Monday, December 9, 2013

Time to dump the Tubes

9 Dec 2013: It has been an expensive experiment, these evacuated tubes. My two Sunboxes continue to turn in good performance figures, but the tubes have never been good, even in the height of summer - 7kWh is the best ever (after two summers of testing), but the problems are greater than the benefit.
    On a summer's day, the tube ends can reach 130ÂșC and I dread to think of the effect this can have on the rest of the system if, for some reason, the pump isn't pushing liquid round to cool them. I don't want to build up pressure cooker pressures. All my problems with leaks started after these tubes were fitted. The simpler system of 40mm hose-joints etc cannot cope with sudden rises in pressure. Even after I fitted a heat exchanger, it worked better as two distinctive circuits, but the daily harvest was not improved.
   The performance boost from fitting ETFE to the main Sunbox was so big that it eclipses any benefits from fitting the tubes. The annual solar capture with the Sunbox 3 and 4 combined is now up to 3,470 kWh/annum, which is a better deal than the range  of 2400-3000 I used to get before.
   It is clear that tubes can get to higher temperatures if facing south, and should be used to heat hot water in a tank in a system that can cope with high temperatures and pressure. In a low pressure, high volume, low temperature circuit like the Sunboxes, the tubes are a hazard, and would be a greater one if facing south.

Postscript: as an end of the year action, I disconnected the electricity to the tubes on New Year's day, and will drain the pipes soon. 

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