Resource Assessment – The First Step


Resource Assessment – The name might sound unfamiliar to some, but if one plans on investing in a Solar plant for a farm or roof this is one of the pre-requisites to be considered. It is as good as ensuring that there is enough fuel in ones car to estimate how far it will go before setting out. Well, not sure if this was a good example but hope the below explanation makes it clear.

So what exactly is Solar Resource Assessment? A simple definition would be, the systematic collection of site-specific meteorological data for the purpose of accurately estimating a prospective solar farm’s annual energy production (Source: Renewable NRG Systems).

Now let me explain – Well, like the weather, solar radiation varies with location and time so it is important to measure factors such as solar radiation, wind speed, air temperature, etc. to ensure that your site receives enough solar radiation. This in turn would help you choose the most optimum configuration of solar plant for your site resulting in maximum electricity production.

How is Resource Assessment done?
While there are various tools / models to assess solar resource, for simplicity I have classified it broadly into 2 categories:

• Ground mounted measuring instruments
• Satellite derived data

In the first method, i.e., Ground mounted measurements, instruments such as pyranometers or pyrheiometers are used for measurement of solar irradiation and other meteorological data. In India the Ministry of New and Renewable Energy has setup 115 automatic solar and meteorological measuring stations known as SRRA stations all over the country in Phase I and II of JNNSM. While without any doubt the data from such ground mounted instruments can be highly accurate, there are some limitations such as limited time of measurements, unknown accuracy, regular maintenance of the instruments, high cost of operation, etc.

The second method, i.e., Satellite derived data is what Helical Power uses in Resource assessment. Helical Power uses data from SolarGIS which uses statistically aggregated solar and temperature data stored in the database with a time step of 15/30 minutes. Solar radiation is calculated from IODC satellite data covering a period from 1999 to 2011. Derived solar parameters are calculated for any fixed-mounted or sun-tracking PV. Air temperature is derived from atmospheric models from ECMWF and NCEP. Solar GIS database has been independently identified as the most reliable irradiation database on multiple occasions.

To summarize, both the above methods have their pros and cons. However the importance of either cannot be discounted as both are complementary to each other. While the Satellite derived data method provides global coverage with a reasonable quality, the ground mounted systems provide high-quality ground measurements for local conditions. (Source: SolarGIS)

While resource assessment is just the initial step, there are a list of other factors such as direction of solar PV panels, angle of mounting, losses which occur due to shading, wiring, soling, DC/AC conversion, etc. which play a key role in optimizing electricity yield of a solar plant. However as these are mainly system specific factors I would rate them after resource assessment.

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