for those who are interested in building a cheap heat buffer tank that works passively. to buy one, they are about $1000 - $1500… here’s how you build one for $50 plus tank. I build my first one about ten years ago and it still going strong…
parts you will need:
one new/use hot water tank
two watts a-524 compression fitting or similar (plumbing and/or automotive supply)
two peices of 3/4" copper pipe at 4-6 inches long
and one roll 50’ of soft 1/4 ACR copper tubing
cut the roll of ACR into 7 equal peices and insert into rigid copper pipe like this;
butt the flat end against a block of wood or soft pottery clay, and then fill in from the back side with solider - like this:
when done it should look something like this:
bore out the watts a-524 fitting with a 7/8’’ hole saw so that it slips over the 3/4 copper pipe:
once cleaned up, the finished product should look something like this:
now simply insert it into the hotwater tank either through where the heating element go or other unused 3/4 inlet on the tank . to prevent accidental kinking insert 1/8 aircraft wire in the copper tubing ( it will also help facilitate fishing the copper pipe through the tank … thread the fitting down… to connect your solar thermal to them. simple use a “shark bite” fitting or another compression fitting of some form
the heat transfer is pretty good as it has the same surface area and volume as a 20 feet of 1/2 line .
other option is a flat plate heat exchanger and a secondary pump which is simpler, if but a little bit more expensive route ~$100 to $200 plus tank.
good luck have fun 
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