[Stoves] Looking for stove designþ

[Crispin Pemberton-Pigott]

Dear Vetle

 

This is exactly the sort of information that builds the case for a new stove
or a new approach to fuelling and operation.

 

Use acceptance is a major issue and the follow-up is important – meaning do
the new stoves remain in use. When they do, it is often for reasons that
have to do with fuel availability in my limited experience. Not have to go
as far or spend as much for fuel is a strong incentive. The only thing that
mitigates hard against it is if the stove is inconvenient to use or won’t
cook properly. African women tend to like a high heat (high firepower) to
‘cook fast’ so speed to boil is important.

 

>The 6 liter Peko Pe stove can be filled with 600 grams of Maize cobs, with 
the sizes of 40-60 mm, it will burn for about 30 minutes and boil one 
liter of water within 9 minutes after ignition (6,5 minutes after the pot 
is put on the "flame"). 

Is 1 litre large enough to interest a family? How much to people cook at a
time? The answer might be ‘a little in the morning and a lot in the
evening’. It is quite reasonable for there to be two stove sizes or for one
to be used only for certain tasks. If the fit is good, people will adopt
them.

If I estimate 600 g @ 15% moisture (air dry in summer) I get about 7.8 MJ of
potential energy. Subtracting for 20% char (125 g) I get 4.1 MJ of heat over
that 30 minutes. That is 2.3 kW average (with perhaps a peak of 3 kW?).
Does that sound reasonable? Is the heat fairly constant during that time? If
so, it is going to have a pretty constant thermal efficiency.

If the fire is low for 2.5 minutes then the pot goes on, and the boiling is
6.5 minutes per litre, that is an overall efficiency of about (0.335 MJ heat
absorbed / 0.975 MJ heat applied) = 34%.  If the power is not up to that 2.5
kW average at the time, perhaps it is higher. If the fire is really large at
that time, perhaps the efficiency is In the high 20’s. Does any of this
sound like what you are seeing?

30% is pretty good. 

So the information I will be looking for relates to the cooking power in the
first phase (pyrolysis), the duration of the burn and the heating power that
is available if one wanted to continue to simmer on the remaining char.

There is a test which would be very valuable and it is a water heating (not
boiling) test.

Put on a large pot with a lot of water, large enough that it will not boil
for at least 40 minutes. Record the temperature every 20 seconds or so (be
regular to make it easier to analyse) using a thermometer that gives 0.1
Degrees.

As the thermal efficiency will probably not change a great deal during the
cooking session, the temperature rise rate is an indicator of power output.
It can be calculated for various intervals based on time or temperature. I
have attached a typical result of such a test for an old Panda paraffin
stove using a heterogeneous test protocol. The top blue line is the mass of
water remaining. You can see when the stove was changed from High to Medium
power as the boiling rate (water mass loss) changes. The thermal efficiency
line (green) detects a change in efficiency with the turn-down.

If the power level changes the slope of the water heating line will change.
This can tell you what the power change is as the stove goes from ignition
to a fully developed fire to a late-pyrolysis burn. This information is very
valuable for you. It would be very helpful if you share the result. Using
this approach, you can tune the performance to a cooking task in a given
community.

If the pot boils during this type of test the result is harder to get
because it has to be done on a scale but it can be done if you have to. 

>For normal cooking of Nshima and similar food 
preparing, 30 minutes would be enough for each meal in everyday use. 

For South Africa and vicinity it would take 45 minutes after boiling to cook
‘pap’ which is much longer.

>Cooking Nshima need less than 10 minutes so usually they can prepare most
of their additional food with one filling. 

That is a very good fit then, especially if adding some chopped wood will
extend it by a few minutes if needed. Do people object to chipping/chopping
the wood? Zimbabwe (rural) had difficult with splitting wood for cooking
because the trees are so hard. That problem is avoided it no wood or little
wood is involved. As the maize cobs collect around the house it should be
used first I guess. Hopefully.


>We do feel that 6 liter Peko Pe stove is a suitable size for unprocessed 
BIOmass wich are most available around the farms. 

Agreed, by the sound of it. You have a good match between the stove, fuel
and the cooking task.

>Since they already are cooking with one pot at the time, we don’t think 
they need to have more than one stove,. 

Good point. How do they prepare the ‘relish’ to go with the nshima? I have
seen it prepared in a smaller separate pot.

>This is off course an investment cost up for discussion for every
household. When less cooking time are needed they can 
just use less fuel. 

Exactly.

>With beans, the farmers will spend more time for cooking. This can either 
be done by empty the stove and refilling it with maize or use smaller cobs 
(>10 - 20mm) to get more mass into the stove (800g = <40 min "flame time")
or also by 
using other kind of denser BIOmass. Chopped wood will burn for more than 1 
hour. 

Is the burn rate (mass per minute) the same with all the fuels or do see a
noticeable difference?

So far it seems that the farmers are satisfied with both the size and
cooking time. Time will show if adjustment of stove size are required. 

Agreed. 

>Since the farmers in this project are targeted to collect BIOchar from
maize cobs used for household energy purpose, they will refill the stove
when needed to get sufficient cooking/energy time. 

They will probably do that until they run low on fuel and there is nothing
wrong with that.

>The BIOchar will be stored for later to be used as soil improvement instead
of using it as glowing charcoal. 

I understand that process. The soils of Zambia are already famously
productive so any effect will be important to document.

I really appreciate you taking the time to provide hard data for us who are
disconnected from the work on the ground. I would have shown one to the
Chinese rural energy delegation Roger Samson is bring around this week if I
had a working sample. We are going to demonstrate several stoves that can
burn pelletized (and chunked) switchgrass to try to generate interest in
TLUD technologies.

Thanks

Crispin

 

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