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Active Solar Heating Systems Performance/Reliability

After the energy crises of the 1970s and the subsequent increases in the cost of petroleum-based fuels, interest in active solar energy systems surged. Thousands of systems were installed from the late 1970s through middle 1980s. Federal and local governments, utilities, and private homeowners participated in the installation of these systems. During this period, hundreds of solar systems were monitored, producing a great deal of documented performance data. The majority of the data collected was on the performance of solar domestic hot water systems. The following is a summary of the effectiveness of the more common types of solar domestic hot water systems from data collected from performance monitoring activities of the 1980s.

These studies show that solar domestic hot water systems are more reliable than combined hot water/space heating systems. Solar air space heating systems are generally more reliable than solar domestic hot water systems. This is mostly due to leaks in the liquid-based systems. While air leaks in solar air heating systems are difficult to detect, they generally cause no damage. They only result in lower system efficiencies and performance. If the air system is not monitored closely, the defects in the system often go undetected.

Of all the liquid systems evaluated, drain-back and recirculation systems are the most reliable. Antifreeze and oil systems are more reliable than drain-down systems, as well as systems that use electric resistance heating to prevent freezing. This is primarily due to failures in the sensors and automatic valves that made up their freeze detection circuits. Some of the other failures were attributed to heating circuits that did not work, usually during electric utility power failures.

Domestic solar hot water systems that use a photovoltaic (PV) module to power pumps are generally more reliable than other solar domestic hot water systems. Although they initially cost more to install, they are very simple and require little maintenance. In addition, they are not affected by power outages.

Reliability of Solar Heating System Components: The primary components of solar heating systems are the collectors, heat transfer systems, heat storage, and controls. Based on results from the National Solar Demonstration Program (NSDP), collector and heat transfer components were the most failure-prone. Of all the solar systems tracked in the NSDP, about 30% had problems with these components. Problems with the storage and controller systems occurred 20% of the time. Almost 60% of all the systems experienced some sort of difficulty.

Collector Problems: The greatest problem in all types of collectors is leakage. Leaks can occur from improper fabrication or installation or as the result of freeze damage. Other problems are damage to collector glazing, breakage of seals and gaskets, buildup of mineral deposits (when water high in mineral content is used as the heat transfer fluid), and corrosion (when dissimilar metals such as aluminum and copper come into contact).

Heat Transfer Subsystem Problems: The largest potential problem in heat transfer systems is freeze protection failure. Freeze protection tends to be unreliable in water-based systems. Freezing can even occur in the water side of the heat exchanger in systems that use an antifreeze solution as the heat transfer fluid, if the antifreeze solution thermosiphons between cold collectors and a warm heat exchanger. Another problem results when antifreeze solutions become acidic, due to the breakdown over time of the antifreeze corrosion inhibitors. This eventually results in internal damage to the pipes, collectors, and heat exchangers, which can cause leaks.

Freeze-related failures also occur in certain types of air systems using antifreeze solutions as part of their heat transfer system. If a back-draft damper is not installed in the air ducts, cold air may thermosiphon back to the air-to-water heat exchanger and freeze the water.

Domestic water that is high in mineral content (or "hard") may cause the buildup (or scaling) of mineral deposits in liquid based solar heating systems. If water is used as the heat transfer fluid, scaling can occur in the collector, distribution, and heat exchanger piping. In systems that use glycol (an anti-freeze) as the heat transfer fluid, scaling can occur on the surface of the heat exchanger that transfers heat from the solar collector to the domestic water. 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 with a wire brush.

Many of the problems mentioned above can be eliminated with proper installation, insulation, and maintenance practices.

Storage Problems: Storage subsystems are storage tanks in liquid systems, and rock bins or phase-change materials in air systems. A NSDP study showed that 19% of the systems experienced storage system problems. For both water and air storage systems, leakage is the greatest problem. Leaks in air storage systems are hard to detect, so rock bins must be securely sealed during construction to prevent them. Generally, the problems in storage systems are not severe, and do not have a significant impact on the overall operation of the system.

Control Problems: Temperature sensor placement and installation are critical to the operation of the system. Most control failures are due to improper installation or abuse of the sensors.

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