Solar Radiation measuring techniques
In any PV system design, it is essential to know the amount of sunlight available at a particular location at a given time.
The two common methods which characterise solar radiation are the solar radiance (or radiation) and solar insolation.
Solar radiance is an instantaneous power density in units of kW/m2. The solar radiance varies throughout the day from 0 kW/m2 at night to a maximum of about 1 kW/m2. The solar radiance is strongly dependant on location and local weather. Solar radiance measurements consist of global and/or direct radiation measurements taken periodically throughout the day. The measurements are taken using either a pyranometer (measuring global radiation) and/or a pyrheliometer (measuring direct radiation).
An alternative method of measuring solar radiation, which is less accurate but also less expensive, is using a sunshine recorder. These sunshine recorders (also known as Campbell-Stokes recorders), measure the number of hours in the day during which the sunshine is above a certain level (typically 200 mW/cm2). Data collected in this way can be used to determine the solar insolation by comparing the measured number of sunshine hours to those based on calculations and including several correction factors.
A pyranometer is a type of actinometer that gives information about solar irradiance in the desired location by measuring solar radiation flux density. Pyranometers measure solar radiation flux density in watts per square meter (W/m2) within a wavelength range of 300 nm to 3000 nm from a fixed plane at a specific orientation with a hemispherical field of view. The pyranometer uses flat spectral sensitivity to cover this spectrum.
Pyranometers are used to assess the efficiency of a solar power system. These devices can be used to compare the actual output of a solar power system to the expected output. If a significant deficit is detected, it may indicate that service is needed. Pyranometers can also be used to determine the suitability of a location for a potential solar power plant.
Based on the Seebeck- or thermoelectric effect, a pyranometer is operated based on the measurement of a temperature difference between a clear surface and a dark surface.
The black coating on the thermopile sensor absorbs solar radiation, while the clear surface reflects it. Hence, less heat is absorbed on the clear surface.
The potential difference created in the thermopile owing to the temperature gradient between the two surfaces reveals information about the amount of solar radiation.
The voltage produced by the thermopile can also be measured using a potentiometer. While most pyranometers generate an analogue signal, some come with a smart digital interface.
Solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of kWh/(m2 day). While the units of solar insolation and solar irradiance are both a power density, solar insolation is quite different than the solar irradiance as the solar insolation is the instantaneous solar irradiance averaged over a given time period. Solar insolation data is commonly used for simple PV system design while solar radiance is used in more complicated PV system performance which calculates the system performance at each point in the day. Solar insolation can also be expressed in units of MJ/m2 per year.