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In this NEW series, we introduce you to solar irradiance. In the first instalment, we touched on what affects irradiance, how it is measured and the types of irradiance. In this article, the second and final instalment in the series, we give an overview of solar potential maps and applications of solar irradiance.

Solar irradiance is a very important quantity that is studied extensively especially in the design of solar technologies. Understanding and measuring solar irradiance has important implications, such as the heating and cooling loads of buildings, the prediction of energy generation from solar power plants, and weather forecasting.

Applications of Solar Irradiance Measurement

1. Solar power

Sunlight carries radiant energy in the wavelengths of visible light. Radiant energy may be developed for solar power generation.

Solar irradiation figures are used to plan the deployment of solar power systems. In many countries, the figures can be obtained from an insolation map or from insolation tables that reflect data over the prior 30–50 years. Different solar power technologies are able to use different components of the total irradiation. While solar photovoltaics panels are able to convert to electricity both direct irradiation and diffuse irradiation, concentrated solar power is only able to operate efficiently with direct irradiation, thus making these systems suitable only in locations with relatively low cloud cover.

Because solar collector’s panels are almost always mounted at an angle towards the sun, insolation must be adjusted to prevent estimates that are inaccurately low for winter and inaccurately high for summer. This also means that the amount of sun falling on a solar panel at high latitude is not as low compared to one at the equator, as would appear from just considering insolation on a horizontal surface.

Photovoltaic panels are rated under standard conditions to determine the Wp (watt peak) rating, which can then be used with insolation to determine the expected output, adjusted by factors such as tilt, tracking and shading (which can be included to create the installed Wp rating). For example, insolation values range 800–950 kWh/(kWp·y) in Norway to up to 2,900 kWh/(kWp·y) in Australia.

2. Buildings

In construction, insolation is an important consideration when designing a building for a particular site.

The projection effect can be used to design buildings that are cool in summer and warm in winter, by providing vertical windows on the equator-facing side of the building (the south face in the northern hemisphere, or the north face in the southern hemisphere). This maximizes insolation in the winter months when the Sun is low in the sky and minimizes it in the summer when the Sun is high. (The Sun's north/south path through the sky spans 47° through the year).

3. Civil engineering

In civil engineering and hydrology, numerical models of snowmelt runoff use observations of insolation. This permits estimation of the rate at which water is released from a melting snowpack. Field measurement is accomplished using a pyranometer.

4. Climate research

Irradiance plays a part in climate modeling and weather forecasting. A non-zero average global net radiation at the top of the atmosphere is indicative of Earth's thermal disequilibrium as imposed by climate forcing.

5. Space

Insolation is the primary variable affecting equilibrium temperature in spacecraft design and planetology.

Solar activity and irradiance measurement is a concern for space travel. For example, the American space agency, NASA, launched its Solar Radiation and Climate Experiment (SORCE) satellite with Solar Irradiance Monitors.

Solar Potential Maps    

Assessment and mapping of solar potential at the global, regional and country levels have been the subject of significant academic and commercial interest. One of the earliest attempts to carry out comprehensive mapping of solar potential for individual countries was the Solar & Wind Resource Assessment (SWERA) project, funded by the United Nations Environment Program and carried out by the US National Renewable Energy Laboratory. Other examples include global mapping by the National Aeronautics and Space Administration and other similar institutes, many of which are available on the Global Atlas for Renewable Energy provided by the International Renewable Energy Agency. A number of commercial firms now exist to provide solar resource data to solar power developers, including 3E, Clean Power Research, SoDa Solar Radiation Data, Solargis, Vaisala (previously 3Tier), and Vortex, and these firms have often provided solar potential maps for free. In January 2017 the Global Solar Atlas was launched by the World Bank, using data provided by Solargis, to provide a single source for high-quality solar data, maps, and GIS layers covering all countries.

Solar maps

Solar radiation maps are built using databases derived from satellite imagery, as for example using visible images from Meteosat Prime satellite. A method is applied to the images to determine solar radiation. One well validated satellite-to-irradiance model is the SUNY model. The accuracy of this model is well evaluated. In general, solar irradiance maps are accurate, especially for Global Horizontal Irradiance.


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