Sunday, April 3, 2011

Include hot water in Carbon zero equation?

2 April '11: I have to write this on the second to avoid the reader thinking this is an April Fool. I am now convinced we have achieved Carbon Zero for the Space Heating AND the Hot water (DHW).
    PV roofs are generally limited to 4 kW by the Feed in tariff system, and with a south facing roof, this might capture 3,600 kWh at best, and for our house with an east facing roof, we seem to be averaging 3,250 kWh, so this provides the target to beat.
   Following the Transition movement's motto of "Power Up, Power Down" we need to do both - to achieve a good power generation up to a target figure, and to reduce our heating requirements down to or below the same figure.  The efficiency of the GSHP has improved now that the deep soil below is warmer, so there has been a continuous improvement in performance, week by week and the figures are converging. (and we are being helped by the Spring weather being unusually benevolent!).

   Sometime last year, beginning of October, the annual totals of the PV roof managed 3,325 kWh and the GSHP consumed just under 4,000 kWh. Assuming that the space heating is 75% of the effort, that would be 3,000 kWh, which is below the capture of the PV roof, ergo, the space heating was carbon zero - we put electricity into the Grid, and we draw less than that total from the Grid. The Sunboxes had 6 months of work to this point, so I didn't really expect it to get better.

I found some more ways to make improvements in reducing power consumption:
At that point, I fitted the aluminium mirrors and voltage regulation. Earlier, I had added more insulation to the internal water tank in the GSHP. During the winter, I fitted a cylinder thermostat to stop the IVT's persistent water overheating problem. My wife decided that we would have the thermostat turned up for the winter by a degree, but in return agreed that if she wants to sleep in the bedroom with the window open even on the coldest nights, the bedroom door could be closed (bargain!)

  We then had the coldest November and December in weather history, and somewhat depressingly, the annual PV fell to 3,236 and the annual GSHP rose to 4,245, and for a while we had zero input from the Sunboxes (and the meter went wrong for a short while). Our 'annum' included the double winters of 2010, the unusually cold Jan, Feb and Mar, and the exceptionally cold Nov and Dec.

During the Spring of 2011, we have had mild weather and things cheered up. At this moment, we are getting close to 3,500 for the GSHP and the PV seems to have settled to a safe annual average of 3,250. My dissertation student Lyndsey pointed out that I should deduct the consumption of the floor pump to arrive at a more accurate figure for the GSHP. In fact, I propose to buy a meter specially for this pump, so it can be deducted precisely.
  I calculate that the underfloor heating circulating pump uses 250-300 kWh during a winter. Deducting that now brings the consumption of the GSHP itself down to equal to or less than the PV roof. That is its entire effort, including the domestic hot water.  3,250 equals 3,250 !
   There is another 6 months before the anniversary of the mirrors, so I hope that further improvements in efficiency will be achieved.

1 comment:

  1. Its fair enough to say that the circulating pump is part of the Heating system... although Gas users never include the cost of the central heating circulating pump.... but if we get the total for the heat pump (including floor pump) down to 3250, we have a wrap!

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