SOME ISSUES IN TESTING OF BIOMASS COOKSTOVES

K. B. Sutar, M. Ilyash, S. Kohli* & M. R. Ravi

Department of Mechanical Engineering

Indian Institute of Technology

Delhi, India

Introduction

Methodology used

Uncertainty in cookstove testing

Results of tests on cookstoves

Implication of the analysis on protocol design

Outline of the Presentation

Testing of stoves continues to be an important area for scientists and researchersTwo conflicting requirements of a good testing protocol:

• Repeatability in the lab measurements• Need for the test results to be representative of the

field performance

Introduction

The approach suggested by K. Krishna Prasad (1985) to resolve the conflict• Ensuring repeatable measurements in lab.• Determining efficiency (η) versus fire power (P) as

performance characteristics.• Combination of the above can be used for performance

prediction in the field.

Focus of the present work• Identifying the main contributors to the high uncertainty in

cookstove test results with a view to improve repeatability.• Developing a methodology for obtaining η Vs P curve

Introduction

A commercially available forced draught stove with fan regulator was tested using BIS (with minor modifications), WBT 3.0 and EPTP protocols.

Each test was repeated three to four times.Only thermal performance was measured (No emission measurements).Detailed uncertainty analysis was carried out to identify the main contributors to uncertainty.

Steps identified to reduce the uncertainty

Methodology Used

Source of uncertainty • due to measuring instruments

• due to inherent variability in the basic phenomena

(combustion, heat transfer, etc.) and method of conducting

experiments

•Baldwin (1988) first reported statistical analysis using student's t-test

in cookstove testing.

Uncertainty in Cookstove Testing

Student's t-test analysis

•Let, 1, 2, 3…….n be the efficiencies at cold start phase

of WBT for the cookstove.

•Arithmetic mean of efficiency,

•Unbiased or sample standard deviation

•Confidence interval gives the probability that the mean

value of efficiency lies within a certain number (t) of

sample standard deviation (s) which gives,

1 2 ..... n

n

1/22

1

( )

( 1)

ni m

i

sn

1, /2 [at (1- ) 100% confidence level]nt s

n

•The variable t, for different degrees of freedom

(n- 1) and levels of confidence can be found in the

tables available in literature on statistics.

•The number of degrees of freedom is given by,

= n-1. For example, at 95% confidence interval and

at n = 3 i.e. for = 2, the value of t is 4.303.

Student's t-test analysis

•Tests were conducted using

modified BIS protocol.

•Cookstove was run at different

fire powers by regulating fan

speed and corresponding fuel

burning rate.

•Each test was conducted

thrice.

Maximum stove efficiency in the range of 2.3 kW to

3.5 kW fire power.

Results of Tests on Cookstove

•To determine whether the test should end:• when the water reaches at a specified temperature (WBT 3.0) or when

the fuel burns completely (BIS).

• WBT shows higher uncertainty in thermal performance than BIS• WBT results must be reported separately for different phases:

(cold start, hot start and simmering)

When Should a Test End?

•Efficiency of the cookstove, η = f (mi, mf, Ti, Tf, mb, mc, CVb, CVc)

•Uncertainties in masses of water, biomass and charcoal and temperatures of

water viz. dmi, dmf, dmc, dmb (kg); dTi, dTf (C); CVb, CVc (kJ/kg)

•Efficiency of the cookstove, = Eout/Ein

•Differential uncertainty,

•Where,

•Energy supplied by combustion of fuel, Ein = mbCVb – mcCVc

•Differential uncertainty in energy supplied,

1/22 2

out inout in

d dE dEE E

1

out inE E

2out

in in

E

E E

1/22 2 2 2

in in in inin b b c c

b b c c

E E E EdE dm dCV dm dCV

m CV m CV

Contributions to Uncertainty in Efficiency

•Energy utilized for boiling of water, Eout = miCp,w(Tf – Ti) + (mi - mf)hfg

•Differential uncertainty in energy utilized,

•Experiment: Cold Start Phase, WBT: Vessel with 2.5 Liters of water.

•Contributions to uncertainty in thermal efficiency : (29.95 7.96)%

1/22 22 2

out out out outout i i f f

i i f f

E E E EdE dm dT dT dm

m T T m

•Fire power of the cookstove, P = f (mb, mc, CVb, CVc, t) also P = Ein/t

•Differential uncertainty in fire power,

Where,

•Experiment: Cold Start Phase, WBT: Vessel with 2.5 Liters of water.

•Contributions to uncertainty in fire power: (2.94 1.52) kW

1/22 2

inin

P PdP dE dt

E t

1

in

P

E t

2inEP

t t

Contributions to Uncertainty in Fire Power

•About 92 % of the uncertainty in efficiency is contributed by the

uncertainty in the final mass of water .

• Water should not be allowed to vaporize, i.e. heat the water

well below boiling point and use a pot lid.

•To reduce uncertainty in fire power, fuel should be consumed fully

or till only charcoal is left.

• Leftover charcoal must be accounted for.

•Rather than heating fixed quantity of water to a fixed temperature,

it is better to burn a fixed quantity of fuel completely and transfer

heat to vessels with known quantity of water.

Implications of Analysis on Protocol Design

•Tests conducted on cookstove using WBT 3.0 protocol.

•Higher stove efficiency for vessel with 5L as compared to that with 2.5L .

•Fire power should be independent of quantity of water.

• To know the reason behind lower fire power during 2.5L WBT, more

tests required to be conducted.

Effect of Quantity of Water

•Tests conducted on cookstove using EPTP protocol at different fuel feeding rates.

•No clear conclusion can be drawn due to large uncertainties associated with

the results. Need for more tests.

•In general, feeding the fuel disturbs the combustion, and larger the feed,

greater is the disturbance i.e. lower uncertainty is expected for continuous

feed.

Effect of Fuel Feeding Rate

•This statistical technique can be used to identify the better one from a

pair of samples subjected to identical operating conditions.

•Let Xi and Yi be the efficiencies of cookstove at X and Y feeding rates

respectively for ith test.

•(X1,Y1), (X2,Y2)……..(Xn, Yn) be the n pairs of the efficiency data.

•Let Di = Xi –Yi … difference between the efficiencies for ith test.

•Let D1,…. Di…. Dn be a small random sample of differences of pairs.

•If the number n is small (<30), then the level 100(1-)% confidence

interval for the mean difference D is given by

Paired Data Analysis

1, /2D

D n

sD t

n

The effect of fuel feeding rate in WBT on cookstove efficiency :• Pairs compared: (continuous feed Vs 100g)

(continuous feed Vs 50g)

(100g Vs 50g)

Comparison between 100g and 50g fuel feeing ratesn = 4 Cold Start Hot

Start Simmering WBT Cycle

η100g (X) 27.83 23.03 35.26 28.84η50g (Y) 21.43 27.36 32.89 27.35

Diff. (D = X-Y) 6.41 -4.33 2.37 1.491.484

Sample Std. Dev. sD 4.427

tn-1, 0.025 3.182tn-1, 0.025sD/sqrt(n) 7.043

D @ 95% CI 1.484 ± 7.043 i.e. (-5.559, +8.527)

D

•At 95% CI, mean difference in efficiencies is more on positive side hence efficiency of cookstove is better with 100g fuel feeding rate than with 50g fuel feeding rate.

Paired Data Analysis

Comparison between continuous and 50g fuel feeing rates

n = 4 Cold Start Hot Start Simmering WBT Cycle

ηcontinuous (X) 26.86 29.07 32.38 29.78η50g (Y) 21.43 27.36 32.89 27.35

Difference (D = X - Y) 5.43 1.71 -0.51 2.432.266

Sample Standard Dev. sD 2.451

tn-1, 0.025 3.182

tn-1, 0.025sD/sqrt(n) 3.900

D @ 95% CI (2.266 ± 3.9 ) i.e. (+6.165, -1.634)

D

•At 95% CI, mean difference in efficiencies is more on positive side hence efficiency of cookstove is better with continuous fuel feeding rate than with 50g fuel feeding rate.

Paired Data Analysis

Comparison between continuous and 100g fuel feeing rates

n = 4 Cold Start Hot Start Simmering WBT Cycle

ηcontinuous (X) 26.86 29.07 32.38 29.78η50g (Y) 27.83 23.03 35.26 28.84

Difference (D = X - Y) -0.98 6.04 -2.87 0.940.782

Sample Standard Dev. sD 3.835

tn-1, 0.025 3.182

tn-1, 0.025sD/sqrt(n) 6.101

D @ 95% CI (0.782 ± 6.101 ) i.e. (6.883, -5.319)

D

•At 95% CI, mean difference in efficiencies is slightly on positive side hence efficiency of cookstove is marginally better with continuous fuel feeding rate than with 100g fuel feeding rate.

Paired Data Analysis

Uncertainty in cookstove performance data can be minimized:•by not allowing water to vaporize, and by using a lid during testing.•by completely consuming the fuel, and accounting for any remaining charcoal.

Statistical analysis of stove test data helps in•identifying the largest contributor(s) to the uncertainty and hence minimizing the contributions(s)•comparing the influence of different parameters on the cookstove performance.

Conclusions

Feasibility of determining η Vs P characteristics for the stove has been demonstrated

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