Crispin and list:
Thanks for the updated cite. Makes at least two that are not behind
paywalls. Hope we can find some that are even more current and easy
obtainable. But there is a big literature on heat pipes outside the cooking
arena.
I think we can learn a lot from the solar cooker side - but my interest
is on biomass stoves and the ability to feed multiple pots from a single flame.
You talk also about thermo-siphoning and pumps. I am describing neither of
those - a heat pipe operates on a very different principle as I know you know.
The word “diode” is important here.
I talked with one of the last references in your cite - Mr. C J Swet
at about the time he wrote that 1974 article - on (I recall) a thermo siphon
principle.
Ron
On Jan 29, 2014, at 12:13 PM, Crispin Pemberton-Pigott
<[email protected]> wrote:
> Dear Ron
>
> Good find.
>
> This is basically identical to a system built by Mercedes in Germany that was
> shopped to South Africa for testing. It cost an absolute fortune so there was
> no plan to make a commercial version at that stage. I understand it had been
> fabricated by the apprentices working at Mercedes as a development project
> for high solar insolation countries.
>
> There is a paper Stumpf, P., Balzar, A., Eisenmann, W., Wendt, S., Ackermann,
> H., Vajen, K., 2001. Comparative measurements and theoretical modelling of
> single- and double-stage heat pipe coupled solar cooking systems for high
> temperatures. Solar Energy 71 (1), 1–10, that might be relevant (it is cited
> at the end). I am wondering if there is a connection between Mercedes and
> those guys. Just a thought.
>
> So eventually this pair (was it three?) devices made its way in not very
> perfect condition to St Joseph’s Missions east of Manzini, Swaziland and I
> was called in to see if I could get it working. The working fluid was peanut
> oil which has the highest practical operating temperature, they said. I think
> marula oil is higher but you can check.
>
> The construction was identical to that linked above. Each unit was quite
> large. There were two cooking areas which were really stainless steel bowls
> that were sunken below the liftable, reasonably insulated wooden lid.
>
> There were connector problems (the collectors are separable) and that was
> easy to fix. The problem, according to the cooks, is that they did not cook
> properly. They were good at heating water but not good at ‘cooking’. I traced
> the problem eventually to the fact that the thermosiphon system that
> circulated hot oil from the reservoir to the cooking pot was circulating too
> slowly. It was hot enough, but it was short of power, meaning the flow rate
> of the oil could not be increased to the point that a pot would ‘look like it
> is cooking’ even though the oil temperature was above 150 C. That was
> disappointing and systemic.
>
> We had email correspondence in those days with the factory and some comments
> were traded back and forth about what to do. Having been given the final
> verdict that the problem was the heat transfer rate being limited by the
> piping restricting the flow rate between the reservoir and the pots directly
> above it, and the lack of a circulation pump of any kind to increase it, the
> reply came that the real problem was the connections between the panels and
> the cooking unit. In other words they blamed me for their design errors.
>
> The thing definitely collected a lot of heat and stored it. Anyone wanting to
> build one should pay close attention to the heat transfer rate (in Watts)
> between the heat store and the cooking vessel. It is a good example of the
> difference between temperature and a quantity of heating.
>
> Peanut oil is a good heat transfer and storage medium. The thermosiphon
> method is good for getting heat into the system.
>
> Remember everyone, that Prof Bernhard Scheffler (Solar Energy Society of
> South Africa) proved mathematically in 1982 that you get the maximum heat
> gain per day by passing the volume of the working fluid through the collector
> exactly once during the solar heating period, say, 8 hours. That fixes lots
> of design parameters.
>
> Regards
> Crispin
>
>
> Tom: cc List, Crispin (new thread name)
>
> The number one new thought I want to emphasize from my brief
> report is that of the heat pipe/thermal diode - which I have not seen
> mentioned on this list. Googling found this (no fee) paper on solar cookers
> that is relevant:
>
>
> http://cfc.kscia.or.kr/new/wwwboard/admin/wwwboard/attach/1087363006/26.pdf
>
> Several other references to cooking, but this the only no-fee
> paper in the first ten Google pages for the search I used (with “stove”).
>
>
> This is to ask if anyone has been trying this out with biomass
> cook stoves. What working "fluid" is appropriate (maybe lead)? A high
> temperature oil? Note this could also be very appropriate for ovens, where
> we mostly want no smoke.
>
> One cook stove could feed several ovens (or any cooking surface).
> Eliminates the soot problem, also. We could totally insulate the cook pot
> while cooking.
>
> This could be relatively low cost - just a closed pipe. Nothing
> to wear out.
>
> This did not come up at ETHOS. Anyone needing the 3-pager
> report noted below can get from the stoves archives or write me.
>
> Ron
>
>
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