Using Thermal Imaging to Troubleshoot Solar Water Heating Systems
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Thermal imaging cameras, also referred to as infrared (IR) cameras, are becoming common tools for PV system troubleshooting. The devices are also very well suited for diagnosing performance issues in solar water heating (SWH) systems. After all, SWH systems collect, transport and store heat. The ability to accurately determine temperature differentials between points in the system provides crucial visibility into an installation’s performance. In the past, I relied on noncontact infrared thermometers for recording equipment temperatures during system commissioning and troubleshooting. IR thermometers are limited, however: You measure the temperature of the specific point you are aiming at, and it is easy to miss hot spots. An IR camera allows you to capture a thermal image of a larger area and visually displays the temperature and temperature gradients of objects in the image.
While several good IR cameras are available, I use a Fluke TiR32. With a list price of approximately $8,000, it is expensive, but it provides information that is otherwise impossible to capture in the field. The TiR32 has a 2% accuracy range, which is more than adequate for measuring temperatures in solar heating systems. The following case study illustrates the effectiveness of using a thermal imaging camera for SWH system troubleshooting.
The Ramsey County Law Enforcement Center is a pretrial holding center for approximately 400 inmates in downtown St. Paul, Minnesota. A 35-collector solar heating array was installed on the facility’s roof in October 2012. The project was one of several SWH installations completed by the City of St. Paul and funded in part with an American Recovery and Reinvestment Act (ARRA) grant. Prior to the system’s installation, the center’s potable water was heated by a hot water loop supplied by District Energy, a nonprofit company that operates the largest biomass-fueled hot water district heating system in North America. It supplies heat for more than 185 buildings and cooling for 100 more in downtown St. Paul.
Westwood Professional Systems managed and Karges-Faulconbridge engineered the construction of the Ramsey County Law Enforcement Center installation. With an estimated peak production of approximately 75 kW (93 MWh annually), the solar heating project was expected to reduce the center’s dependency on the city’s hot water system and the related monthly expense by 40% to 50%. The array’s 35 Solar Skies NSC-40 collectors and array-side piping are filled with a 50% glycol solution to withstand St. Paul’s frigid winters. Each collector has an absorber plate area of 36.9 ft2 and holds 1.21 gallons of glycol solution. The system uses two Sondex A/S PHE heat exchangers. The SWH system transfers heat to a branch of the District Energy hot water line, and that branch then flows through a second heat exchanger to heat the correctional facility’s domestic water.
A LI-COR LI-200SA pyranometer mounted at the array measures solar irradiance. An Alerton BACtalk controller continuously calculates the amount of heat that the system should produce based on the current level of irradiance and compares it to the expected heat loss within the collectors due to the ambient temperature at the array. As soon as this number is greater than 0, a pump turns on to circulate glycol through the collectors. The heated fluid flows through the first heat exchanger that is installed in line with the District Energy heat system. A temperature sensor in this line activates a second pump when the temperature reaches 100° F, allowing domestic water to flow through the second heat exchanger. A temperature sensor on the domestic side of the second heat exchanger signals the controller to stop circulation of the domestic hot water when temperatures reach 140°F and allows the District Energy heat line to absorb the heat from the glycol. Because the building is located on the farthest end of the City’s hot water line, it should always be able to absorb any excess heat. In case it does not, an alarm signals the hot water on the secondary side of the first heat exchanger to flow directly from the District Energy return header if the glycol system exceeds 200°F.
Commissioning and Troubleshooting
During the system’s initial testing and commissioning, the commissioning agent raised concerns about the circulating pumps: They were allowing the domestic water system to heat to approximately 129°F rather than the 140°F design temperature. The source of the problem was difficult to track. All gauges and sensors on the new system appeared to be operating at the preset 140°F, but the existing water storage tank was reaching only 129°F. An extremely congested mechanical room and a lack of labeling on the piping (custom labels had not yet been applied) complicated troubleshooting.
During the troubleshooting process, I used the Fluke thermal imaging camera to verify that the solar collectors were operating correctly, with no impedance to fluid flow. I did not identify any problems, which pointed to an issue elsewhere in the system. Using the camera, I was able to quickly trace which of the two pump systems was running at a specific time and identify the subsystem’s associated piping. Heat loss was evident around valves that could not be fully insulated and several pipe joints that still needed insulation. Using the IR camera, I verified that the temperature of the piping near the point of the control sensor for the domestic water loop was 129°F. However, the sensor was reading 140°F and signaling the system to divert heat to the District Energy hot water system, away from the domestic system. The IR camera allowed me to quickly identify this faulty sensor and then document the issue.
With the faulty temperature sensor replaced, on mostly clear days the system now produces hot water at the expected design temperature of 140°F.
—Cari Williamette / EcoVision Electric / Minneapolis, MN / ecovisionelectric.com