Wednesday, May 19, 2010

Can we be sure of the Boreholes?

19 May : I am enjoying this deeper investigation into numbers, and wish I had done it before. I am particularly prone to work on the basis of enthusiasm and hope, and it can be healthy to do some numerical checks. Now that I have dissertation students working on it, we need more calculated numbers.

Had a long telephone call with David Atkins of Ice Energy, checking my sums for the borehole and for the house heat loss. It seems that the borehole estimate was about right - i.e. we should not expect the earth down there to rise noticeably in temperature, as a whole. We could not be sure if it had, unless we had some very accurate measuring equipment down there. There is a thermocouple down there, but I forget now how deep it is, and how well taped it was to the black borehole pipe.
    Any attempt to measure this (e.g. to find out if it takes 200 days to charge it up 1 degree) is complexified immediately by some significant variables:
• we are putting energy down there, daily, at amounts determined by the weather.
• we are getting getting energy back up, every day, dependent on the season and heating demand.
• the putting and getting of energy causes 'thermal rings' around the pipes and with two pipes, we have overlapping rings.
• we do not really know how large the 'effective radius' is - we estimate the 5 metres, but the immediately effective radius may only be 1.
• the 'bottles' are not contained in an insulated jacket - they are infinite.
• the soil below may be stratified - if there were some air pockets, or gravel layers, the calculation would change.

Diurnally, we observe that we are getting good performance from the immediate clay around the plastic pipes. Perhaps we should no longer measure the ground temperature in the evenings because it is too 'ringed' with cold and warm rings as a result of daytime charging and evening heating. We should do a Morning Ground test after it has spent a night resting, and not expect it to rise dramatically. Or, we could measure the ground at midnight, about 2 hrs after it is turned off for the night.
    It is written that the COP of the heatpump is estimated to rise 3% with every additional degree C in the warming medium - so if our heating and cooling is working to a more limited radius, eg one metre around the pipes, we are seeing that at least 5 degrees warmer than in March just before the sunboxes started work.

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