Types of solar water heating systems

Solar water heaters can be either active or passive. An active system uses an electric pump to circulate the heat-transfer fluid; a passive system has no pump. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors.

Solar water heaters are also characterized as open loop (also called "direct") or closed loop (also called "indirect"). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze, for example) to collect heat and a heat exchanger to transfer the heat to household water.

Direct or open loop systems circulate potable water through the collectors. They are cheaper than but can have drawbacks:

• They offer little or no overheat protection unless they have a heat export pump.

• They offer little or no freeze protection, unless the collectors are freeze-tolerant.

• Collectors accumulate scale in hard water areas, unless an ion-exchange softener is used.

Until the advent of freeze-tolerant solar collectors, they were not considered suitable for cold climates since, in the event of the collector being damaged by a freeze, pressurized water lines will force water to gush from the freeze-damaged collector until the problem is noticed and rectified.

Indirect or closed loop systems use a heat exchanger that separates the potable water from the fluid, known as the "heat-transfer fluid" (HTF), that circulates through the collector. The two most common HTFs are water and an antifreeze/water mix that typically uses non-toxic propylene glycol. After being heated in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable water. Though slightly more expensive, indirect systems offer freeze protection and typically offer overheat protection as well.

Passive systems rely on heat-driven convection or heat pipes to circulate water or heating fluid in the system. Passive solar water heating systems cost less and have extremely low or no maintenance, but the efficiency of a passive system is significantly lower than that of an active system, and overheating and freezing are major concerns.

Active systems use one or more pumps to circulate water and/or heating fluid in the system.

Though slightly more expensive, active systems offer several advantages:

• The storage tank can be situated lower than the collectors, allowing increased freedom in system design and allowing pre-existing storage tanks to be used.

• The storage tank can always be hidden from view.

• The storage tank can be placed in conditioned or semi-conditioned space, reducing heat loss.

• Drainback tanks can be used.

• Superior efficiency.

• Increased control over the system.

Modern active solar water systems have electronic controllers that offer a wide-range of functionality, such as the modification of settings that control the system, interaction with a backup electric or gas-driven water heater, calculation and logging of the energy saved by a SWH system, safety functions, remote access, and various informative displays, such as temperature readings. A typical programmable differential controller

The most popular pump controller is a differential controller that senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. In a typical active system, the controller turns the pump on when the water in the collector is about 8–10 °C warmer than the water in the tank, and it turns the pump off when the temperature difference approaches 3–5 °C. This ensures the water always gains heat from the collector when the pump operates and prevents the pump from cycling on and off too often. (In direct systems this "on differential" can be reduced to around 4C because there is no heat exchanger impediment.)