Thursday, January 19, 2012

Kjellsson thesis on domestic solar charging

Front cover of Elisabeth's thesis
18 Jan 2012: I've had an email exchange with Goran Hellstrom of NeoEnergy, and he has sent me a copy of Elisabeth Kjellsson's PhD thesis 2009 on Solar Collectors Combined with Ground-Source Heat Pumps in Dwellings. It's great to find that this has been properly written up. It is on the internet, with this link above.
   It appears to be based mainly on simulation, using TRNSYS software, which according to her paper, "has the capacity to simulate the whole energy flow with the basis in the climate, heat losses, transmission, ventilation, domestic hot water and demand of cooling." I would like to meet them one day, perhaps at Ecobuild. It would be interesting to run TRNSYS with the actual size of this house and the local climate and the capacity of the GSHP and see how the simulation matches reality.

Could I simulate the Peveril house system?
  I tried writing my own simulation using GDL, and really ought to go back to it: I should spend more time on it. I burnt out, slightly, after a day, but realised at the time that a more systematic approach would be better than diving in and trying it without deeper systems analysis first. See:
   The biggest problem for me was in trying to predict how much natural heat will return to the borehole. This is influenced by several things, most notably which part of the year it is, and the delta-T between the borehole zone and the space around - this changes every week. To make it more difficult there are contours around the borehole moving outwards (rings) and contours depending on the depth of the hole (distance from the surface). In addition, if you have twin boreholes, the rate at which natural heat is restored to the borehole is non-regular. Between the boreholes, the solar heat remains longer, but in winter, the natural heat finds it more difficult to return. In winter and equinox, the solar heat put down on one day is closely around the pipe and will be drawn up the same evening, without flowing out to the surroundings.

a screen capture of my effort at animation.
This image was from running the simulation
without solar input.
   Another problem is that the COP of the heat pump changes as the solar heat is provided. One could calculate the heat withdrawn from the ground by taking the electricity used and merely multiplying that by a COP index, say, 3.0. Or one could calculate the heat by looking at degree days for the week, and relating that to the theoretical heating and DHW needs of the house.
   The simulation has to work with and without the solar input, and it has to work for varying quantities of solar capture. If I increase the area of the Suryas to 12 square metres there would be a point where it would lose more than it gained. This may happen when I fit the evacuated tubes.
   By comparison, a simple calculation based on inputs and outputs would be easy.
  Another problem was how to represent it. I showed the borehole as something like a giant pair of lungs, with inflation and deflation, based on REAL figures recorded from the meters. Perhaps if I just showed it as numbers first, that would be enough, and then consider the visual metaphor of bottles or lungs later. 

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