EXPERIMENTAL SOLAR CHIMNEY DATA WITH ANALYTICAL MODEL PREDICTION

Transcription

1 EXPERIMENTAL SOLAR CHIMNEY DATA WITH ANALYTICAL MODEL PREDICTION Mohammad O. Hamdan Department of Mechanical Engineering United Arab Emirates University P.O.Box17555 Al-Ain, Abu Dhabi, UAE Obada Rabbata Department of Mechanical Engineering United Arab Emirates University P.O.Box17555 Al-Ain, Abu Dhabi, UAE ABSTRACT The paper presents an experimental data that is collected for a solar chimney draft tower that is constructed in AlAin in United Arab Emirates. The solar chimney data are collected over the period of three days on December The size of the tested chimney is 8 m high with 100 m 2 collector area. The collected data include air inlet temperature, air temperature before entering the chimney, the solar flux inside/outside the collector, and wind speed inside the chimney. The collected data are compared against an analytical model which shows very good agreement with the experimental data. The mathematical thermal model is created under the assumption of steady airflow with modified Bernoulli equation, buoyancy effect and ideal gas equation. The developed analytical model was used to evaluate the effect of geometric parameters on the solar plant power generation. The results show that the chimney height, the collector radius, the solar irradiance, and the turbine head are essential parameters for the design of solar chimneys. Keywords: Solar chimney; Buoyancy effect; Draft tower; Renewable energy. 1. INTRODUCTION A solar chimney power plant (SCPP) combines the concept of solar air collector and central updraft chimney to generate a solar induced convective flow which drives wind turbines to generate electricity. The SCPP consists of a greenhouse roof collector and updraft chimney that is located at the center of the greenhouse roof collector. The greenhouse roof collector is usually made of plastic sheet or glass plate which traps the solar energy and elevates the air temperature. The chimney is used to direct and vent the hot air through the wind turbine. The wind turbine is used to convert the air kinetic energy into mechanical work. The main advantages of a solar chimney system are (1) the low maintenance cost, (2) the simplicity of operation and (3) the durability of the system. Solar chimneys have been conventionally used in agriculture for air replenishment in barns, silos, greenhouses, etc. as well as in drying of crops [1], grains, fruits or wood [2]. Natural ventilation in buildings is another popular application of solar chimney where natural passive ventilation is used to improve the quality of indoor air and to increase the comfort index for inhabitants [3]. Recently, more interests are developed to utilize the solar chimney to harvest solar energy [4]-[16]. The shortage of available energy resources and the continuous growing energy demands have drove the energy cost to record high levels and fostered the search for more reliable renewable energy. Scientists are exploring several techniques focusing on different aspects including minimizing operational costs, simplifying and lowering maintenance cost, minimizing the use of toxic materials due to health and environmental concerns, and increasing reliability. The solar chimney is one of the techniques that have a strong potential as a green source of energy which has many advantages and has huge potential in energy generation. Therefore it has a broad range of applications and can contribute substantially to our future energy needs. In the 1980s, a pilot plant was built and tested in Manzanares,

2 Spain and data collected from this pilot plant were published by Haaf [4], [5] in which a brief discussion of the energy balance, design criteria, and cost analysis was presented. The Manzanares pilot plant was rated at 36-kW and produced electricity for 8 years which was used to prove the efficiency and reliability of this novel technology. No full scale solar chimney power plant has been built to date however many proposal have been investigated in different parts of the world. The cost of chimney construction is directly affect the cost of energy produced which needs more investigation since it depends on location, labor cost and material cost which vary dramatically based on region. It was reported that the price of the electricity produced by a solar chimney power plant in the Mediterranean region is considerably higher compared to the other power sources [6]. A recent of review study [7] found that solar chimney power generation is a promising approach for future applications. Different studies have evaluated and modeled the performance of SCPP. Using a detailed thermal analytical model, Bernardes et al. [8] showed that the height of chimney, the factor of pressure drop at the turbine, the diameter and the optical properties of the collector are the most important parameters for the solar chimney design. Zhou et al. [9] reported that the maximum height for convection and the optimal height for maximum power output increases with larger collector radius. Hamdan [10] presented a thermal mathematical model which related the power generation by SCPP to the geometry and size of the chimney and collector. The feasibility of SCPP was evaluated by different experimental study [11], [12]. Different scientist evaluated the SCPP numerically [13]- [15]. Maia et al. [13] numerical results showed good quantitative agreement with earlier experimental work which also indicated that the height and diameter of the tower are the most important physical variables for solar chimney design. Different approaches were proposed to improving the performance of SCPP including double glass collector, phase change material, thermal storage [11], tilted chimney [16], swirl generation [17], and system integration to generate fresh water [17]. The heat transfer coefficients have direct effect on performance of the solar chimney since it affects the air temperature and draft [18]. The purpose of this work is to evaluate experimentally the performance of solar chimney and compare it with the mathematical thermal model prediction [10].The study report the effect of internal collector dynamic temperature, the amount of solar energy trapped within the collector, and expected wind draft inside the chimney. 2. EXPERIMENTAL SETUP 2.1 Building of a pilot setup In order to evaluate the solar chimney model, a prototype is constructed in AlAin, UAE as shown in Fig. 1 which consists of a 10m x10m collector and an 8.25 m tall chimney. A steel pipe with diameter of 24 cm and height of 8.25 m is used to construct the solar chimney tower which is supported by steel beams as shown in figure 1a. The collector is constructed using steel beams with steel wired network use to support a semi-clear plastic cover as shown in figure 1b. The collector surface is designed with inclination to allow water drainage in the event of rain with collector height from the ground of 0.5m near the collector edge and 0.75m at the collector center. (a) (b) Fig. 1: Pictures of solar chimney prototype with some dimensions at UAE University, UAE; (a) from outside, (b) under the plastic cover collector.

3 2.2 Measurements The temperature and relative humidity (RH) data are collected using data recorder from ACR Smart Reader Plus 2 which has uncertainty of ±0.2 o C and ±3%RH, respectively. The data are recorded every 10 minutes which allows 10 months period of data collection. The temperature in the shadow and under the collector near the entrance of the tower is recorded. The air speed inside the tower is measured at the bottom of the tower using anemometer which has uncertainty of ±(2%+0.2 m/s). The solar flux is measured using solar flux meter with uncertainty of ±0.1 W/m 2. The solar flux meter is providing reading through calibrated solar photo voltaic cell. 3. MATHEMATICAL MODEL In this study the experimental data are compared with earlier mathematical model developed by the author [10]. In this section only the assumptions of the model is presented and the mathematical equations are listed. More details regarding the model is presented in earlier literature [10]. A schematic diagram of the solar chimney power plant is presented in Fig.2. A simplified model is used to describe the entire power plant including the three major components, which are the solar concentrator, the chimney, and the wind turbine. The physical dimensions investigated in this model focus on the chimney height, the chimney diameter, the concentrator s coverage area (diameter), and the turbine pressure head. In order to simplify the problem, the following assumptions are adopted: 1) Heat is added to the solar collector through solar radiations while heat is lost through natural convection to the surrounding environment. The natural convicted heat loss to is calculated using as the heat convection coefficient. 2) The chimney is assumed insulated and hence no heat loss through the chimney. 3) As reported by Zhou et al. [12] and Koonsrisuk and Chitsomboon [14] for the CFD solar chimney the temperature change across the chimney is small, hence the flow inside the chimney is assumed isothermal ( ). 4) The process across the turbine is assumed isentropic (reversible and adiabatic). The mechanical loss in the turbine is assumed minimal and hence is ignored. 5) The analysis is based on steady flow assumption which is an approximation because solar radiation is transient in nature. This assumption is very useful to investigate the main parameters overall effect that influence the solar chimney performance. 6) The chimney is discretized to multiple series pieces to allow density changes. Modified Bernoulli equation is applied across each series of pipes and density is allowed to change across the chimney. 7) The measured atmosphere temperature and solar heat flux are used in the calculation to obtain the velocity of air inside the chimney. The set of equations resulted from these assumption are listed below: For the surrounding atmospheric conditions: (1) (2) For the Solar Collector: (3a) For the turbine (5) For the chimney (8) (3b) (6) The air draft velocity inside the chimney is calculated by solving the above algebraic set of equations for all the unknowns,,,,,, and using EES solver. (4) (7)

4 4 Chimney D chimney Z 1 Collector 2 3 D Collector 2 "2 Fig. 3: Variation of Temperature with time measured inside the collector and in the shadow outside the collector. Fig. 2: A schematic diagram of the solar chimney tower power plant with infinitesimal control volume inside the collector that is used in modeling temperature rise in the collector. 4. RESULTS AND DISCUSSION The temperature of air in the shadow and under the collector is presented in figure 3. The data for the first three days in December 2011 are presented. Due to greenhouse effect the temperature inside the collector is higher than the temperature in the shadow. This temperature difference is the one generating the wind draft and pushes the low density air to rise by high density air. The peak of maximum temperature for the collector is around 1:30 PM while for the atmosphere the maximum temperature occurs later by one hour and half which is due the air draft inside the chimney. Since as temperature inside collector rise the wind speed inside the system raises too causing more heat to scape with air draft and forcing peak temperature to occur earlier than atmosphere peak temperature. The heat fluxes for the first three days in December 2011 are presented in figure 4. The heat fluxes data are collected manually every one hour starting for the three days from 7AM till 7PM. Three values of solar flux discussed here are (1) the solar flux reading inside the collected while keeping the solar meter is perpendicular to the ground, (2) the solar flux reading outside the collected while keeping the solar meter is perpendicular to the ground, and (3) the solar flux reading outside the collected while tilting the solar meter to obtain maximum possible solar flux. By comparing the solar flux reading inside and outside the collector it is clear that some solar radiation is not reaching inside the collector which could be due to (1) some solar radiation already turned to heat at the surface of the collector, and (2) also it means that some solar radiation does get reflected or distorted due to collector cover material and clarity. As indicated in the figure more than 25% of the solar radiation does not make inside the collector which indicate the importance of selecting the collector material properties. For the outside the collector reading, the big discrepancy in the solar flux values between inclined and perpendicular readings is indicating that tracking the sun is very critical to collect maximum power, however this is less important for solar chimney applications specially if will clear material used for the collector with light reflecting properties.

5 shown in figure 6, the mathematical model velocity value agrees with the experimental one which gives more confident in the mathematical model which later can be used to extrapolate results for bigger scale power plant. Fig. 4: Variation of solar radiation with time measured inside the collector and in the shadow outside the collector. Fig. 6: Experimental and analytical velocity value inside the solar tower taken on December 2 nd CONCLUSION Fig. 5: Experimental temperatures and velocity value variation with time taken on December 2 nd As shown in figure 5, the air speed inside the solar chimney is measured using anemometer and recorded every one hour starting 7AM till 7PM. As shown in figure 5, the peak of maximum velocity does not occur at the maximum temperature recorded inside the collector or the temperature recorded outside the collector. Instead it occur between these peak which is expect since the mean driving force for air draft is the temperature difference between collector internal temperature and atmosphere temperature. Hence the peak of the velocity occurs at the peak of the temperature difference between collector and atmosphere as shown in figure 5. The experimental data is used to validate the mathematical model reported by Hamdan [10]. Using the experimental data mainly atmosphere temperature and solar flux, the velocity of air inside of the solar tower is calculated and plotted versus the experimental one as shown in figure 6. As A pilot solar chimney is constructed in AlAin, UAE and a series of data have been collected over period of three days to investigate the performance of the solar chimney power plant and to compare such results with a proposed mathematical model. The following are concluded from the experimental data: 1) The maximum peak temperature inside the chimney occurs earlier that atmosphere peak temperature. 2) The collector material properties such as clarity, reflectivity, and thermal conductivity can directly affect the performance of the solar chimney by limiting amount of solar radiation entering the collector and amount of heat lost to the surrounding. 3) The air speed peak is directly related to the temperature difference between collector internal temperature and atmosphere temperature. NOMENCLATURES area, thermal capacity, diameter, Fanning friction factor gravity acceleration, enthalpy, kj/kg head, m

6 SCPP loss coefficient mass flow rate, pressure, Pa heat flux, radius, air gas constant, kj/kg K entropy, kj/kg K Solar Chimney Power Plant temperature, K velocity, m/s Overall natural convection coefficient, United Arab Emirates Power, W height, m Greek symbols laps rate, K/m density, kg/m3 Subscripts inlet of the solar collector inlet of the wind turbine inlet of solar chimney/outlet of the wind turbine outlet of the solar chimney ambient condition 6. ACKNOWLEDGEMENTS The author would like to acknowledge the support provided by United Arab Emirates University. This work was financially supported by the Faculty of Engineering in the United Arab Emirates University through seeds project funding. 7. REFERENCES (1) J.K. Afriyie, H. Rajakaruna, M.A.A. Nazha, F.K. Forson, Simulation and optimisation of the ventilation in a chimney-dependent solar crop dryer, Solar Energy, 2011, 85: (2) N.A. Vlachos, T.D. Karapantsios, A.I. Balouktsis, D. Chassapis, Design and testing of a new solar tray dryer, Drying Technology, 2002, 20: (3) N.K. Bansal, J. Mathur, S. Mathur, M. Jain, Modeling of window-sized solar chimneys for ventilation. Building and Environment, 2005, 40: (4) W. Haaf, K. Friedrich, G. Mayr, J. Schlaich, Solar chimneys, Part I: principle and construction of the pilot plant in Manzanares, Solar Energy, 1983, 2:3-20. (5) W. Haaf, Solar towers, Part II: preliminary test results from the Manzanares pilot plant, Solar Energy, 1984, 2: (6) S. Nizetic, N. Ninic, B. Klarin, Analysis and feasibility of implementing solar chimney power plants in the Mediterranean region, Energy, 2008, 33: (7) X. Zhou, F. Wang, R. M. Ochieng, A review of solar chimney power technology, Renewable and Sustainable Energy Reviews, 2010, 14: (8) M.A. dos S. Bernardes, A. Voß, G. Weinrebe, Thermal and technical analyses of solar chimneys, Solar Energy 2003, 75: (9) X. Zhou, J. Yang, B. Xiao, G. Hou, F. Xing, Analysis of chimney height for solar chimney power plant, Applied Thermal Engineering, 2009, 29: (10) M. O. Hamdan, Analysis of a solar chimney power plant in the Arabian Gulf region, Renewable Energy, 2011, 36: (11) J. Schlaich, R. Bergermann, W. Schiel, G. Weinrebe, Design of commercial solar updraft tower systems utilization of solar induced convective flows for power generation, Trans ASME: Journal of Solar Energy Engineering, 2005, 127: (12) X. Zhou, J. Yang, B. Xiao, G. Hou, Experimental study of temperature field in a solar chimney power setup, Applied Thermal Engineering, 2007, 27: (13) C.B. Maia, A.G. Ferreira, R.M. Valle, M. F. B Cortez, Theoretical evaluation of the influence of geometric parameters and materials on the behavior of the airflow in a solar chimney, Computers and Fluids 2009, 38: (14) A. Koonsrisuk, T. Chitsomboon, Dynamic similarity in solar chimney modeling, Solar Energy, 2007, 81: (15) R. Sangi, M. Amidpour, B. Hosseinizadeh, Modeling and numerical simulation of solar chimney power plants, Solar Energy, 2011, 85: (16) E. P. Sakonidou, T. D. Karapantsios, A. I. Balouktsis, D. Chassapis, Modeling of the optimum tilt of a solar chimney for maximum air flow, Solar Energy, 2008, 82: (17) B. A. Kashiwa, C. B. Kashiwa, The solar cyclone: A solar chimney for harvesting atmospheric water, Energy, 2008, 33(2): (18) M. A. S. Bernardes, T. W. V. Backström, D. G. Kröger, Analysis of some available heat transfer coefficients applicable to solar chimney power plant collectors, Solar Energy, 2009, 83: