1r.sun(1) Grass User's Manual r.sun(1)
2
6 r.sun - Solar irradiance and irradiation model.
7 Computes direct (beam), diffuse and reflected solar irradiation raster
8 maps for given day, latitude, surface and atmospheric conditions. Solar
9 parameters (e.g. sunrise, sunset times, declination, extraterrestrial
10 irradiance, daylight length) are saved in the map history file. Alter‐
11 natively, a local time can be specified to compute solar incidence
12 angle and/or irradiance raster maps. The shadowing effect of the topog‐
13 raphy is optionally incorporated.
14
16 raster
17
19 r.sun
20 r.sun help
21 r.sun [-s] elevin=string aspin=string slopein=string [linkein=string]
22 [lin=float] [albedo=string] [alb=float] [latin=string]
23 [lat=float] [coefbh=string] [coefdh=string] [incidout=string]
24 [beam_rad=string] [insol_time=string] [diff_rad=string]
25 [refl_rad=string] day=integer [step=float] [declin=float]
26 [time=float] [--overwrite] [--verbose] [--quiet]
27 Flags:
29 -s
30 Incorporate the shadowing effect of terrain
31 --overwrite
33 Allow output files to overwrite existing files
34 --verbose
36 Verbose module output
37 --quiet
39 Quiet module output
40 Parameters:
42 elevin=string
43 Name of the input elevation raster map [meters]
44 aspin=string
46 Name of the input aspect map (terrain aspect or azimuth of the
47 solar panel) [decimal degrees]
48 slopein=string
50 Name of the input slope raster map (terrain slope or solar panel
51 inclination) [decimal degrees]
52 linkein=string
54 Name of the Linke atmospheric turbidity coefficient input raster
55 map [-]
56 lin=float
58 A single value of the Linke atmospheric turbidity coefficient [-]
59 Default: 3.0
60 albedo=string
62 Name of the ground albedo coefficient input raster map [-]
63 alb=float
65 A single value of the ground albedo coefficient [-]
66 Default: 0.2
67 latin=string
69 Name of the latitudes input raster map [decimal degrees]
70 lat=float
72 A single value of latitude [decimal degrees]
73 coefbh=string
75 Name of real-sky beam radiation coefficient raster map [-]
76 coefdh=string
78 Name of real-sky diffuse radiation coefficient raster map [-]
79 incidout=string
81 Output incidence angle raster map (mode 1 only)
82 beam_rad=string
84 Output beam irradiance [W.m-2] (mode 1) or irradiation raster map
85 [Wh.m-2.day-1] (mode 2)
86 insol_time=string
88 Output insolation time raster map [h] (mode 2 only)
89 diff_rad=string
91 Output diffuse irradiance [W.m-2] (mode 1) or irradiation raster
92 map [Wh.m-2.day-1] (mode 2)
93 refl_rad=string
95 Output ground reflected irradiance [W.m-2] (mode 1) or irradiation
96 raster map [Wh.m-2.day-1] (mode 2)
97 day=integer
99 No. of day of the year (1-365)
100 step=float
102 Time step when computing all-day radiation sums [decimal hours]
103 Default: 0.5
104 declin=float
106 Declination value (overriding the internally computed value) [radi‐
107 ans]
108 time=float
110 Local (solar) time (to be set for mode 1 only) [decimal hours]
111
113 r.sun computes beam (direct), diffuse and ground reflected solar irra‐
114 diation raster maps for given day, latitude, surface and atmospheric
115 conditions. Solar parameters (e.g. time of sunrise and sunset, declina‐
116 tion, extraterrestrial irradiance, daylight length) are stored in the
117 resultant maps' history files. Alternatively, the local time can be
118 specified to compute solar incidence angle and/or irradiance raster
119 maps. The shadowing effect of the topography is optionally incorpo‐
120 rated, a correction factor for shadowing to account for the earth cur‐
121 vature is internally calucated.
122 The units of the parameters are specified in brackets, a hyphen in the
123 brackets explains that the parameter has no units.
124 For latitude-longitude coordinates it requires that the elevation map
126 is in meters. The rules are:
127 lat/lon coordinates: elevation in meters;
129 Other coordinates: elevation in the same unit as the
131 easting-northing coordinates.
132 The solar geometry of the model is based on the works of Krcho (1990),
134 later improved by Jenco (1992). The equations describing Sun –
135 Earth position as well as an interaction of the solar radiation with
136 atmosphere were originally based on the formulas suggested by Kitler
137 and Mikler (1986). This component was considerably updated by the
138 results and suggestions of the working group co-ordinated by Scharmer
139 and Greif (2000) (this algorithm might be replaced by SOLPOS algorithm-
140 library included in GRASS within r.sunmask command). The model computes
141 all three components of global radiation (beam, diffuse and reflected)
142 for the clear sky conditions, i.e. not taking into consideration the
143 spatial and temporal variation of clouds. The extent and spatial reso‐
144 lution of the modelled area, as well as integration over time, are lim‐
145 ited only by the memory and data storage resources. The model is built
146 to fulfil user needs in various fields of science (hydrology, climatol‐
147 ogy, ecology and environmental sciences, photovoltaics, engineering,
148 etc.) for continental, regional up to the landscape scales.
149 As an option the model considers a shadowing effect of the local topog‐
151 raphy. The r.sun program works in two modes. In the first mode it cal‐
152 culates for the set local time a solar incidence angle [degrees] and
153 solar irradiance values [W.m-2]. In the second mode daily sums of
154 solar radiation [Wh.m-2.day-1] are computed within a set day. By a
155 scripting the two modes can be used separately or in a combination to
156 provide estimates for any desired time interval. The model accounts for
157 sky obstruction by local relief features. Several solar parameters are
158 saved in the resultant maps' history files, which may be viewed with
159 the r.info command.
160 The solar incidence angle raster map incidout is computed specifying
162 elevation raster map elevin, aspect raster map aspin, slope steepness
163 raster map slopin, given the day day and local time time. There is no
164 need to define latitude for locations with known and defined projec‐
165 tion/coordinate system (check it with the g.proj command). If you have
166 undefined projection, (x,y) system, etc. then the latitude can be
167 defined explicitely for large areas by input raster map latin with
168 interpolated latitude values or, for smaller areas, a single region
169 latitude value lat can be used instead. All input raster maps must be
170 floating point (FCELL) raster maps. Null data in maps are excluded from
171 the computation (and also speeding-up the computation), so each output
172 raster map will contain null data in cells according to all input
173 raster maps. The user can use r.null command to create/reset null file
174 for your input raster maps.
175 The specified day day is the number of the day of the general year
176 where January 1 is day no.1 and December 31 is 365. Time time must be a
177 local (solar) time (i.e. NOT a zone time, e.g. GMT, CET) in decimal
178 system, e.g. 7.5 (= 7h 30m A.M.), 16.1 = 4h 6m P.M..
179 Setting the solar declination declin by user is an option to override
181 the value computed by the internal routine for the day of the year. The
182 value of geographical latitude can be set as a constant for the whole
183 computed region or, as an option, a grid representing spatially dis‐
184 tributed values over a large region. The geographical latitude must be
185 also in decimal system with positive values for northern hemisphere and
186 negative for southern one. In similar principle the Linke turbidity
187 factor (linkein, lin ) and ground albedo (albedo, alb) can be set.
188 Besides clear-sky radiations, user can compute a real-sky radiation
190 (beam, diffuse) using coefbh and coefdh input raster maps defining the
191 fraction of the respective clear-sky radiations reduced by atmospheric
192 factors (e.g. cloudiness). The value is between 0-1. Usually these
193 coefficients can be obtained from a long-terms meteorological measure‐
194 ments.
195 The solar irradiation or irradiance raster maps beam_rad, diff_rad ,
197 refl_rad are computed for a given day day, latitude lat (latin), eleva‐
198 tion elevin, slope slopein and aspect aspin raster maps. The program
199 uses the Linke atmosphere turbidity factor and ground albedo coeffi‐
200 cient. A default, single value of Linke factor is lin=3.0 and is near
201 the annual average for rural-city areas. The Linke factor for an abso‐
202 lutely clear atmosphere is lin=1.0. See notes below to learn more about
203 this factor. The incidence solar angle is the angle between horizon and
204 solar beam vector. The solar radiation maps for given day are computed
205 integrating the relevant irradiance between sunrise and sunset times
206 for given day. The user can set finer or coarser time step step used
207 for all-day radiation calculations. A default value of step is 0.5
208 hour. Larger steps (e.g. 1.0-2.0) can speed-up calculations but produce
209 less reliable results. The output units are in Wh per squared meter per
210 given day [Wh/(m*m)/day]. The incidence angle and irradiance/irradia‐
211 tion maps can be computed without shadowing influence of relief by
212 default or they can be computed with this influence using the flag -s.
213 In mountainous areas this can lead to very different results! The user
214 should be aware that taken into account the shadowing effect of relief
215 can slow down the speed of computing especially when the sun altitude
216 is low. When considering shadowing effect (flag -s) speed and preci‐
217 sion computing can be controlled by a parameter dist which defines the
218 sampling density at which the visibility of a grid cell is computed in
219 the direction of a path of the solar flow. It also defines the method
220 by which the obstacle's altitude is computed. When choosing dist less
221 than 1.0 (i.e. sampling points will be computed at dist * cellsize dis‐
222 tance), r.sun takes altitude from the nearest grid point. Values above
223 1.0 will use the maximum altitude value found in the nearest 4 sur‐
224 rounding grid points. The default value dist=1.0 should give reasonable
225 results for most cases (e.g. on DEM). Dist value defines a multiplying
226 coefficient for sampling distance. This basic sampling distance equals
227 to the arithmetic average of both cell sizes. The reasonable values are
228 in the range 0.5-1.5. The values below 0.5 will decrease and values
229 above 1.0 will increase the computing speed. Values greater than 2.0
230 may produce estimates with lower accuracy in highly dissected relief.
231 The fully shadowed areas are written to the ouput maps as zero values.
232 Areas with NULL data are considered as no barrier with shadowing effect
233 .
234 The maps' history files are generated containing the following listed
236 parameters used in the computation:
237 - Solar constant 1367 W.m-2
238 - Extraterrestrial irradiance on a plane perpendicular to the solar
239 beam [W.m-2]
240 - Day of the year
241 - Declination [radians]
242 - Decimal hour (Alternative 1 only)
243 - Sunrise and sunset (min-max) over a horizontal plane
244 - Daylight lengths
245 - Geographical latitude (min-max)
246 - Linke turbidity factor (min-max)
247 - Ground albedo (min-max)
248 The user can use a nice shellcript with variable day to compute radia‐
250 tion for some time interval within the year (e.g. vegetation or winter
251 period). Elevation, aspect and slope input values should not be reclas‐
252 sified into coarser categories. This could lead to incorrect results.
253
255 Currently, there are two modes of r.sun. In the first mode it calcu‐
256 lates solar incidence angle and solar irradiance raster maps using the
257 set local time. In the second mode daily sums of solar irradiation
258 [Wh.m-2.day-1] are computed for a specified day.
259
261 Solar energy is an important input parameter in different models con‐
262 cerning energy industry, landscape, vegetation, evapotranspiration,
263 snowmelt or remote sensing. Solar rays incidence angle maps can be
264 effectively used in radiometric and topographic corrections in moun‐
265 tainous and hilly terrain where very accurate investigations should be
266 performed.
267 The clear-sky solar radiation model applied in the r.sun is based on
269 the work undertaken for development of European Solar Radiation Atlas
270 (Scharmer and Greif 2000, Page et al. 2001, Rigollier 2001). The clear
271 sky model estimates the global radiation from the sum of its beam, dif‐
272 fuse and reflected components. The main difference between solar radi‐
273 ation models for inclined surfaces in Europe is the treatment of the
274 diffuse component. In the European climate this component is often the
275 largest source of estimation error. Taking into consideration the
276 existing models and their limitation the European Solar Radiation Atlas
277 team selected the Muneer (1990) model as it has a sound theoretical
278 basis and thus more potential for later improvement.
279 Details of underlying equations used in this program can be found in
281 the reference literature cited below or book published by Neteler and
282 Mitasova: Open Source GIS: A GRASS GIS Approach (published in Kluwer
283 Academic Publishers in 2002).
284 Average monthly values of the Linke turbidity coefficient for a mild
286 climate (see reference literature for your study area):
287 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
288 annual
289 mountains 1.5 1.6 1.8 1.9 2.0 2.3 2.3 2.3 2.1 1.8 1.6 1.5
290 1.90
291 rural 2.1 2.2 2.5 2.9 3.2 3.4 3.5 3.3 2.9 2.6 2.3 2.2
292 2.75
293 city 3.1 3.2 3.5 4.0 4.2 4.3 4.4 4.3 4.0 3.6 3.3 3.1
294 3.75
295 industrial 4.1 4.3 4.7 5.3 5.5 5.7 5.8 5.7 5.3 4.9 4.5 4.2
296 5.00
297 Planned improvements include the use of the SOLPOS algorithm for solar
300 geometry calculations and internal computation of aspect and slope.
301 Shadow maps
303 A map of shadows can be extracted from the solar incidence angle map
304 (incidout). Areas with zero values are shadowed. The -s flag has to be
305 used.
306
308 Nice looking maps can be created with the model's output as follows:
309 g.region rast=elevation.dem
310 r.sun -s elev=elevation.dem slop=slope asp=aspect beam=beam_map day=180
311 r.colors beam_map col=grey
312 d.his i_map=beam_map h_map=elevation.dem
313
316 r.slope.aspect, r.sunmask, g.proj, r.null, v.surf.rst
317
319 Hofierka, J., Suri, M. (2002): The solar radiation model for Open
320 source GIS: implementation and applications. Manuscript submitted to
321 the International GRASS users conference in Trento, Italy, September
322 2002.
323 Hofierka, J. (1997). Direct solar radiation modelling within an open
325 GIS environment. Proceedings of JEC-GI'97 conference in Vienna, Aus‐
326 tria, IOS Press Amsterdam, 575-584.
327 Jenco, M. (1992). Distribution of direct solar radiation on georelief
329 and its modelling by means of complex digital model of terrain (in Slo‐
330 vak). Geograficky casopis, 44, 342-355.
331 Kasten, F. (1996). The Linke turbidity factor based on improved values
333 of the integral Rayleigh optical thickness. Solar Energy, 56 (3),
334 239-244.
335 Kasten, F., Young, A. T. (1989). Revised optical air mass tables and
337 approximation formula. Applied Optics, 28, 4735-4738.
338 Kittler, R., Mikler, J. (1986): Basis of the utilization of solar radi‐
340 ation (in Slovak). VEDA, Bratislava, p. 150.
341 Krcho, J. (1990). Morfometrická analza a digitálne modely
343 georeliéfu (Morphometric analysis and digital models of geore‐
344 lief, in Slovak). VEDA, Bratislava.
345 Muneer, T. (1990). Solar radiation model for Europe. Building services
347 engineering research and technology, 11, 4, 153-163.
348 Neteler, M., Mitasova, H. (2002): Open Source GIS: A GRASS GIS
350 Approach, Kluwer Academic Publishers.
351 Page, J. ed. (1986). Prediction of solar radiation on inclined sur‐
353 faces. Solar energy R&D in the European Community, series F –
354 Solar radiation data, Dordrecht (D. Reidel), 3, 71, 81-83.
355 Page, J., Albuisson, M., Wald, L. (2001). The European solar radiation
357 atlas: a valuable digital tool. Solar Energy, 71, 81-83.
358 Rigollier, Ch., Bauer, O., Wald, L. (2000). On the clear sky model of
360 the ESRA - European Solar radiation Atlas - with respect to the
361 Heliosat method. Solar energy, 68, 33-48.
362 Scharmer, K., Greif, J., eds., (2000). The European solar radiation
364 atlas, Vol. 2: Database and exploitation software. Paris (Les Presses
365 de l’ École des Mines).
366 Joint Research Centre: GIS solar radiation database for Europe and
368 Solar radiation and GIS
369
371 Jaroslav Hofierka, GeoModel, s.r.o. Bratislava, Slovakia
372 Marcel Suri, GeoModel, s.r.o. Bratislava, Slovakia
373 Thomas Huld, JRC, Italy
374 © 2002, Jaroslav Hofierka, Marcel Suri hofierka@geomodel.sk suri@geo‐
375 model.sk
376 Last changed: $Date: 2008-03-21 11:18:05 +0100 (Fri, 21 Mar 2008) $
378 Full index
380 © 2003-2008 GRASS Development Team
382GRASS 6.3.0 r.sun(1)