Alex and Fred,
I've been designing and installing solar
how water systems in VT for almost 9 years now. I think
I can answer your question, at least in terms of what can be done to minimize bacterial problems. If you set up the solar tank as a pre-heating tank for your existing hot water tank, then whenever you turn on the hot water tap, fresh water is added and old water is taken away from both the solar tank and the back-up heating tank. So with a pre-heating arrangement, cold water from the well or town, enters the solar tank first. The hot water outlet of the solar tank is plumbed into the cold water inlet of the electric hot water tank, (or indirectly heated from oil or nat. gas hot water tank). Water flows through both whenever anyone turns on the hot water tap, so I doubt bacteria would have much chance to colonize and contaminate.
The temp that the plumbing codes say a tank should
be kept at is 140 deg F . Because this could scald you, you should have a mixing valve on the output of the water heater to mix a little cold water with the hot coming out. These cost about $100.
A tank thats only heated by solar might go for long periods at temps which are below 140, other times it might get up to its top temp (I set that at 175). So on a routine basis the solar tank is sterilized by high temp, but even if it weren't it wouldn't pose a problem in my opinion because as long as people
are using hot water, then the water in the tank is refreshed daily. If people aren't using hot water then the solar system should get it above 140 on a periodic basis, unless it were poorly installed or designed. The systems I generally install have 2 flat plate collectors and 80 gallons of storage. With one day of sun they usually can go from 50 deg to 140. It depends on the circumstances of the site, but thats typical.
In tanks which have a coil in the bottom, and an electric element half way up, you can have the same tank heated with solar and electric. The one drawback to this is that you only effectively have half the tank to store captured solar energy because the top half is already heated with the electric element. Solar can heat the top up to a hotter temp even if its already at 140, but the collectors will lose efficiency and energy is lost. You can put the electric element on a timer, so that it only comes on at about 4pm and goes off at 10:00 in case the sun hadn't already heated it up because it was a cloudy day. Or you can get a larger tank. For a 2 flat plate system (generally) a 100 gal tank is needed if you want to heat it with electric and solar.
hope this helps