Friday, April 9, 2010

Solar heat working late at night!

April 8: One of the things I hoped for from the sunboxes was that the brickwork inside the box would warm up on sunny days, and prolong the heating benefit into the evening. That was a hypothesis... now it's a theory... and will be proved fully when we get the datalogger working.
   Today was the sunniest day of the year, with 20 kilowatt hours recorded by our photovoltaic roof. The sunboxes captured about 18kWhrs too in that time. We were in Norwich for an overnight stay, so the heatpump was off for 33 hours, but the PV and sunboxes continued to work.
On Thursday, Sunset was 730pm (BST), and when arriving home at 930pm, and later at 1030pm, the sunboxes were still working, amazingly. Although cold glycol was being pumped round the black collectors, the sunbox air temperature was holding remarkably steady. The heatpump was turned on and worked hard to bring the house temperature up to level (it brought the water to 51.0 remarkably quickly). So it was pumping out cold glycol into the loop. The temperature difference only has to be 3 degsC for the sunboxes to come on.
  • Example: 930pm (working) - 13.7º in sunbox, 11.2º external air, pump pushing out 4.2º, coming back from boxes at 5.5º, and coming back up from the borehole at 7.2º.
  • Example: 1030pm (still working!) - 13.3º in sunbox, 11.1º external air, pump pushing out 2.9º, coming back from boxes at 4.0º, and coming back up from the borehole at 5.8º.
  So it's clear from these that the GSHP gets a substantial amount of heat directly from the sunboxes and as the glycol is cold, it does not give heat to the soil. Instead, it just reduces the amount of heat that the soil has to give back to the glycol. So this is a direct 'real-time' return from the sunboxes to the heatpump. This is an unexpected benefit, in addition to the already observed diurnial benefit.
  Back to the boxes: despite the very cold glycol circulating through the large black collectors, the air temperature in the boxes was remaining remarkably steady. As the exterior temperature was falling, the only thing in the boxes with the thermal capacity to keep the temperature high must be the brickwork.
  This would never have worked as well if I had simply fitted a pro-flat plate panel to the wall - they have no thermal capacity.

  In our visit to Norwich, we were surveying the Norwich house, and are planning to put 1.88 kW worth of PV panels on the south facing roof.
 However, one thing that very much alarmed me was that the Circuit breaker for the OfGem meter popped sometime during the day, so the roof would go on generating, but none of it would go to the grid or show up on our meter. If we were on summer holiday, it would be very annoying to find that thecircuit breaker had been popped for several days and generated nothing useful.

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