Wednesday, January 5, 2011

Comparing Degree Days and GSHP

4 Jan 2011: This Degree Day exercise has been very useful in reinforcing my confidence in this project.
  The chart compares the kWh consumption of the heat pump and the degree days of the same month. They are not the same units, but there is a linear proportionality between them.
  The GSHP meter was fitted in the last week of Sept 2009, so I have records from 1 Oct 2009, the same date as the start of the PV roof.

The Winter of 2009-2010 shows that the heat pump consumption for that winter was much greater in proportion to the degree days, compared with the winter we are now going through.
  The Surya Sunboxes started working in the first week of March 2010, and March was quite sunny. There is a steep fall in the GSHP consumption to bring it much much closer to the degree day curve. There was a brief cold spell in May 2010 which is reflected in both curves. During the cold months, the GSHP figures also include about 60 kWh per month for the circulating underfloor heating pump.
  During the summer, there was no space heating requirement, but the GSHP has to heat water, so it has a small workload. Other houses may have needed a small amount of heating in summer, but ours did not as the good insulation and thermal capacity ensure that it performed passively.
   In late September, the heating season started. The Oct-Dec'10 degree days curve reveals a much colder half winter that in Oct-Dec'09, but the heat pump's consumption is tracking it more smoothly, and the consumption peak at the end of the year is never as high as it was at the start of the year.
Weather data from
   Our energy efficient lifestyle hasn't changed, and if anything, we use more energy because we start the GSHP in the mornings nearly 2 hours earlier than during the winter of 09-10. May I be permitted to interpret this improved efficiency as being a sign of better heat resource in the boreholes?
  The graphs on the right show temperature records for Nov and Dec 2010.


  1. Great figure. I´m eager to see how it goes March-April.

  2. Thanks Sami! It is very encouraging. We have a mild week forecast here, but no Sun likely to be visible until Sunday. But I hope that with the mirrors helping, we will have a higher solar thermal harvest in the coming year.
    I havent posted this graph yet on the Navitron forum. Do you think I should? i dont want to attract another wave of scornful disbelief.

  3. Just post, and post next one in end of Feb.


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