Part 3—Compare Two Locations and Download Data

Note: This chapter was retired in July 2015 as the tools and data are no longer available. The pages are available here for reference.

Step 1-Compare a Southern (Lower Latitude) Location to a Northern One

We look at the power data from Abilene, TX and Petersham, MA (Harvard Forest).

Power output numbers alone is not a basis for comparison of two sites because each site has different equipment, such as the number of solar panels, and a different maximum capacity for generating power. If you could find two identical maximum KW generating sites, you might be able to compare just the power numbers. What may be done is to compare the actual generated power to the potential generated power modeled by NREL's PVWatts v 2.0 model. Soltrex has published monthly and annual power from this model for each of its systems. Look at Soltrex's "Explore Systems" menu or from the graphing page, click on the "System Details".

One solar power monitoring program that has data going back almost 10 years is from some schools in Texas, Louisiana, and Arkansas in a project called Watts on Schools. The Abilene, Texas planetarium has been part of this project. All of the information here is available in the "system details" link on each Soltrex system. Below, we see that the Abilene facility has a maximum power capacity of 5 kWac and the Harvard one has a 12 kWac power capacity.
In the table below are the actual solar power data and the modeled potential power monthly and annually at the two sites. The modeled power numbers are not available as a download. Focus on the ratios annually, or monthly.

Abilene TX record of solar data
HF power record and modeled
Each system is performing near its potential indicated by most of the ratios being near 1.0. The only handicaps are weather phenomena which can cover its panels (such as snow or dust) or break components.

Step 2-Download Soltrex Data, Save it in Excel

  1. On the Soltrex site, you would chose your location, specify a time range and time interval, and choose a parameter (such as Power) to graph. Click on "Refresh Graph" at the bottom of the orange graphing controls box. The download icon and link appears below each graph.
  2. Click on the download link. A file called "soltrexdata.xls" downloads to your Desktop. You can rename this to be able to download several parameters or sites or time ranges.
  3. You will download the Abilene Texas data - daily interval for all of 2008 and rename the dafault file that appears on your computer's Desktop as "soltrexdata.xls" to "TX-2008_daily.xls".

    Download icon

    Note At the Soltrex website, you are limited to 1000 Excel data points in your download file. This is 2.7 years of daily time interval data, 42 days of hourly data or 10 days of 15 minute interval data.

  4. Download from Soltrex the same time range for the Petersham, MA. and name if HF-2008.xls
  5. If you had trouble downloading the data or renaming it, here is the Abilene data Excel file (Excel 119kB Apr13 11) and here is the Harvard Forest data Excel file ( 116kB Apr13 11) to continue with the graph. Right-click (PC) or control-click (Mac) the links to download the files.

  6. Copy the column of power data from the Harvard Forest data to the Abilene data and graph in Excel.
    1. Duplicate the file "TX-2008_daily.xls" and rename it to "Compare-TX-MA.xls"
    2. Open Excel and choose the menu File>Open and choose "Compare-TX-MA.xls". Keep columns A and B, but clear the columns with data from C on. Provide a new header in column C labeled "HF Power". Leave the file open.
    3. Open HF-2008.xls (Harvard Forest), select the column of Power data and copy it. (Edit>Copy)
    4. Copy HF power data
    5. Click on the column C header cell in the "Compare-TX-MA.xls" file. Paste (Edit>Paste) the HF Power column into column C .
    6. select 2 cols - to graph

Step 3-Make a Graph in Excel

  1. Select both columns of data, including headers, that you wish to graph. The two columns should be highlighted.
  2. To graph data, choose the Excel menu item Insert>Chart,
  3. Choose a basic file type Line
  4. choose line type Excel 2008
  5. Choose a specific line type with markers
  6. excel 2008 specific line type
  7. The first graph appears, small and in need of labeling usually.
  8. You can pull on the corners of the chart to enlarge it. Clicking on the frame away from the corner will let you move it around.

    Excel2008 first view graph

    Add the Formatting Palette to the screen by clicking on View>Formatting Palette

    Excel2008 formatting palette

    Use the Chart Options section of the Formatting Palette to add titles, or make other adjustments to your chart.

    Excel 2008 Formatting Palette
  9. Here is the resulting Excel graph. What can you say about this graph?
  10. Analyze the HF and Abilene on same axes
Here are some things you will notice about the two locations:
  • The vertical height of the Harvard Forest power is about twice as much as the Texas installation because there is a bigger system at the Harvard Forest. Harvard Forest has a potential for 12 Kw, But Abilene's maximum is 5 Kw.
  • The general shape of the two lines is similar in its seasonal output. This has to do with daylength and the location's geography (valley settings, plateaus, etc) and therefore probablity of fully sunny days.
  • There are great daily variations in power in both locations, likely due to weather differences - cloudy days, windy days that break components, snowy days that cover the panels.

  • Step 4- How to Import File Type csv into Excel

    You could also use the Solar School House or Heliotronics Sunviewer websites and download data in comma-separated variable (csv) format. At the Solar Schoolhouse, you can download data for more than one school simultaneously. The data is downloaded as "SSHDownload.csv" to your Desktop. You would rename it with the names of schools. Similarly, if you download data from the Heliotronics Sunviewer website, the default name is "sunlogger.csv", which you could rename with the system location identification. If you have multiple schools data in one file, you would still have to consider the solar panel specifications of the two (or more) installations and realize that the maximum power potential may be different for each school.

    The data download at the Solar Schoolhouse is "raw data" that gives readouts every 15 minutes. This is difficult to aggregate into daily or monthly data. The temperatures must be averaged, and the power summed. There are negative values in the power data that should be ignored. The online tools for graphing are the best choice here for seeing trends over time.

    You can import a csv file into Excel by opening the Excel application and going to File>Import.
    1. Choose File>Import in the Excel menu, and in the next window, choose "csv" type.
    2. file-import Excel 2008
      Choose import data type Excel 2008
    3. Next, choose the SSHDownload.csv or your renamed file. Here my file is renamed to barnstableHS-Arpil2011.csv. Select it and click on Get Data,
    4. load a csv
    5. Excel will now step though a 3-step wizard as indicated below.
    6. Excel import file wiz 1
      Excel 2008 import wiz 2
      Excel 2008 imprt wiz 3
    7. You can choose to have your chart appear within the same data sheet, or in a new sheet.
    8. Excel 2008 csv import last step
    9. The loaded file will be data in watts every 15 minutes.

    10. Excel loaded from csv, 2 schools
    Here are two schools on Cape Cod in Massachusetts with the same potential for solar power output: 2.04 kW. One is Mattacheese MIddle School in Yarmouth and the other is Morse Pond School in Falmouth. Here is theSolar Schoolhouse online graph of their data for two weeks in January, 2007. What can you say about it? Compare it to a table of the data online.
    graph compare - 2 similar solar installations, nearby schools
    The lines fall on top of each other, so the weather and power output must have been pretty similar at each location. The two schools are about 30 miles apart and remember, they have the same solar panel set-up. Here is the online table that Solar Schoolhouse generates. It is easier to compare numbers on the table than on the graph, but the dates begin with the end date and go backward in time. Does the peak power agree on January 10th with the graph?
    a table to show power over 2 weeks - aggregates days
    Here is the online graph of data for three years at the Morse Pond School for temperature, irradiance (sunshine) and Photovoltaic power output. The X-axis labels the months for each year, but it is not spaced well. What can you say about these three graphs?
    Morse Pond School - temp-PV-irradiance
    1. The shapes are similar. When one is high, so are the others.
    2. The irradiance and PV output are much more variable than the temperature
    3. More energy is able to be generated in summer than winter.
    Now let's think in Part 4 about what these numbers can mean. How much power is this? Is it enough for all the lights in the school? Why would we want to use solar panels instead of the electricity from an oil or coal burning power plant or even a nuclear power plant?

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