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MODELING a SOLAR CHIMNEY for MAXIMUM SOLAR IRRADIATION and MAXIMUM AIRFLOW, for LOW LATITUDE LOCATIONS Leticia Neves1, Maurício

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MODELING a SOLAR CHIMNEY for MAXIMUM SOLAR IRRADIATION and MAXIMUM AIRFLOW, for LOW LATITUDE LOCATIONS Leticia Neves1, Maurício Proceedings of Building Simulation 2011: 12th Conference of International Building Performance Simulation Association, Sydney, 14-16 November. MODELING A SOLAR CHIMNEY FOR MAXIMUM SOLAR IRRADIATION AND MAXIMUM AIRFLOW, FOR LOW LATITUDE LOCATIONS Leticia Neves1, Maurício Roriz2 and Fernando Marques da Silva3 1State University of Campinas, Brazil 2Federal University of Sao Carlos, Brazil 3National Laboratory of Civil Engineering, Lisbon, Portugal example. ABSTRACT The solar chimney is composed by a solar collector Numerous researches have shown the possibility to with two parallel plates – a glass cover and an enhance indoor natural ventilation in buildings by absorber plate – that heats up the air, enhancing the using inclined solar chimneys, although most of them greenhouse effect and inducing it to move upwards. usually include determining an optimum tilt for the Consequently, air from the internal space moves absorber, considering an inclination angle between towards the chimney inlet and outdoor air enters the maximum solar irradiation and best stack height. space through the inlet openings of the room (Figure This paper resumes the investigation of a possibility 1). to optimize solar irradiation on the absorber plane The efficiency of solar chimneys depends, among and also guarantee a considerable stack height, by other factors, on the intensity of solar radiation that using a chimney extension, which would be reaches the glass cover. Since the angle of incidence responsible to maintain a minimum height for the of solar radiation on a surface varies with latitude and system, independently from the absorber inclination. time of the year, it becomes advantageous using Results of a building simulation were compared with inclined solar chimneys at locations near the Equator, outputs from an experimental test cell for calibration. due to the reduction of the incidence angle and Theoretical analyses were developed, aiming to consequent increase of irradiance on the glass cover. evaluate the proposed system’s performance. Results testify a significant airflow enhancement, demonstrating its viability to use as a natural ventilation strategy in a Brazilian low latitude location. INTRODUCTION Natural ventilation in buildings is an important strategy to remove indoor air pollution and dilute contamination. For Brazilian tropical climate, it is also a recommended strategy to provide thermal comfort through passive cooling in most of the territory (ABNT, 2005). The strategy is suitable to use when indoor air temperature is higher than outdoor, which could occur due to high internal loads and/ or gains through Figure 1 Schematic section of a solar chimney solar radiation. When this situation occurs, natural ventilation is a strategy that helps obtaining thermal One of the first works to take into consideration the comfort, by evaporation of skin’s sweat and possibility of varying the chimney’s tilt is from consequent cooling effect. An airflow of 0.5m/s, for Bansal et al (1993), performed in Jaipur, India example, causes a cooling effect of 1.2°C on an (26°53’ latitude north). They developed a theoretical indoor occupant (Freixanet and Viqueira, 2004). study through a steady state mathematical model. A way of naturally induce air movement inside The study included different chimney sizes and buildings is by using the solar chimney principle. It different discharge coefficients. The best results consists of using energy available from solar indicated an induced airflow of 331m³/h for a solar radiation to heat up the air and induce stack effect, collector area of 2.25m², 0.15m depth of air channel, through the enhancement of pressure and temperature 30° of inclination of the collector with horizontal and differences between inlet and outlet openings. It is an 1000W/m² of solar irradiation. alternative system to provide natural ventilation Also in India, Mathur et al (2006) used experimental inside a building, in locations where winds are not and theoretical methods to investigate the effects of frequent, as it can occur in densely urban areas, for the solar chimney’s inclination angle in the - 1119 - Proceedings of Building Simulation 2011: 12th Conference of International Building Performance Simulation Association, Sydney, 14-16 November. ventilation rates, during summer months. The the following dimensions: 2.7m x 1.6m x 2.3m mathematical modeling consisted of developing three (Figure 2). The inlet opening was located 0.3m above steady state equations of energy balance for glass, air the floor, at the south facade, with an effective open and black plate. The experiment aimed to validate the area of 0.14m2. theoretical solution. A cubical wooden chamber of The experimental chimney had a narrow 1m edge was built, with a 45° tilt solar chimney with parallelepiped shape solar collector of variable tilt, 2 2 1m of solar collector area and 0.35m of air gap. facing north, with dimensions: 1m length, 1m width Experimental and theoretical studies had a good and 0.18m air gap. The black absorber consisted of a agreement. Results have shown that solar chimney’s 1mm thick aluminum sheet painted black and was optimum tilt varies from 45° to 60°, depending upon provided with rectangular fins in the air passage the latitude. The theoretical model indicated an (facing down), in order to increase the heat transfer average air velocity of 0.18m/s inside the chimney between absorber and air, as recommended by Garg for a solar irradiation of 750W/m2 and ambient et al (1991). The absorber was covered with a 6mm temperature of 40°C. For the same conditions, the thick clear glass. All exposed sides of the solar experiment indicated a velocity of 0.22m/s. collector were insulated by a 5cm thick polyurethane Bassiouny and Korah (2009) studied the effects of layer (Figure 3). solar chimney’s inclination angle on ventilation rates and how it affects the airflow pattern inside a space. chimney extension The study was developed through Computational and outlet openings Fluid Dynamics simulation using Ansys software. solar collector Results showed an optimum flow pattern and (facing north) increased velocities for solar collector inclination angles between 45° and 75° with the horizontal, for 28.4° latitude north. Maximum air velocity was obtained for 45° tilt. Sakonidou et al (2008) developed a mathematical model, aiming to determine the solar collector’s tilt that maximizes airflow inside a chimney. The analysis included theoretical predictions and an experimental model. The authors concluded that, when the inclination varies, two things occur in Figure 2 Experimental set-up of inclined solar opposite directions: a lower inclination with the chimney horizontal results in higher exposure to solar irradiation, but reduces stack heitght and, consequently, reduces effective pressure head of the chimney and so diminishes airflow. They obtained an optimum tilt between 65° and 76° for maximum glass cover airflow, and between 12 and 44° for maximum black absorber thermal insulation fins irradiation. The objective of the presented research is to Figure 3 Cross section of the solar collector investigate a possibility to optimize solar irradiation on the solar collector of a solar chimney and also With the intention of optimizing the solar radiation guarantee a considerable stack height, aiming to incidence angle on the glass cover throughout the increase the system’s performance in Brazilian’s low year, the solar collector had an adjustable tilt, varying latitude tropical climate. It consists in using a from 0° to 45° relative to the horizontal plane, chimney extension, which would be responsible to covering the best range of angles for the current maintain a minimum height for the solar chimney, latitude. A chimney extension, made of black independently from the absorber inclination, aluminum sheets, was used to maintain a 1.80m total allowing the use of lower inclination angles for the height for the solar chimney, independently from the solar collector. This is an important subject for solar collector inclination. Outlet openings were designing solar chimneys in buildings located near located in both north and south facades, at the end of the Equator region. the chimney extension. Contact-type sensors were used to measure surface EXPERIMENT temperatures in points located on the absorber An experimental study was developed through the surface, fin surface and inner surface of glazing. Hot- construction of a test cell in the city of Sao Carlos, wire thermo-anemometers were used to measure air Sao Paulo state, Brazil (22° latitude south), under temperature and velocity inside the channel between real weather conditions. absorber and glass and inside the channel between The test cell was built with ceramic bricks and had fins. - 1120 - Proceedings of Building Simulation 2011: 12th Conference of International Building Performance Simulation Association, Sydney, 14-16 November. Outdoor weather data – including dry bulb The computer model had the same geometric and temperature (DBT), relative humidity, wind velocity constructive characteristics as the experimental test and direction, horizontal solar irradiation, pluviosity cell, except that the fins inside the solar collector and atmospheric pressure – were obtained from the were not modelled, due to software limitations. closest weather station of Brazilian’s National Instead, the solar collector was composed by a simple Institute of Meteorology (INMET), situated 200m box, where the black absorber had direct contact with from the test cell location. Direct and diffuse solar thermal insulation (Figure 5). To account for local irradiation were calculated from a recognized resistances existing in the experimental solar theoretical model, published by Muneer (2004). collector, pressure losses of the rectangular fins were According to the law of conservation of mass, it has calculated and added to the simulation model. been assumed that net volume flow rate through the Differences on solar absorption and thermal solar chimney channel is given by stream velocity resistance due to the absence of fins were taken into times the cross sectional area of the channel between consideration in the result analysis.
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