Solar hot water analysis with SAM (system advisor model)

BY BRISTOL STICKNEY
contributing writer

Active solar hot water systems operate in a world of relatively complex interactions. The weather, hot water usage patterns, shifting temperatures, the type of equipment, its configuration and size all have a profound effect on the actual solar benefits. I have mentioned in previous columns that there are computer simulation models available to help sort out this complexity. I recently turned to one myself to help make some comparisons.

I have been working with two different solar heating designs, one in Texas and one in Oregon. In both cases, the design process began with the idea that solar domestic hot water (SDHW) was desired, and the question was whether to use vacuum tube collectors or flat plate panels to provide the solar heat. Since the climate and the location of these jobs is very different, one might expect the answer to the collector question to be different as well. I decided to use the system advisor model (SAM) solar simulation program to help evaluate these situations in a manner that is (presumably) fast, impartial and accurate.

SAM: What it is and how it works

I like to think of SAM as the solar analysis tool provided by “Uncle SAM.” It was created by the U.S. government at the national labs, so it has been placed in the public domain. The program can be downloaded free at the National Renewable Energy Lab (NREL) website. The program puts a lot of emphasis on photovoltaic analysis and other even more exotic energy technologies. If you look through the menus carefully, you will find solar water heating included, almost as an afterthought.

A sample hot water system that you can use as a starting point includes reasonable default values for a typical system, including the collector, tank, heat exchanger, backup tank, backup fuel and all the other important details. Most of these details can be alternately supplied or changed by the user, including costs, hot water usage hour by hour and all the other variables of which you can think.

A location must be specified; the program will help you find weather data for any location from a number of sources. Run the program while you are connected to the Internet, and it will find weather data very quickly. Once located and installed in the program, a weather file will provide typical weather for that location, hour by hour, for one year. The weather data and the solar heating system inputs are used by the simulation engine (TRNSYS) to produce results that can be summarized by hour, day, month or year.

Useful features and results

Here is a brief list of the things that SAM can do:

• Weather data, compatible with SAM, is plentiful and easy to locate live on the Internet.

• Solar availability is computed based on the weather data and the collector properties, which are both easy to change.

• All results can be displayed on graphs, hourly, daily and monthly.

• Any data that can be graphed can also be exported to a spreadsheet for calculations and graphics created by the user.

• Collector data is built into the program and can easily be chosen or changed using a drop-down list.

• SRCC test data is built into the collector data, so heat output is based on actual test results.

• A comprehensive financial analysis is included with the program, but using it is not mandatory for running the performance simulation.

A quick SDHW comparison

In the final analysis, what you want from a solar water heater is low fuel consumption and plenty of hot water at a reasonable cost. If the equipment is the right size for the water heating job, then very little conventional fuel will be consumed each year. So in that case, the annual fuel savings provides a good measurement of the success of a solar water heater.

With that idea in mind, I created several identical solar water heater models, using identical default values for many of the inputs. Then I changed the type of collector from a flat plate to a vacuum tube of comparable size in each location, Corvallis, Oregon and Marfa, Texas. All of the collectors modeled were nominal 40 square feet, tilted at latitude plus 15 degrees, which tends to maximize annual solar output. The water usage patterns were identical in all of the models, with just over 60 gallons of hot water consumed each day from an 80 gallon tank. The water usage was relatively continuous, consumed steadily but with varying flow rates throughout most of the day and night. This simulates a building with a high occupancy rate. A two-tank system was provided by SAM as default, with the solar heated tank feeding a conventional tank.

As the simulation engine tallies up the solar provided, the hot water consumed and the backup heat required over the year, the question is, which collector provides better fuel savings and does climate matter?

Results can be unexpected

In a complex system, there are many variables in play, and the outcome may not be obvious or intuitive. This is a good reason to employ simulation software to inspect and analyze a system as thoroughly aspossible. In the case of my simulation experiment, the results were unexpected.

I would have guessed that vacuum tube collectors would be more important in Oregon than in Texas. SAM did not agree. In these simulations, no additional savings came as a result of using vacuum tubes in either location.

I would have also assumed that, with identical collectors, more solar savings can be achieved in Texas than in Oregon. Again SAM disagrees. The results show a little more savings is achieved consistently in Oregon using this configuration. A solar water heater that saved 2632 kWh in Texas annually was saving 2886 kWh in Oregon. Figure 36-1 shows a screen shot of the SAM “dashboard” showing the monthly savings in Texas.

Conclusion

The model used in these simulations seems reasonable in terms of hot water consumption, equipment sizing and performance calculations. The vacuum tube collectors I have been looking at tend to cost about 50% more than flat plate panels per unit area. Since the simulation model does not indicate anywhere near a 50% improvement in savings, I can conclude that, under these conditions, the extra cost is not justified by performance.

In other climates and conditions of water usage and temperature, the results could be quite different. Changing the size of the collectors and tanks could also change the outcome. SAM makes it easy to inspect and study these multifaceted interactions.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 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.