Bird Solar Model Source Creator

Transcription

1 Bird Solar Model Source Creator INTRODUCTION This knowledge base article describes a script that generates a FRED source that models the properties of solar light incident on a tilted or solar-tracking surface. The calculations are based on the National Renewable Energy Laboratory (NREL) Bird Simple Spectral Model. The model, created by Drs. Richard Bird and Carol Riordan, determines the irradiance incident on a tilted plane based on the sun s position at the time and location of interest and a series of environmental factors such as aerosol optical depth, precipitable water vapor, and surface pressure, among others. The script included in this knowledge base article communicates with an Excel spreadsheet where the calculations are performed. It then extracts the resulting direct, diffuse, and total spectral irradiances and uses the desired spectrum to create a source. The spreadsheet can be found on the NREL website at the following location: Note that the script will not work unless the SPCTRAL2.xls spreadsheet is downloaded and saved. USING THE SOLAR MODEL SOURCE CREATOR Some useful information and tips about the script is included below. For more information about the Bird Simple Spectral Model and its inputs, please refer to the NREL page listed in the Introduction. Before running the script, make sure that: 1. The NREL Excel spreadsheet SPCTRAL2.xls is saved in the FRED Current Directory. If it isn t, the following error will occur when the script is run: (1004) C:\...\SPCTRAL2.xls could not be found. Check the spelling of the file name, and verify that the file location is correct 2. There is a FRED file open. When the script is run a dialog will prompt for the type of surface being modeled, as shown in Figure 1. The Bird Simple Spectral Model can be used to model the solar spectral irradiance incident on one of the following surfaces: 1. Sun tracking: The angle of incidence of the sun onto the surface is always Fixed tilt: The surface is oriented at a fixed location and the angle of incidence of the sun onto the surface depends on the sun s position.

2 Figure 1. Dialog 1: Surface Type Depending on the surface type chosen, either Dialog 2 or 3 is displayed next. As seen in Figures 2 and 3, the two dialogs are almost identical, with the fixed tilt surface dialog requiring a few extra parameters relating to the surface orientation. Figure 2. Dialog 2: Parameters and properties if the surface is sun-tracking.

3 Figure 3. Dialog 3: Parameters and properties if the surface is fixed-tilt. The following are some points and clarifications about Dialogs 2 and 3: The defaults are approximate values for Tucson, AZ in the summer. The year, month, and day are used to determine the day of the year input for the spreadsheet. This is the number of days that have elapsed since January 1 st in a given year. The year is used to adjust for leap years. Latitude is +N, -S; Longitude is +E, -W Albedo (surface reflectance) is used in the diffuse irradiance calculations. The surface area is used to determine the total integrated power incident on the surface. This is set as the source power. The orientation of the fixed tilt surface is defined by two parameters: o The surface slope/tilt is the angle in degrees the collector surface is tilted. Flat/horizontal (facing straight up) is 0 degrees. Vertical is 90 degrees. o The surface azimuth direction in degrees. This is the direction of the surface tilt. North is 0, east is 90, south is 180, and west is 270.

4 The spreadsheet contains a formula for estimating the total column ozone (O3) based on the latitude, longitude, and day of year. The user is given the choice to use this estimation or specify a value. Pressing Print Results displays a summary in the Output Window of some values calculated based on the inputs. The dialog re-appears, keeping the values last entered. The following parameters are displayed: o Sun zenith and azimuth angles in degrees. o Angle of incidence of the sun on the surface. o Integrated direct, diffuse, and total irradiances in W/m 2. Total irradiance is direct + diffuse. o The total integrated power (W) incident on the surface. This is (total irradiance) x (surface area) Pressing Create Source saves all the values and displays the next dialog. Dialog 4 is used to create the source model and associated spectra, as shown in Figure 4. Figure 4. Dialog 4: Source and Spectra Parameters The source that is generated is a Detailed Source with ray positions in a grid plane of the specified size, number of rays, and shape. The rays point in a single direction, based on the angle of incidence (θ) of the sun onto the collector surface as follows: X component: 0 Y component: sin(θ)

5 Z component: cos(θ) The user may adjust the location and orientation of the model source manually after it is created. The user is given the choice of adding between one and six relevant solar spectra, all spanning 0.3 μm - 4 μm. The direct/diffuse/total spectral irradiances (W/m 2 /μm) are derived in the spreadsheet using the Bird solar model. The direct/diffuse/total irradiances (W/m 2 ) are the spectral irradiance values integrated over wavelength intervals (also calculated in the spreadsheet). The source wavelengths are defined Randomly according to spectrum using the spectrum chosen in the wavelength spectrum drop-down menu in the dialog. The chosen spectrum will be added to the Spectra folder even if it is not checked on the right-hand side as one of the spectra to be generated. The check boxes allow for additional spectra to be added if desired. THE SCRIPT The script is structured as follows: 1. Open Excel spreadsheet. 2. Display Dialog 1 prompting for surface type (sun-tracking or fixed tilt). 3. Start while loop that repeats if the Print Results button is pressed in Dialog 2 or Dialog Display Dialog 2 if the surface is sun-tracking or Dialog 3 if the surface is at a fixed tilt angle. a. Insert default values if it s the first run. Otherwise the values last entered are kept in memory. b. Gather information about the date and time, location, surface of interest, and environmental properties. c. Close Excel and terminate script if Cancel button is pressed. d. Assign values from the dialog to variables. 5. Use year, day, and month to calculate the day of the year (i.e. how many days have elapsed since the start of the year), which is the value needed for the spreadsheet. Adjust for leap years. 6. Insert values into the corresponding fields in the Excel spreadsheet. 7. Extract output values from the spreadsheet.

6 a. There are 122 entries for the wavelength and spectral irradiances (direct, diffuse, and total). The irradiances (spectral irradiance values integrated over wavelength intervals to convert from W/m 2 /μm to W/m 2 ) have 121 entries, so 0 is entered as the first value. b. Close Excel and terminate script if Cancel button was pressed. 8. Print derived values in the Output Window if Print Results was pressed. 9. End while loop. a. Go back to Step 4 if Print Results button was pressed. b. Move to Step 10 if Cancel or Create Source pressed 10. Close Excel. 11. Make an array containing names of possible spectra. This is what the drop-down menu in Dialog 4 will display. 12. Display Dialog 4: Source Parameters. a. Insert default values. b. Get user inputs on the number of rays, aperture, and spectrum of the source to be created as well as which spectra to generate and add to the Spectra tree folder. c. Terminate script if Cancel button is pressed. d. Assign values from dialog to variables. 13. Add each spectrum that was either checked or chosen as the source spectrum in Dialog 4 to the Spectra folder. 14. Create the source using input parameters from Dialog Print some derived values that may be useful in the Output Window. 16. Print name of newly created source. 17. Done!