The "re-done" solar hot water system. The original system was put together many years ago using some broken panels and an old tank from a different supplier. It all worked well, better and longer than expected, but the tank finally gave way. Preferring a DIY solution, we looked around for possible alternative tanks. Surplus 200L plastic and steel drums from local food processors seemed the best solution.
This meant a change from the commercial static "sealed/pressurised" tank to an open tank supply. ( A sealed tank requires a pressure release valve) The most efficient approach was to use the tank as a water bath for a copper coil heat exchanger. These tanks are not lined to be suitable for human consumption, so the heat exchanger approach ensured no contamination from the tank, or microbial growth in an inadequately heated static tank.
We had tested the approach with an old 200L fuel drum - it worked well, but the old drum rusted quickly. So...the next question was whether the 200L plastic drums would survive as a hot water bath. We expected them to be OK due to the high temperatures used in their manufacture, but tested anyway by boiling a piece of the drum. Reassured, we went ahead.
We also acquired a 200L steel drum from the same food processor. The steel drum was slightly taller than a standard 200L fuel drum and worked perfectly as an external skin to protect the plastic drum and insulation... just needs a nice looking lid!
This was a try-as-you-go development, as while there were plenty of descriptions on the internet, we still had a few questions.
The water in the tank cycles through the panels through the "heat/thermal syphon" effect and is independent of the supply which goes from the system to the house.
There are 2 pipes involved in cycling through the panels, and an overflow drain at the top (can also serve as a pressure relief if the tank is fully closed/sealed) and an inlet at the top (to keep the tank full) which is operated by a simple float valve as used in a cattle trough. The plastic float does not appear to be affected by the hot water bath, which is great as it is much cheaper than a brass float option.
Noting that hot water rises to the top, the float valve can have a drop pipe to ensure new cold water is fed to the bottom of the tank (we did not bother with that)
The bottom pipe takes the "colder" water from the bottom of the tank and feeds it into the bottom of the panel. The heated water from the panel at the top opposite to the inlet and enters the tank below the water level. This allows a thermal syphon effect to circulate the water. It is important that the tank inlet from the panel is lower than the tank water level - We just moved the pipe up and down beside the tank, to locate the best flow, and marked the position to attach the inlet fittings.
There are two connections to feed the heater exchanger. The heat exchanger is made from 2 rolls of 12mm copper tube. In theory it is better for the heat exchanger to be near the hotter water at the upper levels of the tank - in practice, the best you can do is push the coils in and have the connections near the top.
As the water flowing through these pipes is from your pressurised house supply (town water, pressure pump, or header tank), you can reduce possible leaks from these connections (holes through the tank), by feeding the connections to the heat exchanger through the top opening. We kept the heater exchanger at 12mm. While it slows the water flow a bit, it reduces hot water use and allows it to heat better. If flow is a problem, just use larger tube for all or the first a section of the heat exchanger.
It would have been nice to use proper tank flange fittings, as they give a better seal, but these were not available to us. The home made fittings, using threaded brass pipe, threaded flange nuts , large washers and car tyre tube washers, with plenty of thread tape, have worked well. There is no pressure in the panel/tank water cycling. The only pressure is through the heat exchanger which uses standard plumbing compression fittings.
Cold water supply pipes are all poly tubing. The hot water supply pipe is copper, insulated with poly pipe. The hot water pipe connection from the panel outlet to the tank is poly - was temporary as we were out of 20mm copper, but it is not for consumption, and still working well after more than 12 months, so there is no reason to change.
Insulation was a combination of whitegoods packing materials and "sisalation" roofing insulation sheet.
The final consideration was to use a strip of aluminium to take the strain of the pipe connections from the plastic tank.
We have had a few earthquake tremors and the tank has not fallen through the roof yet, but we'll look for a way to lay the tank horizontally, to reduce the earthquake risk. We didn't do this at first as we were not sure the screw fittings at the top of the tank would not leak... but with a bit more confidence now... we'll look at that as an alternative approach.