 |
|
Shading Analysis
Shade can have a dramatic impact on solar production. Evaluating it is critical before getting too far into the system design process. Various on-site analysis tools and techniques can be used, including viewing reflections from a mirror dome (Solar Pathfinder), multiple digital pictures (Wiley Electronics ASSET) or using a fish-eye lens and digital camera to capture the whole sky in a single image (Solmetric Sun Eye). The result is information about the shading obstacles? elevation versus azimuth. The sun path information described earlier can be overlaid directly onto these views, so that the impact of shading can be determined either graphically or numerically. An example of a polar chart taken with a fish-eye lens is shown in Image 1.
Even for small residential arrays, shading analysis typically requires taking multiple readings at various positions. These readings can then be averaged or processed in simulation programs to modify the energy production estimates for the entire array. For larger commercial and utility scale projects, readings from various locations can be tagged with their GPS coordinates and then compared on a map, like the Google Earth plot map shown in Image 2. Shading data can also be shown as elevation versus azimuth as depicted in Graph 3, using the same data as Image 1. Sun elevation and azimuth are also shown.
Extending Point Measurements
This article focuses on ways of characterizing the solar access from point measurements, for example with the fish-eye lens. Specific modules in an array will experience shading at different times of day and year. Typically, to get a good estimate of a system?s performance, multiple points should be measured in and around the array, such as at each corner of the array. Some techniques and trends for combining multiple point readings are listed below. The different techniques vary in accuracy and complexity.
Interpolation Techniques
Use linear interpolation to estimate the solar access at locations in between measurement points. Precise measurements of the relative locations are necessary to enable accurate interpolation.
3-D Modeling From On-Site Data
In this case, the height and elevation of each obstruction must be known. This can provide some measurement challenges. With few obstructions, this approach is practical, but the complexity increases with many obstructions.
3-D Modeling From Aerial/Satellite Imaging
GIS and mapping technologies are advancing rapidly. Tools like Google Earth, Microsoft Virtual Earth, and ArcGI Explorer are extending our ability to view buildings and obstructions online. In the future, these technologies may provide the 3-D details necessary for initial estimates and may provide a useful complement to on-site evaluations.
ELEVATION ANGLES AD EXTRAPOLATING MEASUREMENTS
Elevation angle is a very useful way to describe obstructions. However, the angle alone may be insufficient to describe the shape and direction of the shade on an array. For a more complete analysis, the distance to the obstruction and its height can be measured. Shading is sometimes quantified in this way. The relationship between elevation angle (θ), distance to the obstruction (D) and height above the measurement plane (H) is shown in Illustration 1.
The California Solar Initiative requires that the shading ratio (D:H) must be at least 2:1. This is equivalent to an elevation angle of less than 26.6°. If a site meets these requirements, it is deemed to have good solar access, and a detailed shade analysis is not required.
An alternative way to specify shading is to determine a site?s shade-free hours, such as 10am?2pm or 9am?3pm. In this case, obstructions are allowed to cast shadows only before or after the specified time period.
The minimum D:H ratio can be specified for the shade- free time periods for a given location. Some example calculations are shown in Table 1. Note that this requirement is worst-case and applies only at the lowest sun elevation of the year within those time windows. This may be too conservative and restrictive for typical pitched roof applications, but it may be useful when considering row spacing in flat-roof or ground-mount system installations.
When collecting shading data, it is possible to take the data in one location and extrapolate it for another. This can allow an analysis using ground-level data by extrapolating up a distance H and over a distance X. This approach is useful and often necessary when taking measurements at a location where the building is not yet constructed or when it is not practical to get shading data from the true height of the proposed array. The calculations can be complex for the full 3-D analysis. For reference, a 2-D equation is shown in Illustration 2. For this equation to apply, the lines must all be coplanar.
Even for small residential arrays, shading analysis typically requires taking multiple readings at various positions. These readings can then be averaged or processed in simulation programs to modify the energy production estimates for the entire array. For larger commercial and utility scale projects, readings from various locations can be tagged with their GPS coordinates and then compared on a map, like the Google Earth plot map shown in Image 2. Shading data can also be shown as elevation versus azimuth as depicted in Graph 3, using the same data as Image 1. Sun elevation and azimuth are also shown.Extending Point Measurements
This article focuses on ways of characterizing the solar access from point measurements, for example with the fish-eye lens. Specific modules in an array will experience shading at different times of day and year. Typically, to get a good estimate of a system?s performance, multiple points should be measured in and around the array, such as at each corner of the array. Some techniques and trends for combining multiple point readings are listed below. The different techniques vary in accuracy and complexity.
Average Multiple Point Measurements
Average the monthly solar access values from each reading to generate 12 numbers that reflect the average monthly solar access for the entire array. The California Solar Initiative program requires that the measurements be taken at the four corners of the array and averaged in this manner. More points can improve accuracy but can be time consuming.
Interpolation Techniques
Use linear interpolation to estimate the solar access at locations in between measurement points. Precise measurements of the relative locations are necessary to enable accurate interpolation.
3-D Modeling From On-Site Data
In this case, the height and elevation of each obstruction must be known. This can provide some measurement challenges. With few obstructions, this approach is practical, but the complexity increases with many obstructions.
3-D Modeling From Aerial/Satellite Imaging
GIS and mapping technologies are advancing rapidly. Tools like Google Earth, Microsoft Virtual Earth, and ArcGI Explorer are extending our ability to view buildings and obstructions online. In the future, these technologies may provide the 3-D details necessary for initial estimates and may provide a useful complement to on-site evaluations.
ELEVATION ANGLES AD EXTRAPOLATING MEASUREMENTS
Elevation angle is a very useful way to describe obstructions. However, the angle alone may be insufficient to describe the shape and direction of the shade on an array. For a more complete analysis, the distance to the obstruction and its height can be measured. Shading is sometimes quantified in this way. The relationship between elevation angle (θ), distance to the obstruction (D) and height above the measurement plane (H) is shown in Illustration 1.
The California Solar Initiative requires that the shading ratio (D:H) must be at least 2:1. This is equivalent to an elevation angle of less than 26.6°. If a site meets these requirements, it is deemed to have good solar access, and a detailed shade analysis is not required.
An alternative way to specify shading is to determine a site?s shade-free hours, such as 10am?2pm or 9am?3pm. In this case, obstructions are allowed to cast shadows only before or after the specified time period.
The minimum D:H ratio can be specified for the shade- free time periods for a given location. Some example calculations are shown in Table 1. Note that this requirement is worst-case and applies only at the lowest sun elevation of the year within those time windows. This may be too conservative and restrictive for typical pitched roof applications, but it may be useful when considering row spacing in flat-roof or ground-mount system installations.
When collecting shading data, it is possible to take the data in one location and extrapolate it for another. This can allow an analysis using ground-level data by extrapolating up a distance H and over a distance X. This approach is useful and often necessary when taking measurements at a location where the building is not yet constructed or when it is not practical to get shading data from the true height of the proposed array. The calculations can be complex for the full 3-D analysis. For reference, a 2-D equation is shown in Illustration 2. For this equation to apply, the lines must all be coplanar.