3
<b>r.sun</b> computes beam (direct), diffuse and ground reflected solar
4
irradiation raster maps for given day, latitude, surface and atmospheric
5
conditions. Solar parameters (e.g. time of sunrise and sunset, declination,
6
extraterrestrial irradiance, daylight length) are stored in the resultant maps'
7
history files. Alternatively, the local time can be specified to compute solar
8
incidence angle and/or irradiance raster maps. The shadowing effect of the
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topography is optionally incorporated, a correction factor for shadowing to account
10
for the earth curvature is internally calucated.<br>
11
The units of the parameters are specified in brackets, a hyphen in the brackets
12
explains that the parameter has no units.
14
For latitude-longitude coordinates it requires that the elevation map is in meters.
17
<li> lat/lon coordinates: elevation in meters;
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<li> Other coordinates: elevation in the same unit as the easting-northing coordinates.
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The solar geometry of the model is based on the works of Krcho (1990), later
23
improved by Jenco (1992). The equations describing Sun – Earth position as
24
well as an interaction of the solar radiation with atmosphere were originally
25
based on the formulas suggested by Kitler and Mikler (1986). This component
26
was considerably updated by the results and suggestions of the working group
27
co-ordinated by Scharmer and Greif (2000) (this algorithm might be replaced
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by SOLPOS algorithm-library included in GRASS within <a href="r.sunmask.html">
30
command). The model computes all three components of global radiation (beam,
31
diffuse and reflected) for the clear sky conditions, i.e. not taking into
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consideration the spatial and temporal variation of clouds. The extent and
33
spatial resolution of the modelled area, as well as integration over time,
34
are limited only by the memory and data storage resources. The model is built
35
to fulfil user needs in various fields of science (hydrology, climatology,
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ecology and environmental sciences, photovoltaics, engineering, etc.) for
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continental, regional up to the landscape scales.
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<p>As an option the model considers a shadowing effect of the local topography.
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The r.sun program works in two modes. In the first mode it calculates for the set
40
local time a solar incidence angle [degrees] and solar irradiance values [W.m-2].
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In the second mode daily sums of solar radiation [Wh.m-2.day-1] are computed
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within a set day. By a scripting the two modes can be used separately or
43
in a combination to provide estimates for any desired time interval. The
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model accounts for sky obstruction by local relief features. Several solar
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parameters are saved in the resultant maps' history files, which may be viewed
46
with the <a href="r.info.html">r.info</a> command.
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<p>The solar incidence angle raster map <i>incidout</i> is computed specifying
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elevation raster map <i>elevin</i>, aspect raster map <i>aspin</i>, slope
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steepness raster map <i>slopin,</i> given the day <i>day</i> and local time
51
<i>time</i>. There is no need to define latitude for locations with known
52
and defined projection/coordinate system (check it with the <a href="g.proj.html">
54
command). If you have undefined projection, (x,y) system, etc. then the latitude
55
can be defined explicitly for large areas by input raster map <i>latin</i>
56
with interpolated latitude values or, for smaller areas, a single region
57
latitude value <i>lat</i> can be used instead. All input raster maps must
58
be floating point (FCELL) raster maps. Null data in maps are excluded from
59
the computation (and also speeding-up the computation), so each output raster
60
map will contain null data in cells according to all input raster maps. The
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user can use <a href="r.null.html">r.null</a>
62
command to create/reset null file for your input raster maps. <br>
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The specified day <i>day</i> is the number of the day of the general year
64
where January 1 is day no.1 and December 31 is 365. Time <i>time</i> must
65
be a local (solar) time (i.e. NOT a zone time, e.g. GMT, CET) in decimal system,
66
e.g. 7.5 (= 7h 30m A.M.), 16.1 = 4h 6m P.M.. </p>
67
<p>Setting the solar declination <i>declin</i> by user is an option to override
68
the value computed by the internal routine for the day of the year. The value
69
of geographical latitude can be set as a constant for the whole computed
70
region or, as an option, a grid representing spatially distributed values
71
over a large region. The geographical latitude must be also in decimal system
72
with positive values for northern hemisphere and negative for southern one.
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In similar principle the Linke turbidity factor (<i>linkein</i>, <i>lin</i>
74
) and ground albedo (<i>albedo</i>, <i>alb</i>) can be set. </p>
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<p>Besides clear-sky radiations, user can compute a real-sky radiation (beam,
76
diffuse) using <i>coefbh</i> and <i>coefdh </i>input raster maps defining
77
the fraction of the respective clear-sky radiations reduced by atmospheric
78
factors (e.g. cloudiness). The value is between 0-1. Usually these
79
coefficients can be obtained from a long-terms meteorological measurements.</p>
80
<p>The solar irradiation or irradiance raster maps <i>beam_rad</i>, <i>diff_rad</i>
81
, <i>refl_rad</i> are computed for a given day <i>day,</i> latitude <i>lat
82
(latin), </i>elevation <i>elevin</i>, slope <i>slopein</i> and aspect <i>
83
aspin</i> raster maps. The program uses the Linke atmosphere turbidity factor
84
and ground albedo coefficient. A default, single value of Linke factor is
85
<i>lin</i>=3.0 and is near the annual average for rural-city areas. The Linke
86
factor for an absolutely clear atmosphere is <i>lin</i>=1.0. See notes below
87
to learn more about this factor. The incidence solar angle is the angle between
88
horizon and solar beam vector. The solar radiation maps for given day are
89
computed integrating the relevant irradiance between sunrise and sunset times
90
for given day. The user can set finer or coarser time step <i>step</i> used
91
for all-day radiation calculations. A default value of <i>step</i> is 0.5
92
hour. Larger steps (e.g. 1.0-2.0) can speed-up calculations but produce less
93
reliable results. The output units are in Wh per squared meter per given
94
day [Wh/(m*m)/day]. The incidence angle and irradiance/irradiation maps can
95
be computed without shadowing influence of relief by default or they can
96
be computed with this influence using the flag <i>-s</i>. In mountainous areas
97
this can lead to very different results! The user should be aware that taken
98
into account the shadowing effect of relief can slow
99
down the speed of computing especially when the sun altitude is low.
100
When considering shadowing effect (flag <i>-s</i>) speed and precision computing
101
can be controlled by a parameter <i>dist</i> which defines the sampling density
102
at which the visibility of a grid cell is computed in the direction of a
103
path of the solar flow. It also defines the method by which the obstacle's
104
altitude is computed. When choosing <i>dist</i> less than 1.0 (i.e. sampling
105
points will be computed at <i>dist</i> * cellsize distance), r.sun takes
106
altitude from the nearest grid point. Values above 1.0 will use the maximum
107
altitude value found in the nearest 4 surrounding grid points. The default
108
value <i>dist</i>=1.0 should give reasonable results for most cases (e.g.
109
on DEM). <i>Dist</i> value defines a multiplying coefficient for sampling
110
distance. This basic sampling distance equals to the arithmetic average of
111
both cell sizes. The reasonable values are in the range 0.5-1.5. The values
112
below 0.5 will decrease and values above 1.0 will increase the computing
113
speed. Values greater than 2.0 may produce estimates with lower accuracy
114
in highly dissected relief. The fully shadowed areas are written to the ouput
115
maps as zero values. Areas with NULL data are considered as no barrier with
116
shadowing effect .</p>
117
<p>The maps' history files are generated containing the following listed
118
parameters used in the computation: <br>
119
- Solar constant 1367 W.m-2 <br>
120
- Extraterrestrial irradiance on a plane perpendicular to the solar beam
122
- Day of the year <br>
123
- Declination [radians] <br>
124
- Decimal hour (Alternative 1 only) <br>
125
- Sunrise and sunset (min-max) over a horizontal plane <br>
126
- Daylight lengths <br>
127
- Geographical latitude (min-max) <br>
128
- Linke turbidity factor (min-max) <br>
129
- Ground albedo (min-max) </p>
130
<p>The user can use a nice shellcript with variable
131
day to compute radiation for some time interval within the year (e.g. vegetation
132
or winter period). Elevation, aspect and slope input values should not be
133
reclassified into coarser categories. This could lead to incorrect results.
137
<p>Currently, there are two modes of r.sun.
138
In the first mode it calculates solar incidence angle and solar irradiance
139
raster maps using the set local time. In the second mode daily sums of solar
140
irradiation [Wh.m-2.day-1] are computed for a specified day.</p>
145
Solar energy is an important input parameter in different models concerning
146
energy industry, landscape, vegetation, evapotranspiration, snowmelt or remote
147
sensing. Solar rays incidence angle maps can be effectively used in radiometric
148
and topographic corrections in mountainous and hilly terrain where very accurate
149
investigations should be performed.
151
The clear-sky solar radiation model applied in the r.sun is based on the
152
work undertaken for development of European Solar Radiation Atlas (Scharmer
153
and Greif 2000, Page et al. 2001, Rigollier 2001). The clear sky model estimates
154
the global radiation from the sum of its beam, diffuse and reflected components.
155
The main difference between solar radiation models for inclined surfaces
156
in Europe is the treatment of the diffuse component. In the European climate
157
this component is often the largest source of estimation error. Taking into
158
consideration the existing models and their limitation the European Solar
159
Radiation Atlas team selected the Muneer (1990) model as it has a sound theoretical
160
basis and thus more potential for later improvement. </p>
162
Details of underlying equations used in this program can be found in the
163
reference literature cited below or book published by Neteler and Mitasova:
164
Open Source GIS: A GRASS GIS Approach (published in Kluwer Academic Publishers
167
Average monthly values of the Linke turbidity coefficient for a mild climate
168
(see reference literature for your study area): </p>
170
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec annual<br>
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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 1.90 <br>
172
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 2.75 <br>
173
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 3.75 <br>
174
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 5.00
177
Planned improvements include the use of the SOLPOS algorithm for solar
178
geometry calculations and internal computation of aspect and slope.
181
A map of shadows can be extracted from the solar incidence angle map
182
(incidout). Areas with zero values are shadowed. The <em>-s</em> flag
187
Nice looking maps can be created with the model's output as follows:
188
<div class="code"><pre>
190
g.region rast=elevation.dem
191
r.sun -s elev=elevation.dem slop=slope asp=aspect beam=beam_map day=180
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r.colors beam_map col=grey
193
d.his i_map=beam_map h_map=elevation.dem
199
<a href="r.slope.aspect.html">r.slope.aspect</a>,
200
<a href="r.sunmask.html">r.sunmask</a>,
201
<a href="g.proj.html">g.proj</a>,
202
<a href="r.null.html">r.null</a>,
203
<a href="v.surf.rst.html">v.surf.rst</a>
208
Hofierka, J., Suri, M. (2002): The solar radiation model for Open source
209
GIS: implementation and applications. Manuscript submitted to the International
210
GRASS users conference in Trento, Italy, September 2002.
212
Hofierka, J. (1997). Direct solar radiation modelling within an open GIS
213
environment. Proceedings of JEC-GI'97 conference in Vienna, Austria, IOS
214
Press Amsterdam, 575-584. </p>
216
Jenco, M. (1992). Distribution of direct solar radiation on georelief and
217
its modelling by means of complex digital model of terrain (in Slovak). Geograficky
218
casopis, 44, 342-355. </p>
220
Kasten, F. (1996). The Linke turbidity factor based on improved values of
221
the integral Rayleigh optical thickness. Solar Energy, 56 (3), 239-244. </p>
223
Kasten, F., Young, A. T. (1989). Revised optical air mass tables and approximation
224
formula. Applied Optics, 28, 4735-4738. </p>
226
Kittler, R., Mikler, J. (1986): Basis of the utilization of solar radiation
227
(in Slovak). VEDA, Bratislava, p. 150. </p>
229
Krcho, J. (1990). Morfometrická analza a digitálne modely georeliéfu
230
(Morphometric analysis and digital models of georelief, in Slovak).
231
VEDA, Bratislava.</p>
233
Muneer, T. (1990). Solar radiation model for Europe. Building services engineering
234
research and technology, 11, 4, 153-163. </p>
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Neteler, M., Mitasova, H. (2002): Open Source GIS: A GRASS GIS Approach, Kluwer
237
Academic Publishers. </p>
239
Page, J. ed. (1986). Prediction of solar radiation on inclined surfaces. Solar
240
energy R&D in the European Community, series F – Solar radiation data,
241
Dordrecht (D. Reidel), 3, 71, 81-83. </p>
243
Page, J., Albuisson, M., Wald, L. (2001). The European solar radiation atlas:
244
a valuable digital tool. Solar Energy, 71, 81-83. </p>
246
Rigollier, Ch., Bauer, O., Wald, L. (2000). On the clear sky model of the
247
ESRA - European Solar radiation Atlas - with respect to the Heliosat method.
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Solar energy, 68, 33-48. </p>
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Scharmer, K., Greif, J., eds., (2000). The European solar radiation atlas,
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Vol. 2: Database and exploitation software. Paris (Les Presses de l’ École
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<p>Joint Research Centre: <a href="http://re.jrc.ec.europa.eu/pvgis/">GIS solar radiation database for Europe</a> and
255
<a href="http://re.jrc.ec.europa.eu/pvgis/solres/solmod3.htm">Solar radiation and GIS</a>
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Jaroslav Hofierka, GeoModel, s.r.o. Bratislava, Slovakia <br>
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Marcel Suri, GeoModel, s.r.o. Bratislava, Slovakia <br>
264
Thomas Huld, JRC, Italy <br>
266
© 2002, Jaroslav Hofierka, Marcel Suri
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<a href="MAILTO:hofi@geomodel.sk">hofierka@geomodel.sk</a>
269
<a href="MAILTO:suri@geomodel.sk">suri@geomodel.sk</a>
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<p><i>Last changed: $Date: 2010-01-15 15:57:25 +0100 (Fri, 15 Jan 2010) $</i> </p>