Stagnation

The heat transfer fluid used in the collector must also withstand the high stagnation temperatures. Stagnation occurs when the heat transfer fluid stays in the collector too long and a high temperature is reached greater than the normal due to the heat from solar radiation. This could happen when there is a pump or control failure, when the heating demand of the users and the storage tank are satisfied or when the system is down for maintenance. Under normal operation the heat transfer fluid is under a pressure to avoid vaporization. Most collector systems are designed to operate at pressures below 125 psig (833 kPa). This is to allow the use of standard piping components (class 125) and avoid using more costly components. Since the solar collector is generally placed at a higher elevation than other parts of the system the static head of the fluid column must be added to the operating pressure in the collector when determining the pressure on components. This means pressures at or just below 75 psig (500kPa) are common in the solar collector. At this pressure, a 60/40 mixture of water/glycol will begin to vaporize at 320 °F (160 °C). If the pressure is dropped to 45 psig (310 kPa), then vaporization would begin at a temperature of 284 °F (140 °C). Vaporization of water at these pressures is 307 °F and 275 °F (153 and 135 °C), respectively.

When vaporization begins to occur, the resulting gas displaces the liquid in the collector and to some degree in the nearby piping. This displaced fluid should be directed to a recapturing tank so that it can be used again to fill the system when the temperature cools down. There are safety valves in the piping system that must be certified for the highest temperature that may occur. They should be placed on connections to the lines leading to the recapture tanks.

The disadvantages of frequent vaporizations are:

• Water/glycol mixtures will have a shorten life when exposed to at temperatures in the 300 °F –320 °F (149 – 160 °C) range.
• Anti-corrosion additives and contaminants may stick to the interior of pipes and absorbers flow channels.
• Operating personnel need to spend time refilling the system.

All glycol systems should use the Pressure Stagnation Protection (PSP) method. This method allows over sizing of the pressure relief valve to 150 psi (1034 kPa), which allows the system pressure to rise with stagnation temperature. This protects the fluid from overheating and preserves the properties of the glycol by keeping it in a liquid form at all times.

Since glycols begin to break down and start to become corrosive when heated to temperatures greater than 240 °F (116 °C). One way to avoid this condition is to send excess heat to a nearby low priority heat user such as a swimming pool when the temperature in the storage tank is satisfied. Such a connection to a swimming pool is established using another heat exchanger with redundant, multiple pumps. Another solution may be to route external fluid to air heat exchangers, or in some cases, to a ventilated recooling device. For smaller applications (up to several hundred m2) it is sufficient to design the solar loop expansion vessel in a way that both the additional volume of the heat transfer medium (due to the decreasing density), and the volume of the evaporated heat transfer medium (in case of stagnation) can be absorbed.