Dear Massive Friends
There is a strong preference to put mass walls into homes in Mongolia as spare
income becomes available. Something like 60% of people who used to live in a
ger (yurt) have built permanent structures and many of the small homes have a
mass wall in them.
There is a significant risk with these devices and that is the problem of
controlling the air flow through them once the fire dies down. The amount of
heat lost up the chimney can be very significant when the wall is hot and there
are open holes. It is not possible to close the chimney completely because it
is never certain that a fire is completely out so CO would get into the home if
the ventilation of the wall is blocked.
When checking the excess air ratio of a couple of these ‘wall stoves’ as they
call them, I found a lot of air is being pulled through, driven by the retained
heat. It is not only possible, but a surety that the thermal efficiency drops
below 0% under late-fire and fire-out conditions. There are a few things one
can do with the design, but keep in mind that having an episodic fire instead
of a continuous one exacerbates the problem. A continuous fire for a long time
is inevitably going to have different firepower at different stages so it is
not a complete cure either.
The big advantage of a mass wall is not that it smooths the heat delivery
(which is does) but that it will deliver heat throughout the night. That is the
biggest issue. 10 hours of heating. No one wants to get up on the night and
tend a stove.
There is a lot of work needing to be done on this to get systems that drain
water effectively (there is a huge amount of condensation inside such devices)
and to work out the best mass. I did a little work with (then) GTZ modelling
this and we arrived at a number of 500 bricks as being the minimum number to be
effective in that climate. Maybe 1-2 tons.
The brick layout determines the effectiveness at picking up heat so
possibilities abound. It is expected that these devices will approach the
efficiency of a high efficiency condensing natural gas furnace (92%). At
present there are huge issues with the condensation because it appears not to
have been seriously considered when building them. The amount of water to be
handled is large: perhaps 450 litres per month. Only some of it passes through
as water vapour.
I look forward to seeing this application applied with rigour.
We tested a double TLUD stove in Ulaanbaatar made by a local artisan. It worked
in sequence so there was always a fire going. Whether the fire is continuous or
episodic, there is still a large advantage to a well-build mass wall: it turns
the stove into a condensing heat exchanger! That is a gain of at least 15% in
efficiency, more likely 60% compared with the baseline devices. The downside
risk is that the chimney will run all the heat out of the system by pulling
cold air into the home and shoving it up the chimney when the fire is out, or
nearly out, at a (negative) efficiency of -250% or even more. What we know for
sure is that people who have a heating wall use about 1.5 times as much fuel as
those who do not (for a complex of reasons).
If you want to build suitable ones in which we would be interested, go for 12
and 25 kW sizes.
It may surprise you to know that the major application of new stoves this year
(season starting Aug 2011) is TLUD stoves from “Silver” being installed on
heating walls. The market is sometimes ahead of the stovers! They are
installing a couple of thousand a week, many connected to heating walls.
Regards
Crispin
+++++++++
Alex: I believe the most appropriate answer to your final question below
("What do you see as the advantages of combining these two approaches?") is one
you gave yourself: ("...a desire for char ...").
The beauty of high mass heaters is that they mesh well with a batch process
- the main (only?) drawback of char-making stoves.
Ron
++++++
Paul,
The TLUD configuration has it's advantages and disadvantages, like any system.
One advantage is it's steady state energy release. High mass heaters strive for
a similar result. It seems a bit redundant to pair the two. Perhaps it depends
on other considerations like a desire for char or an available particulate fuel
supply.
There are some folks doing fine work at the Masonry Heater Association.
http://mha-net.org/
They have done a lot on emissions, efficiency and durability. They have looked
into Down feed rocket configurations.
I'm not sure if they have quantified losses between firings, or seasonal
efficiency.
Its a simple concept with lots of technical details.
What do you see as the advantages of combining these two approaches?
Alex
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