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Solar Energy Measurements - Pyrheliometer

Objective 1

Learn how to assess the solar energy potential of a site using a pyrheliometer. Pyrheliometers are used to measure Direct Normal Irradiance or DNI, which is the component of solar energy used in concentrating solar energy systems. The user should assess the feasibility of Concentrating Solar Power(CSP) systems by calculating the total wattage requirement of the proposed system and assessing the feasibility by finding out the total irradiance in a specific area and multiplying with the area of the proposed system

Objective 2

Assess the solar energy potential of a site with DNI and calculate the feasibility of a Concentrating Solar Photovoltaic (CPV) system , with tracking option

(Hint : Use the DNI study in reference 1 - 2000 kWh per m2 per year is the minimum DNI requirement for installing a CSP. Also, areas with annual average direct normal irradiation values of greater than 2.2 MWh/m2/year or 6.0 kWh/m2/day can easily have a good CSP or CPV system. Most of solar radiations received on erath surface is within the range of 0.25 - 0.04 micron)

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.

Solar Radiation Measurements

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

  • Irradiance – the instantaneous quantity of solar radiant energy incident on a surface per unit area.
  • 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.
  • 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.
  • Global solar irradiance, commonly called the “global radiation” – the solar irradiance on a horizontal surface which includes both direct sunrays and diffuse sky radiation.
  • Reflected solar irradiance – the short-wave radiation which is reflected upward from the Earth’s surface.
  • 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.
  • 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.

A graph indicating the power ouput from a PV module based on different irradiance levels is given below [3]

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.

DNI is the "DIRECT NORMAL" radiation measured on a tilted plane. This is the direct radiation used by converters such as flat plate and parabolic collectors to convert the direct normal solar component to heat which can be transfered to the HTF (heat transfer fluid). The DNI can be related to the Direct Horizontal component by a cosine of the angle of the tilted plane. The Global Horizontal component is measured by summing up the Direct Horizontal and Diffuse Horizontal components, to be measured in the pyranometer experiment, coming next. The absorber or receiver of a collector is to have very high absorptance in the 300-2000 nm range, to make maximum conversion use of the DNI.

DNI is normally measured with a pyrheliometer. An image of the pyrheliometer used in this experiment is below.

A standard pyrheliometer has about a 5 degree cone of acceptance. Practically, this means that the pyrheliometer must always be pointed nearly directly at the sun for it to measure the intensity of the radiation coming directly from the sun. Therefore, the pyrheliometer is attached to a tracking device to make it always point at the sun. 

In order to point the pyrheliometer directly at the sun regardless of the time of day or time of year, the tracking devce must have at least two degrees of freedom. The tracking unit we use for this experiment can rotate about the azimuth and altitude angles. The altitude is the angular distance of the sun above the horizon. The altitude angle is also called the elevation angle. It is complemented by the zenith angle, such that altitude angle + zenith angle = 90 degrees. When the sun is setting or rising, the altitude angle is zero. When the sun is directly overhead, the altitude angle is 90 degrees. 

The azimuth angle is the angle of the projection of the sun on the Earth's surface, when referenced from North or South. The diagram below illustrates these concepts. (Image)




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