The plumbing of the solar system consists of three basic components:
the solar storage tank,
A closed-loop system (the most common in Canada) has two pipes connecting the solar panels to the solar storage tank. One carries hot glycol to the storage tank, and the other returns the cooler fluid to the collectors to gain more energy. A heat exchanger transfers the heat from the hot glycol to the potable water. A heat transfer fluid is circulated between the solar collectors and the heat exchanger by a circulator activated by the sun.
A controller compares the temperature of the collectors to the temperature of the storage, and starts the circulating pump when the collector is 8°C warmer than the water in the storage tank. When the collector temperature is less than 3°C higher than the storage tank temperature, the pump shuts off.
A closed loop system is designed for year-round use, so there are no freezing problems. There are also several non-freeze protected systems such as drainback and thermosiphon systems, and various configurations for getting the heat transferred from the solar collectors into the heat storage.
A closed loop is under pressure and needs an expansion tank to accommodate expansion of liquid. A drainback system is similar to a closed loop system, with collectors, a pump, a drainback tank, a heat exchanger within the tank and a controller that monitors the operation. When the collector heats up, the pump moves water from the drainback tank through the collectors and back to the tank by gravity.
A passive system is different, in that the hot water storage is elevated above the collector. The sun shines on the collectors and heats the fluid so that it circulates continuously. The hot water enters the existing hot water tank from the solar tank. Its obvious advantage is the lack of need for any power or moving parts, but it is a seasonal system and has to be drained in winter.
Water is the best heat transfer fluid for use in heat exchangers because it is non-toxic and has a high specific heat, but most units use propylene glycol to provide corrosion protection and freeze protection.
©2004 Nexus Solar Corporation.