Design of the linkages type tracking mechanisms of the solar energy conversion systems by using Multi Body Systems Method

Transcription

1 Design of the linkages type tracking mechanisms of the solar energy conversion systems by using Multi Body Systems Method M.Comsit * I.Visa Transilvania University of Brasov Transilvania University of Brasov Brasov, Romania Brasov, Romania Abstract Nowadays the field of applied mechanical systems opens new horizons for the use of orientation mechanisms. The opportunity to use mechanisms with a sustainable purpose leads to new approaches in the development of renewable energy systems design. The evaluation of the existing products shows that the tracking mechanisms for solar energy conversion systems may improve the efficiency of the solar energy conversion systems may increase their efficiency up to 30% - 50%. Considering this facts the paper aims to identify all the possible orientation mechanisms type linkages for solar energy conversion systems designed by using Multi-Body System Method. Keywords: mechanism design, tracking systems, Multi- Body Systems, linkages I. Introduction Nowadays the development of the solar radiation conversion systems is focused mainly on aspects related to materials development and solar energy conversion processes. In the design process of the solar trackers the input data is the direct solar radiation that may be converted in thermal energy (by using solar collectors) or electrical energy (by use of the photovoltaic panels) []. The competitiveness of the solar energy conversion system on the market deals with their efficiency and an alternative solution for improving their efficiency is the use of the tracking systems so called solar trackers or sun - tracking systems. According to the scientific literature, by increasing the incident radiation rate with solar trackers, in order to maximize the degree of direct (and diffuse) solar radiation collection, the efficiency of the solar radiation conversion systems may be increased up to 50%[,9,3]. II. About sun - tracking systems The sun tracking systems are, most of them, mechatronic devices used for the orientation of the solar energy conversions systems. The input data that determines the orientation principle is provided by the position of the Sun on the celestial sphere [6]. In order to reach the highest conversion degree the sunrays has to fall perpendicularly onto the receiver surface. The periodically adjustment of the receiver is determined by the astronomical information related to the Sun position on the sky dome. Observing the geometrical relation Sun-Earth there are identified motions that has to be considered: the Earth describes along one year a rotational motion on an elliptical trajectory around the Sun; combined with the precession motion this rotation generates the seasons and is responsible for the altitude variation of the Sun on the celestial sphere during one year; but the Earth has also a daily motion around its own axes that is responsible for the succession of the days and the nights and more concluding for the east - west daily path of the Sun. Fig.. The principle of sun - tracking The scientific literature and the market investigations show that the field of solar trackers knows a slow but certain development [, ]. Regarding the actuating system there are types of solar trackers: passive trackers - systems that are not using classical mechanisms; they are based on thermo sensitive fluids that that adjust the position of the receiver in agreement with the position of the sun. * comsit@unitbv.ro visaion@unitbv.ro MultiBody

2 Fig.. Passive solar tracker and the structural scheme active trackers - mechatronic devices based on actuators, gears or other mechanical configurations. Fig. 3. Active solar tracker and the structural scheme Usually, the existing tracking systems and the patents mentioned in the literature are showing that the active trackers are using mechatronic configurations based on gears, chain or belt transmissions, linear actuators [8] or combinations of them. Considering the configuration of the conversion system there are two fundamental ways to track the sun, by one axis or by two axes [7]: the single axes tracking systems pivot on their axis facing east in the morning and west in the afternoon [0]. The tilt angle of the system is equal with the latitude angle of the loco because the revolution axis has to be always parallel with the polar axis Fig. 5. Dual axis tracking system In the scientific literature all this types of existing trackers are presented but they are not synthesized. The paper aims to impose a synthesis method in order identify all the possible solutions and to select the reliable ones. III. The structural synthesis method The synthesis is based on MultiBody Systems Method (MBS) according to a mechanical system is defined as a collection of bodies with large translational and rotational motions, linked by simple or composite joints [4]. The interest elements in MBS theory are: fixed bodies, drive bodies, driven bodies, complex bodies (more than connexions) and bodies with applied forces [4,5]. The functional design process at structural level consists of the following stages: Identification of all possible graphs based on the following input data: - spatiality of the multibody system, S; - type of the geometrical constraints g c (simple or/and compound); - number of bodies n b; - the mobility of the multibody system M. Selection from multitude of the identified graphs, the graphs that are admitting supplementary conditions imposed by the specific field of application. Successive transformation of the selected graphs into mechanisms by: - mentioning the fixed body and the role of the other bodies(ex.-fixed body, -input body, 3-output body - identification of distinct graphs versions based on the previous particularization; - transformation of these graphs versions into mechanisms by mentioning the types of constraints g c (rotation, translation etc.) [ 5, 6]. The MBS frequently uses simplified graphical representations of the mechanisms so called graphs. Fig. 4. Single axis tracking system the dual axis solar trackers combine two motions so they are able to follow precisely the sun trajectory along the whole year. Consequently the dual axis solar trackers are more efficient than the single axis ones but also more expensive because they are using an extra-actuating system for the second axis Fig. 6. Basic modules of the restriction types

3 The graphs of the multibody system are defined as features based on the modules introduced in the previous figure and are considering the number of bodies and the relationships between them. The notations R and T represent rotation type restriction and respectively translation type restriction. All the other notations represent composite joints as combinations of the ones mentioned before. The identification of all possible graphs starts with definition of the types of the geometrical constraints between the bodies considering the chosen space S (g c,min =, g c,max = S-) [7]. For example, in the planar space (S = 3), all the possible graphs can be designed using the restrictions types from Fig,6 where g c = (Fig 6.a), g c = + ( Fig 6.b), g c = (Fig 6.c) and considering the correlations between the number of bodies n b, the mobility M and the sum of the geometrical constraints Σg c. This correlation is give by the equation: M=S(n b -)-Σg c () Considering TABLE I that correlates the number of bodies n b, with mobility M and the sum of geometrical constraints Σg c., all the possible graphs can be generated. n b = n b =3 n b =4 M g c M g c M g c TABLE I The correlation between mobility M and number of bodies n b On the basis of these correlations, all possible graphs that can be obtained are transformed into mechanisms. IV. Implementation and results In the near future the sustainable energy systems are expected to become the main providers of energy. Considering the use of such systems at large scale, the design of new, efficient and cost effective renewable energy systems is compulsory. The mass production of the solar energy conversion devices and implicit of the solar trackers requires computer aided design, prototyping and optimisation. The field of applications is relatively new, but it is obvious that the mechanical area of research may identify and solve problems that occur in the conceptual design and also the optimisation stages of the product development. Thus, rises the necessity of unitary modelling methods of the tracking mechanisms of the solar energy conversion systems. This method, the Structural synthesis and is based on the MultiBody Systems theory (MBS). In this paper this method is applied in the design of the dual axis solar trackers. In this application, it has to be considered some aspects dealing with the geometrical relations between the source (the Sun) and the conversion system (which is grounded the earth). In agreement with the relative motion of the Sun on the sky dome, the tracking system must have two degrees of freedom. According to the algorithm of the structural synthesis method previously described there are established criteria specific to the field of application. General criteria: M = or n b = 3 or 4, S = 6. The scientific literature referring to the multi-axes orientation mechanisms shows that it is more reliable to use decoupled motions for these systems in order to achieve the desired positions of the output element [6]. The decoupled motions facilitate also a simplified control scheme and a reliable solution for the implementation of a control system. Considering these issues specific criteria are introduced as input data in the structural synthesis Specific criteria: The daily motion (rotation) has to have the revolution axis perpendicular on the equatorial plane of the earth For the simplicity of the analysis, the daily rotation will be introduced by a rotational constrain (kinematical constrain). The specific criteria reduce the synthesis to a mechanism with mobility M= because a driving constrain is introduced by a specific criterion. Thus, there will be generated mechanisms type linkages for the motion that covers the altitude variation of the Sun on the sky along the year. In this situation the synthesis is reduced to the planar space S=3 while the mobility will be considered M= for a number of bodies n b = or 3. The planar space of the mechanism represents the plane of the elevation motion of the solar tracker and is perpendicular on the ecuatorial plane of the earth. Considering fundamental structures with n b =, M= or n b =3, M= possible graphs are identified (TABLE II). 3

4 n b =, g c =, M= n b =3, g c =5, M= + From functional, economical and efficiency reasons the dual axes solar orientation systems, are using mechanisms with decupled motions. Excluding the possibility of a driven rotational joint (kinematical constraint) used for the main motion (the daily motion) there is the alternative to use linkages in order to achieve the daily positions of the solar energy conversion system. The most adequate way to follow the variable trajectory of the sun along the year is to use mono-contour planar mechanisms disposed in perpendicular planes. These mechanisms generated by the previous structural synthesis may be easily identified from the Figures 7. They may be combined in order to generate a multitude of new configurations for the orientation system. Four examples are presented as follows: TABLE II Possible graphs for the orientation mechanisms According to the structural synthesis algorithm the graphs were transformed into mechanisms and in respect with the specific criteria the reliable solutions were selected and presented in the figure below. The mechanism from Figure 8 is based on mono-contour chains type RRTR perpendicularly disposed, where the kinematical constraints are introduced by using a piston type element. This combination is obtained by repeatability of a mechanism identified in the structural synthesis (see fig. 7.) Fig. 8. Orientation mechanism based on perpendicular mono-contour chains type RRTR Fig. 7. Structural schemes for the identified graphs The mechanism has degrees of freedom so M= and the number of bodies is n b =3 in the space S=6. The mechanism presented in figure 9 is based on a combination between mono-contour chains type RRTR and RRRR perpendicularly disposed, where the kinematical constraints are introduced by using a piston type element for the first chain and a rotational kinematical constraint in one of the rotational joints for the second chain. 4

5 This configuration is obtained by combining the solutions from the Figures 7.. and 7.6. modular configuration that is more reliable in the perspective of implementation. Fig. 9. Orientation mechanism based on perpendicular mono-contour chains type RRTR RRRR The mechanism has degrees of freedom so the mobility M=, the number of bodies is n b =4 in the space S=6. The mechanism presented in the figure 0 is based on a combination between mono-contour chains type TRRR and RRTR perpendicularly disposed, where the kinematical constraints are introduced by using a piston type element for the first chain and a translational kinematical constraint in the translation joint for the second chain. This configuration is obtained by combining the solutions from the figures 7.. and 7.7. Basically the solution from figure 8 is similar with the solution from figure 0. The difference is the position of the translational joint from the chain that is designated to orient the conversion system in agreement with the altitude variation of the sun. Apparently, at structural level this does not affect substantially the configuration of the orientation mechanisms but may have important implication when we are dealing with implementation aspects. This disposure of the translational joint provides a big flexibility in adopting different actuating system facilitating the easy mounting and dismounting of the driving aggregate. The mechanism has degrees of freedom M= the number of bodies is n b =4 in the space S=6. Even the mechanisms from figure 9 and are similar it can be easily observed that the second mentioned configuration has four bodies while the first one has three bodies. Also the mechanism from Figure 8 is based on a Fig. 0. Orientation mechanism based on perpendicular mono-contour chains type TRRR RRTR The mechanism presented in the figure 8 is based on a combination between mono-contour chains type RRRR and RRTR perpendicularly disposed, where the kinematical constraints are introduced by rotational kinematical constraint for the first chain in one of the rotational joints and a translational kinematical constraint in the translation joint for the second chain. This configuration is obtained by combining the solutions from the figures 4.7. and 4.9. Fig.. Orientation mechanism based on perpendicular mono-contour chains type TRRR RRRR 5 5

6 The mechanism has degrees of freedom M= the number of bodies is n b =5 in the space S=6. The advantage of this system consist in the fact that for the daily motion the driving motion may be introduced in any of the four rotational joints of the RRRR monocontour. The inconvenient is that this configuration has a larger number of bodies. In the MultiBody theory the number of bodies has to be minimal [8]. V. Conclusion The paper presents an applied Structural synthesis method on the planar linkages type mechanisms used for the orientation of the solar energy conversion systems. The method may be extended on spatial linkages, cams or gear mechanisms. By using the MultyBody theory all the possible graphs were identified. After a selection based on the particularities of the solar trackers, the graphs have been transformed into mechanisms. The presented application has as result possible versions (existing and new ones) for the imposed input data: for example, Fig. 7. represents an existent configuration for this kind of devices; fig.7., 7.3, 7. represent new systems based on the same graph where the driving motion is introduced in a different way; figures from 7.5 to 7. show different configurations with 3 bodies that allow many possibilities to introduce the driving motion; solutions as described in Fig. 7.4, 7.6, 7.8, may be reliable because of their compact structure; also spherical mechanisms as in Fig.7. may offer an accurate orientation in order to follow the sun path According to the specific criteria there were obtained structural configurations of planar mechanisms for the elevation motion considering that the daily motion is induced by a kinematical constraint placed in a rotational joint. Combining these planar mono-contours there were obtained new mechanism solutions reliable for bi-mobile orientation systems. The reputability or the combinations of the mono-contour modules disposed in perpendicular planes allow decupled motions and simplify the kinematical and dynamical analysis of the mechanical systems for the orientation of the conversion systems. These solutions present also advantages concerning the control, prototyping and implementation issues. [3] W. Schihlen, Multybody System Handbook, Springer-Verlag, Germany, (999) [4] P. Alexandru, I. Visa, Functional design of the mechanisms(ro), Editura Lux Libris, Brasov, (998) [5] I. Visa, Cs. Antonya, Modeling the structure of mechanical multybody systems. The fourth International Conference IDMME 00 (Integrated Design and Manufacturing in Mechanical Engineering),), pages -0, 4-6 may, Clermont-Ferrand, France, (00. [6] I. Visa, Cs. Antonya, Structural synthesis of planar linkages as multybody systems, Proceedings of the Eighth IFToMM International Symposium on TMM, Bucharest, Vol. I, (00), [7] I. Visa, Structural analysis of linkages as multybody systems- Proceedings of the Eighth IFToMM International Symposium on TMM, Bucharest, Vol. I, (00), [8] Hiwin Lineartecnologie Company Catalog, Hiwin Elektrohubzylinder, (004) [9] P. Baltas, M. Tortoreli, P. Russel, Evaluation of power output for fixed and step tracking photovoltaic arrays, Solar Energy 37, (986), [0] M. Brunotte, A. Goetzberger, U. Blieske, Two-stage concentrator permitting concentration factors up to 300X with one-axis tracking, Solar Energy, 56, pages (996),. [] S. Shinni, N.Rumala, A shadow method for automatic tracking, Solar Energy, 37, (986), [] R. Zogbit, D.Laplaze, Design and construction of a sun tracker, Solar Energy 33, pages (984),. [3] S. Abdallah, S. Nijmeh, Two axes sun tracking system with PLC control, Energy Conversion and Management, pages , (004),. [4] Haug E. J., and others. Virtual prototyping simulation for design of mechanical systems. Transaction of ASME, 7:63 70, 995. [5] Haug E.J. Computer aided kinematics and dynamics of mechanical systems. Allyn and Bacon, 989. [6] Odeh S., and others. Design of a single-axis tracking collector for moderate temperature applications. In Proceedings of the 4-th ISES Conference EUROSUN, pages 57 53, Freiburg, 004. [7]Visa I., and Comsit M. Tracking systems for solar energy conversion devices. In Proceedings of the 4-th ISES International Conference EUROSUN, pages , Freiburg, 004. [8]Visa I. Mechanical systems modelling as multibody systems in product design. In Proceedings of PRASIC 0, pages 55 63, Brasov, 00. References [] J.A. Duffie, W.A., Beckman, Solar Engineering of Thermal Processes Second Edition -A Willey Interscience Publication, New York, (99) [] G.N. Tiwari, Solar Energy-Fundamentals, Design, Modelling and Applications, Alpha Science International Ltd., Pangbourne, England, (00). 6