[Stoves] "78% Efficient"

Thu May 7 16:22:42 CDT 2015

Frank Shields
franke at cruzio.com

> On May 7, 2015, at 1:37 PM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
> 
> Dear Frank
> 
> Good questions. 
> 
>> >>The method is not utterly precise ‎because it uses a hood with volume and pitot tube velocity with compensations for temperature and pressure. When emissions are low, fixed flow systems have difficulty measuring anything. 
> 
> >I’m thinking the accuracy of measuring velocity of a gas moving through a pipe would be highly suspect. That because taking a cross section finds the flow differs from the outside to the center. A slight movement of probes and one gets a different reading that when multiplied up to determine the amount of gas per hour (for example) will differ a lot. 
> 
> The calibration standard is 'within 5%'. That is the velocity. You can imagine what happens with the air density in a hot, humid climate with a large fire or small, boiling pot or not. Etc. The density of the gas (which directly affects the detected speed) values from >1.4 to <1.1. 5% is unlikely.

So a stove is calibrated using a standout measured gas flow from a nitrogen tank(?) plotting against different temperatures and increasing gas speeds? Lots of work. 

> 
> >I would like to see this replaced with the surrogate helium standard I mentioned before. 

Bleed in helium at a slow, known rate and take He readings at points of interest to determine constituents (chemical and physical like soot) calculated as per mass of helium. 

> ‎You had better point me to that again. 
>> >>The alternatives are carbon mass balance or chemical mass balance. The former is used by the EPA for vehicles and the latter is used by SeTAR methods. 
>> 
>> >>The efficiency (as a heater) can be determined using the Siegert Formula which is a chemical mass balance approach with various iterations depending on the ‎data available. 
> 
> >Neither will work as I see it. 
> 
> They both work but there are ways to make it better or worse, hard or easy. 

How can a carbon mass balance be used to determine the energy without knowing the form and energy the carbon has left in it? Unless you think all stoves are proper burning stoves only releasing CO2.
Chemical mass balance (for energy?) can be done if all the many, many organic structures are determined and each energy determined - unless you think the stove is properly burning everything to CO2? 

All the same arguments I have had with the United States Composting Council. : )

> 
> Sooner or later you have to come up with the volume of gas and its average concentration of gases and soot. The issue is how well can one do that, and under what conditions. Biomass stoves have a huge range of emission concentrations and therein lies the challenge. 
> 

Therefore lies the Helium surrogate to the rescue!

> >Like measuring temperature or CO2 produced in a compost pile to determine if the pile has stabilized only works with a properly formulated and working compost. 
> 
> That is quite a good analogy. The method may work well when the pile is working well. If the point is to find bad piles the method may not be good at all unless it was designed to find bad, not good piles!

And like stoves what is the point of testing if all compost and stoves are working just fine?

> 
> >...Unless these are determined we do not know the energy value to assign. That was the purpose of my idea of the heated tube with oxygen input. Measuring CO2 before and after to get an idea (not exact) of the energy remaining in the outflow gas from the combustion chamber.
> 
> Measuring CO2 is unfortunately expensive and strongly affected by the pressure. Lot of care needed. Suppose you knew there was X amount of uncombusted gas in the tube. How do you know what it was after it is burned in the tube.

I was thinking CO2 before and CO2 after to get an idea (only an idea) of the unburned gases left. This old idea of mine is now replaced with Helium idea.  

> 
> There are cells that detect CxHy that way, measuring the heat created which is pretty cool. The CO is detected separately and it's potential heat subtracted from the total. It gets pretty complicated.

This I like if works - 

Helium surrogate if we can get it to work. Not cheap. 

>> >>If a stove has a firepower of 5 kW then it is assumed it can 'cook a pot' of a certain size. If it is 10 kW it is assumed it can cook a pot of twice the capacity. 
> 
> >In this case is the ‘fire power’ defined from the rate the fuel (of known energy)  is consumed and without heating water in the pot? 
> 
> Yes, and stoves with no pot on do not behave the same as with a pot so that is a problem too. The method assumes it makes no difference.

Why bother going down these paths when we know they don’t work?

> 
> >Do the chart of pot sizes really display a linear response? 
> 
> No but that is not the issue. Assigning any standard pot size from the list of 28 standard pots assumes a standard heat transfer efficiency.

I am not in favor to assign a standard pot because that implies stove designers are to design their stoves around one pot. They should design stoves and pots most liked for their users.  Test the stove for efficiency using the pot designed for the stove is how it should be done. Using a Standard Pot will give completely meaningless results pertaining real world use and restrict creativity of the designer.  

>> >Think about this for a moment: what does 'cook' mean?
>> 
>> That doesn't worry me as much. It means heat the contented at a culturally acceptable rate, if you really get down to it.
>> 
>> >...So creating a stove that will cook an 80 litre pot with a '20 litre' fire is unexpected. 
>> 
> >Interesting. There are so many variables in our small combustion chambers relating to stove construction, design and air flow that these test methods seem to provide no room for improvement - if I understand this?
> 
> The assumption is I think that emissions might be improved but that no one will make a stove with four times the heat transfer efficiency. 

>> >>So in the EPA methods there is in the background calculations some things to do with emissions and performance that are based on a 78% efficiency. 
> 
> >Really? What could these possible be? And why?
> 
>> I will answer that separately. It is deep in the math.

Probably well over my head.  

> >Biomass combustion is used in many different applications and, as I see it, they need to be grouped with each group having their own set of tests. Do we stovers care about condensing furnaces? 
> 
> Good stoves that space heat will one day soon be condensing. They also cook. We have to be prepared to rate them correctly. 

Lets start with the simple stoves first. 

> 
> Regards 
> Crispin 
> 
> 
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