Didn't know where exactly to put this; a few categories potentially applied. I settled on water since that's the target use for the energy.
I toyed with this idea a while back, but couldn't figure out how exactly I would come up with the source for the compost, and now, after digging around in the back 'yard', it became clear. Now I know many are skeptical, and more or less have shot this down in the past, but...
I have this annoying tree that grows in the form of several (many several?) saplings called a "black locust". This thing has a few annoying traits, the least of which is the sapling stage's one inch (ish) thorns that grow out of the thing every few inches.
The other is the one I intend to exploit; I figured I had killed a few of them by removing all of the foliage from the main 'trunk' (about 3/4" diameters currently), but the damn things just sprouted more, as if I had pruned it, and of a substantial amount in just a few weeks.
At the time, I was only concerned with regaining access to my bug zapper which the foliage had obscured. Being lazy, I only had a pair of dikes with me and hacked up what I could, with a willy-nilly attitude. BUT - I figure with optimum pruning, these things can really put out, and I'd just have to wear chain mail or something to harvest them.
The pile of 'branches' that I pulled off of the main trunks were all left in a pile, and they are composting into a fine powder rather readily. I think I'm on to something here.
So what I need to know is, how do I (roughly) calculate how much heat I can get out of a given amount of this stuff, and what might the critical mass be?
I realize there are several variables to this, but I'm thinking it will be useful as a preheat for DHW at the very least. I'm thinking something like a 6 or 8 foot diameter 'bin' made out of brick(?) and then insulated, to contain the compost. Maybe 8 feet tall? Run some tubing through it running to a heat exchanger in a closed loop under the house.
I wouldn't necessarily be all that worried about overall efficiency, but just use common sense efficiency (insulation, etc). I'd use it on a "get what it can" approach, maybe having a recirc pump that only runs when triggered by a pressure switch that senses the change in pressure or something when a hot valve is opened. That part is still grinding amongst the gears at this point...
Whatcha think?
Steve