Modified Matlab Simulation for a Solar Chimney Passive Ventilation System for the City of Tehran Alberte Erantis Kofoed Lauridsen [email protected] under the direction of Prof. Hamed Hamid Muhammed Medical Engineering School of Technology and Health, KTH Research Academy for Young Scientists July 8, 2015 Abstract Air pollution is the cause of numerous deaths every year and cities where the airflow is restricted by mountain ranges are especially threatened. Tehran is such a city, transcending the air quality standards many days a year. Solar chimneys are constructions which rely on wind and stack effect. Earlier studies have been made about solar chimneys for power production and room ventilation. The aim of our study is to modify a Matlab script in order to simulate a large scale sloped solar chimney with the purpose of passively ventilating the city of Tehran. The results from the study show that there is airflow through the chimneys during the sunlight hours, however it is very limited. The percentage of ventilated air per chimney is too low for it to be an effective solution to Tehrans pollution problem, but it could be valuable solution to smaller scale areas such as factories. Contents 1 Introduction 1 1.1 Solar Chimneys . .1 1.2 Mathematical Model . .3 1.3 Purpose . .6 2 Method 7 2.1 Topography . .7 2.2 Meterological Data . .8 2.3 Simulation . .9 3 Results 13 3.1 Ambient Conditions . 13 3.2 Volume Flow Rate . 14 3.3 Air Changes per Hour . 15 4 Discussion 16 5 Acknowledgements 18 A Appedix 20 B Matlab code for linear approximation of ambient conditions 20 C Matlab code for iterative loop 21 1 Introduction High concentrations of air pollution is the cause of numerous deaths every year. Being exposed to toxic gases can cause both acute and chronic damage [1]. An important thing to take into account when looking at health effects of air pollution is alterations in the composition and size distribution of particulate matter [2]. Tehran, the capital city of Iran, is strongly affected by pollution such as PM10, SO2, NO2, HC, O3 and CO [2]. The topography of Tehran, surrounded by 1000- 3800 mountain ranges in the east, southeast, north, and northwest, complicates and partly restricts the airflow both in and out of the city [2]. The air is particularly constrained when there is inadequacy of wind and cold air amid the winter season. In regards to atmospheric pollution, Tehran is one of the worst areas in the world, transcending the air quality standards many days yearly and generally having higher concentrations of the aforementioned pollutants than the standard level [2]. Morbidity, mortality and other symptoms increase as an effect of the air pollution [3], hence solving this issue is essential. 1.1 Solar Chimneys Solar chimney passive ventilation systems are environmentally friendly and easy to manage [4]. The chimneys consist of a solar collector, which is an air inlet made of glass, a chimney, and a most often a turbine as well. The chimneys rely on wind and on stack effect. As the chimney is heated by solar radiation, the temperature of the air rises and the density of the air is reduced. This causes air to flow upward through the chimney. 1 Previous studies and simulations have been made for small scale vertical chimneys used for room ventilation [5]. Larger scale chimneys have also been researched and simulated for the purpose of producing energy and electricity. A simulation of a sloped solar chimney has been made for a power plant in Lanzhou, China, [4] and in Manzanares, Spain, moreover a prototype of a vertical solar chimney for high latitudes has been constructed [6]. 2 1.2 Mathematical Model Nomenclature 2 Aab Area of absorber (m ) 2 Ac Area of cover (m ) 2 Ai Cross sectional area of chimney inlet to air flow channel (m ) 2 Ao Cross sectional area of chimney outlet to air flow channel (m ) Ar Ratio of Ao to Ai ACH Number of air changes per hour Cd coefficient of discharge of air channel inlet (0.57)) Cfl Specific heat of air (J/kg K) −2 hab Conductive heat transfer coefficient for absorber (W m K) −2 hc Conductive heat transfer coefficient for cover (W m K) −2 hr;av;c Conductive heat transfer coefficient between absorber and cover (W m K) Ls Stack height (m) 1 m_ Mass flow rate (kg s ) −2 Sc Solar radiation heat flux absorbed by cover (W m ) − Sab Solar radiation heat flux absorbed by absorber (W m 2) T a Ambient temperature (K) Tab Mean temperature of absorber (K) Tc Mean temperature of cover (K) Tf Mean temperature of air in chimney (K) Tr Room temperature (K) −2 Ub Overall heat transfer coefficient between vertical wall and room (W m K) − Ut Overall heat transfer coefficient from top of cover (W m 2) v Air velocity at outlet of chimney (m s−1) V_ Volume flow rate (m3 s1) −3 ρf Density of air flow in chimney (kg m ) γ Constant for mean temperature approximation According to the energy balance equation for the glass cover, the incident solar radiation and the radiative heat gained by the glass cover from the absorber wall must equal the convective heat loss to air in the flow channel and the overall heat loss coefficient from the glass to ambient. It can be presented mathematically as [ScAc] + [hr;ab;cAab(Tab − Tf )] = hcAc(Tc − Tf )] + [UtAc(Tc − Ta)]: (1) 3 The energy balance equation for the absorber is based on that the solar radiation is equal to the convection to airflow, the long wave re-radiation to cover and the conduction to the main room. Similarly it can be written mathematically as [SabAab] = [habAab(Tab − Tf )] + hr;ab;cAab(Tab − Tc)] + [UbAab(Tab − Tr)]: (2) The energy balance equation for the airflow can be expressed as: convection from the absorber is equal to the convection from the cover added with the useful heat gained by the air. Mathematically it can be expressed as follows hcAcTc − (hcAc + habAab +mC _ fl/γ)Tf + habAabTab = −(_mCfl/γ)Tr; (3) where the mean temperature approximation coefficient chosen at 0.74, as suggested by Ong and Chow [7], is represented by γ. In the simulation the Gauss Seidel method is used with Successive Over Relax- ation Method for solving linear equation systems. The codes can be found in the appendix. Different files, the main named EISCRV, are used for the code and their main functions are: 1. Implementing the calculation part of temperature, ACH, efficiency, volume flow rate, mass flow rate, and iteration performed. 2. Implementing the calculation of assorted properties of the surface of the glass surface and the absorber wall. This is done in order to retrieve the mean glass temperature, the mean temperature of air and the mean temperature of the 4 vertical wall. 3. Creating the matrix, that is to say the coefficients of temperature and right hand side vector, furthermore, some dimensional attributes. 4. Calculating the volume flow rate, mass flow rate, efficiency and number of air changes per hour (ACH), by alternating different parameters, such as the height of the wall opening and gap between the glass, and plotting them with regard of solar intensity. For a chimney, with two openings and a consistent room air temperature, designed with the purpose of room ventilation, the mass flow rate can be written as an equation as follows, s ρf A0 2gLs(Tf − Ta) m_ = Cd p : (4) 1 + Ar Ta However, when this is calculated it is assumed that there is no effect of room volume on the volumetric flow rate obtained through the following equation, m_ V_ = : (5) ρf Calculating the number of air changes per hours is done by using the following equation, V_ · 3600 ACH = : (6) v The theoretical value of air velocity through Equations 5 and 6 provide the theo- retical air change rate. In order to obtain velocity of air through the flow channel, the temperatures Tf and Ta can be used together with Equation 1. The volume 5 flow rate of air and the number of air changes per hour can be obtained once the value of mass flow rate has been calculated in Equation 5 and 6. 1.3 Purpose The purpose of this study is to simulate a sloped large scale chimney which is to rest on the surrounding mountainsides. The purpose of the chimney is to ventilate the air in the city of Tehran. The Matlab scripts and information from a former study about vertical solar chimneys for room ventilation is modified and adjusted in order to gather information about the effectiveness [5]. The purpose of our study is to evaluate whether or not a large scale sloped solar chimney ventilation system could be a possible temporary solution to Tehran’s pollution problem. Figure 1: Desired airflow through Tehran 6 2 Method 2.1 Topography In order to calculate the amount of solar radiation and approximate the volume of the most polluted part of the city, coordinates of were chosen, see Figure 2 and Table 1. The total area chosen was 498.6 km2. In order to estimate the volume of chosen part of the city, the height difference between the lowest and the highest coordinate, and the total area of the coordinates were multiplied. Height differene : 1615 − 1048 = 0:567km (7) Volume : 498:6 · 0:567 = 282; 7km3 (8) Table 1: Coordinates of the area chosen and height of coordinates in meters above sea level (MSL). Coordinate Height (MSL) 1 35o44’19.3”N 51o06’32.3”E 1214 2 35o47’26.6 ”N 51o23’31.6”E 1613 3 35o47’29.1”N 51o28’40.1”E 1518 4 35o43’50.3”N 51o38’06.3”E 1615 5 35o43’16.1”N 51o30’51.2”E 1287 6 35o34’49.0”N 51o26’13.6”E 1048 7 Figure 2: Sattelite image of Tehran [8], with the investigated area marked out.