Design of Low Cost Parabolic Solar Dish Concentrator

Size: px

Start display at page:

Download "Design of Low Cost Parabolic Solar Dish Concentrator"

Merry Lindsey
6 years ago
Views:

1 ABSTRACT Design of Low Cost Parabolic Solar Dish Concentrator Hamza Hijazi, Ossama Mokhiamar Mechanical Engineering Department, Faculty of Engineering Beirut Arab University Beirut, P.O. Box Reyad El Solh, , LEBANON Osama Elsamni Mechanical Engineering Department, Faculty of Engineering Beirut Arab University Beirut, P.O. Box Reyad El Solh, , LEBANON Solar energy is one of the widely spreading applications especially in Middle East. It has been used for water heating, direct electricity generation by means of photovoltaic, and for steam generation using parabolic trough solar collectors. Such utilities require large parcels of land in order to install tremendous arrays of panels or reflectors of the power needed of the order of MWs. Fabricating and constructing of thousands of reflectors necessitate the unit to be simple, reliable and with the lowest possible cost. This paper presents the design of a low cost solar dish with small-to-moderate size for direct electricity generation. Such model can be installed in rural areas which are not covered by the governmental grid. The study considers different dish diameters with full stress analyses to the dish components using Autodesk Inventor. One of the most important ways to reduce the cost is to find the optimum design of reflecting sheets, and robust structure with low weight, also a compact power transmission with minimum torque required to track the sun rays. The results show that the wider the angle between the ribs, the minimum cost would be. The unfolding of the sheets shows that landscape distribution will give lower number of sheet hence lower cost. 1 INTRODUCTION Many types of solar dish concentrators have been designed and installed many years ago, they are classified according to size, frame design, converting unit, solar tracking system and so on. The issue is: which one of them has the best efficiency and the lowest cost? Efficiency and cost differ in meaning but can be two faces of a coin; efficiency depends on the reflecting surfaces, tracking system and the converting unit, but in order to have an efficient solar system we must lower the cost compared to other energy sources. Therefore, the objective of this paper is to get an optimum design of low cost solar dish system. 2 DISCRIPTION OF THE SOLAR DISH CONCENTRATOR 2.1 Parabolic dish The solar dish system mainly consists of the base support, the concave dish frame, the reflecting material, the conversion unit and the tracking system. To have an efficient system all of XYZ-1

2 these parts must work together like one unit regardless of the cost, but each of them must be of low cost to get the chance to be used. Most of the cost for the system goes to the dish frame part, so lowering this cost can decrease the total cost. Available dish frames differ from small dish diameter to large ones. Most of the small diameter use a single curved beam arrayed in 360 degree angle, the large diameters use the truss structure in order to have a robust design. Hence, if a truss structure is used for small diameters and vice versa, who decides which one is the best choice for the system? Of course design constraint and cost are two important guides for the right design. Design constraints are the deflection, the load of weight and wind force. The cost depends on manufacturing, assembling and weight of the frame. Truss structure is more robust than single beam but it is more complex in manufacturing and assembling and its weight hence increases cost. So, the simple structure is selected over the complex one but in the limits of design conditions as it will be discussed later. Reflecting material is one of the important parts of the dish, mirrors are the famous known reflecting material around the world but have many problems such as; weight, easy to crack, and can't be of concave shape. On the other hand, stretch membrane coated with reflected material can be used, although it is light but has deformation problem and generates wind vibrations due to wind forces (mainly in large diameters formats). in this research, aluminium and polished stainless steel sheets can be used, they are light in weight and have high reflectivity as mirrors and can be easily fixed to the frame. The supporting system of the dish is linked directly to the size of the dish and the tracking system. The tracking system controls the power transmission system to follow the sun movement. In order to lower the cost, the driving power must be minimized. To achieve this goal, the system must be well balanced. So, the rotating mechanism must be supported at the centre of gravity of the dish both with the conversion unit as shown in the Fig. 1.The tracking system will be designed to rotate the in two directions. The sun-tracking system consists of an electronic circuit that processes the input signals from a set of sensors and outputs the control signal to the relays that drive the dc motors coupled to gear boxes at the pivot centre. The sensors are a set of photo resistors operating on 5 V power supply. When the sensors are exactly opposite the sun, the two photo resistors have the same resistance value as shown in Fig. 2. Most solar dishes converting units are sterling engines or steam turbine engines, both can be used in our design analysis. XYZ-2

3 2.2 Selection of the focal point The focal point (FP) is a point at where the reflected sun rays intersect, the focal point distance depends on the dish diameter and the concavity of the dish. The more concave dish has, the closest focal point will be. Focal point to diameter ratio (FD) is the ratio that represents the focal point distance with respect to the dish diameter (D). FP=FD*D To know which focal point is suitable for our design a Matlab program is conducted with parameters of FD= 0.1 to 0.6 and D= 5 m. The results are shown in Fig. 3. Drawing each frame model for its FD ratio using Autodesk inventor helps us to know the weight of each model and the location of its center of gravity with conversion unit mass that varies from 100 to 300 Kg as shown in Fig. 4. XYZ-3

4 From Fig. 4, it is seen that the mass of the dish decreases as the FD ratio increases and below FD=0.2 the focal point will be below the centre of gravity, hence the distance of center of gravity increases as the FD ratio increases, the conversion unit placement go far and dish gets flattened so FD=0.3 is a compromise value and has been selected for this design. 2.3 Selection of ribs and rings The dish frame structure consists of ribs in the radial direction connected to a rounded U- shape beam at the centre. These ribs are connected by rings in the angular direction. The shape of the ribs and the rings can be any type of bars found in the market where its dimensions must satisfy the design criteria. The hollow round bars are good in constructing to give the concavity but with low resistance to deflection, same as the hollow rectangular bars. The T shape bars are good for supporting the sheets over its flat surface and prevent metal deflection, but has more weight when compared to rectangular bars which are lighter in weight and can resist deflection. Rectangular bars will be used in this design and the reflecting facets can be supported to it by clamps bolted to the vertical side of the bars as shown in Fig. 5. XYZ-4

5 3 REFLECTING FACETS, UNFOLDING AND OPTIMUM SHEETS DISTRIBUTION The intersection of ribs and rings form variable sizes of reflecting facets, the largest facet is formed at the tip of the ribs. The reflecting facets are unfolded using Advanced Smart Unfolding software integrated with Autodesk Inventor. The reflecting facets have been cut from stainless steel sheets which are available in the market in two sizes 1 m width and 2 m length or 1.25 m width and 2.5 m length. The small size will be used. The size of the largest facet when unfolded must fit within the sheet boundary area; hence the ribs angle and the rings distance must not exceed that limits. For example the rib angle (α) and the rings distance (d) of dish diameters D=5, 10 & 20 m must not exceed 24, 12 & 6 degree for ribs angle and 1030, 980 & 950 mm for rings distance respectively. The ribs angle are to be varied from 3 degree to the maximum limitation angle and the rings distance is taken one time equal to the limitation distance and another time equal half of this distance in such a way to have multiple set of facets. These facets is then unfolded and distributed over horizontal and vertical aligned sheets as shown in Fig. 6. The number of sheets required to be cut and the perimeter length of the facets is calculated, then the cost of laser cutting is added to the sheets cost to know which set has the lower cost as shown in tables 1, 2, & 3. XYZ-5

6 XYZ-6

7 XYZ-7

We can deduce from the previous tables that the lowest cost occur in the wide angle and rings distance zone and for horizontal sheets alignment highlighted in red colour.

8 We can deduce from the previous tables that the lowest cost occur in the wide angle and rings distance zone and for horizontal sheets alignment highlighted in red colour. The optimized result is summarized in the following table, table 4. 4 STRESS ANALYSIS OF THE FRAME The design criteria is to lower the cost, lowering the cost can achieved by lowering the manufacturing cost as well as by lowering the initial cost of materials. The first issue done by optimizing the correct way in cutting the reflecting facets and the best angle for each sector and the number of ribs and rings used as well. The second issue done by lowering the weight of the structure through minimizing the dimensions of its parts, but there is a limit of dimension that we can't take values below it. Stress analysis can tell us which dimension will stand for the loads and give a robust body. It has been decided to use rectangular bars which have wide range of sizes and can be manufactured more easily but it has no sufficient area to fix the reflecting facets over it. This problem has been fixed by using L-shape clamp bolted to the side of ribs and hold the reflecting facets over it as shown in Fig. 5. The stresses on the dish frame are generated due the weight of the structure and the wind forces induced on the reflecting facets. The wind forces are proportion to the reflecting facets projected area and to the wind speed as clear in the equation: F A P (1) C d Where P is the wind pressure and can be calculated from this equation: P V (2) The area of the reflect facets (A) is known from the drawing model. C d is the drag coefficient and is equal to 2 for flat surfaces. The wind speed (V) is set to 60 mph. The stress analysis is done over one sector of the dish since the other sectors is similar to each other and to minimize software running time. The wind pressure on the facets is directly transferred to the supporting clamp and to the ribs as bearing forces from the bolts that fix the clamps to the ribs. XYZ-8

The forces on the ribs and the rings neither equal nor linear like the facets area too. Table 5 shows the facets area and the corresponding forces of it.

In the first trial, the dimension of the rib was 80*6 mm and the dimension of the ring was 30*4 mm. The max stress on ribs was found to be 98.7 Mpa and for ring dimension 20*2.

9 The forces on the ribs and the rings neither equal nor linear like the facets area too. Table 5 shows the facets area and the corresponding forces of it. Figure 7 illustrate the stress analysis environment for Autodesk Inventor. Many trials have conducted to know the effect of rings on the ribs. In the first trial, the dimension of the rib was 80*6 mm and the dimension of the ring was 30*4 mm. The max stress on ribs was found to be 98.7 Mpa and for ring dimension 20*2.5 & 80*4 the results as follow 99.8 & 98.4 then I change the rib dimension to 40*5 and ring dimension 40*4 the max stress was MPa. Then I change the ring dimension to 30*3 & 20*2.5 then the results are & MPa as shown in table 6. Then, the size of the rings has been assigned and the optimum dimension of the ribs is obtained by varying the width and height, then solving for the maximum stresses, the maximum deflection, the factor of safety and the frame masses in each try. The results are represented in figures 8, 9, and 10 for dish diameters 5, 10 and 20 m respectively. To find the optimum height, specify a desired factor of safety. Then draw a line corresponding to the specific factor of safety, this line intersects the curves (which represent the rib width at different values) at points. Then the intersected points is projected to the abscissa line to get the height values of the rib correspond to their width values, by getting height and width values in hand we enter the masses curves in order to know which dimensions satisfy the lowest frame mass. Finally check the deflection value, if it is within the reasonable range or not, if not another selection with higher factor of safety is repeated and so on. The optimized dimension and their corresponding data are presented in table 6. XYZ-9

10 XYZ-10

11 XYZ-11

12 XYZ-12

13 5 CONCLUSION The present study aims at designing a low-cost solar parabolic dish concentrator for direct electricity generation. Minimizing the cost necessitates the reduction of the entire structure weight and the required torque needed for driving the system. For simplicity, super mirror aluminium sheets are used for sun rays reflection onto the energy converting unit. The focal to diameter ratio is selected to be 0.3 in order to avoid using counter weight and make the dish self balanced. In this phase of the research, the distribution of the sheets on portrait or landscape orientation is investigated in order to minimize the number of sheets and the cutting and assembly costs. The angle between the ribs carrying the sheets is changed between 6 and 24 degrees. Three different diameters; 5, 10, and 20 meters are investigated. Numbers of rings are linked with the ribs at distances 50 or 100 cm to support the entire dish truss. Full stress analyses have been carried out using Autodesk Inventor for the optimizing the rib dimensions under the effect of the dish weight and the wind. For factor of safety of 1.5 the deflection limitation of 3 cm, the optimization shows that the wider the angle between the ribs, the minimum cost would be. The unfolding of the sheets shows that landscape distribution will give lower number of sheet hence lower cost. The second phase of the research will continue the stress analysis of the entire system, power transmission, solar tracking micro-controller and full details of the cost as function of the diameter. REFERENCES [1] K. Lovegrove. A new 500 m paraboloidal dish solar concentrator, Solar Energy 85 (2011) [2] N.D. Kaushika*, K.S. Reddy, Performance of a low cost solar paraboloidal dish steam generating system, Energy Conversion & Management 41 (2000) 713±726 [3] I. Palavras, G.C. Bakos, Development of a low-cost dish solar concentrator and its application in zeolite desorption, Renewable Energy 31 (2006) [4] H.Sofrata,A.Hussein, Proposed mwthod for optimum focal length estimation for a seventeen meter in diameter solar dish, the fourth Saudi engineering conference,nov.1995-volume 5. [5] J. A. Peterka, Z.Tan, B. Bienkiwicz, J. E. Cermak, Wind Loads on Heliostats and Parabolic Dish Collectors, Fluid Mechanics and Wind Engineering Program Colorado State University, Fort Collins, Colorado,November 1988 XYZ-13

Lecture Notes (Reflection & Mirrors)

Lecture Notes (Reflection & Mirrors) Intro: - plane mirrors are flat, smooth surfaces from which light is reflected by regular reflection - light rays are reflected with equal angles of incidence and reflection