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Solar Energy Measurements



Learn how to assess the solar energy potential of a site using a pyrheliometer, an unshaded pyranometers and a shaded pyranometer. These measure respectively, the Direct Normal Irradiation (DNI), the Global Horizontal Irradiation (GHI), and the Diffuse Horizontal Irradiation (DHI). These three values can be used to characterize a site for a variety of solar energy uses. 

GHI= DNI cosØ +DHI where Ø  is the solar azimuthal angle

Objective 1

Find the separate values of DNI, GHI and DHI and evaluate the feasibility of installing a large fixed photovoltaic system.

Objective 2

Find the separate values of DNI, GHI and DHI and evaluate the feasibility of installing a CPV (Concentrating photovoltaic ) system

Objective 3

Find the separate values of DNI, GHI and DHI and evaluate the feasibility of installing a CSP(Concentrating solar power) system.

(Hint Reference #10 has the map which is a result of the study on the global annual solar radiation. If the radiation is falling within the feasible range of more than 1700kWh/m2 per year, it is possible to have a solar installation )

Background and Theory

The background for the different types of solar radiation was already given in the Pyranometer and Pyrheliometer experiments. If you have not completed those experiments, please do so before attempting this experiment. 

There are two primary types of solar energy systems - non-concentrating and concentrating. Non-concentrating solar energy systems are those you typically see around you. Examples include most photovoltaic (PV) solar systems and flat plate collectors for heating water for domestic home use. These systems are commonly placed on rooftops or other areas which receive good sunlight. The rays from the sun are received on the system and converted to heat or electricity. These systems are able to collect solar energy which comes from a wide variety of directions. As a result, non-concentrating solar energy systems are able to utilize both direct (DNI) and diffuse (DHI) radiation. This means that they will even work in cloudy weather.  A sample graph with different radiations is given below[10]:

Concentrating solar energy systems, on the other hand, take the rays which come from the sun and focus them on a smaller area. They frequently use mirrors or other optical devices to achieve this goal. However, the geometry of these systems of mirrors and reflectors requires that they are only able to utilize sunlight which is coming directly from the sun, and they must frequently be pointed towards the sun. In other words, they are only able to use DNI solar energy and they require tracking. 

Concentrating solar energy systems can be further broken down into 2-D and 3-D concentrating systems. A 2-D concentrating solar thermal system is able to track the sun through one axis of freedom. They concentrate the sun's energy onto a line. Examples of this type of system include parabolic troughs and linear Fresnel collectors. The theoretical maximum concentration ratio of a 2-D concentrating system is 212:1 and can achieve temperatures into the hundreds of degrees Celsius (see Power from the Sun in the Reference section for a complete derivation of this proof). A 3-D concentrating system, on the other hand is able to track the sun through two axes of freedom. As a result, 3-D systems able to focus the sun's energy to a point. Examples include the "Power Tower" Central Receiver system and Sterling Dishes. They are able to achieve concentration ratios as high as 45,000:1 and temperatures into the thousands of degrees Celsium (see Power from the Sun in the Reference section for a complete derivation of this proof). 

Solar Radiation

 • Direct solar radiation is the radiation that comes directly from the sun, with minimal attenuation by the Earth’s atmosphere or obstacles.

• Diffuse solar radiation is that which is scattered, absorbed, and reflected within the atmosphere, mostly by clouds, but also by particulate matter and gas molecules.

•The direct and diffuse components together are referred to as total or global radiation

Necessity of ground measurements

 •Datasets are considered adequate for planning purposes, but planners should be aware of the uncertainty associated to the data. Annual DNI sums and yearly distribution differ very much among datasets for the same specific sites since the models apply different atmospheric corrections.

•Locations with similar average DNIs can see variations of up to ± 9% in annual electricity production due to differences in DNI frequency distribution. (IEA, 2010).

•Ambient temperature, wind speed and direction and relative humidity conditions at the site AFFECT the performance.Therefore, satellite based datasets must be scaled with ground measurements in order to obtain reliable and “bankable resource assessments” during the project development phase.

•Solar resource uncertainty risk is perceived as one of the highest by financiers. A minimum of one year of on-site measurements is required. The information obtained, together with satellite and historic data, must be analyzed to produce long term estimates of the solar resource.


Cite this Simulator:

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