Recently, I was asked by a company whether two loops or four loops can be used in each borehole. Has anyone had experience using four loops in one hole?? Based on past experience, I understand two loops to be a reasonable limit for looping within one borehole. As systems are used dominantly to handle the summer peaks, the ground temperatures have shown some upward creep of the soil temperatures. Since heat transfer occurs in both directions, it is important to maximize surface area of the loops for conductivity through grouting and soils. By adding loops, within one borehole, the contact with the soil is cut in half (one loop to two loops) or in quarters (one loop to four loops). I believe the cost savings of few boreholes compared to the reduced effectiveness of the system's heat transfer, the upward creep of ground temperatures, and increased pumping to attain the same BTU transfer from the loop system does not sound justified. Can anyone explain their experience?
Just to clarify... one loop has two pipes...one down,one back. So two loops in one hole would give you four pipes. I have heard of two loops in one bore hole, never four. The majority of jobs are one loop per bore hole. You do gain some small benefit from having two loops in the same bore. The Ball State project which has 4100 bore holes is putting two loops in each borehole. I think the only time it makes sense is if you are really limited on ground space for drilling and the ground is highly conductive.
I will add to what Dewayne said, as the amount of things you put into a bushell basket approaches 2 bushels you must increase the size of the borehole to accomadate more loops. The cost of drilling per foot is what dictates the cost of a loop install. If you increase the diameter of the borehole the cost per foot to drill may go up faster than any calculated increase. The cost of the loops will also increase as you are putting four in one hole. Eric
# of Loops I have had several 2-loop systems. I believe your assessment to be generally true. When you model a multi-loop system using commercial software you do not double the heat exchange when you add the second loop. All is dependent on borehole configuration and thermal resistance but, gennerally, I get about 2/3 more transfer. I guess you could put as many as you could stuff into the ground but as I look at it, you really approach the casing/dip-tube configuration. (Casing in the hole with one pipe going to bottom and circulating back up the inside of the casing.) I believe that Dr. Spitler was working on this configuration. I would be interested to hear his comments.
