[Stoves] Stove Definition - controllability

Sat May 4 08:44:12 CDT 2013

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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