Evaluation Criteria for Improved Cookstove Programs - The Assessment of Fuel Savings

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  • 7/28/2019 Evaluation Criteria for Improved Cookstove Programs - The Assessment of Fuel Savings




    Jan B ia ly

    EWC/ESMAP/UNDP E v a l u a t i o n o f Imp ro ved C ook sto ve Pro grams

    F in a l Rev ised Repor tNovember 4 , 1 991

    Risk and Development ProgramE nv iro nm en t a nd P o l i c y I n s t i t u t eEast-West CenterHono lulu , Ha wa i i 96848

    Pro j ec t Background Repor t #2

  • 7/28/2019 Evaluation Criteria for Improved Cookstove Programs - The Assessment of Fuel Savings




    32 Methods of assessing fuel consumption 43 Questionnaire surveys 6

    4 Water heating tests 85 Cooking tests 106 Field trials 117 Expression of results 138 Conclusions 169 Recommendations for further research 17

    Selected bibliography 19


  • 7/28/2019 Evaluation Criteria for Improved Cookstove Programs - The Assessment of Fuel Savings


    I) Introduction


    The principal aim of the majority of improved cookstove programmes (ICPs) which havebeen introduced in the developing world over the past decade is a reduction in the domesticconsumption of wood. The reason for identifying this as a major objective has not always beenclearly stated. A direct association between domestic fuelwood consumption, as cause, anddeforestation, as effect, has been widely promulgated but rarely substantiated as a justification onenvironmental grounds.' Reducing the amount of time spent collecting wood or the quantity ofmoney required to pay for it have also been given as reasons on economic grounds. In nearly allcases it has been expected that the successful dissemination of an improved stove, which used lessfuel for cooking, would lead directly to a reduction in total fuel consumption.

    There has been an implicit assumption that there would be no significant change in theoperations performed on the stove or the quantities of food cooked. In addition, little account hasbeen taken, in areas where fuelwood is bought, of the possible market interactions that wouldfollow any reduction in demand, specifically a fall in price and a consequent rise in demand to anew equilibrium level. Finally, in the attempt to design improved stoves for cooking, lit ttleattention has been paid to the use of fuelwood for space heating in many parts of the world. Theconsumption of fuel for the latter purpose may be significantly larger than for cooking in areas ofhigh altitude or latitude. It follows that ICPs might have little effect on total fuel consumption inthese areas.

    The current lack of clarity in defining fuelwood savings criteria for ICPs owes much to theconfusion surrounding the measurement of domestic fuelwood consumption. Many differentmethods have been used and only recently has any consensus emerged on what constitutes anappropriate assessment technique in most cases. In general, the choice of technique will dependon three factors:

    I) The purpose for which the information on fuel consumption is to be collected (eg. theactual / effective / perceived change in total / cooking fuel consumption)

    2) The time, labour and money available to perform the assessment3) The pattern of domestic fuel consumption in the area under consideration (i.e. the extent

    of space heating, water heating or boiling and changes in the types and quantities of food cookedduring the year).In many instances, it will be found that the first of these is inadequately defined, the second is at aless that desirable level and the third is largely unknown.

    See, for example, VITA / ITDG (1980), p.1; Micuta (1981), p.7; TINS() (1981), pp. 1 - 3; Kaufman (1983),p.1 and Aprovecho Institute (1984), pp.6 - 9.

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    2) Methods of assessing fuel consumptionAll methods of assessing fuel consumption will involve a comparison between some

    measure of the various domestic heating operations performed, primarily cooking, water heatingor boiling, and space heating, and the quantity of fuel consumed. Fuel consumption is thenexpressed as the quantity of fuel consumed per unit measure of operation performed, however thatmeasure is defined. In other words.

    Fuel consumption = Measure of fuel inputMeasure of operational output

    The choice of measure wil l depend on the technique used to assess fuel consumption.' Fourprincipal techniques, all of which are in current use, may be distinguished:1) The questionnaire survey involv es the estimation o f fuel consumption by the

    householders interviewed. The results are usually expressed as kg per capita-thy o r perhousehold-thy. The operations performed are not described specifically and it is assumed that theaverage operational output per capita-day does not vary much.

    2) The water heating test measures the quantity of wood required to heat a specifiedquantity of water to a given temperature, usually, but not always, the boiling point. The test isusually carried out under controlled conditions in the laboratory. The results are generallyexpressed as a thermodynamic efficiency, that is the ratio of the heat energy absorbed by the waterto the chemical energy in the wood. This is effectively the reciprocal of a fuel consumptionmeasurement. In many cases the ratio is expressed as a percentage, which in some instances isreferred to as the percentage heat unused (PHU).

    3) The cooking test measures the quantity of wood required to cook a meal or a singledish. The test may be earned out either in the laboratory, in which case the quantities and types offood cooked are carefully specified, or in the field, in which case they are chosen by the hosthousehold. The results are expressed as a specific fuel consumption (SFC), that is the ratio of themass of food cooked (the mass of water is sometimes included) to the mass of fuelwoodconsumed.

    4) The field trial involves the actual measurement of the total quantity of wood used on a2These techniques and the measures used t o express the results are discussed in more detail in Bialy (1986),pp. 153 - 172 and pp. 215 - 342.3The description -k g p e rc a p i t a -d a y "

    considered preferable as it avoids doubt over whether the second -p e r " t a k e s t h e -d a y " b a c k

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    daily basis by each of several households. As with the questionnaire survey the results areexpressed in kg per capita-day or per household-day.

    The order in which these four techniques of assessment have been placed reflects theaccuracy with which they may be used to predict actual fuel consumption when properlyconducted. The least accurate of the four methods is the questionnaire survey. The field trial is themost accurate method when properly carried out but is also the most time-consuming andexpensive.

    Ideally, in measuring the fuel savings resulting from the introduction of an improvedstove, all four techniques would be used. Each provides an independent method for assessing fuelsavings and comparison of the results could be of considerable use in analysing the effect ofintroducing a new stove.

    The terminology used by some writers differs from the above. Reference is notinfrequently made to the provisional international standards for testing the efficiency of wood-burning stoves which were proposed by a meeting of experts organised by VITA in December1982.4T he ses tanda rd sactuallyreferto

    are distinguished, the water boiling test (WBT), the controlled cooking test (CCT) and the kitchenperformance test (KVF). These equate roughly to the second, third and fourth techniquesdescribed above, but are much more closely specified. The calculation of efficiencies is not, infact, recommended by the standard; the results of both the WBT and CCT are to be calculatedinstead as a standard specific consumption (SSC), which is the same as the SFC defined above.

    Although the VITA tests might permit international comparisons to be made (this was,indeed, their primary objective), it was considered in this document that greater latitude should beallowed in the definition of the test procedures and factor specifications.' There is no evidence thatthe VITA standards constitute a necessary or even sufficient set of tests to allow the right choice oftechnology to be made, or the right action to be taken, at each stage in the development of an ICP.

    Even as a basis for international comparison, the VITA protocol leaves much to be desiredand it has not been as widely used as was originally intended. Thus the WBT is very carefullydefined in a number of respects but fails to specify what types of vessel are to be used or howexactly the wood should be prepared. The use of heavy ceramic vessels and large sticks of a highdensity wood would undoubtedly give a much higher SFC than the use of light aluminium vessels

    See Wood et al. (1982).sThe tenu 'factor specification" refers collectively to the levels of all the factors that are controlled and

    specified within the test, such as the type of vessel used and the size to which the fuelwood is cut. This is discussedin more detail in Bialy (1986), pp. 256 261 and 266 267.

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    3) Questionnaire surveys


    and small cubes of a low density wood. The difference could easily obscure differences inperformance capacity between two stoves.

    The CCT is only very loosely defined, perhaps necessarily so, and no restrictions areplaced on the kind or quantity of foods that are to be cooked 7, e x c e p t t h a t t h e yf a l l w i t h i nt h e"range of meals normally prepared in the area where (the stove) is to introduced". I t isrecommended that the tests be repeated 5 times (compared to only 4 times for the WBTs) and thatthe results for two different stoves, one possibly being a traditional design, be compared by usinga t test. It is not known of any ICP where this procedure has been followed, although the use ofat least some form of statistical analysis is certainly to be encouraged.

    Finally, in the KPT, no recommendation is made to measure or record the operationaloutput of the stove in terms of the quantity of food cooked or the number of vessels used. Theresults are expressed as a specific daily consumption, that is, in kg per capita-day.

    It is believed by some that because a questionnaire survey can be carried out in a muchlarger number of households, it will necessarily produce a much more accurate estimate of averagefuel consumption than a field trial. Much will depend on how carefully the survey and trial arecarried out but since the results of a questionnaire survey are based on the opinions of thehouseholders interviewed, there is the possibility of a consistent bias in the results. There arevarious reasons why domestic fuel consumption may be deliberately overestimated orunderestimated. If, for example, fuel is scarce or expensive, and it is desired to give animpression of poverty in the hope of obtaining some help then the level of fuel consumption maybe exaggerated. If, on the other hand, much of the fuel has been obtained illegally, or it is fearedthat some form of tax may be imposed, then the level of consumption may be understated.

    This error of reporting, as it may be called, is one of four possible types of error whichmay occur when a questionnaire survey is carried out. These are as follows:

    1) Errors of comprehension: the householder may not correctly comprehend questionsabout his fuel consumption and the interviewer may not correctly comprehend the answers. Thus

    The standard only specifies loca lly dominant wood species. air dried , preferably 2 to 3 cm diameter" (seeWood et al. (1982), p. 5). The length of each piece is not specified, although reference to the wood elsewhere in thetext suggests that sticks, that is, of length several times greater than 3 cm, are intended. The use of small cubes ofwood, of side 2 cm, is not precluded.7Wood et al. (1982), pp. 15 - 17.

    See Bialy (1986), pp. 70 - 78.

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    there may be certain domestic operations. such as food processing. which do not occur dAily butwhich involve a relatively high level of fuel consumption. The interviewer wil l generally beinterested in the total or average fuel consumption but the interviwee may perceive that it is onlythe day-to-day fuel consumption which is of interest and may fail to report any extraordinary use.The same may happen with space heating if the survey is carried Out at a time of year when spaceheating is not required.

    2) Errors of perception: the householder may not perceive accurately how much fuel isconsumed by the household. Much will depend on the manner in which the fuel is obtained andthe frequency with which it is delivered to the household. The response is also likely to depend onwho in the household is interviewed.

    3) Errors of reporting: as pointed out above, the householder may deliberately understateor overstate his perception of the quantity of fuel consumed.

    4) Errors o f conversion: t h e interviewer may not correctly convert the reportedconsumption into the desired units, normally kg of air-dry wood. This is particularly a problemwhen consumption is reported in non-standard units such as a headload or cartioad, or isdemonstrated as an approximately equivalent pile of wood. Failure to account for variations inmoisture content can also be a significant source of error where wood is purchased by weight andis not properly dried at the time of delivery.

    Errors of perception are likely to be random but the other three types of error may all bethe subject of a consistent bias. Thus consistent errors of comprehension may arise i f thequestionnaire does not specifically address the problem of extraordinary fuel use. The possibilityof a bias in errors of reporting has already been discussed, and consistent errors of conversionmay occur if the moisture content of the wood is not measured.

    One way of minimising the effect of non-random sources of error would be to approachthe question of fuel consumption in two different ways. For example, questions could be askedabout the quantity of fuel used per day or per week and about the length of time for which a givenload brought to the household would last. With questionnaire surveys there is also the possibilityof asking questions directly about fuel savings as well as comparing the reported levels ofconsumption before and after the introduction of the improved stove. Unfortunately, in manycases, questionnaire surveys are only carried after the new stove has been introduced and thisvaluable means of checking reported savings is lost. It is also frequently observed that there is apersistent bias in reporting fuel savings; many people feel obliged to report favourably on a stovethey have been told will save wood, especially if they have not had to pay for the stove.

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    4) Water heating tests


    The water heating test has been by far the most common method of assessing relative fueleconomy in improved stoves over the past decade. There are several good reasons for this.although there is still no clear evidence that the results of water heating tests, whether carried outin the laboratory or the field, can be used to give a reliable estimate of fuel savings in practice. Atone time it was not uncommon for those promoting improved stoves to predict, on the basis ofwater heating tests, that domestic fuel consumption could be reduced to half or even a third of itsinitial level through the dissemination of the new stoves. Estimates of potential savings are nowmuch more cautious and it is more widely accepted that water heating tests alone are not a reliableindicator of the extent of these savings.'

    Relative levels of fuel consumption predicted for different stoves will often vary accordingto the form of the test carried out., o n e t e s tm a yp r e d i c tt h a ta ni m p r o ve ds t ov eu se s4 0%l es sf ue l

    than the traditional method of cooking, another test only 20%. Furthermore, operations other thanwater heating will generally account for a significant proportion of total fuel consumption. Testswhich indicate savings in water heating operations will not necessarily guarantee savings in theseother operations.

    The principal reason for the widespread use of water heating tests is that they are relativelyeasy to carry out Cooking tests, which would give a more accurate estimate of savings in cookingoperations, inevitably require more preparation and operational control than water heating tests. Itis therefore more difficult to standardise the conditions under which they are carried out. Theresults of cooking tests are also likely to vary more because of the problems associated withdefining the end-point of the test, that is, when the food is cooked. For this reason water heatingtests are often carried out to simulate cooking tests. Typically, such a test would involve bringing aspecified quantity of water, usually l or 2 kg, to the boil and then simmering it at a reduced ratefor a fixed period of between 30 and 60 minutes. This procedure would be intended to simulatethe cooking of foods such as rice and stews.

    Ultimately, the function of water heating tests is to provide some measure of relative fueleconomy which is quick, repeatable and reliable. It is unfortunate that the first two qualities areincompatible with the third. A single test of short duration cannot be representative of all thedifferent types of operation performed in practice. A test which gives repeatable results wil l need

    As far as is known, no systematic attempt has ever been made to measure or assess the accuracy with whichthe results of water heating tests may be used to predict the results of field trials. The relationship between the twois important in that a knowledge of how closely they are correlated could save much time currently spent on bothtypes o f assessment. I t is probable, as with other important areas of research in this field, that the work isconsidered by those who have the power to fund or authorise it, to be too academic,

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    to be carried out under carefully controlled conditions. On both counts the results of the test areunlikely to be a reliable indicator of fuel savings in practice. In contrast to the water heating test,the field trial, which is discussed below, is much more reliable as an indicator of savings, but isneither quick nor usually repeatable.

    A compromise, namely the development of a limited but sufficiently reliable spectrum ofshort tests, the results of which could then be appropriately weighted, might be possible.Extensive research would be required, however, to determine the appropriate weights. It would benecessary first to carry out a survey of fuel use patterns in order to estimate the relative quantitiesof fuel used for different types of operation. A range of WHTs would then have to be devised tosimulate each of these types of operation and the results of these tests weighted accordingly. Inorder for this method to give a reasonably accurate estimate of total fuel savings, the survey offuel use patterns would need to take account of seasonal variations. This would be especiallyimportant where space heating were required during the cooler months of the year.

    The results of a water heating test are often expressed as a thermodynamic efficiency bycomparing the energy output with the energy input. It is a widely held view that the efficiency isan intrinsic property of the stove being tested, and this is the reason why water heating tests arebelieved in some quarters to be more accurate' than other methods of assessment. The concept ofefficiency is common to many disciplines, but the precise definition varies considerably, evenwithin a single discipline and in relation to a particular type of process. Efficiency is not afundamental physical quantity but an artificially created ratio and its definition is, of necessity, asubjective one.

    The efficiency of a stove, as measured in a particular water heating test, is no more afunction of the stove than it is of the vessel in which the water is heated, or the fuel, or even thetest procedure. Variations in any of these factors may cause the efficiency to vary. Only by holdingthese other factors at fixed levels, sometimes very different from those observed in practice, canthe efficiency be made to appear as a constant. By appropriate choice of the factor levels in thelaboratory, the efficiency can also be made to increase to much higher levels than those whichmight be measured in the field. Burning small cubes of oven-dry wood is one example which hasbeen used by some promoters of improved stoves. Of course these factors do not all have an equaleffect on the efficiency, but, cumulatively, factors other than the design of the stove can easilyhave as much effect as the design. Thus the open fire is frequently found to have an efficiency of5 to 10%, but by careful control of other factors 0 this can be increased to almost 30%, which issimilar to the efficiencies measured for improved stoves under less carefully controlled conditions.

    10See Bialy (1986), p.,5.


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    5) Cooking tests


    The major advantage of cooking tests over water heating tests is that they relate moreclosely to certain cooking operations which often account for the major part of domestic fuelconsumption. The major disadvantage, as pointed out above, is that they are more difficult tostandardise and control, and so the results are more likely to vary than with water heating tests.Because of the wide range of foods and dishes that may be cooked, it is also impossible to definea single cooking test that would apply in all situations. Even in a specific community, there wil lusually be a considerable variety of foods and dishes cooked, and these may also change with theseasons. Accordingly, no cooking test can ever be more than representative of certain commonlycooked dishes or meals.

    As with water heating tests, cooking tests can be carried Out under either laboratory orfield conditions. Under laboratory conditions the quantities of food cooked will be maintained atfixed levels whereas in the field they wil l normally be determined individually by the host cook.For this reason, it is possible in the laboratory to compare the performance of an improved andtraditional stove directly, simply by comparing the quantities of wood consumed in the test. In thefield, however, it is necessary to take account of the variations in the quantities of food cooked bycalculating the specific fuel consumption (SFC). Inevitably there will be more variation in theresults and so it will be necessary to carry out more tests in order to determine how much fuel issaved by the improved stove. The advantage of carrying out the tests under field conditions is thatit is not necessary to impose any control on the quantities of food cooked. This is important in thecooking of rice, for instance, where the cook may wish to add varying amounts of extra waterduring the cooking process according to how rapidly the water evaporates.

    Given the variety of domestic heating operations performed in most households, a singlecooking test is very unlikely to prove a reliable indicator of potential fuel savings any more than asingle water heating test. Most likely a range of tests would need to be devised and the results ofthese would have to be weighted according to the overall importance of the cooking operationswhich the various tests represented. As before, it would be necessary to carry out a preliminarysurvey of fuel use patterns in cooking in order to determine the appropriate weighting factors.Once carried out, however, the survey would not need to be repeated; the same weighting factorscould be used to interpret the results of cooking and water heating tests carried out on all theimproved stoves being considered for the area concerned. Using this procedure, the effects ofsmall variations in the design of a stove could also be determined more accurately than would bepossible without the use of weighting factors and more quickly than would be possible bycarrying out a field trial.

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    6) Field trials


    In a field trial, the total level of domestic fuel consumption associated with the use of aparticular stove is determined by direct measurement and not, as in the questionnaire survey, byinterviewing the householders. The cooperation of the householders is obviously required, buttheir estimates of fuel consumption are no longer relied upon. In this way, errors of perceptionand reporting are avoided. Errors of comprehension may still occur if the survey procedure is notadequately explained; indeed this is the main source of error in field trials. Errors of conversion,on the other hand, 'nay be largely avoided by measuring the fuel consumption in the desired unitsand checking the moisture content where necessary.

    Field trials have been carried out in various different ways. Since this method ofassessment is so important, it is worth examining some of the factors that may affect the resultsand how the methodology may be developed to take account of these factors. The normal methodof carrying Out a field trial is to measure the total consumption of fuel both before and after theintroduction of the improved stove. The householders participating in the survey are persuaded touse fuel only from a single pile or reservoir, which is reweighed and replenished at regularintervals. The accurate estimation of fuel consumption by this reservoir method, as it may becalled, depends on how the fuel is collected or delivered to the house. If the fuel arrives onceevery week or two, it is far easier to monitor than if it arrives practically every day. Unfortunately,the latter situation is very common amongst poor families and in many rural areas where firewoodis collected from nearby forest lands.In either situation, there are four ways in which the reservoir method may be rendered ineffective:

    1) Fuel may be added to the reservoir without being weighed first2) Fuel from some source other than the reservoir may be consumed by the household.3) Fuel taken from the reservoir may not be used for domestic operations in the household

    concerned.4) The fuel in the reservoir may decrease in weight due to evaporation.The first two of these unaccountable transfers of fuel are most likely to result simply

    through carelessness or through the household not understanding the survey technique. It is veryeasy, especially where fuel is brought to the house every day, for someone to accidentally add thefuel to reservoir without allowing it to be weighed first. Equally well the fuel may be addeddirectly to the fire i f the reservoir is forgotten or is seen as an inconvenience. The other twounaccountable transfers are less likely to occur. Furthermore, both of them would result in thelevel of fuel consumption being overestimated, whereas the first two would result in the levelbeing underestimated. Overall, therefore, these errors are most likely to lead to an underestimation

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    of the total fuel consumption.


    The greater the period elapsing between successive weighings of the reservoir, the morelikely are the above sources of error to occur or to go unnoticed. It is best, therefore, if thereservoir is reweighed and replenished daily. Care must obviously be taken when the reservoir isreplenished to ensure that enough fuel is le ft for the following day. A two-day supply, theapproximate size of which may be estimated by consulting the cook, is adequate to cover mostvariations. The measurements should be continued for at least seven days at a time, and if possiblerepeated at different seasons during the year. This is obviously particularly important where thereis a marked cold season during which space beating is required.

    The principal difference between a field trial and the two types of test described above isthat the latter are limited to a single specified type of operation. Thus a water heating or cookingtest will not give any information on how much fuel would be used in space heating operations.There are also many different types of cooking operation, and the conditions under which bothcooking and water heating operations are carried out in practice may differ considerably fromthose under which a test is carried out. Cooking and water heating tests are therefore of limitedvalidity. A field trial is the most reliable method of estimating the fuel-saving potential of anyimproved stove since it measures the relative levels of fuel consumption over all domestic heatingoperations.

    The relevance of the results of a field trial to the actual conditions of use of an improvedstove is inevitably accompanied by a high degree of variability when the trial is repeated. Thisvariability arises because of unavoidable differences in the pattern of fuel use between onehousehold and another and, even in the same household, from one day to another. It is essential,therefore, that a statistical approach be adopted not only in interpreting the results but also inplanning the trial. Ideally, a factorial design should be used and the results analysed byconstructing an ANOVA (analysis of variance) table. Otherwise, daily or weekly consumptionfigures for an improved and traditional design of stove may be compared over a random sample ofhouseholds by carrying out a Student t test.

    An important part of carrying out a field trial is the appropriate choice of an adequatesample. The sample should clearly be random, although stratified samples will often be necessaryat a national level to take account of variations in patterns of fuel use existing in different areas. Inthe same way, care should be taken to ensure that seasonal variations in fuel use patterns,particularly where space heating is required, are taken into account by repeating the trial atdifferent times of year. The trial should be carried Out at least four times, the choice of datesdepending on the timing of the climatic and agricultural seasons. The results of this stratified form

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  • 7/28/2019 Evaluation Criteria for Improved Cookstove Programs - The Assessment of Fuel Savings



    may not be true, especially in areas where there has been an increasing shortage of fuel over along period. It is quite likely in such areas that the number of meals cooked may have beenreduced to cope with the shortage. The boiling and heating of water may also have been drasticallycut.

    If this form of adaptation has taken place, it is very likely that the introduction of animproved stove will result in an increased consumption of cooked food and heated water, but verylittle reduction in the quantity of wood consumed. The stove may stil l be considered to savewood, but the saving will be at least partly offset by an increased level of use of the stove. Inorder to detect this kind of response it would be necessary, when a field trial was carried out, tomonitor not only the consumption of fuel but also the domestic cooking and heating operationsperformed. It would be extremely difficult to measure the actual quantities of food cooked orwater heated throughout the entire trial period without a major disruption in the activities of thecook. The number of items or pots of food or water placed on the stove each day could berecorded, however, and would provide some measure of any change in the output of domesticheating operations performed by the stove.

    Accordingly, i t is suggested that, where possible, a record be kept of the number ofvessels of food or water placed on the stove every time it is used. The size of the vessels used indifferent households will generally increase as the household size increases and for this reason,even though the increase will not be proportional, the number of vessels used for any given meal(or other domestic heating operation) during the day should be multiplied by the number of peopleconsuming the meal (or benefitting from the operation). The resulting measure of the operationaloutput may be described as the v e s s e l-c a p i t a-d a y o r ,s t r i c t l ys p e a k i n g ,s i n c et h ev e s s e l -c a p i t a

    product may be summed over the entire trial period, the vessel- c a p i t a T h e r e s u l t so f t h ef i e l dt r i a l

    may then be expressed as kg / vessel-capitaThe change in the kg / vessel-capita statistic may be calculated for each household in the

    trial and a statistical analysis performed as described above. Alternatively, since fuel consumptionis unlikely to be directly proportional to the vessel-capita measure of output, a linear regressionmay be carried out and the coefficients of the regression lines for the improved and traditionalstoves compared.

    It is not easy, on theoretical grounds alone, to justify a particular choice of measure for theoperational output. One way of deciding which measure to choose is to correlate the observedmeasurements of fuel consumption with different measures of output and then choose thatmeasure for which the correlation coefficient is highest. Unfortunately, relatively few field trialshave been carried and few of these give any measure of operational output other than the

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    household size and the duration of the trial.


    One trial was carried out recently in Sri Lanka, however, in which records were kept overthe duration of the trial of the number of meals cooked, the number of items (or vessels) cookedand the number of people partaking in each meal. 2 A preliminary analysis of the data given,which cover a sample of 65 households over a trial period of 10 days, shows that the vessel-capitameasure of output gives the highest correlation. Further tests would obviously need to be carriedout to confirm the usefulness of this measure, but it merits being recommended at present, giventhat it is relatively easy to compute and at least allows some account to be taken of changes inoutput.

    Finally, in relation to the measurement of changes in output, some mention should bemade of a very simple, but compelling mathematical argument which emphasises the need tomeasure these changes. In general, the total consumption of fuel (F) of a household over a givenperiod may be expressed as the product of the fuel consumption per operation, or per unit ofoutput, (S). and the number of operations or measure of output (P). In other words,

    Total fuel consumption (F) = fuel consumption per unit of output (S) x output (P).If S. P and F now change to new levels, S', P and F' respectively, then clearly

    S' = S + AS,P' = P + APand F ' F + AF,where AS, AP and AF are the increments, whether positive or negative.It follows from the original relation thatF' = S' x P'which gives F + AF = (S + AS) (P + AP)

    = SP + P AS + S AP + ASAP.Subtracting the original relation now leaves

    AF = PAS + SAP + ASAP.If P does not change, as is normally assumed when a new stove is introduced, then AP will bezero and the last two terms on the right will disappear. The change in S resulting from theintroduction of a new technology, such as an improved stove, will be negative and hence AF willalso be negative, that is, there wil l be a reduction in the total consumption of fuel. In practice, as

    2See Atapat-t t i e ta l

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    pointed out above, P may well increase. The signs of AP and AS wi ll then be opposite and AFmay be either positive or negative, or possibly zero. In other words, the introduction of a fuel-efficient stove may or may not result in a reduction in fuel consumption.8) Conclusions


    The following major conclusions may be drawn from the above discussion about theassessment of the fuel savings resulting from the dissemination of improved stoves:

    I) Fuel savings should be measured by carrying out a field trial to determine the total levelof fuel consumption before and after the introduction of the improved stove. Questionnairesurveys, water heating tests and cooking tests are not considered to be reliable enough asindicators of actual fuel savings.

    2) During the performance of the field trial a record should be kept of the number ofvessels of food or water placed on the stove during the trial period and the number of peoplesharing the contents of each vessel T he output from the stove may then be measured as vessel-capita and the effective fuel consumption calculated as kg of air-dry wood per vessel-capita.

    3) A statistical analysis should be carried out on the results of the trial to determinewhether the average change in effective fuel consumption over all the households included in thetrial is significant or not. The significance level should be recorded. Significance at the 5% level issuggested as a minimum requirement.

    4) It is suggested that in interpreting the results of a field trial the minimum reduction in theaverage effective fuel consumption necessary to claim that a stove saves fuel should be 20%. Thisfigure is based on the assumption that any smaller reduction is unlikely to be perceived by mostpeople using the stove. Since individual water heating tests are not an accurate indicator of actualsavings, it is recommended that where such tests offer the only basis for comparison, testing becontinued only if the results indicate savings of at least 30%.

    5) Finally, some attempt must be made to take account of the use of fuel for space heatingwhere this is required. Ideally, any field trial should be carried out both during the colder seasonsand the hotter seasons.

    These conclusions refer primarily to the dissemination phase of an ICP. During the designand pilot phases, less rigorous standards might be applied. Thus in examining the effects of smallalterations to the design of an improved stove, the above methodology would be unnecessarily

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    cumbersome. Instead, as suggested in Sections 4 and 5 above, weighting factors could bedetermined from a preliminary survey of fuel use patterns in the area concerned and these couldthen be applied to the results of water heating and cooking tests. Once the optimal stove designhad been determined on the basis of this simplified procedure, the above methodology could thenbe implemented to evaluate the stove prior to widespread dissemination.

    It has been suggested by some that the criteria for evaluating fuel savings could beprogramme specific, that different programmes might be evaluated according to different criteriachosen from a recommended range. There is a problem, however, if the user of these criteria ispresented with too many choices and the recommendations become too general to be effective.Besides, the purpose of Phase I is to develop a set of criteria which can be used to evaluatedifferent programmes on a comparable basis. The reason for recommending the use of the vessel-capita measure of consumption is that even where there is no actual reduction in the quantity offuel used, the stove may still be an improvement because of an increased output. The vessel-capitameasure takes account of this possibility whereas the capita-clay measure does not.

    On the question of deforestation, it has to be said that there does not appear to be any wayof directly assessing the impact of an ICP on the rate of deforestation. The latter is affected bymany factors, agricultural expansion, population growth and bush fires being amongst the mostcommon. As pointed out above, it is difficult enough even to assess how much wood an ICP maysave or, more precisely, how much less wood is estimated to be used for domestic purposesfollowing the introduction of an ICP. A ll that may be said about the impact on deforestation isthat, other factors being equal, the rate may be reduced at most by the quantity of wood saved peryear by the ICP.

    Apart from the effects of the other factors, the rate wil l be determined, in particular, byhow the fuelwood used for domestic purposes is collected. Urban fuelwood consumption, whichoften involves the destruction of entire trees in the rural areas, will generally have more impact ondeforestation than rural consumption, which more often involves the collection only of dead woodor the cutting of a few branches from a tree rather than the felling of the whole tree. For thisreason, urban ICPs are more likely to have an impact on the deforestation process.9) Recommendations for further research

    Although there are doubtless many who would say that more than enough research hasalready been carried out on the fuel savings resulting from the introduction of improved stoves,there is still much to be done if an efficient and effective methodology is to be developed. Thefollowing important areas remain largely unexplored and should receive attention.

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    I) The proportion of domestic fuelwood consumption in different parts of the developingworld that is required for space heating, as opposed to cooking. Most ICPs attempt to reduce onlythe consumption of fuel for cooking; China appears to be one of the few countries where the useof fuel for space heating has been examined at the same time. Although the use of fuel for cookingis certainly more universal, there are large numbers of people in the world who use far more fuelfor space heating. One way of developing a global perspective of the importance of space heatingwould be to combine maps of thermal data integrated over all the seasons with maps of populationdensity. In addition to this, measurements need to be made of the relationship between temperatureand the consumption of fuel for space heating.

    2) The performance of a properly designed field trial for an improved stove, withmeasurements being made in different communities, and at different times of year, both of the fuelinput and the operational output of an improved and a traditional stove. As far as is known thishas not yet been carried out in any ICP. A comprehensive statistical analysis of the resultswould reveal what measure of operational output was correlated most closely wi th themeasurements of fuel consumption, as well as the variability in the results caused by various otherfactors. This information would be of great value in the design of later field trials.

    3) An analysis of the relationship between the results of water heating tests, cooking testsand field trials. If it could be shown in a number of cases that, properly interpreted, and possiblyweighted, the results of water heating tests or cooking tests could provide a reliable indication ofthe results o f a field trial, then much time could be saved. At present all that is known is thatlaboratory tests always indicate much larger savings than ever seem to be achieved in practice.'4

    13A few field trials have been carried out, but apart from that referred to in #7 above, which was reported byAtapattu et al (1989) in Sri Lanka, there do not appear to be any which have attempted to record changes in output.An extensive field trial reported by the World Bank (1987) was carried out in Niger, and although an average savingof 12% was calculated for the improved stove, no proper statistical analysis of the data was carried out. As far as isknown, no attempt has yet been made anywhere to measure the variation in fuel consumption and savings withseasonal changes.The correlation between the results of WHTs, cooking tests and field trials will vary between regions andcommunities.11 is not being suggested here that the same weighting factors be applied universally. If, however, areasonably close correlation can be found in a number of communities between the results of field trials and theweighted results of specific tests, then the method could be applied more widely and much time saved in the designand pilot phases of ICPs. It is important to emphasise that this is being recommended as a topic for further researchand not, at least at present, as a method of evaluating fuel savings in ICPs.

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