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.