SAM Clarifications

I have the following questions regarding SAM:

•How do we run the model if there is more than one inverter type installed in the site (e.g. one 500 kW and one 135 kW inverters)? SAM assumes that the system consists of one or more of the same type of inverter and we only have the option of specifying the number of inverters in the system. Similarly, if the projects have single array at one or more different azimuth angles, how do we take this into account?.

I understand that there is option of modeling multiple subsystems in SAM. I was thinking of running simulations for different inverter types and determining the cumulative energy output. However, this is limited because we do not have specific information for each inverter type in terms of DC capacity, no. of modules etc. Is there another approach we can take?

•Assuming that there is no solar radiation incident on the array between 7 PM and 4 AM in most cases, Is there a way to estimate the shading factor for sky diffuse radiation when there is no scope of using PVSyst or SunEye.

•How can we download TMY weather data specific to each location?

SAM models systems with one type of inverter. You could use the Multiple Subsystem feature to model a system that consists of different arrays, each with its own inverter type. However, in SAM 2011.12.2, the feature is not working correctly (it will be fixed in the Spring 2012 version).

SAM models sky diffuse shading using a single value that you specify on the Shading page. The sky diffuse shading factor applies to all hours of the year. The shading data that you can import from Sometric SunEye or Solar Pathfinder software are beam shading factors, not the sky diffuse factor.

In the continental United States, you can use the Location Lookup feature on the Climate page to download weather data for specific years in the TMY format. You can also download typical-year data in TMY3 format for many locations in the U.S. (including Hawaii and Alaska) from the NREL NSRDB website . For locations around the world, you can purchase weather data from commercial providers. See Sources Non US Weather Data . See the Help topic Weather Data, Weather Files on the Web for details.

Thanks Paul for your prompt reply. Thanks for clarifying all my questions. I needed some further information.

1) Since the Multiple Subsystem feature is not working correctly in SAM 2011.12.2, do you think it's a good idea to use PVWatts for calculating the annual energy output for modeling PV projects with multiple subsystems?

2) In terms of location lookup feature on the Climate page,

I tried to locate a sample address "101 College Parkway, Arnold, MD 21012"
and SAM reported Lat: 39.0; Long: -76.5, Elev: 22 m whereas PVSYST V5.3 reported
latitude - 39.2, longitude - 76.7, elevation - 47 m. Can I consider SAM values here or are there any other factors I should took into consideration?

1) If your goal is to calculate the annual output of the system, you may want to consider modeling each subsystem as a separate case in SAM (using either the PVWatts or PV Component model option), and then manually adding up the annual output of each case to determine the system's total output.

2) SAM uses Google's Geocoding API to determine the latitude and longitude from the street address. I don't know how PVSyst converts the address to a latitude and longitude, but the results from SAM and PVSyst seem close enough. The difference in elevation should not have a significant impact on results.

When you use Location Lookup in SAM, please be aware that the "TDY" option is appropriate for concentrating solar power applications (CSP). For PV systems, you should use the specific year weather files.

Best regards,
Paul.

Hi Paul,

Thanks for answering my questions. The location lookup feature in SAM is really helpful. I believe for this feature, SAM simulates the PV system using a file from NREL's database of satellite derived solar resource data. I'm assuming this is same as the data Solar Prospector tool generates. Can you confirm this?

Yes. SAM's Location Lookup feature downloads data from the NREL Solar Prospector, maps.nrel.gov/prospector .

Please note that the "TDY" file option is for a "Typical DNI Year" file, which is appropriate for concentrating solar power (CSP) and concentrating PV (CPV) systems, but not for flat-plate PV systems.

Also note that Solar Prospector data is only available for the continental United States.

Best regards,
Paul.