1624 Control Systems

Solar systems should operate automatically with little attention from operating personnel. A good control system will optimize the performance of the system with reliability and at a reasonable cost. The heart of any solar thermal collecting system is a device to turn on the collector fluid circulating pump (and other necessary devices) when the sun is providing sufficient insolation so that energy can be collected and stored, or used. With flat-plate collectors it is common to use a differential temperature controller (Figure 16.16), a device with two temperature sensors. One sensor is normally located on the collector fluid outlet and the other in the storage tank near the outlet to the heat exchanger (or at the level of the internal heat exchanger). When the sun is out, the fluid in the collector is heated. When a prescribed temperature difference (about 20°F) exists between the two sensors, the controller turns on the collector pump and other necessary devices. If the temperature difference drops below some other prescribed difference (about 3 to 5°F), the controller turns off the necessary devices. Thus clouds or sundown will cause the system to shut down and prevent not only the unnecessary loss of heat to the collectors but also the unnecessary use of electric

Figure 16.16 Installation of a differential temperature controller in a liquid heating system.

ity. The distinct temperature difference to start and to stop is to prevent excessive cycling.

Differential temperature controllers are available with adjustable temperature difference settings and can also be obtained to modulate the flow of the collector fluid, depending upon the solar energy available.

Controllers for high-temperature collectors, such as evacuated tubes and tracking concentrators, sometimes use a light meter to sense the level of sunlight and turn on the pumps. Some concentrating collectors are inverted for protection when light levels go below a predetermined value.

In some systems the storage fluid must be kept above some minimum value (e.g., to prevent freezing). In such cases a low-temperature controller is needed to turn on auxiliary heaters if necessary. A high-temperature controller may also be needed to bypass the collector fluid or to turn off the system so that the storage fluid is not overheated.

Figure 16.17 shows a control diagram for a solar-heated asphalt storage system (Figure 16.18) in which the fluid must be kept between two specified temperatures. Solar heat is used whenever it is available (collector pump on). If the storage temperature drops below the specified minimum, the pump and an electric heater are turned on to circulate electrically heated fluid to the tank. If the tank fluid gets too warm, the system shuts off. Almost any required control pattern can be developed for solar systems using the proper arrangement of a differential temperature controller, high- and low-temperature controllers, relays, and electrically operated valves.

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