During the long and hot summer days, solar water heating systems generate a lot of heat. At the same time, the demand for hot water is often at its lowest point. The combination of these two factors is a condition commonly referred to as "stagnation."
Basically, when the solar water storage tank is heated to the highest temperature early in the day, there will be stagnation; the movement of the solar collector stops, and the liquid in the system becomes hotter and hotter while sitting in the sun. The result is a high-pressure, high-temperature state, over time, the system will be damaged by extreme expansion and contraction. In addition, when the antifreeze is overheated every day for several weeks, it tends to decompose and become acidic, thereby turning into a corrosive substance that circulates in your system and slowly damages its components.
In order to prevent this, when the hot water capacity exceeds your needs, some methods of overheating protection are needed. Solutions range from basic, cheap, but (possibly) cumbersome, to fully automatic, but slightly expensive.
In the basic, but (possibly) clumsy category, we recommend.
Cover your panel
Depending on the accessibility of your flat panel, you can cover all or part of the panel with prefabricated plywood. Obviously, this is the easiest in ground installation or flat roof installation. The plywood should not only be cut to fit the panel perfectly, but it should also be fixed to the panel in some way to prevent the wind from taking it away or damaging the panel.
Another option (especially in the case of evacuation tube solar collectors) is to make a custom shade net (or tarp) that tightly surrounds the flat panel or tube array.