SHINING ON

Chapter 1: What are solar radiation data?

Chapter 2

Why do we need solar radiation data?

The earth receives a vast amount of energy from the sun in the form of solar radiation. If we converted to usable energy just 0.2% of the solar radiation that falls on our nation, we would meet the energy demand of the entire United States. A variety of solar energy technologies are being developed to harness the sun's energy, including:

solar electric (photovoltaic) for converting sunlight directly into electricity
solar heat (thermal) for heating water for industrial and household uses
solar thermal electric for producing steam to run turbines that generate electricity
solar fuel technologies for converting biomass (plants, crops, and trees) into fuels and by-products
passive solar for lighting and heating buildings
solar detoxification for destroying hazardous waste with concentrated sunlight.

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Fig. 1: These technologies convert sunlight into usable forms of energy.

The economics of these technologies depend on the equipment and operating costs, the percentage of the solar radiation that can be converted into the desired energy product, and the amount of solar radiation available. Users of these technologies need high-quality solar radiation data. If the actual solar radiation for a location is less than indicated by available data, the performance and the economic goals for the system will not be met. On the other hand, if the actual solar energy at a location is greater than indicated by the data, the performance and economic projections may be too conservative and prevent a viable technology from being used.

"The more accurately we know the solar resource, the better we can optimize the system. Therefore, accurate solar radiation data are an important factor in solar system design."
--David F. Menicucci, Sandia National Laboratories

To minimize energy consumption, heating and air-conditioning engineers also use solar radiation data to select building configurations, orientations, and air-conditioning systems. Because energy costs are a significant expense in building ownership, an energy-efficient design can significantly reduce the life-cycle cost of a building.

The amount of solar radiation received changes throughout the day and year due to weather patterns and the changing position of the sun, and solar radiation data reflect this variability. By knowing the variability, we can size storage systems so they can provide energy at night and during cloudy periods. For technologies with no energy storage, we can evaluate load matching by comparing the profile of the available solar radiation throughout the day with a profile of the energy required by the load. Solar radiation data also help determine the best geographic locations for solar energy technologies. Other factors being equal, a site receiving more solar radiation will be more economical.

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Fig. 2: Because of absorption and scattering by the atmosphere, the spectral distribution of solar radiation outside the atmosphere differs significantly from that on earth. Also, the spectral distribution on earth changes throughout the day and year and is influenced by location, climate, and atmospheric conditions. Consequently, the percentage of energy that is composed of UV, visible, or near-infrared radiation, or portions thereof, also varies by location, time of day, and year.

For certain technologies, we also need to know the spectral, or wavelength, distribution of the solar radiation. For example, photovoltaic devices respond primarily to wavelengths in the visible and near-infrared regions of the spectrum, while solar detoxification uses energy from the ultraviolet (UV) region. Location, climate, and atmospheric conditions influence the spectral distribution of solar radiation.

Chapter 3: What influences the amount of solar radiation?

Table of Contents

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