Passive house cooling was mentioned in another thread and I thought it would be good to have its own thread.

Here are some questions:

Can passive cooling be done with a horizontal closed loop?
Can systems not originally designed to be passive be modified to be selectively passive or run with the compressor?
What would be involved and what would be the ballpark cost be to do this?
Would this void the manufacturer's geothermal warranty?
Does passive cooling do anything for humidity removal?

Passsive cooling simply runs water through a fan coil and blows air across it. This fan coil could be installed in ductwork (generally) downstream of a furnace, air handler, or geo heat pump.
Dehumidification would occur two some degree as long as the delta t is such that condensation will form.
You could use the brine from a closed loop system in early spring but once ground temp rose to as little as 55 or so, you would not be thrilled with the results, nor would you continue to dehumidify.
For brief spans a passive cooling system can work satisfactorily, but if you noticed, when it was mentioned the poster said "if you are going open loop anyway". Curiously during the times when passive might work with your closed loop brine, a 2 stage closed loop unit could be rockin with EERS in the 30's because of the low EWT. IOW it might cost less to run than a pump and dump passive.
Joe

Thanks Joe. I was mainly thinking of using the existing fan and coil in the geothermal unit, just not the compressor. Could this even work? It's probably not worth it for us based on what you said on the EERS, but I am still curious.

Your existing fan coil is filled with refrigerant. A passive coil; with water. Don't want to mix the two so a second coil would have to be added. It has been done, but not as you envision it.
Joe

Cool (pun intended). Thanks again.

We have a passive option with our setup. Basically our geothermal system only heats our indoor pool and dumps the cold side to the field. The field is in parallel with a water coil in the furnace. I can set it so if the house calls for cooling it turns the “furnace” fan on and a circulation pump in the field side. If the geothermal is running to heat the pool, it is plenty cool, about 40F. If the geothermal isn't running then we are at whatever the field is at. It starts in spring about 45F and by mid August the field is about 60F. While 60F helps a bit, the house will raise the about 1-2F over the day, but with it off it will raise about 8-10F over the day (off peak period)(9a to 7p). As Joe said there is very little dehumidification at 55F and at 60F there is really none. But it warms up the field a bit which helps once it starts trying to heat the pool once we hit off peak. In summer I close off the field during off peak times to dump all the cooling from the geothermal to the house.

Is it worth it? I like to tinker and it requires more manual input, but there is an advantage.

Off the top of my head I can envision an arrangement wherein systems in northern areas with sub 60 entering water direct that water first through a coil placed in the return air path. Under typical operating conditions that would mostly result in removing some sensible heat. The unit's evaporator coil sensible heat ratio would then skew more toward latent. I guess there would be a slight improvement in overall efficiency, and better dehu but at the cost of additional complexity, first cost, and increased blower power overcoming the additional coil.

Brock's system accentuates the possibilities, depressing the EWT via pool heat, which is desireable during all but the warmest few weeks in the northeast, upper midwest, and northern plains.

Again (lest we forget the point) with a closed loop system (op's question), the time when you might be able to passive cool are also times when EERS will be highest with the compressor running....
j

Passive cooling? True passive cooling doesn't require power, when you add in a fan to circulate the air as well as a pump to circulate the water, it's really not passive then is it.

How about a hamster on a wheel with a belt (or direct) drive to a fan blade?