Clear Sky Model and Measured Irradiance Data

Tasks

The goal of this activity is to use the BIRD Clear Sky Model to predict the solar irradiance at a specific time at a specific locale and further compare those predictions to actual irradiance measurements.

So there are two main tasks in this activity:

1. Download and learn how to use the Bird Clear Sky Model (predicted irradiance)
2. Download and plot the SURFRAD Measured Data (measured irradiance)

These two tasks will be performed for a specific date - July 31, 2007 (in this assignment) for the specific locale - SURFRAD Meteorological Station at Rock Springs

In the end, you will need to assess how the modeled (predicted) and measured data fit and provide some discussion on it.

Part 1. Bird Clear Sky Model - Instructions

The Bird model is an old but advanced clear sky model used by solar professionals to estimate clear sky conditions at a locale. The clear sky model for ahorizontal surfaceis the basis for almost all modern comparisons of "ideal skies" to actual skies (with clouds and dynamic light interference effects). You will see what the global, beam horizontal, and diffuse components of solar light should look like when you set the meteorological parameters affecting the sky clearness. The model could also provide DNI (Direct Normal Irradiance), which would show irradiation values higher than the global horizontal values for the mornings and evenings.

• Go tohttp://rredc.nrel.gov/solar/models/clearsky/(link is external)to download the Bird Clear Sky Model. It is a free download, and the tool comes in the form of an Excel spreadsheet. I suggest that you rename the original file for any case study you perform further.
• Study the instructions on the spreadsheet to learn how to use the tool and make a note of the input and output parameters.
• Modify the inputs for the State College, PA, locale. The following parameters can be obtained from the SURFRAD site (see part 2):
  • Latitude 40.72
  • Longitude -77.93
  • Time zone UTC -5
  • Pressure (mbar) - average for the day
  • Aerosol optical depth (AOD)

Note: access the AOD data through thisgraphical page(link is external). Take the daily average as input for the Bird model. In this case, 415nm AOD can be used as a proxy for 380nm AOD input and 500nm AOD data can be used as is.

• You can leave other inputs at their default values
• Expand the spreadsheet (as explained in the instructions) to generate output for the entire year, then extract the data for July 31st (day # 212) and copy values to a separate file.
• Plot global horizontal irradiance (GHI), direct horizontal irradiance (DHI), and diffuse horizontal component (DIF) versus time for the day

The model plot will look similar to the one below. This is what you need to produce.