Heat Exchangers in Solar Water Heating Systems

Green or sustainable building and construction is becoming more important and prevalent every day. Heat exchangers play an important role in green building design by reducing the environmental impact of buildings through increased efficiency and use of space. One way heat exchangers are transforming residential and industrial life is through solar water heating systems.


In a solar water heating system, a heat exchanger connected to solar panels is used to heat water by increasing the temperature of the liquid or air used to heat the water. While this may not sound like much, reducing the cost of heating water is very important. According to Green Building Advisor, “Even with the most efficient methods of generating hot water, we still lose the vast majority of that heat down the drain … hot water is a once-through product … 90% of the heat in hot water is lost down the drain.”


In other words, we don’t want to spend any more money or resources heating water than we have to because most of that energy is lost. In fact, the money spent on water heating is probably generating the least return for investment in your home or business.


The process of using a solar water heater is fairly simple. A heat-transfer liquid is heated by the solar collector. The liquid then travels through the heat exchanger and heats the water. The heat-transfer fluid then returns to the solar collection device to be reheated. Heat exchangers are typically made of copper due to the metal’s ability to conduct thermal heat. Copper also resists corrosion and fouling. In instances when the heat transfer liquid is corrosive or temperatures may be very high, iron or steel may be selected instead of copper.


While a single-wall system may be used, it is wise to consider a double-wall heat exchanger when the water being heated is potable. This is even more important if the heat-transfer liquid is caustic or toxic. Because the thickness of the metal is increased, and the heat must pass through extra layers, double-wall exchangers are not as efficient as a single-wall model. The extra safety should offset any concerns about lost efficiency, however.


Maintenance and Repair


As with anything that includes moving parts, wear and tear occurs. Adding to the potential issues, water and other liquids tend to wear on pipes, and may even cause chemical reactions that lead to corrosion, fouling, or failure.


The frequency and type of maintenance required depends on the heat exchanger type, the metal used in the heat exchanger, your climate, and the type of heat-transfer fluid used.


In very hot climates, solar panels may cause water temperatures to exceed the boiling point. In this case, a heat-transfer liquid with a higher boiling point must be used. These liquids break down over time and must be replaced. It is also important to inspect for corrosion and fouling on a regular basis, especially around joints. In cold climates where freezing is likely to occur, the heat-transfer liquid is an antifreeze solution which must be changed out every three to five years.


If water is used as your heat-transfer liquid, there are a number of factors to consider. If the system is closed, the water will likely be fine; however, if new water is added and more oxygen is introduced into the system, there is a higher likelihood of corrosion. Additionally, the mineral content of the water makes a difference. In areas with hard water supplies, scaling may reduce efficiency.


Scaling may be reduced or avoided by using a water softener, but you will still want to run a de-scaling agent through the pipes every few years. You will want to inspect every surface that the water has contact with, and not just the piping. This includes the outsides of the heat exchanger piping (which may need to be sanded down), the valves, and the pumps.


Corrosion is the destruction of metal through a chemical reaction. We most often think of corrosion as oxidation, though there are other forms. In solar water heat exchangers, oxidation is a real concern if the pipes are steel or iron, and oxygen has a way to enter (such as through an open loop system).


In hot or cold climates where antifreeze or other corrosive chemicals are used as a heat-transfer liquid, the system is pressurized. If the pressure regulator goes bad, so does the whole system. The sensors and regulator should be inspected and maintained regularly. Sensors that are not properly placed or functioning will not alert the system to a potential freeze and not be able to take corrective action.


If there is an air vent, make certain that it is not blocked, and that it can open and close freely. Many times safety features are not used (thankfully) but the hinges, springs, and other parts become stuck if they are not maintained. Air vents are frequently forgotten, but in the event the collection loop needs to drain, that air vent is the only thing preventing a vacuum from forming within the loop as it drains.


It should be mentioned that there are two variants of the solar water heater: passive and active. The passive systems are used in moderate climates not prone to freezing. They are less expensive and not as efficient, but also require very little maintenance.


Active systems are used in climates with extreme temperatures. Because they require different heat-transfer liquids, pumps, and controllers, these systems require more frequent maintenance. Granted, they are also more efficient, so the extra maintenance costs are usually offset by the savings obtained.
Many times a homeowner or business will want to save some money by maintaining the system themselves. Several of the tasks associated with annual maintenance can certainly be done without professional assistance; however, the inspections are not just about efficiency, but about safety and preventative maintenance. The risks associated with a system failure far outweigh any potential savings from forgoing or skimping on proper and professional oversight and maintenance.


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