Hmmmm, have you overlooked the idea of digging a "cave" while you have the
backhoe? Instead of building a room, dig a hole. Line it with concrete block
walls. Excavated dirt mounded over the top. If your water temperature is 50
degrees, presuming Farenheit, the subterreanean temperature should be pretty
close. And very stable, summer or winter. Earth is a good insulator.
Don't forget solar energy to power pumps and stuff. Wait, you said
Washington. My experience with Washington is the sun only shines one day out
of 20, solar might not be cost effective. --LOL--
A half-horse pump will easily raise a 16-foot water head. Two-stages work
best -- that is, pump the water into a pressure holding tank.
Take a serious look at using convection to move air. Think about going to
the local junk yard for automobile heater "radiators" and build your own
heat exchangers with low power/cost fans.
Insufficient input for BTU calcs. Complicated process.
Sounds like a fun project. Oh yeah, do a serious cost comparison against
refrigerator style "cellars." Mine is great. Did I mention that the diurnal
temperature here in the Mojave Desert can swing more than your annual
temperature variation. The winters here get down to the mid-teens and the
summers go into the mid-hundred-teens.
Keep us posted on your progress step-by-step. Sounds interesting.
Seems to me that in the gardenweb greenhouse? section there was discussion
of such things- obviously taylored to a different direction. Maybe some good
info. I'm also in western WA.