Adding the glycol’s thermal expansion to the steam expansion equals the total amount of fluid that the expansion tank must be able to absorb. The membrane in the expansion tank must be able to stretch enough to accept this fluid volume. Expansion tanks have an acceptance volume specification. Even if an expansion tank is rated as an solar expansion tank by the manufacturer, it is important to verify that its acceptance volume is large enough—some larger solar expansion tanks have larger air cushions than smaller models, but have the same acceptance volume.
Any reserve volume that is stored in the solar expansion tank when the system is initially charged must be factored into the acceptance volume. In cold climates, it is common for installers to inflate the air cushion to 2 to 3 psi below the initial system pressure. When the system is filled with glycol and pressurized, the air cushion will compress a bit and allow the solar expansion tank to hold a small amount of glycol. On cold nights, when the collector fluid cools, some of this reserve volume of glycol will enter the system piping to accommodate any glycol contraction. You may need an additional 0.1 to 0.2 gallons of acceptance volume for each collector in the system to account for the reserve volume.
Once you have identified a solar expansion tank with a sufficient acceptance volume, it is important to ensure that the air cushion is large enough. As the fluid expands, the air cushion in the solar expansion tank will compress. If there is a relatively small range between the system’s normal operating pressure and the pressure at which the pressure-relief valve opens, a larger air cushion is needed to accommodate the expanded volume of fluid. If the difference between the normal operating pressure and the relief-valve rating is high, then the air cushion can be smaller.
If you multiply the factor in the “Solar XTank Pressure Factors” table by the minimum acceptance volume due to thermal expansion, steam expansion, and the reserve volume of glycol in the solar expansion tank, you can determine the minimum tank volume required. For example, if the initial system pressure is 25 psi and the pressure-relief valve is rated at 87 psi, a system with a minimum acceptance volume of 2 gallons will require an expansion tank with a nominal volume of 2 gallons times 2.4, or 4.8 gallons.
Installation considerations. Since solar expansion tank sizing depends upon the system’s initial pressure, the air cushion’s charge pressure, and the pressure-relief valve’s pressure rating, be sure to pay attention to these details during the system’s installation.
The initial system pressure is influenced by the difference in height between the top of the system and the pressure gauge, which is typically located near the solar storage tank. You can select an initial pressure that exceeds the values in the “Solar XTank Pressure Factors” table, since a precise height can be difficult to ascertain (and it’s better to err on the high side), but keep in mind that higher initial pressures will reduce the allowable pressure fluctuation in the system during stagnation.
Selecting an initial pressure below these values may affect system operation. If the pressure is too low, fluid contraction due to cooler system temperatures—such as during winter—could lead to negative pressures at the top of the system. This can encourage air to enter the system and can cause an air lock that will affect pump operation.
The air in an expansion tank is pressurized at the factory. You’ll need to check the precharge pressure with a tire pressure gauge, and increase or decrease it to avoid affecting system performance. For example, if the initial air cushion pressure is 10 psi below the system’s initial operating pressure, the excess glycol could take up one-third to one-half of the acceptance volume of the expansion tank. This will effectively reduce the expansion tank’s capacity.
In SWH systems, common pressure-relief valve ratings include 75, 87, 100, 145, and 150 psi. It is important to verify that the solar expansion tank’s pressure rating is equal to or exceeds the relief valve’s rating. For example, some solar expansion tanks are only rated to 100 psi and would be unsuitable for use in systems with a 145 or 150 psi-rated relief valve.
Also examine the compatibility of the materials used to connect the tank to the piping. If a solar expansion tank with a steel nipple is inserted into a copper female adapter, galvanic action may accelerate the corrosion of the steel. In this case, a brass fitting can be used to separate the steel and the copper, or a stainless steel nipple could be connected directly to the copper piping.
Finally, consider the absorber type in your collector and the collectors’ orientation, since both can influence whether or not fluid can be easily pushed out of the array when steam is created. If the collectors or absorbers act as a trap, the system pressure will increase as the residual liquid turns to steam and the amount of fluid that is forced into the expansion tank will increase. A study performed by the International Energy Agency discusses absorber configurations that promote the emptying of glycol from the collector array during stagnation (see Web Extra).
Web Extra: International Energy Agency • Stagnation Behaviour of Solar Thermal Systems • bit.ly/StagnationPDF