Our condo HOA had a meeting last fall, and somebody brought up selling the flat plate collectors on the roof that are part of our defunct solar thermal hot water system. The 750 gallon cylindrical storage tank rusted out in 2003 after 20 years of service. The outbuilding that houses it was basically built over the tank, so swapping it out for a new one would have meant either chopping the thing up in place with a cutting torch and building a new one on site, or removing the roof, which nobody was keen on. Some plumbing got re-routed and the tank sits there still, derelict. It was also mentioned that the main boiler for our hydronic district heating might be nearing the end of its days. I volunteered to look into whether it would make economic sense to repair the solar thermal system, and what the options were for the boiler.
Given that flat plate solar thermal collectors generate an average of about 1kBTU worth of heat per day per square foot (according to the US EIA), and given that we have about 250 square feet of collecting area (nine 28 square foot panels), the current system ought to collect something like 250kBTU/day. Our current boiler consumes 520kBTU/hr worth of gas, meaning that the solar thermal system could at best displace a half hour’s worth of operation each day. Gas costs about $8/million BTUs, so the boiler costs about $4/hr to run. If we assume optimistically that system losses are negligible, and that the boiler runs at least half an hour a day 250 days a year (it was only hooked up to the baseboard heating, not the domestic hot water) then the solar thermal system is capable of displacing something like $500 worth of gas each year. This is a best case scenario though, since the hydronic system needs water that’s hotter than the flat plate collectors can make it (so the boiler will have to do some work to boost the temperature) and because the system losses are almost certainly non-negligible.
Still, $500/year might be a significant savings. To know whether it’s really worthwhile, we need to know how much it will cost up front to get this savings, and how long we ought to expect to be able to collect it (i.e. what’s the system’s expected lifetime). I got wildly varying estimates of the cost to get the system up and running again. At the low end it was $5000, to leave the rusty tank where it is and put a collapsible storage bladder in the crawlspace. At the high end it was $20,000 to remove the old tank and build a new spray-foam insulated stainless steel one in its place. I used this calculator to sanity check my energy numbers above (which don’t seem crazy), as well as the estimates. It suggests that all in, the total system cost including installation would be something like $28,000. I suspect that a plastic bladder in the crawlspace wouldn’t be as efficient or as durable as the new stainless tank. For the sake of argument, let’s say the cheap option will only last 5 years, and the expensive one will last 30 years. The original tank lasted about 20 years. Here’s what it looks like today: