Measurement of solar Radiation

Solarmeter: A Solarmeter is an instrument used for measuring the solar radiation. It uses the photovoltaic effect to measure the amount of solar radiation reaching a given surface.

Pyranometers :The Pyranometers are instruments used to measure the global radiation on a surface (direct and diffuse radiation).

Pyrheliometer: A Pyrheliometer is an instrument for direct measurement of solar irradiance

Sunshine Recorder: A sunshine recorder is a device that records the amount of sunshine at a given location.

SOLARMETER

# Uses the photovoltaic effect to measure the amount of solar radiation

reaching a given surface.

# It produces an electrical signal as a function of incident light, especially

responds to visible light and its response depends on the temperature of the

cell.

# Solarmeter with a silicon cell is able to capture light waves with a

spectrum range of approximately 330nm to 1100nm.

# In order to obtain a measure not influenced by the temperature, the values

measured by a solarmeter using the photovoltaic effect must be corrected

according to the temperature of the photovoltaic cell.

# This measurement can be done thanks to a thermocouple, and the fix

factor should have a level of precision is not easy to achieve.

PYRANOMETER♣ The operating principle is generally based on the measurement of the difference in temperature between a clear surface and a dark one.

♣ A dark surface can absorb most of the solar radiation, while a clear surface tends to reflect, and so it absorbs less heat.

♣ The potential difference that is generated in the thermopile due to the temperature gradient between the two surfaces, allows to measure the value of global solar radiation incident.

♣ It responds to radiation of all wavelengths, hence measures total power in the incident spectrum accurately.

♣ It has a thermopile whose sensitive surface consists of circular, blackened, hot jns. Exposed to sun, the cold jns. Being completely shaded.

♣The temp. difference between hot and cold jns. Is the fn. of radiation falling on the sensitive surface.

♣ The sensing element is covered with two hemispherical glass domes to shield from wind & dust and reduce convection currents.

♣ A radiation shield is provided coplanar with the outer dome to prevent direct solar radiation from heating the base of the instrument.

♣ The instrument has a voltage output of approximately 9µV/W/m2 and has an output impedance of 650Ω .

♣ When provided with shadow band to prevent beam radiation from reaching the sensing element, measures the diffused radiation only.

♣ Inexpensive instruments based on cadmium-sulphide photocells and silicon photodiodes can also be used but they cannot be accurately

♣ Pyranometers are frequently used in meteorology, climatology, solar energy studies and building physics

Precision Pyranometer

Arrangement of pyranometer at solar panels

Pyranometer with shadow band

It is used with a solar tracking system to keep the instrument aimed at the sun and to get continuous readings.

It uses long collimator tube to collect beam radiation whose field of view is limited to a solid angle of 5.50 by appropriate diaphragms inside the tube.

The inside of the tube is blackened to absorb any radiation incident at angles outside the collection solid angle.

At the base of the tube a wire wound thermopile is placed which converts heat to an electrical signal that can be recorded.

The signal voltage is converted via a formula to measure watts per square metre.

The tube is sealed with dry air to eliminate absorption of beam radiation with in the tube by water vapour.

The instrument has a voltage output of approximately 8µV/W/m2 and has an output impedance of 200Ω .

PYRHELIOMETER

A pyrheliometer is often used in the same setup with a pyranometer.

Typical pyrheliometer measurement applications include scientific meteorological and climate observations, material testing research, and assessment of the efficiency of solar collectors and photovoltaic devices.

SUN SHINE RECORDER

A sunshine recorder is a device that records the amount of sunshine at a given location.

The results provide information about the weather and climate of a geographical area. This information is useful in meteorology, science, agriculture, tourism and other fields.

This recorder consists essentially of a glass sphere mounted concentrially in a section of a spherical metal bowl, the diameter of which is such that the sun`s rays are focused sharply on a card held in the grooves, in the bowl.

Three overlapping pairs of grooves are provided in the bowl to taken up cards suitable for the different seasons of the year.

This instrument measures the duration in hours of bright sunshine during the course of a day.

The card is prepared from a special paper bearing a time scale.

As the sun moves, the focused bright sunshine burns a path along this paper.

The length of the trace thus obtained on the paper is a measure of the duration of the bright sunshine.