I'm building an off grid solar heated shower. The system uses 5 evacuated tubes. I have built a manifold using a 750mm long copper pipe with a diameter of 54mm. The manifold will run fluid to a coil of 10mm copper pipe in a large, insulated box of kiln dried sand. A separate 15mm copper pipe heat exchange will heat cold water for the shower from the (hopefully) warm sand. I'm using sand to avoid the danger of legionnaires disease in standing warm water. The system will be 12 volt with a separate PV panel charging a battery and powering a small pump (only need a slow flow rate)

So, a number of questions.

1. What liquid is best for the closed loop system - I'm told glycol is good, but there are many different types available and does it need diluting etc? I'm looking at a system of about 3 litres.

2. I believe I will need an expansion vessel to cope with pressure changes - but not sure which is going to be most suitable - is there a good make people recommend and are there specifications according to the volume in the system and the likely heat levels - I presume installation is at the top of the system.

3. It is very difficult to find the right 12v pump - anyone got any ideas - the max height of the manifold above the lowest point of the heat exchange is around 1.6m.

4. I'm also struggling with finding the most suitable thermostatic control switch - Ideally I want to switch on the system at about 25 - 30 degrees c - anything below that and it should be off, although ideally the system would measure the residual heat of the sand and not pump until the manifold was hotter.

Any other advice on efficiency, heat storage, reliability, managing pressure etc gratefully received - I'm not a scientist - just an over ambitious DIYer! Thanks