Chapter 4: Why do we need solar radiation data?
To determine the performance and economics of solar conversion technologies, designers and engineers use solar radiation data to estimate how much solar energy is available for a site. Depending on the particular technology, the solar collector might be a photovoltaic array, a concentrating parabolic trough, a domestic hot-water collector, a window, a skylight, or a canopy of foliage. Designers and engineers use hand calculations or computer simulations to estimate the solar radiation striking a collector.
Hand calculations are appropriate when using solar radiation data that represent an average for an extended period. For example, designers of remote photovoltaic-powered systems for charging batteries use average daily solar radiation for the month to determine the size of the photovoltaic array. The criterion for this application is not the amount of solar radiation for a given hour or day but whether or not the average daily solar radiation for the month is sufficient to prevent the batteries from becoming discharged over several days.
The month used in the design process depends on the relative amount of solar radiation available compared to the energy required by the load. For a system in which the load is constant throughout the year, solar radiation data for December or January are usually used for the Northern Hemisphere.
Computer simulations are an effective tool when an hour-by-hour performance analysis is needed. Utility engineers may want to know if the output of a solar electric power plant could reliably and economically help meet their daytime electric demand. (One of the potential benefits of a solar electric power plant is that its output may coincide with the utility peak electric demand for summertime air-conditioning loads.) By using the hourly solar radiation data for its location, the utility can run computer programs that show how much energy could be produced on an hour-by-hour basis throughout the year by the solar electric power plant.
Some solar energy conversion technologies require a threshold value of solar radiation before certain operations can begin or be sustained. As an example, a central receiver solar thermal electric power plant may require direct normal solar radiation values above 450 W/sq m to produce steam for the turbine generator. Consequently, to evaluate a site's potential for solar thermal electric production, a designer examines the solar radiation data to determine the times of day when the solar radiation exceeds the threshold value.
Fig. 4: Computer simulation using solar radiation data shows how the output of two photovoltaic power systems cold be added to the utility's generation to help meet peak electric demand in the summer. The fixed-tilt array faces south and is tilted from the horizontal at an angle equal to the site's latitude. The tracking array uses motors and gear drives to point the array at the sun throughout the day. Depending on location, the photovoltaic system with the 2-axis tracking array receives annually 25% to 40% more global solar radiation than the fixed-tilt photovoltaic system and provides more power for longer periods. This must be weighed against the higher initial cost and maintenance required for the tracker.
"Because the solar load is the largest component for building exterior surfaces, and because windows are the most sensitive to the solar load, solar radiation data are essential for the accurate and energy efficient design of buildings and their air conditioning systems."
--Jack F. Roberts, P.E.
American Society of Heating, Refrigerating and Air-Conditioning Engineers
Except for concentrator systems, solar radiation data cannot be used without first accounting for the orientation of the solar collector. Concentrators track the sun and focus only direct beam radiation, but flat-plate collectors receive a combination of direct beam radiation, diffuse (sky) radiation, and radiation reflected from the ground in front of the collector. Depending on the direction the collector is facing and its tilt from the horizontal, flat-plate collectors receive different amounts of direct beam radiation, diffuse radiation, and ground-reflected radiation. Designers employ equations to calculate the total or global radiation on a flat-plate collector. The equations use values of the direct beam radiation, the diffuse radiation on a horizontal surface, and the orientation of the collector.
To maximize the amount of solar radiation received during the year, flat-plate collectors in the Northern Hemisphere face south and tilt from the horizontal at an angle approximately equal to the site's latitude. The annual energy production is not very sensitive to the tilt angle as long as it is within plus or minus 15° of the latitude. As a general rule, to optimize the performance in the winter, the collector can be tilted 15° greater than the latitude. To optimize performance in the summer, the collector can be tilted 15° less than the latitude. Solar radiation data combined with computer simulations can define these relationships more precisely.
In the initial design stage, designers of cells used in photovoltaic modules can use spectral solar radiation data bases and models to optimize the cells for maximum energy production. Because the spectral content of solar radiation changes throughout the day and season, photovoltaic cells are tailored for a specific range of solar radiation wavelengths that will produce the most energy. Different photovoltaic materials have different peak responses; performance models using spectral solar radiation data bases can compare two or more photovoltaic materials operating under a range of seasons and climates. This results in optimizing the design early and eliminates the expense and time that would otherwise be needed for preliminary field testing.
"For sizing stand-alone PV systems, we calculate the number of PV modules required to keep the batteries charged by using the average daily solar radiation incident on the collector for the month of the year with the smallest ratio of solar radiation to electric load demand."
--Richard N. Chapman, Sandia National Laboratories
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