Stove Design & Performance Testing Workshop

Transcription

1 The University of California at Berkeley s Energy and Resources Group and School of Public Health Stove Design & Performance Testing Workshop WHO IAP Workshop Kampala, Uganda June 17 th, 2005 Presented by Peter Scott ( apropeter@hotmail.com) With special thanks to Mike Hatfield and Rob Bailis With protocols developed by Rob Bailis, Dean Still, and Damon Ogle with input from Dr. Kirk Smith and Dr. Rufus Edwards

2 Simplified stove theory Wood doesn t burn Wood gets hot and releases volatile gases that then combust For this to happen we need to have sufficient time temperature and turbulence If wood is heated to 650 degrees Celsius (and sufficient oxygen is mixed with the volatile gases) the result is complete combustion. The products of clean combustion are CO2, water vapour and heat. A lot of heat, roughly speaking, dry wood has half the energy per kg as gasoline, Smoke is wasted energy

3 What are limiting factors to high temperatures? Challenge # 1 Cool stove body Cool earth the body of the stove or of the earth robs heat from the fire which lowers combustion temperatures which decreases efficiency and increases smoke

4 What are limiting factors to high temperatures? Challenge # 1 Cool stove body Cool earth the body of the stove or of the earth robs heat from the fire which lowers combustion temperatures which decreases efficiency and increases smoke Solution? Insulate the stove with low mass, heat resistant materials in order to keep the fire as hot as possible Remember mass is the opposite of insulation Effective stove insulators are pumice, vermiculite, and wood ash Dense things such as earth,sand, cement, water and cast iron are poor insulators

5 Maximizing combustion efficiency Challenge #2 Cool wood which lowers combustion temperatures which decreases efficiency And increases smoke

6 Maximizing combustion efficiency Challenge #2 Cool wood which lowers combustion temperatures which decreases efficiency And increases smoke Solution? Meter the fuel! Use small sticks whenever possible Maximize the surface area of the wood exposed to coals Heat only the fuel that is burning Burn the tips of sticks only as they enter the combustion chamber

7 Maximizing combustion efficiency Challenge # 3 Cool air/ Too much air which lowers combustion temperatures which decreases efficiency And increases smoke Note: an open fire can draw 20 times more than is required for stochiometric (chemically ideal) combustion

8 Maximizing combustion efficiency Challenge # 3 Cool air/ Too much air which lowers combustion temperatures which decreases efficiency And increases smoke Solution? Do not allow too much or too little air to enter the combustion chamber. there should be a minimum excess of air supporting clean burning. Note: an open fire can draw 20 times more than is required for stochiometric (chemically ideal) combustion

9 Maximizing combustion efficiency Challenge # 4 Cool cooking pot The cooking pot is generally no more than a degrees Celsius Flames touching the pot? Soot and smoke!

10 Maximizing combustion efficiency Challenge # 4 Cool cooking pot The cooking pot is generally no more than a degrees Celsius Flames touching the pot? Soot and smoke! Solution? Elevate the pot above the height of the flames This creates an internal chimney which increases draft And gives time for improved air/ fuel mixing

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12 Optimising heat transfer Maximize surface area of pot that is exposed to hot flue gases Maximize velocity of hot flue gases to disturb boundary layer Maximize Delta T. Temp difference between hot gases and pot. Not all fires are the same temperature Exit temperatures not to exceed 180C With a heat exchanger, overall efficiency can be improved by 50% or more

13 Rocket stove heat exchanger/skirt Minimize the gap between the skirt and the pot while maintaining the cross sectional area of the combustion chamber ( for average size pots 1cm is good rule of thumb) Make it as tall as feasibly possible

14 Rocket Stove Combustion Chamber One of the keys to producing a smokeless Rocket stove is to find inexpensive, local, and durable materials for the combustion chamber. In Malawi, we have been blessed to work with Dedza Pottery. They have helped us produce an insulative refractory brick from sawdust grog and clay that is light (0.67 g/cc) and durable. In Lesotho we are using cement fondue with ground and graded waste pumice. In Uganda we are using cut pumice stone.

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16 This open fires use 170 kg of wood to cook corn porridge for 110 people (approx 80.5 kg of cooked food).

17 A visual comparison between the quantity of wood used (170kg) for the open fire vs. the amount of wood used (13kg) by the 100L Rocket stove. Independently tested by EP Lauderdale Tea Estates (Malawi)

18 200L Rocket Stove uses 9.5 kg of wood to cook enough Nsima (corn porridge) for 225 people; Approximately 160 kg less wood to cook twice as much food.

19 A few rocket stove design possibilities

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21 Can chimneys increase IAP? Chimneys burn out or get clogged This will decrease efficiency and eventually will increase IAP. If we have poor combustion with a chimney are we just exporting the smoke to our neighbors? What can be trained? What can t be trained

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23 Rocket L (Uganda) 100Litre WBT PHU Efficiency 49% without chimney Boiled 75 Litres of water in 52 min (no lid) approx 6 kg of wood 100Litre WBT 36% with chimney

24 X Basic Rocket Stove Geometry 1.5-2X TH= X + 1.5X + 5 cm X 2X

25 More household Rocket prototypes In Kenya, a number of modified rocket stoves were made with a shorter internal chimney. More smoke, but guaranteed heat transfer with a pot that fits directly into the stove body In Malawi, it also comes in a 100% ceramic version for the home.

26 Before implementing a improved cook stove project Questions to ask Are people cooking outside or inside? Inside Leave them there! Outside. Leave them there! Prioritize Heat Transfer Eff. C Eff is less important

27 Before implementing a improved cook stove project Questions to ask Are they presently using biomass fuel for cooking? If no or have other fuel options Don t promote biomass ICS! If yes Are they paying for fuel? If no or are very low income Disseminate via teaching circles If yes consider commercial approach Consider User priority e.g. Refugees fuel saving? Smokeless? Clean pots? Social status?

28 Rationale for the SPT Demonstrate impact of ICS projects using methods that are Standardized and repeatable Comparable within and across projects Statistically sound but still appropriate and flexible enough to adapt to local circumstances and constraints! Caveat: Monitoring is important but question of allocation of resources

29 And because everything works

30 Stove Performance Testing (SPT) Past and present In 1985 Vita developed a set of protocols for testing stove performance Functional yet somewhat cumbersome. Not generally used In 2003 Shell/EPA request UC Berkeley and Aprovecho to develop a new set of universally adopted SPT protocols

31 What is Stove Performance? Measures of Stove Performance 1. Efficiency/exit temp 2. Fuel consumption 3. Turn-down ratio (TDR) 4. Speed of cooking 5. User satisfaction 6. Emissions (?) Combustion efficiency Fuel consumption Efficiency Net stove efficiency (PHU) Turn-down ratio User satisfaction Heat transfer efficiency Speed of cooking Ease of use Durability Flexibility Aesthetic appeal

32 Measures of performance 1. Efficiency Entirely lab-based Combustion efficiency - Difficult to measure directly - Can be approximated by measuring PICs (e.g. Smith, Uma et al. 2000) Heat transfer efficiency - Very difficult to measure directly PHU ( Percentage Heat Utilized) - Combustion efficiency CE Overall Efficiency Overall efficiency (PHU = CE x HTE) Heat transfer efficiency (HTE)

33 Combustion efficiency and PHU 1.00 Combustion efficiencies and PHU for 28 stove-fuel combinations in India (each data point is the mean of three tests) Nominal Combustion Efficiency Charcoal Net stove efficiency (PHU) Liquid and gaseous fuels Solid biomass fuels

34 Measures of performance 1. PHU - Fairly easy to measure directly (assuming fuel HV is known) - Energy into the food Fuel energy consumed It is measured by the change in the temperature of the water and the quantity of water that is evaporated So if we compare 2 stoves, the one with the higher PHU is the better stove right? Not necessarily! Because PHU rewards the stove that makes a lot of steam

35 Specific Consumption vs. PHU Stove 1 Time to boil 10 minutes Wood burned 1000 grams Water vaporized 100 grams Water remaining 4.9 liters Stove 2 Time to boil 100 minutes Wood burned 1000 grams Water vaporized 1000 grams Water remaining 4.0 liters Eff= 9.9 % Eff= 21.3%

36 Specific Consumption vs. PHU Stove 1 Time to boil 10 minutes Wood burned 1000 grams Water vaporized 100 grams Water remaining 4.9 liters Stove 2 Time to boil 100 minutes Wood burned 1000 grams Water vaporized 1000 grams Water remaining 4.0 liters SC = 204 grams/litre SC = 250 grams/liter

37 Measures of performance 2. Fuel Consumption Lab-based (WBT) - Very easy to measure - Multiple definitions possible Field-based (KPT) - Ranges from easy to difficult to measure depending on study design and desired outcome. 3. Turn-down ratio (TDR) or Control efficiency Ratio of high to low power Stoves with large TDRs can be operated more efficiently and may be more preferable to users

38 Measures of performance 4. Speed of cooking Can be either lab or field-based Lab-based, easy to measure - Cooking is simulated (not directly predictive of real household use) Field-based - Can be measured directly, but better to rely on surveys

39 Measures of performance 5. User satisfaction Hard to measure, subjective, and dependent on many factors Fuel consumption User satisfaction Speed of cooking Ease of use Durability Flexibility Aesthetic appeal

40 Water Boiling Test 1 - Based largely on VITA (1985) and Baldwin (1986) with small modifications - Limits Variables - Transferable between various projects Lab-based test provides 4 of the 5 indicators of SP: 1. PHU 2. Specific Consumption 3. TDR 4. Time to boil -But its difficult to extrapolate these results to actual field performance without complimentary data from actual users (the CCT and KPT).

41 Overview of the WBT Modifications to VITA s WBT: Specific Consumption defined as the ratio of the total amount of wood used to the amount of water boiled, but was modified for multi-pot stoves. Low power test is done as a separate test, which allows for a more relaxed procedure than VITA s test with minimal loss in accuracy. For the low-power test, the tester keeps the water temperature as close as possible to 3 C below boiling in order to reduce variation in test results (earlier tests used a 5 degree range). Hot and cold starts are used in the high power phase to account for differential performance of stoves that are kept hot throughout the day (important for massive stoves with performance that varies between cold and hot starting conditions). Simmering time in low-power test is longer than VITA, but shorter than Baldwin (45 minutes rather than 30 or 60). This is a compromise: long enough for the stove at low power to establish equilibrium but reduce the time required for the test.

42 Controlled Cooking Test (CCT) - lab controlled test with added variables of an actual cook cooking real food - Only can be used to compare two stoves from a particular - Compares fuel consumption (specific consumption), and speed of cooking - Much better at predicting actual stove performance and fuel consumption in the field

43 Kitchen Performance Test KPT More complex than WBT: - Both qualitative survey and quantitative measurements Takes stove testers into peoples households Sampling procedure and study design are critical Variability in real-world setting increases the number of samples needed to make results statistically valid (more later). Gives daily wood consumption and gauges user satisfaction

44 Overview of the WBT Each WBT consists of 3 parts: High-power cold start High-power hot start Low-power (simmer) And takes roughly 2 hours to complete We recommend 3 tests of each type of stove Sufficient to detect a 30% improvement with 95% confidence if the pooled CV of measurements is 15% and a 20% improvement in PHU if the pooled CV is 10% (more on this later).

45 Over view of Excel sheet

46 Overview of the KPT Also based largely on VITA (1985) with inputs from FAO (1983) and others. Field-based test provides two indicators of SP: 1. Fuel consumption 2. User satisfaction Based on actual measurements from households using the ICS. Principal procedure for Shell HEH projects to demonstrate that they have met their stove performance objectives.

47 Review of KPT procedures Qualitative surveys Include two surveys Pretreatment designed to assess situation in households before use of improved stoves Post-treatment designed to assess qualitative impact of stove on household If group already has surveys that they use, these should provide additional ideas for questions

48 Review of KPT procedures What is included in qualitative surveys? Preliminary survey: Household characteristics Current cooking patterns Who cooks? What type(s) of stove and fuel are used? Who collects fuel? How much fuel is used? Is the family interested in receiving a new stove and/or participating in future surveys

49 Review of KPT procedures What is included in qualitative surveys? Post-treatment survey: Any changes in household characteristics since receiving the stove Use of the new stove and other stoves Stove maintenance Stove performance and user satisfaction Time to cook, smokiness, ease of use, etc.

50 Review of KPT procedures Sample selection for qualitative surveys Issues to consider: Clustering Randomization Number of households Size of community ( or group of communities) Small (less than 300 households) Medium ( ) Large (more than 1000 households) Number of households to be surveyed At least 30 ~10% 100

51 Review of KPT procedures The quantitative field test Main output is average fuel consumption per person Sample selection Similar issues as in qualitative survey but quantitative nature of data makes study design very critical. Clustering? Are there qualities in different communities that affect fuel consumption (climate, wealth, fuel availability, etc.)? If yes, communities should be treated as different populations and tested separately! Randomization? Should be done if possible important to generalize results of tests. Requires census of households within target communities Number of households?

52 Review of KPT procedures Number of households Must decide on cross-sectional or paired sample design Cross-sectional different groups of households are tested using TCS and ICS. Paired sample the same group of households is tested first using TCS and later using ICS Each method has advantages and disadvantages

53 Review of KPT procedures Deciding on the number of households: The variability of the data (how scattered is it?) The difference that you want to detect in average fuel consumption between TCS and ICS The study design (cross-sectional or paired-sample)

54 Review of KPT procedures Deciding on the number of households: Hypothetical results (in kg-wood per person-day) of 3 sets of measurements Traditional stove Improved stove 1 Improved stove 2 Household Household Household Mean = Std Dev = CV = 20% 31% 58%

55 Review of KPT procedures (sample) Traditional stove Improved stove Traditional stove Improved stove 2 Pooled data Pooled Mean = Pooled Std Dev = Pooled CV = % difference in means Sample size (paired test) Sample size (cross-sectional test) TS and IS % -30% 7 14 TS and IS % -30% 14 28

56 Wrap-up of SPT Training Results of our trial WBT COLD START Thermal efficiency PHU% Temp-corr sp consumption grams/litre Firepower kw Open fire Rocket Stove Charcoal HOT START Thermal efficiency Temp-corr sp consumption Firepower Open fire Rocket stove Charcoal SIMMER Thermal efficiency Specific fuel consumption Firepower TDR Open fire Rocket Charcoal

57 Wrap-up of SPT Training Results of our trial WBT Comparison of Thermal efficiency COLD START HOT START SIMMER Comparison of Specific consumption COLD START HOT START SIMMER Comparison of Time to boil COLD START HOT START open fire rocket Charcoal Open fire Rocket charcoal 10 - open fire SuROcket charcoal

58 Rocket Bread Oven 200 kg of wood for 17 kg of bread 5 kg of wood for 17 kg of bread