沙漠研究 19-1, 105- 108 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Solar Resource Potentials of a Very Large Scale PV System
in Sahara DesertMasakazu ITO1, Yuki HAMANO2, Kosuke KUROKAWA1
Abstract: A desert has abundant irradiation and plenty of land. Especially, a gravel desert issuitable for a PV system installation, because it is consisted of flat hard soil. To find suitable
places such as the flat gravel desert for the Very Large Scale PV (VLS-PV) system, the authors
have been developed remote sensing method with satellite images. Land condition isevaluated by using three analyses: ground cover classification, which is statistical calculation;
undulating hills classification, which is extraction of edges of satellite images; and vegetationindex. Then, we applied the method to Sahara desert, and found that 373 TW VLS-PV
systems are possible to be installed, and it can generate 626 PWh/year. This is four times asworld energy demand in 2010.
Keywords: Desert, Remote sensing, Satellite image, VLS-PV

1. Introduction
The Very Large Scale Photovoltaic power generation (VLS-PV) system has been studied in order toresolve the world energy and environmental issues in the Task8 in the Photovoltaic Power Systems
Programme (PVPS) in the International Energy Agency (IEA). The objective is to examine and evaluate
the feasibility of the VLS-PV System, which have a capacity ranging from over multi Megawatt toGigawatt, and develop practical project proposals for demonstrative research toward realization of theVLS-PV Systems in the future. The desert has very large energy resource, because the desert hasabundant solar irradiation and plenty of land. However, the VLS-PV system can be installed in not allarea in desert. For example, sand dune is difficult for installation, and trees should not be damaged. Thegravel desert is suitable, because it consists of small locks and is flat land area.
The authors have been developed a method which extracts suitable areas for the VLS-PV by using
remote sensing with satellite images. The result of the method shows not only suitable area but also
potentials of PV power generations in the desert. This method has been updated from last method(Sakakibara et al., 2005) to give improved accuracy.
2. Satellite images and the method
In this paper, we use two types of satellite images for the method. One is LANDSAT-7/ETM+
images which are available at a web site of the Global Land Cover Facility (GLCF) in University ofMarylandA).
It provides satellite
images for free, however, there is somelimitation.
Another is NOAA/AVHRR images to obtain Normalized
Difference Vegetation Index (NDVI).
This NDVI dataset are possible todownload at web site of the Center for
Environmental
RemoteSensing(CEReS) in Chiba UniversityB). The
yearly maximum NDVI (NDVIymax)
was calculated from the NDVI dataset.
Forsmoothing
yearlyclimatevariability,
averageof
five-year
Fig. 1. Analysis area of Sahara desert in this study.
1

Integrated Research Institute, Tokyo Institute of Technology (Tokyo Tech), 2-12-1 Ookayama, Meguro-ku, Tokyo,152-8550, Japan, E-mail: ito@iri.titech.ac.jp
2
Solar Energy Group, KYOCERA Corporation, Mie Ise Plant 600-10 Shimono-cho, Ise-shi, Mie 516-8510 Japan,
E-mail: yuuki.hamano.ke@kyocera.jp

106

NDVIymax is utilised, since the amount ofNOAA
Landsat 7
precipitation of desert differs widely each year.
NDVI Data set
Satellite images
An analysis area in this study is the Sahara
Conversion bydesert in North Africa as shown in Figure 1.
reflection ratio
2
The analysis area covers 12.5 million km
which is approximately 8 percent of whole land
Ground cover
Undulating hills
Vegetation index
classification
classification
area on the Earth.
To find suitable area for the VLS-PV
system, three analysis methods were developed
Weight and integrate
as shown in Figure 2. We used vegetationindex developed by using NOAA. Landsat
satellite images are also investigated for
estimations of ground cover classification andFig. 2. Scheme of the analysis method.
undulating hills classification. At first, theLandsat satellite images were converted byusing reflection ratio. The ground coverclassification was done by maximum likelihood
procedure.
And the rolling groundclassification was done by Laplacian filter.
The result of three analyses were weighted andintegrated, and we obtained suitable areas for
the VLS-PV systems. In addition, we didground truth to evaluate the method.
For the ground cover classification, weapplied a maximum likelihood estimation
method. Band 2, 3 and 4 were utilised, andFig. 3. Undulating hills extraction.
classified as sand dune, gravel, Steppe, trees,and water area. Training data from the testsite were prepared in advance. Average and variance were used for the calculation. And by using themajority filtering, noises in the results were reduced.
The algorithm of extracting undulating hills was done by filtering which emphasize edges, reducenoises, and cut and emphasize specific frequency. We did the filtering to band 3, because it is data of thenear-infrared and it is suitable for making out geological structures. We used the Laplacian filter for thealgorithm because it can extract not only vertical and horizontal edge but also slanting edge. After that,
we used dilation and erosion. An example of the result is shown in Figure 3.
The Landsat 7 has high resolution, and is good for evaluate geographic condition. However, if we
use the free images, it is difficult to get its images in same season, and image size is small. Therefore, wehave to use satellite images of dry and rainy season at same area’s evaluation. We made a collection
method of seasonal differences, but it is not very good. On the other hand, NOAA AVHRR is low
resolution, but one image cover large
area. In addition, CEReS, Centerfor Environmental Remote Sensing,Gobiin Chiba University is distributing aNegev
NDVI database which consists of 36Sonora
Thardata which are 12 months data andSahara
three 10-days data in a month. AndGreat sandy
the data is from 1980 to 2000.
Figure 4 shows an annual maximum
NDVI. Dark gray shows lower
highvegetationlowvegetations, and white shows higher
Crop
SteppeDesert
0.55
0.45
0.35
0.25
0.15 NDVIvegetation.
We referred to a paper (Ma andFig. 4. The analysis result of annual maximum NDVI.
ymax

107
Ground coverclassification

GravelVegetation
index

NDVI<0.55

SteppeSand dune

Forest,Water

0.15≤NDVI<0.55

NDVI<0.15

Landsat 7
ETM+

NOAA
NDVI dataset

Undulating hills
extraction

0.55<NDVI

Flat

Undulating hills

Suitable land

Not suitable land

Fig. 5. The integration of three classification analyses.

highvegetation
Cropn.a.

0.550.45

Steppe0.35

0.25

lowDesert

0.15

NDVIymax

Fig. 6. Result of the evaluation of the Sahara area.

Veroustraete, 2006), we define three vegetation levels. Desert level (NDVIymax < 0.15): Very littlevegetations. It may seem that almost areas are available for the VLS-PV. However, this very small
NDVIymax is made by mobile sand. Therefore these areas are not gravel areas but sand dune areas.
Steppe level (0.15 ≤ NDVIymax < 0.35): Vegetations can grow in rainy season. The condition is notharder than desert level. Therefore, there are some villages in these areas. Crop level (0.35 ≤NDVIymax < 0.55): This level is crop or grass area. The vegetation level gets higher in rainy season.
But it reduces to steppe level in dry season.
Figure 5 shows the summary of the integration algorithm. At first, the Landsat satellite images weredivided to four categories; gravel, steppe, sand dune, and forest and water. Forest and water areas areevaluated at not suitable land. Second, we evaluate the areas classified as gravel, steppe and sand dune.
The gravel and steppe areas are evaluated by NDVI. If it is lower than 0.55, they go to next step. If not,they are not suitable land. The sand dune areas are also evaluated. If the NDVI is between 0.15 and0.55, they go to next step. Finally, the passed areas are evaluated by the undulating hills extraction. Ifthey are classified as flat land, they are suitable land for VLS-PV. If not, they are not suitable land.
3. Results and Discussion
Figure 6 shows evaluation results by the three analyses. The image area is 12.5 million km2.
Suitable land area for the VLS-PV was: 4.5 thousand km2 at desert level; 5.3 thousand km2 at desert andsteppe level; and 5.5 thousand km2 at desert, steppe and crop level. They were about 36 %, 43 % and
44 % of analyzed area.
To evaluate the accuracy of the algorithm, we did the ground truth. It is comparison between actual
land condition and the results of the evaluation. We went to the Sahara desert, and took photos withlocation data by GPS.
The ground truth in Sahara desert was investigated in September in 2007. Its root was from Djerba
island to Tozeur through Ksar Ghilane, Douz, salt lake, and mountain area. A left figure of Figure 7 isconsists of 4 images which were taken on December, April, May and June. A right figure shows a result
and a track line showing white line. We checked 135 points. The accuracy was 86 %. However, area
ratio which is actual data divided by suitable area was 75 %. We suppose that there are areas of combined
sand dune and gravel area which the algorithm evaluates as sand dune. And because salt lake is an easy
place to evaluate, accuracy there is high.

108vegetation
Cropn.a.

0.55

0.45

0.35

0.25

Desert0.15

210 km

Djerba island

low

SteppeSalt lake

NDVIymax

210 kmhighTrack line

Sand dune
330 ｋｍ

330 kmFig. 7. The ground truth in Tunisia.
Table 1. Analysis result of PV potential.

Area

Analysis area
[103 km2]

Vegetation level

Area ratio[%]

PV capacity[TW]

Sahara12,514

DesertSteppe and lower
Crop and lower

35.9
42.6
43.9

314.2
373.1
384.9

Anuralgeneration[103TWh]
527.3
626.0
645.0

4. Solar energy potential
The 14 % efficiency is the typical efficiency of a multi crystalline silicon PV module. Its output is140 W at standard test conditions which are 1000 W, 25 centigrade, AM1.5. 50 % space factor was set
for distance because of shadow of arrays. The performance ratio of 0.7 is also typical or lower than average
considering desert conditions. Annual yield means the amount of irradiation, which can be calculatedusing an irradiation database. We used NASA’s irradiation database which was prepared by the project
SeaWiFS, Sea-viewing Wide Field-of-view SensorC). This data was evaluated by remote sensing.
Resolution is 2.5˚ × 2.5˚, 280 km × 280 km. Error is ±5W/ m2. The data in 1990 was used. Our
program calculates the irradiation and PV output of each pixel, and annual yield was calculated as 2684 h.
Table 1 shows analysis result of PV potential. For steppe vegetation level, 373 TW PV systems can beinstalled and it generates 626 PWh. It is 4 times as world energy demand in 2010.
5. Conclusions
The suitable area and resources for the VLS-PV systems in deserts have been estimated by the remotesensing approach. The algorithm has been improved by the integrated evaluation approach with threekinds of layers. The PV capacity is calculated with 373 TW in desert and steppe level, and the quantity ofannual generation is 626 PWh. It is enormous and has a potential to mitigate the global warming andenergy issues.
Acknowledgement
The authors thank Mr. Sakakibara who developed basic method of this research when he was in master course.
Annotation
A) Global land Cover Facility web site, http://www.landcover.org/index.shtml
B) Center for Environmental Remote Sensing (CEReS) Chiba University: Twenty-year Global 4-minute
AVHRR NDVI Dataset of Chiba UniversityC) Goddard Institute for Space Studies, New York, N.Y.: SeaWiFS Surface Solar Irradiance
References
Ma Mingguo, Veroustraete Frank (2006): Reconstructing pathfinder AVHRR land NDVI time-series data for the Northwest of China.
Advances in Space Research, 37: 835–840.
Sakakibara K., Ito M., Kurokawa K. (2005): Results of PV Resource Survey for World 6 desert by a Modefied Remote Sensing
Approach. Proc. of the 31th IEEE PVSC, 1808-1811.