Solar Heat Collectors: Area versus aperture

Every solar heating project inevitably starts with an estimate of collector size. For example, a commonly used “Rule of Thumb” for the heating conditions in Santa Fe is 10 percent of the heated floor area plus 1 square foot per every 2 gallons in the Solar Domestic Hot Water (SDHW) tank.

An educated guess can be done even during an informal phone call with a quick calculation based on the floor area of the building and the size of a SDHW tank. This gives a reasonable size for the minimum number of collectors needed in total combined square feet that is probably in the right “ballpark” for discussion purposes.

The two most common kinds of solar heat collectors used to provide heat for buildings today are the flat plate type and the evacuated tube type. They both have glazed (glass covered) heat absorber surfaces, but have very different surface area configurations. Did you know that there are three different kinds of surface area on a solar heat collector? Let’s take a closer look at how this plays out in these types of collectors.

SRCC collector area specifications

In the U.S., the Solar Rating & Certification Corporation (SRCC) provides a standard procedure for testing solar heat collectors. A collector that has passed the SRCC certification process (OG-100) meets certain reasonable standards of collector construction and operation. The test results can be used to compare one collector to another with the understanding that they were all tested the same way.

SRCC certification is often required in order for a collector to qualify for tax credits and other subsidies promoting alternative energy. So, you will find an SRCC certification sticker on nearly all of the solar heat collectors installed today in the U.S. These ratings can be seen on the SRCC web page at www.solar-rating.org.

When an SRCC certification is issued, the collector specifications are listed along with the other results. The collector area is not listed as a single value, but rather as three possible values: Gross Area, Net Aperture Area and Absorber Area.

Gross Area is the total size of the surface of the collector that faces the sun. This includes any part of the collector construction that is integral to its proper functioning that cannot be removed or separated from the collector itself. So, glass (glazing), frames, spaces between components, integrated plumbing and hardware may be included in the Gross Area depending on the technical design of the collector.

Net Aperture Area typically includes only the glazed (glass covered) area of the collectors. The area of the glazing is the part of the collector that is designed to trap solar radiation.

Absorber Area includes only the size of the black absorber surface inside the glass cover. The black surface is actually the place where the solar radiation is intercepted and converted to heat. The size of the absorber may be smaller or may be the same as the glazed aperture area.

An evacuated tube example

The photo in Figure 75-1 shows a house near Denver with a recently completed New Standard solar hydronic heating system. The solar collectors that can be seen on the roof are Apricus AP-30 models.

This is an evacuated tube collector that consists of 30 glass tubes plugged into a top header (the light colored cap above the dark glass tubes). Each glass tube has a long narrow flat plate absorber surface inside that provides solar heat to a heat pipe which, in turn, delivers all the heat to the top header. A vacuum inside the glass tube surrounds the black surface acting as thermal insulation to prevent heat loss from the hot absorber. Hydronic fluid is pumped through the top header to carry the heat away from all the tubes.

The SRCC certification for the AP-30 collector can be seen on the SRCC website. It lists the area specifications for this collector as follows.

  • Gross Area – 44.76 Square Feet
  • Net Aperture Area – 30.05 Square Feet
  • Absorber Area – 26.35 Square Feet

Notice that the difference between the gross area and the net area is 14.7 square feet. This is made up mostly by the header on top of the tubes, which is an important part of the connective plumbing, but not part of the glazed aperture.

Collectors are often compared based on the Category C rating under ”medium” solar radiation conditions. This represents the heat output of the collector under typical hot water temperature conditions found in home heating systems. This value can be found on the SRCC sticker on the collector itself. For the AP-30 collector this comparative heat output rating is as follows.

TABLE 75-1:
SRCC Heat Output Rating for AP-30 Collector
Category C, Medium Solar Radiation
Per Collector – 29,300 Btu/Day
Per Gross Square Foot - 654.6 Btu/Day
Per Net Square Foot - 972.0 Btu/Day

A flat plate example

The photo in Figure 75-2 shows another house near Colorado Springs with a recently completed New Standard solar hydronic heating system. The Solar Collectors that can be seen on the roof are Solar Skies SS-32 models. This is a Flat Plate Collector that is nominally 4 feet by 8 feet. A tempered glass cover is mounted over a black absorber plate which heats up when exposed to solar radiation. The glass cover helps to trap the heat inside the collector while hydronic liquid is pumped through the collector to carry away the heat to the house.

The SRCC certification for the SS-32 collector can be seen on the SRCC website. The area specifications for this collector as follows.

  • Gross Area – 31.91 Square Feet
  • Net Aperture Area – 29.93 Square Feet
  • Absorber Area – 29.93 Square Feet

Notice that the difference between the gross area and the net area is 1.98 square feet. This is the surface area of the metal frames that surround the glazing and hold the glass in place.

The comparative Heat Output (SRCC rating) of the SS-32 collector is:

TABLE 75-2:
SRCC Heat Output Rating for SS-32 Collector
Category C, Medium Solar Radiation
Per Collector – 21,800 Btu/Day
Per Gross Square Foot - 683.1 Btu/Day
Per Net Square Foot - 728.4 Btu/Day

Reality check

The SRCC standard procedure uses the gross area of the collector when calculating efficiency and performance values on a ”per unit area” basis (per square foot). So, to do this, we compare the “per gross square foot” value in Table 75-1 to Table 75-2.

You may be surprised to see that the Flat Plate Collector is rated slightly higher (4 percent) than the Vacuum Tube Collector with 683.1 Btu/day/square foot versus 654.6 Btu/Day/square foot respectively under these SRCC standard test conditions. The advanced technology of the vacuum collector does not seem to produce a clear performance benefit under these typical home heating conditions using this method of analysis.

The glazed aperture areas of both these collectors are essentially the same size, with around 30 square feet of glazed area each. And, if we compare the performance of the two collectors based on the net glazed aperture area we find that the Vacuum Tubes perform better (25 percent) than the flat plate under these conditions with a score of 972.0 to 728.4 Btu/day/square foot, respectively.

When we make our comparison based on the net glazed aperture, there is a clear advantage in the thermal performance of the vacuum tube system. Under more severe conditions (e.g. SRCC Category D), the vacuum tube system will also tend to shine.

This is an example of how the same data can be used to support two opposite conclusions. So, if we were going to choose one of these collectors of similar size over the other, under these climate and temperature conditions, what really matters? The gross collector’s performance or some other issue?

The answer is up to the designer, owner, solar budget, and the compatibility with each individual installation. There may be other important factors involved in the final choice, such as compatibility with thermosyphon self-cooling, snow shedding, hail resistance, low profile capability, local climate extremes, and high temperature requirements.

But, quite often the final choice is driven by cost. After a reasonable analysis of all the issues above, the bottom line may come down to a simple question. For example, if one collector costs 30 percent more than the other, is it likely to produce 30 percent more savings (or other real benefits) in this installation or not? The proper interpretation of standardized performance data like the SRCC data can be a good starting point in answering this kind of question once the price of the collectors has been determined.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.

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