Solar hot water is water heated by the use of solar energy. It is a system generally composed of solar thermal collectors and a fluid system to move heat from the collector to the point of usage. (A collector is fastened to the roof of the building or on a wall facing the sun. In some cases, it may be freestanding.) Solar hot water may use electricity for pumping fluid and the system may have a reservoir or tank for heat storage. Systems run in two ways: active (which requires a pump) and passive (which relies on convection or a heat-pipe).
Solar hot water harvests more energy than solar photovoltaic and has up to three times the efficiency producing energy from the sun. It can also be substantially cheaper and can offer long-term performance with low maintenance. Solar hot water heaters can shave up to 60-70% off water heating costs. Another benefit may be federal, state, and utility incentives.
Solar Hot Water System Types:
Batch
Batch systems have two types of storage units: integrated collector storage, which means the collector and storage tank are all one unit, or convection heat storage units, which are plate type intense collectors with built-in insulated tanks. Since portable water is heated directly, this is an open-loop system. No pump is used to move water, so this system is considered “passive.”
Thermosyphon
Thermosyphon systems are the most popular solar hot water heater used worldwide. It works on the principle of heat rising. In nonfreezing climates, it functions as an open-loop system (water enters the bottom of the collector and rises to the tank as it warms). In colder climates (like our beloved Tahoe), an antifreeze solution is used in a closed solar loop with freeze-tolerant piping. The advantage of thermosyphon is that the water is stored in a well-insulated tank, enabling use anytime without overnight heat losses.
Open-Loop Direct
The open-loop direct solar hot water system is used primarily in tropical settings where freezing never occurs. It is the simplest of the active systems. Normally, an electrical element is not hooked up, so the tank becomes storage only for preheated water, feeding an existing backup water heater.
Pressurized Glycol
Pressurized glycol is a closed-loop active system. Incoming potable water is routed to a solar storage tank, but never into the collector. This system requires an expansion tank and a few other auxiliary components for filling, venting, and maintaining. The collectors for this system can be mounted anywhere. Pressured glycol is the only system available for a very cold climate.
Closed-Loop Drainback
The closed-loop drainback system requires the least routine service of any active system because the heat transfer fluid is distilled water, which is seldom changed. In this system, the collector must always be higher than the storage tank and there must be a sufficient, continuous slop in piping to ensure against freezing. The closed-loop drainback system is very effective and reliable as it works well on hot or cold days.
