[Stoves] Heat-pipes/thermal-diodes

[Crispin Pemberton-Pigott]

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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