OG-300 solar water heater system comparisons

 

BY BRISTOL STICKNEY

contributing writer

 

Solar water heater (SHW) systems are the low hanging fruit for designers, installers and building owners who want to deploy solar heating equipment. Virtually every occupied building has a water heater, and this hot water is needed all year round not just in winter. Solar water heater systems tend to be small and therefore more affordable and easier to install than space heating systems. So anyone who is attracted to solar heating can potentially start small by installing a solar water heater.

 

Packaged solar water heater systems are becoming more commonplace and more standardized, which makes specification and installation faster and easier as well. It stands to reason that increasing numbers of packaged systems will be installed in the future as solar water heating becomes more commonplace.

 

When choosing a SHW system, it is a good idea to compare one to another to be sure that when installed, it will meet the needs of the user. In the U.S., the effort to compare solar water heating systems in a standardized way is being lead by the SRCC (Solar Rating and Certification Corporation) using a method they call OG-300. I introduced the SRCC and OG-300 water heater test standards in earlier articles in the spring of 2009. See the web archives at Plumbing Engineer and Phc News magazines for copies of these articles.

 

Since that time, the SRCC website has improved with searchable test results and performance comparisons for various city locations. Their ratings and comparisons used to be presented as pdf publications with long tables of data, but now, it is more interactive with system types and city locations chosen by the user.

 

When viewing the results, the solar water heaters are labeled according to their “system type” using the following shorthand:

 

DF= Direct Forced; IF= Indirect Forced; IT= Indirect Thermosyphon; DI= Direct Integral

(These are the types I look up most often. Others are also available as well.)

 

Also, while we are clarifying the SRCC shorthand:

 

OG= Operating Guidelines; Q= Energy; ef= Energy Factor; sef= Solar Energy Factor (sef)

 

Qsolar= (Annual) Solar Savings is presented in KiloWatt Hours (kWhr) when compared to conventional Electric water heaters and Therms when compared to Gas.

 

SF= (Annual) Solar Fraction is presented for various city locations as a Decimal value.

(e.g. 0.65 means 65% of the annual hot water load provided by solar.)

 

The fact that a SHW system has been certified by the OG-300 test is in itself a testament that the components and construction of the system comply with certain standards of safety, performance and durability that are part of the testing and inspection procedure. Each SHW system that is listed by the SRCC is presented with a one-line piping diagram, and the study of these piping diagrams alone can be a worthwhile education for anyone interested in the proper placement of collectors, storage tanks, pumps, heat exchangers, valves and other components in a large variety of configurations.

 

The SEF rating

 

SRCC uses the Solar Energy Factor (sef) as its primary performance rating for solar domestic water heating systems. The sef is defined as the energy delivered by the system (as useful hot water) divided by the electrical or gas energy put into the system. And while this rating is modeled after the Energy Factor (ef) used by the gas water heater industry, it more closely resembles a Coefficient of Performance (cop) used to rate refrigeration heat pumps where the value is generally always greater than one. It is calculated using the formula:

 

sef = Qdel/(Qaux + Qpar)

 

Where:

 

Qdel = Energy delivered to the hot water load: Using the SRCC rating conditions, this value is 41,045 Btu/day. This is a reasonable amount of hot water heat for a residential application when about 64.3 gallons are provided by a conventional water heater each day. A 50-gallon conventional water heater tank, for example, could provide this amount of hot water easily under these test conditions.

 

Qaux = Daily amount of energy used by the auxiliary water heater or backup element with a solar system operating, (Btu/day). To convert to kWh, divide this value by 3,412. To convert to therms, divide this value by 100,000.

 

Qpar = Parasitic energy: Daily amounts of AC electrical energy used to power pumps, controllers, shutters, trackers, or any other item needed to operate the SHW system, (Btu/day). To convert to kWh, divide this value by 3,412.

Using this rating method, there is a direct relationship between solar collector size and sef test results. The bigger the collectors, the larger the sef rating value will be. Also, the highest possible rating on the sef scale would be a system that has no Qaux and no Qpar. In other words divide Qdel by zero. This can occur when a solar water heater provides 100% solar heat to the heating load, using 100% solar electric pumps and controls requiring no conventional energy. There are some test results that, indeed, show sef values as high as 999.9 (which imply a sef rating of infinity)! This indicates that the solar water heater is much larger than it needs to be to provide the heat required from a 50-gallon conventional water heater under these standard test conditions. But a rating of infinity is off the scale, and so does not clearly indicate the magnitude of the oversizing.

 

SEF rating and its limitations

 

The sef rating is defined as a comparison of any solar water heater to an equivalent 50-gallon conventional water heater that consumes 41,045 Btus per day (where most of the hot water is used during the sunny part of the day). For a quick comparison of a conventional residential water heater replacement in this size range, the sef can be useful. For example, if one 65 gallon SHW system is rated at sef=2 and another is rated at sef=4 then you may assume that the second system will provide a greater solar contribution than the first.

 

As a general rule, when using sef, it is best to compare solar water heaters of the same size in the same location. The sef rating tends to reward solar component size. As a single rating, sef tends to promote large collectors with large tanks, rewarding them with higher rating values without regard for cost effectiveness, proper sizing or other considerations. Therefore, the sef rating when taken by itself, may send the wrong message when comparing a large SHW to a smaller one, or when trying to compare any water heater installation that does not match the standard OG-300 test profile. Keep in mind that the more your local conditions differ from the OG-300 test conditions, the less accurate and meaningful the sef rating will be.

 

Other useful comparisons

 

In the Thermal Performance Rating section of the SRCC Web pages, they explain that the Solar Energy Factor (sef) can be converted to an equivalent Solar Fraction (sf) using the following formula:

 

sf = 1 – (ef÷sef)

 

The Energy Factor (ef) for the SRCC standard electric auxiliary tank is 0.9 and for the gas tank is 0.6. The Solar Fraction, as considered here, is the portion of the total conventional hot water heating load (delivered energy and tank standby losses) provided by solar energy.

 

Sef can also be used to calculate the “solar savings” (Qsolar), again described on the SRCC Web pages. Luckily you do not have to do these calculations yourself, since the SF and the Qsolar can be seen in the Annual Performance listings once you have chosen a SHW system (make or model) and a city location to be displayed.

 

I prefer looking at comparisons of Qsolar & Solar Fraction since these values are more intuitive for me. These ratings allow a direct comparison of energy savings or percent solar contribution respectively, and those concepts seem somehow more familiar and easier to visualize. For example, using the formula above, a solar fraction (sf) of 99% can be expressed as a solar energy factor (sef) of “infinity.” Which rating would you rather try to explain to a client? Solar Fraction, right?

 

What is missing

 

When comparing one SHW system to another for installation in the “real world,” the bottom line always involves a cost comparison. The cost versus solar fraction or the cost versus energy savings in the same location must be compared before a prudent choice can be made. The SRCC literature includes some discussion about how to use their test results to compare cost effectiveness, but there is no cost information included in any of the system descriptions or test data. Accurate costs can only be obtained from local equipment suppliers, and labor costs can only predicted from local experience. So gathering meaningful cost data is up to you.

 

Also, there are many new solar water heaters and components entering the market every year, and so there are always new SHW systems missing from the SRCC data base.

 

This does not necessarily mean that new brands, or new technology should be avoided. It just means they haven’t been rated yet, so it is up to you to compare these new solar collectors, tanks, heat exchangers, pumps and controls to other more familiar systems.

 

Final notes

 

This has been an abbreviated discussion of information available from the SRCC. This can be studied in depth on their web site at http://www.solar-rating.org. Brand names, organizations 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, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, 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 and is involved in training programs for solar heating professionals (visit online at www.cedarmountainsolar.com for more training information.)