Fridge is a web service that on the HOMER website that serves up an XML file containing monthly solar radiation data for a specified latitude and longitude.
HOMER hits that site when the user clicks the "Get Data Via Internet" button in the Solar Resources window. Fridge decides which data set to refer to, and HOMER just accepts whatever Fridge gives it.
At the moment, Fridge has access to two data sets, the Climatological Solar Radiation (CSR) data set and the NASA Surface meteorology and Solar Energy (SSE) data set. The National Renewable Energy Laboratory for the U.S. Department of Energy developed the CSR model; the CSR data set contains monthly solar data covering approximately 25% of the earth's land surface at a resolution of 40km. The NASA SSE data set has a coarser spatial resolution of 1° or 2.5° depending on who you ask, but it covers the entire earth.
Fridge stores the CSR data set itself in an SQL database. The NASA SSE data set sits on a NASA website but they have written an XML interface that Fridge uses to get data from it. Fridge supplies CSR data for any location covered by that data set, and SSE data everywhere else. We prefer the CSR data set because it is NREL's, it is a higher resolution, and because the NASA SSE dataset requires a registered username.
To clarify that last point, when we use the XML interface to the NASA SSE data set, we have to supply a username, which is an email address, and that username must be registered with the NASA SSE website. So depending on what location you request data for, HOMER might ask you to specify a username for NASA on their SSE website.
Please use email@example.com as your NASA SSE username in HOMER2. NASA recently re-configured their website and eliminated the need to validate specific email addresses. To enter this in the HOMER software, open HOMER2, select File >> Preferences, and enter firstname.lastname@example.org into the field for NASA SSE username.
Here is what Fridge does when it gets hit with a lat/long:
1. It calculates (in a few lines of ASP code) the number of the CSR grid cell that covers the specifies lat/long.
2. It looks in the database to see if the CSR table contains a record with that grid cell number.
3. If the CSR data is available, Fridge spits it out in XML format. If not, it asks the NASA SSE site for the data for the lat/long in question, then it reformats it in XML and spits it out.