Solar Energy 147 (2017) 68–82 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Numerical analysis on a solar chimney with an inverted U-type cooling tower to mitigate urban air pollution ⇑ ⇑ Tingrui Gong a, Tingzhen Ming b, , Xiaoming Huang a, , Renaud K. de Richter c, Yongjia Wu d, Wei Liu a a School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b School of Civil Engineering and Architecture, Wuhan University of Technology, No. 122, Luoshi Road, Wuhan 430070, China c Tour-Solaire.Fr, 8 Impasse des Papillons, F34090 Montpellier, France d School of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg 24060, USA article info abstract Article history: The acceleration of urbanization process, which triggered a series of environmental issues, has become Received 7 October 2016 increasingly prominent. Air pollution, especially atmospheric fine particulate matter (PM2.5), is one of Received in revised form 7 March 2017 the most severe pollutions affecting the human health and living standard. In this article, a novel solar Accepted 10 March 2017 chimney with an inverted U-type cooling tower and a water spraying system (SCIUCTWSS) was proposed to mitigate the urban air pollution. In this system, an inverted U-type cooling tower was used to take place of the traditional chimney erected in the center of the collector; a water spraying system was Keywords: installed at the turning point of the inverted U-type cooling tower to enhance the driving force; a filtra- Air pollution tion screen is placed near the entrance of the collector to filter out PM and large particulate matter from Solar chimney 2.5 U-type cooling tower the airflow. The clean air out of the system’s outlet can immediately improve the air quality in the spec- Water spraying system trum of human activity. A mathematical model to describe the fluid flow, heat transfer of the system was further developed. Influence of injected water from the water spraying system on the pressure, velocity, temperature, and air density distributions were analyzed. The numerical simulation results indicated that water injection is efficient that can strengthen the natural convection and has a positive influence on the heat transfer process within the system. This proposed SCIUCTWSS in this article is able to process atmo- spheric air at a volume flow rate of 810 m3/s, corresponding to the volume of 69,984,000 m3 of air to be cleaned in one day. However, in view of the economic and human comfort, the amount of injected water should be taken into comprehensive consideration. Ó 2017 Elsevier Ltd. All rights reserved. 3 1. Introduction of PM2.5 was measured often exceeded 200 lg/m and reached up to 600 lg/m3 on Jan. 13, 2013 in Beijing (Quan et al., 2014). In 3 The rapid economic development with a dramatic growth of Xi’an, the average PM2.5 mass is 142.6 ± 102.7 lg/m during the urbanization has induced many environmental issues. The emis- whole measurement period, which is more than four times that sion of various pollutants into the atmospheric is one of the most of the Chinese national ambient air quality standard (Wang et al., severe forms of pollutions. Controlling air pollution remains a great 2010). Besides, Xi’an, Tianjin, and Chengdu became the most pol- challenge because of the diversity of sources and the complex evo- luted megacities in the world, all of which had an annual average 3 lution of aerosol particles (de Richter et al., 2017). Atmospheric fine concentration of PM2.5 over 89 lg/m (Zhen et al., 2016). PM2.5 particulate matter (PM2.5) with the aerodynamic equivalent diam- exposure is considered to be the most significant known cause of eters equal or less 2.5 lm is an important determinant of air qual- daily mortality related to poor air quality (U.S. EPA, 2011a; 3 ity which adversely affects human health (WHO, 2006). However, Schwartz and Zanobetti, 2002). A 10 lg/m increase in PM2.5 was research has shown that PM2.5 often exceeds the new National associated to a 1.5% increase in daily mortality (Schwartz et al., Ambient Air Quality Standards of China (75 lg/m3 for 24 h aver- 1996). More than 3.7 million people were killed worldwide yearly age) (Sun et al., 2013). Especially, the hourly mean concentrations due to the air pollution (WHO, 2014a). Therefore, it is imperative to develop strategies to reduce the pollutant emission and to control the air pollution dispersion. ⇑ Corresponding authors. Recently, Cao et al. (2015) proposed a solar-assisted large-scale E-mail addresses: [email protected] (T. Ming), [email protected] (X. Huang). cleaning system for air pollution. The system consists of a http://dx.doi.org/10.1016/j.solener.2017.03.030 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved. T. Gong et al. / Solar Energy 147 (2017) 68–82 69 Nomenclature 2 Ra Rayleigh number DH2O diffusion coefficient of water vapor into air [m /s] cp specific heat capacity [J/(KÁkg)] v local air velocity [m/s] L characteristic length [m] q the amount of injected water [kg/s] T temperature [K] h convective heat transfer coefficient [W/(m2ÁK)] u velocity [m/s] RH relative humidity [%] g gravitational acceleration, 9.8 [m/s2] t time [s] Greek symbols Á 3 Sm mass source term [kg/(s m )] D difference or increase 3 F external body force [N/m ] a thermal diffusivity [m2/s] p pressure [Pa] b thermal expansion coefficient [1/K] E instantaneous energy inside the control volume [J] q air density [kg/m3] Á keff the effective conductivity [W/(m K)] e turbulence kinetic energy dissipation rate [W/kg] J the diffusion flux of species l dynamic viscosity [kg/(mÁs)] S the heat of chemical reaction or any other volumetric h lt turbulent dynamic viscosity coefficient 3 heat sources [W/m ] m kinematic viscosity [m2/s] h sensible enthalpy [m/s2] rk turbulent Prandtl number for k Y mass fraction of species re turbulent Prandtl number for e k turbulence kinetic energy [J/kg] s stress tensor [N/m2] Gk the generation of turbulence kinetic energy due to mean C diffusion efficient velocity gradients [J] / scalar Gb the generation of turbulence kinetic energy due to buoyancy [J] Subscripts C1e, C2e, C3e constants for turbulent model i,j any directions of x, y and z Sct turbulent Schmidt number SH2O water vapor added to or removed from the air [kg/ (sÁm2)] large-scale solar collector with the radius of 2500 m, and a chim- the performance of a glazed SC for heat recovery in naturally- ney with the height of 500 m. There is a filter bank placed near ventilated buildings, the results showed that the predicted ventila- the entrance of the chimney, thus the PM2.5 and larger particulate tion rate increases with the chimney wall temperature. Nouanégué matter is separated from the air. Zhou et al. (2015) proposed high and Bilgen (2009) used numerical method to study the conjugate SCs to drive the warm air containing haze up to higher altitude and heat transfer in solar chimney systems for heating and ventilation enhance the dispersion of dense haze. They made creative use of of dwellings, the results showed that the surface radiation urban heat island instead of a vast and expensive solar collector improves the ventilation performance. Arce et al. (2009) built an to provide warm air. Besides, Ming et al. (2014) also suggested that experimental model of SC in order to investigate the thermal per- the SC technology is able to transfer heat from the Earth surface to formance for natural ventilation. They observed that the air flow the upper layers of the troposphere, thus could cool down the rate is influenced by the differential pressure between input and Earth and combat climate change. Based on these ideas, it seems output, which mainly caused by the thermal gradients and wind that the application of SC is a feasible approach to control the air velocity. Due to small temperature difference between the inside pollution. and outside of the SC, natural ventilation is inefficient in hot and In fact, the idea of solar chimney power plant (SCPP) was first humid climates. Yusoff et al. (2010) proposed a solar induced ven- put forward by Schlaich et al. (2005). It is based on the utilization tilation strategy, which combines a roof solar collector and a verti- of the air density decrease with increasing temperature. The air is cal stack. The findings showed that the proposed strategy is able to heated in a solar collector, then it rises inside a chimney driven by enhance the natural ventilation. The ventilation performance of a buoyancy, and it drives turbines to generate electricity. In 1983, series of connected SCs integrated with a typical two-floor building the world’s first SCPP was built in Manzanares, Spain. This experi- were numerically studied by Wei et al. (2011). The results showed mental SCPP with 194.6 m chimney height and 5.08 m radius was that there exists an optimal chimney length to width ratio and an fully tested and validated till 1989. The relevant experimental optimal inclined angle of second floor chimney inlet, which are results and a scientific description were given by Haaf et al. 12:1 and 4°, respectively. Jing et al. (2015) carried out an experi- (1983) and Haaf (1984). After that, more and more researchers mental SC study with large gap-to-height ratios, they found that engaged in the research of SCPP (Pasumarthi and Sherif, 1998a, the temperature and velocity distribution of airflow in chimney 1998b; Pastohr et al., 2004; Maia et al., 2009; Patel et al., 2014; are highly dependent on heat flux and chimney gap.