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Procedia Engineering 16 (2011) 407 - 413

International Workshop of Automobile, Power and Energy Engineering, APEE2011

Thermo-hydrodynamic analysis of the flowfield in an underground concentrating solar tower under unsteady solar radiation

Uzu-Kuei Hsua, Liang-Ji Changb, Ying-Ji Wangb , Chang-Hsien Tai b*

aDepartment of Aircraft Engineering, Air Force Institute of Technology, Gangshan, Kaohsiung 82063, Taiwan bDepartment of Vehicles Engineering, National Pingtung University of Science and Technology, Pingtung County 91201, Taiwan

Abstract

The purpose of the work presented in this study is related to the optimal design of an underground concentrating solar tower with sunlight using CFD (Computational Fluid Dynamics) method to analyze the matching in chimney height and heat collector size. The present study applied the finite volume method, pressure-based algorithms, and coupled with P-1 radiation model to investigate the inflow effect under the solar radiation through glasses roof. Thereafter, design and simulation of different chimney height have been presented to study the greenhouse effect and application. Analysis of the results from CFD show that the flow can reach to higher temperature in the bottom of a solar chimney with N2O filled. The matching of the height of chimney and temperature of collector remain the optimal aspect value. © 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Automobile, Power and Energy Engineering

Keywords: finite volume method, concentrating solar tower, thermal radiation, thermal convection

1. Introduction

The underground concentrating system is a hemispheric shell which setup under the ground. It is coved by the glasses on the top on the ground surface, and this system can let the sunshine incident and induce the greenhouse to raise the ground surface temperature. There is a wind turbine mounted in the tower that is operated by the nature convection due to the effect of solar radiation in the tower. Greenhouse effect causes many countries to concern, and developed countries make many policies to reduce greenhouse gas emissions measures [1], decreasing high-carbon machinery and developing low-carbon emission technologies. Most of the planted forests or use of renewable energy to reduce carbon emissions, renewable energy at present there are hydropower, wind power, biomass power generation, and solar power. The sunshine disappears every day, so solar energy has unlimited development potential.

In 1903, Leonardo da Vinci and Cabanyes proposed solar chimney concept of heat convection to generate electricity, and a German scientist, Gunter, writing in the form to describe the work of its

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.08.1103

principle in 1931[2]. In 1978, Schlaich carried out the concept of a solar tower again [3]. The concept is successfully induced three technologies: solar collector technology, chimney technology and wind turbine technology into one. Heat convectional solar chimney power generation system includes a huge collector, the center base of a chimney with wind turbine. There is a distance between the collector roof and the ground in order to form an air intake, and the tower is an effective heat convectional device which is a collect and storage systems. The air is accelerated by heating from collector, and flowing into the chimney. Then, the convection flow pushes up the wind turbine rolling in tower, and drive the generator. In 1981 to 1982, a Spain's company and German government cooperated to build the first solar chimney power plant in Manzanares, Spain [4]. Advantages of solar energy: power generation technology for building hot air in the sparsely populated desert or wilderness areas. Collector surface can absorb and store a small portion of solar radiation, the electric field can not only generate electricity during the day and night, will release energy that can generate electricity continuously day and night, good thermal convection effect will not pollute the environment, no harmful objects, it does not produce CO2 and other greenhouse gases. In 2010, Chergui et al.[5] carried out that it has good convection state while the thermo-hydrodynamic aspect e/H=0.1 Rayleigh numbers. Numerical simulations have been carried out by Tingzhen et al.[6] on solar chimny power plant system couple with turbine in 2008.

Solar power system for the tower there are many imperfections, how to develop a low cost, easy maintenance and high performance solar power plants, is of great significance. The construction of innovative and different from the previous solar tower, results, and the solar tower and the condenser to the effect of the tower. Mainly analog-style sink to the greenhouse effect of concentrating solar tower and natural convection, and enhanced high-temperature furnace temperature. In a set of solar chimney turbine generator, the temperature due to high exports of flat glass chimney temperature is lower, while the natural convection driven turbine to generate electricity, while the chamber there is a high-temperature furnace, the temperature is set in the greenhouse one location, so that production of zinc oxide reduction furnace temperature of 1200 degrees to meet the sub-project in the design and development of zinc oxide temperature furnace.

2. Research Methods

2.1. Physical Model

Size used in the simulation shape (figure 1), the space can be divided into two layers, one below ground level is lower, and second, the upper above ground. Top of solar chimney type, the diameter of the chimney height of 10 m x 1 m, solar tower base diameter of 16 m x high 0.3 m, in the lower space for the closed diameter of 16m x high 6.4 m, in order to prevent reflected in the high-temperature furnace distributing too much radiation, so the entire space closed to prevent any air conduction, so that the cooling furnace. Numerical simulation of computational fluid dynamics program, the high quality of the computing grid systems to produce results is to determine the correctness of the key technologies. This study is the use of CFD software Gambit pre-drawn type condenser can sink to the tower model, the grid computing research and testing by the previous [7].

2.2. Numerical Methods

In this study, computational fluid dynamics CFD calculation is based finite volume method, using Pressure Based Scheme algorithms, the governing equations of the spatial dispersion in the convective terms of physical quantities on the Control Surface second-order upwind method to calculate the diffusion are the use of central difference Law discrete, and time step calculated using Implicit Method; its internal

air convection heat transfer problem at the same time avoid the numerical error caused by the orthogonal grid is too large, so it is collocated gird [8] of the SIMPLE (Semi-Implicit Method for Pressure -Linked Equation) algorithm [9-11], that the momentum Momentum Interpolation Method / MIM manner [12], and in rotary sliding grid interface method to steady-state approach to thermal radiation of the P-1 model simulation of concentrating solar power systems to solve scattering problems relative to the opposite sex. Solar system contains a number of energy conversion. In all the calculation software and can not accurately calculate the solar radiation reflected the effect of concentration at one point, so the role of written reflection equation code to simulate the entire system, to improve simulation accuracy, ease of analysis.

Figure 1. dimensions of solar tower

2.3. Governing Equations

In recent years, the tower air convection to natural convection and forced convection of the mixed flow into the main 1997 Kreetz [13] the first time the heat balance calculation using the hybrid formula:

NUmx =

■x0.25

Among them, the forced convection heat transfer coefficient is expressed as: Nu

= 0.024Re08 Pr04

Re = -

npFvr r (3)

Pastohr [13] and so the results using Fluent software is used to calculate the mixed flow model (Eq. 1) calculated results are basically consistent.

Assuming the tower when the air flow of current between parallel plates, the collector plate and the tower of air forced convection heat transfer coefficient between the available Gnielinski formula is:

h = (f /SXRe- 1000)Pr

1 +12.7(/8)1/2 (Pr2/3-1)

2.4. Boundary Conditions

Boundary conditions of this study is shown in figure 2:

A) inlet boundary conditions: the initial operation, simulation of air flow through the solar tower entrance to the chimney, taking the pressure in the inlet boundary conditions.

B) outlet boundary conditions: simulation of air flow through the sink to export solar tower in the flow of outside air space, so the chimney set the outlet pressure outlet boundary conditions are the conditions, set to form the pressure-200pa.

C) sliding moving mesh: For simulation fans, like with the average fan outside world, so set the sliding surface state (Slip Boundary Condition).

D) as a glass collector, set to a transparent state, requires the simulation mode in the sun.

Figure 2. boundary conditions of the computational domain Figure 3. closed system without chimney

3. Results and Discussion

Figure 3 shows the solar radiation system without chimney. Because of the sun stuffy, the temperature is increasing according to time. The final temperature is 490K from simulation closed to the lecture result.[14] Figure 4 shows the wind speed of the wind turbine in chimney bottom. From 6:00 to 18:00, the max. wind speed, 12m/s, is induced by convection flow of the solar radiation in the noon. There is a high temperature furnace under the center of the glasses roof in solar chimney. In unsteady studies, the temperature change is more while the chimney tower is higher as shown in figure 5.

Figure 4. wind speed of the wind turbine in chimney root The heat convection is good in higher chimney. There is more heat to take away because of the convection velocity. On the other hand, there is less heat to take away in lower chimney height. Figure 6 shows pressure and velocity of the solar chimney system in different height. The data show the values in top, root, and furnace receptivity. The underground greenhouse filled with N2O, and it can absorb infrared to heat the ground temperature. Then, it is augment the convection flow speed, and advance the wind turbine power. The height of chimney is a key point for convection speed, but the high temperature keeping is importance. The matching of the height of chimney and temperature of collector are considered. In this study, we are concerned about the high temperature keeping in the furnace. The highest temperature of the furnace is occurred in the chimney height = 10m, 1080K at 08:00am, as shown in figure 6.

¡■t ■

— — - Hjjh Chimney 5 m h igfi LMmney i om — - (-ligti CHImrwy 15m —f--High ChlmriyiOm h igh Chimney 25m -— Hiflh IJhimrcyypm

4-ura soou Iteration

Figure 5. temperature fluctuation of the furnace in different chimney height

Chimney height

Structural shape

r-psKiirF:

Velocity:

om 380

4.6 1109

6. 1 11)80

15m 349

7. 2 11)60

25m 3J9

8.8 1039

30m №

Figure 6. pressure and velocity of the solar chimney system in different height at 09:00am

Figure 7 shows the furnace temperature temperature reach to 2070K in the case filled

profile in air and N2O filled from in 24 hours. The with N2O. There are only three hours, 11a.m. to 1p.m.,

above 1500K in the case with air filled. However, there are about five hours, 10a.m. to 2p.m., keeping high temperature in the case with N2Q filled.

06:Hfl 09 12:0(1 15:00 IR:(I(1 21:01) 00:00 03:1)0 06:00 Nmp f t II d,!

Figure 7. the furnace temperature profile in air and N2O filled during 24 hours 4. Conclusions

According to the investigation of the inflow effect with the glass solar heat radiation, we found the flow can reach to higher temperature in the bottom of a solar chimney with N2O filled. It can absorb infrared to raise the ground temperature, and induce the faster convection flow to drive wind turbine in center of chimney button. The matching of the height of chimney and temperature of collector are considered, and it remains the optimal aspect value.

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