[Stoves] Question about the turn down ratio

[Crispin Pemberton-Pigott]

Dear Friends

I am asking that you think about the meaning of the 'turn down ratio' (TDR)
of a stove.  The testing protocols call for the inclusion of a calculated
TDR.

Normally this is the ratio between the power available to boil (maximum
power if you follow the UCB-WBT protocol) divided by the power needed to
simmer a pot.

It is quite possible that the power used to boil a pot might be tuned to the
pot used during a test or when cooking.  The maximum power of a stove may
not be a fair reflection of the appropriate power level for use during a
water boiling test when the stove will be rated on its performance and
emissions profile. It would be quite possible to build a stove that was
designed to perform well on particular tests rather than to perform well in
real life.

Stove A has very good combustion but a low heat transfer capability. It is
equal in fuel and emission performance to Stove B with lower combustion
efficiency and better heat transfer. Stove A will get a lower TDR 'rating'
even though Stove A would be able to warm food for a long time without
burning it while Stove B cannot do that. If that is not clear I can explain
it better another time.

It is analogous to testing a car's performance.  One does not conduct a fuel
efficiency test with the pedal to the metal.  But the maximum acceleration
is measured that way.

So, is it not wise that a stove be tested in 'normal operating conditions'
for boiling and simmering?  Operating a stove at its maximum possible power
is not how to investigate a stove's performance. This may, if I guess
correctly, reflect on the poor performance of the 6 improved stoves being
promoted in western Ugandan refugee camps (recently reported here) none of
which performed better than an open fire, two of them worse.  The test
protocol calls for the stove to be operated at it maximum power during
boiling. That may not reflect the manufacturer's recommendations nor what
the people in the refugee camp do either.

Next, there can be quite a difference between the power at which a stove is
operated when simmering and the lowest power.  If the pot was simmered with
the lid off, there is much more power required than if it is on, and it
still may be quite capable of warming a pot at an even lower level. The
TDR's lid-off and lid-on will be quite different in a
high-heat-transfer-efficiency stove.

What do you think about getting the TDR by dividing the maximum possible
power by the minimum sustainable power level? How should it be determined?

I use as examples an LPG or wood gasifier stoves with a maximum power of
2300 watts and a minimum of 350. That is a TDR of 6.57.  

During a WBT the power required used might be 2000 and 650, giving TDR of
3.08 which misrepresents the capabilities of the technology. It seems to me
the TDR should report the actual turn down capability of the appliance.

As we draw closer to having agreed international stove performance test
protocols and ultimately, performance benchmarks, issues like this should be
commented on by all.

Regards
Crispin in JHB