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Solar Radiation Measurement

 Learn how you can benefit from accurate radiation data in your project - starting in the planning phase of the solar power plant and finally in the operation of the power plant. Design a profitable solar power plant by analysing radiation and meteorological data. You will benefit in the operation by reduced maintenance costs and high efficiency. We explain the difference between radiation data from ground-based measurement stations and the data based on weather satellites and weather stations. Additionally, we recommend measurement instruments related to your planned or operating solar power plant. For further details, refer also to our information brochures on radiation measurement:
Download: Information brochure on solar measurement (English, PDF)
Download: Ammonit Solar Resource Assessment Systems (English, PDF)

 

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
ammonit scheme ghi
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
ammonit scheme gti
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
ammonit scheme dni
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
ammonit scheme dhi
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.

ammonit solar correlation

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 in Europe, Africa and South-West Asia.

PVGIS for Europe
PVGIS for Africa and South-West Asia

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

WorldBank-GlobalSolarAtlas

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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


ISO 9060 PYRANOMETER SPECIFICATIONS


SECONDARY STANDARD


FIRST CLASS


SECOND CLASS

Response time: time to reach 95% response  < 15s  < 30s  < 60s
Zero-offset:
Offset-A: response to 200 W/m² net thermal radiation, ventilated
Offset-B: response to 5 K/h change in ambient temperature

 + 7 W/m²
 ± 2 W/m²
 
 + 7 W/m²
 ± 2 W/m²
 
 + 7 W/m²
 ± 2 W/m²
Non-stability: % change in responsivity per year  ± 0.8%  ± 1.5%  ± 3%
Non-linearity: % deviation from responsivity at 500 W/m² due to change in irradiance from 100 ... 1000 W/m²  ± 0.5%  ± 1%  ± 3%
Directional response (for beam irradiance): the range of errors caused by assuming that the normal incidence responsivity is valid for all directions when measuring from any direction, a beam radiation whose normal incidence irradiance is 1000 W/m²  ± 10 W/m²  ± 20 W/m²  ± 20 W/m²
Spectral selectivity: % deviation of the product of spectral absorbance and transmittance from the corresponding mean, from 0.35 ... 1.5 μm  ± 3%  ± 5%  ± 10%
Temperature response: % deviation due to change in ambient within an interval of 50K, (e.g. -10 ... +40°C typical)  2%  4%  8%
Tilt response: % deviation in responsivity relative to 0 ... 90° tilt at 1000 W/m² beam irradiance  ± 0.5%  ± 2%  ± 5%
Achievable uncertainty (95% confidence level)
Hourly totals
Daily totals


 3%
 2%


 8%
 5%

 20%
 10%

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Applications and required measurement instruments

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

  Recommended measurements System components
Small PV power plant GHI and GTI
  • Data logger
  • Steel cabinet with solar power supply and communication system
  • Pyranometer

Optional:

  • Pyranometer tilted in the same angle as solar module to measure GTI
  • 2 solar reference cells (horizontal and tilted)
Medium PV power plant GHI, DHI and calculated DNI
  • Data logger
  • Steel cabinet with solar power supply and communication system
  • Rotating Shadowband Irradiometer to measure GHI, DHI and DNI (calculated)

Optional:

  • 2 solar reference cells (horizontal and tilted)
  • Delta-T SPN1 Pyranometer to measure GHI, DHI and DNI (calculated)
  • Pyranometer tilted in the same angle as solar module to measure GTI
Large PV and CPV power plant GHI and DNI
  • Data logger
  • Steel cabinet with solar power supply and communication system
  • Pyranometer
  • Pyrheliometer

Optional:

  • 2 solar reference cells (horizontal and tilted)
  • Pyranometer tilted in the same angle as solar module to measure GTI
  • Rotating Shadowband Irradiometer to measure GHI, DHI and DNI (calculated)
  • Delta-T SPN1 Pyranometer to measure GHI, DHI and DNI (calculated)
CSP power plant GHI and DNI
  • Data logger
  • Steel cabinet with solar power supply and communication system
  • Sun tracker
  • Pyranometer
  • Pyrheliometer

Optional:

  • Pyranometer with shadow ring or shadow ball
  • Rotating Shadowband Irradiometer

 

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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How to contact us

Ammonit Measurement GmbH
Wrangelstrasse 100
10997 Berlin
Germany

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T: +49-30-6003188-0

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