Friday, December 10, 2010

Effect of Pump speed

8 Dec '10: In further discussion of the increased performance, I must say that the total quantity of heat downloaded is of course smaller in Winter. We sometimes have sequences of days with no heat from the boxes at all. The difference has been in the heat PER hour. Part of this is down to mirrors, but part is down to pump speed and hours of operating.

  • In the Winter, the GSHP is much more proactive, on for longer hours, driving down the glycol temperature, pushing liquid through at 18 litres/min, getting the heat in fewer hours, then closing the valve.  
  • In Summer, the GSHP is largely asleep, leaving it to Sun and air temperature to drive the boxes - far longer hours of operating, and only 5 l/min flow rate. It does get more heat in summer, but needs more hours to get it. 

  The only way to know is to continue, and then test the daily harvest of heat during Summer 2011. As we all know that Solar Cookers work best in Africa and India, it's fair to hope that the daily thermal harvest will be higher in the summer sun with the mirrors diverting extra heat onto the collectors - higher than the unmirrored ones were in Summer 2010.
    Logically, I should continue with the high delta-T requirement (6.0ยบ) that I am now using, so that it really only runs when there is something to run for.... and when it does run, have a higher pump speed to increase capture - when this occurs in summer, there is likely to be plenty of PV power to match the pump requirements. For last summer, the summer pumping rate was 30W, approx 5 litre/min - next spring, it will be time to try the middle speed, which I think is 45W.

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