While most antifreeze systems rely only upon a properly sized solar expansion tank to accommodate the expansion cause by the formation of steam, there are other approaches. By utilizing a pressure-relief valve with a 150 psi rating, Heliodyne’s system is able to accommodate larger swings in system pressure. Since higher pressures raise the boiling temperature of the glycol solution, Heliodyne’s approach is to permit higher pressures and prevent steam formation. For example, a 50:50 glycol-and-water solution boils at 220°F at atmospheric pressure (14.69 psi). In a system that reads 65 psi on the pressure gauge at sea level, the boiling temperature of the same glycol solution is approximately 320°F. In a Heliodyne system, since the system pressures are able to increase up to 150 psi, the boiling temperature of the fluid remains higher than the system’s stagnation temperature. This is a delicate balance—if the expansion tank is too small, the pressure will climb too fast and the pressure-relief valve will open; if the expansion tank is too large, the pressures will not be high enough to keep the glycol from boiling. As a result, it is critical to follow the expansion tank sizing guidelines provided by Heliodyne.
A few other companies utilize methods that provide passive cooling of the collectors or provide a reservoir where fluid can be held during overheating conditions. When installing a system, it is critical to understand the manufacturer’s approach to addressing overheating and solar expansion tank sizing. While many systems will allow erring on the side of oversizing the solar expansion tank, there are a few where installing a larger solar expansion tank may affect system operation and performance.