Wednesday, July 13, 2011

How does 15% become 30%?

12 July 2011: It has troubled me, that there seems to be a mathematical puzzle. How does a 15% improvement in COP give a 30% improvement in electricity consumption?
  At New Year's Eve, the deep temperature of the deep boreholes were 5 degs warmer than in the winter of 2009-2010. On this basis, I expected the improvement in GSHP efficiency to be in the region of 15% at best - the rule of thumb is that each extra degree of warmth in the heat source gives 3-4% increase in COP. So 30% should not be possible, but this is the evidence of our electricity readings.
  It struck me of course, and I don't know why it took so long. Most good quality Heat pumps on the market have a protective algorithm, that if they cannot get enough heat from the ground, they change over to direct heating. Our GSHP is nominally 6 kW and for the first two years of existence it used 222 hours per year. In the first test year from Oct'09 to Oct'10, it used 111 hours (strange mathematical coincidence there). That makes a difference of several hundred kilowatt hours every year. If the COP improves significantly, it tips the GSHP over into the happy state of not using the emergency 1:1 heating. In the whole of last winter, the coldest on record, it used 1 hour.

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