Analyzing European Influence on American Solar Hot Water Systems
Heliodyne panels being installed
As a thirty-plus year veteran of the solar thermal industry, I still make a conscious effort to stay on top of the latest developments in solar hot water systems. One good way for me to keep informed about the latest trends (good and bad) is by reading SolarPro magazine. Although the publication is typically dominated by PV articles and advertisements, occasionally I’ll find something of interest on solar thermal.
So I was certainly curious when I saw that the August/September 2011 issue had an interview with Ole Pilgaard, President of Heliodyne. The article, entitled Ole Pilgaard, Heliodyne: Bringing European Experience to the US Solar Thermal Industry can be read in its entirety by clicking here (free subscription required).
In the interview, Mr. Pilgaard discusses differences in history and philosophy between European solar thermal development and American.
He states for example, “The European solar thermal industry developed faster in the northern countries, like Germany and Austria, where freeze protection is more of an issue than stagnation. Some malfunctioning drainback systems received bad publicity in Europe when solar thermal grew into the mainstream contractor business. As a result, it became more apparent that drainback systems are extremely sensitive to correct installation. The European solar thermal technology platform therefore developed in the direction of fully flooded glycol systems, which are more reliable than drainback systems and less sensitive to correct installation.”
What is of interest to me is his statement about malfunctioning drainback solar hot water systems and sensitive installation. Of course, I have no idea what the designs of the drainback systems were, but in the late ’70s and early ’80s, people were trying many different designs for drainback, most of which were not well thought out.
Drainback is not one particular design. That is why my solar thermal systems are sometimes called GRC drainback systems. GRC stands for Gravely Research Corporation, founded in 1977, and holder of the drainback patents.
I am not sure if Mr. Pilgaard is referring to drain down, a pressurized open loop system that has been a disaster.
Also, Mr. Pilgaard’s comments about northern Europe are key to understanding the European approach to solar thermal. Having abandoned drainback (or, never having gotten the right design), they adopted glycol systems. As he said, stagnation is much less of a problem in the colder climate of northern Europe.
In the USA, stagnation is a real problem, and I am afraid all the European systems coming into this country are going to have some surprises. America is littered with failed and poorly performing glycol systems.
The “sensitive installation” comment is also interesting. It turns out that glycol solar water heating systems are far more sensitive to components and technique than drainback systems. There should be no traps in the lines, and a gentle slope of the supply and return lines to the tank. These techniques should be followed on a glycol system because one problem with them is purging all the air from the lines. The bubbles need to go to a high point and out the air vent. Since (GRC) drainback systems have no air vents, expansion tanks, check valves, pop off valves, and pressure gauges, and don’t require a mechanical pump and reservoir of glycol mix to pressurize the system, I don’t think I can agree that they are more sensitive than a glycol system. You have one thing to get right in a drainback system – the slopes mentioned above. Once done, they never need touching again. On a glycol system, you have many things to get right, and they have to be checked, maintained, and replaced over and over again. So much for superior reliability. Even the NABCEP training information states that glycol systems are 15% less efficient and have 30% shorter lifetimes than drainback systems. This information comes from experience.
On another subject, Pilgaard comments on collector design. In Europe, the serpentine design for the absorber plate is common. In America, that design was abandoned for the parallel riser design (also called harp design) in the early ’80s. He says:
“The serpentine concept, contrary to the harp design, is based on one riser and consequently has a lot fewer joints to braze. It’s therefore cheaper to produce. US manufacturers have been hesitant to move to serpentine collector design because of the larger pressure drop inherent in this design approach and the resulting decrease in flow rates and performance because the collectors operate at a higher delta T.”
Basically, he says it is cheaper to produce, but has decreased performance vs the standard harp design. I don’t see how attaching a serpentine piece of copper tube to a plate is easier than brazing ten risers in a jig.
Serpentine designs cannot be used in drainback systems because the geometry will not drain. I also know one major European solar manufacturer that uses an aluminum absorber on copper water tubes. This choice of materials was abandoned in American in the early ’80s due to galvanic corrosion between the copper and aluminum. The rule of thumb ever since has been to never use aluminum for the absorber plate.
In summary, Mr. Pilgaard describes system designs and approaches that have failed or been problematical in the US. The US pioneered highly efficient, durable solar hot water systems in the early ’80s, only to have the industry destroyed by federal government energy policies. Europe was lucky that their governments were more far sighted and encouraged the industry to thrive. As the US industry begins to grow again, we need to remember that we developed the best technologies a long time ago, now we need to rebuild with American know how.
I hope someday we can bring our high efficiency, rock solid American solar thermal technology to the Europeans.