Silicone heat transfer fluids have many favorable properties that make them prime candidates for collector fluids. They do not freeze, boil, degrade, or corrode common metals, including aluminum. They have excellent stability in solar systems stagnating under 400F. Silicone fluids are also virtually nontoxic and have high flash and fire points. Current evidence suggests that silicone fluids should last the life of a closed-loop collector system with stagnation temperatures below 350F to 400F. The flash point is high, 450F, but since HUD standards say that heat transfer fluids must not be used in systems whose maximum stagnation temperature is less than 100F lower than the flash point of the fluid, this limits most silicone oils to systems with a maximum temperature of 350F or less. Also, silicones do not form sludge or scale, so system performance does not decrease with time.
The main drawback of silicone fluids is their cost. The cost of the 20 to 30 gallons of collector fluid required for a typical 500 ft collector system becomes considerable. As with hydrocarbon oils, the lower heat capacity and higher viscosity of silicone fluid requires larger diameter and more expensive piping. Because of the higher viscosity, larger pumps are required and subsequently, higher pumping costs. One other problem with silicone fluids is the seepage of fluid at pipe joints. This problem can be prevented by proper piping installation and by pressurizing the system with air to test for leaks. There have also been reports of seepage past the mechanical seals of circulating pumps.
Silicones have the advantage of lasting the life of the system with little maintenance. The high initial cost of silicone heat transfer fluid may be less than the savings that result from minimum maintenance and no replacement of collector fluid. The use of silicone fluid allows aluminum absorbers to be used without fear of corrosion. Hydrocarbon oils, like silicones, also have a long service life, but cost less. They are relatively noncorrosive, nonvolatile, environmentally safe, and most are nontoxic. They are designed for use in systems with lower operating temperatures, since some brands break down at higher temperature to form sludge and corrosive organic acids.
Distilled water has been suggested for use in solar collectors, since it avoids some of the problems of untreated potable water. However, distilled water is still subject to freezing and boiling. Therefore, an antifieeze/antiboil agent, such as ethylene glycol, should be added.
Nonfreezing liquids can also be used to provide freeze protection. These fluids are circulated in a closed loop with a double-wall heat exchanger between the collector loop and the storage tank (fig. 15-5.)
Water/antifreeze solutions are most commonly used. Ethylene glycol and propylene glycol are the two most commonly used antifreezes. A 50-50 water/glycol solution provides freeze protection down to about -30F and also raises the boiling point to about 230F.
The three most common types of solar collectors are flat-plate collectors, evacuated tube collectors, and concentrating collectors. Because of certain cost and performance advantages, flat-plate collectors have been used extensively for domestic water heating and space heating. Evacuated tube and concentrating collectors are used mostly in solar applications requiring high temperatures. A brief description follows.Continue Reading