[Stoves] The need to continue the discussion: simmer efficiency

Thu Feb 19 02:42:11 CST 2015

Thank you, Jiddu

You suggest “I believe an external review (scientific: engineers, physicists, mathematicians) would be the most suitable option.” I originally took an interest in this question as an external reviewer, who had worked on liquid-fuelled appliances but had had no experience of solid fuelled stoves except coal stoves, and in that area I had only been involved in emissions reduction. So I looked at the curious results from the low-power tests and identified the fundamental problem – which, as you so succinctly and correctly say, yield a figure which is “the energy released by the combustion divided by a random number. “ This is the primary problem with the WBT – it is absolutely and fundamentally incorrect in this area. I have some problem with other aspects, but they are practical problems not fundamental.

Prof Philip Lloyd

Energy Institute

Cape Peninsula University of Technology

PO Box 652, Cape Town 8000

Tel:021 460 4216

Fax:021 460 3828

Cell: 083 441 5247

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Jiddu Broersma
Sent: 19 February 2015 09:57
To: stoves at lists.bioenergylists.org
Subject: [Stoves] The need to continue the discussion: simmer efficiency

Dear All,

First,

Paul, thank you for keeping the conversation going with great intentions. Much appreciated.

I have been a quiet follower of the stovelist and I just caught up on the simmering discussion. I believe it is a necessary discussion.

I would like to share my opinion. It is one from the perspective of a stove manufacturer and a theoretical physicist. 
I do this purely to contribute to the discussion. Although I will formulate my opinion direct with strong language, I want to assure you all that I value everyone's opinion equally.

Now, I will jump straight into the low power metrics.

Simmering
As has been explained, simmering is a 'zero work' task, it does not have an efficiency. Evaporating water and overcoming heat losses require work, they can have an efficiency. However, is anyone interested in knowing the efficiency of evaporating water? Or is anyone calculating how much energy is lost from the pot?

Either way, simmering is only a term that we will hopefully agree on. Most important is to understand the meaning of the metrics that are calculated: Low power specific fuel consumption, Low power CO and Low power PM.

Low Power Specific Fuel Consumption

>From the WBT sheet I find that (equation)

Low power specific fuel consumption = (weight fuel consumed * calorific value of fuel ) / (weight of water remaining * minutes of simmering * 1000)

To explain in steps: 

1.	Weight of fuel consumed divided by minutes simmering is the burn rate.
2.	If we multiply this by calorific value we get the energy released by the combustion per minute. Let's call this the 'energy rate'.
3.	If we divide this weight of water remaining we just divide it by a random number that has no meaning. Keeping more food/water hot does not require more energy. (I believe it does the contrary, because volume grows faster than surface when you increase quantities)

-> We have the energy released by the combustion divided by a random number.

If we wish to calculate some kind of efficiency number we require useful energy (into pot) divided by used energy (from combustion). What we have calculated is nothing close to this.

We could possibly calculate the energy into evaporation and heat loss from the pot, but we have no interest in this. 

Hence, a simmering task can't give us a useful efficiency type of number!

That was me as a physicist, as a employee of a manufacturer I am concerned that stoves are unfairly compared. Because the system can be manipulated to gain better results by using a pot with larger volume capacity and higher insulating properties. 

Also another big issue is that more efficient stoves can have lower rating at the same power output. I'll explain by example (as others have done before me):

Two stoves are equal except that one has better heat transfer efficiency. When they both run at the lowest power possible (which is the same power for both), the one with the better heat transfer efficiency will have likely evaporated more water because more energy went into the pot. Do to more evaporation it will end up with a rating that is worse!

Low power Emissions
Both PM and CO are given in 

weight  / (minute * liters of water)

Similar breakdown:

Weight of emissions per minute is straightforward.

Dividing this by number of liters in the pot is simply dividing the emissions by a number of your choice (the liters you fill in the pot). It has no value to stove rating.

Generally I think that weight of emissions per minute is not a bad metric. However, simmering is not a task that can be compared fairly between two stoves because it is not a specific task! If we can't compare the way two stoves simmer because we know nothing about the useful energy that went into the pot we are not allowed to compare the results because it means nothing without enough information.

Wrap-up

The list of complications that result from these invalid simmering metrics goes on. Please read again Crispin's and Philip Lloyd's comments for a more comprehensive list. 

Worst is that many manufacturers are optimizing their products using the WBT and that stoves are not actually improving in the field. The WBT can result into the production of bad stoves for millions of already suffering women! 

I find myself in the position where I have to tell my company to produce stoves with lower rating because it will be better for women that we build them for.

It is not a matter of right or wrong, it is a matter of how we stimulate the production of stoves that are really improving lives. 

My opinion is that we need to review the meaning of metrics at fundamental level. (Ie. Boiling and simmering are not scientific ways of describing a state of cooking for stoves).

Due to the differences in opinion in the stove world, I believe an external review (scientific: engineers, physicists, mathematicians) would be the most suitable option. The review could simply explain the physical meaning of all calculated results. 

Best regards,

Jiddu

Jiddu Broersma

Technology and Organization Officer

www.praktidesign.com <http://www.praktidesign.com/> 

Spirit Sense, Old Auroville Road

Bommiyarpalayam

605104 Tamil Nadu

INDIA

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