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/(K kg)] 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/(m2 K)] u velocity [m/s] RH relative humidity [%] g gravitational acceleration, 9.8 [m/s2] t time [s] Greek symbols 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] 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/(m s)] 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/ (s m2)]
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. Haghighi Koonsrisuk and Chitsomboon, 2009; Fasel et al., 2013; Krätzig, and Maerefat (2014) investigated the capability of SC to meet the 2013; Cao et al., 2013; Bernardes et al., 2008). Some researchers required thermal and ventilation needs of individuals in winter also have proposed a series of novel SCPP systems (Koonsrisuk, days. The results showed that the system is even capable of provid- 2012; Cao et al., 2011; Kalash et al., 2013; Kashiwa and Kashiwa, ing good indoor air condition with poor solar intensity of 215 W/ 2008; Ming et al., 2016). However, it’s worth mentioning that most m2 and low ambient temperature of 5 °C. Khanal and Lei (2015) researchers are more focused on how to improve the efficiency of carried out a numerical investigation of the buoyancy induced tur- the SC power generation. bulent air flow in an inclined passive wall SC for ventilation appli- Motivated by the diversity development of SC technology, there cations. They showed that the inclined passive wall SC is able to are numerous studies of small SCs for enhancing the natural venti- enhance the thermally driven ventilation. The heat transfer process lation. Gan and Riffat (1998) developed CFD program to investigate and fluid flow in a SC for natural ventilation were numerically and 70 T. Gong et al. / Solar Energy 147 (2017) 68–82 experimentally studied by Amori and Mohammed (2012) in Iraq. immediately improve the air quality in the spectrum of human Meanwhile, they investigated the effect of integrating paraffin activity. (phase change material) in the chimney, and found it could extend the ventilation period after the sunset. Iraqi researchers Imran 2. Model description et al. (2015) built an experimental and numerical model of a SC to study its performance in local environmental conditions. Thus 2.1. System mechanism the two dimensional steady turbulent flow was developed inside an inclined SC. The results indicated that the induced air stream Actually, an attractive approach that using a tower built as an by SC is able to be utilized for natural ventilation and cooling in inverted U-tube to implement such an expansion-compression a natural way (passive). cycle was reported by Oliver et al. (1979). From Fig. 1, the humid In hot and arid climates, there is a huge reliance on air condi- air expands and rises on the left side of the tower, when it past tioners to provide human comfort. Iranian researchers are devoted the top of the tower, cooling (evaporative cooling utilized in this to use conventional wind towers to achieve natural ventilation article) is introduced. As a result, the air on the right side is cooler through living spaces. Rabani et al. (2015) carried out an experi- and denser than that of the left side. Thus compression takes mental study of a new designed Trombe wall in combination with place as the air descends on the right side; an expansion- a solar chimney and water spraying system during the summer in compression cycle could be realized via hydrostatics. In addition, Yazd (Iran). The results showed that the energy storage of the they suggested that a large amount of power could be extracted Trombe wall plays an important role during non-sunny periods. during the expansion which represents the benefit. Conversely, Moreover, the water spraying system was able to decrease indoor a large but less amount of power is put back into the air during ° air temperature and increase relative humidity by about 8 C and compression which represents the cost. The difference is the net 17%, it even enhances thermal efficiency of the system by approx- output aiming to obtain. However, as they pointed, the scheme imately 30%. Maerefat and Haghighi (2010) proposed a system is only a concept due to it is not economically practical in con- consisting of a SC and an evaporative cooling cavity to enhance struction of such a very high tower. In fact, this configuration is passive cooling and natural ventilation in a solar house. The a form of a natural draft by the difference of hydrostatic pressure. required thermal needs of individuals and the effects of geometric If we can find some ways to augment this pressure difference, the parameters on performance were investigated. It found that the cycle can still work as the tower is not very high. Benefit from the system is capable of providing good indoor air condition even with development of SC technology, which is able to produce a strong 2 poor solar intensity of 200 W/m . A higher performance is achieved natural convection inside the tower and provide a promising by using the evaporative cooling cavity. Kalantar (2009) investi- approach. Based on this idea, we propose a solar chimney with gated the cooling performance of a wind tower by the relevant an inverted U-type cooling tower and a water spraying system experiments and numerical studies in Yazd, Iran. He indicated that (SCIUCTWSS). Unlike the reported expansion-compression cycle, the role of SC is considered to develop a natural flow of air without we do not need to extract output power in the expansion process blowing wind. Three-dimensional and steady conditions were used due to the clear air through the processes of this system is the to analyze the air flow with water spraying at the top of the wind desired product. tower. The results showed that the evaporative cooling is very effective as the temperature inside the wind tower decreases con- 2.2. Geometric model siderably. In addition, Guan et al. (2015) also studied the potential use of direct evaporative cooling and found it was very significant To investigate the effectiveness of the SCIUCTWSS, a simplified in Australian climates. model is adopted for the numerical analysis. As shown in Fig. 2, the It is obvious that, the majority of previous SC studies have model has an inverted U-type tower with 200-m-height SC and a focused on the indoor ventilation, few studies on the outdoor ven- 200-m-height cooling tower; both of them have a radius of 5 m. tilation. More importantly, when SC is used in outdoor ventilation, There is a simplified collector with a radius of 120 m and a con- the size should be very large which would raise some economic stant height of 2 m which covers the ground in a round shape, thus and engineering problem. Meanwhile, it’s worth noting that the the collector inclination is not considered. Since solar radiation thermal airflow with a high temperature flows out chimney outlet heats the air inside the ground collector, airflow is driven by the can’t directly improve the air conditions in the spectrum of human buoyancy generated in the system and moves closer to the center activity, so the efficiency of ventilation is difficult to verify. Based of the collector where it goes upwards inside the chimney due to on these issues, we cannot help but raise a question: is there some way to reduce the size of SC and increase the efficiency of air pol- lution control? To answer this question, we propose in this article a novel solar chimney with an inverted U-type cooling tower and a water spraying system (SCIUCTWSS). The scale of the SCIUCTWSS is almost the same as the experimental SCPP in Manzanares (Haaf, 1984). The differences are: (1) an upside down U-shaped tower is used to replace the traditional chimney; (2) a water spray- ing system is installed at the turning point of the U-shaped tower, which will enhance the stack effect; and (3) a filtrating screen is placed near the entrance of the collector due to the low velocity of airflow in this position, and it is assumed to be helpful for the filtration process. The water spray method is utilized and it is very efficient in reducing PM2.5 pollution. Moreover, it has excellent advantages such as rapidity, an already available technology, low cost, and a nature-like process (Yu, 2014). The air which first enters the SC is filtered, then when it goes updraft and gets out at the top of the chimney where it is cooled down thanks to water Fig. 1. Sketch of an inverted U-tube mechanism of expansion-compression cycle evaporation, so that it can go downdraft, then the clean air can (Oliver et al., 1979). T. Gong et al. / Solar Energy 147 (2017) 68–82 71
Continuity equation: @q @ þ ðquiÞ¼Sm ð2Þ @t @xi
where the mass source term Sm is added to or removed from the continuous phase due to evaporation or condensation of the liquid droplets. Navier-Stokes equation: @ @ @ @s ðq Þþ ðq Þ¼ p þ ij þ q þ ð Þ ui uiuj gi Fi 3 @t @xj @xi @xj
where the stress tensor sij is defined as: @ui @uj 2 @ui sij ¼ l þ l dij ð4Þ @xj @xi 3 @xi Energy equation: ! @ @ @ @T X ðqEÞþ ðu ðqE þ pÞÞ ¼ k h J þ u ðs Þ þ S @t @x i @x eff @x j j j ij eff h i j j j ð5Þ
Fig. 2. 3-D geometrical model of the whole SC system. where keff is the effective conductivity (k+kt, where kt is the turbu-
lent thermal conductivity); Jj is the diffusion flux of species j; Sh includes the heat of chemical reaction or any other volumetric heat the stack effect. Near the collector inlet there is a filtrating screen ðs Þ 2 sources. ij eff is the deviatoric stress tensor, defined as: placed vertically with the total area being 1382 m and the thick- ness being 2 m. When the airflow passes through the filtrating @ @ @ ðs Þ ¼ l uj þ ui 2 l uk d ð Þ ij eff eff eff ij 6 screen, PM2.5 and large particulate matter are absorbed by the fil- @xi @xj 3 @xk trating system and are removed from the air. Assuming the sym- metric property to be perpendicular to the z-axis direction, only In Eq. (5): half of the whole system is displayed in the model. This assump- p v2 E ¼ h þ ð7Þ tion is acceptable with steady numerical simulation. The influence q 2 of energy storage layer was not considered and was not included within the geometrical model. The position of water injection is where sensible enthalpy h is defined for incompressible flows as: at the top cooling tower. In this article, the main purpose is to X ¼ þ p ð Þ study the performance of such a SC system with evaporative cool- h Yjhj q 8 j ing, thus the energy required to pump the water to the top of the tower is not considered. In short, pushing volumes of water to where Yj is the mass fraction of species j. the top cooling tower is not difficult. When the water is injected Equation for the turbulent kinetic energy k: into the system, evaporative cooling occurs. @ @ @ l @k ðqkÞþ ðqku Þ¼ l þ t þ G þ G qe Y þ S @t @x i @x r @x k b M k 2.3. Mathematical model i j k j ð9Þ For a conventional SCPP, the airflow inside the system is consid- Equation for the energy dissipation: ered to be natural convection induced by solar radiation heating @ @ @ l @e e e2 the ground wall. Thus the Rayleigh number is introduced to char- ðqeÞþ ðqe Þ¼ l þ t þ ð þ Þ q þ @ @ ui @ r @ C1e Gk C3eGb C2e Se acterize the buoyancy-induced flow in the collector and the t xi xj e xj k k chimney: ð10Þ
3 gbDTL where Gk represents the generation of turbulence kinetic energy Ra ¼ ð1Þ av because of the mean velocity gradients and can be defined as 0 0 Gk ¼ qu u ð@uj=@xiÞ; Gb is the generation of turbulence kinetic where g is the gravitational acceleration, b is the thermal expansion i j energy due to buoyancy; r and re are the turbulent Prandtl num- coefficient, DT is the maximum temperature increase within the k bers for k and e: r = 1.0, re = 1.3. C e, C e are constants: C e = 1.44, system, L is the collector height, a is the thermal diffusivity, v is k 1 2 1 2 e l ¼ð lq =eÞ l the kinematic viscosity. The preliminary resulting values of Ray- C2 = 1.92. t C k and C = 0.09. leigh number are higher than 1010 for the whole system. Therefore, Species transport equation: the turbulent mathematical model needs to be selected to describe @qY @ @ l @Y H2O þ ðqY u Þ¼ qD þ t H2O þ S ð11Þ fluid flow within the system. The standard k e turbulent model is H2O i H2O H2O @t @xj @xj Sct @xj chosen as an economic approach because of its robustness at a rel- atively low computational cost. The density variation of the air is where SH2O is the water vapor added to or removed from the air caused by temperature changes, rather than that of the pressure. because of evaporation or condensation. The incompressible flow is assumed and we use the ideal gas law Scalar quantities: to express the relationship between density and temperature for @ @ @ @/ natural convection (FlunetInc, 2006). As a result, the transport ðq/Þþ ðq/uiÞ¼ C þ S/ ð12Þ @t @x @x @x equations for incompressible turbulent flow can be written as j j j follows: where / is an arbitrary scalar, C is a diffusion coefficient. 72 T. Gong et al. / Solar Energy 147 (2017) 68–82
2.4. Boundary conditions ent air, the heat transfer coefficient is set as 8 W/(m2 K) Bernardes et al., 2008 which can be accepted when the ambient air velocity is The boundary conditions of the computations are shown in not very large. Radiation heat transfer among the canopy and other Table 1. walls of this system in the model is not considered. The solar radi- In this article, the airflow is assumed fully developed and the ation is assumed uniform vertical incident rays, the ground layer is ambient air temperature constantly at 293 K. The ambient relative assumed homogeneous and isotropic. Therefore, the solar radiation humidity is set as 58.5%. Relative static pressure is used for the is set at 857 W/m2, the corresponding heat flux on the ground sur- simulation to analyze the whole pressure distribution of the sys- face is set at 600 W/m2 due to the energy loss through thermal tem, which is the static pressure difference between the radiation and conduction (Ming et al., 2012). As the chimney is SCIUCTWSS and the environment at the same height (set as 0 in not too high, the pressure at both the entrance of the collector this article) (Tingzhen et al., 2006), also used by Pastohr et al. and the exit of the chimney are set equal to the standard atmo- (2004), Ming et al. (2013) and Sangi et al. (2011). Convection heat spheric pressure. The filtrating screen adopts fan boundary condi- transfer occurs between the canopy of the collector and the ambi- tion to simulate the pressure drop. The pressure drop across the filtrating screen can approximate it by Lackner et al. (2012): Table 1 D ¼ qv2 ð Þ Boundary conditions. p 13
Place Boundary Value Thus the pressure drop is much small that can be neglected. type Collector inlet Pressure inlet p = 0 Pa, T = 293 K, 2.5. Meshing method RH = 58.5% Chimney outlet Pressure p=0Pa Fig. 3 shows the grid distribution of the geometric model. Hex- outlet Surface of the chimney No-slip wall q = 0 W/m2 ahedral (HEX) meshing method is applied in the model due to its Surface of the tower No-slip wall q = 0 W/m2 economic and it can reduce false diffusion. Fig. 3(a), it reveals the Filtrating screen Fan 0 Pa local grid distributions within the chimney and its outside ambi- 2 Ground wall under the Heat flux 600 W/m ence. In order to reduce the grid number and improve the compu- canopy Collector canopy No-slip wall T = 293 K, h = 8 W/(m2 K) tational accuracy, the fined grids for the boundary layer were Chimney and tower surface No-slip wall q = 0 W/m2 adopted near the chimney wall, collector wall and ground wall. Symmetry surface Symmetry Moreover, the structured quadrilateral grids were adopted near the joint of the collector and the chimney to minimize errors by
Fig. 3. Grid distributions of the geometric model: (a) local grid distribution near the chimney, (b) local grid distribution near the chimney bottom and (c) local grid distribution near the cooling tower outlet. T. Gong et al. / Solar Energy 147 (2017) 68–82 73
Fig. 4. The grid-independent performance of the numerical simulation results.
Fig. 5. Comparison between numerical simulation results and experimental results.
Fig. 6. Influence of injected water on relative static pressure distributions in the symmetry plane. (a) q = 0 kg/s, (b) q = 1 kg/s, (c) q = 2 kg/s and (d) q = 3 kg/s. 74 T. Gong et al. / Solar Energy 147 (2017) 68–82 the complex flow of air in this part of the model. Fig. 3(b) displays To verify the grid-independent performance of the numerical the local grid distribution near the cooling tower outlet. The fined simulation results, three test cases of the model under the same grids for the boundary layer also were adopted in the tower to sim- conditions (solar radiation is 857 W/m2, injected water is 0 kg/s) ulate the tube flow, because of the relatively steep gradients in with grid numbers being 1,989,315; 2,344,117; 2,665,165 were velocity and temperature near the wall. verified, as shown in Fig. 4. Numerical simulation results showed that the volume flow rates of the system outlet are 105.76 m3/s; 3 3 2.6. Numerical method 106.81 m /s; 106.82 m /s, and the average temperature of the sys- tem outlet are 339.69 K; 339.78 K; 339.45 K, respectively. From the The governing equations are solved by the finite volume comparison between the numerical simulation results, we found method in the general purpose CFD program ANSYS Fluent. SIMPLE that there was only a deviation of approximately 1.0% between algorithm is applied as the pressure-velocity coupling scheme. The these three results, which demonstrated the solutions in this arti- QUICK scheme is used to discretize the convective terms and the cle are grid-independent. The grid number of 2,344,117 is selected second order upwind scheme is chosen as the spatial discretization in this paper. method for the diffusion terms. Besides, the numerical calculation is performed with the double precision solver due to the disparate 3. Validation length scales in the model. To monitor the solution convergence, the iterations were continued until the relative errors for all In order to verify the validity of the numerical codes and variables were below 10 4. procedures, the experimental results collected on 2nd September
Fig. 7. Relative static pressure distributions on four heights of cooling tower with q = 0 kg/s. (a) H = 0 m, (b) H = 50 m, (c) H = 100 m and (d) H = 150 m. T. Gong et al. / Solar Energy 147 (2017) 68–82 75
1982 of the Spanish prototype are compared to the numerical sim- equally and uniformly along the injection surface. The diameter ulation results without cooling tower. The computation parame- and the temperature of the water liquid are assumed to be ters and operation conditions are set according to literature 30 10 6 m and 280 K respectively. Because the air is dry, liquid (Haaf, 1984). The turbine is regarded as a reverse fan with pressure water once injected, evaporative cooling would occur, then the drop at the exit of collector which same used by Xu et al. (2011) air becomes heavier so that a downdraft is formed. and Ming et al. (2013). As shown in Fig. 5, the simulation results To determine the effectiveness of air pollution mitigation, the are quite consistent with the experimental results. According to flow performances of the SC system is investigated. Studying the the air temperature, it shows difference of less than 1%, which relative static pressure changes within the system which is the proved the simulation method is effective. Hence it can be con- cause of the driving force is necessary. Besides, the temperature cluded that the numerical codes and procedures applied in this characteristics of the system play a crucial role for the natural flow. paper is feasible. Air changes in properties caused by temperature variation also provided the basis to measure the flow performances, such as 4. Results and analysis the air density and the air relative humidity distribution in the system. When the solar radiation through the transparent canopy of the collector is absorbed by the ground, the ground temperature rises, 4.1. Comparison of flow performances then heating the air inside the collector. Resulting in the density and the relative humidity of air reduced, strong updrafts of natural Fig. 6 displays the static pressure when the amount of injected convection are formed by air buoyancy difference. When the air water differs from each other. From this figure, it is obvious that flows to the top of the cooling tower, liquid water is injected different amounts of injected water affect the relative static
Fig. 8. Influence of injected water on velocity distributions in the symmetry plane. (a) q = 0 kg/s, (b) q = 1 kg/s, (c) q = 2 kg/s and (d) q = 3 kg/s. 76 T. Gong et al. / Solar Energy 147 (2017) 68–82 pressure distribution of the SC system. A similarity between these produced by the thermal expansion and other factors, thus the cor- figures is that the maximum pressure appears at the top of the U- ner joint loading is complex in the inverted U-type cooling tower. shaped structure. The minimum pressure always presents at the Once liquid water is injected, part of the airflow flows downward, bottom of the chimney, and then the relative pressure increases so the value of the maximum pressure decreases with the increas- gradually through the chimney. These two can be attributed to ing injected liquid water, as shown in Fig. 6(a–d). Besides, as the the updraft. To clarify the maximum pressure presented at the solar radiation heating the air inside the collector, thermal airflow top of the system, Fig. 7 shows the relative static pressure distribu- is generated and vertical natural convection formed. Because of the tions on four heights of cooling tower with q = 0 kg/s. Thus, we stack effect, strong vertical natural convection is produced inside seem to see that, when the thermal airflow rises to the top of the the chimney. Correspondingly, it means that there is greater buoy- system, it will not drop immediately but come together, thus the ancy, therefore the negative pressure or the minimum pressure maximum pressure presented here. On the other hand, corner joint represents the pressure difference between the airflow within usually is a major component of the pipeline system, which not the system and the stable atmosphere outside. Ambient air is only change the direction of the pipeline, but also through the sucked into the bottom of the chimney. In some ways, the value elastic deformation to absorb the force and bending moment of the minimum pressure can reflect the strength of natural
Fig. 9. Influence of injected water on velocity distributions in the collector cross section (half the collector height). (a) q = 0 kg/s, (b) q = 1 kg/s, (c) q = 2 kg/s and (d) q = 3 kg/s. T. Gong et al. / Solar Energy 147 (2017) 68–82 77 convection (Tingzhen et al., 2006). The greater the negative pressure, the stronger the natural flow. By contrasting the relative static pressure distributions as shown in Fig. 6(a–d), we can find that the minimum pressure at the chimney bottom are 13.50 Pa, 68.50 Pa, 105.00 Pa, 136.00 Pa respectively corre- sponding to the injected water: q = 0 kg/s, q = 1 kg/s, q = 2 kg/s, q = 3 kg/s. It is evident that the natural flow undergoes a promotion by liquid water injected in the system. Fig. 8 shows the comparison of contours of velocity distribu- tions at the symmetry plane when the injected water increased from 0 kg/s to 3 kg/s. It is apparent that when no liquid water is injected into the system, as shown in Fig. 8(a), the natural flow inside the system is not strong. If no measures are taken to make the air flow to go downward, the volume flow rate of airflow moved to the outlet is low. Meanwhile, the reversed flow phe- nomenon is found near the system outlet. This means that the flow inside the system is very weak at this time, needed to take mea- Fig. 10. Variations of average velocity of airflow from the collector center to the sures to strengthen the natural flow. By water injection in the sec- collector inlet in the collector cross section (half the collector height). ond part of the inverted U shaped chimney, the velocity of airflow increases with the increasing amount of injected water. As air nearby flows into the collector, heated by the solar radiation and is sucked into the chimney because of the negative pressure. The updraft reaches its peak speed from 9.50 m/s to 13.50 m/s at the chimney bottom, as shown in Fig. 8(b–d). Velocity distributions within the system are very similar, even at the very place where the most intense flow shows up. The only difference is that the magnitude of the velocity is gradually increasing. The increase of velocity is due to the absorption of liquid water by the airflow, which also becomes heavier. The driving force represented by the negative pressure is getting stronger. Strong natural flow formed inside the system. What is noteworthy is that the reversed flow phenomenon disappears in these three figures, which provides an additional evidence for the strong natural flow. In addition, we found that when air flows through the U-turn joint, there will be some flow losses. It indicates that wall friction mainly con- tributes to the depletion of kinetic energy of airflow as the air flows inside the system. In order to further illustrate the flow performances in the sys- Fig. 11. Influence of injected water on the average velocity and volume flow rate of tem, Fig. 9 shows the comparison of contours of velocity distribu- airflow. tions inside the collector. As it is seen, airflow nearby is sucked into the chimney, and the closer the chimney, the greater the speed. Therefore the structured mesh in this area is considered above stagnation area due to the airflow expansion has also been small enough to have acceptable results. At the very center of verified, which is corresponding to the chimney radius. At the the collector, there is a small area of stagnation, where the mag- same time, we noted that the average velocity of airflow increases nitude of speed is relatively small. It is because of the airflow with the increasing amount of injected water, especially at the expansion. Moreover, as the water into the airflow in the cooling transitional corner between the collector and the chimney. When tower, the velocity distribution in the collector also happened to the amount of injected water is 2 kg/s, there is a great increase in change. With the increase of water injection, the natural flow average velocity which proved the evaporative cooling is an effec- near the chimney is getting stronger, as shown in Fig. 9(a–d). This tive method to strengthen natural convection. Then, the increase shows again that water injection is useful which can strengthen in velocity by water injection gradually decreases. Therefore, it is the natural convection, whether it is for SC, cooling tower or col- concluded that the effectiveness of evaporative cooling is sup- lector. Whereas, the influence of ambient crosswind is not taken posed to be weakened with the increasing amount of injected into account in this paper, future studies should consider this water in the collector. influential factor. Fig. 11 denotes the influence of injected water on the average Fig. 10 displays the average velocity of airflow of different velocity and volume flow rate of airflow. It is evident that with water injection changes with the position from the collector cen- the increasing amount of injected water, the velocity of airflow ter to the collector inlet. As shown in this figure, the average increases not only in the collector but also in the chimney. In fact, velocity of airflow begins to increase gradually from the collector the natural flow throughout the system has been strengthened, as inlet, and reaches a maximum at about 5 m away from the center shown in Fig. 11. As for the reason that there is little variation of of collector, and then it drops quickly which is because of the air- velocity in the collector inlet, it is mainly because of the large flow expansion and there is a small area of stagnation as shown radius of the collector, thus the effect on the flow at this place is in Fig. 9. It is a quantitative description of the variations of the relatively small. But even so, we can still see a slight growth trend average velocity of airflow in the collector. The velocity of the from the curve of velocity at the collector inlet. When thermal air- place near the collector inlet is very small. It is the validation flow is sucked into the chimney, reaches a maximum speed at the whether the filtrating screen placed near the collector inlet, the chimney inlet or the chimney bottom, which is consistent with our pressure drop across was neglected by Eq. (13). The scope of above analysis. However, it is apparent that the velocity at the 78 T. Gong et al. / Solar Energy 147 (2017) 68–82 system outlet is lower than that of the chimney outlet. With the since it will not immediately be cooled, it will gradually accumu- increase of water injection, this comparative decrease is also grow- late at the top so that the local pressure is relatively large. To solve ing. It is because on one hand, with the increasing velocity, the flow this problem, it should be found a way to make it cool instantly and losses are become larger when the thermal air flows through the drop. Therefore, evaporating cooling is an effective method but not chimney, corner joint and the cooling tower also increased. On the only method of cooling, the flow performance of the system is the other hand, when thermal air rises to the top of the system, expected to improve in future studies. But even so, from the figure,
Fig. 12. Influence of injected water on temperature distributions in the collector cross section (half the collector height) and in the symmetry plane. (a) q = 0 kg/s, (b) q = 1 kg/s, (c) q = 2 kg/s and (d) q = 3 kg/s. T. Gong et al. / Solar Energy 147 (2017) 68–82 79 the volume flow rate of airflow in the system is remarkable. If the amount of injected water is 9 kg/s, this system is able to process atmospheric air at a volume flow rate of 810 m3/s, corresponding to the volume of 69,984,000 m3 (nearly 0.07 km3) of air to be cleaned in one day. Of course, the efficiency of the performance and the economy should also be taken into comprehensive consid- eration into an actual situation.
4.2. Comparison of temperature characteristics
Fig. 12 shows the influence of injected water on temperature distributions in the collector cross section (y = 1 m) and in the symmetry plane. As seen in this figure, the air temperature gradu- ally rises from the collector inlet to the chimney inlet. Then it will expand and flows upward under the effect of buoyancy inside the chimney by absorbed the solar radiation. The reversed flow appears in Fig. 12(a), which represented the natural flow is rela- Fig. 13. Variations of local air temperature from the collector center to the collector tively weak and causes the value of air temperature outlet is about inlet. 339.78 K at the system outlet. The value of air temperature for the other three conditions is 365.43 K, 356.18 K, 348.22 K, respectively. Meanwhile, the temperature distributions with the increased water injection are different through the system. Firstly, with the increasing injected water, the air temperature decreases in the col- lector. It is because that the injected water strengthens the natural flow in the system, includes the collector. Thus the local convective heat transfer rate is also increased, resulting in the low air temper- ature. The more water injected, the lower the air temperature in the collector. Then, as the airflow sucked into the chimney, there is a flow stagnation zone where the air velocity drops quickly due to the air expansion. Therefore, the local high temperature caused by the local convective heat transfer rate reduced in the center of collector, as it clear shows in Fig. 12. In addition, it is apparent that the air temperature inside the cooling tower decreases with the increasing liquid water injected. This may on one hand explain why the natural flow within the system can be strengthened. The temperature of airflow drops rapidly through evaporative cooling by water injected, which undoubtedly reduces Fig. 14. Influence of injected water on the local air temperature of airflow. the system’s cold junction temperature. Obviously, it increases the buoyancy force originating from the temperature difference between the system and the ambience. From what the comparison after the evaporative cooling, as it is shown in Fig. 14. With the displayed, it is safe to draw the conclusion that water injection has increasing water injected, both of them continue to lower. The only a positive influence on the heat transfer process within the system. difference is that the air temperature of the chimney outlet drops Fig. 13 displays the variations of local air temperature from the more slowly than that of the tower outlet. It is mainly because collector center to the collector inlet. From the figure, the air tem- there is a steady stream of thermal airflow rises to the chimney perature in the collector increases gradually from the inlet to the outlet, and it is not directly affected by evaporative cooling effect, center of the collector. Then there is a significant increase of air but affected by the strengthened natural convection. Besides, the temperature at the chimney bottom, where within the about 5 m temperature drop between them is increasing obviously. From of the center of the collector. The reason for the sharp rise in air what the temperature drop reflected, it can be concluded that temperature is because the reduction of the local convective heat the cooling performance of evaporative cooling increases with transfer rate with a relatively low air velocity. Moreover, it can the increasing amount of injected water, which corresponding to be seen that increasing the amount of water injected, the greater the increasing volume flow rate of airflow. the temperature rise. When the injected water is only 1 kg/s, the decrease of air temperature of the entire collector is most signifi- 4.3. Comparison of air properties cant compared to the condition of no water is injected. It indicates that the cooling efficiency of evaporative cooling may be reduced During the heat transfer process of thermal airflow, air density with the increasing amount of water. Still, we can easily come to changes due to the variation of air temperature. Coupled with the the conclusion that the evaporative cooling is still very effective effect of gravity resulting in buoyancy driven flow, which also is in the collector from the figure. the formation process of natural convection. Therefore the air den- To determine the efficiency of evaporative cooling to the sys- sity plays an important role in the flow process of the system. tem, Fig. 14 denotes the influence of injected water on the local Fig. 15 shows the influence of injected water on air density distri- air temperature. Because of the airflow is relatively intense and butions in the collector cross section (y = 1 m) and in the symme- disordered at the bottom of the chimney, and the adiabatic bound- try plane. From Fig. 15(a), it can be seen that the air density ary condition applied at the chimney wall, so we can use the aver- gradually decreases from the collector inlet to the chimney inlet, age air temperature of the chimney exit on behalf of the thermal which is due to the air temperature variation after the air heated airflow temperature before the cooling. Similarly, the average air by the solar radiation. Accordingly, the density of air inside the temperature of the tower outlet represents the air temperature chimney and the tower is relatively lower corresponding to the 80 T. Gong et al. / Solar Energy 147 (2017) 68–82
Fig. 15. Influence of injected water on air density distributions in the collector cross section (half the collector height) and in the symmetry plane. (a) q = 0 kg/s, (b) q = 1 kg/s, (c) q = 2 kg/s and (d) q = 3 kg/s.
high air temperature. Meanwhile, the reflux phenomenon appears corresponds to the slight air temperature increase in the collector. at the system outlet is still clearly visible, local air density at this Despite this, it does not make the temperature difference or the place is relatively large. From what the figure displayed, it is obvi- density difference of natural convection decrease. The low air ous that the air density distributions are different with the amount density regions caused by the local high air temperature is also of liquid water for evaporative cooling. There is a slight positive apparent from Fig. 15 (b-d) at the very center of the collector. effect of water injection on the air density increase, which Besides, it is clear that the air density increases rapidly by the T. Gong et al. / Solar Energy 147 (2017) 68–82 81
Based on such considerations, we propose in this article a novel SCIUCTWSS. Firstly, the biggest difference is that an upside down U-shaped tower is used to replace the traditional chimney, thus the clean air can be conducted back to ground level and immedi- ately improve the air quality in the spectrum of human activity. Secondly, a water spraying system is installed at the turning point of the U-shaped tower, which will enhance the stack effect and the natural convection, thus the efficiency of this system can be opti- mized. In addition, the water spray method is proved to be very
efficient in reducing PM2.5 pollution or other air pollution (Yu, 2014). Then it is a filtrating screen placed near the entrance of the collector, which is assumed to be helpful for the filtration process. It is necessary to mention that, we propose a new type of con- cept device to mitigate air pollution, and perform numerical inves- tigation, which proves that it is a feasible and promising approach. Nonetheless, some aspects such as the engineering and installa- tions of water spray system and filtrating screen, or whether there is a recirculation of the air that comes out of the reversed U shape Fig. 16. Influence of injected water on the relative humidity of airflow. chimney and then starts a second circulation cycle, are not yet fully explored. That is to say that there is still work to perform the development from this conceptual device to an engineering appli- liquid water added to the airflow in the cooling tower, where the cable one, and in future studies we will overcome these questions evaporative cooling occurs. From the above analysis, we can easily and aspects. come to the conclusion that the density change in the system caused by the evaporative cooling is conductive to the develop- ment of natural convection. 6. Conclusion Fig. 16 denotes the influence of injected water on the relative humidity of airflow at the system outlet and at the cooling tower, In this article, a novel solar chimney with an inverted U-type respectively. Generally, the air relative humidity increases with the cooling tower and a water spraying system (SCIUCTWSS) is pro- increasing amount of injected water at the system outlet, except posed to mitigate urban air pollution. A filtrating screen is the case of no liquid water injected, which the reversed flow placed near the entrance of the collector, PM2.5 and large partic- appears. Moreover, the air relative humidity at the system outlet ulate matter is removed from the air in the natural flow. The is sensitive to the injection of liquid water; it is able to reach to water spray method is utilized at the top of cooling tower for 72.2% with the amount of 9 kg/s water into the airflow. As the evaporative cooling, it causes the air to become heavier and a clean air flows out the system is directly supplied to the people, natural air downdraft to be produced, then the clean air out this sensitivity analysis can provide a reference for adjusting the of the system outlet can immediately improve the air quality relative humidity of the air. On the other hand, it can also be seen in the spectrum of human activity. To investigate the effective- from the figure, with the increase of water for evaporative cooling, ness of this novel chimney system on air pollution control, the air relative humidity inside the cooling tower is also increasing. numerical simulations for performance analyses including com- When the liquid water is injected to the thermal airflow, it vapor- parative analyses and sensitivity analyses in different amount of izes quickly, so there is a sharp increase in the relative humidity of injected water were carried out. The mathematical model to air in a very short period of time. Then the air relative humidity describe the fluid flow, heat transfer of the system was further slightly decreases with the reducing height as the air flows down- developed. The influence of injected water on pressure, velocity, ward. Obviously, the increasing amount of water added, the temperature and air properties distributions was analyzed. The greater the air relative humidity at the system. However, in view numerical simulation results can be draw from the above of the economic and human comfort, we should consider these fac- analyses: tors in an integrated manner. (1) Water injection is efficient to strengthen the natural convec- 5. Further discussion tion, whether it is for SC, cooling tower or collector. Based on this way, the volume flow rate of airflow in the system is SC technology is a method of solar thermal utilization, which remarkable. If the amount of injected water is 9 kg/s, this among its advantages does not require an external driving force system is able to process atmospheric air at a volume flow and can produce strong natural updraft convection. Previous SC rate of 810 m3/s, corresponding to the volume of technology studies on natural ventilation more focused on the 69,984,000 m3 of air to be cleaned in one day. indoor ventilation. When it comes to outdoor ventilation, very (2) Water injection has a positive influence on the heat transfer few investigations are related to it. Even if the conventional SC process within the system, the cooling performance of evap- is utilized for outdoor ventilation, the size should be very large orative cooling increases with the increasing amount of to transfer air pollution from ground level to the upper layers injected water, which corresponds to increasing the volume of the troposphere, which would raise some economic and engi- flow rate of the airflow. neering problems. And more notably, the thermal airflow with (3) The air density variation in the system caused by the evap- higher temperature flows out of chimney outlet can’t directly orative cooling is conductive to the development of natural improve the air conditions in the spectrum of human activity. convection. The greater the amount of water added, the So, in case of intent to spend huge costs to build this giant struc- greater the air relative humidity at the system. However, ture, the efficiency of ventilation is difficult to verify, this is a in view of the economic and human comfort, in future we question worth pondering. should consider these factors in an integrated manner. 82 T. Gong et al. / Solar Energy 147 (2017) 68–82
Acknowledgements Ming, T., Wang, X., de Richter, R.K., Liu, W., Wu, T., Pan, Y., 2012. Numerical analysis on the influence of ambient crosswind on the performance of solar updraft power plant system. Renew. Sustain. Energy Rev. 16, 5567–5583. This research was supported by the National Natural Science Ming, T., Richter, R.K., Meng, F., Pan, Y., Liu, W., 2013. Chimney shape numerical Foundation of China (Nos. 51576077 and 51106060), the Scientific study for solar chimney power generating systems. Int. J. Energy Res. 37, 310– Research Foundation of Wuhan University of Technology (No. 322. Ming, T., Liu, W., Caillol, S., 2014. Fighting global warming by climate engineering: is 40120237), the ESI Discipline Promotion Foundation of WUT (No. the Earth radiation management and the solar radiation management any 35400664), and the Fundamental Research Funds for the Central option for fighting climate change? Renew. Sustain. Energy Rev. 31, 792–834. Universities (WUT Grant No. 2016IVA029). Ming, T., Gong, T., de Richter, R.K., Liu, W., Koonsrisuk, A., 2016. Freshwater generation from a solar chimney power plant. Energy Convers. Manage. 113, 189–200. References Nouanégué, H., Bilgen, E., 2009. Heat transfer by convection, conduction and radiation in solar chimney systems for ventilation of dwellings. Int. J. Heat Fluid Amori, K.E., Mohammed, S.W., 2012. Experimental and numerical studies of solar Flow 30, 150–157. chimney for natural ventilation in Iraq. Energy Build. 47, 450–457. Oliver, T.K., Groves, W.N., Gruber, C.L., Cheung, A., 1979. Energy from humid air. Arce, J., Jiménez, M., Guzmán, J., Heras, M., Alvarez, G., Xamán, J., 2009. Final report. http://dx.doi.org/10.2172/5715593.