In indirect systems, the expansion tank must be properly sized for proper system operation. For customised thermal expansion tanks, the receiving volume must be sufficient to accommodate the expansion of the heat transfer fluid when the solar circuit goes into stasis. The following provides basic sizing rules for selecting and sizing a solar thermal expansion tank in an indirect solar system.
Expansion tanks for potable water differ from expansion tanks for glycol-based non-potable water. Potable water expansion tanks are designed to resist corrosion that may occur due to oxygen in the water. Non-potable expansion tanks have air pockets designed to resist the chemicals normally found in indirect applications. For indirect solar systems, the expansion tank must be usable with propylene glycol or other heat transfer fluids used in the collector circuit. In addition, the temperatures and pressures in indirect solar systems may be significantly higher than those found in typical hydronic systems. Care must be taken that the expansion tank selected will meet the requirements of the solar application.
As shown in the diagram below, the expansion tank contains an air cushion which is separated from the glycol by a butyl airbag. The compression and expansion of this air cushion regulates the pressure within the indirect glycol system. The "tank volume" is the total volume of the tank. "Acceptance" is the volume of fluid that the expansion tank can accept, i.e. how much fluid can be pushed into the expansion tank as the fluid in the indirect heats up and expands. As the fluid is pushed into the expansion tank, the air cushion is compressed and therefore the pressure in the tank increases. The pressure of the air cushion in the expansion tank regulates the pressure of the fluid in the entire indirect system. When the air cushion is compressed to half its original volume, the absolute pressure in the tank is doubled. Large expansion tanks will maintain small pressure fluctuations as the fluid expands and contracts, while smaller expansion tanks will cause larger pressure fluctuations as the system heats up and cools.
If the expansion tank is undersized, the system pressure may increase enough to open the pressure relief valve, resulting in a loss of glycol and the need to refill the system.