SolarUS, a manufacturer and distributor of solar collectors and solar thermal products, has been in the solar thermal industry for more than 30 years. During that time, the company’s experienced team has considered many technologies in pursuit of maximizing the amount of energy its equipment can extract from the sun.

SolarUS designs and installs closed-loop glycol systems, the most widely used solar thermal system design. In the past, other solar thermal system technologies such as drain-back equipment offered users better heat transfer characteristics than closed-loop systems. To improve efficiency, SolarUS uses flat plate and evacuated tube collectors, which have strong heat transfer characteristics. Also, unlike steam-back and drain-back equipment, the closed-loop glycol systems have heat transfer fluid loops that offer freeze protection and corrosion protection. Properly designed closed-loop systems also offer users simple, cost-effective installations with protection against steam expansion.

As SolarUS introduced new technology in its collectors, the company encountered a problem with the closed-loop glycol systems. High performing collectors can reach 485°F (252°C) due to the advanced design and materials. These collectors are well suited for maximizing the extraction of the sun’s energy, yet this ability highlighted the temperature limitations of the heat transfer fluid used in its closed-loop glycol systems. The company noted that over time, the heat transfer fluid tended to break down. This can lead to several problems, including a significant decline in freeze protection. The team also noted that fluid breakdown resulted in the buildup of corrosive acids in the fluid as well as rust deposits in the closed-loop system piping.

The team was concerned that the fluid degradation would lead to a decrease in the efficiency of the company’s equipment. This was particularly alarming because many of SolarUS’s customers keep a close watch on system efficiency. If efficiency decreased, SolarUS’s customers would call upon the company more frequently to perform system maintenance, which would likely result in fluid replacement.

In addition to the fluid degradation issues, the team was concerned that the buildup of rust in the piping would shorten the lifespan of solar thermal equipment. This effect would decrease the value proposition the company could offer prospective users.

To combat the fluid degradation problem, engineers at SolarUS looked at alternatives to the propylene glycol (PG)-based fluids they were currently using. One of the options considered was a glycol called Susterra 1,3 propanediol (PDO) made by DuPont Tate & Lyle Bio Products Co., a DuPont joint venture. The PDO fluid offers better freeze protection and thermal stability than PG — benefits that the solar thermal company valued. The PDO fluid is certified as 100 percent bio-based by the U.S. Department of Agriculture.

While investigating the performance of the PDO-based fluid versus a PG-based fluid, the SolarUS team used data from a study comparing inhibited PG-based fluid to inhibited PDO-based fluid. They noted that after both fluids were subjected to long periods of extreme temperatures, there was a visible difference in their appearances. The PG-based fluid appeared muddy compared to the PDO-based fluid. Additional testing revealed that the inhibited PDO-based fluid exhibited a reduction in organic acid production when compared to the PG fluid (figure 1). The team also noted that the PDO-based fluid retained more of the inhibiting nitrites required to protect the pipes from damaging corrosion (figure 2). That fluid quality helps SolarUS protect its equipment against rusting in the heat transfer pipes.

Based upon the test results, SolarUS took an unusual action: working with DuPont Tate & Lyle, SolarUS launched its own fluid, called So-Blu. The fluid tolerates extreme temperatures for extended periods of time. It extends the frequency interval between required equipment maintenance and fluid replacement because it prevents the buildup of sludge in pipes.

 SolarUS plans to install the fluid in all of its closed-loop glycol solar thermal equipment. The company will continue to monitor the performance of its fluid in its equipment and compare it to previous installations in which they used PG-based fluid. The business believes that the fluid’s stability at high temperatures will increase the lifetime of its solar thermal equipment. That performance benefit is expected to help the company achieve energy conservation while reducing the environmental footprint.