Monday, October 3, 2011

Question on thermal storage

2 October 2011: I will moot a point now that perhaps will be prophetic when I look back on it, or simply expresses anxiety.... or invites comment. Looking at a the graph above, here's a number of comments.
• Realtime mode: It's clear that the Sunbox is beneficial in supporting the GSHP in winter, so we can surmise that the contribution of the Realtime mode is working.  We are successfully defrosting the deep ground in the worst part of the Winter. It never got below 10.0ºC. Also, during Summer, the hot water heating almost always makes use of the Sunboxes in Realtime, and keeps its consumption super low, compared with the previous years. This is working!
• Diurnial mode: The descent of the curve and the climb back-up in spring shows that the speed of recovery is aided by the Sunbox, working in its Diurnial mode (bright days providing top-up heat for the cold evenings) This is working!
• Interseasonal storage - I am having slight herby-jeebies, for the first time, even though this is the central pre-occupation of the whole project. The graph of ground temperature is turning down slightly even though we haven't started heating yet. I have Questions...

So, let's discuss that:
We are still in the Summer season as far as the GSHP is concerned. There was a two Cold-snap a week ago, and we had the heating on (for evening only briefly) and the ground temperature dipped slightly to 12.9ºC - a distressingly quick response to a very small demand.
    We have had a fabulously sunny week since then and the solar capture has been 123 kWh, more than twice the normal weekly capture at this time, in fact the highest ever in a week since the Sunboxes were installed. Using the same measuring method, the ground temp has returned to 13.4ºC. Nice, but could it not have gone higher after all that input? I really was hoping for a 14 !
    It seems to me that there is an autumnal downturn in the ground curve, even though the heating season hasn't started yet. Is this just a result of heat moving away from the borehole to the region around (levelling out the thermal contours) ? I recognise that with a tarmac road running north south next to the house, we are well positioned to pick up solar heat from nature, but surely, it takes decades for that heat to reach down 48 meters? The heat down there comes from all around, not just from the immediate road. Also, 123 kWh may not be very much in the context of 3,600 cubic metres of compacted dirt, but in diurnal mode, while the GSHP is running daily, this heat is very close to the borehole pipes and therefore most useful. Interseasonal heat dumping moves away from the borehole into the vastness around.
   I have estimated that Annually, we need to draw about 9,000 kWh from the ground. We help it by putting 3,000 kWh down with the Sunbox. So the greater ground and the natural forces which put heat down there are the dominant entity, the Sunbox being only a helper. From this, you would think that with the simple equation of 9k - 3k = 6k, the ground still needs to yield a large amount, 6,000 kWh. Because Sunbox heat is mostly closest to the pipe (in Diurnial and Realtime mode) the effect of the Sunbox is Realtime and Diurnal mode is greater than this simple linear approach suggests.

No comments:

Post a Comment

Comments will be moderated before showing. Please make them relevant to the subject of the posting. Comments which advertise commercial products will usually be deleted.

Popular Posts