The optimization in virtual environment of the mechatronic tracking systems used for improving the photovoltaic conversion.

Alexandru, Catalin ; Pozna, Claudiu

Abstract: In this paper, we present a solution for increasing the efficiency of the photovoltaic systems. The idea is to design a dual-axis tracking system, which changes the position of the photovoltaic panel for maximizing the incident radiation on panel. The tracking system is approached in mechatronic concept, integrating the control system in the mechanical model of the tracking system. The virtual model is a control loop composed by the multibody mechanical model connected with the dynamic model of the actuators and with the controller dynamical model. Using the virtual prototype, we are able to optimize the tracking mechanism, choose the appropriate actuators, and design the optimal controller.

Key words: photovoltaic panel, tracking mechanism, mechatronic system, virtual prototype, optimization.

1. PROBLEM STATEMENT

Solar energy conversion is one of the most addressed topics in the field of renewable energy systems. The technical solution for converting the solar energy in electricity is well-known: the photovoltaic systems. The energetic efficiency of the photovoltaic systems depends on the degree of use and conversion of the solar radiation (Meliss, 1997). There are two ways for maximizing the rate of useful energy: optimizing the conversion to the absorber level, and increasing the incident radiation rate by using mechanical tracking systems.

The key word for the design process of the tracking systems is the energetic efficiency; using the tracking system, the photovoltaic panel follows the sun and increase the collected energy, but the driving motors & actuators consume a part of this energy. The tracking system is efficient if the following condition is achieved: [epsilon] = [DELTA][E.sub.P] - [E.sub.T] >> 0, where [DELTA][E.sub.P] = [E.sub.PO] - [E.sub.PF] is the difference among the electric energy produced by the photovoltaic panel with tracking ([E.sub.PO]), and the same panel without tracking/fixed ([E.sub.PF]), and [E.sub.T] represents the energy that is consumed for tracking the PV panel. The maximization of the parameter [epsilon] became an important provocation in the modern research and technology.

2. ACTUAL STAGE AND ORIGINAL CONTRIBUTIONS

In literature, the increasing of the photovoltaic efficiency is approached mainly through the optimization of the conversion to the absorber level, and this because the subject is considered a "monopoly" of the chemical and electrical engineering. In fact, this is an interdisciplinary subject on the border between the chemical, electrical and mechanical engineering (Goswami et al., 2000). The connection with the mechanical engineering is made through the orientation devices (i.e. the tracking systems), which are in fact mechatronic systems (i.e. mechanisms driven by controlled motors & actuators).

For the design process of the tracking systems, there are taken into consideration two rotational motions, the daily motion and the yearly precession motion, so that there are two fundamental ways for tracking the sun: by one axis or by two axes (Visa & Comsit, 2004).

The single-axis systems pivot on their axis to track the sun, facing east in the morning and west in the afternoon. The two-axes tracking systems combine the two rotational motions, so that they are able to follow very precisely the sun path. Depending on the mode in which the panel is rotated, there are two types of dual-axes systems: azimuthal systems (the main motion is made around the vertical axis), and polar systems (the main motion is made around the polar axis). The literature present constructive solutions, mainly for the azimuthal trackers, which are simpler from constructive point of view, but there is necessary a continuous correlation between the daily and the elevation motions, and this fact generates the increasing of the control system complexity (Abdallah & Nijmeh, 2004).

Depending on the mode in which the driving elements are controlled, the panels can be rotated without brakes, or can be discontinuously driven, usually using 10-15 steps on daylight. The controlling is achieved offline--based on the statistic meteorological data, or online--using light sensors (Odeh, 2004). Unfortunately, in literature, the data regarding the controlling process are insignificant. At the same time, when the energetic balance is performed, there is not detailed the power consumption for realizing the orientation, which has a major contribution on the energetic efficiency of the system.

Determining the real behavior of the tracking system is a priority in the design stage since the emergence of the computer graphic simulation. Important publications (Bedford & Fowler, 2002; Schiehlen, 1993) reveal a growing interest on analysis methods for multi-body systems that may facilitate the self-formulating algorithms, having as main goal the reducing of the processing time in order to make possible real-time simulation (this technique is called Virtual Prototyping).

In the above-presented conditions, our contributions can be structured in the following directions: developing a general & unitary method for the structural synthesis, which is based on the MBS (Multi-Body Systems) theory; developing a mathematic model for computing the incident solar radiation, depending on the total direct radiation and the angle of incidence; developing the analysis & optimization flow-chart of the tracking systems, based on the main mechanical models (kinematic, inverse dynamic and dynamic), in MBS concept; developing the virtual prototyping platform for simulating the tracking systems in "real" operating conditions, by integrating specific software solutions; developing the control system, and integrating the control in the mechanical model of the tracking system at the virtual prototype level, in the concurrent engineering concept; developing and testing different models for moving the panel in both daily and seasonal directions; establishing the energetic efficiency of the tracking systems.

3. RESULTS

Our researches in the field of photovoltaic tracking systems are focused on solutions for increasing the efficiency through the maximization of the solar radiation degree of use, and the minimization the power consumption for orientation.

For this paper, we propose a design strategy that involves two steps: designing an optimal tracking system that intends to minimize the actuating torques & forces that are needed for tracking the sun movements, and designing the optimal control law in order to minimize the energy consumption for orientation. The key idea is to maximize the energy gained through the step-by-step orientation, for absorbing a quantity of solar energy closed by the ideal case (continuous orientation), and to minimize the energy consumption for realizing this orientation. The optimization is made by reducing the angular field of the axes and the number of actuating operations, without significantly affecting the incident radiation.

This strategy is possible by developing the virtual prototype of the tracking system, which is a complex dynamical model, composed by the multibody mechanical model connected with the dynamic model of the actuators and with the controller model. As example, we considered a polar dual-axes tracking system (fig. 1). The daily motion is directly driven by a rotary motor, and for the elevation motion a linear actuator is used. For developing this prototype we used a virtual prototyping platform that includes CAD, MBS and Command & Control software solutions. The CAD environment (CATIA) was used to create the 3D solid model of the system. The MBS software (ADAMS) was used for analyzing, optimizing and simulating the virtual prototype. The integration of the control system in the mechanical model of the tracking system was made by using specific control software (MATLAB/Simulink).

The numeric simulations were performed considering the input data specific for the summer solstice day. The optimization of the mechanical model is focused-on the minimization of the motor torque (for the daily motion) and force (for the elevation motion), over a selection of design variables, while satisfying various constraints on the design. For optimizing the control law, we propose a solution in which the daily motion is made in steps, in the angular field [beta]*[member of][-80[degrees], +80[degrees]]; the operating time for a step is 0.1 h. For decreasing the number of actuating operations, the panel is fixed maintained in the morning (4.26-6.91) and in the evening (17.01-22.00). Finally, the panel returns in the initial position (-80[degrees]), with continuous motion, in 0.2 h. Regarding the elevation motion, the panel remains in the specific position for the summer solstice day ([gamma]*= 22.05[degrees]) in the time period 9.01-14.91; excepting this period, for increasing the incident solar radiation, the panel is supplementary inclined with 11.05[degrees] ([gamma]*=11[degrees]); the operating time of the actuator is 0.1 h.

With these motions, we obtained the angle of incidence and, consequently, the incident radiation. At the same time, we determined the incident radiation obtained in the case of the fixed panel, considering the panel in the solar noon position ([beta]*=0[degrees], [gamma]*=22.05[degrees]). Integrating the radiation curves, and taking into consideration the active surface and the panel's efficiency, we obtained the energy (mechanical work) produced by panel, as follows: panel with tracking--1755 J; fixed panel--1231 J.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

For realizing the imposed trajectory and computing the energy consumption, we developed the control system of the mechanism, using ADAMS/Controls and MATLAB/Simulink, which communicate by passing state variables back and forth (fig. 2). The motor torque and force represent the input state variables in the mechanical model; the outputs for controller are the daily and elevation angles of the panel. The mechatronic tracking system is a nonlinear system, which is controlled using two PID controllers, with the following performance indices: overshot--10%, settling time--10 s.

In this way, we obtained the energy consumption / mechanical work for tracking the panel--106 J (including the both motions). With this value, we performed the energetic balance, as follows: [epsilon] = ([E.sub.PO] - [E.sub.PF]) - [E.sub.T] = (1755 - 1231) - 106 = 418 J. This means that the tracking system is efficient from energy balance point of view (the energy contribution obtained through orientation is approx. 34%, relative to the fixed panel).

The application is a relevant example regarding the implementation of the virtual prototyping tools in the design process of the tracking systems. One of the most important advantages is the possibility to perform virtual measurements in any point/area and for any parameter (motion, force, energy). This helps us to make quick decisions on any design changes without going through expensive prototype building & testing.

In the future, we intend to develop new types of tracking systems, to improve the input database (direct radiation, incident radiation), to develop new models for controlling the tracking systems, in order to obtain as much as possible incident radiation with a minimum energy consumption, and to develop experimental models & stands.

4. REFERENCES

Abdallah, S.; Nijmeh, S. (2004). Two-Axis Sun Tracking with PLC Control, Energy Conversion and Management, No. 45, 31-39, ISSN 01968904

Bedford, A.; Fowler, W. (2002). Engineering Mechanics: Dynamics, Prentice Hall, ISBN 0130200042, New Jersey

Goswami, Y.; Kreith, F.; Kreider, J. (2000). Principles of Solar Engineering, Taylor&Francis, ISBN 1560327146, London

Meliss, M. (1997). Regenerative Energiequellen--Praktikum, Springer-Verlag, ISBN 3540632182, Berlin

Odeh, S. (2004). Design of a Single-Axis Tracking Collector, Proceedings of the 14-th Conference EUROSUN'04, ISES, pp. 527-532, ISBN 3980965643, Dorint Hotel, June 2004, DGS Munich Publisher, Freiburg

Schiehlen, W.O. (1993). Advanced Multibody Systems Dynamics, Springer, ISBN 0792321928, London

Visa, I.; Comsit, M. (2004). Tracking Systems for Solar Energy Conversion Devices, Proceedings of the 14-th Conference EUROSUN'04, ISES, pp. 783-788, ISBN 3980965643, Dorint Hotel, June 2004, DGS Munich Publisher, Freiburg