Design and Construction of a Dual Axis Passive Solar ...

Proceedings of the ASME 2011 5th International Conference on Energy Sustainability ES2011 August 7-10, 2011, Washington, DC, USA

ES2011-54 DESIGN AND CONSTRUCTION OF A DUAL AXIS PASSIVE SOLAR TRACKER, FOR USE ON YUCATÁN María M. Pérez Sánchez Universidad Autónoma de Yucatán Facultad de Ingeniería Av. Industrias no contaminantes s/n x Periférico norte Mérida, Yucatán, México Tel (99) 9300550 Fax (99)9300589 email: [email protected]

David Balam Tamayo Universidad Autónoma de Yucatán Facultad de Ingeniería Av. Industrias no contaminantes s/n x Periférico norte Mérida, Yucatán, México Tel (99) 9300550 Fax (99)9300589 email: [email protected]

Ricardo H. Cruz Estrada Centro de investigación Científica de Yucatán Calle 43 No 130 Col. Chuburná Mérida, Yucatán [email protected] ABSTRACT

INTRODUCTION

In this investigation, we propose to use the thermal expansion properties of metals in a bimetallic strip as a base of operation of a passive solar tracker. The design process involved the determination of all aspects necessary to make a first prototype based on requirements and operating conditions previously identified. Predictive mathematical models were used to decide critical aspects. Certainly, some aspects of the design were determined experimentally to ensure the proper functioning of the solar tracker. The product of this research was the construction of a prototype with the ability to be placed with an average angular difference of 25 degrees to the position of the sun, under controlled conditions. The device created is a passive solar tracker with two degrees of freedom, one used to track the sun daily, operates automatically actuated by the bimetallic strip, the other one is manually adjusted in seasonal changes to compensate the variation in the decline of the sun along the year. Although the accuracy of the system is low, the cost of production is well below the purchase price of any commercial solar tracker, and its construction is simple, making it an economical alternative to increase the production of photovoltaic energy on a PV panel currently fixed.

Solar energy systems are continuously being improved, ensuring more efficient and economical operation. The power of a PV panel depends fundamentally upon the amount of radiation that can capture. Several factors cause significant reductions on PV's efficiency, such as temperature, dirt, and the incidence angle of solar radiation [1]. Research has shown that placing the panels in the direction normal to sunlight maximizes captured radiation, raising the efficiency about 25% to 40% with respect to static panel. For this reason it has attached great importance to the development of devices able to track the sun [2], [3]. In the last decades have been developed electromechanical and thermal systems to accomplish this task, and have emerged different types of trackers that are produced commercially, but their prices are very expensive, so that many facilities do not have positioning mechanisms. The bimetallic strip The bimetallic strip is a simple device that can be used to convert temperature changes in a mechanical displacement, is the longitudinal joint of two metal strips of different materials, each of these materials has a different expansion coefficient. For greater difference between the coefficients of expansion,

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Copyright © 2011 by ASME

greater deflection of the strips is obtained. This simple technology is a cheap alternative to build functional solar tracker [4]. When using materials with elastic moduli and thicknesses similar (relatively easy and commonly used) the term that describes the radius of curvature is shown in equation 1. Ec. 1

TECHNICAL PROPOSAL The proposed solar tracker operate using two bimetallic strips of steel and aluminum, it is placed in a framework that supports the photovoltaic panels too, the frame has one freedom degree that is operate by the bimetallic strips, the movement will be dependent on the difference in deflection between strips. The other freedom degree is operated manually. On mobile parts of each strip, have to be placed a weights. it will a function as a factor of unbalance when changing its position with respect to the axis of rotation. Deflection difference is caused by a temperature difference that will depend of the amount of sunlight and therefore heat receiving by the strips, is a mechanism similar to the shading system, used on commercial solar trackers, but with significant differences. The shading mechanism is composed by a solar collector, located at the ends of the framework, to one side of the bimetallic strips; these collectors shall be positioned so that when the sun falls on them, focuses radiation to the bimetallic strip, it is placed near the focus of collectors. The collectors play a dual function: as a system of solar concentration and as shading system.

When is the bimetallic strip thickness. are the linear expansion coefficients. are the difference between actual temperature and relax temperature of the strip [5]. DESIGN CONSIDERATIONS Localization and geographical features The Yucatan State, in Mexico, is located in the northern hemisphere, between latitudes 21 ° and 19 °. The average annual radiation received in the State of Yucatan is about 600 W/m² [6]. The weather of this State is generally hot and humid. Climatic conditions are very stable throughout the year, including temperature. Temperature average fluctuates around 27 ° C, the hottest months being April and May, in which temperatures have been recorded around 40 degrees Celsius, the days are considerably hotter than nights, minimal temperatures are about 7 ºC. This thermal gradient can be used to implement a system to return the device at night. Hurricanes and storms are common in this region.1

Processes sequence Fig. 1 show the algorithm that describes the operation processes of the solar tracker proposed.

Social conditions In Yucatan, there are regions of extreme poverty and have limited access to technology, as there are places that are lagging behind due to their remoteness from major urban centers, so that communication is not efficient In this respect, the solar tracker design presented in this paper, can be very appropriate. Since by using simple technologies are not necessary technical specialists for installation and repair of the system. Also the tracker is robust, since it does not have parts that are easily damaged, as well as the maintenance is simple, inexpensive and requires infrequently. Technical requirements This solar tracker will be operated, in a humid environment, corrosion protection must be considerate. This track must be able to move at least a single solar panel; features from a commercial solar panel were considered (dimensions, weight, etc.). [7].

Fig. 1. Solar tracker description of operation processes.

1 Data collected by the National Water Commission of México in 50 meteorological stations in Yucatán from 1995 to 2005.

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THEORETICAL MODEL

Material 1

Material 2

Cuvature radius (m)

Heat transfer

Structural iron

Aluminum

9.7222

Heat transfer processes function as a primary drivers of the system, the heat flow are continuously change modifying the system configuration, there is a closed loop conditions. Both bimetallic strips, attempt to reach the thermal equilibrium, radiation, convection and conduction processes are involved on this dynamic interaction. A gain of heat occur when the solar radiation is lighting strips, however there are also losses of heat by convection and conduction mechanisms principally, the heat flow directs to the environment.[8] To increase the deflection of the strips can achieve, one side of which can be painted using paint with a high absorptivity coefficient (the metal side of higher coefficient of expansion) [9].

Structural iron

Stainless iron

12.6557

Structural iron

Cooper

20.5671

Table 2: Curvature radius of bimetallic strips for different combinations of materials.

Mass displacement The linear displacement of bodies located at the ends of the bimetallic strip is essential to the tracker operation, this movement will produce the change in mass distribution, amplifying the effect of the bimetallic strip torque, resulting in a net torque on the tracker and hence its movement. This shift depends directly on the curvature of the strips and the length of the strip. The geometrical arrangement that was taken as the basis for calculating the linear displacement of the masses is presented in Fig 2, where is considerate almost lineal displacement due to the largest curvature radius on comparison to strip longitude.

Bimetallic strip analysis There are several desirable features in a bimetallic strip: it wants to achieve a curvature that causes a linear displacement sufficient to cause a significant imbalance in the distribution of mass, must be flexible and resilient to mechanical fatigue, and finally the cost and methods of production must be cheap. To reach this conditions, there are numerous materials and methods for conform the bimetallic strip. We considerate only the materials available in the region, observing their characteristics and evaluating the curvature radius can be produce combinations of them. Table 1, gives the mechanical properties of our interest. Table 2, show the pairs that was evaluating and the theoretical curvature radius.

Material

Linear expansion coefficient (°C-1)

Elasticity modulus (N/m2)

Structural iron

1.2 x 10-5

21

Cooper

1.7 x 10-5

8.2

Aluminum

2.4 x 10-5

7.0

Stainless steel

1.73 x 10-5

20.5

Fig. 2. Geometrical arrangement of lineal displacement of block mass. Where, r, is the curvature radius of bimetallic strip s, is the longitude of both strip, considerate than the increment of long is depreciate. L, is h is the variable longitude between relax position of the mass and stress position. According to fig. 2, arc longitude L, can be calculated using equation 2.

Table 1. Mechanical properties of the selected materials.

For reach the most satisfactory performance, the materials to use will be structural iron and aluminum.

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Ec. 2 By their gravitational nature, applied forces always act in the vertical direction, so, according to equation 6 the torque is maximum when the angle θ is 0°, which implies that the follower is completely horizontal. We can see that the moment value decreases to be zero as the lever arm about the vertical. It is therefore necessary to restrict the range of angular motion of the follower, so that they can’t reach angles for which the moving mechanism inoperable.

The angle can be obtained from this, and shows in ec. 3. Ec. 3

PROTOTYPE The length of the segment x taking into account one of the triangles with hypotenuse value, r, is given in eq. 4. Ec. 4

In order to describe the components that make up the solar tracker, we explain in detail the role that each part will integrate the system, dimensions and other relevant technical aspects such as the construction method was used. Fixed base

The relation between h y x can be established using the triangle whit hypotenuse 2x from Fig. 2, using ec. 3 and ec. 4 can be express the lineal displacement as is showed in Ec. 5. Ec. 5

Consists of a tubular of iron of 0.04 m. in diameter is soldered onto a square iron plate of 0.4 m. side and 0.5cm thick, reinforced with four angle formed by segments of pipe welded to the plate by way of straps. The tube has three holes on the tire diametrical serve to adjust the height of the follower The function of this element is to provide the support necessary to keep in place full system, must be anchored to the floor or surface on which the tracker will be installed to provide the stability needed to counteract the forces exerted by wind and other disturbances, the alignment of the base should be such that the main axis is perpendicular to the follower North.

This expression describes the linear translation of the bodies on ends of bimetallic strips.

Net torque Mobile base

The torque produced by mass unbalanced, is the responsible of move result. The torque is dependent of the state of bimetallic strips and of the solar tracker angle. For a given position the maximum torque obtains when one of bimetallic strip is on relax state, and the other one obtains the maximum deflection as possible for determinate ranges of temperature.

The mobile base consists of a galvanized steel tube, which diameter is just 3mm inner than the diameter of the tube that forms the base. This base is inserted above the fixed base, and has the option to rotate and move up and down it. It has a hole on the bottom that goes through its diameter, to be matched with the holes of the fixed base and passed through a screw then you can adjust the height of the follower at three levels. Its construction was simple, since only consisted of cutting the tube to proper length and diametrically perforate one end.

The torque relation is given by the equation 6. Ec. 6

Variable declination mount The mount is a slider that slides along the length of the mobile base, in its extreme is one of the hinges that hold the principal axis of the tracker, so that by sliding the slide and lock into a certain position, then the axis has multiple tilt angles. This part is to adjust the solar tracker according to seasonal changes in the sun's declination.

Where is the longitude between the axis and the base point of bimetallic strips. is the deflection longitudinal for the strip n . This is the point of location of the heavy bodies respect to the axis. When y be equals, the net torque is zero. The block mass collocated at the ends of the strips, were identified with gravity acceleration is θ is the angle formed by the horizon with tilt of the tracker

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Mobile chassis This part of the solar tracker, is itself a moving part on which to place the devices that need to be perpendicular to the sun, also supports solar concentrators and bimetal strips including the weights on the ends. It is composed of two parts: the axis of rotation and the rigid frame. The construction of this element was conformed by six square tubular, joint were welded.

PROTOTYPE TESTING Tests performed: Measurement of linear displacement was achieved by the bimetallic strip to raise its temperature and suffer deflection. Follower response time and angular displacement in a semi controlled. Torque needed to overcome the rotational inertia and friction of the shaft and start the movement.

The angular range of motion is 70 degrees so that the tracker can get light on its upper face during the day as to that range of motion his journey covers 290 º on it where it can present a positive angle with respect the sun. Shock absorber As previously mentioned, it is important to have a damping system to perturbations do not significantly affect the tracker, wind or other violent movements may even damage some parts of the tracker. The inclusion of a buffer in the system represents a complication more difficult to solve, because if you use a very low level of damping the system could keep swinging too, in the opposite case the answer would be too slow follower, another complication to respect is to the point where the shock is positioned as being a rotating system then the system is sensitive to where you set the buffer. All these problems are compounded by the fact that it is not easy to get shocks to specific damping coefficients, nor the specific with technical features, so it is necessary to use trade dampers such as those used for automobiles, doors or another. Among the available dampers was selected one taking into consideration the price, physical characteristics and has a "hardness" low, required for this application due to the low load that move, we chose a rear shock for a light sedan car. For placement was necessary to design pieces that can sustain the shock to firmness and in turn to rotate to compensate for movement caused change the tilt of the tracker at different times of the year. The piece was designed to a "C" square, which can rotate about its axis of symmetry, the shock is held using screws that pass through its extension

Chart 1. Linear displacement vs. bimetallic strip temperature. CONCLUSIONS AND RECOMENDATIONS It was determined that the torque required to initiate the movement of the tracker is 0.138Nm, which is equivalent to placing a weight of about 20g at the end of the tracker or move the masses of a bimetallic strip 1.1 cm more than the other . The loss in traction as a result of convective processes, suggests the need to add protection to the tracker gusts of wind that dissipates heat by convection. It suggests creating a transparent and light to allow the passage of radiation and slow down the direct impact of wind, of course without causing greenhouse gases and would slow the cooling of the strips when required. We observed a decline in speed (not quantified) of rotation of the tracker when the tilt angle is close to 60 degrees. This decrease can be attributed to three factors that affect the action of the bimetal strips: the torque due to the application of the weight of mass at large angles decreases due to the cosine of the angle factor which increases the weight, another factor is that the more the frame is vertical, the greater the vertical component of gravity, which causes the weights applied in bimetallic strips tend to move in or against deflection, whereby the displacement of the mass is lower in the top and bottom increases, it is not known if the buffer has a linear behavior, since its hardness can be variable and non-linear along its length. It was not possible to determine the functional form of the

Fig. 3 .Prototype perspective.

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angular velocity under the applied torque, since they do not know what kind of response from the shock. So the behavior is not known to be the tracker due to variations of torque produced by the difference in displacement of the masses. The final prototype photo is shown in fig. 4.

REFERENCES [1] California Energy Commision. (2001). A guide to photovoltaic (PV) system design and installation. Consultant report Version 1.0, pp. 8-9. [2] Pérez Richard. (2004). To track or not to track. Home power No. 101, pp. 60-63. [3] Clifford M.J. , Eastwood D. (2004).Design of a novel passive solar tracker. Solar Energy No. 77, pp. 269–280. [4] McPhee Erin, Park Kristina, Rost Philip, Sefzik Travis. (2008).Design of a Bi-Metallic Strip for a Thermal Switch. Reporte de proyecto. Universidad de Pittsburgh. Recuperado el 20 de mayo de 2010 de http://www.pitt.edu/~ths26/ [5] Pallas, Ramón. (2008).Sensores acondicionadores de señal. Alfaomega. Cuarta edición. México. [6] Fernández José, Estrada Vicente. (1983). Cálculo de la radiación solar instantánea en la República Mexicana. Series del instituto de ingeniería. No. 472.

Fig. 4: Picture of the dual axis passive solar tracker prototype.

[7] Shell Solar. Product Information Sheet Shell SM 100-12PObtainedfrom: www.meet-egypt.com/Downloads/SOLARM/MONO /SHELL100-12p.PDF

ACKNOWLEDGMENTS The authors want to thank to the Mexican Council for Science and Technology and to the Government of the Yucatan State for the financial support granted to carry out this study through the project YUC-2008-C06-107327 (Fondo Mixto CONACyT-Gobierno del Estado de Yucatán).

[8] Cengel, Yunus A; Boles, Michael.(2003). Heat transfer. Mc Graw Hill. Segunda edición. EUA. [9] Hetnarski, Richard; Eslami, Reza.(2009) Thermal Stresses Advanced Theory and Applications. Rochester Institute of Technology. ISBN 978-1-4020-9246-6

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