[Stoves] Calculation help

[Crispin Pemberton-Pigott]

Dear Frank

 

>It takes [Joules of] energy to go from 20c to 100c and from 100c liquid to
100c vapor. 

 

Correct.

 

>But doesn't it also take in more energy when  water vapor is heated from
100 to 450 (?) 

 

Also correct.

 

>.and that is removed before it is used for work?

 

The 'work' is heating the pot and that drops the temperature of the gases
and vapours. At some point the gases are pretty much ineffective at raising
the temperature of the outer skin of the pot so it is literally wasted.

 

A good example to think about is a gas oven for firing ceramics. It produces
a huge amount of heat, and the temperature of the flame is high, but they
are very inefficient. The gases pass out of the furnace at about the
temperature of the inside. If you are firing to 1100 C then the wasted gas
is >1100 coming out. That is an enormous loss. A gas oven is only about 18%
efficient at that temperature! Electric ovens are much more efficient, but
you have to generate the electricity first. Turning gas into electricity at
50% efficiency then feeding it into electric elements uses less total
energy.

 

The stove is akin to a kiln. There is always a loss because the pot when
boiling is never much below 100C at habitable altitudes. The convention is
to use LHV (lower heating value) as the measure, assuming that heat below
100C is always going to be wasted. 

 

The heat it takes to go from 0 to 20 to 100 is for some reason not included
in the calculation, just the latent heat of evaporation. I believe it is for
simplicity and convenience. It is not exact but the error is small -
typically 60,000 Joules per kg of biomass (0.06 MJ).

 

A small measurement error in fuel moisture has more influence than that dose
of energy.

 

Regards

Crispin

 

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