I’ve heard that the U.S. Department of Energy determined that loose tube collectors are best for pool heating when they selected this type of solar pool heater for the Atlanta Olympic Games. Is this true?
No. This particular system was actually selected to cool the Olympic venue pool. Here’s what happened. The swimming and diving competitions of the 1996 Summer Olympic Games were held at the Georgia Tech Aquatic Center, which was designed as an outdoor facility in order to increase spectator seating capacity from 2,000 to 15,000. (The Aquatic Center was converted to an indoor facility about five years after the Atlanta Summer Games.)
Pool water temperature for competitive swimming events must be maintained between 25° and 28° Celsius (77° and 82.4° Fahrenheit). An outdoor pool’s temperature tends to match the 24-hour average air temperature for the preceding week, and Atlanta’s average daily air temperature in late July is 88°F. It isn’t unusual for temperatures to climb into the high 90s.
This meant the Aquatic Center pool water would be at least 10 degrees too warm for competition. So the primary goal of the architects and engineers was to come up with an effective method of cooling the pool.
Most non-metal solar pool heating collectors, including loose tube designs, radiate energy to a clear night sky at roughly the same rate. However, loose tube collectors have much greater convective heat transfer rates than flat plate designs because air is able to flow freely around and between the fluid passageways.
This is a really bad thing when you’re trying to heat a pool and the surrounding air is cooler than the water circulating through the collector’s fluid passageways. And even worse when the wind picks up.
On the other hand, a higher rate of convective heat transfer is great if your primary goal is to cool a pool at night. Thus, a loose tube heat exchanger design was a perfect choice to cool the Atlanta Olympic pool during the hot Georgia summer.
The manufacturer and dealers of this particular loose tube collector system promote the Georgia Tech Aquatic Center project as their flagship large-scale “solar heating” installation, and often point to the Department of Energy selection process for this project as evidence of the product’s solar heating superiority. Nevertheless, the fact remains that the loose tube collector array installed at the Georgia Tech Aquatic Center was specifically designed for cooling. The pool has a steam heat exchange system for primary heating.
A salesman told me that solar collector panels with two-inch header pipes are better than models with 1-1/2 inch headers. Is this true?
No. The argument is that a two-inch solar collector panel header improves efficiency by allowing more water per minute to flow into the fluid passages of the heating surface. While it is true that two-inch pipe has a higher saturation (maximum) flow rate than 1-1/2 inch pipe, a single bank of solar panels is never installed with more than about 480 square feet of total solar collector panel area. (Larger solar systems are broken into multiple panel banks.) Solar panels designed for swimming pool heating temperatures function best at a water flow rate of about 1/10 gallon per square foot of solar panel surface area per minute. So for the best thermal performance, we would never want to flow more than about 48 gallons per minute (1/10 gpm per square foot x 480 square feet) through a single panel bank, regardless of the pipe size. 48 gallons per minute is well below the saturation flow rate of 1-1/2 inch pipe.
Some solar collector panels require larger headers to partially offset the increased back pressure created by a plenum chamber design (see below). Unfortunately, some of the companies that sell plenum chamber collectors teach their salespeople to compare the costs of 1-1/2 inch and two-inch Schedule 40 PVC pipe at building supply outlets like Home Depot and Lowe’s, to justify higher prices for their solar pool heaters. But this is a meaningless comparison because headers comprise only a fraction of the material in a solar collector panel.
Is it true that solar collector panels with “flow-balancing” plenum chambers perform better than other solar collector designs?
No. The idea is that flow-balancing plenums—secondary water chambers between a solar collector’s headers and the flow passageways of its heating surface—provide more balanced water flow throughout a bank of solar collector panels, and thus more efficient heat transfer.
But this is a solution to a non-existent problem. A basic rule of fluid hydraulics is that flow rates will vary in parallel pipes so that the head losses are equalized through each flow path. In plain English, if the diameters and lengths of the individual parallel flow passages in a solar collector are identical, the flow rates through these passages will be identical. This is always the case in a correctly installed solar pool heating system.
The only practical effect of additional flow restriction in a properly installed solar pool heating system is increased workload for the pump.
Here’s how this idea got started. During the 1970s, a solar collector manufacturer developed a process for heat-welding the heating surface of a polypropylene solar collector to the collector’s header pipes. The patented process involved fusing strips of plastic—called flanges in the patent application—along the length of each header pipe, encasing the the ends of the heating surface. The flange design cut manufacturing costs by reducing the number of steps needed to attach the heating surface—and its many individual fluid passageways—to the headers. Unfortunately, the new process created a secondary water chamber along the length of each header, which significantly increased flow restriction through the collector.
An old saying holds that you should find ways to turn your weaknesses into strengths. At some point, someone came up with the idea that the additional flow restriction would ensure that water spread out more evenly among all of the fluid passageways in the solar collector’s heating surface. And so it was that the drawback of substantially increased flow restriction was magically transformed into “flow metering” and “flow balancing.”
Two major solar pool heater manufacturers use the plenum chamber design in their solar collector panels today. Their dealer salespeople sometimes use the example of house central air conditioning ducts to illustrate the need for a flow-balancing plenum chamber. But this is a poor analogy because there is usually great variation in the length and size of air conditioning ducts branching to the different rooms within a house, so there is indeed a need to balance the air flow between the rooms. This variation does not exist within a bank of solar collector panels.