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Site Specific Weather
SITE SPECIFIC WEATHER PATTERN QUANTIFICATION
Resources:
The insolation values for specific sites are heavily influenced by the weather patterns for that particular location. Various weather and insolation data is available for locations worldwide.
The National Renewable Energy Laboratory recently released updated weather data for 1,020 locations in the US. The database, known as TMY3, records insolation during all hours of the year and can be very helpful in estimating weather-corrected insolation for a given site.
The effects of shading and weather can be observed using an annual insolation chart. Graph 4a shows a plot of annual insolation vs. tilt and azimuth values for Sacramento, California, without shading. Graph 4b incorporates significant shade to the east, as in the Skyline shown in Image
1. Notice that the optimum value shifts in azimuth and tilt, and the optimum available insolation is reduced. Sacramento's insolation chart is close to symmetrical around the southern direction, which indicates that morning and afternoon insolation is similar. Honolulu, Hawaii, shown in Graph 5, reveals a significant eastward shift in the optimum values, indicating significantly less insolation in the afternoon than in the morning, most likely due to patterns of afternoon clouds and rain. The impact of climate change on insolation levels could be significant over an array's 30-year operational life. Although general trends may be clear, precise modeling is difficult and results may be controversial. General trends, such as drier or wetter, could provide a qualitative indicator of either higher or lower solar production.
THE BOTTOM LINE
The National Renewable Energy Laboratory recently released updated weather data for 1,020 locations in the US. The database, known as TMY3, records insolation during all hours of the year and can be very helpful in estimating weather-corrected insolation for a given site.
The effects of shading and weather can be observed using an annual insolation chart. Graph 4a shows a plot of annual insolation vs. tilt and azimuth values for Sacramento, California, without shading. Graph 4b incorporates significant shade to the east, as in the Skyline shown in Image
1. Notice that the optimum value shifts in azimuth and tilt, and the optimum available insolation is reduced. Sacramento's insolation chart is close to symmetrical around the southern direction, which indicates that morning and afternoon insolation is similar. Honolulu, Hawaii, shown in Graph 5, reveals a significant eastward shift in the optimum values, indicating significantly less insolation in the afternoon than in the morning, most likely due to patterns of afternoon clouds and rain. The impact of climate change on insolation levels could be significant over an array's 30-year operational life. Although general trends may be clear, precise modeling is difficult and results may be controversial. General trends, such as drier or wetter, could provide a qualitative indicator of either higher or lower solar production.
THE BOTTOM LINE
The financial plan for a photovoltaic or solar thermal project involves costs and benefits, and a detailed site evaluation can make or break a project's success. Contractors, investors and clients will all benefit from an open, honest review of the solar access data upfront to avoid unwanted surprises during or after project implementation. The site assessment itself represents a project cost.
Depending on the size of the array and complexity of the site a thorough site assessment can add a few hundred to a few thousand dollars to the project. In the precontract phase, contractors are typically not getting paid for this work, so quick results with sufficient accuracy are critical. Often a preliminary site analysis is performed as part of the sales process with a more detailed analysis after a contract is signed. Solar energy project success requires a good site assessment. Location, panel orientation, weather and shading all interact to influence solar access and therefore energy production potential. With the right tools and techniques, solar installers can dramatically improve the chances of their project's success.
Mark Galli / Solmetric / Bolinas, CA / mark@solmetric.com
Peter Hoberg / Solmetric / Bolinas, CA / peter@solmetric.com /
Depending on the size of the array and complexity of the site a thorough site assessment can add a few hundred to a few thousand dollars to the project. In the precontract phase, contractors are typically not getting paid for this work, so quick results with sufficient accuracy are critical. Often a preliminary site analysis is performed as part of the sales process with a more detailed analysis after a contract is signed. Solar energy project success requires a good site assessment. Location, panel orientation, weather and shading all interact to influence solar access and therefore energy production potential. With the right tools and techniques, solar installers can dramatically improve the chances of their project's success.
Mark Galli / Solmetric / Bolinas, CA / mark@solmetric.com
Peter Hoberg / Solmetric / Bolinas, CA / peter@solmetric.com /
Resources:
- California Solar Initiative
- National Renewable Energy Laboratory /(photovoltaic performance calculator)
- National Renewable Energy Laboratory, TMY 3 /(weather data)
- University of Oregon Solar Radiation Monitoring Laboratory / (sun path chart program)
- University of Oregon Solar Radiation Monitoring Laboratory / (polar sun path chart program)