[Stoves] High mass space heating options Re: Rocket Stove for the PLACE

Sun Oct 9 23:12:55 CDT 2011

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

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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
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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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