Joe,
Welcome to the conversation.
Geothermal pretty much started with cooling, and only later began to use ground for heating.
Here's a link to the main
Geothermal page at Oklahoma State University, where it all started.
Here's a link to the
page where they offer manuals. They are expensive, but there is no place I know of where you can get this information any cheaper.
I own a copy of CLGS Installation Guide (#21020):
(The "original" Installation Guide), and I think it is well worth the price.
From what you have said, it looks like you have a pretty good idea what you're doing.
I say full speed on the project!
It would be very helpful if you documented your project here on this site, with pictures and plenty of description of what you did.
Just imagine how helpful it would be if you found a thread right here where somebody had already done just the project you want to undertake!
That's how it works... only you get to be the first one to do a Trench Air Conditioning Project.
A couple of thoughts I have...
A single 750' run of 3/4 HDPE is going to develop a fair amount of friction. You can get the required 3 gallons of water per minute per Ton to flow thru the pipe (6 gpm for your application), but you'll have to use more pump power than otherwise. This extra power will come off of your efficiency.
If you run your two loops in series, you will totally have too much friction.
If you run the loops in parallel, it will be better, because of less friction.
This is the kind of thing that you would find in the CLGS Installation Guide. There you will find formulas for calculating optimum trench/pipe sizes, pump sizes, etc. You can do it by trial & error, but it doesn't hurt a bit to be informed before you begin to dig.
Personally, I think you have a winner on your hands.
Take lots of pictures so we can all benefit from your experience.
> my concerns really are with adapting my existing a/c unit any thoughts?
You indicated you have questions about converting your existing heat pump, but you weren't so specific what those questions were.
Please clarify...
Also, as Xringer correctly identified:
So even though you don't have plans to heat with your GSHP in the winter, any removal of heat from the ground will be better you in the summer cooling cycle.
For instance,
- use some of the ground heat to heat your hot water, or better yet, divert the heat to a water heater before it gets to the ground. (get your water heater outside the house envelope. it is a summer heat source.)
- if you have the space, you could pump hot air through a small spare room where you have clothes hanging for drying, giving you a GSHP dryer.
- not unrelated, if you are very clever, you could divert some of the cold refrigerant to chill your food, and eliminate the refrigerator as a summer heat source.
...additionally, I would imagine that your house is single story, slab on grade, without a lot of natural shading, dark colored comp roof, am I close? If you painted the roof with a white heat-reflecting coating it would help you, if you put solar panels on the roof, they would offset some of your electricity use and provide some shade, reducing the amount of work your heat pump will be doing. Along these lines, there is a type of solar collector called 'thin film' that costs a good bit less per watt to install, but occupies twice the surface. In your case, this might be an additional benefit, because it would offer even more sun shading.
...if your house happens to be cinder block, and you're thinking about insulating, put it on the outside of the masonry walls... 2 inches of foam would work wonders.
Right now, it is cold in Oregon, I envy you your cooling problems!
Best of Luck,
-AC_Hacker