Improving the energetic efficiency of the photovoltaic arrays using mechatronic solar trackers.

Alexandru, Catalin ; Pozna, Claudiu

1. INTRODUCTION

The paper is approaching a theme that belongs to a very important field: renewable sources for energy production increasing the efficiency of the photovoltaic (PV) conversion. "The PV systems can deliver energy on large-scale to a competitive price" is the conclusion of the European Commission for Energy in the report "A Vision for Photovoltaic Technology for 2030 and Beyond" (2004). In this frame, the realization of the PV arrays (system of PV panels that function as a single electricity-producing unit) appeared as a necessity for the development of large systems for producing electric energy. The energetic efficiency of the PV arrays depends on the degree of use of the solar radiation, which can be maximized by use of tracking systems for the orientation of the panels in accordance with the paths of the Sun.

In practice, there are two solutions for developing the PV arrays with tracking, depending on the existence (or not) of a common frame: (a) PV platforms, where the panels are mounted on a common frame, the orientation being simultaneously realized by moving the entire platform; (b) array with individual panels, which are separately mounted on individual sustaining structures. In the second case, the orientation can be realized in two ways: (b.1) independent orientation for each panel (panel with own tracking system--self motor source); (b.2) simultaneous orientation of all panels of the array by using single driving source and mechanisms that transmit the motion to the panels (this solution, even is more complex by constructive aspects, ensures a greater energetic & economic efficiency because of the minimization of the consumers--motor sources). The simultaneous orientation, with the predicted advantages and the characteristic problems, opens a research area insufficiently explored since now, fact sustained by the literature and practical developments in the field that refer almost entirely to the individual orientation of the panels.

2. ACTUAL STAGE AND CONTRIBUTIONS

The bibliography research reveals a series of aspects and hardships in the literature concerning the tracking systems for PV arrays. In the literature there are no unitary models for the tracking mechanisms of the PV arrays referring to the structural, kinematical and dynamical issues. In the same time, there is no general approach for conceptual design and structural synthesis of these mechanisms. Thereby becomes obviously the necessity a method for the unitary modeling of the mechanisms and according to our strategy this method is based on the Multi-Body Systems (MBS) theory, which may facilitates the self-formulating algorithms for making possible the real-time simulation (Schiehlen, 1993).

The issue concerning the control of the tracking systems is approached mostly for the tracking systems of the individual panels, using different techniques (closed loop systems with photo sensors, opened loop systems based on astronomical computerized systems, or hybrid combinations), and types of controllers - PID, FNC, FNLC (Abdallah & Nijmeh, 2004; Chojnacki, 2005). The research is focused mostly on the quantity of the energy achieved by tracking, but there is no evaluation on the energy consumption for orientation. A possible cause of this case is due to the fact that the issue is not approached as an integrated assembly.

The disposing of the panels in the array configuration is approached in the literature mainly from the point of view of the self-shading avoiding (Perez et al., 2007; Kaushika et al., 2005). Complementary, our research approaches the issue regarding also the simplification of the solution for transmitting the motion from the driving source to the panels of the array, in order to obtain efficient and feasible solutions. Following this trend, we also investigate a modular approaching of the PV array by designing modules of panels with tracking which can transmit (or receive) the motion to (from) other modules.

Therefore, the paper is approaching the improvement of the energetic efficiency of the PV arrays by designing a tracking mechanism that simultaneously changes the daily position of the panels with a single driving source. The main task in optimizing the mechanism is to maximize the energetic gain by increasing the solar input and minimizing the energy consumption for tracking. The paper proposes the integration of the mechanic and control components at the virtual prototype level (modeling in mechatronic concept), which allows performing the energy balance: energy gain by orientation versus energy consumption for realizing the motion. Thus, the physical testing is greatly simplified, and the risk of the control law being poorly matched to the real system is eliminated.

3. RESULTS AND CONCLUSIONS

The application is made for a single-axis tracking system, which simultaneously change the daily position of the panels. The mechanism is designed for a string configuration array (panels disposed in line), but it supports the adaptation for other configurations (in the general case, the matrix configuration). The motion is transmitted from the driving source (i.e. the DC rotary motor), which directly drives the first panel, with a multi-parallelogram mechanism, the revolute axes of the panels being parallel with the polar axis. The geometric constraints between parts are revolute joints. Each panel rotates around its own support, which is mounted on the sustaining pillar.

[FIGURE 1 OMITTED]

For establishing the solution, the structural synthesis was made using a method based-on the MBS theory. In this way, a collection of possible schemes have been obtained. The solution for system used in the study was selected from the multitude of the structural solutions by using of the Multi Criteria Analysis. The evaluation criteria were referring to the tracking precision, the amplitude of the motion, the complexity and the reability of the system.

For simulating the dynamic behavior of the PV system, we have developed the virtual prototype (fig. 1), by using a digital prototyping platform that includes the following software solutions: CAD (CATIA)--to create the solid model, which contains information about the mass & inertia properties of the parts, MBS (ADAMS/View)--for analyzing the tracking system, and C&C (ADAMS/Controls & MATLAB/Simulink) for modeling the control system.

For connecting the mechanical model and the electronic control system, the input & output parameters have been defined. The angular velocity of the rotor, which determines the daily position of the panels (there is the same angle for all panels because the motion is transmitted with a parallelogram mechanism, at which the input and output motions are identical) represents the input parameter in the mechanical model. The output transmitted to the controller is the control torque generated by the DC motor. The philosophy is to control the angular velocity of the rotor, which is perturbed with the control torque (the mechanism is considered as a perturbation for the DC motor). The computation of the control torque is based on the dynamic model of the tracking mechanism, including parts, constraints, internal & external forces.

The ADAMS and MATLAB models communicate by passing state variables back and forth. ADAMS/Controls save the input and output information in a specific file for MATLAB (*.m); it also generates a command file (*.cmd) and a dataset file (*.adm), which are used during the simulation. With these files, the control diagram has been developed in order to complete the connection between the mechanical device and the actuating & control system (fig. 2). The input signal block represents the database of the daily angle (the position angle of the rotor), which defines the motion law of the tracking system.

The electric energy produced by the PV array depends on the quantity of incident solar radiation, the panel efficiency, and the number of panels. The direct radiation is empirical established depending on the extraterrestrial radiation, the medium solar constant, the day number during a year, the distortion factor, the solar altitude angle, the solar declination, the latitude angle, the solar hour angle, and the local solar time. The incidence angle is determined from the scalar product of the sunray vector and the normal vector on panel. Thus, we are able to estimate the incident radiation in every day during a year, for different locations, and tracking strategies. In paper, the results correspond to the summer solstice day, which is a relevant situation for evaluating the energetic efficiency.

[FIGURE 2 OMITTED]

The panels can be rotated without brakes during the daylight, or can be discontinuously driven (step-by-step motion). The key idea for optimizing the motion law is to reduce the angular field of the motion, and the number of actuating operations, without significantly affecting the incoming energy, and with minimum energy consumption. Thus, we obtained the optimum angular field for the daily motion, [-60[degrees], +60[degrees]], with 6 motion steps (after the sunset, the system returns in the initial position). In this way, the energy (mechanical work) balance can be performed, considering the energy produced by the PV array with tracking--[E.sub.T], the energy produced by the same array without tracking (fixed) - [E.sub.F], and the energy consumption for orientation--[E.sub.C]. For example, considering a string with 6 panels, there are the following values: [E.sub.T] = 10358.658 J, [E.sub.F] = 7384.356 J, [E.sub.C] = 52.697 J; this means an energy contribution of 39.57 % by orienting the panels, relative to the fixed array case.

Concluding, the optimization strategy leads to an efficient PV system, without developing expensive hardware prototypes. In this way, the behavioral 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.

The tracking system will be manufactured and tested in the Centre Product Design for Sustainable Development from Transilvania University, creating a real perspective for the research in the field. This allows a relevant comparison between the virtual prototype analysis and the data achieved by measurements. At the same time, we intend to develop new types of tracking mechanisms for different topologies of PV array, and new models for controlling the tracking systems.

4. REFERENCES

Abdallah, S. & Nijmeh, S. (2004). Two-Axis Sun Tracking with PLC Control. Energy Conversion and Management, Vol. 45, No. 11-12, 31-39, ISSN 01968904

Chojnacki, J.A. (2005). Application of Fuzzy Logic Neural Network Controllers in PV Systems, Proceedings of the 20th European Photovoltaic Solar Energy Conference, WIP-Renewable Energies, pp. 2269-2272, ISBN 3936338191, June 2005, Published by WIP Munich, Barcelona

Kaushika, N.D.; Gautam, N.K. & Kaushik, K. (2005). Simulation Model for Sizing of Stand-Alone Solar PV System with Interconnected Array. Solar Energy Materials and Solar Cells, Vol. 85, No. 4, 499-519, ISSN 09270248

Perez, P.J.; Almonacid, G. & Vidal, P.G. (2007). Estimation of Shading Losses in Multi-Trackers PV Systems, Proceedings of the 22-nd European Photovoltaic Solar Energy Conference, WIP-Renewable Energies, pp. 2295-2298, ISBN 3936338221, September 2007, Published by WIP Munich, Milan

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