[Stoves] Stove Definition - controllability

[rongretlarson at comcast.net]

Paul, Crispin, list 

First - to agree with Paul totally. Were the Indonesian cooks asked whether they would prefer a turn down ratio of 2 with a response time of 1 second vs a traditional stove with TDR of 10 or 20 and a response time of 2 minutes. Better, were they given a chance to use or see each in action? 

I've been having some fun reading about cooking for the last hour (can you believe deep-fried hamburgers?). Several thoughts popped out of the reading: 
a. Rayleigh's Law of radiative heat transfer has power (losses) going as the absolute (add 273 to degrees C) temperature to the 4th power. Try calculating some temperature changes in degrees C when the power levels change by a factor of 4. (to be realistic you also have to have some fraction of the losses related to evaporation and condensation. Also conductive and convective losses. Beyond this response. 

b. Cooks everywhere are going to think in temperature terms, not power level changes. If you want high temperature frying you can use a pan with less surface area and less T^4 losses. You can also change colors of the utensils perhaps. I'll bet if you ask 99% of the cooks we are trying to reach, they never heard of the exponent "4", nor its meaning . 

c. The average water temperature for simmering can be hugely different from that for a vigorous boil - and many foods should not be cooked anywhere near 100C. And sometimes the converse - needing a pressure cooker. 

d. I found a site selling a product that changes the stove/utensil power level to achieve the correct cooking temperature by using the sound of the bubbling. I can conceive (Chinese ingenuity) changing the fan speed on a char-making stove to achieve the desired simmer temperature that way for $1. Claimed to save 35% on energy bill. Can anyone conceive that for the average stove? The average cook can tell a lot about temperatures by looking (maybe listening) and the power ratio needed is way different than four. 

To repeat - Paul Olivier has the right issue in talking about times. 

Ron 
----- Original Message -----
From: "Paul Olivier" <paul.olivier at esrla.com> 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
Sent: Saturday, May 4, 2013 5:04:14 PM 
Subject: Re: [Stoves] Stove Definition - controllability 



Crispin, 

You say: The time taken to increase or reduce the heat in the pot (which I will call the ‘cooking power’ because it is the heat available to the cook inside the pot) is not defined or restricted. 

It makes no sense to talk about a turn -down ratio of 4 to 1, if the time needed to turn down or turn up is not clearly defined. If turning up and turning down cannot be done quickly and easily, it becomes exceedingly difficult to cook a meal. The ease and speed at which heat is adjusted should be part of a minimal set of standards. 


The perfect stove in all respects is a modern gas stove. It's got everything: low emissions, marvelous adjustability, instantaneous start-up and shut down, and so forth. In terms of ease of cooking, it beats any biomass stove out there. In terms of ease of cooking, why not use it as the standard against which all biomass stoves are judged? Of course, in terms of its carbon footprint, in terms of carbon sequestration, in terms of sustainability, the modern gas stove is a big zero. 



Thanks. 

Paul 



On Sat, May 4, 2013 at 8:44 PM, Crispin Pemberton-Pigott < crispinpigott at gmail.com > wrote: 






Dear Paul and Lanny and everyone else who is considering this matter 





Paul O > I agree: it is hard to cook without controlling heat. Your call for a level of control from 100% to 25% sounds reasonable. 

This request for controllable heat is not originating from me speculating. The feedback from the social science team in Indonesia is that the controllability of the cooking power is a pre-requisite for acceptance. A ‘cooking stove’ has a definition supplied by the users, in other words. I can relay to you the various opinions but I will filter them while trying simultaneously to develop a test method for meeting their requirements. 

There are several relevant points raise by Paul so I will base my reply to everyone on this set of comments. 


> Let us take the example of someone using a direct combustion wood stove. This person might start out at 25% and add on more wood to achieve 100%. But it will take time to reach 100%, and it will also take time to reduce heat back down to the original starting point of 25%, if need be. 

The analysis must remain cognizant of the uses to which the stove is put. The time taken to increase or reduce the heat in the pot (which I will call the ‘cooking power’ because it is the heat available to the cook inside the pot) is not defined or restricted. The cooking power is different for different meal types, and sometimes it varies during the preparation of a single food. Rice is a good example, but only in some cases. In Central Java rice is steamed which requires high cooking power and a high-medium power. If the rice is not steamed it requires high power followed by a very low power. We cannot say ‘cook rice’ and then decide if it is a good stove because people cook rice in many different ways. 

I am trying to set minimum standards which if not met, will mean the product is ‘not able to do that’ or perhaps ‘is not a cooking stove’ though it could do other tasks like heating water or space heating. Because of the minimum demands of the end users, there is a need for some way of assessing the controllable from the uncontrollable stoves. 



> Trying to establish criteria by which to judge and compare stoves is awfully complex. 

Be that as it may, there is a job to be done and we have to start somewhere. The best we can do is survey the opinions of the stove makers (you guys and gals) so you know there is a minimum set of performance criteria that will be applied when testing day comes. And it is coming soon. 


> For example, so much depends on the type of fuel that is available in a given area. 

The fuel or fuels will be specified, for example teak. However if a manufacturer has a stove that requires a particular fuel, they can say so and it will be tested with that fuel. If you offer a sawdust stove, it will be tested with sawdust. For example a stove may require wood pellets of 8mm diameter with a moisture content under 10%. No problem, you can specify that, and it is up to your company to try to make that fuel available. Ethanol stove and fuel people do that all the time. 

Better cooking solutions can include a specific fuel and it will be tested using that fuel. Getting a ‘passing grade’ does not mean someone else will develop the market. But the gatekeepers will have a pass/fail stamp on what is a cooking stove. Reasonable, no? However it will also be started that for a particular are the fuels available are xx and yy and zz. The test will be conducted with those fuels; here is the baseline emission level from the existing stoves. In many cases the manufacturer may not have the fuel so they will have to take a guess and the stove tested a few times by the rating laboratory to see how it performs and feedback given to the producer. No problem. The point is to get better products. 

>From the cooks’ point of view, claiming that a stove can cook when it can’t be used for most cooking tasks is misleading and the dissemination plan will fail. Customers insist on a turn down ability. The discussion then turns to ‘how much’, not ‘whether that is a real need’ or ‘but my stove can’t do that – make an exception’. The customer wants power control over the heat in the pot. End of short story. 


> The perfect stove might demand a perfect fuel, and if this perfect fuel does not exist in a given area, one has to choose from an array of imperfect stoves. 

Fortunately this is not a contest to find the ‘best stove’ as the customers will pass their opinion and even when shown a ‘perfect’ stove from the inventor’s point of view, might reject it. ‘Imperfect’ is in the eyes of the buyer. 


> The perfect stove might be too expensive for a particular poor corner of our planet. So once again we have to choose from an array of cheap and imperfect stoves. 

Again, fortunately, that is not for a regulator to decide. There is a big broad market out there. People can sell what they want provided it meets certain minimum criteria. Today we are talking about the turn down ratio (TDR) and what constitutes a fair requirement for a minimum level of repeatable control. 



> Funding agencies come along and demand criteria by which to judge stoves. 

Now this is an interesting point. A project can support whatever they want and have project criteria that meet their own agenda – like solar cookers or pellet stoves or whatever else (like, 100% locally produced and so on). A national regulator might set certain minimum performance standards but a project may have much tighter criteria or higher expectations. That is up to them and their project team. 


> But I profoundly mistrust the role of funding agencies. They, with massive inputs of capital from the outside, can easily distort the normal evolution of cook stove technology in a given area. 

Funding inappropriately can upset the production systems and destroy a lot of businesses, but that may be the intention – to get rid of a host of really bad products and replace them with a major improvement. I can think of a couple of cases. Trust? We are entitled to our opinions. The government’s opinion matters most and they regulate. So what should the minimum regulations say? 



> Fuel preparation needs funding (the before), and biochar research needs funding (the after). 

I will restrict this to the turndown ratio. Fuel we can discuss later and in relation to fuel standards, OK? 

So, the demand, the requirement from the field is for a measure of control. This is not the opinion of project people, funders, analysts and technicians, it is from the customers who are supposed to be the beneficiaries. No turn down, no sale. 

Let’s say our target is 10 million homes. We survey the typical cooking styles and ask the typical users their opinion and find that they use a TRD during a typical week of 4:1. When setting a national standard for performance the widest possible accommodation must be made so as not to limit innovation, to provide for an array of cooking needs, and to still provide a guarantee to the public that if it says ‘Passed’ it means something real. 

If we test a stove product that is designed for small pots in an urban setting, it may have an upper power limit if 1.5 kW. That means it can reasonably cook a smallish pot in a reasonable time. It is clear already how to describe that so I will write it now: 

The cooking experience is related to the time it takes to heat things. Typical of this is boiling water – a common task. Asking for input on the matter, it seems that a 5 litre boiling time of 25 minutes is considered ‘good’. To produce a boiling time of 25 minutes for 5 litres in a 400g pot with a lid, 254mm in diameter on it takes a heat transfer rate of about 2 watts per sq cm. If you are not familiar with this approach, it means that 2 Joules enters the outer pot surface per second over the whole bottom of the pot. The ‘area’ of the pot can be calculated from its outer diameter. 

Heat enters a pot primarily through the bottom and nearly nothing enters (net) from the side. The sides are in fact a source of net cooling almost all the time. So the relationship is between the pot bottom and the fire. If the heat getting into the pot (counting for the thermal mass of the pot as well) reaches 2 watts per sq cm, it will have ‘an acceptable cooking power’, at least it will in Indonesian households, at least in Central Java, at least in the homes we asked, at least that is what they said. 

So starting with this as a concept to measure cooking satisfaction, we also start with the figure of 2 watts/sq cm. For a 25 cm diameter pot it is 1 kW gained by the pot, i.e. 1000 Joules gained per second, with the lid on. If a stove can induce that much cooking power, the stove can be considered ‘improved’ or ‘modern’ or ‘acceptable’, at least in that set of communities. In other places the number may be different. 

Given this heating rate, what then shall be the definition of a ‘controllable fire’? With charcoal people turn down the air supply, remove charcoal, splash on a little water or move the pot (is that cheating?). There is a clear demand for controllability. How shall it be determined? 

We can ask the operator to turn down the heat (by any means) and demonstrate a cooking power of 1 watt per sq cm. That would be ½ power. There is no implication about how many kW the fire is, just that for a pot the manufacturer says “this stove can cook it”, it must be able to provide 2 watts per sq cm and it can turn it down to 1 watt. 

Again we can ask that it be reduced to ½ a watt per sq cm and see if it can do that too. If all stoves claimed to be ‘cooking stoves’ can demonstrate this level of control, no matter the size of the stove, we have a simple rule that can apply to anyone’s product, for any stove size, and for any pot size, even if it is a frying wok or a steaming pan. 

I noted more than one comment that some stoves do not turn down well. That is a fact and a reality for the inventor. If a stove can’t be turned down, it will be rejected by the cooks we meet because they demand a minimum turndown. They do not express it in watts, but I can translate. We measure what they do and find out what the ‘cooking power’ is (as defined above) then set a standard for the power per sq cm in the pot. If a stove is large, it can ‘properly cook’ a larger pot. But it still gets the same minimum power and performance test of a 50% and 25% power level, defined by the expected cooking experience. 

I am hoping that many of the people reading this list who do not usually make comments will arise on this occasion and give some feedback on this issue. If we are promoting cooking stoves, from the point of view of the customers in your area, what constitutes a reasonable level of control over the cooking power, below which they are not interested in buying it? 

I am not directing developers as to how they control the cooking power, only that the control has to be real – i.e. sustained for 20 or 30 minutes – and that it cover a minimum range stated in a standard. We can set a level with some permissible variability. 

Stoves used for water boiling, continuous heat applications and so on which do not require control can be classified as ‘stoves’, but not ‘cooking stoves’. That again does not originate from my opinion but from the field and is the opinion of the customers who we are trying to satisfy. They are happy with water heating stoves, but for cooking are more demanding. 

As a brief aside the use of LPG in Indonesia (another thread) is widely promoted (and subsidised) however field research shows that nearly all households that use LPG also use biomass to heat water. If the LPG price rises many will return to biomass for more cooking tasks. LPG is highly controllable – more than 8:1 TDR. People like that. 

Final proposition: Do you agree that products meeting the minimum standard to be labelled an improved ‘cooking stove’ can be required to show the product can deliver 2 watts per sq cm into a pot (as defined) – which allows the manufacturer to define the stove they think they can sell and pot sizes it can cook ‘properly’) – and that the requirement for the stove to be turned down for 20 to 30 minutes to 1 watt and 0.5 watts per sq cm – for a TDR of 4:1 – are a reasonable minimum requirements ? 

Thanks everyone 

Crispin 
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Paul A. Olivier PhD 
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