As was seen in the previous sections of this chapter, the control system should be capable of handling all possible system operating modes, such as the heat collection, heat rejection, power failure, freeze protection, and auxiliary heating. The basis of solar energy system control is the (DTC). This is simply a fixed temperature difference thermostat with hysteresis. The differential temperature controller is a comparing controller with at l east two temperature sensors that controls one or more devices. Typically, one of the sensors is located at the top side of the solar collector array and the second at the storage tank as shown in Fig. 32. Most other controls used in solar energy systems are similar to those for building services systems.

　　The differential temperature controller monitors the temperature difference between the collectors and the storage tank. When the temperature of the so lar collectors exceeds that of the storage by the predetermined amount (usually 4–11 °C), the differential temperature controller switches the circulating pump ON. When the temperature of the solar collectors drops to 2–5 °C above the storage temperature, the differential temperature controller stops the pump. Instead of directly controlling the solar pump, the differential temperature controller can operate indirectly through a control relay in order to operate one or more pumps and possibly perform othe r control functions such as the actuation of control valves.

　　The differential temperature set point of the controller may be fixed or adjustable. If the controller set point is fixed, the selected controller should correspond to the requirements of the solar system. An adjustable differential set point makes the controller more flex ible and allows it to be adjusted to the specific system or specific condition of the solar system. The optimum differential ON set point is difficult to calculate because of the changing variables and conditions. Typically, the ON set point is 5–9 °C above the OFF set point. The optimum ON set point is a balance between optimum energy collection and the avoidance of short starts and stops of the pump. The optimum OFF temperature differential should be the minimum possible, which depends on whether there is a heat exchanger between the collectors and the storage.

　　Frequent starts and stops of the pump, (called short cycling) must be minimized because it can lead to premature pump failure. Short cycling depends on how quickly and how often the solar collector sensor tempera ture exceeds the ON set point and drops below the OFF set point. This is influenced by the insolation intensity, the pump flow rate, the solar collector thermal mass, the response of the sensor and the temperature of the fluid entering the collector. The most common method of avoiding short cycling is the use of wide temperature difference between the ON and OFF set points. This however will lead to the requirement of a lot of insolation to switch the pump ON which will loose energy in the collector and may never reach the ON set point in periods of low insolation. Therefore, the guidelines given below must be followed for deciding the correct setting.

　　If the system does not have a heat exchanger, a range of 1–4 °C is acceptable for the OFF set point. If the system incorporates a heat exchanger, a higher differential temperature set point is used so as to have an effective heat transfer. The minimum, or OFF temperature differential is the point at which the cost for pumping the energy is equal to the cost of the energy being pumped in which case th e heat lost in the piping should also be considered. For systems with heat exchangers, the OFF set point is generally between 3 and 6 °C.

　　In closed loop systems, a second temperature sensor may be used in the tank above the heat exchanger to switch the pump between low and high speed and hence provide some cont rol of the return temperature to the tank heat exchanger.