[Stoves] Heat-pipes/thermal-diodes

[Ronal W. Larson]

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 <crispinpigott at gmail.com> 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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