Our panels corroded but we had a stainless tank and simply replaced
the panels and they have been fine for 7 years now but only after
research. (Touch Wood)
Solar thermal systems need to be very well designed to last well past
the financial and energy payback period. They are quite vulnerable to
corrosion if the incorret materials are used or not well designed.
It's clear that a great many systems that were sold lacked long term
experience and testing. I guess some kind of statuatory warranty
extending out to over 7 years is required.
The problem around here was climate, and a general lack of common sense.
They never lasted long enough to corrode.
The climate problem is the fact that it's the state of Indiana, and gets
down to
-10 F or lower for long periods of time during some winters.
The common sense problem was that people fell for the hype and bought them,
and they were not able to see the primary difficulty in putting a box full
of water
filled pipes out in subfreezing weather.
Some had systems to drain the water during the winter.
But the systems were not 100% effective, and it would
sometimes leave enough water in the panel to bust it open.
Other systems had continuous flow.
The water pump would be left on and continuously pump
water from the internal holding take during the nights.
Even though it did waste some energy in the nights, it didn't freeze..
Until.....
Well.. Until there was a power outage in the middle
of a subzero (on the F scale) night.
No power to run the pump....
The solar water heater then becomes a solar ice heater....
In the day, you had a ice shelf hanging off the side of the house.
I never seen anyone that was trying to sell a dual loop system.
But if one was going to go with solar water heating around here,
they would need a working fluid that would be unfreezable down to -20F.
That way, you could shut off the pump for the cold times, without anything
freezing.
And during the hot sunny days, turn the pump on and transfer the heat to the
water
in the holding tank.