| Closed loop systems are used in areas where freezing may occur. Closed loop systems usually use an anti-freeze solution and incorporate a heat exchanger. Systems installed in warmer climates may use water as a transfer fluid and may or may not incorporate a heat exhanger. |
| Thermosyphon systems use natural convection to circulate water within a closed loop. These systems can be used in all type of climates. Heat exchangers with non freezing fluids are used in colder climates. Consideration must be take when locating the system components to ensure proper operation. |
Solar Water Heating Systems
| Flat plate collectors are commonly used in areas where freezing weather can occur. Flat plate collectors, constructed using copper risers and absorber, can raise water temperatures up to boiling if the water is left stagnant. Flat plate collectors made using poly only achieve temperatures close to 100 F if left stagnant. Constant expansion and contraction of tubing and connections can result in failure. Poly collectors are not suitable for freezing climates and may only be used during summer months in such areas. |
| Trough collectors make transfer fluids reach high temperatures quickly and are usually used in large commercial heating operations. Transfer fluids such as oil are used to gain higher temperatures over water and glycol. Many troughs are needed to compensate for the low flow required to achieve such high temperatures and the volume of fluid normally needed to be effective in a system. |
| Batch collectors heat water in a tank directly without the use of a heat exhanger or circulation of fluid. |
| Evacuated tube collectors are similar to flat plate collectors in operation.Though they have the ability to heat water to high temperatures, flow rates must be reduced due to the smaller tubing arraingments. They are not highly recommend for areas where freezing can occur especially those types that use sealed tubes full of water that exchange their heat with the fluid passing through the header. Since they are heat exchangers themselves, overall system efficiency is lowered. |
| Active, closed loop systems incorporate a pump to circulate heat transfer fluid between the collector and the tank. Differential temperature controllers or PV are use to determine when the pump operates. |
| Drainback systems can use either anti-freeze with a heat exchanger, or water and are normally considered to be an open system. Proper piping and controls are needed to ensure that pumps do not run dry and that syphoning cannot occur. |