I next needed a heat storage tank, and to my dismay the tank cost more than the collector. An 80 gallon tank with two heat exchanger coils cost over $1000, and a whole house storage with 1000 gal of capacity was going to cost $12-15K. Being a yankee inventor, I figured there was a cheaper way. I designed modular solar heat storage system that could be expanded to any size in 55 gal increments. The tank is non corroding HDPE that is rated for stacking at 150F and gets soft at 253F. The removable cover is stiffened with thoroughly painted plywood, and has an upper 30 foot of 1/2"OD copper tube heat exchanger, and an identical lower coil. The coils are fed with 3/4" copper tube and coupled with a custom copper fitting that I made. The drums were filled with 53 gallons of water and 2 gallons of EV anti freeze. The drums are connected in series with unions for easy maintenance if replacement is needed. The two coils position on the cover is alternated to simplify piping. Since the collector stagnation temp is over 212F, pressure relief valve and expansion tank were needed to protect the solar loop from explosion. The drums are sitting on an insulated base , and are surrounded by a thermomax aluminum faced insulation . The air space between and around the drums is filled with celulose attic insulation. The addition of flow control valves allows me to select the size of the storage, and heat 1, 2, or 4 of the drums. This allows me to get one drum very hot quickly, or more drums heated to a lower temp.
The mixing of flows and control of the three heating loops is with a custom Labview program and servo controlled valves.The hot water tank is keep at the storage tank temperature with an attached tube in tube heat exchanger.