Which solar collector is rated number one by FSEC?
The answer to this question is provided by James Roland, former Director of FSEC's Testing & Operations Division, and John Harrison, Senior Research Analyst.
Simply stated, none. FSEC does not rank collectors. We do, however, provide information which consumers can use to make informed, intelligent decisions when purchasing a solar water heating system.
First, here's an explanation of what FSEC certification means. Solar collectors may be tested at FSEC or several other FSEC-accredited laboratories. If the collector successfully undergoes the prescribed tests without degrading, and if it meets all the required quality standards, it is certified by FSEC. Although certification is no guarantee of long-term durability, it does mean the collector has held up under relatively severe conditions. Florida law requires that all collectors sold or manufactured in the state be certified to FSEC standards.
Once a collector meets FSEC standards, an energy rating is produced. The rating is calculated based on the measured efficiency of the collector.
Operating efficiency of a solar collector varies during the day as changes in solar radiation, ambient temperature, and collector temperature. To give the consumer a usable rating, we establish the variable conditions for an average Florida day, which is a profile of solar radiation and air temperature. The collector temperature depends on the task for which the collector is intended. For example, collectors for heating swimming pools will operate at a relatively low temperature (95° F); domestic hot water collectors must be hotter (122° F); and collectors providing energy for industrial applications still hotter (212° F). Solar collector certification indicates how much energy the collector will produce at each of the three temperatures.
Unglazed collectors (swimming pool collectors) produce high energy ratings at 95° F, low ratings at 122° F and usually zero at 212° F. This means they are very efficient at low temperatures but, because they are subject to a high heat factor loss, their efficiency declines dramatically as the temperature increases. Collectors with single and double covers have better energy ratings at 122° F. Evacuated tube collectors are designed specifically to operate at higher temperatures and perform well at 212° F.
Another type of "collector" rated by FSEC is an integral collector storage unit (ICS). An ICS system heats and stores the water all in one unit, unlike a system with a flat-plate collector that heats the water which is then stored in a separate tank. ICS ratings are developed from a system test that differs significantly from the flat-plate collector test. For this reason, the ratings for ICS systems cannot be directly compared to the ratings for flat-plate collectors. It would be like comparing apples to oranges. An overall solar system performance comparison will provide a valid comparison of systems using flat-plate versus ICS collectors.
FSEC has normalized collector performance to allow a consumer to make direct performance comparisons. Total energy output is divided by the collector area to produce a number representing energy output per unit (Btu per square foot). These thermal performance ratings are published and distributed by FSEC.
Often we hear of a manufacturer claiming an advantage of one Btu per square foot over another. This simply doesn't make sense. Relative accuracy of the original efficiency test and rounding off of the energy ratings make such fine-tooth comparisons meaningless. The consumer should consider criteria such as cost, warranties, dealer reputation, etc., in making a decision. The ratings should not be a critical factor unless there is a substantial rating difference (at least 25 Btu per square foot) between the competing collectors.
The FSEC collector test program has helped manufacturers as well as consumers. The ratings have prompted manufacturers to seek improvements in materials and manufacturing processes. These improvements account for a 30 percent increase in the average thermal performance ratings for collectors since the program was established.