As I mentioned in the weather thread
, I'm always amazed that in the middle of our cold winter here in N IL, the surface of the south-facing siding on our house will be over 100F. Seems like we ought to be able to capture that heat.
linked this article:
The $1000 Solar Water Heating System
and of course, I spent a good part of the day on the site instead of doing my taxes. I agree with samclem that there are some good aspects to the design (the 12 Gallon capacity PEX heat exchanger inside, near the standard water heater).
Like samclem says, though it is tempting to use this heat for the house, it probably makes better payback sense to heat water with it, as that is a year-round need. A few things that are de-motivating me though...
1) I analyzed my gas bill for a few summer months - after you subtract out the fixed costs, I only spend ~ $13-~19 per month on gas to heat our water. And I'm overdue for a replacement water heater, which will be slightly more efficient (and this is the lower cost, lower efficiency model, no premium for efficiency that I wouldn't need with the solar system anyhow).
2) That article mentions $1,000 cost, not including time. If I assume it provides 90% of my hot water needs, that is an over 7 year payback (assigning 4% opportunity cost to the $1,000). Not too bad really, on pure economic terms except...
3) you end up with a custom, somewhat 'tweaky' system. Who really knows how reliable this will be? Who is going to fix it if something goes wrong? Unless I'm there to do it, I can't imagine what a HVAC guy would charge. I imagine most of these guys are replacing parts on units they are familiar with - being faced with a system like this would surely throw them for a loop.
4) Resale value - I expect this would decrease the value of my house. I can't imagine that too many buyers would like the idea of a handmade 150 gallon tank in the crawlspace, a bunch of wiring, pumps and plumbing, and a big panel on the side of the house. All to save $15 a month?
5) While I think the article is on solid ground with their heat exchanger design (the PEX tube holds 12 gallons of domestic water, so it's OK if it takes some time for it to absorb the heat from the circulating solar heated water), I really wonder about the engineering of those solar panels. As samclem mentions, aluminum in contact with copper over the long run? And how do they determine the optimum pipe diameter, spacing, etc. If this isn't engineered properly, you are wasting money on this part of the unit. Maybe there is info elsewhere.
6) Freezing - the thing is a pretty good 'failsafe' design, but as samclem points out, a stuck relay could keep that pump going and allow things to freeze. A series relay on a timer so can only run during daylight hours would provide some added protection. One system used a solar PV panel to run the pump - that seems pretty good (cost?) - it can only run when the sun is shining.
Now, if this was sized to provide 90% domestic water heat in winter, there would be excess heat in the warm months. If that could be turned into electricity with a stirling engine, maybe it could all work out. More complications/cost.
To avoid all those issues - I'm more attracted to my original thoughts - just a hot air collector to warm the house. No liquids, fewer complications, but I'm not sure of the payback (only used part of the year), which I think would be OK if done on new construction.
$350 Solar Heating Thermosyphon Collector
Bottom line, I ought to just figure out how to conserve $15/month in utility costs. That would probably be far easier.