[Stoves] Examples of results of simmer efficiency Re:[Ethos] Additional presentations at ETHOS 2015

kgharris kgharris at sonic.net
Mon Feb 16 02:29:12 CST 2015


Crispin,

Thank you for the effort you put into your response.

If I put a pot of water on the stove and turn the heat all the way up, the water boils.  If I turn the power level down the water stops boiling and cools down to a point of equalibrium, heat in equals heat out.  If I put a pot skirt around the pot it increases the heat transfer into the pot and I can turn the power level down further to maintain the same temperature.  This I have done and it is the real world, not theory, mathematics or other calculations, models or descriptions.  This is what the cook in Africa, China, India or anywhere else will experience.  I respect your discussion but I see nothing which tells me to reject the reality that I see with my eyes and measure with my thermometer.  Have you not done these things also in all of your cooking experiences?  Have you not turned the heat down when it was too hot?  One does not need high powered mathematics to do that.

Respectfully,

Kirk Harris
----- Original Message ----- 
  From: Crispin Pemberton-Pigott 
  To: 'Discussion of biomass cooking stoves' 
  Sent: Sunday, February 15, 2015 10:06 PM
  Subject: Re: [Stoves] Examples of results of simmer efficiency Re:[Ethos] Additional presentations at ETHOS 2015


  Dear Kirk

   

  You cannot make a determination of the heat transfer efficiency to a hot pot at low power using evaporated water and fire heat only. 

   

  Any twiddling of the numbers will not make it possible. A determination of the heat transfer efficiency at low power, yes you make it, but not if the pot is already boiling hot.

   

  Here is why:

   

  A cold pot on a fire absorbs heat and the losses from the sides are minimal and have a very small influence on the actual result. The actual result is the heat available to the pot divided by the heat received by the pot. The heat getting into the pot includes heat lost from the sides and the heat that changes the temperature of the water and pot material, and the heat used to evaporate water. 

   

  If the pot is cold, the temperature rise is a very accurate measure of heat gained. The loss from evaporation (which starts at about 70 C) can be added, but the losses from the side start to rise in proportion to the temperature of the pot. 

   

  The WBT's approach is a problem:

  When the pot is already boiling, the losses from the sides are at maximum.

  The loss from temperature change is nil.

  The loss to evaporation is measured by determining a change in mass and calculating the number of Joules needed to evaporate the water.  The change in temperature is considered, but the side losses are not.  

   

  When the stove is simmering, especially when it is simmering at just the right energy level to offset the losses from the sides of the pot, the size of the side losses are the majority of all heat gained by the pot, and the evaporative losses are minimal. The unmeasured component greatly exceeds the measured component. That is BAD.

   

  You have described the stove with a better turn-down ability as a 'better stove' operating at a lower power level.

   

  But calculating the 'efficiency' or 'heat transfer efficiency' using the missing water and the fire heat gives a completely misleading result. The better the turn-down and the more closely the fire matches the losses from the sides of the pot, in other words what you described as a 'better stove', the worse the reported efficiency.  That is REALLY BAD.

   

  The less the stove can be turned down, the higher the fuel consumption. If the heat transfer efficiency is actually the same, the WBT will report an increase over the efficiency that is reported for lower power. That is ALSO BAD. The fact is we don't know what the heat transfer efficiency is because we are not measuring it - only the heat lost to evaporation divided by fire heat. 

   

  Thermal efficiency can be determined with reasonable precision for a hot fire and a cold pot. Measuring it with a hot pot at low power gives very misleading results.

   

  Combining the two, reporting the overall thermal efficiency for boiling and simmering a hot pot is also misleading because of the influence of the invalid simmering number on the valid boiling number. It is easy to determine the heat transfer rate to a pot at high and low power, but not while simmering a hot pot. Just use a cold pot and low power, or any power for that matter. That is what the CSI-WHT does. It does not (yet) use the losses from the sides of the pot but that is coming too. Then the heat transfer efficiency number you like so much will be reported accurately.

   

  The variable 'heat transfer efficiency' is valid, but how it is calculated - not so much these days.

   

  Just because one method miscalculates something does not mean everyone should. And it does not mean it cannot be determined correctly. It just needs a conceptual re-think.

   

  Thanks for raising the questions. More are welcome. We should all understand this.

   

  Regards
  Crispin

   

  From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of kgharris
  Sent: Monday, February 16, 2015 12:24
  To: Discussion of biomass cooking stoves
  Subject: Re: [Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

   

  Paul,

   

  I respectfully disagree.  Two stoves are burning at the same fire power and one has better heat transfer to the pot.  The water in the better heat transfer stove will be hotter because less heat is lost, producing more steam and lower scores.  The better heat transfer stove will have to be turned down to a lower power level to keep the pot the same temperature as the worse heat transfer stove.  That is the whole reason for improving heat transfer into the pot, to allow a lower power level for the same cooking ability.  

   

  The thing which has been shown about the WB simmering test is that it does not work if each stove has it's own seperate simmering temperature and steam production.  It will work if all stoves are adjusted to a fire power that produces the same simmering temperature and steam production for all tests.  Then the lower a stoves fire power and fuel use the better the score, and no punishment for a more efficient stove.  Keeping the simmering temperature and steam production the same for all stove tests allows meaningful comparisons between stoves and good protocol for tier ratings.  The results of keeping the temperature the same will help the stove designer because different designs can then be compared with each other on equal basis, and the more efficient design, balanced with cost, can be chosen.

   

  I must agree with Dean Still on this.

   

  Kirk Harris

   

    ----- Original Message ----- 

    From: Paul Anderson 

    To: Discussion of biomass cooking stoves 

    Sent: Sunday, February 15, 2015 7:30 PM

    Subject: Re: [Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

     

    Kirk,

    Your comment contains one incorrect statement: 

      The stove which gets more transfer of the heat into the pot may cause more steam if it cannot turn down to a lower power level than the lesser stove, but that means that the two simmering temperatures are different.

    Essentially there is only one simmering temperature that is allowed, which is to be never more than 6 deg C lower than the boiling point.   If local boiling point is 100 C, then never less than 94 C, and probably best to keep the temperature of the water at about 97 C.   

    But even a roaring fire cannot raise the temperature above the boiling point.   So the difference in the TEMPERATURE OF THE WATER is of little consequence, being about 3 deg C.   And remember that the purpose or objective of simmering is to maintain the temperature.   The purpose is NOT to minimize the amount of evaporated water, which is only a poor reflection of how much fuel is used.   

    Please also see my next message, that is addressed to Phillip and Dean who have exchanged messages recently.

    Paul



Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  Email:  psanders at ilstu.edu   Skype: paultlud      Phone: +1-309-452-7072Website:  www.drtlud.comOn 2/15/2015 6:41 PM, kgharris wrote:

      All,

       

      I am not understanding why the WBT is invalid if the simmer temperature is held the same for all stoves.  The stove which gets more transfer of the heat into the pot may cause more steam if it cannot turn down to a lower power level than the lesser stove, but that means that the two simmering temperatures are different.  Dean is talking about the test simmering temperature being the same for all stoves.  The stove that gets more heat transfer into the pot will need more turn-down than the lesser stove in order to simmer at the pre-choosen test temperature.  That is the whole reason for improving the heat transfer into the food.  To be able to use lower power levels, less fuel, and fewer emmissions to cook with.  The two capabilities need to evolve together, and both are improvements which can enhance a good field usable stove.  Also simplicity of construction and ease of use are important qualities which need consideration and perhaps some kind of metrics for measurements.

       

      Kirk Harris

      Santa Rosa, CA. USA

       

       

      ----- Original Message ----- 

        From: Dean Still 

        To: Discussion of biomass cooking stoves 

        Sent: Sunday, February 15, 2015 1:35 PM

        Subject: Re: [Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

         

        Dear Philip, 

         

        The Low Power test works well when the fuel use is normalized using a set simmering temperature. 

         

        Sam and I are writing up some characteristics of the WBT and I'll post the paper here. Lots of work to do and I look forward to our continued collaboration.

         

        Best,

         

        Dean

         

        On Sun, Feb 15, 2015 at 11:43 AM, Philip Lloyd <plloyd at mweb.co.za> wrote:

          Dear Dean

           

          Crispin said it well:
          "The three low power metrics are invalid. The variables selected are inappropriately chosen. The calculated results are misleading and contrary to any claim [that] they provide guidance for product development or selection. We have to move on." 

           

          I have looked at the simmering metrics in WBT 4.3.2 and can only concur.  That is why I do not think we should waste much more time arguing about them - they are fundamentally wrong. Yes, stove designers need to be concerned with simmering and turndown; no, the WBT simmering metrics do not provide them with guidance, and can be positively misleading, which is worse.

           

          Kind regards

           

          Philip Lloyd

           

           

          From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Dean Still
          Sent: 15 February 2015 06:38
          To: Discussion of biomass cooking stoves
          Subject: Re: [Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

           

          Dear Prof Loyd,

           

          As I pointed out, when the stoves do the same work (hold the water at 97 C, for example) the stove with greater heat transfer efficiency scores better. Simmering tests are important and simmering is an important part of cooking.

           

          The ISO process is creating new history and approaches to old problems. Whatever emerges will certainly be defensible as the new approaches are forged by consensus.

           

          Best,

           

          Dean

           

          On Sun, Feb 15, 2015 at 12:58 AM, Philip Lloyd <plloyd at mweb.co.za> wrote:

          I am concerned that this is turning into a very fruitless discussion.

           

          On fundamental grounds the simmering test does not provide anything meaningful.  Crispin has demonstrated that rigorously, and others have pointed out that the test can score an efficient stove poorly and an inefficient stove well, so it does not provide any useful measure.  To go on defending the indefensible does not make sense, even if it did accentuate the need for turndown - but that need was always there, it was not the product of the WBT.

           

          We need defensible measures of stove performance.  Can we please turn our attention to developing those, and leave the indefensible to history?

           

          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 Paul Anderson
          Sent: 15 February 2015 02:26
          To: Discussion of biomass cooking stoves
          Subject: Re: [Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

           

          Dear Dean,    my reply is below:

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  Email:  psanders at ilstu.edu   Skype: paultlud      Phone: +1-309-452-7072Website:  www.drtlud.comOn 2/14/2015 1:06 PM, Dean Still wrote:

            Dear Paul, 

             

            To do well on the Low Power Specific Consumption metrics the stove has to have a good Turn Down Ratio. In other words, the stove has to have high power and low power.

          I totally agree with this.   But it is not the whole story of LPSC.   Other factors influence LPSC, especially concerning the measurement of the variables that are used to make the calculation.   These can include the insulation of the pot (incl. skirts), lid on pot, pot characteristics such as size, quantity of water in the pot at the start, and at the finish.

           

          Specific Consumption is based on how much energy was used to create simmered water. 

          Simmered water is not created.   It was already hot at the start of the simmer phase of testing.   We are interested in how much energy is used to MAINTAIN the required temperature near boiling, but preferable about 3 degrees C lower than that boiling temperature.   In fact, a super-insulative pot could need barely a flicker of a flame, and therefore even a well turned-down stove could cause the water to boil and evaporate.   

          If the stove only operates at high power there is more steam made and [at the end of testing] less simmered water remains....

          that is true.   but continue.

          ..... so energy was used to create less product.

          Stove simmering is not creating a product.   It is maintaining a temperature.   The steam that is driven off does not represent loss of "product" which should be understood to be "cooked food" (and not meaning water that can be added to the pot by any attentive cook in a household.)

           

          I like Specific Consumption because it forces stove designers to make stoves that simmer successfully, not just boil water. 

          I agree.   But the measurement procedures need to accurately document the ability to have that strong turn-down ratio, without calculations that can yield ambiguous or mis-leading results.

          For example, new TLUDs are better stoves because they have both high power and low power. In my opinion, the WBT 4.2.3 helped to create these more successful TLUDs.

          The cause-and-effect relationship is not totally clear.   We have wanted turn-down capabilities in TLUDs for many years.   

           

          As Sam says, we are working on a paper showing characteristics of the WBT 4.2.3 for the ISO work. Knowing the characteristics lets folks evolve a perfect test. 

          I question the above wording to "evolve a perfect test" (which is a test run, not the test procedures.)   Maybe the statement should be that "knowing the characteristics let's folks operate their stoves in special ways to obtain superior results that are not realistic for average users."  OR "... let's folks 'game the metrics' to present 'perfected' test-results BASED ON OPERATIONAL PROCEDURES AND NOT ON IMPROVEMENTS TO THE STOVES THEMSELVES." 

          OR it could be that flawed protocols /procedures (such as dividing by the volume of remaining water after simmering) can yield numerical results that are questionable and perhaps disadvantageous to the development of clean cookstoves. 

           

          Sam is doing great work as he crunches all the data....

          absolutely.   But we are questioning if the numbers are as valid and useful as claimed.

           and gives ISO real numbers to work with in their discussions.


          Concluding statement:   The topic of Low Power Specific Consumption is too important to just brush aside the stated issues.   More "expert testimony" would be useful, including a mathematical analysis of the impact of the parts of the calculations.   

          Paul



           

          Best,

           

          Dean

           

          On Sat, Feb 14, 2015 at 8:18 AM, Paul Anderson <psanders at ilstu.edu> wrote:

          Dear Tom H.,         and to all who are interested in proper testing of stoves.

          Your reply about your experiences is helpful.   Sounds like you had qualified testing center do the testing, in accordance with the procedures that Crispin is questioning.   Please send to me the full details.   Could be off-list, but this is sufficiently important that we will want the full results known.

          I have a specific case of official testing of one of my stoves with unfavorable results for Low-Heat Efficiency (simmering).   I will add that into the list of examples and provide the details very soon.

          I invite anyone else who has something to report about simmering efficiency to also send details of their experiences, either favorable or unfavorable or neutral.  

          The examination of the questionable methods about simmer efficiency might take some days, maybe weeks.   But not the months or years that this debate has been "simmering".   

          Remember:  A testing center that properly conducts testing using an endorsed but possibly flawed procedure is NOT a culprit.  The culprit is the testing protocols, IF found to be faulty.   And we hope that the testing center people (employees and leaders) who understand the technical aspects of the calculations will be among those who can help resolve these serious issues.

          Even those who developed protocols that are eventually shown to be faulty are not culprits.   Mistakes can be made.    However, the culprits can include those who advocate a protocol that he or she knows (or reasonably suspects) to be faulty.

          Paul 

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD  Email:  psanders at ilstu.edu   Skype: paultlud      Phone: +1-309-452-7072Website:  www.drtlud.com 

           


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