Scaling And Corrosion In Solar Heating Systems
The two major factors affecting the performance of (a properly sited and installed) solar heating system are scaling (in liquid or hydronic based systems) and corrosion (in hydronic and air systems).
Domestic water that is high in mineral content (or "hard") may cause the buildup (or scaling) of mineral (calcium) deposits in hydronic solar heating systems. Scale buildup results in reduces system performance in a number of ways. If water is used as the heat transfer fluid, scaling can occur in the collector, distribution piping, and heat exchanger. In systems that use other types of heat transfer fluids (such as glycol, an anti-freeze), scaling can occur on the surface of the heat exchanger that transfers heat from the solar collector to the domestic water. Scaling may also cause valve and pump failures on the potable water loop.
Scaling can be avoided with the use of water softeners, or corrected by circulating a mild acidic solution (such as vinegar) through the collector or domestic hot water loop every three to five years, or as necessary depending on water conditions. Heat exchanger surfaces may need to be cleaned (carefully) with medium grain sandpaper. A "wrap-around," external heat exchanger is an alternative to a heat exchanger located inside a storage tank.
Most well designed solar systems experience minimal corrosion. When they do, it is usually "galvanic corrosion." This is an electrolytic process caused by two dissimilar metals coming into contact with each other. One metal has a stronger positive electrical charge than the other, which pulls electrons from the other, causing one of the metals to corrode. The heat transfer fluid in some solar energy systems sometimes provides the bridge over which this exchange of electrons occurs.
Oxygen entering into an open loop hydronic solar system will cause rust in any iron or steel component. Such systems should have copper, bronze, brass, stainless steel, plastic, rubber components in the plumbing loop, and plastic or glass lined storage tanks.
Problems with Aluminum Collectors: In many early types of solar collectors, the heat transfer fluid ran though aluminum tubing. Improper design and installation caused corrosion in aluminum collectors. In some cases, copper tubing was connected to aluminum collectors, which caused massive galvanic corrosion of the aluminum, and eventually system failure. Aluminum tubing for solar collectors is no longer made, and most collectors now use copper tubing or channels.
If you must connect aluminum to copper or other dissimilar metals, use dielectric couplings (plastic or rubber tubing). Aluminum collectors require a water-based fluid with corrosion inhibitors (such as glycol) or a non-water-based fluid such as a silicone or hydrocarbon oil. You can also place a container of aluminum shavings in the liquid loop ahead of the collector. This becomes a "sacrificial anode," with the aluminum shavings corroding instead of the collectors.
Corrosion in Copper Collectors: Collectors with copper tubing and a water-based fluid can also corrode if the system design or fluid additives are improper. In closed-loop systems that use ethylene or propylene glycol anti-freeze, air and heat may, over time, cause acids to form that can attack metals. Check the collector fluid periodically, and if needed, adjust it to the proper pH (acidity-alkalinity) level.
A combination of heat, water, and air may cause corrosion of the copper collector tubes in open-loop systems that drain back to a storage tank. Some recent drain-back systems have a layer of nitrogen gas above the water reservoir, which displaces the water in the collectors during drain-back. The nitrogen excludes air and reduces corrosion.
External Corrosion: Corrosion can also occur on external parts of a solar system. In larger systems, steel storage tanks and other steel parts can rust. Using concrete, fiberglass, or plastic tanks prevents this. Collectors near the ocean are exposed to salt air; collectors in urban areas are exposed to industrial pollutants. These environments can cause pitting of unprotected collector parts. As long as the pitting is not severe, a good cleaning and the application of a suitable paint can repair minor damage.
Passive Systems: Passive solar energy systems sometimes store heat in unlined, water-filled steel drums. Inhibitors should be added to the water to prevent corrosion inside the drums. If the pH of the water is less than 7 (acidic water), corrosion can become severe. You can obtain commercial corrosion inhibitors from refrigeration supply houses. Kits to test the pH are available from pet stores that sell tropical fish, swimming pool dealers or from swimming pool dealers and home supply/gardening centers.
Recommendations: Since the potential for corrosion exists, the design of the solar energy system should allow for easy inspection of corrosion-prone components. Periodic sampling of the collector fluid is also helpful. If high or low pH levels are found, you should conduct a closer inspection of the whole system. With meticulous maintenance, care in operation, and the proper matching of system materials, your solar system should perform for 25 years or longer.