I had the opportunity yesterday to spend the day at a plant which manufactures solar thermal water heating system, and learned a lot about those systems. Learned there are a lot of systems installed which don't work...can't work! A solar system really calls for an engineer-prepared design, and of course a well-qualified installer. They do provide engineering drawings with their system.
First thing is sizing. Traditional data like recovery rate, gallons per hour, etc. are NOT where you have to focus. You have to focus on total gallons per day of hot water use. This company installs a temp. flow meter with internet data logger. That give them the baseline to work off. Ideally, the system should be designed to provide well over 90% of the hot water demand, in the warm summer months. Naturally it will then provide down below 60%, and even as low as 25% or even 0 %, in the winter months depending on where you are!
A solar system will tend to need much more storage capacity then you might assume.
Another feature that I was not aware of...the most common system type today does not circulate the potable through the collector...they use a glycol loop with a heat exchanger.
A key point if you are selling these systems.....the rebate/tax credits/depreciation picture is very complicated, and the system design will be carefully scrutinized. If it it does not meet criteria...no rebate! The California Solar Initiative group has a mandatory one day class before you can sell these system, mainly for that reason.
One final point which I had not penciled out.....Solar thermal installed where you are heating water with electricity will have a payback in as little as 3 years, depending on electric rates. Apparently the cost of NG is so low that the pay back for solar would be over 30 years before rebates, but on a commercial project where there are rebates and depreciation, it could be about 12 years. That one will be a tough sell. It is great when a company just wants to go green, but if they are just penciling out the bottom line, it is a wobbler.
I think you're spot-on considering how cheap NG is, and payback times getting longer and longer. I'd think though that given the huge amount of solar equipment on the used market -- and the way you see more do-it-yourselfers in a down economy anyhow -- there are going to be a lot of people looking at super-simple setups, done themselves, and on the cheap.
I've sort of become one of those by accident; I couldn't pass up a pair of 20-year-old collectors that didn't leak for $50, so I'm probably going to do a pre-heat DHW tank just as you describe, with glycol and a heat exchanger. "Free" BTUs are hard to pass up.
But new equipment, pro install... it's going to be a tough sell indeed. Probably the growth market on all this is going to be post-DIY service calls.