SCALABLE SOLAR TOWER FOR RURAL AREAS

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1 3 rd International Conference on Energy Systems and Technologies 6 9 Feb. 25, Cairo, Egypt SCALABLE SOLAR TOWER FOR RURAL AREAS Experimental Study and CFD Analysis Comparison Basil Abdel-Megied and Mohamed El-Anwar 2 Visitor Researcher, Mechanical Engineering Dept., National Research Centre, Egypt 2 Associate Professor, Mechanical Engineering Dept., National Research Centre, Egypt Rural areas are in essential need for energy, and there is an obstacle to connect it with the national energy network. Consequently, a CFD model was implemented to investigate and evaluate the power gained from a mini-tower. On the other hand, experimental tower was built to find out real measurements to verify the CFD results. The CFD model results were compared with experimental measurements and showed good agreement. Therefore, it is recommended to scale the experimental tower up, for higher energy gain. Keywords: CFD, Experimental, Renewable energy and rural areas INTRODUCTION The fossil fuel reserve is going on a depletion curve through the last few decades. Solar and wind energies are considered as the most obvious source of renewable energy. The first discussion of solar tower was made theoretically; this was the initial start of this trend of energy generation []. Based on the theoretical idea, an experimental tower was built in China which has 244m diameter collector and 94.6 m chimney height and.6 m in diameter, by inping hou et al. It was named solar chimney because it has not heat addition area at its walls, but it can gain a lot of energy throughout the area located at the bottom of the tower [2]. Another tower was studied in Australia theoretically. This tower was 2 m tall and large shaft diameter of 4 m. This one was totally opposite to the pervious tower because they use the hot air from the top of the tower to spray water to it, in consequence the air will cool down and its humidity increases which will increase its weight. Therefore it will create a back draft air flow going back to the bottom of the tower, and this will operate the turbine located at the lower exit [3]. -3-

2 Another CFD study was also making for the solar chimney to optimize its dimensions, to find out the best dimension combination which would generate the highest flow speed. This study was initially made on a relatively large scale; collector area was 7, meters in diameter and a larger chimney height of, meters [4]. Another theoretical study on a large circular greenhouse, and a tall cylinder in the center of the greenhouse named solar chimney, studied the three main parameters that control its performance. Like greenhouse height at inlet and connection with the solar chimney, chimney diameter was 24 m and 5 m high, and collector diameter 2 m [5]. The most recent and promising projects based on the solar towers/chimney are [5]:. Enviromission Ltd. which took interest in building the 2 MW plant for Australia is presently working on building 2 solar updraft towers in California with 2 MW capacities each. The power purchase agreement for the first plant has been approved by the Southern california public power Authority in October 2 2. Green Tower Ltd, a company authorized for constructing SCPPs had a proposal for building a 4 MW plant in Namibia. In 28 Namibian Government approved this proposal and the preliminary power purchase agreement has been issued by Nampower. 3. Green Tower Ltd. has a proposal to build a green tower in Rajasthan, India and the Power purchase agreements are being negotiated at present. Negotiations with the UAE for cheap desalination, power + CO 2 sequestration have been initiated As a continuation of a previous work [6], it was planned in this research to get benefit from solar energy beside the wind by replacing the wind turbine tower by small sized (mini) solar tower, which is convenient to be used in rural areas. It was planned to place a mini-tower (as in Figure ) above a small building to generate electricity from two turbines. One vertical axis wind turbine will be placed above the tower (not included in this study). While the second turbine, will be placed at the mini-tower top exit. The mini-tower walls (sides) were covered by trapezoidal Aluminum sheets (.5 mm thickness), painted in black. Gap distance was maintained by wooden strips between the Aluminum sheet and a glass (or plastic) cover. Such glass (or plastic) cover traps solar heat in between, which rise the temperature of Aluminum sheets to highest possible value (to create a temporary constant temperature at the walls. This creates a constant heat transfer from the walls into the tower). The heated Aluminum sheets will transfer heat to the air inside the tower by convection. The heated air will rush up to tower top opening (air turbine), while fresh air will replace it through the inlet opening at the button of the tower. The air flow will operate an air turbine located near the top of the tower, driving an electrical generator (see Figure ). -4-

3 a. b. c. d. Figure. (a. Actual and CAD structure, b. Outlet section, c. Experimental tower after setup and d. Recommended vertical axis wind turbine on the tower as future work.) MATERIALS AND METHODS A three dimensional CFD model was specially prepared to evaluate the gained energy along a complete year from a mini tower with one vertical air turbine placed at the top of the mini tower. Experimental trials were carried out to verify the CFD model with experimental measurements. CFD Analysis Under commercial finite element package environment (ANSS ver. 4.5, ANSS Inc., Canonsburg, PA, USA), a 3D CFD model was established, simulating the proposed study, as illustrated in Figure 2. Fluid 42, is the used element to mesh the tower geometry, assuming incompressible flow (density change causes buoyancy), in addition to thermal heat addition via constant wall temperature. The mesh density was 55,383 nodes and 37,456 elements. Boundary conditions were set for all walls as zero velocity (no slip condition), in addition to constant temperature on the walls -5-

4 depending on the measurement. Ambient pressure was set as a boundary condition for the inlet and the exit. The analysis was made on Dell Inspiron, with Intel Core 2 Duo CPU GHz processors (with 4MB L2 cache), 2GB RAM. The analysis took about 8 minutes to complete 2 iteration. Figure 2. Meshed model (ANSS screen shot) Experimental Study A tower of two meters height can reach a high temperature in the air gap, and increase the inside flow temperature by a significant amount to lighten the density, with one turbine at the tower top exit. This experiment was carried out at Dokki, Giza, Egypt. The experimental analysis was carried out using a set of J type thermocouples. These set of thermocouples were distributed to monitor the change in temperature with respect to height and time. Five thermocouples were distributed along the height of the tower, one at the entrance to measure the inlet temperature and one at the exit to measure the outlet temperature, in addition to, three thermocouple distributed along the height (8 and 9 cm from ground). Another thermocouple was placed in the air gap to measure the temperature rise of the gap. These set was used to acquire data for about six hours for each measuring day. Measurement was made in separate days/months to resemble the whole year. The temperature measurements were acquired and saved by data logger (OMEGA DAQPRO-53) at sampling frequency. Hz. On the other hand the air velocity at the outlet and the pressure difference were measured by TESTO 4 (multi-functional measuring instrument) RESULTS AND DISCUSSIONS Figures 3 to 6 demonstrate samples from the CFD Analysis represent average of one month in each season: a. Spring season: SUB = VSUM (AVG) NOV :55:53 SUB = PRES (AVG) NOV :56:4 SM =.639 S = SM = M M

5 Figure 3. Velocity and pressure plot for spring season (sample on March) b. Summer season: SUB = VSUM (AVG) SM = NOV :23:46 SUB = PRES (AVG) S = SM =.325 M NOV :24:8 M Figure 4. Velocity and pressure plot for summer season (sample on August) c. Autumn season: SUB = VSUM (AVG) SM = NOV :9:23 SUB = PRES (AVG) S = SM = M NOV :2:9 M Figure 5. Velocity and pressure plot for autumn season (sample on October) d. Winter season: SUB = VSUM (AVG) SM = NOV :37:49 SUB = PRES (AVG) S = SM =.7573 M NOV :49:2 M Figure 6. Velocity and pressure plot for winter season (sample on December) While figures 7 and 8, illustrate sample of temperature measurements and hourly average temperature measured by each thermocouple in March 2 th,

6 Figure 7. Temperature measurement along a day in March Figure 8. Hourly average temperature on the same day to be used in verification The results of the CFD analysis indicated that such mini tower can generate power about one watt. The heat addition areas of the experimental tower are relevant to the amount of power gain, which match the genera regime of other researches mentioned before. CONCLUSIONS The implemented CFD model can be used as a design tool for scaling up such type of solar towers. The CFD model results was compared with experimental measurements and showed good agreement. This agreement should be followed to trace this conceptual design at relatively hot rural areas, to be scaled up for gaining higher energy. However, the output power was low. Therefore, it was recommended to increase the solar addition energy. The increase was introduced as illustrated in figure

7 N O D A L S O L U T I O N S T E P = S U B = V S U M ( A V G ) R S S = S M = M M N D E C : 4 7 : Figure 9. Model after adding solar collection area SUB = VSUM (AVG) SM = DEC :36:35 M Figure. Velocity at exit after one meter of the solar addition area -9-

8 N O D A L S O L U T I O N S T E P = S U B = V S U M ( A V G ) R S S = S M = D E C : 4 9 : 7 M Figure. Velocity at exit after one meter of the solar addition area The previous model (figure 9) was simulated with a solar addition area of radius one meter, two meters, three meters and four meters added to the radius of the tower. The one meter radius and two meter radius (figure and ) gives promising results, although, at three and four meters there was a flow choking. Consequently, a dimension optimization will be made for the whole tower to avoid flow choking and increase the amount of energy output. REFERENCES. M.M. El-Wakel, Power Planet Technology, Second edition, McGraw Hill, (989). 2. inping hou, Jiakuan ang, Bo iao and Guoxiang Hou, "Experimental study of temperature field in a solar chimney power setup", Applied Thermal Engineering, Vol. 27, , (27). 3. T. Altmann,. Carmel, R. Guetta, D. aslavsky and. Doytsher, "Assessment of an Energy Tower potential in Australia using a mathematical model and GIS", Solar Energy, Vol. 78, , (25). 4. Lavish Ordia, "Parametric Study and Designing of Solar Up Draft Tower Using Computational Fluid Dynamics", Indian Institute of Technology Bombay and Larsen and Toubro Limited, Mumbai, India, May June Jagadeesh, S. Pattanashetti, and Madhukeshwara, N., "Numerical Investigation and Optimization of Solar Tower Power Plant", International Journal of Research in Aeronautical and Mechanical Engineering, Vol. 2(), 92 4, Jan Basil Abdel-Megied, and Mohamed El-Anwar, "Solar Tower Experimental Study and CFD Analysis Comparison, SET24-E8", Proceeding of 3 th International Conference on Sustainable Energy technologies (SET24), August 24, Geneva, Switzerland. -2-