Virtual Prototyping of a New Tracking System

Transcription

1 Virtual Prototyping of a New Tracking System DANIELA CIOBANU, ION VISA, DORIN DIACONESCU, RADU SAULESCU Product Design Center for Sustainable Development Transilvania University of Brasov Eroilor Street, No. 28, Brasov ROMANIA daniela.ciobanu@unitbv.ro / Abstract: - Concentrator collectors are generally used in order to increase the amount of energy absorbed from the Sun. These collectors use only the direct radiation, being equipped with a single axis tracking system. The paper refers to parabolic trough collectors, whose geometry imposes one axis tracking system..in order to set the dynamic condition for the tracking system motion, the month of March is considered as a math example, when the average day time is of 12 hours. Thus, the reflector should rotate during the 15 hour. The maximization of system efficiency is obtained by operating the collector at specific moments aiming to low energy consumption during the tracking generation. The dynamic behavior of the tracking system is obtained based on the modeling and simulation of the mechanism by using Solid Works, ADAMS and Matlab/Simulink software. Key-Words: - Solar-thermal energy conversion, parabolic trough collector, tracking system, cam mechanism 1 Introduction Concentrator collectors are generally used in order to increase the amount of energy absorbed from the Sun. The most common concentrator collectors are parabolic trough, central receiver and parabolic dish. A parabolic trough concentrates the incoming solar radiation into a line running along the length of the trough (fig.1). Parabolic Reflector Parallel Rays of Sunlight Glass Envelope Absorber Tube Receiver Fig.1 Operating principle for parabolic trough collector For a better concentration of the solar radiation, due to the change of the sun position on the sky, these collectors are equipped with tracking systems. Tracking systems are classified by their motions: rotation can be around a single axis or can be around two axes. The parabolic trough collectors are designed to operate with tracking rotation round one axis due to its geometry [1, 7, 8]. Most common tracking systems use a gear box and a belt, [3,4, 6], rope or chain transmission. Belt and rope transmission tracking systems require accurate maintenance and could generate errors; chain transmission cannot be used for large dimension collectors. The paper presents a tracking system derived from a rotational (cardioids), [2] cam mechanism and oscillating role follower. A complex mechanism is generated by this, similar with a bolts gear. The mechanism uses two opposite cams and multifollowers generated by a wheel with two rows of bolts with half step delay. The advantages of this mechanism are: low costs due to a simple design, smooth motion and high efficiency due to the cycloidal gear, generating a more accurate operation comparing with other tracking systems. Equipped with a servo-engine and an adequate temporization, this mechanism can generate a controllable collector orientation. 2 Tracking system Design The monthly average of daylight for Brasov area is presented in Fig. 2. In order to determine the kinematics and dynamic condition for the tracking system, a math example is considered for the month of March when the average day time is of 12 hours. Thus, the reflector should rotate during the 12 hours. The maximization ISSN: ISBN:

2 of the system efficiency is obtained by step operating the collector at specific time steps, the energy spent for collector orientation being consequently lowered. Considering that the tracking mechanism is driven by a gear box at each 18 minutes, 40 steps per day are necessary. The collector should rotate in 12 hours, which means 4,5 deg/step Hours of daylight No. of day Fig. 2 Daylight for Brasov area a. Front view of a cam mechanism b. Detail view of cam mechanism Fig. 3 Tracking mechanism for a parabolic trough collector Based on the above example, the control program has to guarantee 40 steps during 12 hours of daylight, every 18 minutes, to rotate the reflector by In order to transmit the movement, two opposite cams with cardioids shape are used. In Fig. 3 there is presented the tracking mechanism for a parabolic trough collector. The mechanism driving is performed using a gear box, equipped with an control system. Number of bolts n corresponding to a cam, is calculated considering the conditions: 2 * π * r2 n = = Integer number (1) 2 * π * r1 Based on these conditions, the radius r 2 is chosen. Thus, the tracking cam mechanism has the following dimensions: centroid radius: r 1 =15 mm (cams centroid), r 2 =600 mm (wheel centroid); speed ratio: i 12 =40; n=40 bolts/cam; the angular step between bolts for one cam: Virtual prototype In order to build the virtual prototype of the tracking system consisting of a rotational cam and multiple follower mechanism type, the following modeling software were used: a. Solid Works, for generating the geometrical model of the tracking system and the through; b. ADAMS, for analysis, optimizing and kinematic-dynamics simulation; c. MATLAB/Simulink, for the command and control phase; ISSN: ISBN:

3 The analysis algorithm of the tracking system supposes the development of two specific models using MBS: a). kinematical model composed by the bodies connected by of kinematical joints and the geometrical parameters of the mechanism (joint placement); the input is generated using kinematical constraints (driving movements) applied in the rotational/translational joints of the driving elements which usually control the displacement and angular velocity; b) dynamic model composed by the kinematical model and the external/internal forces that act on the tracking system (e.g. mass forces, wind, etc.). The model is used to determine the driving torque (for operating with a rotational engine) or the driving force (for operating with a linear actuator) which generates the kinematical movement of the tracking system. The inertial-mass properties (mass, location of mass center, inertial torques) of the tracking system bodies necessary for the dynamic model were established by means of a solid 3D modeling using Solid Works software, Fig. 3. The transfer between the CAD and MBS software (ADAMS/View) was performed using of a STEP format file (Standard for the Exchange of Product Model Data). Using ADAMS, materials were associated for each body, the software automatically generating the inertial-mass properties of the bodies (Fig. 4). The total angle reached by the collector is ( ), with the zero position at noon ( 12 o clock), the return to the initial position (sunrise) being performed at o clock. Simulation is performed considering the specific conditions of the Summer Solstice (sunrise hour 4.27 solar time, sunset hour ) and the latitude of Brasov area. The modeling of the dynamic answer, regarding the collector orientation, was performed considering simplified conditions: the collector operating is performed using a mecatronic moto-reductor equipped with brake system which allows achievement of an output torque approximately constant: a motor torque +1,5Nm and a braking torque of -1.5Nm. Considering the previous conditions, the tracking system answer was simulated, using both ADAMS and MATLAB/Simulink software, getting similar results. For exemplification, in fig. 5 is illustrated the dynamic answer described by the variations in time of the following parameters: resulting torque, angular acceleration, angular velocity and angular displacement, afferent to the collector during the 12 hours-light of the summer solstice; the specified variations are established considering a step by step orientation program, one step being performed one hour after the previous one. In order to mark out better the variation curves, in figure 6 were presented, enlarged, the variations of the parameters afferent to the acceleration and breaking phases during a diurnal orientation step (first step performed in the morning), and in figure 7 were similar illustrated the variation of the parameters afferent to the phase of returning to the initial position. Considering the above premises, according to figure 5.7 (see also fig. 5.6.a), the resulted angular acceleration (fig. 5.6,b) is constant and positive during the acceleration phase (ε =0,35 rad/s 2 ) and negative during the deceleration phase (ε =-0,35 rad/s 2 ), the angular velocity (fig. 5.6,c) has a linear increasing variation during the acceleration phase (ω = 0 0,3 rad/s) and decreasing for the breaking phase (ω = 0,3 0 rad/s), and the angular displacement has a parabolic variation during a step of 15 o /hour; during a step, the acceleration as well as the deceleration are performed in a time range Δt = 0,865s. a. Inertial-mass properties of the trough collector b. Inertial-mass properties of the cams Fig. 4 ISSN: ISBN:

4 Fig. 5 Fig. 6 According to figure 7, the return to the initial position is done performed after the 12 diurnal steps were completed (see fig. 5.6) and it is characterized as follows: a) a resulting torque T=-1.5 Nm during the acceleration phase (for returning to the initial position) and T=+1,5 Nm for the breaking phase, b) ISSN: ISBN:

5 a constant negative angular velocity during the acceleration phase (ε =-0,35 rad/s 2 ) and positive during the breaking (ε =+0,35 rad/s 2 ), c) an angular velocity with a linear increasing variation during acceleration (ω = 0-1,15 rad/s) and decreasing during breaking and d) an angular displacement with a parabolic decreasing variation from to 0 0 (initial position); during the phase of returning to initial position, the acceleration, respectively deceleration are performed in a time range Δt = 3s. 4 Conclusion Tracking step - wise mechanisms based on double cams and multiple followers are presented as a solution for parabolic trough collector. The collector daily motion was performed in steps, the displacement function being established considering a minimum number of actions. By using this tracking system the amount of direct radiation captured by the parabolic trough receiver raises with about 45% than in the case of fix collectors orientated toward to the South [2]. The designed tracking system achieves the imposed movement functions and meets the desired placement of the collector along the operating cycle. Starting from this virtual prototype both the mechanism and the tracking algorithm will be tested in laboratory and field operating conditions. Based on the tests, the tracking algorithm is optimized targeting a minimum number of steps thus, an increased overall energy efficiency. References: [1] Duffie, J.A., Beckman, W.A., Solar engineering of thermal processes, Second edition, A Wiley- Interscience Publication, John Wiley & Sons, 1991, ISBN [2]. CIOBANU, D., Visa, I., New tracking systems for small parabolic trough collectors, 4 th European Solar Thermal Energy Conference, ESTEC 2009, Munich, Germany, pp [3] *** The Russian Patent No: [4].***The World Patent No: WO A. [5] ***The US Patent No: US [6]. ***The US Patent No: US [7]. er1.htm. [8]. Fig. 7 ISSN: ISBN: