A drainback system often uses only one pump (P1). The drainback tank holds 5-10 gallons of water and is often piped to a heat exchanger coil within a solar storage tank. The controller is measuring the temperature of the tank and the solar thermal panels. When the panels are hot and the solar storage tank is below its target temperature, P1 will begin pumping water to the panels and the water returning from the panels will first drain into the drain back tank and then through the heat exchanger coil in the solar storage tank. The heat from the panels will be transferred to the water in the solar storage tank and will keep the conventional gas or electric powered heat from needing to turn on. This minimal usage of the heating element or gas burner results in energy cost savings. When the solar storage tank’s target temperature has been met, the pump turns off and the water that was circulating through the panels drains back to the drainback tank.
Closed Loop Glycol System
The closed loop glycol system is a pressurized system that uses a non-toxic propylene glycol as a heat transfer fluid. On a sunny day, the fluid is pumped by a pump station to the panels and the returning glycol is hot. A very common type of pump station has two pumps and a heat exchanger. The first pump (P1) is moving the glycol to the panels and back through a heat exchanger while the second pump (P2) is drawing water from the solar storage tank through the heat exchanger and returning the heated water back to the top of the tank. This pumping by P2 is what allows the heat from the glycol to transfer heat to the water. The controller measures the temperature of the water in the tank and the temperature of the panels. If heat is available from the panels and the solar storage tank is below its target temperature, both pumps turn on simultaneously and the heat from the panels gets injected into the solar storage tank. Often, the solar storage tank is plumbed as a pre-feed for the home’s conventional water heater. When hot water is used at a faucet somewhere within the home, the cold water that would normally flow into the bottom of that tank instead flows to the solar storage tank. This in turn displaces the solar heated water and has it flow into the bottom of the conventional water heater. The pre-heated water can be hotter than the conventional water heater’s temperature and ultimately results in the conventional water heater rarely engaging its heating element or gas burner. This minimal usage of the heating element or gas burner results in energy cost savings.