If the climate is characterized by frequent sub-freezing temperatures, positive freeze protection with the use of an antifreeze solution in a closed collector loop is necessary. A detailed schematic of such a liquid-based system is shown in Figure 6.14. A collector heat exchanger is used between the collector and the storage tank, which allows the use of antifreeze solutions to the collector circuit. The most usual solution is water plus glycol. Relief valves are also required to dump excess energy if over-temperature conditions exists. To “top off” the energy available from the solar energy system, auxiliary energy is required. It should be noted that the connections to the storage tank should be done in such a way as to enhance stratification, i.e., cold streams to be connected at the bottom and hot streams at the top. In this way, cooler water or fluid is supplied to the collectors to maintain the best possible efficiency. In this type of system, the auxiliary energy is never used directly in the solar storage tank. This is treated in more detail in the next section.