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Solar Energy Measurements - Pyranometer
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Objective 1

Learn how to assess the solar energy potential of a site using a pyranometer.

Objective 2

Find the GHI using the pyranometer data and assess the feasibility of a solar PV station in the area

Objective 3

Find the GHI using the pyranometer and assess the feasibility of a concentrating solar power system in the area. Assess the solar power [11]. If the power is in the range of 500 to 1000 suns [10], the site is suitable for a Concentrator Photo Voltaic(CPV) installation. 

Background and Theory

The first step in assessing a site for suitability for solar power production is to analyze the solar resources available. Since the available solar resources vary greatly with respect to local weather conditions, location, time of day, and time of year, it is generally agreed that accurate site data needs to be taken over an extended period of time in order to determine the amount of energy available in the environment. One year’s data is a common, although even longer time frames are preferred.

While solar resource data is available in various online databases, some of which are given in the Resources tab of this experiment, these databases only cover macro solar data, for example data gleaned from orbiting satellites, or for specific monitoring points, such as from installed metrological weather stations, which do not always provide information relevant to the particular site that is being assessed. For example, buildings, trees, fog, clouds, smoke, smog, pollution and other locally variable factors can greatly influence the amount and quality of solar energy available.

This Virtual Lab experiment is designed to teach the process of actual, applied solar energy site assessment using modern data acquisition systems and the relevant data post-processing techniques. At the end of this experiment, the student will have the necessary skills to analyze solar data for a real-world assessment of a site for solar power production.

GHI of a site can be found out by DNI cos (theta) + DHI. The Pyranometer reading could be verified using the solar tracker module.  The angle theta is as shown in figure below:

Solar Radiation Measurements

There are numerous types and components of solar radiation as shown in the list below. Units are typically in W/m2.

  1. Irradiance – the instantaneous quantity of solar radiant energy incident on a surface per unit area.
  2. Diffuse solar irradiance – the solar irradiance on a horizontal surface due to only sky radiation. This does not include the direct solar irradiance, which is defined next.
  3. Direct solar irradiance, also called the “direct solar irradiation” or the “direct normal irradiation (DNI)” – the solar irradiance on a surface held perpendicular to the sun’s rays while blocking the diffuse sky radiation.
  4. Global solar irradiance, commonly called the “global radiation” – the solar irradiance on a horizontal surface which includes both direct sunrays and diffuse sky radiation.
  5. Reflected solar irradiance – the short-wave radiation which is reflected upward from the Earth’s surface.
  6. Net terrestrial radiation – the long-wave radiation coming off the Earth’s surface minus the upward radiation through a horizontal surface near the Earth’s surface.
  7. Net total irradiance – the downward irradiance minus the upward radiation as measured over the entire spectrum.

In addition, a portion of the incoming solar radiation is absorbed, reflected and reemitted by the atmosphere and the earth. The following figure illustrates this concept.

The following image shows the solar spectral intensity distribution at the top of the Earth’s atmosphere and at the Earth’s surface. The solar irradiance received at the top of the Earth’s atmosphere is about 1367 W/m2; this number is normally termed the “solar constant”.

 

 

Near the equator, on the Earth's surface on a sunny day at solar noon, when the sun is most nearly directly overhead, a maximum of approximately 1000 W/m2 of global radiation is received. This number is commonly called "Standard Solar Radiation" and is the standard that many photovoltaic solar cells are tested to.

Solar irradiance, is measured in terms of power per unit at a particular time. The units can be W/m2, kW/m2. This, measurement, is instantaneous and is only valid for a particular point in time.A more useful measurement is the amount of solar energy received per unit area over a given time frame. This is called irradiation or solar insolation. Typical units for this are Wh/m2, kWh/m2 or MJ/m2.

The irradiation can be found by integrating or performing a numerical “Riemann” sum of the irradiance over time. Standard values that can be found are the average daily insolation, the average monthly insolation, and the average yearly insolation. Average values are very important to gather, as solar radiation can change dramatically over the short term, but are reliable when averaged over the relevant longer time frame. A proper assessment of the energy collected is essential to determining the type of solar energy technology and the capacity installed.

Broad area maps for this information have been compiled by various agencies, usually governmental and are freely available for download from the official sites. This data is frequently compiled from interpolated satellite data. While useful for determining the likelihood of a project’s feasibility, it does not provide the detailed and localized data necessary for a proper site assessment to choose a technology or capacity for installation.

 
 

GHI is the "global" radiation measured on a horizontal plane. It is global in the sense that it includes both direct and diffuse irradiation. As a result GHI is particularly useful for predicting the power production of solar photovoltaic (PV) panels, since solar PV panels can utilize  both direct and diffuse solar radiation. However, most solar PV panels are not oriented in the horizontal plane. Therefore, the irradiance received over a non-horizontal plane can be determined by multiplying it by the cosine of the minor angle between the horizontal and plane of the solar panel. 

GHI is normally measured with a pyranometer. An image of the pyranometer used in this experiment is below.

 

Cite this Simulator:

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