Friday, July 8, 2011

Double width sunbox design

This Surya mark3 would be of twin wall Polycarbonate
with aluminium framing. The upper reflectors
are a wintertime optional booster
7 July 2011: If I rebuild the sunboxes, it may finish up as a large single one - double width.
  Thinking of earlier postings, I remember regretting that I had not made the sunboxes as a larger volume. Although it's OK for the liquid to be fed into the panels in a manifold arrangement, I realise now that one wants a good thermal balance between the volume of air and the volume of liquid in the black panels. If the balance isn't right, then the black panels can chill the air space too quickly.
  There is greater surface area and smaller volume with the two existing rectangular sunboxes, and they don't balance up their temperatures with each other, e.g. the West one is tree shaded in Winter and Equinox, but the thermostat sensor is in the East one.
  Assuming that the air is at about 25ºC and the liquid is a 40-60% glycol water mixture.

 Volume of air = 2.96 m3 (present volume 1.02 m3)
 Volume of liquid = 0.048 m3 (assuming 4 m2 x 12mm)
 Mass of Air = 3.55 kg (present mass 1.22 kg)
 Mass of liquid = 51 kg
 Thermal capacity of air/ºC = 3570 joules (present capacity 1230 joules)
 Thermal capacity of liquid/ºC = 183,500 joules

So, the liquid is unchanged, but the air has almost three times the volume, mass and thermal capacity, the ratio of liquid to air changing from 150x to 50x.
  Question is, do I do it this summer or next? During the summer, the GSHP is virtually inactive, and the ambient air temp is warm enough for bare black panels, so it seems that a vote for this summer would be better. Also... I have some free time at the moment.

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