How to Measure Radiation for Photovoltaic or Thermal Solar Energy

Solar radiation, also known as the solar resource or, more typically, sunlight, is a catchall term for the electromagnetic radiation that the sun emits. It reaches the edge of our atmosphere with an irradiance of around 1400 Watts per W/m². It is reflected partially, scattered and absorbed in the atmosphere by its constituent gasses, water vapor and clouds.

Some of the energy absorbed is re-radiated in the far infrared. Radiation that reaches the Earth’s surface from the sun and sky is separated into short-wave radiation, at several bands in the wavelength range from 300 to 4000 nm (4 µm), and long-wave radiation from 4.5 to in excess of 40 µm (far infrared).

The amount of radiation that reaches the ground is primarily influenced by clouds, particles, pollution and aircraft contrails in the atmosphere.

Why Should Solar Radiation be Measured?

Good quality, dependable solar radiation data is becoming ever more crucial in the field of renewable energy in relation to both thermal and photovoltaic (PV) systems.

It is a crucial input when making decisions on activities such as research and development, production quality control, resolving optimum locations, evaluating the efficiency of installed systems and anticipating the system output under a range of different sky conditions. Errors of just a few percent can have a significant impact on the return on investment.

How to Measure Radiation for Photovoltaic or Thermal Solar Energy

Image Credit: OTT HydroMet

How Can It be Measured?

Photovoltaic materials have the majority of their sensitivity in the near-infared and visible parts of the spectrum, from around 400 to 1100 nm, with a peak immediately following the visible range. There is a lesser response to ultraviolet, and none to long-wave radiation.

For the energy transfer process, commercial-scale thermal energy systems generally utilize reflective solar collectors that concentrate both long-wave and short-wave energy onto the medium that is to be heated. Measurements of solar radiation are typically made utilizing thermopile type radiometers with a flat spectral response.

How to Measure Radiation for Photovoltaic or Thermal Solar Energy

Image Credit: OTT HydroMet

The kinds of instruments, performance specifications and calibration techniques are determined by International Standards Organisation (ISO) and the World Meteorological Organisation (WMO). They offer precise measurements of the total solar energy available under all sky conditions.

The data can be contrasted against measurements from meteorological networks and satellites, spanning a number of locations and for various types of solar energy systems. The ‘global’ short-wave radiation is determined by the long-wave radiation by a Kipp & Zonen CGR Pyrgeometer and a horizontally mounted Kipp & Zonen CMP Pyranometer.

Solar spectrum at sea level, photovoltaic response, visible, pyranometer response, pyrgeometer response, UVA, UVB, near-infrared and far infrared

Solar spectrum at sea level, photovoltaic response, visible, pyranometer response, pyrgeometer response, UVA, UVB, near-infrared and far infrared. Image Credit: OTT HydroMet

What Instruments are Needed?

PV panels possess a wide field of view and are placed in locations that receive the maximum exposure to solar radiation. Therefore, in addition to the horizontally mounted pyranometer, it is generally recommended to have an additional unit fixed to the panel or array to measure the energy available from the hemisphere that the panel can observe.

This enables monitoring of system efficiency as well as helping to schedule maintenance, such as cleaning. Where relative measurements of trends are appropriate, a lower specification and lower cost sensor can be employed.

The SP Lite2 Silicon Pyranometer utilizes a photo-diode detector with a spectral response akin to those found in the majority of solar cells and is widely used for field testing and monitoring applications.

To target focus the sun’s radiation, some PV systems utilize solar concentrators with lenses, and thermal solar energy systems typically have some kind of reflector incorporated. Both have a comparatively small angle of view, and it is crucial to know the amount of radiation available directly from the sun.

This ‘direct’ short-wave radiation is determined using the CHP 1 Pyrheliometer, which has a 5 ° view and is fixed to the SOLYS 2 Automatic Sun Tracker.

How to Measure Radiation for Photovoltaic or Thermal Solar Energy

Image Credit: OTT HydroMet

What is Needed When Researching Solar Prospecting?

As described previously, measurements utilizing WMO/ISO type pyranometers and pyrgeometers can be directly compared across various sites located anywhere in the world, and with data from meteorological networks, satellite information, as well as with solar radiation prediction algorithms.

These parameters are independent and can be called upon for any kind of solar energy system. Therefore it is the optimal solution for solar energy research and site prospecting. The next step is the expansion to a complete Solar Monitoring Station.

How to Measure Radiation for Photovoltaic or Thermal Solar Energy

Image Credit: OTT HydroMet

This system is made up of a sun tracker equipped with a pyranometer for global radiation, a pyrheliometer for direct radiation, and a pyranometer protected from direct sun exposure to measure the ‘diffuse’ sky radiation.

There may also be a tilted pyranometer that can determine the total radiation available to a PV system that has 2-axis tracking arrays. For thermal energy systems, a pyrgeometer is introduced.

Typically, the outputs from all the radiometers are networked to a specific data logger and stored for remote access retrieval. At www.kippzonen.com you can find the various configurations and possibilities for your complete solar energy system.

Why Kipp & Zonen?

Kipp & Zonen is one of OTT HydroMet's brands for professional environmental monitoring. It has been developing and manufacturing solar radiation measurement equipment for more than 75 years and consistently delivers its products to world-leading meteorology and climatology organizations, research institutes and energy companies around the world.

Kipp & Zonen’s worldwide reputation for quality, reliability, expertise and support means that its radiometers can help maximize the performance of a system. In addition to radiometers and sun trackers, Kipp & Zonen has an extensive range of accessories, data loggers and interfacing solutions on offer. All its products come with a 2-year warranty.

1. Project PEGASE of CNRS-PROMES in the Pyrenees of France. 2. Solar thermal collector testing site of the University of Jyväskylä Renewable Energy Research and Education Program. Photo by Jussi Maunuksela (2009). 3. and 4. Solar farms in Palma de Mallorca. Owner: AFFIRMA. Engineered and installed by ELECNOR

1. Project PEGASE of CNRS-PROMES in the Pyrenees of France. 2. Solar thermal collector testing site of the University of Jyväskylä Renewable Energy Research and Education Program. Photo by Jussi Maunuksela (2009). 3. and 4. Solar farms in Palma de Mallorca. Owner: AFFIRMA. Engineered and installed by ELECNOR. Image Credit: OTT HydroMet

Passion for Precision

Kipp & Zonen is a global leader when it comes to measuring solar radiation and atmospheric properties. The company’s passion for precision and accuracy has led to the development of an extensive range of high-quality instruments: from all weather radiometers to complete measurement systems.

Kipp & Zonen’s promise to its customers includes guaranteed performance and quality across a range of diverse markets: Meteorology, Climatology, Hydrology, Industry, Renewable Energy, Agriculture and Public Health.

System Configurations

Basic Solar Monitoring

For fixed (tilted) panels
1 tilted pyranometer for tilted global radiation
1 horizontal pyranometer for global radiation

Recommended Instruments

SP Lite2 / CMP 3 / CMP 6

Advanced Solar Monitoring

For concentrating and/or tracking systems
1 pyrheliometer with sun tracker for direct radiation
1 tilted pyranometer fitted to sun tracker
1 horizontal pyranometer for global radiation

Recommended Instruments

CMP 11, CHP 1, SOLYS 2

Complete Solar Monitoring System

Includes global, direct, diffuse and global tilted measurement
1 tilted pyranometer fitted to sun tracker
1 horizontal pyranometer for global radiation
1 pyrheliometer with sun tracker for direct radiation
1 shaded pyranometer for diffuse radiation (shading assembly on sun tracker)

Recommended Instruments

CMP 11 / CMP 21, CHP 1, SOLYS 2

Relevant IEC Standards for PV Panel Testing

IEC 61215 Design qualification and type approval, crystalline silicon
IEC 60904 (part1/10) Photovoltaic devices, measurements and requirements
IEC 61853 Module performance testing
IEC 61646 Design qualification and type approval, thin film

Relevant ISO Standards for Pyranometers

ISO 9847 Calibration of field pyranometers
ISO 9060 Specifications and classifications of instruments

Traceability

All solar radiation instruments from Kipp & Zonen are fully traceable to the World Radiometric Reference (WRR) in Davos, Switzerland, where Kipp & Zonen instruments form part of the World Standard Groups.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.

Kipp & Zonen is one of OTT HydroMet's strong brands for professional environmental monitoring. For more information on this source, please visit OTT HydroMet.

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