This article describes the need for solar resource assessment and typical methods.
The name might sound unfamiliar to some, but if one plans on investing in a solar plant either for a rooftop or for a farm, this is one of the pre-requisites to be considered. It is as important as assessing how much fuel is in one's car before travelling out. It is also important at the national or regional level, for governments and system planners to understand where the best resources are located, which areas to prioritize for development, and the need for related grid investments.
A simple definition of resource assessment would be, the systematic collection of site-specific meteorological data for the purpose of accurately estimating annual energy production.
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. over a multi-year period to get the best estimate of average resource potential. It is also important to under the temporal characteristics of the resource, such as the monthly and hourly fluctuations, and the sub-hourly variations within one or more "spot" years. This data is particularly important in informing grid integration studies, to understand how the solar power will be absorbed into the national grid and what additional balancing requirements may be needed.
How is Resource Assessment Done?
There are various tools / models to assess solar resource, for simplicity they can be classified into into 2 broad categories:
- Satellite derived data
- Ground-based measurements
Satellite Derived Data
There are multiple commercial providers of solar radiation data, all of whom use satellite derived data fed into proprietary models to generate time series for multiple variables, with increasingly high degrees of uncertainty. An authoritiative tool for obtaining such data is the Global Solar Atlas, which is a free, web-based tool provided by the World Bank with funding from the Energy Sector Management Assistance Program (ESMAP). The data served by Global Solar Atlas is provided by the firm Solargis.
Installation of ground-based instruments such as pyranometers or pyrheiometers is used to obtain high quality solar radiation and other meteorological data with a much lower uncertainty than satelite derived data. Ground-based solar (and wind) measurement campaigns have been carried out by governments and instituions such as the World Bank in many countries, and more limited exercises consisting of a single installation are common among commercial developers as part of their site investigation and due diligence process. While without any doubt the data from such ground mounted instruments can be highly accurate, the high cost (up to US$100,000 per installation), security, and O&M and cleaning requirements can make ground-based measurement campaigns/projects difficult to implement. Increasingly satellite derived data may be sufficient for development of a small or medium-sized solar power project, especially when there is prior experience in the country in question or nearby measurement (or production) data with which to validate the satellite derived estimates.
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.
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, they are rated after resource assessment.