Reducing the effect of urban heat island

U.P.B. Sci. Bull., Series …, Vol. …, Iss. …, 201

ISSN 1223-7027

REDUCING THE EFFECT OF URBAN HEAT ISLAND: A CASE STUDY FOR BUCHAREST, ROUMANIA Adrian CIOCĂNEA1, Andrei DRAGOMIRESCU2, Bogdan TOFAN3, Raluca MANOLACHE4 This paper presents an original solution that aims at reducing the pollution level and the effect of urban heat island (UHI) during the summer season in the city of Bucharest, Roumania, using an outdoor ventilation method. This method is based on lifting air volumes from the free surface of Dâmbovița River that crosses the city center, where the UHI has a significant impact, using cross-flow fans placed on floating structures near the concrete river bank. The electric motors of the crossflow fans are supplied with power by solar cells. A numerical study was performed in order to obtain the flow pattern of the cooler air taken from the water surface and blown towards the concrete embankment and roadway surface. It was observed that at low flow rates the cooler air with higher humidity reaches the roadway surface in a compact jet due to Coanda effect. The height of the coherent air jet is of about 3040 cm. This height equals the mean value of the distance between road surface and vehicle exhaust pipes. Due to the higher humidity of the air jet it is expected that dust and exhaust particles from closely above the road are expected to be better fixed at the ground until street sweepers remove them. In this way, the level of pollution with airborne particles could be reduced. The study opens the possibility to approach such issues at a greater scale in order to assess the viability of appropriate solutions for cooling down the UHI as well.

Keywords: pollution, urban heat island, urban greening, outdoor ventilation, CFD. 1. Introduction Recent studies regarding urbanization issues take more and more into consideration the influence that large urban areas have on climate change, first of all due to the formation of the so called Urban Heat Island (UHI). Consequently, a series of permanently evolving concepts have been defined, such as sustainable development, local sustainability, resiliency, etc. Different strategies were subsequently developed in order to address these issues [1, 2]. The strategies proposed to reduce the influence of UHI were elaborated based on the observed 1

Prof., Dept. of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest, Romania, e-mail: adrian.ciocanea@upb.ro 2 Assist. Prof., Dept. of Hydraulics, Hydraulic Machinery and Environmental Engineering, University POLITEHNICA of Bucharest, Romania, e-mail: andrei.dragomirescu@upb.ro 3 “Ion Mincu” University of Architecture and Urbanism 4 “Ion Mincu” University of Architecture and Urbanism

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

effects: increased heat emissions due to energy consumption, both anthropogenic and industrial, combined with heat retaining of buildings with high thermal retaining characteristics [3, 4]; urban street canyon effect [5-7]; reduced wind speed and absence of green areas due to reducing spaces in a built environment dominated by low albedo and low porous materials for pavement [8-10].

The main methods to improve the micro-climate of an UHI are based on increasing the vegetation quantity and the albedo. Different solutions exist to fulfill these purposes: from green (i.e. covered with vegetation) roofs, facades, or walls [11-14] to porous and cool materials for street and pavement covers [15-18].

Under these circumstances it results that, on the one hand, shading from trees and the evapotranspiration process are the main natural factors that contribute to the cooling effect and, on the other hand, the usage of new materials with porous characteristics, bright colors, and favorable thermal properties is the anthropical factor that must be addressed. Another category of studies established the link between UHI and pollution, which has led to research that aims at reducing the level of emissions, mostly from cars, as a solution for decreasing the temperature in large urban areas [19-21].

It must be noted that all research carried out both to identify, evaluate, and quantify and to propose solutions to the UHI problem aim at minimizing a narrow temperature range caused by urban activity. This range is of maximum 5–10°C, depending on the characteristics of the considered location, and appears during narrow time intervals, especially during the hot season, when the effects are felt stronger. Although this temperature range is narrow, it requires various investments at a constant rate, since the results are time-dependent. In the absence of standardized methods for evaluating the impact on environment, the confidence in the efficiency of investments can only grow if they address simultaneously as many as possible of the UHI problems. For this reason it is desirable that the decrease in temperature in built areas to be accomplished both by increasing green areas and the albedo in conjunction with using porous materials for pavement and by using active methods for reducing pollution.

In this context, the present paper presents a solution that combines the increase in green area, the decrease in pavement temperature, and the reduction in pollution by using the water of a river. The case study is presented for the city of Bucharest in Roumania, which is crossed by Dâmbovița River.

2. Methodology In this paper a working method is proposed, that allows evaluating the viability of an original solution to counteract UHI effects. The solution is based on the controlled transport of air masses from the surface of a river towards the built infrastructure, through the filter of a green barrier consisting in plants with high

Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

level of evapotranspiration, embedded in the riverbank. There are papers in literature that propose working frameworks to prioritize urban public or private open space for microclimate cooling in an attempt both to standardize the decision-making process and to classify the types of future studies [1, 22-24]. The methodology proposed in this paper is consistent with the content of the already known methodology and starts by establishing general conditions, identifying neighborhoods, characterizing the built infrastructure, and evaluating the benefits of the solution for different scenarios, including from an economical point of view. The approach in this paper is focused on highlighting the effects of the solution on the microclimate in the vicinity of the studied area, even though such conclusions are not easy to draw [25-29].

2.1. General conditions The case study refers to a project proposed to Bucharest City Hall, which aims at reducing the effects of UHI. Bucharest is situated at a latitude of 44°24'N and a longitude of 26°05'E. Its altitude above sea level varies between 60 m and 90 m. The population density in Bucharest is of about 9,000 inhabitants per km 2.

To characterize the UHI, records from databases were considered, that refer to soil temperature, main wind direction, and pollution level as presented in Fig. 1. By analyzing this data it can be seen that UHI is most intense in the center of the city, where the soil temperature is the highest, the wind speed is the lowest with long periods of still air (the occurrence of wind speeds below 1.5 m/s is higher than 66%), and the traffic-based pollution reaches maximum levels. In the same time it can be noticed that the city is crossed by Dâmboviţa River from N-W to S-E along 22 km (Fig. 1b). The river has a regulated flow with an average velocity that usually is of about 0.2-0.3 m/s. The concrete riverbed, forming a canal with an average width of about 30 m (Fig. 2), crosses the city center and has 6 canal pounds with heads of 1-2 m along 10 km in the area with high urban density.

Temperature measurements along the concrete riverbed with special focus on central areas, where UHI is more intense, showed that during the summer season (in August) the temperatures are as follows: the temperature at water surface in the canal is of about 24-25°C, the temperature of the concrete bank is of about 36-38°C, and the temperature at road surface in the vicinity of the canal is of about 55-58°C. Under the previously mentioned conditions, it can be noticed that a solution with high impact on reducing UHI in the center of Bucharest could consist in an increased greening of the banks of Dâmbovița River. This is expected to change the local microclimate, the effect being stronger as the solution is applied to a larger scale.

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

a)

b) Fig. 1. Data for average soil July – MODIS sky conditions and day (right) [30], based pollution, c) Bucharest

a) Fig. 2. River section 400 m considered in view, b) canal cross-

c) Bucharest: a) temperature (°C) in satellite under clear during night (left) b) map of trafficwind rose for

b) with a length of the study: a) top section [31].

2.2. Solution characterization The solution consists in the greening of the concrete banks of the canal through which Dâmbovița River flows and which crosses the city center. To accomplish this objective, floating platforms that support plants with a high level of evapotranspiration are intended to be placed on the concrete banks.

Additionally, to further increase the level of evapotranspiration, cross-flow fans are installed on the riverside edges of the platforms. Their air discharge of the fans, directed towards the plants on the platforms, is intended to enhance evapotranspiration, thus having an effect similar to an increase in green area. To power the fans, solar cells are used. Fig. 3 presents the design. The panels form a modular assembly and allow easy standardization and maintenance.

Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

By covering the banks, solar radiation cannot reach anymore the concrete slabs. Consequently, the slabs cannot accumulate heat and their daily average temperature is expected to decrease significantly. Additionally, a change in microclimate is expected due to the respiration in the plants on the floating platforms, plants that can be easily irrigated.

As previously mentioned, the site chosen to analyze the efficiency of the proposed solution is located in the center of Bucharest. Fig. 4 presents the expected urbanistic changes following the project implementation.

2.3. Computational model for evaluating microclimate change The evaluation of microclimate change due to applying the proposed solution is carried out in two steps: 1) calculation of the evapotranspiration and 2) modeling of the flow of air taken from water surface and blown over the green surface of the floating panel. After getting through these steps it results both the humidity surplus due to extending the green surface and the humidity growth potential due to aerating this surface.

a)

b)

Fig. 3. a) Fully equipped floating panel, b) floating scheme.

Fig. 4. Site chosen for the study: before (left) and after (right) implementing the project.

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

1) The calculation of the evapotranspiration can be done using the combined Penman-Monteith relationship:

ET0 

900  U 2 (eS  ea ) Tm  273 ,    (1  0.34U 2 )

0.408  ( Rn  G ) 

(0)

where ET0 is the reference evapotranspired water volume (mm/s),  is the rate of change of saturation specific humidity with air temperature (Pa/K), Rn is the net irradiance of the external source of energy flux (W/m 2), G is the ground heat flux (W/m2),  is the psychrometric constant (≈ 66 Pa/K), U2 is the wind velocity at a height of 2 m (m/s), es is the saturated vapor pressure (Pa), ea is the real pressure of water vapors (Pa), es - ea is the pressure deficit of water vapors (Pa), and Tm is the average air temperature (K).

For the present study, which also considers the irrigation of the green area, the potential evapotranspiration, ETP, is calculated. This is the water consumption of a fully developed grass carpet (perennial grass) boasting abundant water.

Considering the Thornthwaite method, the ETP was calculated based on the relationship between the water consumption of a crop and the air temperature: ETP  160 (10 t / I ) a K

,

(0)

where ETP is the monthly potential evapotranspiration (m3/ha), t is the multiannual (normal) average temperature of each month for which ETP is calculated (°C), I is the annual heat index of the zone where the irrigated terrain is located, calculated as sum of monthly heat indices i, K is the light index corresponding to the geographical location of the studied zone, assessed based on the average day length (in hours) of each month, and a is an empirical coefficient given by the formula a  6.75  107 I 3  7.71  105 I 2  1.792  10 2 I  0.49239 .

(0)

Since the calculation of ETP is cumbersome, a simplified method was used, based on the correlation between air temperature and water consumption of the selected crop for monthly heat indices i of 35, 40, 45, and 50. The real optimal evapotranspiration, ETRO, represents the water consumption due to evapotranspiration of plants and to evaporation at ground surface in case of a crop supplied with the optimum quantity of water. To calculate ETRO, the following relationship is used: ETRO  ETP  K p ,

(0)

Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

where ETRO is the monthly water consumption of the crop (m3/ha) and Kp is a correction coefficient depending on plant and growth zone.

For the present study, the perennial plant chosen to be planted on the floating panels is lucerne (or alfalfa, lat. Medicago Sativa). Lucerne has high water consumption during its growing season. Between April and September, the ETP values of lucerne are the highest among the plant species cultivated in Romania.

Table 1 presents values of the optimal real evapotranspiration of lucerne calculated based on the previously presented methodology and on the statistical climatic data available for the geographic zone where Bucharest is located.

Based on data presented in Table 1, the water volume produced by evapotranspiration at the river section presented in Fig. 2 can be assessed. The river section has a length of 400 m and the total length of its banks is of 800 m.

Each floating panel has a length of 3.5 m and a width of 2.5 m, its surface being of 8.75 m2. The number of panels required to cover the river banks is of 800/2.5 = 320. Consequently, the total surface of the panels covered with irrigated lucerne is of 320Ă—8.75 = 2800 m2 = 0.28 ha.

Table 1 Optimal real evapotranspiration of irrigated lucerne under conditions typical for Bucharest Month

ETP [m3/ha]

March

160

0.100

16.00

April

460

0.183

84.18

May

761

0.122

167.42

June

980

1.130

1107.40

July

1112

1.380

1534.56

August

1085

1.380

1492.50

September

830

1.430

1186.90

October

485

1.230

596.55

TOTAL

Kp

ETRO = ETP¡Kp [m3/ha]

6185.51

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

Table 2 ETRO of irrigated lucerne and non-irrigated vine under conditions typical for Bucharest Month

ETRO of irrigated lucerne [m3/ha]

ETRO of non-irrigated vine [m3/ha]

May

167.42

465.73

June

1107.40

549.80

July

1534.56

915.55

August

1492.50

852.16

September

1186.90

592.62

TOTAL

5488.80

3375.90

Presently, water vapors are supplied to the atmosphere by the green crowns of the trees planted by the river banks close to the concrete slabs. There are about 60 trees with an equivalent total surface of their crowns of roughly 0.16 ha. To make possible a comparison between the present microclimate at the studied site and the microclimate after installing the panels, the ETRO value of the trees on the river banks will be calculated. For this purpose, the crowns of the trees will be treated as grape vine (lat. Vitis Vinifera) which has the same level of evapotranspiration. It should be noted that the trees, and consequently the equivalent vine, are not irrigated. Table 2 presents a comparison between ETRO values of irrigated lucerne and non-irrigated vine under conditions specific to Bucharest.

Using data in Table 2 it results that in July, on the river section considered, having banks with a total length of 800 m and an equivalent surface of 0.28 ha, the daily evapotranspiration will be as follows: 14.3 m3/day for the floating panels with lucerne and 4.9 m3/day for the trees already planted by the river banks. The additional daily water volume of 14.3 m3/day obtained by installing the floating panels with lucerne represents an increase of 292% when compared to the case when only trees are present. The ratio of daily water volumes produced by non-irrigated vine assimilated to the trees and by the irrigated lucerne on the panels is of 34%. Lucerne produces in July on a surface of 1 ha about 40% more water vapors than vine and, consequently, than trees substituted with vine.

It should be considered that, according to the available space, the surface of the panels will be higher than that of the trees. If only the surface of the trees is increased, the difference presented previously will decrease, but the decrease will be limited since the space available is limited by the size of a tree crown. And even when such a choice would be made, the increase in water volume supplied by the evapotranspiration of the increased number of trees will be limited and proportional to the growth time of the trees, while the evapotranspiration potential of lucerne will be almost immediately available.

Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

The model chosen to calculate the ETRO level of lucerne shows a clear improvement in microclimate by an increase in the produced volume of water vapors. To further improve the microclimate, a ventilation of the plants at a low air flow rate is proposed. The effect of air currents on increasing evapotranspiration is known. The evapotranspiration is accelerated due to an enhanced turbulent transfer of water vapors from the moist vegetation (shadowed part of the leaf) to the dry environment. Hence, air currents replace moisture with dry air. An air flow at a velocity of about 2 m/s increases evapotranspiration by roughly 20%, while for 6.5 m/s the increase is of 50%. A numerical simulation was carried out for the site under study, where no horizontal natural air currents are present during the summer season.

Fig. 5. Computational domain. All dimensions are in mm.

2) The numerical simulation of the air flow induced over the plants on a floating panel was carried out with the commercial code Ansys Fluent. At this stage only a two-dimensional simulation was performed aiming at assessing how the artificially induced air flow is influenced by the river bank geometry. Fig. 5 presents the computational domain. Its boundaries are formed by a short portion of the river free surface, the cross-flow fan casing with the fan inlet and outlet, the upper side of the floating panel with plants, the river bank and the neighboring sidewalk, a portion of the neighboring road, and the contour that separates the domain from the surrounding atmosphere. The domain was meshed with an unstructured grid using quadrilateral cells. The mesh was refined at the solid boundaries (road, river bank and sidewalk, panel, fan casing), at fan inlet and outlet, and at the river surface, where higher velocity gradients are expected.

The fluid is air at the reference pressure p0 = 105 Pa and the reference temperature T0 = 308.15 K (35°C). The air flow is induced by the cross-flow fan that absorbs air close to the free surface of the river and blows an air jet over the

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

floating panel. Since the flow velocities are expected to be very low, the order of magnitude being of at most a few m/s, the air was considered incompressible.

However, to catch the effect of possible vortices, the flow was treated as unsteady and turbulent. The flow is governed by the continuity and the Reynolds averaged Navier-Stokes equations. The closure for the turbulent stresses was provided by the Shear Stress Transport (SST) k-ď ˇ model, which is considered to be well suited for jet flows [32]. At the fan outlet an air velocity of 1 m/s was set. The fan inlet was required to evacuate a flow rate equal to that introduced into the computational domain through the fan outlet. The no-slip condition was imposed at all solid walls. At the boundaries that separate the domain from the surrounding atmosphere a gauge pressure of 0 Pa was set.

The equations governing the flow were discretized in space with a secondorder upwind scheme. The first-order implicit unsteady formulation was used for the time discretization. A time step of 0.1 s was used. The simulation was stopped after 120 s of the simulation time. At each time step, the convergence criterion was the drop in all scaled residuals below 10 -3, which is considered to be well suited for most of the technical problems [32].

Fig. 6. Streamlines and constant length velocity vectors superimposed on contours of the absolute velocity at the last time step of the simulations.

The numerical results of the last time step are summarized in Fig. 6 in form of streamlines and velocity vectors superimposed on contours of the absolute velocity. The image in Fig. 6 is typical for the other time steps as well since the flow pattern does not show a significant change over time. The flow obtained is similar to a large extent to a theoretical potential flow induced by a jet in the vicinity of a solid wall [33]. The jet blown by the fan has a typical structure, with a core that narrows and a mixing layer that grows as the jet flows past the panel surface. There is only one large vortex that forms at the left boundary, above the road. The jet does not detach either at the panel surface or at the river bank and sidewalk. It only detaches at the edge of the sidewalk, but then it reattaches at the

Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

road. This flow pattern suggests that a Coanda effect appears and influences the jet flow, hindering its separation. The behavior of the jet could be beneficial from the point of view of reducing the pollution level above the road. Considering the dimensions presented in Fig.5, it can be seen that the estimated height of the coherent air jet that reaches the road is of at least 30-40 cm. This height equals the mean value of the distance between road surface and vehicle exhaust pipes. Thus, the additional moisture produced by evapotranspiration and entrained by the jet could help in aggregating some of the pollutant particles from above the road.

Once aggregated, with their weight increased, the particles could drop and remain at ground level until washed away by street sweepers or by rain.

2.4. Cost assessment For the case study presented, three rigging variants of the floating panels were analyzed. For each rigging, the following elements were considered: 1) The plants: either creeper plants (such as ivy – lat. Hedera Helix) or lucerne. In case of creeper plants, the panels are only required to support the branches and foliage since the roots are on the river banks. In case of lucerne, the panels must support boxes with earth where lucerne is planted.

2) Ancillary equipment: solar panels, cross-flow fans, drip irrigation systems.

3) Devices fulfilling aesthetic purposes: light systems, small artesian fountains, etc.

The three riggings lead to costs between 350 EUR/m 2 and 600 EUR/m2. The cost for the entire site was evaluated to be between 980,000 EUR and 1,680,000 EUR, the evaluation error being of 5–10%. The lifetime of the panels was estimated to be of about 15 years. When calculating costs, maintenance was not considered.

The investment could be seriously taken into consideration since no possibility exists at the studied site to increase the green area. The variant of using green roofs cannot be considered because of the inadequate built structure. To better evaluate the effects of the investment, additional studies are required to quantify the improvement in microclimate. Computational algorithms for such studies are available but not generally accepted. Consequently, the best solution remains a pilot project that must be monitored in order to quantify the effects. Considering that the UHI occurs into heights that are roughly three times higher than the highest building at the studied site, relevant conclusions can only be obtained for a large project scale, as it is proposed in the present study.

3. Conclusions Large urban areas have nowadays a strong impact on climate change, a major source of this impact being the appearance of Urban Heat Islands. To be efficient, any solution to this problem should treat as many as possible of the

Adrian Ciocănea, Andrei Dragomirescu, Bogdan Tofan, Raluca Manolache

issues associated to an UHI. It is desirable to decrease the temperature in build areas by increasing, among others, the green surface. The present paper proposes the greening of the concrete banks of Dâmbovița River that crosses the center of Bucharest. The solution consists in using floating panels that support plants with a high level of evapotranspiration. To further increase the level of evapotranspiration and simulate and increase in green area, cross-flow fans powered by solar cells are installed at the riverside edge of the panels to force an air flow over the plants on these panels. Our study shows that when lucerne is planted on the panels a significant increase in the daily water volume released by evapotranspiration is obtained when compared to the evapotranspiration of the trees existing at the site. Additionally, a numerical study shows that the slow air jets created by the cross-flow fans remain attached to the surface of the panels, of the river bank, and of the nearby sidewalk, reaching the space immediately above the nearby road where the moist produced by evapotranspiration and entrained by the jets could help in diminishing the level of pollutant particles.

However, considering the extent of UHI, a large scale project is required to validate the results of the present study and to realistically assess the effects of the proposed solution on the change of the urban microclimate at the studied site.

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Reducing the Effect of Urban Heat Island: a Case Study for Bucharest, Roumania

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