The majority of systems that use propylene glycol as the HTF are pressurized, closed-loop systems with the following components:
Heat exchanger. Any system using propylene glycol must use a heat exchanger to move heat from the solar loop to the end-use water. Although arguments can be made against the necessity of a double-wall versus a single-wall (more efficient) heat exchanger, in most regions, local building codes specify double-wall exchangers.
Check & clean-out valves. When the collector is colder than the water in the storage tank (at night, for instance), a thermosyphon can start, removing heat from the tank by circulating the HTF loop in reverse and losing heat as radiation from the collector. A check valve allows the HTF to flow only in the correct direction to prevent thermosyphoning. A clean-out valve is also a good addition. It screens out and collects particulates in the HTF, keeping them from damaging seals and components.
Expansion tank. The HTF loop needs a glycol-rated expansion tank—a metal tank with an air bladder inside. When the HTF expands as temperature increases, the air bladder absorbs the pressure, preventing the pressure-relief valve from opening and HTF from being lost.
In addition to the expansion tank on the HTF loop, many local codes also require an expansion tank on the end-use water side of the system.
Pressure-relief valve & air vent. A pressure-relief valve opens when the pressure exceeds its rated amount, often 50 to 75 PSI. When it activates, some fluid is lost, and the system pressure drops slightly. Repeated blow-offs through the pressure-relief valve are an indication of a poorly designed or installed system, as this should not be the normal operating condition. Likely the system will suffer additional problems—such as poor performance and pump failure—as more and more system pressure is lost. The 2009 Uniform Solar Energy Code stipulates that any system using a single-wall heat exchanger shall have a pressure-relief valve of 30 PSI or less.
An air vent is placed at the highest point of the system plumbing—typically at the output of the collector—and can be used to vent air that builds up or was never initially purged from the system.
Differential controller. The controller uses thermistors, whose resistance varies with temperature, to determine the temperature of the storage tank and the collector. The controller then turns the pump(s) on and off as needed. Other features may include vacation mode (see below); the ability to change the off/on temperature differential set points; and a display showing temperatures at different points in the system.
Pressure & temperature gauges. A pressure gauge is needed to monitor system pressure and will indicate if HTF has been lost due through the pressure-relief valve or a leak in the HTF plumbing. Typically installed on either side of the heat exchanger, temperature gauges show HTF temperature as it returns from the collector and after it transfers its heat to the end-use water. This temperature drop indicates that the system is operating and shows how effectively heat is being transferred. Some controllers can provide this information, particularly temperature data.
Circulator pump(s). Closed, pressurized loops have much less resistance (head) to overcome than drainback systems because the HTF fills the entire loop and doesn’t have to be lifted from a resting point as it does in a drainback system. This allows lower-power pumps to be used. A typical residential system pump will use less than 90 watts. In some cases DC pumps that are powered directly by a PV module can be used, eliminating the need for AC power and the differential controller.
The closed loop does not require more expensive stainless steel or brass pumps because the pumps are not subjected to the corrosive effects of air and fresh water being introduced into the system. Most are made of cast iron and steel. Pumps should be plumbed so that they are accessible for maintenance—installing isolation valves on either side makes pump changing easier.
Piping and insulation. Piping must be suitable for use with glycol and the temperatures encountered in SHW systems—usually copper is used. Cross-linked polyethylene (PEX) has too low of a temperature rating. Pipes must also be appropriately sized—too small and flow may be inadequate, reducing the system’s efficiency. Copper and stainless steel tubing are available in rolls, and even preinsulated with an embedded sensor wire, saving installation time. Sizes are 1/2-inch or larger depending on the number of collectors.
Components may be plumbed together ahead of time, on site, or some combination thereof. There are many manufacturers of prebuilt pump stations, with all of the necessary components for a closed-loop, pressurized system already plumbed. Installation can be as simple as mounting the unit and connecting the lines to and from the solar collectors and the tank (see “Solar Hot Water Pump Stations” in HP134).