Analysis of a solarimetric database for Mexico and comparison with the CSR model
Introduction
In recent years the solar industry is experiencing a growth not seen before. Solar energy technologies, both photovoltaic and thermal, are being deployed at accelerated rates around the world. Reliable solar resource information is a crucial input to the successful planning of solar projects, and therefore increased effort is necessary to evaluate this resource where enough quality data is lacking.
It is a common place to say that Mexico has a very strong potential for the utilization of solar technologies due to its high insolation, which is by far its largest renewable energy resource. The country is located in a privileged position in the northern hemisphere, between latitudes 14° 32′ and 32° 43′, including the line of the tropic of Cancer, having nearly 2 million square kilometers area. In early works, it was estimated that average solar irradiance over this territory is higher than the 4.6 KWh/m²/day [1]. However, to date there is a lack of precise knowledge about this resource. Therefore, in order to realize the very important potential of the country, it is essential to gain a better knowledge of the distribution of solar radiation on the territory.
Several maps and tables of solar radiation in Mexico have been published by different authors [1], [2], [3], [4], [5], [6]. However, all of them are based on estimating methods of different types, which have been compared to ground measured solarimetric data for only a handful of locations in the country. Therefore, it is presumed that uncertainties of these models are high [7]. As new and more precise methods are being developed for the indirect evaluation of the solar resource by several researchers [8], [9], [10], [11], [12], there is an increasing need of validated data that can be used for comparison purposes.
The Mexican National Weather Service (SMN; Servicio Meteorológico Nacional) has several networks of sensors to measure climate parameters. Two decades ago SMN initiated an automatic network of atmospheric monitoring stations (EMAS), each of them with a thermopile pyranometer for measuring global solar radiation. The location of these stations is not optimal for the needs of solar resource assessment, because their distribution was made with hydro-meteorological criteria. However, their large number (currently, 136 stations) provides a wide coverage of the different natural regions (Fig. 1) and could fulfill this purpose. Unfortunately, a periodic calibration program was not established for the radiometers in this network, and the equipment has not been calibrated since installation. However, it is to be expected that the first years of operation of each sensor represent reliable information that could be used for the calibration of empirical model parameters related to geographic and climatic conditions for each station [13].
Two years ago, a project to analyze the existing database from the EMAS and to calibrate the radiometers was funded by the Secretaría de Energía (SENER) and the Consejo Nacional de Ciencia y Tecnología (CONACYT). This is a joint project between SMN and the Instituto de Geofísica of Universidad Nacional Autónoma de México (IGF-UNAM), which is recognized as a Regional Radiation Center for the region RA-IV by the World Meteorological Organization, and keeps an absolute radiometer calibrated with the set of radiometers from the world radiation center [14]. One of the main conclusions to date is that the database as a whole shows many problems and inconsistencies. A second project, financed by UNAM is in course, to identify a subset of good quality data from this database that can be used for the purpose of preliminary validation of solar radiation models. In the present work preliminary results from the assessment of the SMN solarimetric database are presented, as well as a comparison with data calculated with the Climatological Solar Radiation model (CSR), which available to the public from the SWERA web site, maintained by NREL [15]. The comparison is made for monthly and yearly averages of daily global horizontal irradiation, for each station.
Section snippets
Analysis of SMN data
The database (from 1998 to 2011) [16] was obtained from a total of 136 solarimetric stations that record global solar irradiance at 10 min intervals. The installed radiometers sensors are mainly Kipp & Zonen CMP11 pyranometers, each one, factory calibrated at the moment of installation. Table 1 shows the names and locations of the stations considered in this paper and Fig. 1 shows the distribution of stations in the Mexican territory. A first step was to identify inconsistent data such as out
Validation of data from SMN
In the initial review of the data contained in the database from SMN, approximately 40 million records of solar radiation were found, from which 45,871 records were found with inconsistent date stamps (0.11%).
Fig. 2 shows the results for 21 stations from the compiled tables. Stations shown are those that had a higher number of data for the time considered in the database, from 1999 to 2011. The graph shows an overall decline in the number of records from the period 2004–2007.
A similar process
Conclusions
A preliminary analysis of the database from the EMAS stations of the SMN was carried out. Large gaps were identified in the database in the period 2004–2007. This problem is generalized in most of stations. From the analysis of daily number of records, for the first three years of operation of every station, those with higher than 80% complete data were identified. The number of stations that fulfilled this requirement was 56 (41%), of a total of 136. After examining the daily irradiance graphs
Acknowledgments
Our thanks to Universidad Nacional Autónoma de México for their financial support through grant PAPITT A100312-2.
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