SOLAR FACTS - COLLECTOR RATINGS
Table of Contents:
- About SRCC Rating and Certification
- About the Rating of Solar Collectors
- Comparing Collector Efficiency and Cost
- Technical Explanation of the Collector Testing and Rating Program
- SRCC Collector Certification Labels
The Solar Rating & Certification Corporation (SRCC) is an independent third-party certification organization that administers national certification and rating programs for solar energy equipment. The SRCC was incorporated in October 1980 as a non-profit corporation. It is governed by a twelve-member board of Directors with representation from the public, private, and generalist sectors.
The SRCC currently operates two major solar programs: collector certification (OG-100), and heating system certification (OG-300). The OG-100 collector certification program applies to that part of a solar energy system that is exposed to the sun and collects the sun's heat. The collectors can be used to heat water, air or other heat transfer media. The OG-300 rating and certification program for solar hot water systems integrates results of collector tests with a performance model for the entire systems and determines whether systems meet minimum standards for system durability, reliability, safety and operation. Factors affecting total system design, installation, maintenance and service are also evaluated.
A direct comparison of an SRCC rated collector to an SRCC rated solar water heating system is not possible. The reason for this is two-fold. First, the collector rating shows the performance of one component in the solar package while the system rating shows the performance of an entire solar package. Second, each rating, whether a collector rating or a system rating, is developed using a separate set of assumed conditions.
The OG-100 directory contains information about solar collectors that have been certified and rated by SRCC.
The information in the directory will provide you with reliable and comparable data for solar water heating collectors you may be considering buying. The rating information is a helpful tool for comparing the efficiency of the various solar collectors on the market. While you can, and should, compare collector ratings, you cannot compare collector ratings with system ratings. All collectors which have been certified by SRCC will bear the SRCC label, which is your assurance that an independent party has verified the performance and basic durability of the solar product you are considering. Copies of SRCC labels are shown in the directory.
The directory contains descriptive information about the solar collectors and also "performance" information about them. "Performance" data relates to the energy output of the collector. The SRCC performance information contained in this directory provides a way to compare the relative performance of different solar water heating collectors, not the actual performance you can expect from a given collector. This is because the collectors and systems are tested under standard laboratory conditions which are certain to be different from those in your home. Think of the SRCC ratings as you do the MPG ratings for cars -- a benchmark, but not necessarily the same performance you will experience. Remember, too, that performance (or energy output) is only one criteria in choosing a solar energy collector. Quality of installation, cost, availability of service and parts, and the expected life of the equipment are also important points to consider. Equipment which is well-designed and well-built, but poorly installed, cannot perform according to the manufacturer's specifications.
Each time SRCC allows a solar manufacturer to attach the SRCC label to its product, very specific steps have been followed to assure consumers that the product meets SRCC's approval and that the performance information provided to you is correct. First, SRCC selects a solar collector at random from the manufacturer's facility. The collector is then sent for testing to an independent laboratory approved by SRCC. When the collector is received by the lab, it is inspected to document the materials used. (You will see much of this information in the directory pages.) Then, the collector is subjected to a variety of durability tests to reveal any leaks, to check the integrity of construction, and to assess the collector's resistance to sudden expansion and contraction and changes in water temperature. Following the durability tests, the energy output of the collector is measured to determine the performance of the collector under the standard laboratory conditions. These measurements result in the performance figures found in the box at the top of each collector's rating page in the directory. Finally, when the testing is complete, the lab partially disassembles the collector and inspects it for any hidden problems.
When the last inspection is completed, the lab sends the test report to the SRCC for review and calculation of the figures which appear in the rating directory. The SRCC also checks the collector design for reliability and durability. When the collector is certified, the manufacturer is notified and required to begin affixing the SRCC label to the solar collector. Also, the manufacturer must provide a copy of the Certification Award with each certified collector.
As you shop for a solar collector, you may see several different types. They are:
- Unglazed liquid-type collectors are those in which a liquid is heated by the sun in a stationary collector which does not have glass or other transparent covering. These collectors are commonly used for swimming pool heating systems, but are also used in domestic water heating systems.
- Glazed liquid-type solar collectors are those in which a liquid is heated by the sun in a stationary collector which has a cover of glass or other transparent material. They are the most common type of collectors, and are often used for domestic water heating and space heating systems.
- Air-type collectors are those in which the sun heats air rather than water in the collector. They are most commonly used for space heating applications.
All three types of collectors work well and can be compared with others of the same type, using the data in this directory.
The performance data about a given collector appears in the box at the top of each rating page. The data on the left is in metric (or SI units) and the data on the right is in English (or Inch-Pound units). The data, whether you read it in metric or English units, provides the total energy produced by that collector in a standard "rating day," that is, under the test conditions used to define a day.
Across the top of the chart are three categories which represent various weather conditions and seasons of the year. See Table 1 for a listing of average daily total solar radiation in several U.S. Cities. The amount of sunlight striking the collector (or "irradiance") is an important factor in how much energy the collector can produce. Also important is how much the energy output of the collector declines as the sunlight declines. Irradiance is measured in megajoules per square meter per day (or in Btu per square foot per day). Generally, a clear sky would be characterized by the 23 MJ/(m2 d) [2,000 Btu/(ft2 day)] column, while a cloudy sky would be characterized by the 11 MJ/(m2 d) [1,000 Btu/(ft2 day)] column. The 17 MJ/(m2 d) [1,500 Btu/(ft2 day)] column characterizes a mildly cloudy conditions.
Once you have determined the correct weather column, you will need to choose the correct category. The categories are listed down the left side of the box, using letters A through E. The accompanying numbers are the difference between the temperature of the water or air entering the collector and the temperature of the air around the collector. These temperature differences are important factors in the ability of the solar collector to produce energy. To use the rating chart, it is easier to refer to the following table for the correct category:
|A||-5°C||(-9°F)||Certain types of solar assisted heat pumps.
Swimming pool heating.
|B||5°C||( 9°F)||Liquid collectors with certain types of solar assisted heat pumps.
Swimming pool heating.
Space heating - air systems.
|C||20°C||(36°F)||Service hot water systems.
Space heating - air systems.
|D||50°C||(90°F)||Service hot water systems.
Space heating - liquid systems.
|E||80°C||(144°F)||Space heating - liquid systems.
Industrial process heat.
The collector with the higher number in the box which reflects your climate and category produces more energy than those with lower numbers. While such a comparison should not be the only basis for your choice of a solar energy system, you may find it helpful. Remember, too, that the energy output of these collectors in the directory has been measured under test conditions, which are almost certainly not the same as the collector will be subjected to in your home. The remainder of the system and the quality of the installation are also critically important factors in how well your solar system works, and how much energy and money you save.
Table 1 Average Daily Total Solar Radiation for U.S. Cities
|23° Tilt||45° Tilt||23° Tilt||45° Tilt|
|Baltimore, MD/ DC||14.79||14.75||1302||1299|
|Dallas/Fort Worth, TX||17.42||17.44||1533||1536|
|Des Moines, IA||14.87||15.25||1310||1343|
|Las Vegas, NV||24.16||24.14||2127||2126|
|Little rock, AR||17.31||16.94||1524||1492|
|Los Angeles, CA||20.18||19.87||1777||1749|
|New Orleans, LA||17.15||16.41||1510||1445|
|Newark, NJ/ New York, NY||14.16||14.12||1247||1244|
|Oklahoma City, OK||18.40||18.16||1620||1599|
|St. Louis, MO||16.10||16.02||1418||1411|
|Salt Lake City, UT||19.06||19.47||1679||1714|
|Sioux Falls, SD||15.12||15.63||1331||1376|
The values listed in this table are based upon TMY data for each of the cities listed. The data for the tilted surface radiation was processed using the TRNSYS 13.1 radiation processor with the Hay and Davies tilted surface radiation model.
Included in the descriptive information is the size of the collector. The Gross Area is the size of the top face of the collector; the Net Aperture is the size of the glass or other glazing material that sunlight can enter. The size of the collector may be relevant when comparing energy output and price.
Also, the "dry weight" of the collector combined with the "fluid capacity" (for liquid systems; a gallon of water weighs 8.3 pounds) will give you a rough idea of how much weight the solar system will be adding to your roof, if that is where the system is to be installed. Remember to multiply the dry weight plus the fluid weight by the number of collectors in the system.
With the ratings discussed above, it is easy to compare the energy output of one collector to another. It can be difficult however, to take into account the price of the different collectors.
One method is to compare the energy output for each dollar spent on different collectors. Or, in other words, how many Btu (or MJ) does a dollar buy if spent on Collector #1 versus Collector #2? This question can be answered by dividing the energy output by the cost of the collector. For example, you are considering a solar water heating application. Collector #1 has a rating in Category C (for water heating) under the correct climate column of 29 MJ (per collector per day) or 21,000 Btu (per collector panel per day). Collector #1 sells for $387. Collector #2 is rated at 35 MJ or 33,000 Btu; it sells for $675. Thus:
Collector #1 is the better buy, based on performance under the test conditions alone. The higher the number of MJs or Btu per dollar, the more cost-effective the collector is...all other things being equal. Remember, though, that the design and quality of the rest of the system and the installation are also critical to a good solar energy system.
The SRCC solar collector thermal performance test is based on the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Standard 96-1980, Methods of Testing to Determine the Thermal Performance of Unglazed Flat-Plate Liquid-Type Solar Collectors, for unglazed liquid collectors and on ASHRAE Standard 93-1986, Methods of Testing to Determine the Thermal Performance of Solar Collectors, for glazed flat-plate liquid collectors, air collectors, linear tracking concentrators, and other collector devices which fall within the scope of the test standard. Based on the thermal performance data derived from the ASHRAE 96-1980 or ASHRAE 93-1986 test methods, SRCC then calculates the collector ratings according to SRCC Document RM-1, Methodology for Determining the Thermal Performance Rating for Solar Collectors. This rating methodology accounts for diffuse irradiance, which is assumed to be distributed isotropically throughout the view of the collector. The methodology is applicable to all non-tracking collector panels.
Before a collector model is issued certification and ratings, SRCC requires that an individual collector be selected at random from the manufacturer's inventory. That unit is then sent to an independent laboratory approved by SRCC for testing according to SRCC Standard 100-81, Test Methods and Minimum Standards for Certifying Solar Collectors. The SRCC test sequence for collectors is a combination of durability and performance tests.
- Receiving Inspection. To inspect and document the condition of the collector prior to formal testing.
- Static Pressure Test. To determine if a loss of pressure occurs or evidence of fluid leakage or fluid path deterioration.
- 30-Day Exposure Test. To verify integrity of construction after at least 30 days exposure to adverse conditions.
- Thermal Shock/Water Spray Test. To verify that the collector structure and performance will not be degraded due to sudden thermal expansion or contraction.
- Thermal Shock/Cold Fill Test. To determine the reaction of a hot collector after the introduction of cold water.
- Post Exposure Static Pressure Test. To determine if a loss of pressure occurs or evidence of fluid leakage or fluid path deterioration after a collector has been stagnated under worst case conditions.
- Time Constant Determination Test. To determine the transient behavior of the collector or the time required to respond to abrupt changes in either insolation or inlet temperature.
- Thermal Performance Test. To determine the instantaneous efficiency of the collector over a wide range of operating temperatures. ("Efficiency" is defined as the ratio of collected energy to the available energy falling on the entire collector area.)
- Incident Angle Modifier Test. The incident angle modifier needs to be determined in order to predict collector performance over a wide range of conditions. The modifier algorithm is used to modify the efficiency curve to account for changes in performance as a function of the sun's incidence angle.
- Disassembly and Final Inspection. To visually inspect the major components and subassemblies and to report their conditions after testing has been completed.
Once the collector test unit has completed the above sequence of tests, the results are sent to SRCC for evaluation and computation of the thermal performance ratings.
A collector is judged by SRCC to have successfully completed the durability-type tests if none of the following conditions occurred during the testing:
- Severe deformation of the absorber.
- Severe deformation of the fluid flow passages.
- Loss of bonding between fluid flow passages and absorber plates.
- Leakage from fluid flow passages or connections.
- Loss of mounting integrity.
- Severe corrosion or other deterioration caused by chemical action.
- Crazing, cracking, blistering or flaking of the absorber coating or delamination of reflective surface.
- Retention of water in the insulation.
- Excessive retention of water anywhere in the collector.
- Swelling, severe outgassing or other detrimental changes in collector insulation which adversely affect the collector performance.
- Leakage or damage to hoses inside the collector enclosure or leakage from mechanical connections.
- Cracking, crazing, permanent warping or buckling of the cover plate.
- Cracking or warping of the collector enclosure material.
In addition, in order to qualify for collector certification and ratings, manufacturers must document to SRCC that their collectors meet the SRCC requirements for durability in design and construction. For examples, all collectors must be designed to prevent condensation build-up and all glass cover plates must be of a non shattering or tempered type.
The SRCC solar collector thermal performance ratings are valid only for the fluid and flow rate used to generate the ASHRAE test data.
Since performance of a collector may vary with changes in flow rate, in order to allow for an even more direct comparison of the thermal performance of various collector models, SRCC adopted the requirement beginning in April of 1983 that all thermal performance testing of solar collectors be conducted at the ASHRAE standard recommended flow rates except as noted below.
For unglazed flat-plate liquid-type solar collectors, the ASHRAE standard flow rate per unit area (transparent frontal or aperture) is 0.07 kg/(s m2) [51.5 lb/(hr ft2)]. For glazed flat-plate liquid-type solar collectors the ASHRAE standard flow rate per unit area (transparent frontal or aperture) is 0.02 kg/(s m2) [14.7 lb/(hr ft2)]. When air is the transfer fluid, the ASHRAE standard flow rate is 0.01 m3/(s m2) [2 cfm/ft2] or 0.03 m3/(s m2) [6 cfm/ft2], inclusive.
For those collectors which have been designed for a specific flow rate other than the ASHRAE standard recommended flow rate, the manufacturer may petition to have the collector rated at its design flow rate. The flow rate at which each solar collector model was tested is provided on each directory listing.
All solar products certified by SRCC are required to be labeled with an approved SRCC certification label within sixty (60) days of receipt of certification. The label shown below should be on each collector certified under SRCC's OG 100 protocol.
|This product certified by the
Solar Rating and Certification Corporation
c/o FSEC, 1679 Clearlake Road
Cocoa, FL 32922
SRCC Document OG-100
|Sample Solar Corporation
P.O. Box 12345
Anytown, CA 97402
Model No.: Super Sample
Gross Area: 3.72 m2 (40.00 ft2)
|Clear Day Rating in