Bob,
We've had solar water heating for around six years. Here are some snippets of info/observations in no special order:
1. Description: It's a dual-coil pressurised tank (at 3bar), 300 litres in total. It serves two bathrooms + one shower room, kitchen, scullery, etc. The system boiler can heat roughly half the tank, the solar, about 85-90%. There are 8sqm of panels (4 units). The panels are flat, black-anodised stainless (not the glass, vacuum-insulated pipe variety). There is an electric immersion heater, low down in the tank, for ultimate emergency heating. It's hardly ever used, but combined with the boiler can provide a really fast reheat in the winter if we have a house full of guests.
The controller is an obsolete one, similar to a Deltasol E (
http://www.resol.de). There is a sensor on the panels' highest point, one on the tank coil's return pipe, and an FYI one on the hot water output (giving highest tank temp). You can set the hysteresis in both directions ('switch on' temp difference and 'switch off' too). It has an insulated drain-back tank (about 20litres, I think) and adjustable over-temp protection (currently at 104degC, IIRC). There's also a pressure safety valve at 3bar.
There are mechanical flow meters/restrictors, used to balance the flow through the four panels (plumbed in parallel). The only powered component is an ordinary CH pump. That's given no trouble at all.
2. Observations/criticisms:
8sqm was considered excessive for our storage capabilities, but fitted at my request, in order to reduce the reheat time in the early part of the day. I was probably wrong: from March to late October we don't store anything like the energy that we might: a longstanding intention is to add an additional tank at some future point to increase capacity, but this is a complex issue, potentially involving changes to the control system, so I haven't pursued it yet*.
The system comes on in direct sunlight at any time in the year. It's even collected useful energy with snow on the ground. I've got the hysteresis set to 6deg for on and 4deg for off: below that the energy transfer is poor and the pump uses more energy than is being collected.
Cloudy days drop its effectiveness markedly though. I believe the vacuum tube collectors are much better in this regard, but they are much more expensive and fragile - we simply couldn't afford them and I didn't/don't believe the claims of reliability made for them (although I have no data either way).
We didn't consider a plate heat exchanger - they weren't available for domestic systems when we were researching, and I'd question the value, given the additional thermal losses from pipework etc. We are in a hard water area and will eventually have to replace the tank, but that's 15 years down the line presently. To be honest I don't believe we have a big problem with limescale, as little precipitates out in the tank, rather at the ends of the pipes (the taps!). When we removed the old copper immersion tank, it was probably 20+ years old and had about 1" or 2" of loose scale in the bottom and hardly anything on the coil. The present one is stainless - I have no idea as to the relative advantages, but if the precipitation is the same, it'll be a long while before it becomes a nuisance. This one is harder to inspect as it would have to be properly drained, whereas the immersion heater was in the traditional top fitting on the old one, so you could remove it and look to the bottom with a torch. Doing it this way - a DHW tank rather than a thermal store - means the limescale isn't the issue it would be if it were clogging pipes.
I should also mention that our CH system is quite unusual in that the boiler is at the top of the house, in a side attic, with the DHW tank next to the boiler and the drain-back tank close by. The pipe runs are fairly short. We also have industrial-strength insulation on both the DHW and CH trunk pipework. Much of it is 28mm and the insulation is Rockwool with an aluminium reflective outer sheath. It's an art putting it on (it's definitely NOT flexible!), but it really works well. I was nervous about the expense when we did it, but I don't regret it at all. We also have an improvised heat recovery system to capture some of the warm air above the boiler (used in bathroom ventilation).
I have a slight leak around one of the pipes going to the solar collectors presently. It appears to be water is getting into the box around the panel and dripping into the attic in hard rainstorms (it really has to rain very hard). The system itself is fine. I called about two weeks before Christmas when I discovered it: Southern Solar came next working day to look at it. They talked sense and it will be repaired when we re-roof our dormers in March (at my suggestion - it's not panic-worthy at all).
It gurgles and the drain-back tank gets a bit hot in the summer, but overall, it's by far the most reliable thing in the DHW system.
I have several criticisms of the control circuit (but overall it's very simple, which is good for reliability):
1. It would be good to have a calendar and to make its sensitivity/hysteresis programmable for different times of the year.
2. The positioning of the thermocouples leaves a bit to be desired. I'm hoping to put the 'tank exit' one and the 'tank top' one in proper pockets, to improve accuracy.
3. There is a potential failure mode involving the heat pump starting up in the middle of a hot summer day. This corner case should be covered in the firmware, with a suitable strategy.
4. The operating hours meter has insufficient digits - ours has been round the clock, probably several times!
Hope some of that may be useful. If you're ever down Bristol way you're welcome to inspect. Drop me a PM.
E.
*the simplest thing might simply be to add a second tank with the DHW and solar coils both in series (water moving in opposite directions), and move the sensor to the return from the cooler tank's coil, but that risks overheating the main tank.
