Saturday, March 31, 2012

Tubes working, heat exchanger needed

30 March 2012: The Tubes have been installed and are working, but we have rapidly discovered some interesting issues. Their performance is very intermittent and it is apparent that we need the equivalent of a tank or a heat exchanger. The existing Surya sunbox has not needed a heat exchanger because the thermostat is comparing two temperatures - the solar/air temperature (immediately near the black panels inside the sunbox) compared with the down flow temperature of the ground loop. Even when the system is running fast, the solar/air temperature does not cool down fast when the liquid is flowing, and it might flow for 10 or even 20 hours a day in the summer.
  The flow rate for the tubes is slower that with the sunbox, and the thermal sensor for the tubes is measuring the flow through them. What happens is that the temperature in the tubes manifold rises to 22ÂșC, is then triggered by a delta-T of 8, and liquid from the ground loop comes up, immediately cools that sensor to the temperature of the ground loop, so then the delta-T is nothing and it turn off again.

Jason Philips who is the solar plumbing contractor is full of ideas and it seems I have two options, because I am determined to make this work. One is to use a water tank, but I am quite against it because I don't want mains water up in the loft, and I don't want a very expensive copper insulated cylinder.
  Another is that a heat exchanger can be used - this is warmed by a flow going through the tubes, and another flow through it is going to the ground loop. The time delay for energy to transfer from one to the other is enough to keep the tubes loop going. The energy is transferred through metal plates touching the pipes.
  I could make an exchanger up, because I am handy with aluminium and copper, but this would be wrong because one major purpose of this second Surya system (using the tubes) is to use industry standard components, not handcrafted products that take weeks to make.
  My early ideas in 2009 with a tank full of PCM balls were an attempt to make a heat exchanger, but it turned out that it was not necessary (but I should have been smart enough to include an expansion vessel.). I still have the balls, and could make up an exchanger, but instead of using water as the matrix, I would probably use dry sand - but this would be very heavy!

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