There are several manufacturers of heat exchange tanks today for use in making hot water from boilers. They can be used in solar applicationsa as well because the heat source is from heated water. Weil McLain boilers, sold at plumbing suppliers, has a stainless steels heat exchanger tank in sizes from 30 to 120 gallons. They also have a heat exchanger you can use between the panels so you can use an ordinary electric heater tank. Other manufacturers of solar tanks are Triangle, Amtrol, Superstore, etc.