[Stoves] Testing Cookstoves: Autocorrelation and White Swans

[Crispin Pemberton-Pigott]

Dear Julien and All

 

Your comments are well understood. 

 

I think it should be borne in mind that there are two different, maybe quite different, purposes for testing. One of the development of a product and the other is for creating a rating for the purposes of regulation and trade.

 

The development of a stove product usually involves performing tests that are narrowly focussed on examining some aspect of the performance. These can be quite general in nature like funding out if a new configuration is ‘much and obviously better’ or it can be a careful examination of something like the heat transfer efficiency from the gases to a receiving vessel or tank.

 

There are many metrics that are only used for product development and would make a poor metric for rating performance for the government to regulate.

 

An example that is commonly misapplied by stove designers is heat transfer efficiency.  Because of the strong influence on the science of stoves by the coal combustion people who operate power stations, there are significant problems with making this determination. The problem is that power stations run continuously and burn all the fuel all the time. The coal is powdered and blown in and all of it burns within a couple of seconds. The fuel analysis in the lab is identical to what is being burned. This is not the case at all with biomass stoves, and also not the case with coal burning stoves. 

 

Small burners are started, run and stopped. Determining the heat transfer efficiency means trying to work out what just burned. The standard fuel analysis and bomb calorimeter are not all that helpful when trying to do this. The fuel is constantly changing and it is difficult to work out ‘what just burned’.  Not known what just burned means not knowing the heat available and therefore the heat transfer efficiency remains a guestimate, not a measurement.

 

Ina power station, heat transfer efficiency equals fuel efficiency because whole fuel is burned all the time.  Increasing the heat transfer efficiency 1% can reduce the fuel consumption 1% (actually 1/101). This is not the case for biomass stoves because they almost never burn the whole fuel and inevitably, some of the fuel is thrown away. In a power station this is called a mechanical loss and they measure the carbon in the ash. It is produced continuously so the loss can be factored into the system efficiency. We can’t do that with biomass stoves so we need to take a fresh look at what to measure.

 

Everyone knows that increasing the heat transfer efficiency could reduce the fuel consumption however that is an assumption that has proven to be untrue too much of the time. In particular the development of stoves that make char, a hot topic on this list for several years now, skews the ‘fuel consumption’ measurement so much as to cause mirth.

 

We have stoves that show up for rating at the lab with claims of fantastic fuel consumption but which use as much fuel or more than an open fire. By that I mean they use more raw fuel than an open fire to cook a standard meal. That is not a fuel saving, so where does the claim originate?

 

We have discussed this at length on this list with Ron Larsen taking the position that the energy used should count, not the mass of raw fuel needed to perform the task. TLUD’s making char are an extreme example of a stove that is energy efficient and fuel inefficient.  It therefore matters a lot what the consumer of the test wants to know.

 

If the consumer is a designer who wants to know if the new architecture is doing a better job of transferring heat, then they want the heat transfer efficiency. Stove promoters want to know the fuel efficiency and it is not the same thing. Not by a long shot, these days.

 

The WBT reports a number that is not the heat transfer efficiency nor is it the fuel consumption. It is similar to the heat transfer efficiency in that it attempts to measure the energy released by the fire in a primitive manner, and the heat gained by the contents of the pot, but not the pot itself, nor the losses from the pot by radiation and convection.

 

What is it measuring? It tries to measure the fire to pot heat transfer efficiency with a lot of caveats for both elements of the fraction. Remember an efficiency is a ratio.

 

In the numerator there is the heat gained. Several test methods use the ‘sensible heat gain’, meaning the heat that caused a change in enthalpy (total energy) detectable using a thermometer. It can be ‘sensed’. Energy than changes food chemically is hard to detect so it can be calculated from a chart, or ignored for testing purposes whatever its magnitude.  The pot absorbs heat and in some places the pots are heavy: copper or brass of cast iron or cast aluminum.

 

The Indian, HTP and probably other tests consider the energy transferred to the pot because it can be used later when the food is simmering. A No 25 cast iron pot common at schools weighs 61 kg empty. That is a considerable thermal mass. Cp means Specific Heat (heat capacity per degree Kelvin) with units ‘Joules [J].

 

The heat gained by a pot (which may have evaporated some liquid we assume to be water) is:

 

Pot Cp [J] x Pot mass [g] x (Final temperature [T2] – Initial temperature [T1]) + Water Cp [J] x Water mass x (Final temperature [T2] – Initial temperature [T1]) + Latent heat of evaporation [J] x evaporated water mass + 2.44 [J] per g of water evaporated per Deg K less than the boiling point when it was evaporated.

 

For a 1000 g aluminum pot with 5000 cc of water in it that evaporated 5 g at 90 degrees and 50 g at 100 deg (approximately) you get:

 

0.9 x 1000 x (100-20) + 4.186 x 5000 x (100-20) + 2257 x (50+5) + 5g x 5° x 2.44 J

 

That is the sensible heat gained. There is, as discussed before, heat that is gained by the pot and lost without being detected. It can be determined by watching a pot cool on an insulated base. It cools according to the rate of heat loss.  Having a ‘good guess’ at this value is helpful. The heat loss is determined with a thermometer and the thermal mass of the pot and contents.

 

The denominator is the heat available to the pot. There are several choices. The heat in the gases passing the pot is how a power station engineer would determine the heat transfer efficiency – it they were looking for an improvement in that aspect alone.  It is hard to measure.

 

The heat available from the fire has to consider the fuel components that were burned at the time – perhaps heavy on H2 in the early stage and heavy on the Carbon towards the end.  Assuming you know what is burning at the time, it should be compensated for carbon monoxide and hydrogen because those are chemical losses and are a direct result of the stove not burning the fuel properly. These losses can be significant. If you really want to know the heat transfer, you have to know how much heat was available.  

 

Next you might want to know, based on the fuel burned, what the potential heat was and then take that potential as the heat ‘available’. The WBT uses this value.

 

Finally there is the heat content in the fuel consumed whether burned or not. That is the value used as the system efficiency denominator in the HTP. In fact it separately reports a reasonably calculated heat transfer efficiency and the system efficiency. I like that because it informs the nit-picky designer and the policy manager who think ‘efficiency’ means fuel efficiency.  Only the last number, heat available in the fuel consumed, is analogous to the power station ‘thermal efficiency’.  

 

Stated clearly: The Heat Transfer Efficiency is not equal to the fuel efficiency. The System Efficiency is equal to the fuel efficiency. In YDD test the HTE was 51% and the SE was 33%. Why such a big difference? Because that stove makes a lot of leftover char it cannot burn and takes a lot of new fuel to cook the next meal.

 

A designer over-focussed on the heat transfer efficiency can be misled into thinking they are ‘winning’ when they are not.  The heat transfer can be improved by lowering the pot and choking the fire creating oxygen starved conditions in the combustion chamber that result in a large increase in the char production even to the point of blocking the flow of air. Calculating the heat transfer efficiency only ‘looks better’ but a simultaneous calculation of the system efficiency will show that the raw fuel consumption increased, not reduced.

 

Some stoves tend to create a relatively large amount of char. This applies not only to TLUD’s but stick burners and top loaders as well. An interesting observation is that the Rocketworx stove and the Rocket Stove make quite different amounts of char when using the same fuel. That is an example worth looking at. The Rocket stove has ‘overhanging fuel’ sitting on a shelf. The Rocketworx stove has a grate in the form of an inverted Vee with slots that allows air to get under the char and burns it almost completely as it is formed.  The Improved Keren being developed at YDD in Yogyakarta uses a similar idea but implemented in a very different way. 

 

For any given cooking task, the system efficiency of the Rocketworx stove will use less fuel than a Rocket stove because it doesn’t make char. However using the WBT as a measurement method, the energy needed is about the same. That is a good case study for this topic.

 

Regards

Crispin

 

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Julien Winter
Sent: Thursday, February 19, 2015 00:42
To: Discussion of biomass cooking stoves
Subject: [Stoves] Testing Cookstoves: Autocorrelation and White Swans

 

I haven't got as far as water-boiling tests yet, because I have been working on fundamental issues of burner design, and that takes a while.

 

However I can see a some problems with the way stoves are being tested.

1)  Measurements made over the course of a run are autocorrelated.

2)  Proper testing of a stove involves a range of fuels.

3)  Test to find the boundaries of failure, not success.

In summary, proper testing of a stove, prior to manufacturing thousands, or making it an exemplar for millions can't be done with one test.  It takes many tests; tests that try to find out not only where the stove succeeds, but most importantly, where it fails.

 

1)  AUTOCORRELATION and Correlation vs. Independent Observations.

 

Autocorrelation, or serial correlation, is a statistical term use to say that two observations, for example, of temperature, are not independent, because they are related in space or time.  Autocorrelation can be a good thing to study if you are looking at spatial patterns in soils, but it can be a problem if you are trying to measure properties of a stove.

 

If we are trying to measure energy transfer during boiling, followed by energy transfer during simmering, all in the same run, then these two measurements of energy transfer will be autocorrelated.  They are autocorrelated, because there history to the combustion reaction, especially in a TLUD.  In a TLUD, the depth of char increases over time, and changes in temperature.  This change can alter the chemical composition of the pyrogas.  In TLUDs and other stoves burning thick pieces of fuel, char combustion can increase over time.

 

Now it may be that boiling followed by simmering is so common that energy transfer from fire to pot over this the sequence should be measured.  However, if we a primarily interested in how efficient energy is transferred at different power levels, then having a separate run for each power level would make the observations at different power levels independent of each other.

 

There are actually two different turndowns to measure:

a) the turndown of fuel consumption rate

b) the turndown of energy transfer rate to a pot

 

2) A RANGE OF FUELS

 

It is important to test stoves over a range of fuels, because they behave quite differently depending on moisture content, volatile content, particle thickness and shape.  In a ND-TLUD the fire in wood chips invariably, channels; with thick fuel (e.g., sticks) there is char combustion on the surface while the interior pyrolysis; and, if air spaces are vertical then a very strong draft develops in the fuel bed.   Channeling of the ignition can increase as primary air is cut back, especially in wood chips.   

 

Across all these fuels there is >5 fold change in ND-TLUD gasification rate.  In other words, turndown is not properly represented by a single fuel.

 

3) TEST FOR FAILURE NOT SUCCESS

 

Critical testing of stove should try to find where it fails.  Although it is useful to see where a stove succeeds, repeated observations of success is not critical testing.  

 

Scientist are encouraged to design critical experiments that reject hypotheses, not confirm them.  If, under critical test, our hypothesis is not rejected then it is probably true.  We owe this line of reasoning to the philosopher, Karl Popper.  Queen Elizabeth (of England) gave him a knighthood, therefore, he must the right!!

 

In the case of cookstoves, we should cover a range of conditions (fuels and turndowns) to see where they fail.  Using a single fuel is not subjecting a stove to critical testing.

 

To use a more simplistic example, let us say that a European looks in the sky and sees only white swans, and comes up with the hypothesis that "all swans are white".   To confirm the hypothesis is not the way to go.  We can count thousands of white swans in the skies of Europe, and we still haven't put our hypothesis to a critical test.  We have to devise a circumstance or an experiment, where our hypothesis could fail.  So lets search all corners of the Planet to see if we can find a non-white swan.  Lo, in Australia, the swans are black.  The crucial point here is that in our critical test, we only had to see one black swan for us to reject our hypothesis that "all swans are white"; all the thousands of white swans, previously seen, now count for naught.  

 

 

To test cookstoves (i.e., ND-TLUDs) on wood pellets is to count white swans.

 

 

To conclude:  before a cookstove is promoted as an exemplar, or sold by the thousands, there is a lot of testing to be done to characterize stove performance over a range of conditions.  One water-boiling test just doesn't make the grade.

 

Cheers,

Julien.

 

P.S.:  Scientists must really get inventors and engineers pissed-off.  Not only do the want to try to break the stove, they want to do it four times so that they can say, "Yup, I am 95% sure that your stove is broken."

 


-- 

Julien Winter
Cobourg, ON, CANADA

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150219/affc4b79/attachment.html>