Technical Consideration

The maximum output of the solar receiver is achieved when the solar receiver is perpendicular to the Sun's rays. Different attempts were made for making the solar receiver utilizing the maximum portion of incident solar radiation. The use of a dual-axis sun tracker versus a fixed-flat position is evidently profitable, but from economic point of view it is questionable. A mathematical conception has been developed and applied in this chapter to determine the energy gain resulted from different installations of PV systems. The experimental measurements and the model results show that, it is not economical to track the sun in hot and sunny regions because of the overheating effect on the PV panels' performance. The provided data, in literature, compare the performance of dual or single axis tracking with fixed solar receiver even the long term solar tracking is possible and effective with a negligible increase of the price of the unit of useful energy. This can be achieved by choosing the best monthly or even seasonally optimum tilts. The introduced concept of energy gain, see chapter 3, is calculated in this chapter all over the world and it was found that it is very useful in evaluating the performance of different types of tracking. This concept allows to evaluate the effectiveness of daily, weekly, fortnightly, monthly, seasonally, biannually and yearly adjustment of the solar receiver tilt angle in relation with the ideal instantaneous dual tracking.

Terrestrial Solar Radiation

After shading a light on the extraterrestrial solar radiation in the chapter 3 it is important to evaluate the global terrestrial solar radiation and its components. The information on terrestrial solar radiation is required in several different forms depending on the kinds of calculations and kind of application that are to be done. Of course, terrestrial solar radiation on the horizontal plane depends on the different weather conditions such as cloud cover, relative humidity, and ambient temperature. Therefore, the impact of the atmosphere on solar radiation should be considered. One of the most important points of terrestrial solar radiation evaluation is its determination during clear sky conditions. Therefore, in this chapter, the equations that determine the air mass basing on available theories are given and the clear sky conditions are introduced with shading a light on the previous work in identifying clear sky conditions. Taking into consideration that, clear sky solar radiation estimation is of great importance for solar tracking, a detailed review of main available models is given in this chapter. As daily, monthly, seasonally, biannually and yearly mean daily solar radiations are required information for designing and installing long term tracking systems, different available methods are commented regarding their applicability for the estimation of solar radiation information in the desired format from the data that are available. An important accent is paid also on the assessment and comparison of monthly mean daily solar radiation estimation models.

Solar Tracking

Basing on the provided information in the first to four chapters where it was clearly demonstrated that, the maximum utilization of solar energy depends upon determining the exact location of the sun position. By proper calculation and computer programs, the solar path for any geographical location can be tracked. Thus, a clear idea about the prospects of solar energy for any location can be obtained and accordingly decisions and, measures can be adopted for harnessing solar energy in that area. Basing on this information a general formula for on-axis sun-tracking system has been derived using coordinate transformation method. The derived sun-tracking formula is the most general form of mathematical solution for various kinds of arbitrarily oriented on-axis sun tracker, where azimuth-elevation and tilt-roll tracking formulas are specific cases. The application of the general formula is to improve the sun tracking accuracy because the misalignment of solar collector from an ideal azimuth-elevation or tilt-roll tracking during the installation can be corrected by a straightforward application of the general formula. Moreover, the rotation angle in a single axis tracker is calculated. The calculated rotation angle can be used to determine the number of motor revolutions to move the tracker to its optimum position.

Optimum Tilt Angle Determine

After treating extraterrestrial and terrestrial solar radiations in the previous chapters, the use of this information in treating an important question regarding the installation of fixed solar systems, namely the tilt and orientation of the solar receivers, becomes possible. There are several rules that guide designers in this field. These rules are called the rules of thumb. There are two rules that are directly related to the subject of this chapter. One of these two rules says that a solar collector should be orientated towards Equator. The other one says that solar collector should have a latitude tilt value. Are these two rules valid all over the world? The present chapter focuses on presenting an algorithm for determining the optimum tilt angle all over the world and for any collector azimuth angle. The Earth surface, located between latitudes 66.45oS and 66.45oN, is divided into 3 characteristic zones. The first zone is the tropical between latitudes 23.45oS and 23.45oN. The second zone is the mid-latitude zone between 23.45oN and 43.45oN and between 23.45oS and 43.45oS. The third zone is the high-latitude zone between 43.45oN and 66.45oN and between 43.45oS and 66.45oS. For each of these zones an adequate method is proposed for calculating the solar collector optimum tilt. Moreover, four simple equations are proposed for predicting daily optimum tilt angle and optimum tilt angle for any number of consecutive days. It is found that the above mentioned rules of thumb are not applicable in the tropical zone while they could be applied with a sufficient accuracy when dealing with fixed installations all over the year in the mid- and high latitude zones.

Extraterrestrial Solar Radiation

Extraterrestrial solar radiation is the main source of terrestrial solar radiation components. Data on the spectrum of this radiation is available and a value of 1367 Wm-2 for the solar constant is accepted in solar literature. The knowledge of extraterrestrial solar radiation is essential for solar applications, within them is the Sun tracking. This radiation on horizontal surface is widely treated and a simple formula, for calculating it, is widely used. On Equator facing solar receivers, the appropriate equations for obtaining this radiation are also available, but the application of these equations by different authors was found to be not evident mainly on calculating the sunset hour angle on such surfaces. This question becomes problematic for some authors when treating surfaces of different orientations. The term of characteristic day is widely used in solar literature. The ambiguity of this term with regard to extraterrestrial solar radiation, declination angle and extraterrestrial solar radiation on a horizontal plane is described. In order to completely solve the above-mentioned problems, extraterrestrial solar radiation is calculated on surfaces of different orientations and the required relations are given for each case. The introduction of the energy gain on the basis of extraterrestrial solar radiation could be formulated mathematically very precisely. Therefore, this question is treated in details in this chapter. The difference between short term and long term Sun tracking is described also and the maximum possible energy gain of these two types of tracking is characterized.

Geographic Orientations

The determination of the true geographic north is essential in many applications. There are different methods for doing that with different levels of complexity. In this chapter, the basic theoretical considerations, for determining the true north basing on shadow treatment, are described. The principles of using fence shadow for determining the true north all over the day are described. Basing on the above information, an instrument (magnetic declination device) is described in detail. This instrument could be used for determining the geographic north of the site where a solar system will be installed. The information provided in chapter 1 was used in this chapter for studying the shadow of different obstacles on the solar systems.

Economic Consideration

After giving a survey on tracking market and introducing the base elements of economic analysis, several examples were studied in this chapter in order to evaluate the economic feasibility of dual tracking systems in comparison with horizontally installed fixed panels and with latitude tilted fixed panels. It was found that tracking is feasible in relation with these two cases at high latitudes and it questionable at sunny belt region. Anyway, the diffusion of photovoltaic systems is hindered until today by high investment costs. Trackers are more expensive because now you have moving parts. Instead of something that is just sitting on the ground you now have a motor that moves the panels. O&M costs will be higher, as well. The motor needs to be maintained throughout the life of the tracker. However, PV power generation is justified for special purposes. It is clearly demonstrated that, the small scale applications such as telecommunication systems, rural electrification, cathode protection and water lifting are economically feasible. Moreover, the comparison of the effectiveness of tracking in relation to monthly adjusted tilt of PV panels where the simplicity and high energy gain is not considered. This will be done in the near future.

Solar and Collector Angles

The Sun position determination is required in several solar applications, within them is the Sun tracking. The Sun position is determined in this chapter with reference to the Earth's center and with reference to an observer on the Earth's surface. This procedure allows determining the possible relationships between different solar angles. The determination of the solar rays' incidence angle on the surface of different orientations is very important for determining sunshine duration on this surface as well as global solar radiation received by this surface. The obtained formulas could be used for determining the optimum tilt angle of solar receiver. Some procedures for measuring site latitude, solar elevation angle, solar zenith angle, hour angle and solar azimuth angle are presented. Some devices used in measuring sunshine duration are also described. The main systems of coordinates used in solar tracking are introduced. The provided information will be essential background for different types of Sun tracking.