Measuring the sun radiation

In order to assure well-founded decisions in designing profitable solar power plants, the sun irradiation should be measured at the planned site. It is also recommended measuring the produced electrical energy to keep the energy yield high. Photovoltaic or CSP – each applications requires specific measurements to get relevant irradiation information. The sun radiation on the earth surface combines Direct Normal Irradiation (DNI) and Diffuse Horizontal Irradiation (DHI). Both are linked in the formula for Global Horizontal Irradiation (GHI):

GHI = DHI + DNI · cos (θ) 
(where θ is the solar zenith angle)

Normally, on a sunny day the insolation is 100% GHI with 20% DHI and 80% DNI  · cos (θ).

The following table indicates the different types of irradiation as well as which measurement instruments are necessary to measure the irradiation.

Type of radiation	Desciption	Measurement instrument
GHI Global Horizontal Irradiation	The total amount of radiation received from above by a horizontal surface. This value includes both Direct Normal Irradiation (DNI) and Diffuse Horizontal Irradiation (DHI). Application: Fixed PV installations Comparisons with solar data bases to perform MCP (Measure Correlate Predict) evaluations	Pyranometer (horizontal) Reference cell
GTI Global Tilted Irradiation	The total amount of direct and diffuse radiation received from above by a tilted surface. GTI is an approximate value for the energy yield calculation of fixed installed tilted PV panels. Applications: Fixed PV installations	Pyranometer tilted in the same angle as the solar module Reference cell
DNI Direct Normal Irradiation	Direct Normal Irradiation is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line form the direction of the sun at its current position in the sky. Applications: Concentrated Solar Power (CSP) Concentrated PV (CPV) Fixed PV installations	Pyrheliometer installed on a sun tracker Rotating Shadowband Irradiometer
DHI Diffuse Horizontal Irradiation	Diffuse Horizontal Irradiation is the amount of radiation received per unit area by a surface (no subject to any shade or shadow) that does not arrive on a direct path from the sun, but has been scattered by molecules and particles in the atmosphere and comes equally from all directions. Applications: Fixed PV installations Redundancy calculations of GHI → GHI = DHI + DNI · cos(θ)	Pyranometer with shadow ball or shadow ring, installed in a sun tracker Rotating Shadowband Irradiometer

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Parameter that influence the solar energy production

Insolation
The higher the insolation of the sun at the location of the planned solar power plant, the higher can be the energy yield. Depending on the type of the solar power plant, different radiation parameters should be measured, e.g., DNI is important for a planned CSP power plant.

Wind speed and wind direction
In order to design and build robust module carriers, the local wind conditions should be measured. Additionally the cooling effect of the wind on the modules can be estimated.

Air temperature
The air temperature has a significant influence on the efficiency of solar modules. The performance of solar modules is temperature-dependent. Higher cell temperatures lead to lower performance and hence to a lower coefficient of efficiency. The coefficient of efficiency indicates how much of the sun light can be converted into usable electrical energy.

Precipitation and soiling (sand storms)
Data about amount and frequency of precipitation as well as soiling can help to explain low energy yields at high sun radiation.

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Comparison: Satellite-based data vs. ground-measured data*

Most of the available radiation data is based on satellite data, which is calculated from sun radiation (satellite) as well as temperature and wind speed (weather stations). However, there is a lot of uncertainty in these calculations. Thus there are significant deviations between satellite-based data and ground-measured data, particularly when the sky is cloudy. Another influencing fact is the area, which is considered for the data; satellite-based data refers to large areas of approx. 1km², ground-measured data are exact.

However, the deviation is smaller when GHI is considered instead of DNI. If data for GHI and temperature from both satellite-based data and ground-measured data is correlated, good regression values can be reached: R²=0.92 and R²=0.97. If DNI and wind speed is considered, regression values show huge deviation between both measurements: R²=0.78 and R²=0.5.

We recommend measuring the sun radiation with ground stations at the location of the planned solar power plant. Only with measurement stations on the ground you get accurate measurement data, which can be used to calculate the annual energy yield at the site.

*The data is based on the case study „Validation of PV Performance Models using Satellite-based Irradiance Measurements“ published by Clean Power Research.

Additionally, the quality of the installed measurement instruments has to be carefully considered to get accurate measurement results, e.g., classification of pyranometers. Benefit from our long-term experience and our know-how from many projects worldwide to get the best measurement system for your solar project.

Our solar measurement systems meet the latest international quality standards. We design your measurement system including suitable radiation sensors, communication and power supply system considering your planned solar installation as well as local weather conditions.

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Solar Database PVGIS and Global Solar Atlas of the World Bank Group

As part of the SOLAREC action to implement renewable energy in the EU as a long-term energy supply, the European Union introduced the PVGIS (Photovoltaic Geographical Information System). The system provides a map-based inventory of solar energy resource and assessment of the electricity generation from PV power plants.

PVGIS

The World Bank Group provides the Global Solar Atlas. The Global Solar Atlas is an online platform developed as part of the Energy Sector Management Assistance Program (ESMAP). It provides solar measurement data for a number of countries for energy yield analysis.

Global Solar Atlas

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Specification of solar radiation sensors according to ISO 9060

Pyranometers are classified according to the ISO 9060 standard: "Solar energy - Specification and classification of instruments for measuring hemispherical solar and direct solar radiation". The standard is officially approved by the World Meteorological Organization (WMO). The standard specifies three classes:

• Secondary Standard: Scientific quality and highest accuracy
  Applications: Meteorology (BSRN Network); Testing in PV, CPV and CSP

• First Class: Good quality
  Applications: Measurements for hydrology networks and greenhouse climate control

• Second Class: Medium quality
  Applications: Economic solution for routine measurements in weather stations and field testing

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Applications and required measurement instruments

Depending on the planned solar power plant, we recommend the following measurements:

In order to perform a complete site assessment, we recommend measuring additional components:

• Ambient temperature
• Wind speed and wind direction
• Air humidity
• Air pressure
• Precipitation amount and frequency
• Soiling

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Standards and Guidelines for Solar Energy Assessment

NREL "Best Practice Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications" (Feb 2015 / 63112)
The NREL handbook is a comprehensive report, which summarizes important information for all steps of a solar energy project - reaching from required measurements and the design of measurement stations to forecasting the potential solar radiation. Additionally, NREL informs about measurement instruments and its application as well as sources for solar measurement data.
Download: NREL Best Practice Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications (Feb 2015 / 63112, English, PDF)

ISO 9060 Solar energy – Specification and classification of instruments for measuring hemispherical solar and direct solar radiation
In the ISO 9060 standard pyranometers are classified in three classes: Secondary Standard for scientific measurement quality, First Class for good measurement quality and Second Class for medium measurement quality. The ISO 9060 is accepted by the WMO (World Meteorological Organisation). See also Pyranometer.

IEC 61724-1:2017 Photovoltaic system performance – Guideline for measurement, data exchange and analysis
This standard decribes measurement system components and processes. It focuses on measurement uncertainties and defines accuracy classes. Additionally, the standard defines cleaning and calibration intervals for pyranometers.

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