Rough estimates

The server has five hard drives and a Core2Duo motherboard. I'll measure the actual current draw later, but for now I'm estimating that it draws less than 150 watts (9 watts per hard drive, plus 80 to 100 watts for the motherboard, CPU, and power supply inefficiency).

The server has capacity for five more hard drives, so we should count on another 50 watts for future growth.

The battery backup for the server should be OK outside the climate-controlled cabinet so there won't be any heat gain from that (I know they really shouldn't be run in a hot environment, but I've seen them do fine in worse conditions, so I'll count it out for now).

There might be a 5-port gigabit unmanaged Ethernet switch that I would want to keep in the cabinet with the server. I'm guessing about 10 or 15 watts for that, but again I'll take actual measurements later.

The circulating pump for the coolant will dump its heat into the coolant, so we should count on another 35 watts from that.

So my initial estimate of the internal heat load is about 250W, or 853 BTU/hr.

There will be heat gain from outside the cabinet during hot weather. Using the heat load calculator at www.cabinetcooler.info (manufacturers of a pretty slick compressed air-powered cabinet cooler!) as a rough estimate, our cabinet (uninsulated) will have a 388 BTU/hr external heat load if we keep the inside at 85F and the outside reaches 125F. I'm planning to insulate and seal the cabinet so it is closed off from the outside air and to minimize this heat load.

So as a first estimate of our cooling requirements, we need about 1250 BTU/hr. Plus any loss from the pipes that run from the cabinet to the ground loop. But I haven't thought through how to calculate that yet, so I'm going to pretend it doesn't exist for now. We'll need to re-visit that later.

From what I've read, a "real" vertical ground loop geothermal system gets about 1 ton (12,000 BTU) of cooling per 200 feet of well. That's 60 BTU per foot. I don't really expect this to scale down directly for a micro-geothermal system, but as long as I space the wells far enough apart, I think it should be pretty close. The first foot or two of each well probably won't have the same 60 BTU per foot capacity, so we'll need an extra well or two to make up for the "end-loss" at the top of the well. But I'm also counting on there being some margin of error built into the "1 ton per 200 feet" guideline.

With a 1250 BTU/hr requirement, we would need 20.8 feet of well. My auger drill has a 32" bit, and I think I can get away with two 18" extensions before the bit torques itself into a pretzel. So I should be able to reach 6 feet with some shop-vac action) which would require 3.5 wells. I'll plan on 4 or 5 for now until I fine-tune some of these calculations and do some tests. With the space available to us, we can fit 5 or 6 wells easily.

So the plan is shaping up: One server with 10 hard drives and an Ethernet switch in an insulated steel cabinet, being cooled by a small fan-coil unit circulating coolant through 4 or 5 wells, 4" in diameter and 6' deep. I think it's do-able!

-Tom Rusnock