my problem has been not having a solar storage tank for my solar system. i have the solar collectors most of the plumbing, pumps and a controller. the system i have in mind is called a drain-back system. basically the water is pumped up through the solar collectors when there is a call for heat. when the pump is off the collectors drain completely into the tank. this serves several purposes. one is that during freezing temps there is nothing to freeze since all exposed piping is drained into the insulated tank. another is that once the water temperature in the tank is hot enough the panels drain and doesn't over heat the system.
anyway i have been planing to build a tank from cement block and hydraulic cement. needless to say my solution would have been time consuming and tons of physical effort. my solution has presented itself and i am excited that my patience and observation has paid off.
i plan to use an intermediate bulk container.
it holds 275 gallons. it is made from high density polyethylene (HDPE). it is a recycled product and is extremely cheap. the product is already plumbed for my needs almost perfectly. (HDPE) can contain fluids at a sustained temperature of 230° indefinitely. the large access port on the top of the tank will allow me to insert my heat exchanger coil into the tank.
so my idea is to plumb and insulate this tank install a long copper coil as a heat exchanger. the household water will travel through a long pipe immersed in the tank. as the water travels through the immersed pipe it'll warm closer to the tank temperature. the water in the tank will remain as a closed system.
this revelation has just bumped my solar hot water system up on my queue. we'd really like to get all of our hot water from the sun.