Why you shouldn t use efficiency numbers to choose a stove Damon Ogle Aprovecho Research Center January 3, 2005

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Why you shouldn t use ecency numbers to choose a stoe Damon Ogle Aproecho Research Center January 3, 2005 People are naturally drawn to the word ecency and understandably thnk that mproed thermal

1 Why you shouldn t use ecency numbers to choose a stoe Damon Ogle Aproecho Research Center January 3, 2005 People are naturally drawn to the word ecency and understandably thnk that mproed thermal ecency predcts decreased uel use when cookng ood. Unortunately choosng a stoe based on thermal ecency can result n the selecton o a stoe whch s nether uel sang nor user rendly. Some sort o water bolng test s usually used to determne ecency. There are many ersons o water bolng test. Obously aryng test methods result n numbers or ecency whch are not readly comparable. A more undamental problem comes n the denton and calculaton o ecency. In stoe work, there are seeral types o ecency, but usually what people are reerrng to s Thermal Ecency. Ths can be thought o as the energy that got nto the pot dded by the energy that was n the wood. The energy that got nto the pot has two parts: (1) energy used to heat water and (2) energy used to aporze water. The problem comes wth the aporzaton o water, whch uses energy but n most cases (unless you are steamng somethng) does not help to cook ood. Makng a lot o steam rases thermal ecency scores but only wastes uel when cookng. Noce stoe testers are oten surprsed when they wegh hot water because the weght dmnshes. The water s losng weght at a arly rapd rate as t aporzes. Water n a standard open 7 lter test pot, held at 5 degrees below the bolng pont, wll lose about 14 grams o weght per mnute. The longer t s held at that temperature, the more weght t wll lose.

2 At recent stoe camps n Inda, the steam lost whle brngng water to a bol n dentcal open pots aeraged about 10 grams per mnute. Derent stoes had derent total tmes to bol but were smlar n ther rate o water loss. The ormula used to compute ecency s usually some close araton o the ollowng: Where; ecency [ * w *( T T ) + [ 2260 * w w ntal weght o water (grams) T nal temperature o water ( C) T ntal temperature o water (ºC) w water aporzed (grams) [ * LHV d d uel consumed (n grams, corrected or mosture content and charcoal let) LHV Lower Heatng Value (energy content o wood n Joules/gram) To sualze the problems o thermal ecency, let s look at two stoes whch perorm the same task. The task s to brng 5 lters (5000 g) o water rom a startng temperature o 25ºC to bol n an open pot. For comparson, we wll assume that both stoes consume 1000 grams o wood to do the task and both stoes aerage 10 grams/mnute o water aporzaton durng the task. Stoe 1 does the task n 10 mnutes whle stoe 2 requres 100 mnutes to do the same task. To smply our calculatons, hypothetcal (but not unrealstc) numbers are gen or the perormance o the two stoes. Actual stoes gng ery smlar test numbers do exst. Stoe 1 Tme to bol 10 mnutes 1000 grams Water aporzed 100 grams Water remanng 4.9 lters Tme to bol Water aporzed Water remanng Stoe mnutes 1000 grams 1000 grams 4.0 lters Calculaton o Thermal Ecency [ 4.186* w ) + [ 2260* w [ 4.186* w ) + [ 2260* w ecency ecency 25) + [ 2260*100 25) + [ 2260*1000 e e [ 1000*18000 [ 1000*18000 [ [ [ [ e e [ [ e 9.9% e 21.3%

3 Stoe 2 would be the obous stoe to choose based on thermal ecency, but s ths an optmal stoe or cookng? Both stoes n the example used the same amount o wood. Stoe 2 takes 100 mnutes to produce 4 lters o the desred product (bolng water/ood). Stoe 1 takes only 10 mnutes to bol and produces almost 5 lters o the same product. Most cooks mght preer Stoe 1, een though t s less ecent. An alternate approach, Specc Consumpton replaced thermal ecency n the 1985 VITA Internatonal Testng Standard. Specc Consumpton s the uel used per unt o product produced. The unt o product could be bowls o beans or loaes o bread or n ths case, lters o bolng water representng cooked ood. Remember, we are talkng about the weght o nshed product, not startng weght! Let s look at Specc Consumpton or the two stoes n the example. Specc Consumpton o Stoe grams/lter Specc Consumpton o Stoe grams/lter Specc Consumpton results ndcate that Stoe 1 would be the better choce snce t uses less uel to produce a lter o bolng water. Ecency rewards the producton o excess steam whle specc consumpton penalzes t. The VITA 1985 Internatonal Testng Standard recommends Specc Consumpton as the more relable ndcator o stoe perormance. The power requred to rase the temperature o water n an open pot ncreases as the water temperature approaches the bolng pont. Ths s especally sgncant n the last 5 degrees below the bolng pont. In a standard test pot, water at ½ degree below bolng wll eaporate water at about 26 grams/mnute whch s 85% greater than the eaporaton rate at 5 degrees below bolng. Stoe power must be sucent to oercome heat losses through the sdes o the pot and to supply the heat requred or aporzaton o water. The addtonal requrement or more energy as the bolng pont s approached creates an energy hump whch low powered stoes oten hae trouble oercomng. Ths condton wll result n long tmes to bol and large losses o water through aporzaton. Increased steam producton produces hgh ecency numbers. Problems wth ecency become een more edent when smmerng water. Smmerng attempts to mantan hot water (or ood) at just under the bolng temperature usng the mnmum amount o uel. The best methods or smmerng water

4 (rangng rom the use o pot lds and nsulaton up through the use o hay boxes whch use no uel) are most penalzed because they lose ery lttle water to aporzaton. Examnng two hypothetcal examples ponts out the derence between ecency and specc consumpton. The task s to mantan 5 lters o 97 degree C water wthn 6º o the bolng pont or 30 mnutes. One o the stoes has a good turndown ablty and s able to mantan the temperature o the water at 97. The other stoe lacks turndown ablty and apples too much power whch causes the water to reach a ull bol and aporzes lots o water. It s assumed that both stoes hae equal uel consumpton per gram o water aporzed. Smmer tme Water aporzed Water remanng Stoe 1 30 mnutes 250 grams 500 grams 4.5 lters Smmer tme Water aporzed Water remanng Stoe 2 30 mnutes 750 grams 1500 grams 3.5 lters n: Calculatng Ecency and Specc Consumpton or the two stoes results Ecency o Stoe * w ) * w ecency 4.186*5000* ( 97 97) *500 e [ 250*18000 [ 0 + [ e [ e 25.1% [ [ [ [ Ecency o Stoe 2 [ 4.186* w ) + [ 2260* w ecency 97) + [ 2260*1500 e [ 750*18000 [ [ e [ e 25.6% Specc Consumpton o Stoe grams/lter Specc Consumpton o Stoe grams/lter The Ecences o these two stoes are rtually dentcal and one would assume that there was ery lttle derence between them. Examnng the Specc Consumptons tells a ery derent story. Stoe 1 would be the better choce snce t uses ¼ as much wood as Stoe 2 to produce the same output (lters o water whch hae been smmered or 30 mnutes). Repeated testng at Aproecho Research Center has shown that Specc Consumpton s the more relable measure o stoe perormance.

5 Problems wth Thermal Ecency hae been recognzed or decades and hae been ponted out by Baldwn, The Endhoen group and others. Pet Vsser has shown that ecency n conjuncton wth power output (at hgh and low power) can be used to make accurate predctons about stoe perormance. By usng the two actors together and denng a cookng process (cookng rce or example) one can calculate cookng tme, uel use, water loss and so orth. Vsser s absolutely rght n hs conclusons. Ecency by tsel s not a relable predctor o stoe perormance. Damon Ogle monogle@oregoncoast.com

THE VAPOR DRYING PROCESS. Robert D. Graham Wood Technologist Oregon Forest Products Laboratory Corvallis, Oregon

THE VAPOR DRYING PROCESS. Robert D. Graham Wood Technologist Oregon Forest Products Laboratory Corvallis, Oregon THE VAPOR DRYING PROCESS by Robert D. Graham Wood Technologst Oregon Forest Products Laboratory Corvalls Oregon Presented at the Annual Meetng of the Western Dry Kln Clubs Corvalls Oregon May -2 0 95 THE