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Solar Radiation Measurements Accuracy Matters

High quality solar radiation data is critical throughout the life-cycle of a solar power plant. Once a plant has been set-up and is operational, high quality solar radiation data is required for maximizing operational efficiency, scheduling maintenance, monitoring performance and for output forecasting. Errors in the solar radiation measurements can significantly impact upon the return on investment for solar projects.

How is Solar Radiation Measured?

All solar power plants require high quality solar radiation measurement instruments in order to accurately measure the amount of solar energy incident at that location. A combination of Pyranometers and Pyrgeometers for measuring GHI, DNI and DHI constitutes a Solar Monitoring Station. High quality instruments provide solar radiation data that is accurate enough to allow comparison with data from other sites as well as with data from other sources such as satellite data.

High quality ground-based measurements of solar radiation are made by radiometers that have a flat spectral response over a wide spectral bandwidth. These radiometers use a thermopile detector that absorbs the incoming radiation and converts the increase in temperature into a small electrical signal. Instruments that employ silicon based detectors instead of a thermopile, are lower cost but highly compromised in accuracy. The case study reported below also illustrates this.

Along with solar radiation parameters, meteorological parameters such as wind speed, wind direction, ambient temperature, relative humidity and other environmental factors such as cloud cover and dust also impact the solar energy at any given site. Consequently, solar plants also require a high quality weather monitoring stations. Solar Weather Monitoring Stations are normally connected to a data logger that records and stores the readings. Data can be forwarded in real-time via telemetry or periodically downloaded for processing and analysis.

A large site may have two or more Solar Weather Monitoring Stations. For one client in Charanka Solar Park, Gujarat, we have installed four high qualities Solar Weather Monitoring Stations at a single plant to accurately capture data that allows for accurate now casting and forecasting of energy output.

World Standards for Solar Radiation Measurement Instruments:

With the wide range of solar radiation measuring instruments available in India, we often get questions from clients enquiring about the standards and price-effectiveness of available technologies. Our recommendation is to use Pyranometers and Pyrheliometers that meet performance specifications calibration standards laid down by the World Meteorological Organisation (WMO) and the International Standards Organisation (ISO). Instruments that comply with WMO and ISO classifications provide accurate measurements of the solar radiation under all weather conditions.

Through partnership with Kipp and Zonen B.V, The Netherlands, other leading OEMs, BKC Weathersys Pvt. Ltd. Provides turn-key solutions for Solar Weather Monitoring Stations and also provides services for data analysis during plant operation. We also correlate ground measurements with satellite data and advanced solar models to ensure.

Solar Weather Monitoring Stations that employ thermopile detector based radiation sensors from Kipp and Zonen, The Netherlands.We installed This type of Stations like, in Jalgaon (Maharashtra), Noida (Uttar Pradesh), Surender Nagar (Gujarat) and many other places in India. The Solar Monitoring Station included a silicon based solar radiation sensor for measuring the Global Horizontal Radiation (GHI). To gauge the operational efficiency of the plant, they wanted to correlate ground-measurements with satellite data. We employed the automated solar forecasting workstation developed at BKC WeatherSys Pvt. Ltd. to carry out this analysis.

After analysis by our team, we found that the correlation factor between the forecasted GHI value and the actual GHI value as measured by the clients silicon based sensor is very poor (0.51). Their current solar monitoring station cannot be relied upon for either monitoring plant efficiency or making output forecasts as ground measured radiation data is in error of over 50%.

In contrast, ground measurements and satellite based forecasts at two other sites, one in Gujarat and the other in Uttar Pradesh, have good correlation factors. These Solar Weather Monitoring stations included various sensors Including a Kipp and Zonen (CMP11) Pyranometer. The CMP11 (traceable to WRC, Davos, Switzerland) is an ISO- Secondary Standard Pyranometer that allows for accurate measurement of GHI.

These results are not a surprise. Silicon based radiation sensors are known to have limitations arising from temperature fluctuation, spectral range and degradation and cannot be relied upon for accurate measurement of solar radiation under all weather conditions.

Impact of Inaccurate Data on Return on Investment:

The use of low quality radiation measurement instruments for reference purposes and for providing data for informing key decisions like scheduling maintenance and cleaning of panels, monitoring performance and even predicting plant output is fraught with error. As solar radiation measurement equipment present less than 1-2 percent of the total cost of a solar power project, investing in high quality instruments makes sound Financial sense. Poor quality data can compound errors in plant design, performance and output forecasting, negatively impacting return on investment. In conclusion, investment in reliable solar radiation instruments pays off during plant design as well as the entire life cycle of the project.

About Author Dr. Jaya Singh :

Dr. Jaya Singh drives strategy and R&D at BKC WeatherSys Pvt. Ltd. with a focus on technologies for disaster management, agriculture, and renewable energy. She has a Ph.D. in Chemistry from Brown University and a B. Sc. in Chemistry from St. Stephen's College at Delhi University. Dr. Singh has over 18 years of cross-functional experience in academia and across a range of industries including biotech, environmental sciences, and public health. Prior to her move to India, Dr. Jaya Singh was faculty at Harvard Medical School for over eight years and was also involved in shaping policies for academic advancement. Dr. Singh has published widely in areas of chemistry, life sciences, environmental sciences, and public health.

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Article Added on Thursday, July 10, 2014
Other Articles by Dr. Jaya Singh

Monitoring Solar Radiation to Improve Plant Performance
Performance Evaluation without Accurate Radiation Measurement is Meaningless: A critical component for any solar power project is solar radiation monitoring and measurement. Although this equipment presents a negligible fraction of total project cost (less than 0.1% of total project cost), it is the component most overlooked and over shadowed over other more capital intensive components. However, in this more mature phase of the Indian solar power industry, we are starting to see a shift in...

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