Solar water heaters perform three basic operations before the hot water comes out of your tap:
— Nanotechnology News (@Nanowerk) December 5, 2013
1. Energy Collection: Sunlight is collected and converted to heat energy.
The solar collector is mounted on or near your home facing south. As the sunlight passes through the collector’s glazing, it strikes an absorbing material. This material converts the sunlight into heat, and the glazing prevents the heat from escaping in a similar manner to leaving a car parked in the sun with its windows rolled up. The temperature inside a glazed solar collector on your roof can easily reach 150°C when there is no heat transfer fluid flowing through it.
The two most common types of solar collectors used in solar water heaters are glazed flat plate and evacuated tube collectors. A glazed flat plate collector consists of a shallow rectangular box with a transparent glass “window” covering a flat black plate. The black plate is attached to a series of parallel tubes or one serpentine tube through which air, water, or other heat transfer fluids pass.
An evacuated tube collector consists of several individual glass tubes, each containing a black metal pipe through which the heat transfer fluid passes. The space between the pipe and the glass tube is “evacuated,” so the air is removed.
Although uncommon, unglazed plastic collectors can be used as a seasonal solar water heating system. However, they are used mostly for pool heating. Each collector has its advantages, and each can perform well if matched with the proper, well-designed, storage unit.
2. Energy Transfer: Circulating fluids transfer the collected energy in the form of heat to a storage tank.
Heat energy is transferred from the collector to the water storage tank. In some water heaters, hot fluid is pumped from the collector to the storage tank. The pump is powered by electricity that either comes from an electrical wall outlet or a small photovoltaic module located beside the collector.
In other types of solar water heaters, the sun heats the storage tank directly, so the fluid in the collector is heated and rises naturally to a storage tank above the collector. This type of solar water heater is often referred to as a thermosiphon system. It does not require an electric pump. Passive solar and solar photovoltaic-powered systems will continue to operate even if there is a power outage. The products that require electricity from a wall outlet will not operate during a power outage.
3. Energy Storage
Solar-heated water is stored in an insulated tank until you need it. Hot water is drawn off the tank when tap water is used, and cold make-up water enters at the bottom of the tank.
Solar water heaters tend to have a slightly larger hot water storage capacity than conventional water heaters. This is because solar heat is available only during the day and sufficient hot water must be collected to meet evening and morning requirements.