A pyranometer is a type of
actinometer
Actinometers are instruments used to measure the heating power of radiation. They are used in meteorology to measure solar radiation as pyranometers, pyrheliometers and net radiometers.
An actinometer is a chemical system or physical device which ...
used for measuring
solar irradiance
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/ ...
on a planar surface and it is designed to measure the solar radiation flux density (W/m
2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm. The name pyranometer stems from the Greek words ''πῦρ'' (''pyr''), meaning "fire", and ''ἄνω'' (''ano''), meaning "above, sky".
A typical pyranometer does not require any power to operate. However, recent technical development includes use of electronics in pyranometers, which do require (low) external power.
Explanation
![Solar Spectrum and Spectral-Response](https://upload.wikimedia.org/wikipedia/commons/9/9d/Solar_Spectrum_and_Spectral-Response.png)
The solar radiation
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
that reaches earth's surface extends its wavelength approximately from 300 nm to 2800 nm.
Depending on the type of pyranometer used, irradiance measurements with different degrees of spectral sensitivity will be obtained.
To make a measurement of
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
, it is required by definition that the response to "beam" radiation varies with the
cosine of the angle of incidence. This ensures a full response when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0° angle of incidence), zero response when the sun is at the horizon (90° angle of incidence, 90° zenith angle), and 0.5 at a 60° angle of incidence. It follows that a pyranometer should have a so-called "directional response" or "cosine response" that is as close as possible to the ideal cosine characteristic.
Types
Following the definitions noted in the ISO 9060, three types of pyranometer can be recognized and grouped in two different technologies:
thermopile
A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...
technology and silicon semiconductor technology.
The light sensitivity, known as 'spectral response', depends on the type of pyranometer. The figure here above shows the spectral responses of the three types of pyranometer in relation to the solar radiation spectrum. The solar radiation spectrum represents the spectrum of sunlight that reaches the Earth's surface at sea level, at midday with A.M. (
air mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to la ...
) = 1.5.
The latitude and altitude influence this spectrum. The spectrum is influenced also by
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
and pollution.
Thermopile pyranometers
A thermopile pyranometer (also called
thermo-electric pyranometer) is a sensor based on
thermopile
A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...
s designed to measure the broad band of the solar radiation flux density from a 180° field of view angle. A thermopile pyranometer thus usually measures 300 to 2800 nm with a largely flat spectral sensitivity (see the spectral response graph) The first generation of thermopile pyranometers had the active part of the sensor equally divided in black and white sectors. Irradiation was calculated from the differential measure between the temperature of the black sectors, exposed to the sun, and the temperature of the white sectors, sectors not exposed to the sun or better said in the shades.
In all thermopile technology, irradiation is proportional to the difference between the temperature of the sun exposed area and the temperature of the shadow area.
Design
In order to attain the proper directional and spectral characteristics, a thermopile pyranometer is constructed with the following main components:
* A
thermopile
A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Such a device works on the principle of the thermoele ...
sensor with a black coating. It absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nanometer range, and has a near-perfect cosine response.
* A glass dome. It limits the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800 nm), while preserving the 180° field of view. It also shields the thermopile sensor from convection. Many, but not all, first-class and secondary standard pyranometers (see ISO 9060 classification of thermopile pyranometers) include a second glass dome as an additional "radiation shield", resulting in a better thermal equilibrium between the sensor and inner dome, compared to some single dome models by the same manufacturer. The effect of having a second dome, in these cases, is a strong reduction of instrument offsets. Class A, single dome models, with low zero-offset (+/- 1 W/m
2) are available.
In the modern thermopile pyranometers the active (hot) junctions of the thermopile are located beneath the black coating surface and are heated by the radiation absorbed from the black coating. The passive (cold) junctions of the thermopile are fully protected from solar radiation and in thermal contact with the pyranometer housing, which serves as a heat-sink. This prevents any alteration from yellowing or decay when measuring the temperature in the shade, thus impairing the measure of the solar irradiance.
The thermopile generates a small voltage in proportion to the temperature difference between the black coating surface and the instrument housing. This is of the order of 10 μV (microvolts) per W/m2, so on a sunny day the output will be around 10 mV (millivolts). Each pyranometer has a unique sensitivity, unless otherwise equipped with electronics for
signal calibration.
Usage
![Thermopile pyranometer as part of MeteoStation](https://upload.wikimedia.org/wikipedia/commons/6/6d/Thermopile_pyranometer_as_part_of_MeteoStation.jpg)
Thermopile pyranometers are frequently used in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
,
climatology
Climatology (from Greek , ''klima'', "place, zone"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of stud ...
,
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
research,
building engineering physics
The term building engineering physics was introduced in a report released in January 2010 commissioned by The Royal Academy of Engineering (RAeng). The report, entitled ''Engineering a Low Carbon Built Environment: The Discipline of Building Engi ...
,
photovoltaic system
A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and co ...
s, and monitoring of
photovoltaic power stations
A photovoltaic power station, also known as a solar park, solar farm, or solar power plant, is a large-scale grid-connected photovoltaic power system (PV system) designed for the supply of merchant power. They are different from most building- ...
.
They are typically installed horizontally in meteorological stations.
The solar energy industry, in a 2017 standard, IEC 61724-1:2017, has defined what type of pyranometers should be used depending on the size and category of solar power plant. That norm advices to install thermopile pyranometers horizontally (GHI, Global Horizontal Irradiation), and to install photovoltaic pyranometers on plane of PV modules (POA, Plane Of Array) to enhance accuracy in performance ratio calculation.
Photovoltaic pyranometer – silicon photodiode
Also known as a photoelectric pyranometer in the ISO 9060, a photodiode-based pyranometer can detect the portion of the solar spectrum between 400 nm and 1100 nm. The photodiode converts the aforementioned solar spectrum frequencies into current at high speed, thanks to the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
. The conversion is influenced by the temperature with a raise in current produced by the raise in temperature (about 0,1% • °C)
Design
![Photodiode pyranometer](https://upload.wikimedia.org/wikipedia/commons/a/a7/Photodiode_pyranometer.jpg)
A photodiode-based pyranometer is composed by a housing dome, a
photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
, and a diffuser or optical filters. The photodiode has a small surface area and acts as a sensor. The current generated by the photodiode is proportional to irradiance; an output circuit, such as a
transimpedance amplifier
In electronics, a transimpedance amplifier (TIA) is a current to voltage converter, almost exclusively implemented with one or more operational amplifiers. The TIA can be used to amplify the current output of Geiger–Müller tubes, photo multipli ...
, generates a voltage directly proportional to the photocurrent. The output is usually on the order of millivolts, the same order of magnitude as thermopile-type pyranometers.
Usage
Photodiode-based pyranometers are implemented where the quantity of irradiation of the visible solar spectrum, or of certain portions such as UV, IR or PAR (
photosynthetically active radiation Photosynthetically active radiation (PAR) designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more o ...
), needs to be calculated. This is done by using diodes with specific spectral responses.
Photodiode-based pyranometers are the core of
luxmeters used in photography, cinema and lighting technique. Sometimes they are also installed close to modules of photovoltaic systems.
Photovoltaic pyranometer – photovoltaic cell
Built around the 2000s concurrently with the spread of photovoltaic systems, the photovoltaic pyranometer is an evolution of the photodiode pyranometer. It answered the need for a single reference photovoltaic cell when measuring the power of cell and photovoltaic modules. Specifically, each cell and module is tested through flash tests by their respective manufacturers, and thermopile pyranometers do not possess the adequate speed of response nor the same spectral response of a cell. This would create obvious mismatch when measuring power, which would need to be quantified. In the technical documents, this pyranometer is also known as "reference cell".
![Sunmeter-35rid](https://upload.wikimedia.org/wikipedia/commons/5/5b/Sunmeter-35rid.jpg)
The active part of the sensor is composed of a photovoltaic cell working in near short-circuit condition. As such, the generated current is directly proportionate to the solar radiation hitting the cell in a range between 350 nm and 1150 nm. When invested by a luminous radiation in the mentioned range, it produces current as a consequence of the
photovoltaic effect
The photovoltaic effect is the generation of voltage and electric current in a material upon exposure to light. It is a physical property, physical and chemical phenomenon.
The photovoltaic effect is closely related to the photoelectric effect. F ...
. Its sensitivity is not flat, but it is same as that of Silicon photovoltaic cell. See the Spectral Response graph.
Design
A photovoltaic pyranometer is essentially assembled with the following parts:
* A metallic container with a fixing staff
* A small photovoltaic cell
* Signal conditioning electronics
Silicon sensors such as the photodiode and the photovoltaic cell vary the output in function of temperature. In the more recent models, the electronics compensate the signal with the temperature, therefore removing the influence of temperature out of the values of solar irradiance. Inside several models, the case houses a board for the amplification and
conditioning of the signal.
Usage
![Solar Irradiance Sensor, Model Seven Sensor](https://upload.wikimedia.org/wikipedia/commons/3/3f/Solar_Irradiance_Sensor%2C_Model_Seven_Sensor.jpg)
Photovoltaic pyranometers are used in solar simulators and alongside photovoltaic systems for the calculation of photovoltaic module effective power and system performance. Because the spectral response of a photovoltaic pyranometer is similar to that of a photovoltaic module, it may also be used for preliminary diagnosis of malfunction in photovoltaic systems.
Reference PV Cell or Solar Irradiance Sensor may have external inputs ensuring the connection of Module Temperature Sensor, Ambient Temperature Sensor and Wind speed sensor with only one Modbus RTU output connected directly to the Datalogger. These data are suitable for monitoring the Solar PV Plants.
Standardization and calibration
Both thermopile-type and photovoltaic pyranometers are manufactured according to standards.
Thermopile pyranometers
![MS-80 Class A Secondary Standard Pyranometer](https://upload.wikimedia.org/wikipedia/commons/f/ff/MS-80_Class_A_Secondary_Standard_Pyranometer.jpg)
Thermopile pyranometers follow the ISO 9060 standard, which is also adopted by the
World Meteorological Organization
The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics.
The WMO originated from the Internati ...
(WMO). This standard discriminates three classes.
The latest version o
ISO 9060 from 2018 uses the following classification: Class A for best performing, followed by Class B and Class C, while the olde
used ambiguous terms as "secondary standard", "first class" and "second class".,
Differences in classes are due to a certain number of properties in the sensors: response time, thermal offsets, temperature dependence, directional error, non-stability, non-linearity, spectral selectivity and tilt response. These are all defined in ISO 9060. For a sensor to be classified in a certain category, it needs to fulfill all the minimum requirements for these properties.
‘Fast response’ and ‘spectrally flat’ are two sub-classifications, included in ISO 9060:2018. They help to further distinguish and categorise sensors. To gain the ‘fast response’ classification, the response time for 95% of readings must be less than 0.5 seconds; while ‘spectrally flat’ can apply to sensors with a spectral selectivity of less than 3% in the 0,35 to 1,5 μm spectral range. While most Class A pyranometers are ‘spectrally flat’, sensors in the ‘fast response’ sub-classification are much rarer. Most Class A pyranometers have a response time of 5 seconds or more.
The calibration is typically done having the World Radiometric Reference (WRR) as an absolute reference. It is maintained by PMOD in
Davos
, neighboring_municipalities= Arosa, Bergün/Bravuogn, Klosters-Serneus, Langwies, S-chanf, Susch
, twintowns =
}
Davos (, ; or ; rm, ; archaic it, Tavate) is an Alpine resort town and a municipality in the Prättigau/Davos R ...
,
Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
. In addition to the World Radiometric Reference, there are private laboratories such as ISO-Cal North America who have acquired accreditation for these unique calibrations. For the Class A pyranometer, calibration is done following ASTM G167, ISO 9847 or ISO 9846. Class B and class C pyranometers are usually calibrated according to ASTM E824 and ISO 9847.
Photovoltaic pyranometer
Photovoltaic pyranometers are standardized and calibrated under IEC 60904-4 for primary reference samples and under IEC 60904-2 for secondary reference samples and the instruments intended for sale.
In both standards, their respective traceability chain starts with the primary standard known as the group of cavity radiometer by the World Radiometric Reference (WRR).
[IEC 60904-4:Procedures for establishing calibration traceability- Table1 and Fig.1]
Signal conditioning
The natural output value of these pyranometers does not usually exceed tens of millivolt (mV). It is considered a "weak" signal, and as such, rather vulnerable to
electromagnetic interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electros ...
s, especially where the cable runs across decametrical distances or lies in photovoltaic systems. Thus, these sensors are frequently equipped with signal conditioning electronics, giving an output of 4-20 mA or 0-1 V.
Another solution implies greater immunities to noises, like
Modbus
Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a ''de facto'' standard communication protocol and is now a commonl ...
over
RS-485
RS-485, also known as TIA-485(-A) or EIA-485, is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. Electrical signaling is balanced, and multipoint systems are supported. The s ...
, suitable for ambiances with electromagnetic interferences typical of medium-large scale
photovoltaic power station
A photovoltaic power station, also known as a solar park, solar farm, or solar power plant, is a large-scale grid-connected photovoltaic power system (PV system) designed for the supply of merchant power. They are different from most building- ...
s, or
SDI-12 SDI-12 (Serial Digital Interface at 1200 baud) is an asynchronous serial communications protocol for intelligent sensors that monitor environment data. These instruments are typically low-power (12 volts), are used at remote locations, and usually ...
output, where sensors are part of a low power weather station. The equipped electronics often concur to easy integration in the system's
SCADA
Supervisory control and data acquisition (SCADA) is a control system architecture comprising computers, networked data communications and graphical user interfaces for high-level supervision of machines and processes. It also covers sensors and ...
.
Additional information can also be stored in the electronics of the sensor, like calibration history, serial number.
See also
*
Actinometer
Actinometers are instruments used to measure the heating power of radiation. They are used in meteorology to measure solar radiation as pyranometers, pyrheliometers and net radiometers.
An actinometer is a chemical system or physical device which ...
*
Photodiode
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
*
Heat flux sensor
A heat flux sensor is a transducer that generates an electrical signal proportional to the total heat rate applied to the surface of the sensor. The measured heat rate is divided by the surface area of the sensor to determine the heat flux.
The ...
*
Net radiometer
A net radiometer is a type of actinometer used to measure net radiation (NR) at the Earth's surface for meteorological applications.
The name net radiometer reflects the fact that it measures the difference between downward/incoming and upward/o ...
*
Pyrgeometer
*
Pyrheliometer
A pyrheliometer is an instrument for measurement of direct beam solar irradiance. Sunlight enters the instrument through a window and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal v ...
*
Radiometer
A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or an ultraviolet detector. Microwave radiometers operate in the microwave w ...
*
Sunlight
Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
*
Solar constant
The solar constant (''GSC'') is a flux density measuring mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (au) from the Sun (roughly the ...
*
Sun path
References
External links
Meteo-Technology instrumentation website
{{Authority control
Measuring instruments
Meteorological instrumentation and equipment