Pyhreliometer and Pyranometer, Overview:
In this article we will study about Pyhreliometer and Pyranometer. As these instruments are used for the measurement of diffused radiation and beam radiation. So first we will discuss about that what is diffused beam radiation and some other important parameters that are related to it.
The scattered, reflected and refracted radiation that is sent to the earth surface from the sun in all direction (reflected from particles, molecules, clouds etc) is indirect radiation is called diffuse radiation. The intensity of the diffused radiation is represented by Id.
The radiation that comes directly from the sun is direct radiation and is called beam radiation. The intensity of the beam radiation is represented by Ib.
The irradiance is the flux of radiant energy per unit area which is measure in W/m2. The irradiance will be normal to the direction of flow of radiant energy through a medium.
Insolation can be measure in J/m2which is the amount of solar radiation reaching a given area.
Terrestrial and extra-terrestrial radiation:
Terrestrial radiations are those radiations which flow throughout the year.
Extra-terrestrial solar radiation:
The extra-terrestrial radiation is the radiation which is incident outside the earth atmosphere. The extra-terrestrial radiation is 1367wattsm2 .
Actinometer is the instrument which is used to measures heating power of the radiation. There are various types of actinometers which are:
Pyrheliometer meter is an instrument which can be used to measures solar irradiance and circumference. In solar radiation it is used only to measures the beam radiation. It used thermopile to measure emf difference. Its outer structure look like a long tube projecting the image of a telescope and we have to point the lens to the sun to measure the radiance. They are commonly used for climatological research to monitor the weather and recently for solar energy research. It can measure the radiation having wavelength between 280 nm to 3000 nm. The pyrheliometer looks like a telescope. Pyrheliometer is used for climatological and weather monitoring research purpose.
Now we want to measure the solar radiation we will set the Pyrheliometer at specific angle which will measures the radiation coming from the sun. The acceptance angle of the instrument should be close to 0 degrees as possible so that the lens can bend and focus the light into black body. Generally this angle is usually 5°.
Long collimator tube:
In this tube we will set the solar radiation which is lengthy tube.
This will be used for setting the angle. It will align the instrument.
Black absorber plate:
When the rays will come inside the tube there will be black plate which will absorb the radiation.
Thermopile junction is located below the black absorber plate which will be used to measure the emf. It will measures the heat from the black body which is recorded for the system.
Pivot for 2-axis rotation:
The whole system is mounted on it with the help of this it will be able to follow the sun throughout the day. We can change the direction of the instrument according to the direct radiation.
Working of Pyrheliometer:
The sunlight will enter in the long collimator tube and will incident on black absorber plate. The thermopile is in contact with black absorber plate when the black plat will absorb the heat the emf will be generated between the hot and colder surface due to the temperature difference. This emf will be used to measure the value of beam radiation.
Pyrheliometer has very less power consumption
Wide range of voltage can be operated with it
Pyrheliometer is stable and Rigid
Pyreliometer is usually used in solar tracking system as it only measure the direct radiation. It will follows the sun position when the sun will moving the tracking system will follow the sun.
A Pyranometer is a type of actinometer used for measuring solar irradiance on a planer surface. This word come from the greek work pyr means fire and ano means above or sky. As the above heat is measured through this instrument therefore it is known as Pyranometer. The working principal of the Pyranometer is based on the thermoelectric detection.
Pyranometer which is a radio meter with a glass dome that has hemispherical view of the whole sky. Pyranometer do not require much maintenance and are designed for long life service. Pyranometer is used for measuring the diffused radiation and beam radiation which varies with the cosine of the angle of incidence.
0° angle of incidence = full response
90° angle of incidence = zero response
60° angle of incidence = 50% response
The incidence angle ɵ is the angle between a normal to the PV module face and the incoming solar beam irradiance.
A Pyranometer measures solar radiation in Wb/m2 . Solar radiation is the source of energy. The energy flux is expressed in Wb/m2 . It include visible as well as non-visible part of the spectrum. Solar radiation can reach us directly or in other ways filtered by clouds, scattered by the atmosphere and reflected by the ground surface. Spectrally flat pyranometer is often preferred because they can be used in horizontal, inverted and tilted orientations using the same instrument with the same calibration. Pyranometer are also available with digital outputs. In that case the calculation are performed inside the instrument. It is designed to measure the solar radiation flux density (W⁄m2 ) from the hemisphere. It measures the wavelength in the range from 0.3 µm to 3 µm.
Classification of Pyranometers:
There are two types of technologies of pyranometer used with the help of which it can measures the temperature.
Thermopile technology (thermo electric pyranometer)
This type of pyranometer used collection of thermocouples which are connected in series. Where the top surface is exposed to heat flux that is the radiation from the sun and the bottom surface is insulated where there are thermal resistance layers due to which heat will not be transfer from the top layer to bottom layer.
The bottom surface will be cold surface and reference temperature will be there. The top surface will be heated by the solar radiation due to the difference in the temperature and voltage will be generated so the voltage can be measured. ∆V represents the change in voltage and may be measured based on the temperature difference which is represented by ∆T. So higher the temperature difference ∆T higher will be the voltage difference ∆V.
The thermopile pyranometer has a very wide spectrum which can measure the solar radiation of wavelength from 300 nm to 3000 nm as shown if the figure below.
The spectrum of thermopile pyranometer in figure in light sky blue colour
- Silicon semiconductor technology (photo electric pyranometer)
In this technology the photons are directly converted in to electricity.
The spectrum of the photo voltaic pyranometer is shown in dark blue colour which has less spectrum as shown in the above figure.
To facilitate the choice of the instrument the ISO 9060 standard defines 3 different pyranometer classes: A, B and C. The ISO 9060 defines specification and gives specification limit per instrument class.
Class A pyranometer:
Class A pyranometer are subjected to elaborate factory testing of directional response and temperature response. Some pyranometers are heated, heating solves problem with dew and frost deposition.
The solar energy flux varies with the cosine of the angle of the incidence of the radiation. Within its view angle the directional response of a pyranometer should resembled this cosine response.
Solar radiation is transmitted by the pyranometer glass dome and is absorbed by the black coating. There is converted in to heat the combination of the glass and coating creates a sensor with a near perfect directional response. This combination of the glass and coating has a flat spectral response. In the range of 300 to 3000 nanometer.
Parts of the pyranometer contains:
- Black detector
- Glass dome surface
- Sun screen
- Desiccant indicator
- Levelling feet
- Bubble level
A pyranometer has a 180° field of view angle. The glass domes are round in shape which will prevent the instrument from dust particles. The glass domes has usually diameter is about 50 mm outer diameter, the middle diameter of the second glass is about 30 mm. The glass domes are used to limit the radiation. It can be used to limiting the spectrum.
Below the glass domes there is black absorber which will absorb the heat coming from the sun. The circular absorber is about 25 mm in diameter.
The guard plate will save the internal mechanism of the instrument.
Inside a pyranometer we will find a very sensitive temperature difference sensor a thermopile. The thermopile is present inside the guard plate which will used to measure the temperature. We know about thermocouple which consists of two dissimilar materials which will form temperature junction which will help in measuring the temperature. When several thermocouples are combines it will form thermopile. The hot junction of the thermopile is connected with the absorber and cold junction is below it. The emf will be generated by their temperature difference. A thermopile generates an output signal proportional to the solar irradiance from the black coating to the metal instrument body. Thermopile does not require any power supply. Calculating the solar irradiance is easy. Just measure the output signal and divide it by the sensitivity of the sensor.
The levelling screw will help in to keep the instrument in level parallel to the earth.
Shading ring will help in the prevention of the direct radiation. If we want to measure only diffused radiation.
- Material assessment and testing research of the efficiency of solar collectors and photovoltaic devices
SPN1 Sunshine Pyranometer:
SPN1 offers a radically different to solar radiation measurement traditionally. The measurement of global direct and diffuse solar radiation has complex expensive equipment. In contrast SPN1 offers similar performance that has:
- No moving parts
- Simple set up
- No routine adjustment
- Saves time and cost
The SPN1 measures global radiation from the whole sky and simultaneously measures the diffuse radiation excluding the direct solar beam. It also provides sunshine duration and enable calculation of DNI (direct normal irradiance). Its innovative design features seven thermopile sensors, a computer modelled shadow mask, a precision ground glass dome, a robust machined aluminium case and an internal heater.
The positions of the seven thermopiles in combination with the shape of the shadow mask ensures that one thermopile is always in the direct solar beam and one is in shade so at any given moment at least one thermopile is measuring direct radiation and at least one is measuring the diffuse radiation. The SPN1 connects two data logger computers and control system. Data can be viewed remotely using delta link cloud. Delta cloud based data service.
SPN1 has wide range of application:
- It is used in solar PV to compare solar panel efficiency to DNI (direct normal irradiance). Its common practice to use expensive mechanical equipment that tracks the sun
- The SPN1 calculates without the complexity and a lower cost
- Dependable solar radiation data is vital for the selection of the new solar farm sites and for efficiency monitoring at established sites
- The SPN1 high quality and affordability make it an excellent choice for these measurements
- The SPN1 enables solar radiation and DNI to be monitored and at hostile and remote sites where regular shadow band adjustment would be virtually impossible
- The built-in heater keeps the dome clear from condensation and ice down to -20° C
- The SPN1 competence and speed of response has seen it mounted on a variety of craft and it can be used to estimate total optical depth and aerosol optical depth as an affordable alternative to sun photometer.
Advantages of SPN1:
- Multiple outputs
- No moving parts
- Compact and rigid
- Internal heater
Versatile communication option