A heat source is a heat source. The only thing I see as being a problem is collecting the heat from the hay, or compost for that matter. I don't fully understand how you store it, if it is in bales, or just piled in a big heep. Tod's response makes me think the latter in a barn, on a concrete floor, thus the pex in the floor so your tractor could ride over it without damage. That doesn't sound like it would be the most effecient collection of the heat, as the heat would want to go away from the floor, but it should capture some of it. It would be best to have the collection tubes in the middle of it for best collection, but that doesn't sound like a workable solution at all. Or imbedded in something that would naturally absorb the heat and transfer it to the lines. Copper lines, or at least metal of some sort, may be better to collect the heat.
Keep us informed if you proceed or at least do some experimenting with it. Sounds like it has some very workable possibilities if the particulars can be worked out. Good luck with it! Jim.
Home of Irish Setter Rescue of North Texas
When Irish Eyes are smiling, they're usually up to something!!
At a minimum, I'm Pentatoxic...but most likely, I'm a Pentaholic. There seems to be no known cure. Pentatonix, winners of The Sing Off, season 3