Quote:
Originally Posted by AC_Hacker
I came across this very interesting document just now:
Table 1 is called:
Table 1. Recommended Lengths of Trench or Bore Per Ton For GCHPs
It adjusts for ground temperature, soil conditions, the whole thing. |
When I made this post, I was not sure if the bore hole that MEMPHIS91 dug was up to the job of delivering heat to the water tank.
Thanks to all the good advice regarding optimizing the compressor, the bore hole was obviously not the problem, and incremental improvements are likely. But I am still intrigued by the information in the PDF that I found, because it has very useful information for GSHPs in general, and information that is particularly relevant for GSHPs in the South Eastern states.
I was wondering how 'close to the edge' is the bore hole that MEMPHIS91 dug, with regards to its ability to yield heat.
There were two tables that were of interest, Table 1 which indicated the depth of a 'standard' bore hole that will yield 12,000 BTU/hr with regard to soil temperature, and Table 4 which has correction factors for various factors, like water content of the soil. So the idea is to select a 'standard' bore hole length according to the ground temperature, and then correct the length for soil moisture, and other factors.
I ignored the horizontal loop-field data, and just concentrated on the vertical bore hole data. The first thing I notice was that there seemed to be an error in Table 1, because the warmer the soil, the deeper the hole needed to be... should be the other way around.
Apparently this chart is for a GSHP that is used as an air conditioner. Also, the data to the left of the 52 degree F column looked 'out of whack' (apparently Southern soil behaves in mysterious ways), so I ignored it. When I reversed the data left to right in the columns in the '52 degree F' to '68 degree F' range, everything began to make sense, and agreed with common knowledge regarding Oregon's soil temperature, so I decided to make that change, and proceed.
Next thing is that this chart is based on using water in HDPE tubing and MEMPHIS91 went with Direct Expansion, which is generally, 15% more efficient. So I tweaked the data for DX...
Then there is the correction factor from Table 4 that needs to be applied to adjust for soil conditions, and as far as I can tell, MEMPHIS91's soil conditions are a gift from heaven, so I applied the very most favorable possible factor, and came out with this:
I'm runnung close to the limit off images per post, so I'm leaving a few steps out...
This image indicates the sustainable heat extraction from a 30 foot DX hole, given MEMPHIS91's soil conditions.
Notice that I have added a 55.5 temp line because that is what MEMPHIS91 has. I did some regression analysis on the data, so that line is more accurate than interpolation would be.
Here is the heat MEMPHIS91 is actually extracting:
So, if this analysis is true, MEMPHIS91 would seem to be performing a miracle, because he seems to be extracting more heat than is possible to extract. But this is not the case, because the study from which this data comes from assumes continuing heat extraction, and MEMPHIS91's application is intermittent, and the bore hole can 'recover' between run cycles.
Continuous extraction curves have a general shape like this:
Although for GSHP heat extraction, they would not be so steep, but they will approach a limit, which the study has assumed.
CONCLUSION: From MEMPHIS91's performance date, and also from an analysis of the study, there is heat yield to spare,
when the system is run intermittently. It also indicates to me that strapping a bigger compressor onto the existing bore hole might yield a larger heat output, but COP would most likely suffer considerably, because in the grand scheme of things, MEMPHIS91 called it pretty close.
Best,
-AC