Periods of high insolation and low load result in overheating of the solar energy system. Overheating can cause liquid expansion or excessive pressure, which may burst piping or storage tanks. Additionally, systems that use glycols are more problematic, since glycols break down and become corrosive at temperatures greater than 115°C. Therefore, the system requires protection against this condition. The solar system can be protected from overheating by a number of methods, such as:
• Stopping circulation in the collection loop until the storage temperature decreases (in air systems).
• Discharging the overheated water from the system and replacing it with cold make-up water.
• Using a heat exchanger coil for rejecting heat to the ambient air.
As will be seen in the next section, controllers are available that can sense over-temperature. The normal action taken by such a controller is to turn off the solar pump to stop heat collection. In a drain-back system, after the solar collectors are drained, they attain stagnation temperatures; therefore, the collectors used for these systems should be designed and tested to withstand over-temperature. In addition, drain-back panels should withstand the thermal shock of start up when relatively cool water enters the solar collectors while they are at stagnation temperatures.
In a closed loop antifreeze system that has a heat exchanger, if circulation stops, high stagnation temperatures occur. As indicated previously, these temperatures could break down the glycol heat transfer fluid. To prevent damage of equipment or injury due to excessive pressure, a pressure relief valve must be installed in the loop, as indicated in the various system diagrams presented earlier in this chapter, and a means of rejecting heat from the collector loop must be provided. The pressure relief valve should be set to relieve below the operating pressure of the component with the smallest operating pressure in the closed loop system.
It should be noted that, when the pressure relief valve is open, it discharges expensive antifreeze solution, which may damage roof membranes. Therefore, the discharge can be piped to containers to save antifreeze, but the designer of such a system must pay special attention to safety issues because of the high pressures and temperatures involved.
Another point that should be considered is that, if a collector loop containing glycol stagnates, chemical decomposition raises the fusion point of the liquid and the fluid would not be able to protect the system from freezing.
The last option indicated previously is the use of a heat exchanger that dumps heat to the ambient air or other sink. In this system, fluid circulation continues, but this is diverted from storage through a liquid-to-air heat exchanger, as shown in Figure 5.28. For this system, a sensor is used on the solar collector absorber plate that turns on the heat rejection equipment. When the sensor reaches the high-temperature set point, it turns on the pump and the fan. These continue to operate until the over-temperature controller senses that the temperature is within the safety limits and resets the system to its normal operating state.
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