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Old 01-01-12, 12:59 PM   #4
AC_Hacker
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This discussion is really most curious, especially if it relates at all to the paper cited in the beginning...

Quote:
Originally Posted by Xringer View Post
Found this paper about using CO2 as a solar heat-transfer medium..

http://www.ewp.rpi.edu/hartford/~cha...c_tube_CO2.pdf

But, the conclusion of the solar-CO2 study, says it's too complicated (to paraphrase).
It would really help to read and understand, at least a little bit of what the paper actually says.

It does NOT say that using CO2 in a solar heating application is too complicated. It DOES say that because of the transcritical nature (the CO2 is changing state between liquid and gas) the measurement is complicated because of the varying characteristics of the medium.

The most important thing it says is that the "annually-averaged collector efficiency is measured at 60.0%".

Compare this to this graph from BiS:


In the ranges shown in the graph, efficiency of a flat plate is averaging 50% while an evacuated tube is averaging 45%.

The paper says the average efficiency when using CO2 as the heat transport medium is 60%. Which is pretty significant, especially when you realize that CO2 is being used in exactly the same way as water, and is exploited for its phase change characteristics in evacuated tube collectors. So the proper comparison is 45% to 60%. This is a big deal.

It looks like what is missing from this discussion is an understanding of how evacuated tube collectors actually work, and the thermal aspects of phase change (which also happens to be the principle that is the basis for all vapor-compression machines).

You might want to start by understanding the principle behind heat pipes.


Otherwise, you have driven off the road and you are sitting in the ditch with your wheels spinning.

-AC_Hacker
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Last edited by AC_Hacker; 02-09-12 at 08:06 AM..
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