Storage tanks range from 40 to 120 gallons for residential systems. Electric water heaters can easily be adapted to serve as solar storage tanks. Specialty tanks come with or without integrated heat exchangers. Most tanks are glass-lined, but can be stainless steel or lined with high-temperature polybutylene plastic.
Drainback (DB) tanks or reservoirs for residential systems usually have a capacity of 8 to 20 gallons. Large DB tanks serve as both a drainback reservoir and storage for the system. The DB tank holds the collector loop water after it has drained from the collector and piping. DB tanks can simply be a small tank or can contain an internal heat exchanger. A DB tank should hold at least twice the volume of the system (piping, collector, and exchanger).
Expansion tanks. Liquids physically expand when they get hot, and closed-loop systems need some way to absorb this expansion. In drainback systems, the DB tank serves this function. In antifreeze systems, expansion tanks are used. They are usually very small—2 to 4 gallons—and are sized depending on system volume and maximum system temperature.
Backup water heater. Many solar storage tanks have an electric element in the top of the tank, which serves as a backup source to heat the water when the SHW system doesn’t produce enough hot water. Single-tank systems have less heat loss and take up less room than double-tank systems.
Two-tank systems use a solar storage tank and a backup gas or electric water heater, which keeps water hot under all conditions. This backup heater can also be a tankless or instantaneous water heater. Double-tank systems add to the overall system storage capability. However, since the storage tank doesn’t have any conventional heating elements, conventional energy cannot preheat the water as it can in a single-tank system.
Pumps are used in forced-circulation systems to move the collector-loop fluid through the collector to the tank or heat exchanger. When an external heat exchanger is used in an indirect system, a pump is also required on the potable or DHW loop. When a pump is used to circulate potable water (open or DHW loops), it must be constructed of bronze or stainless steel because of the corrosive effects of the dissolved oxygen in potable water.
Drainback systems need a high-head pump large enough to push all the air out of the system on system startup and overcome the height of the collector. Antifreeze systems can use smaller pumps, since there is no air in the piping to overcome and the piping and collectors remain filled with the antifreeze. Because a smaller pump is needed, antifreeze systems more readily use PV-powered DC pumps. The pump industry has a very limited selection of high-head DC pumps suitable for drainback systems.
Forced-circulation systems must have a controller to energize the pump. Since the water temperature in the tank will increase by 60°F or more on a sunny day, a differential control will maximize the system’s efficiency. The control has a sensor to monitor collector temperature and another to monitor tank temperature. The control turns on the pump when the collector is a few degrees warmer than the tank and shuts the pump off when the collector temperature drops below being able to add heat to the tank water. Many controls have digital displays, multiple differentials for multiple outputs, high-limit controls for storage tanks, and vacation modes to help avoid system overheating when a home is unoccupied.