The [Simple/Slick/Stupid/Smart/../Slimy/Sexy] Sky Dome Projector (SSDP) (have not decided yet which adjective fits best, there is just so many) is a library for modeling irradiance taking into account the local topography. It implements a hexagonal mesh for the sky and the Perez all weather sky model. For the topography it works with a point cloud of x,y,z coordinates or a regular grid. It can compute the horizon as seen from a specific location in the topography and can project the modeled sky dome onto a surface with a specified tilt and orientation. The surface location and orientation may be adapted to the topography (e.g. specify height above the surface and adapt the orientation to the surface normal).
- Implements a uniform sky and the Perez all weather sky model [1]
- variable size hexagonal sky-dome mesh
- sun taken separately as a point source a alleviate sky-mesh resolution requirements
- Computes solar position given the latitude and longitude and the
UTC time (specified as Unix Time) using the spa algorithm [2]
as implemented by freespa [3]
- also supports a manual specification of azimuth and zenith angles
- Simple and fast irradiance computation
- easy specification of location (x,y,z) and orientation (azimuth,zenith) of the incident plane
- Traces incident light
- Crude "one bounce" approximation for albedo (no ray tracing, it is a fully "local" model)
- only computes irradiance at specified locations
- r-tree-based cache for sky with similar orientations
- faster approximate horizon algorithm based on the radial Sobol topography samples
- 2.5D topography
- both unstructured and regular meshes for the topography
- topography specification using georasters with GDAL
- r-tree-based cache for horizon in the same locations
- multiple topography with different resolution levels and covered areas
- input/output in tab-separated and HDF5 formats
As a validation of the irradiance and transposition models in ssdp we compare simulated plane of array irradiance values with pvlib. To this end we take measured GHI, DHI, and POA data from a location somewhere in the south of germany, and use the measured GHI and DHI to predict the POA irradiance. The POA in this case is a 40 degrees tilt, facing south. We assume a ground albedo of 0.25. For this simulation we do not take into account any topography (the data is for an open field) so we only look at irradiance and transposition modeling. For both ssdp and pvlib we use a Perez model (ssdp uses [1] and pvlib a slightly different model [4]). The results are shown in Fig. 1 for ssdp (top) and pvlib (bottom). Both models perform well with a coefficient of determination of more than R2=0.995. Small differences (~0.1% range) are observed and expected due to the different methods and models.
Fig. 1 Transposition and irradiance modeling with (top) ssdp, and (bottom) pvlib
In Fig. 2 we show two examples where we compute shading. The topography comes from LIDAR data. We compute the irradiation intensity on the topography surface. In Fig 2 (top) the surface irradiance is computed at some location for the 15th of june 2015 at 08:00 for a GHI of 500 W/m2 and a DHI of 200 W/m2. In Fig. 2 (bottom) we integrated the irradiance for a complete summer week.
Fig. 2 (top) Example irradiance computation at one particular moment. (bottom) Example for the integrated irradiance over one summer week
The SSDP program comes with autotools build scripts, hence the installation procedure follows the standard:
./configuremake(sudo) make install
This installs both the ssdp library and the ssdp command-line program.
The configure command takes several optional arguments. Specific ssdp options are:
- --enable-openmp: Use openmp palatalization (disabled per default)
- --disable_fastatan2: Disables a fast atan2 approximation
- --enable-gendaylit: Enables the gendaylit modifications to the Perez model
Compiler optimizations:
Do not use unsafe math optimizations. With gcc/clang I would recommend '-O3' and '-march=native', e.g.
./configure CFLAGS='-march=native -O3' --enable-openmp
The ssdp program comes with man pages. If you have the man program and ssdp installed you can do
man ssdp
to read the documentation. Otherwise you can produce a readable manual using the groff program and the ssdp.man file, e.g.:
groff -man -T pdf src/ssdp.man > ssdp_manual.pdf
(assuming your working directory is the root source directory of ssdp)
[1] R. Perez, et al. "All-Weather Model for Sky Luminance Distribution -- Preliminary Configuration and Validation." Solar Energy 50.3 (1993):235-245
[2] R. Ibrahim and A. Afshin "Solar position algorithm for solar radiation applications" Solar Energy 76.5 (2004): 577-589
[3] freespa (insert github address as it is available)
[4] R. Perez, et al. "Modeling daylight availability and irradiance components from direct and global irradiance". Solar Energy 44.5 (1990):271-289.