Materials for solar container and heating

To build a DIY solar hot water storage tank, you’ll need materials like a solar collector, an insulated storage tank, copper tubing, and a heat exchanger. The collector will harness the sun’s energy to heat the water, which then moves through the copper tubing and is stored in. What materials are used for storing solar heat, and is there a `best' one? A number of materials will work as storage media in home, farm or small business solar heating systems; but only three are generally recommended at this time--rock, water (or water-antifreeze mixtures) and a phase-change. I plan on regulating the tank to 150 degrees F for now (adding more storage in the future if I am heating water that efficiently). Solar energy is a vast renewable energy source, but uncertainty in the demand and supply of energy due to various geographical regions raises a question mark.