[Stoves] Water heating fuel efficiency formula

Crispin Pemberton-Pigott crispinpigott at gmail.com
Thu Oct 3 12:20:43 CDT 2013


Dear Jock

>How would we measure the efficiency of such a system, an iCan TLUD, that
provides useful heat for about 70 minutes from about 3 pounds of wood
pellets and also harvests almost 20% of the weight of the feed stock as
charcoal?

The system efficiency (which is the work energy divided by the available
fuel energy) is well known for good reasons. It predicts fuel consumption in
future for a similar task, and is a way to rate different technologies with
access to the same amount of the same fuel.

The rating of the energy performance has to consider whether or not the fuel
left over is useable tomorrow. A good example is an open fire burning
sticks. Each morning the fire is lighted using the wood left over from
yesterday. Maybe the charcoal is left to burn out each night. That has to be
considered as well - if it is burned it is not available tomorrow so it is
consumed even if it did no work. Local behaviour matters when considering
what stove to promote.

The actual heat available (the effective heating value) is the potential a
stove could get from a given mass of wood with a given moisture content and
elemental analysis.

The stove may not yield that heat for a variety of reasons which I should
not need to enumerate. If it does not, it is not rewarded with a 'better'
number. If the work done, say, boiling water, remains the same, then it is
not reasonable to reduce the amount of heat available and then say the stove
did a better job because the ratio of the work done to heat yielded is
better. Doing a lousier job of burning the fuel, or making use tomorrow of
what remains today, cannot give a stove a 'better' rating.

For all these reasons, the fact that there is char remaining at the end of a
cooking cycle is not a bonus for the thermal performance of a cooking
system. 

When using a fuel that is a non-woody biomass, there are good arguments for,
not special consideration, but for a reduction in the requirement for
efficiency. This is reasonable if there is a surplus of an unused resource
and a scarcity of a used one. So the argument that there should be a
'special' way to calculate the efficiency will not fly. But there is a
chance that pleading a special case based on fuel availability could in
principle succeed.

In the negotiations on this matter, it is not possible to sell the idea that
a stove that uses more wood than the baseline should be promoted,
particularly on a subsidised basis. It is a much easier sell to show that
there is an unused resource that can 'inefficiently' be used but that will
provide some additional benefits (real, not potential).

As for emissions into the home the usual standard would apply of course. It
has to be clean burning.

Regards
Crispin






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