Gabriel López

On the use of the digital elevation model to estimate the solar radiation in areas of complex topography

The development of solar energy as a power source in the next few years requires reliable estimation of available solar energy resources. At local scales, topography is the most important factor in determining the distribution of solar radiation at the surface. Interpolation techniques are usually employed to estimate solar radiation where stations are not available, but their usefulness is limited where topography is an important source of variability. The use of satellite data and more recently of models based on techniques GIS, have contributed to solve this difficulty. In this work the usefulness of a digital elevation model (DEM) in providing topographic information for the estimation of solar radiation in areas of complex topography is analysed. Daily global radiation values were generated using the Solar Analyst software, which uses topographic information to generate radiation data. The generated data were compared with the experimental data obtained from 14 radiometric stations located within the Sierra Nevada Natural Park (southern Spain), an area of complex topography. Results show the usefulness of the topographic information derived from a DEM to estimate the solar radiation in areas of complex topography. Nevertheless, results depend on the DEM resolution and it is important that other factors, such as the albedo, should also be taken into account to obtain better estimates. Copyright

Atmospheric transmission loss in mirror-to-tower slant ranges due to water vapor

Considering CSP systems of the central tower-receiver type, this study investigates the specific effect of water vapor absorption on the total atmospheric transmission losses that impact direct irradiance along the slant path between a distant mirror and the receiver on the tower. Spectral and broadband calculations of total atmospheric attenuation are made for various water vapor conditions (from dry to humid) with both the rigorous MODTRAN code and the simpler and faster SMARTS code. The use of the latter is made indirectly possible through the “fictitious sun” concept. The MODTRAN and SMARTS results compare reasonably well under the present conditions, which closely echo the conditions used in previous studies, thus allowing instructive comparisons that will be reported later. To study the vertical profile of water vapor between surface and a height of 300 m, the columnar precipitable water at ≈5 m resolution has been derived from special high-resolution radiosonde soundings carried out twice daily at ...

Comparison of Simple Illuminance Model Performances under Clear-Sky Conditions for Athens (Greece)

Solar illuminance has become an important task in recent years as a new way of energy saving within the building sector. Incorporation and employ of new more efficient windows and light pipes in buildings require accurate information of external solar illuminance levels for a proper design. Unfortunately, illuminance data is not measured as often as solar radiation and therefore luminous efficacy models must be used. A large amount of these models are based on the empirical relationship existing between the components of luminous efficacy and the corresponding components of solar radiation, i.e. direct, diffuse and global. However, these relationships seem to be non-linear and site specific. In this article, the performance of five illuminance models to estimate clear-sky solar global and diffuse illuminances is analyzed. Experimental values of irradiance and illuminance have been obtained from a solar radiation station sited at the Institute of Environmental Research and Sustainable Development of the National Observatory of Athens (Athens, Greece). Results have shown that illuminance models based on solar geometry variables alone seem to exhibit a noted locality. Inclusion of radiometric variables is thus needed to improve illuminance estimation. This characteristic is noted for both diffuse and global components.