EuroSun2008-2

The Optimization Of The Single-Axis Tracking System. Used For A Solar Collector

C. Alexandru*, M. Com^if and I. Vi§a

Transilvania University of Bra§ov, Product Design and Robotics Department, 29 Bd. Eroilor, 500036,

Bra§ov, Romania
* calex@unitbv. ro

Abstract

This paper presents researches on increasing the efficiency of the solar energy conversion, by designing and optimizing a single-axis tracking mechanism, which changes the daily position of the collector. The main task in optimizing the active component (the tracked solar collector) is to maximize the energetic gain by increasing the solar input and minimizing the energy consumption for tracking. The tracking system is approached in mechatronic concept, by integrating the electronic control system in the mechanical structure of the solar tracker at the virtual prototype level. Thus, the physical testing process is greatly simplified, and the risk of the control law being poorly matched to the real system is eliminated.

Keywords: solar panel, tracking mechanism, control system, virtual prototype

1. Introduction

There is a fact that the fossil fuels (gas, oil, coal) are limited and hand strong pollutants. In the last 15 years, the price of petroleum had tripled and the previsions on the medium term there are not quite encouraging. The increase of the emissions of carbon dioxide, responsible for the global warming and for the greenhouse effect, may have devastating consequences on the environment. The solution to the previously highlighted problems is the renewable energy, including the energy efficiency, the energy saving and systems based on clean renewable energy sources, like sun, wind and water. The solar energy conversion is one of the most addressed topics in the fields of renewable energy systems. The sun is a giant nuclear fusion reactor and the energy it supplies is equivalent of about 27,000 times the total amount of energy presently produced from all other sources. The present-day techniques allow converting the solar radiation in two basic forms of energy: thermal and electric energy. The technical solution for converting the solar energy in thermal energy is well-known: the solar collectors [1].

The efficiency of the thermal solar systems depends on the degree of use and conversion of the solar radiation. The energy balance refers to the surface that absorbs the incoming radiation and to the balance between energy inflow and energy outflow. The rate of useful energy leaving the absorber is given by the difference between the rate of incident radiation on absorber and the rate of energy loss from the absorber. In literature, the increasing of the efficiency of the solar collectors is approached mainly through the optimization of the conversion to the absorber level. On other hand, the degree of use of the solar radiation can be maximized by use of tracking systems for the orientation of the solar panels in accordance with the paths of the Sun. Basically the tracking systems are mechanical systems that integrate mechanics, electronics, and information

technology. These mechanisms are driven by rotary motors or linear actuators, which are controlled in order to ensure the optimal positioning of the collector relatively to the Sun position.

The orientation principle of the solar collectors is based on the input data referring to the position of the Sun on the sky dome. For the highest conversion efficiency, the sunrays have to fall normal on the receiver so the system must periodically modify its position in order to maintain this relation between the sunrays and the collector. The positions of the Sun on its path along the year represent input data for the design process of the tracking systems. The Earth describes along the year a rotational motion following an elliptical path around the sun. During one day, the Earth also spins around its own axis describing a complete rotation that generates the sunrises and the sunsets. The variation of the altitude of the sun on the celestial sphere during one year is determined by the precession motion, responsible for a declination of the Earth axis in consideration with the plane of the elliptic yearly path. In these conditions, for the design process of the tracking systems there are considered two rotational motions: the daily motion, and the yearly precession motion.

Consequently, there are two basic types of tracking systems: single-axis tracking systems, and dual-axis tracing systems. The single-axis tracking systems spins on their axis to track the sun, facing east in the morning and west in the afternoon. The tilt angle of this axis equals the latitude angle of the loco because this axis has to be always parallel with the polar axis; in consequence for this type of tracking system is necessary a seasonal tilt angle adjustment. The two-axis tracking systems combine two motions, so that they are able to follow very precisely the Sun path along the period of one year; that’s why dual axis tracking systems are more efficient than the single one, but also more expensive because they are using electrical and mechanical parts that determines the usage of expensive components in the system. Generally, for the orientation of the solar collectors, single-axis tracking systems are used.

Determining the real behaviour of the tracking systems is a priority in the design stage since the emergence of the computer graphic simulation. Important publications reveal a growing interest on analysis methods for Multi Body Systems - MBS [2, 3]. In the last decade, a new type of studies was defined through the utilization of the MBS software: Virtual Prototyping [4, 5]. This technique consists mainly in conceiving a detailed model and using it in a virtual experiment, in a similar way with the real case. One of the most important advantages of this kind of simulation is the possibility to perform virtual measurements in any point or area, and for any parameter. Thus, the behavioural performance predictions are obtained much earlier in the design cycle of the tracking systems, thereby allowing more effective and cost efficient design changes and reducing overall risk substantially.

In these conditions, our paper presents researches on increasing the efficiency of the solar collectors by designing and optimizing a single-axis tracking mechanism, which changes the daily position of the collector. The main task in optimizing the tracked solar collector is to maximize the energetic gain by increasing the solar input, and minimizing the energy consumption for tracking. The design is made by developing the virtual prototype of the mechatronic tracking system, which is a complex dynamical model. In fact, the virtual prototype is a control loop composed by the multi-body mechanical model connected with the dynamic model of the motor, and with the controller dynamical model.

For developing the virtual prototype, we used a digital prototyping platform, which includes CAD (CATIA), MBS (ADAMS/View), and C&C (ADAMS/Controls & EASY5) software solutions.

The approach is made in the concurrent engineering concept, by integrating the mechanical device model and the control system model at the virtual prototype level. In this way, the physical testing

process is greatly simplified, and the risk of the control law being poorly matched to the real (hardware) system is eliminated.

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