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Photovoltaic Solar Power Basics
Solar energy is captured via photovoltaic (PV) cells made of semiconductor materials and directly converted into direct current (DC) electricity. Photovoltaic literally means: capable of producing a voltage when exposed to radiant energy, especially light. The basic unit of PV technology that captures radiant photon energy is the flat-plate solar cell made of a glass cover to protect from weathering, an antireflection coating to keep 95% of photons from being reflected away from the semiconductor surface and an electrical system that takes emitted electrons from a negative semiconductor (n-type) material through a contact into a current and back through a contact to the positive semiconductor (p-type). 36 of these cells typically create one module and modules are placed into an array based on a desired capacity. Each PV cell provides 1-2 watts of power.
  3 factors define PV systems: DC nameplate ratings of the array, the type of array positioning, and the path of electical transmission.
PV Data
For current simulation models, the National Renewable Energy Laboratory provides data from 239 sites across the U.S. for solar radiation and meteorological conditions in its National Solar Radiation Database (NSRDB). In the NSRDB, hourly data from 30 years (1961-1990) is collected for direct normal radiation, diffuse horizontal radiation, dry bulb temperature, dew point temperature, wind speed, snow depth (albedo) and many other elements. Radiation is collected during all 60 mins of an hour and other values are measured every hour. To understand how incident global radiation or insolation is calculated refer to NREL User Guides found in the Resources for Developers page or in the report linked to the main page. This data is compiled and concatenated into one statistically characteristic year in the Typical Meteorological Year 2 (TMY2) database. A new database has been created (TMY3) for data from years 1991-2005 collected at 1020 sites.
Limitations in Solar Data
Solar data is currently inadequate because it inherently lacks accuracy and is very hard to access in its the way it is currently presented. Inaccuracy of the data comes from the variability of weather conditions such as temperature fluctuations, cloudiness, and storms on a site and thus it is difficult to use current data to predict the future. NREL clearly states in its database "The TMY should not be used to predict weather for a particular period of time" and instead can only be used to "typify conditions at a specific location over a longer period of time, such as 30 years."
The second problem with the data set is that it is presented in an inefficient manner for analysis. Both PVWatts and the Solar Advisor Model software models are currently used to check solar feasibility in a specific location for a specific application. They offer great variability through a sundry of inputs, but they lack coding to extract the data for multiple sites and multiple types of PV systems at once. This results in time-consuming data collection processes and multiple file creations for one type of data. For example, PVWatts must be cycled through by clicking each location for each type of array positioning and saving it from a text to an Excel document. |
