Beckett SolarHot - Manual (Page 23)

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© SOLARHOT 2006-2009
Appendix B: Evacuated Tube Collectors
Many tmes I have been asked, “What about evacuated tube collectors?”
Evacuated tube collectors are long cylindrical glass vacuum tubes that have a copper filament that
extends down the length of them. A collection of these is connected to a separate header. The collection
fluid flows through the header, which is coupled to each of the vacuum tubes.
The theory behnd an evacuated tube s that snce t s a vacuum t wll resst sheddng ts ganed
heat back to the atmosphere. The argument is that it is much more efficient than flat plate collectors
because of the vacuum. There s some truth to that clam.
SRCC has tested and certified a number of evacuated tube collectors. In the rating process they
perform much better than flat plate collectors when the difference between the water’s temperature
gong nto the collector and the outsde ar s very hgh (144
0 F) and when that temperature dfferental
s very hgh and the weather s cloudy. When the dfference between the outsde temperature and the
water going into the collectors is not great, evacuated tube collectors perform much worse than flat
plate collectors. Those are the publshed facts of the testng.
Studes have been run on the performance of evacuated tube collectors n Germany and the tests have
shown that because of the superor vacuum that they possess they don’t slough frost or snow well at
all.
When you combne these publshed test results you end up wth a conundrum. Evacuated tube collectors
outperform flat plate collectors during the same conditions when evacuated tube collectors are likely to
not perform at all. Huh? That’s rght. They perform best on the coldest of wnter days, whch happens
to be the tme when they are lkely to be covered n frost and not perform at all. Even f they ddn’t have
frost on them, thnk about t ths way. Wth a temperature dfferental of 144
0 F between the outsde ar
and the water nlet temperature (generally around 120 0 F), the outsde temperature would have to be
-24 0 F for evacuated tube collectors to outperform flat plate collectors. Don’t forget that would need to
be the temperature when the sun s shnng. No solar collector collects any heat at nght. Based on the
realty of when t s coldest (at nght) plus the problems wth frost, I can’t see evacuated tube collectors
outperforming flat plate collectors.
Therefore, I can’t recommend them to anybody for use n hot water or space heat applcatons. If
somebody s nterested n usng them to power an adsorpton chller, then they mght be a good
selecton.
Appendix C: Environmental Impact
These numbers represent the environmental impact of installing two 35,000 BTU per hour 4’x8’ black
chrome collectors.
Per year, ths system wll elmnate the producton of:
8,496 lbs. of CO
2
20.4 NOX
34.8 SO
4
Over a 20-year lfetme, ths system wll elmnate the producton of:
169,920 lbs. of CO
2
408.0 NOX
696.0 SO
4