When stagnation occurs, the fluid temperature in flat-plate collectors can reach temperatures of roughly 350°F. In evacuated-tube collectors, the fluid temperature can exceed 400°F. In most antifreeze systems, these temperatures will cause steam to form in the collector array. When water turns to steam, its volume increases by a factor of 1,700. As a result, a teaspoon of water that turns to steam will force out the total fluid volume of two standard-sized flat plate collectors.
This fluid must go somewhere, otherwise the system pressure will increase and the pressure-relief valve will open, releasing fluid from the system. If this occurs, glycol will need to be added to the system and the system will need to be repressurized to ensure proper operation. But an appropriately sized and properly located solar expansion tank can maintain system pressures and avoid activating the relief valve.
Solar expansion tank location. To handle stagnation, the expansion tank must be placed where it can prevent the expelled fluid from pushing more fluid into the collectors and generating more steam. By locating the solar expansion tank between the check valve and the collector inlet, glycol can be pushed out of the collector inlet and outlet to the solar expansion tank, and the steam will remain at the top of the system. If the check valve is located between the expansion tank and the collector inlet, the fluid is only able to exit the collector array through the collector outlet, which then forces more fluid into the collectors.
It is also important to install the solar expansion tank where its bladder will be protected from exposure to steam. The bladders of solar expansion tanks are typically rated for maximum temperatures of 200°F to 210°F and can fail if exposed to higher temperatures. If the piping between the solar expansion tank and the collectors is short, then a prevessel (aka cooling vessel)—a small tank installed on the piping stem leading to the solar expansion tank—may be required. The prevessel holds enough room-temperature glycol solution to offset that which would be forced into the solar expansion tank if steam forms in the collectors, and protects the solar expansion tank from excessive temperatures. But for most domestic SWH systems, 50 feet of 3/4-inch piping will contain enough fluid to avoid needing a prevessel.
If the solar expansion tank is able to maintain adequate system pressures during a steam event, the system should return to normal once the temperature in the collector decreases and the steam condenses back into liquid. Using a high-temperature propylene glycol will help prevent the heat-transfer fluid from degrading.
Solar expansion tank sizing. By sizing the solar expansion tank to accommodate steam formation in the system, some system designers are able to eliminate the need for any type of heat dump or other overheating protection. Thus, taking some time to familiarize yourself with the proper solar expansion tank selection can reduce these costs and simplify the system (and its installation), as well as reduce the system’s maintenance requirements.
To size a solar expansion tank so that system pressure stays below the pressure-relief valve’s rating, you must calculate the maximum amount of fluid that could be forced into the solar expansion tank. For most systems, the worst-case scenario occurs when the system stagnates due to reaching the solar storage tank’s high-temperature limit. At this point, all of the glycol in the system has reached its maximum thermal expansion, but stagnation has not yet occurred.
The glycol’s expansion (prior to steam formation) can be estimated by multiplying the glycol solution’s coefficient of expansion by the change in temperature. If the initial fluid temperature in the system was 60°F and the fluid is expected to boil at 240°F, then the glycol can be expected to increase in volume by 7%. For a system with a fluid capacity of 8 gallons, this would be an increase in volume of 0.6 gallon (see “Thermal Expansion of Glycol” table, previous page).
The collectors’ fluid capacity is the amount of fluid that is expelled from the collectors when steam forms. For certain heat-pipe-style evacuated-tube and serpentine-style flat-plate collectors, this volume may be as low as 0.3 gallons per collector. For individual harp-style flat-plate collectors, this volume may exceed 1 gallon.