Solar radiation

Correlation between Measured and Calculated Solar Radiation Data in Compiègne, France
E. ANTALUCA, L. MERINO, B. BECKERS, Avenues - Urban Systems Engineering Department Compiègne University of Technology Compiègne FRANCE eduard.antaluca@utc.fr, luis-enrique.merino-quilodran@utc.fr, benoit.beckers@utc.fr http://www.utc.fr http://www.heliodon.net
Abstract: - Heliodon software has been developedand used as a decision tool of designing with daylight availability by architects and urban designers. The present calculation algorithm of solar radiation of Heliodon is based only on the direct component of the solar radiation corresponding to clear sky days. In order to provide realistic simulations of the energy balance in the cities, the influence of the clouds needs to be taken intoconsideration. This article is a preliminary work in order to obtain a robust method to correlate the experimental data with the results obtained with the program Heliodon, for the city of Compiègne, France. Key-Words: solar radiation modeling, daylight design, sky model

1 Introduction
Nowadays almost 50% of the world population lives in the cities and according to UN studies they will exceed 60% in2030. So the cities will grow up and they will need more and more energy. Today, more than 3% of the land surface of our planet is covered by streets and urban areas. Moreover, for a number of urban areas it will be physically impossible to increase due to objective reasons. So the densification is the only available solution [1, 2]. In these conditions, in the design process, the consideration ofnatural light will improve the energy balance of buildings and minimize the negative impact of the cities on the environment. It becomes a fundamental requisite for architects and urban designers. In the near future, solar radiation studies will be extended to large urban areas. The collaboration between geographers, climatologists and physicists will be crucial in order to define the appropriatenumerical and modeling approaches at different scales. Sun is not only the source of life on earth but also an important source of energy. The total amount of energy emitted from the sun and reaching earth’s surface is about 1.2x105 TW (3.6x104 TW on land). Total worldwide energy consumption in 2008 was about 15 TW with more than 80% derived from the combustion of fossil fuels. The amount ofrenewable energy (geothermal, wind, solar) was only 0.16 TW.

In the proper design of buildings and/or other systems with the energy of the sun, solar radiations models conform to reality are required. Extraterrestrial solar radiation can be described, in a deterministic way, for any space and time position on earth, as a function of the sun-earth distance, the earth’s inclination and the sun’s zenithangle. The ground-level solar radiation is attenuated by atmosphere conditions, clouds distribution, climate type etc. So, for a given constant spatiotemporal position, the ground-level radiation is very difficult to predict. However, some statistical approximations for a specific period of the year can be carried out. In a preliminary research, the purpose of this work is a preliminary search tofind a method to approximate the ground-level solar radiation for a given space position on earth, during a specific period, using a deterministic solar radiation computation and available experimental data.

2 Experimental data
Experimental data were recorded every five minutes from November 2008 to October 2009 at Compiègne, France using a SMA pyranometer (Fig. 1). Compiègne is located at 75km (North) from Paris, France (49°24’54” North latitude and 2°49’23” East longitude). Its elevation is between 31 and 134 meters over mean sea level.

The global radiation energy is the amount of the solar flux that is received by a surface in a specific time interval. Daily global radiation energy per unit area, E, is given by time integration of global radiation per unit area from sunrise...