A Concept of a Novel Solar-Assisted Large-Scale Cleaning System ( SALSCS ) for Urban Air Remediation

A solar-assisted large-scale cleaning system (SALSCS) is proposed for air pollution abatement. The system consists of a large-scale flat-plate solar collector, a chimney, and a filter bank. In the basic configuration, an air flow is driven exclusively by buoyancy generated in the collector–chimney system, and PM2.5 and larger particulate matter is separated from the air in the filter bank. The proof of concept is obtained by means of a transient 3D numerical simulation of fluid and heat flow for a full-scale system, implemented in ANSYS Fluent and performed using high-performance computing techniques. The effects of the filter bank on the system performance including heat transfer and fluid flow characteristics are evaluated by studying two system configurations, without and with a filter bank, respectively. The proposed SALSCS system is predicted to be capable of processing atmospheric air at a flow rate of 2.64 × 10 m/s, corresponding to the volume of 22.4 km of polluted air remediated in 24 hours.


INTRODUCTION
Effective approaches to abatement of air pollution are required in the context of the world-wide growth of particulate matter emissions from mobile and stationary sources such as transportation and industrial processing for energy conversion and extraction of natural resources.Many large urban areas experience unacceptably high local concentration of particulates, resulting in health hazards and lower living standards (Monks et al., 2009).The United States Environmental Protection Agency (USEPA) established PM 2.5 standards for particulate matter (PM) in air of less than 2.5 µm in aerodynamic diameter.In January 2013, PM 2.5 was measured to be around 500-800 µg/m 3 in Beijing (Pui et al., 2014).Between 2008 and 2011, the daily average of PM 2.5 in Delhi was 123 ± 87 µg/m 3 (Guttikunda and Calori, 2013).The concentrations at both locations significantly exceed the USEPA standards of 35 µg/m 3 and 12 µg/m 3 for daily and annual averages, respectively (Pui et al., 2014).Air pollution has been shown to severely affect human health through respiratory and cardiovascular diseases, which leads to increased mortality rate (Seaton et al., 1995;Bernstein et al., 2004;Pöschl, 2005;Pope III and Dockery, 2006;Miller et al., 2007;Pope III et al., 2009;Chen et al., 2013;Shah et al., 2013;Pui et al., 2014).Exposure to finer particle fractions such as PM 2.5 has a more pronounced effect on health deterioration than to coarser fractions such as PM 10 (Schwartz et al., 1996;Cifuentes et al., 2000).Furthermore, the presence of particulate matter in the atmospheric air has been shown to impact the local and global climate through dimming, retarding monsoon circulation, changing the hydrological cycle and reduction of local precipitation (Ramanathan and Feng, 2009;Pöschl, 2005).
The idea of creating an air draft by solar heating is implemented in solar chimney power plants (Schlaich et al., 2005;Zhou et al., 2010;Dhahri and Omri, 2013).The first solar chimney power plant in the world was commissioned in Manzanares, Spain, in 1983(Haaf et al., 1983, 1984).Laboratory-scale prototypes (Pasumarthi and Sherif, 1998b;Zhou et al., 2007) and new designs (Bilgen and Rheault, 2005;Cao et al., 2011;Panse et al., 2011) were developed in follow-up studies.Besides experimental investigations, mathematical and numerical models have been employed to evaluate plant performance.Pasumarthi and Sherif (1998a) developed a mathematical model to obtain a relationship between the system parameters and its performance such as air temperature, velocity and power output.The model was validated using the experimental results from the Manzanares system (Pasumarthi and Sherif, 1998b).Bernardes and Weinrebe (2003) used a mathematical model to study the effects of geometry and ambient conditions on the power output of solar chimney power plants.Zhou et al. (2007) studied the effects of solar radiation intensity, solar collector area and chimney height on the system power output using a mathematical model validated using experimental results from their pilot prototype.Pastohr et al. (2004) conducted a 2D numerical simulation by solving the Reynolds-averaged Navier-Stokes equations (RANS) with a standard k-ε turbulence model.A theoretical analysis, validated using a numerical simulation, was carried out by Ming et al. (2006) to evaluate the performance of a solar chimney power plant.Numerical models were also used to study the effects of energy storage layer in the ground (Ming et al., 2008a), rotation speed of the turbine (Ming et al., 2008b), a strong ambient crosswind (Ming et al., 2012) and chimney configurations (Ming et al., 2013) on heat transfer and air flow and system power output.Sangi et al. (2011) developed both a mathematical model and a numerical model to conduct a detailed 2D analysis on the relationship between the solar radiation and the temperature, pressure and velocity profiles in the solar collector.Simulation results from the two models agreed well with the experimental data of the Manzanares prototype.A 3D numerical model developed by Guo et al. (2014) combined the radiation, solar load and turbine models to investigate the effects of different parameters including pressure drop across the turbine, solar radiation and ambient temperature, on the system performance.Fasel et al. (2013) conducted numerical simulations with ANSYS Fluent to verify analytical scaling laws, and obtained high-resolution timedependent numerical results for a solar chimney power plant.High quality flow visualization was presented and flow instabilities were predicted.
Motivated by the developments of the solar chimney technology for power generation, we propose a solar-assisted large-scale cleaning system (SALSCS).This novel system consists of a large solar collector, a chimney and a filter bank and aims at generating air flows in large quantities to facilitate separation of particulate matter from polluted atmospheric air of urban areas.In this paper, a detailed system configuration is presented for an initial design.Heat and fluid flows are analyzed for the concept system using a transient 3D numerical model developed in ANSYS Fluent.Two configurations of the SALSCS, without and with a filter bank, are studied to evaluate its ability for generating air flow and the effect of the filter bank on the flow field.

SYSTEM DESCRIPTION
Fig. 1 shows a schematic diagram of the proposed SALSCS system.The system consists of a large glass collector (1), a chimney (2) and a filter bank (3).Buoyancy- driven flow is induced by solar heating of the collector.Atmospheric air enters the system at the aperture, passes through the collector while heated by contact with the ground and the cover glass plate, and further flows through the filter bank for separation of particulates.The filtered air enters the chimney and is drafted by buoyancy to the exit at the top.This chimney can be located either at the center of the collector or asymmetrically with respect to the collector.The filter bank can be built vertically in the collector along the circumference of the chimney base.To address solar intermittency, energy storage in soil or water under the collector can be employed.As an option, an air fan/turbine (4) can be installed inside the chimney to generate electricity during sunny days and induce air flow using grid-supplied electricity when solar radiation is not available for continuous 24-hour filtering.
To evaluate the ability of the SALSCS for air cleaning, a numerical simulation is conducted for a design with the chimney located at the center of the collector as depicted in Fig. 1.The shape of the chimney wall is given by two hyperbolas of revolution intersecting at the throat area of the chimney with the narrowest cross section.The chimney configuration has been adopted from the paper by Krätzig et al. (2011), which provides a detailed design to address flow instabilities and wind loads.In the baseline configuration, the height of the chimney measured with respect to the ground level is 500 m.The chimney diameter is 200 m at the base, 120 m at the throat and 136 m at its outlet.The collector occupies a circular area with a radius of 2500 m.Its height increases from 8 m to 20 m from the inlet to the bottom of the chimney supported by a circular plain area with a diameter of 332 m and uniform height of 20 m.The filter bank is located along the edge of this plain area.The main geometric parameters of the SALSCS system are listed in Table 1.

ANALYSIS
Since the geometry of the system configuration is axisymmetric, the 3D computational domain is reduced to a 45° segment of the full geometry with periodic boundary conditions applied at the two boundaries in the azimuthal direction (Fig. 2).To simulate the buoyancy-driven flow, the Boussinesq approximation is applied (Spiegel and Veronis, 1960).Rayleigh number is introduced to characterize the natural convection in the collector and the chimney, where g is the gravitational acceleration, β is the thermal expansion coefficient, L is the characteristic length taken equal to either the collector height or the chimney diameter, ν is the kinematic viscosity, α is the thermal diffusivity, and T max and T min are the maximum and minimum temperatures of the system, respectively.The resulting values of Rayleigh number are greater than 10 10 for both the collector and the chimney, indicating that the air flow in the SALSCS system is turbulent.The standard k-ε turbulent model available in ANSYS Fluent is chosen as an economic approach for complex turbulent flow modeling.Reynolds-averaged Navier-Stokes (RANS) equations are solved along with two additional transport equations for the turbulence kinetic energy (k) and turbulence kinetic energy dissipation rate (ε) (ANSYS, 2012).

Governing Equations
The numerical model is based on the continuity equation, the momentum equations for an incompressible flow with the additional buoyancy term, the energy equation, and the k-ε equations.
Continuity equation: Momentum equations: 0 ( ) Energy equation: Transport equation for the turbulent kinetic energy k: Transport equation for the energy dissipation rate ε: In Eqs.(2-6), β and α are the air thermal expansion coefficient and thermal diffusivity, respectively.α is defined as α = K/ρc p , where K is the thermal conductivity, ρ is the air density and c p is the specific heat capacity at constant pressure.G k represents the generation of turbulence kinetic energy caused by the mean velocity gradients, where ' '

Boundary and Initial Conditions
The boundaries of the computation domain are shown in Fig. 2. The outer temperature at both the glass roof (1) and the inlet (4) of the solar collector is assumed equal to ambient temperature.Convection heat transfer occurs between the collector and the above ambient air as described by Eq. ( 9): where K is the thermal conductivity, n sc is the local coordinate normal to the solar collector roof, and h is the convective heat transfer coefficient adopted from the study by Bernardes et al. (2007).The chimney wall ( 2) is assumed to be adiabatic as: where n ch is the local coordinate normal to the chimney wall.The fan boundary condition provided by ANSYS Fluent is utilized to simulate the pressure drop across the filter bank (3).The pressure drop typical of commercial large-scale filters varies from 200 Pa for a new filter to 800 Pa for a used filter (Donaldson, 2014).In this study, the pressure drop across the filter bank is assumed to be 498.3Pa (which translates to 2 inches of water).The pressure at both the inlet (4) and outlet (5) of the system are set equal to the standard atmospheric pressure.In reality, the pressure at the chimney outlet (5) should be slightly lower than the pressure at the collector inlet (4) due to altitude difference.Periodic boundary conditions are applied at the two boundaries ( 6) and (7) in the azimuthal direction.For the boundary condition at the ground surface (8), a constant wall roughness is assumed.For simplicity, we apply a constant heat flux from the ground surface to the flowing air in the collector: where q'' is the heat flux at the ground surface.The no-slip boundary condition is employed at all the wall-type boundaries of the computation domain.The initial conditions for the model can be expressed by Eqs. ( 12a) and (12b):

Numerical Solution
The finite volume method and the implicit Euler time integration scheme are employed to solve the governing equations as implemented in ANSYS Fluent.The pressurebased solver is used.SIMPLE algorithm is applied as the pressure-velocity coupling scheme, and PRESTO!algorithm is selected as the pressure interpolation scheme.The second order upwind scheme is chosen as the spatial discretization method for all governing equations.The numerical calculation is carried out with the double precision solver considering that our geometry has features of very disparate length scales.To monitor the solution convergence, the absolute convergence criterion of 10 -8 is set for the energy equation and of 10 -6 for all other equations.The time step size is set to be 0.005 s at the beginning and slightly increases to 0.05 s when the model shows a good convergence for each time step.
A hybrid meshing strategy is employed.Most of the computational domain is discretized with structured grids while unstructured grids are applied only in the inner region of the chimney and a part of the collector adjacent to the filter bank.1,400,000+ cells are used in the numerical grid.To address the challenge of an excessive computational time associated with modeling the turbulent flow in the large geometry, parallel solution techniques with high-performance computing resources are employed.A simulation job requires 16 nodes each with 8 processor cores per node, and 2500 MB of RAM per processor core.Each node has two quad-core 2.8 GHz Intel Xeon X5560 "Nehalem EP"-class processors (Itasca, 2014).

RESULTS
Fluid flow and heat transfer are modeled for two configurations of SALSCS, without and with a filter bank, respectively, to evaluate its ability for generating air flow and identify the effect of the filter bank on the flow field.Air properties at the temperature of 20°C are used in the simulations and system input parameters for this study are listed in Table 2.
The gauge pressure contours on the computational domain boundaries of the two systems are shown in Fig. 3.A constant pressure drop of 498.3 Pa (2'' H 2 O) prescribed in the model along the filter bank is observed by comparing Figs.3(a) and 3(b).According to the figures, the lowest gauge pressure appears near the bottom of the chimney, which equals to -920.4 Pa for the system without a filter bank and -931.5 Pa for the system including the filter bank.For both the two configurations, this low-pressure region results from vortices generated when the air enters the chimney with a relatively high velocity.The local pressure values along the centerlines of both the collector and the chimney are plotted in Figs.4(a) and 4(b), respectively.Fig. 4(a) shows that the static pressure decreases from the inlet of the collector to the bottom of the chimney.There is also a pressure increase at the radial position from 100 m to 0 m resulting from air flow expansion at this location.Fig. 4(b) indicates the pressure profiles along the centerlines (y-axis) of the two systems' chimneys.The pressure in general increases back to the atmospheric pressure from the chimney bottom to the outlet.Obviously, a lower overall pressure drop in the system is obtained when the filter bank is not included in the system.
The velocity fields for the two SALSCS configurations are presented in Fig. 5.The highest velocity is found at the throat area of the chimney, 38.3 m/s and 26.3 m/s for the system without and with the filter bank, respectively.In addition, vortices appear near the low-pressure region at the bottom of the chimney as discussed before.The local velocity magnitudes along the centerlines of the collector and the chimney are plotted in Figs.6(a) and 6(b), respectively.According to Fig. 6(a), the inlet velocity of the solar collector equals to 3.05 m/s for the system without a filter bank and 2.12 m/s as the filter bank is implemented.The velocity increases from the inlet to the outlet of the collector.At the radial position from 100 m to 0 m, it drops quickly, which is caused by the air flow expansion.Fig. 6(b) shows that the velocity magnitudes in the chimneys increases at first and decreases after the air passes through the throat of the   chimney with the narrowest cross-section area.Comparing the velocity profiles in Fig. 6, we know that the air flow speed is reduced with the filter bank implemented in SALSCS.
The local temperature values along the centerlines of the collector and the chimney are plotted in Figs.7(a) and 7(b) respectively.According to Fig. 7(a), the temperature in the collector increases gradually from the inlet to the outlet of the collector.At the chimney bottom, there is a significant temperature increase by about 10 degrees, which is caused by the reduction of the local convective heat transfer rate with a relatively low air speed.Fig. 7(b) shows that the temperature in the chimney only decreases slightly from the chimney bottom to its outlet.This result can be referred to the adiabatic boundary condition applied at the chimney wall.In general, Fig. 7 indicates that a higher overall temperature in SALSCS is achieved if a filter bank is included in the system.As an explanation, the filter bank decreases the average velocity of the flow field and reduces the convective heat transfer effect in the whole system, leading to the increase of the overall system temperature.
The system volumetric flow rates are given in Table 3.Another factor named flow-rate/solar-collector-area ratio, γ, is also defined as follows, for evaluating the efficiency of the system: where Q  is the volumetric flow rate and A is the area occupied by the solar collector.As indicated by Table 3, the filter bank reduces the flow rate of a SALSCS by 30.5%, and a system with the filter bank implemented can generate a volumetric flow rate of 2.64 × 10 5 m 3 /s.Thus the system is able to clean 22.4 km 3 volume of air in 24 hours, and we believe that it can be more efficient with some optimizations in the future.
As a case study, a simple estimate is obtained for the city of Beijing, China.The municipality of Beijing has a total area of 16,801 km 2 including the suburban districts and rural counties.The urban area enclosed by the 6 th Ring Road of the city, of approximately 300 km 2 and the volume of 750 km 3 (250 m height) is considered.A single SALSCS system can process about 3.04% of air in the considered volume for 24-hour operation.With multiple of the SALSCS systems employed, the approach proposed in this study is promising for urban air remediation in major cities around the world.

SUMMARY
In this paper, a Solar-Assisted Large-Scale Cleaning System (SALSCS) is presented for urban air remediation.By using numerical simulation as a tool, we have designed a SALSCS in ANSYS Fluent which is composed of a fullcircle solar collector of 2500 m in diameter, a chimney of 500 m in height and a filter bank implemented vertically at a location in the collector.Based on the designed system geometry, the numerical simulation is conducted on two system configurations: a system with a filter bank and a system without a filter bank.According to our simulation results, the impacts of the filter bank on the system flow field has been analyzed and discussed.In general, with the  filter bank implemented, there is a decrease in system air flow velocity, and an increase for both the overall system pressure drop and temperature.In addition, our simulation also tells that a SALSCS of our design presented in this paper is able to generate a volumetric flow rate of 2.64 × 10 5 m 3 /s, meaning that 22.4 km 3 of air can be cleaned by the system in 24 hours.We believe that SALSCS will be a promising method for solving air pollutions problems in urban areas worldwide.

Fig. 3 .Fig. 4 .
Fig. 3. Gauge pressure distribution at the boundaries of the computational domain for (a) no filter bank in the system, and (b) the filter bank present in the system.

Fig. 5 .Fig. 6 .
Fig. 5. Streamlines in SALCS for (a) no filter bank in the system, and (b) the filter bank present in the system.

Fig. 7 .
Fig. 7. Temperature profiles in SALSCS along the centerlines of (a) the collector, and (b) the chimney.

Table 1 .
Main geometric parameters of the SALSCS system.
is a constant.Pr t is the turbulent Prandtl number for energy and is modeled as a constant in the standard k-ε model in ANSYS Fluent.C 1ε , C 2ε and C 3ε are constants.σ k and σ ε are the turbulent Prandtl numbers for k and ε, respectively (ANSYS, 2012).

Table 3 .
Volumetric flow rates and flow-rate/green-house-area ratios of SALSCSs.System Volumetric Flow Rate (m 3 /s) Ratio of Flow-Rate/Solar-Collector-Area (m/s) Aarea occupied by the solar collector, m 2 C µ , C 1ε , C 2ε , C 3ε constants for turbulent model