How to Use Solar Water Heating
Using solar hot water is easily integrated into most people’s lifestyles. Once a system is installed and commissioned, it rarely takes any maintenance at all. The three main uses for SHW are:
Domestic hot water is perhaps the best known of solar water heating applications. Nearly every home uses hot water every day, with the typical U.S. household being around 30 gallons per person per day. This covers showers, dish washing, and clothes washing. Hot water is stored in a tank for when it is needed. An energy efficient household might use less than half of that amount.
Space heating is a lesser-used application for solar hot water. Similar to domestic solar hot water heating, except that non-potable hot water is pumped into either in-floor hydronic tubing, to heat registers or radiators on the walls or baseboards, or to a heat exchanger inside a forced-air heater.
Pool heating is a very easy use of solar water heating technology, and because its equipment is minimal and less expensive, is practically a no-brainer—if you have a heated pool.
There are several system types for turning solar energy into hot water, and each has its purpose or advantages and disadvantages:
Batch heaters are the most basic—a tank in the sun, though usually in an insulated box with a glass or plastic cover. These are plumbed inline with the domestic water system they serve, no pumps or controls are needed. They make great DIY projects that can be made from any old conventional hot water heater tank. But they do not do well in freezing temperatures—most people will route the water supply around them, and empty them in the winter.
Thermosyphon is the next simplest system. These entirely passive (meaning no pumps) systems rely on the collector being below the tank, so that natural convection moves the hot water out of the collector and to the tank for household use. These systems are also inappropriate for freezing climes.
Direct forced-circulation is similar to the thermosyphon system, but adds a pump and controller. The pump makes it possible to pick up more heat out of the collector. The controller measures when the collector is hotter than the tank and turns the pump on or off as appropriate. These systems also have no freeze protection.
Drainback systems add a tank and a heat exchanger. The drainback tank is sized so that all the water in the collector and lines can drain down into it, when the pump turns off. That way there is no water left in the collector at times when it could otherwise freeze. The water in the system is separate from the household water, so it needs a heat exchanger to transfer the heat from the collector water to the domestic water.
Antifreeze systems are the most common type in climates that regularly freeze. It is has no drainback tank, so the fluid in the collector cannot drain out of the outdoor plumbing. The freeze protection comes from polypropylene glycol antifreeze, so it can stay in the collector after the controller shuts the pump off.
Pool heaters are the final system, and are not used for heating domestic water. Pool collectors are usually made up of many parallel poly pipes sitting on a roof near the pool. The pool water is circulated through the pipes using the pool’s circulation pump, which would be running anyway. The controller operates a valve to divert some of the pool pump’s water through the collector. When not operating, the collector water drains back into the pool.