stove project.Preventing burns is quite possibly one of themost important functions of an improved stove.Burns are quite common in homes using fire andcan be fatal or horribly disfiguring. To protect thefamily the stove body should not be hot enough tocause harm. Stoves and pots should be stable.Surround the fire with the stove body so thatchildren cannot be burnt. Injuries from fire are amajor problem that stoves can remedy.Baldwin includes tests to determine whetherconsumers are happy with the product, if firewoodis being saved, and how lifestyle issues are affected.Without continual testing, a stove project operatesin the dark; it lacks essential technological,sociological, and business information. Reading thestove testing chapter in Biomass Cookstoves is highlyrecommended.Careful testing of stoves has resulted in a moreaccurate understanding of how to make betterstoves. Without experimentation and testing, thedevelopment of a stove is based on conjecture.Careful investigation can quickly separate truthfrom opinion. Testing has a twofold function: toidentify problems and to point out solutions. It isan essential ingredient for progress. A simple waterboiling test is included in Chapter 5 on page 30.Chimneys or smoke hoods can be used to getsmoke out of the kitchen. According to recentestimates by the World Health Organization, up to1.6 million women and children die every yearfrom breathing polluted air in their houses.Pneumonia and other respiratory diseases inchildren are caused by breathing smoke. Unventedstoves can be used outdoors, under a roof, or atleast near a large window. Operational chimneysand airtight stoves can remove essentially allpollution from the indoor environment. Chimneysare used in industrialized countries and arerequired for protecting families from dangerousemissions. Shouldn’t people in poorer countries beprovided with the same protection?11

Design Principles for Wood Burning Cook StovesTen Design PrinciplesChapter 2Ten Design PrinciplesDr. Larry Winiarski’s design principles have been used by many organizations to create successful stoves. TheHELPS plancha stove in Guatemala, the PROLENA EcoStove in Nicaragua, the Trees, Water and PeopleJusta stove in Honduras, the ProBec stoves in South Africa, the new generation of GTZ cooking stoves inAfrica, and the famous Rocket stove are all designed using his principles. Winiarski’s design approachcombines both clean burning and optimized heat transfer characteristics. Any type of intermittently fedwood burning stove can first be designed by locals to meet their needs and then finished by adapting theseprinciples.Batch fed and fan assisted stoves operate differently. These alternative stove design methods can be used assuccessfully to improve wood burning stoves. While many experts are working on these two approaches,both Crispin Pemberton-Pigott and Dr. Tom Reed have developed excellent working models, both of whichare for sale. For more information on the Vesto stove please contact: Crispin Pemberton-Pigott atvesto@newdawn.sz or VESTO, P.O. Box 85274 Emmarentia, Republic of South Africa 2029. Dr. TomReed has spent decades experimenting with wood burning. His fan-assisted stoves are wonderful inventions.He currently markets them under the name “Wood Gas Camp Stoves.” Dr. Reed can be reached throughthe Biomass Energy Foundation Press or at tombreed@comcast.net.PRINCIPLE ONE:Whenever possible, insulate around the fireusing lightweight, heat-resistant materials. Ifpossible, do not use heavy materials like sand andclay; insulation should be light and full of smallpockets of air. Natural examples of insulationinclude pumice rock, vermiculite, perlite, andwood ash. Lightweight refractory brick (brick thathas been fired and is resistant to cracking at hightemperatures) can be made from locally availablesources (for recipes see Chapter 4, Option #2:Insulative Ceramics, page 27).Insulation around the fire keeps it hot, which helpsto reduce smoke and harmful emissions. Also,insulation around the fire keeps the heat fromgoing into the stove body instead of into the pot.Unfortunately, metal does not last very longnear a hot fire. However, locally made ceramic tilescan be found that are durable when used as walls for acombustion chamber. Loose insulation can surroundthis type of construction. (See Chapter 4, Option #1:Floor Tiles, page 26.)Insulative brickPockets of air which slow thetransfer of heat to the brickFigure 7 - Insulation around the fire12

Design Principles for Wood Burning Cook StovesTen Design PrinciplesPRINCIPLE TWO:Place an insulated short chimney right above the fire. The combustionchamber chimney should be about three times taller than its diameter.Placing a short chimney above the fire increases draft and helps the fireburn hot and fierce. Smoke will contact flame in the chimney andcombust, reducing emissions. Pots or surfaces to be heated are placed abovethe short chimney. A taller combustion chamber chimney, more than threetimes the width, will clean up more smoke, but a shorter chimney willbring hotter gases to the pot. The very tall combustion chamber chimneycan develop too much draft bringing in too much cold air that willdecrease heat transfer.Figure 8 - An insulatedshort chimney above the firePRINCIPLE THREE:Heat and burn the tips of the sticks as they enter the fire. If only the wood that is burning is hot there willbe much less smoke. Try to keep the rest of the stick cold enough that it does not smolder and make smoke.The goal is to make the proper amount of gas so that it can be cleanly burned without making charcoal orsmoke. Smoke is un-burnt gas! It is harmful to breathe. Even cleaner looking combustion contains harmfulemissions.Figure 9 - Cleaner BurningFigure 10 - Smoldering Wood Makes SmokePRINCIPLE FOUR:High and low heat arecreated by how many sticksare pushed into the fire.Adjust the amount of gas madeand fire created to suit thecooking task. (Wood gets hotand releases gas. The gascatches fire and makes heat.)Figure 11 - Low HeatFigure 12 - High Heat13

Design Principles for Wood Burning Cook StovesTen Design PrinciplesPRINCIPLE FIVE:Maintain a good fast draft through the burningfuel. Just as blowing on a fire and charcoal canmake it hotter, having the proper amount of draftwill help to keep high temperatures in your stove.A hot fire is a clean fire.Figure 13 - Maintaining a Good DraftPRINCIPLE SIX:Too little draft being pulled into the fire willresult in smoke and excess charcoal. But toomuch air just cools the fire and is not helpful.Smaller openings into the fire help to reduce excessair. Improving heat transfer to the pot or griddle isthe most important factor that will reduce fuel usein a cooking stove. Improving combustionefficiency reduces pollution but is less importantwhen trying to save firewood.Figure 14 - Balancing the air flow in a multipot stovePRINCIPLE SEVEN:The opening into the fire, the size of the spaceswithin the stove through which hot air flows,and the chimney should all be about the samesize. This is called maintaining constant crosssectional area, and helps to keep good draftthroughout the stove. Good draft not only keepsthe fire hot; it is also essential so that the hot aircreated by the fire can effectively transfer its heatinto the pot. Air does not carry very much energy,so a lot of it needs to go through the stove in orderto accomplish the task of heating food or water.Figure 15 - Maintaining Constant Cross-Sectional Area14The size of the openings is larger in more powerfulstoves that burn more wood and make more heat.As a general rule, a door into the fire with asquare opening of twelve centimeters per sideand equally sized chimney and tunnels in thestove will result in a fire suited to familycooking. Commercial stoves need bigger openings,tunnels, and chimneys because bigger fires requiremore air. For more information, please see thechapter Designing Stoves with Baldwin andWiniarski on page 17.

Design Principles for Wood Burning Cook StovesTen Design PrinciplesPRINCIPLE EIGHT:Use a grate under the fire. Do not put the sticks onthe floor of the combustion chamber. Air needs topass under the burning sticks, up through thecharcoal, and into the fire. A shelf in the stoveopening also lifts up sticks so air can passunderneath them. When burning sticks, it is best tohave them close together and flat on the shelf, withan air space in between each stick. The burningsticks keep the fire hot, each fire reinforcing theother to burn more completely. It is optimum if theair passes under the shelf and through the coals sothat when it reaches the fire it is preheated to helpthe gases reach complete combustion. Air that passesabove the sticks is not as helpful because it is colderand cools the fire. A hot raging fire is clean, but acold fire can be very dirty.Figure 16 - Use of a Grate Under the FirePRINCIPLE NINE:Insulate the heat flow path. Cooks tend to likestoves that boil water quickly. This can be especiallyimportant in the morning when family membersneed to get to work. If heat goes into the body ofthe stove, the pot boils less quickly. Why heat upfifty or one hundred kilograms of stove eachmorning when the desired result is to heat up akilogram of food or a liter of water? Usinginsulative materials in the stove keeps the flue gaseshot so that they can more effectively heat the panor griddle. Insulation is

Design Principles for Wood Burning Cook Stoves 7 Chapter 1 Stove Theory Even an open fire is oft