1r.sun(1) Grass User's Manual r.sun(1)
2
6 r.sun - Computes direct (beam), diffuse and reflected solar irradia‐
7 tion raster maps for given day, latitude, surface and atmospheric con‐
8 ditions. Solar parameters (e.g. sunrise, sunset times, declination,
9 extraterrestrial irradiance, daylight length) are saved in the map his‐
10 tory file. Alternatively, a local time can be specified to compute
11 solar incidence angle and/or irradiance raster maps. The shadowing
12 effect of the topography is optionally incorporated.
13
15 raster
16
18 r.sun
19 r.sun help
20 r.sun [-s] elevin=string aspin=string slopein=string [linkein=string]
21 [lin=float] [albedo=string] [alb=float] [latin=string]
22 [lat=float] [coefbh=string] [coefdh=string] [incidout=string]
23 [beam_rad=string] [insol_time=string] [diff_rad=string]
24 [refl_rad=string] day=integer [step=float] [declin=float]
25 [time=float] [--overwrite]
26 Flags:
28 -s Incorporate the shadowing effect of terrain
29 --overwrite
31 Parameters:
33 elevin=string
34 Name of the input elevation raster map [meters]
35 aspin=string
37 Name of the input aspect map (terrain aspect or azimuth of the
38 solar panel) [decimal degrees]
39 slopein=string
41 Name of the input slope raster map (terrain slope or solar panel
42 inclination) [decimal degrees]
43 linkein=string
45 Name of the Linke atmospheric turbidity coefficient input raster
46 map [-]
47 lin=float
49 A single value of the Linke atmospheric turbidity coefficient [-]
50 Default: 3.0
51 albedo=string
53 Name of the ground albedo coefficient input raster map [-]
54 alb=float
56 A single value of the ground albedo coefficient [-] Default: 0.2
57 latin=string
59 Name of the latitudes input raster map [decimal degrees]
60 lat=float
62 A single value of latitude [decimal degrees]
63 coefbh=string
65 Name of real-sky beam radiation coefficient raster map [-]
66 coefdh=string
68 Name of real-sky diffuse radiation coefficient raster map [-]
69 incidout=string
71 Output incidence angle raster map (mode 1 only)
72 beam_rad=string
74 Output beam irradiance [W.m-2] (mode 1) or irradiation raster file
75 [Wh.m-2.day-1] (mode 2)
76 insol_time=string
78 Output insolation time raster map [h] (mode 2 only)
79 diff_rad=string
81 Output diffuse irradiance [W.m-2] (mode 1) or irradiation raster
82 map [Wh.m-2.day-1] (mode 2)
83 refl_rad=string
85 Output ground reflected irradiance [W.m-2] (mode 1) or irradiation
86 raster map [Wh.m-2.day-1] (mode 2)
87 day=integer
89 No. of day of the year (1-365)
90 step=float
92 Time step when computing all-day radiation sums [decimal hours]
93 Default: 0.5
94 declin=float
96 Declination value (overriding the internally computed value) [radi‐
97 ans]
98 time=float
100 Local (solar) time (to be set for mode 1 only) [decimal hours]
101
103 r.sun computes beam (direct), diffuse and ground reflected solar irra‐
104 diation raster maps for given day, latitude, surface and atmospheric
105 conditions. Solar parameters (e.g. time of sunrise and sunset, declina‐
106 tion, extraterrestrial irradiance, daylight length) are stored in the
107 resultant maps' history files. Alternatively, the local time can be
108 specified to compute solar incidence angle and/or irradiance raster
109 maps. The shadowing effect of the topography is optionally incorpo‐
110 rated, a correction factor for shadowing to account for the earth cur‐
111 vature is internally calucated.
112 The units of the parameters are specified in brackets, a hyphen in the
113 brackets explains that the parameter has no units.
114 The solar geometry of the model is based on the works of Krcho (1990),
116 later improved by Jenco (1992). The equations describing Sun –
117 Earth position as well as an interaction of the solar radiation with
118 atmosphere were originally based on the formulas suggested by Kitler
119 and Mikler (1986). This component was considerably updated by the
120 results and suggestions of the working group co-ordinated by Scharmer
121 and Greif (2000) (this algorithm might be replaced by SOLPOS algorithm-
122 library included in GRASS within r.sunmask command). The model computes
123 all three components of global radiation (beam, diffuse and reflected)
124 for the clear sky conditions, i.e. not taking into consideration the
125 spatial and temporal variation of clouds. The extent and spatial reso‐
126 lution of the modelled area, as well as integration over time, are lim‐
127 ited only by the memory and data storage resources. The model is built
128 to fulfil user needs in various fields of science (hydrology, climatol‐
129 ogy, ecology and environmental sciences, photovoltaics, engineering,
130 etc.) for continental, regional up to the landscape scales.
131 As an option the model considers a shadowing effect of the local topog‐
133 raphy. The r.sun program works in two modes. In the first mode it cal‐
134 culates for the set local time a solar incidence angle [degrees] and
135 solar irradiance values [W.m-2]. In the second mode daily sums of
136 solar radiation [Wh.m-2.day-1] are computed within a set day. By a
137 scripting the two modes can be used separately or in a combination to
138 provide estimates for any desired time interval. The model accounts for
139 sky obstruction by local relief features. Several solar parameters are
140 saved in the resultant maps' history files, which may be viewed with
141 the r.info command.
142 The solar incidence angle raster map incidout is computed specifying
144 elevation raster map elevin, aspect raster map aspin, slope steepness
145 raster map slopin, given the day day and local time time. There is no
146 need to define latitude for locations with known and defined projec‐
147 tion/coordinate system (check it with the g.proj command). If you have
148 undefined projection, (x,y) system, etc. then the latitude can be
149 defined explicitely for large areas by input raster map latin with
150 interpolated latitude values or, for smaller areas, a single region
151 latitude value lat can be used instead. All input raster maps must be
152 floating point (FCELL) raster maps. Null data in maps are excluded from
153 the computation (and also speeding-up the computation), so each output
154 raster map will contain null data in cells according to all input
155 raster maps. The user can use r.null command to create/reset null file
156 for your input raster maps.
157 The specified day day is the number of the day of the general year
158 where January 1 is day no.1 and December 31 is 365. Time time must be a
159 local (solar) time (i.e. NOT a zone time, e.g. GMT, CET) in decimal
160 system, e.g. 7.5 (= 7h 30m A.M.), 16.1 = 4h 6m P.M..
161 Setting the solar declination declin by user is an option to override
163 the value computed by the internal routine for the day of the year. The
164 value of geographical latitude can be set as a constant for the whole
165 computed region or, as an option, a grid representing spatially dis‐
166 tributed values over a large region. The geographical latitude must be
167 also in decimal system with positive values for northern hemisphere and
168 negative for southern one. In similar principle the Linke turbidity
169 factor (linkein, lin ) and ground albedo (albedo, alb) can be set.
170 Besides clear-sky radiations, user can compute a real-sky radiation
172 (beam, diffuse) using coefbh and coefdh input raster maps defining the
173 fraction of the respective clear-sky radiations reduced by atmospheric
174 factors (e.g. cloudiness). The value is between 0-1. Usually these
175 coefficients can be obtained from a long-terms meteorological measure‐
176 ments.
177 The solar irradiation or irradiance raster maps beam_rad, diff_rad ,
179 refl_rad are computed for a given day day, latitude lat (latin), eleva‐
180 tion elevin, slope slopein and aspect aspin raster files. The program
181 uses the Linke atmosphere turbidity factor and ground albedo coeffi‐
182 cient. A default, single value of Linke factor is lin=3.0 and is near
183 the annual average for rural-city areas. The Linke factor for an abso‐
184 lutely clear atmosphere is lin=1.0. See notes below to learn more about
185 this factor. The incidence solar angle is the angle between horizon and
186 solar beam vector. The solar radiation maps for given day are computed
187 integrating the relevant irradiance between sunrise and sunset times
188 for given day. The user can set finer or coarser time step step used
189 for all-day radiation calculations. A default value of step is 0.5
190 hour. Larger steps (e.g. 1.0-2.0) can speed-up calculations but produce
191 less reliable results. The output units are in Wh per squared meter per
192 given day [Wh/(m*m)/day]. The incidence angle and irradiance/irradia‐
193 tion maps can be computed without shadowing influence of relief by
194 default or they can be computed with this influence using the flag -s.
195 In mountainous areas this can lead to very different results! The user
196 should be aware that taken into account the shadowing effect of relief
197 can slow down the speed of computing especially when the sun altitude
198 is low. When considering shadowing effect (flag -s) speed and preci‐
199 sion computing can be controlled by a parameter dist which defines the
200 sampling density at which the visibility of a grid cell is computed in
201 the direction of a path of the solar flow. It also defines the method
202 by which the obstacle's altitude is computed. When choosing dist less
203 than 1.0 (i.e. sampling points will be computed at dist * cellsize dis‐
204 tance), r.sun takes altitude from the nearest grid point. Values above
205 1.0 will use the maximum altitude value found in the nearest 4 sur‐
206 rounding grid points. The default value dist=1.0 should give reasonable
207 results for most cases (e.g. on DEM). Dist value defines a multiplying
208 coefficient for sampling distance. This basic sampling distance equals
209 to the arithmetic average of both cell sizes. The reasonable values are
210 in the range 0.5-1.5. The values below 0.5 will decrease and values
211 above 1.0 will increase the computing speed. Values greater than 2.0
212 may produce estimates with lower accuracy in highly dissected relief.
213 The fully shadowed areas are written to the ouput maps as zero values.
214 Areas with NULL data are considered as no barrier with shadowing effect
215 .
216 The maps' history files are generated containing the following listed
218 parameters used in the computation:
219 - Solar constant 1367 W.m-2
220 - Extraterrestrial irradiance on a plane perpendicular to the solar
221 beam [W.m-2]
222 - Day of the year
223 - Declination [radians]
224 - Decimal hour (Alternative 1 only)
225 - Sunrise and sunset (min-max) over a horizontal plane
226 - Daylight lengths
227 - Geographical latitude (min-max)
228 - Linke turbidity factor (min-max)
229 - Ground albedo (min-max)
230 The user can use a nice shellcript with variable day to compute radia‐
232 tion for some time interval within the year (e.g. vegetation or winter
233 period). Elevation, aspect and slope input values should not be reclas‐
234 sified into coarser categories. This could lead to incorrect results.
235
237 Currently, there are two modes of r.sun. In the first mode it calcu‐
238 lates solar incidence angle and solar irradiance raster maps using the
239 set local time. In the second mode daily sums of solar irradiation
240 [Wh.m-2.day-1] are computed for a specified day.
241
243 Solar energy is an important input parameter in different models con‐
244 cerning energy industry, landscape, vegetation, evapotranspiration,
245 snowmelt or remote sensing. Solar rays incidence angle maps can be
246 effectively used in radiometric and topographic corrections in moun‐
247 tainous and hilly terrain where very accurate investigations should be
248 performed.
249 The clear-sky solar radiation model applied in the r.sun is based on
251 the work undertaken for development of European Solar Radiation Atlas
252 (Scharmer and Greif 2000, Page et al. 2001, Rigollier 2001). The clear
253 sky model estimates the global radiation from the sum of its beam, dif‐
254 fuse and reflected components. The main difference between solar radi‐
255 ation models for inclined surfaces in Europe is the treatment of the
256 diffuse component. In the European climate this component is often the
257 largest source of estimation error. Taking into consideration the
258 existing models and their limitation the European Solar Radiation Atlas
259 team selected the Muneer (1990) model as it has a sound theoretical
260 basis and thus more potential for later improvement.
261 Details of underlying equations used in this program can be found in
263 the reference literature cited below or book published by Neteler and
264 Mitasova: Open Source GIS: A GRASS GIS Approach (published in Kluwer
265 Academic Publishers in 2002).
266 Average monthly values of the Linke turbidity coefficient for a mild
268 climate (see reference literature for your study area):
269 Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
270 annual
271 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
272 1.90
273 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
274 2.75
275 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
276 3.75
277 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
278 5.00
279 Planned improvements include the use of the SOLPOS algorithm for solar
282 geometry calculations and internal computation of aspect and slope.
283 Shadow maps
285 A map of shadows can be extracted from the solar incidence angle map
286 (incidout). Areas with zero values are shadowed. The -s flag has to be
287 used.
288
290 Nice looking maps can be created with the model's output as follows:
291 g.region rast=elevation.dem
292 r.sun -s elev=elevation.dem slop=slope asp=aspect beam=beam_map day=180
293 r.colors beam_map col=grey
294 d.his i_map=beam_map h_map=elevation.dem
295
298 r.slope.aspect, r.sunmask, g.proj, r.null, v.surf.rst
299
301 Hofierka, J., Suri, M. (2002): The solar radiation model for Open
302 source GIS: implementation and applications. Manuscript submitted to
303 the International GRASS users conference in Trento, Italy, September
304 2002.
305 Hofierka, J. (1997). Direct solar radiation modelling within an open
307 GIS environment. Proceedings of JEC-GI'97 conference in Vienna, Aus‐
308 tria, IOS Press Amsterdam, 575-584.
309 Jenco, M. (1992). Distribution of direct solar radiation on georelief
311 and its modelling by means of complex digital model of terrain (in Slo‐
312 vak). Geograficky casopis, 44, 342-355.
313 Kasten, F. (1996). The Linke turbidity factor based on improved values
315 of the integral Rayleigh optical thickness. Solar Energy, 56 (3),
316 239-244.
317 Kasten, F., Young, A. T. (1989). Revised optical air mass tables and
319 approximation formula. Applied Optics, 28, 4735-4738.
320 Kittler, R., Mikler, J. (1986): Basis of the utilization of solar radi‐
322 ation (in Slovak). VEDA, Bratislava, p. 150.
323 Krcho, J. (1990). Morfometrická analza a digitálne modely
325 georeliéfu (Morphometric analysis and digital models of geore‐
326 lief, in Slovak). VEDA, Bratislava.
327 Muneer, T. (1990). Solar radiation model for Europe. Building services
329 engineering research and technology, 11, 4, 153-163.
330 Neteler, M., Mitasova, H. (2002): Open Source GIS: A GRASS GIS
332 Approach, Kluwer Academic Publishers.
333 Page, J. ed. (1986). Prediction of solar radiation on inclined sur‐
335 faces. Solar energy R&D in the European Community, series F –
336 Solar radiation data, Dordrecht (D. Reidel), 3, 71, 81-83.
337 Page, J., Albuisson, M., Wald, L. (2001). The European solar radiation
339 atlas: a valuable digital tool. Solar Energy, 71, 81-83.
340 Rigollier, Ch., Bauer, O., Wald, L. (2000). On the clear sky model of
342 the ESRA - European Solar radiation Atlas - with respect to the
343 Heliosat method. Solar energy, 68, 33-48.
344 Scharmer, K., Greif, J., eds., (2000). The European solar radiation
346 atlas, Vol. 2: Database and exploitation software. Paris (Les Presses
347 de l’ École des Mines).
348 Joint Research Centre: GIS solar radiation database for Europe and
350 Solar radiation and GIS
351
353 Jaroslav Hofierka, GeoModel, s.r.o. Bratislava, Slovakia
354 Marcel Suri, GeoModel, s.r.o. Bratislava, Slovakia
355 Š 2002, Jaroslav Hofierka, Marcel Suri hofierka@geomodel.sk suri@geo‐
356 model.sk
357 Last changed: $Date: 2006/05/24 02:49:34 $
359 Full index
361GRASS 6.2.2 r.sun(1)