Jonathan Mau wrote:

I have been giving consideration to a big water thermal store also. The
55 gallon plastic drums can be had for CAN$10. What I don't know is the
temperature characteristics. I think these are HDPE. The tanks I have
found in HDPE are only rated at 120 degrees F with a liquid of S.G. 1.7 I
think. I wonder if one could get a few more degrees with a S.G 1 liquid?

Sure. Then again, maybe that rating only applies for rail or truck transport.

I suspect this is an issue of wall thickness, temperature, and pressure
and not an absolute limit of the material.

You might wrap a drum with chicken wire.

My calculations show so far that the lower cost of 55 gallon drums outweighs
the higher cost of insulation as compared to one large tank only considering
the cost of tank(s) and insulation. Throw in all the bits and pieces to run
piping into say between 4 and 18 55 gallon drum tanks and things might change.

You might arrange plastic drums in a square or hex grid and connect them with
short threaded nipples through adjacent holes in the walls near the bottoms
or bulkhead fittings with barb or garden hose adapters. If the drums are
cylindrical, a single PVC bulkhead fitting might connect two of them.

Low-pressure ag applications use "push plumbing"--drill a smooth hole
in a drum with a step bit, and push some smooth tubing into the hole.

Nick