IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 6 Issue 5, May 2019 ISSN (Online) 2348 – 7968 www.ijiset.com Estimation of Optimum Tilt Angles for Solar Collector and Gained Energy at Cairo, Egypt Aiat Hegazy National Research Centre, Solar Energy Department, El Behouth St., 12622, Dokki, Giza, Egypt Abstract The solar energy incident on any fixed solar collector, for thermal or electrical purposes, is highly affected by the tilt angle of the collector over the horizontal surface. The tilt angle of any solar system is a function of the site parameters and the time through the year due to the continuous daily and seasonally movements of the earth with respect to the sun. This paper presents a theoretical study for detecting the effect of tilt angle on the incident solar energy on a solar collector and consequently determines the optimum tilt angles for each month, season and through the year. The mathematical model is based on the monthly average hourly measured values for diffuse and global solar radiation on horizontal surface in the test field of Solar Energy Department, NRC, Cairo, Egypt. The calculations also use the beam radiation on the horizontal as the difference between the measured global and diffuse values. The results showed that optimum fixed tilt angle through the year ranges from 28 o -30 o and the corresponding solar energy gain over that of the horizontal is 10%. For non-fixed systems, changing the tilt angle monthly, seasonally, two times per year and yearly can increase the collected solar energy over that of the optimal fixed tilt (30 o ) by 5.5%, 4.6% and 4%, respectively. Keywords: Solar radiation, Radiation conversion models, Collector tilt angles, Solar angles, Solar energy gains. 1. Introduction Due to the continuous diminishes in the world conventional energy with the great demand of energy for development strategies and the world concerns about environmental pollution, the solar energy as a renewable source has been utilized and become one of the most important alternative safe sources for solving these problems [1]. In this manner, solar energy can help in supporting the required energy in many applications either thermal or electrical. In thermal applications, it can be used in water heating, agriculture and medical products drying, air heating and conditioning, solar cooking, solar desalination, water treatment and greenhouse applications [2]. Electricity also can be generated using solar energy as direct generation in photovoltaic systems or as indirect generation such as in concentrated solar power stations [3]. In all applications, solar as a source of energy has many advantages such as; clean without any impact on the environment, renewable, safe, low maintenance, technology development and the generated energy can be used directly at the site without need any expensive and complicated transmissions [4]. In all solar installations, the orientation of the solar system with respect to the sun’s position is the major factor affecting the output energy and hence the overall system performance. As the performance of the solar system depends on the amount of solar energy reached to its surface, maximizing the energy received by the system is very important [5]. The amount of solar irradiation incident on an arbitrary solar system is a function of several parameters such as; total radiation on a horizontal surface, diffuse radiation, surround ground reflected radiation, system location, day number and system orientation. Orientation which is very important factor for designing and installating the solar systems is mainly a function of the system tilt angle and facing [6]. Solar system can be installed either fixed or movable to maximize the amount of solar energy received via keeping the incident solar beam radiation normal to the system surface as possible. In the fixed system, the solar collector is mounted on a fixed structure facing directly south or north according to the geographic location in northern or southern hemisphere, respectively. Also, it can be tilted over the horizontal surface by a tilt angle according to the site latitude and the time. This orientation can help in reducing the incident angle of the solar beam according to the orientation position [7]. The advantages of the fixed 249