- Email: [email protected]
How high albedo and traditional buildings’ materials and vegetation affect the quality of urban microclimate. A case study
Accepted Manuscript Title: HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY Author: Ferdinando Salata Iacopo Golasi Andrea de Lieto Vollaro Roberto de Lieto Vollaro PII: DOI: Reference:
S0378-7788(15)00309-6 http://dx.doi.org/doi:10.1016/j.enbuild.2015.04.010 ENB 5802
To appear in:
ENB
Received date: Revised date: Accepted date:
3-12-2014 8-4-2015 9-4-2015
Please cite this article as: F. Salata, I. Golasi, A.L. Vollaro, R.L. Vollaro, HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY., Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Title: HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY.
Author names and affiliations:
DIAEE - Area Fisica Tecnica, Università degli Studi di Roma “Sapienza”.
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DIMI - Università degli Studi “Roma TRE”.
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Ferdinando Salata1,*, Iacopo Golasi1,@, Andrea de Lieto Vollaro1,#, Roberto de Lieto Vollaro2,§
* Corresponding author: Ph.D. Ferdinando Salata - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy;
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Phone: +390644585661; Fax. 064880120; Email: [email protected]
@ Ing. Iacopo Golasi - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy; Phone: +390644585661; Fax.
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064880120;Email: [email protected]
# Prof. Andrea de Lieto Vollaro - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy; Phone:
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+390644585720; Fax. 064880120; Email: [email protected] § Ph.D. Roberto de Lieto Vollaro - Postal address: Via Vito Volterra, 62 Rome 00146, Italy, Phone:
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Highlights:
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+390657333505; Email: [email protected]
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Microclimate analysis of enclosed urban context in the Mediterranean area; Experimental calibration of simulation in ENVI-met; Influence of urban green and high albedo materials on outdoor thermal comfort; High albedo materials should be used considering the sky view factor; Urban green could lead to a decrease of 1.5 of the Predicted Mean Vote.
Keywords: urban heat island; outdoor thermal comfort; microclimate mitigation strategies; urban requalification; Predicted Mean Vote; ENVI-met.
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HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY
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ABSTRACT The wellbeing and life quality also depend on the climatic conditions of the surrounding environment. In this
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case study the focus is on those interventions that can be performed, especially on enclosed urban contexts, to control the thermal environment. It pays attention on the effect of the vegetation and high albedo materials
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characterizing horizontal and vertical boundaries of the site and the Cloister by Giuliano da Sangallo, a historical site in Rome, is taken as case study. The model of the site was simulated with the software ENVI-met
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and it was verified thanks to a measurement campaign in situ. Five scenarios with different vertical and horizontal materials of the present buildings were simulated together with an analysis of the variations of physi-
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cal quantities (air temperature, mean radiant temperature, relative humidity, wind speed) affecting the perception of environmental comfort (calculated through the Predicted Mean Vote).
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The result is that in those areas characterized by a Mediterranean climate, where the summer months with high temperatures must be mitigated, the vegetation can be a significant benefit to the environment, and high
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albedo materials can ease the thermal load of the buildings with a higher thermal stress for the pedestrians.
1. INTRODUCTION
Examining the urban microclimate is really important in order to establish the life quality of an urban context, which in turn affects the image of a city. Since the ancient times suitable public spaces were designed to attract as many people as possible and make them more livable spaces [1]. During the years the techniques used for these purposes improved, but only today we have the sophisticated means for an optimized designing of outdoor spaces. In other words it can be possible to improve the quality of an outdoor public space by taking care of those factors influencing the thermohygrometric comfort [2-6]: actually, spaces characterized by a discomfort tend to be avoided or poorly exploited.
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-4It should be kept in mind that recently the chaotic development of cities led to different and unwanted microclimatic changes which in turn determined that temperatures, in the urban setting, were higher than those characterizing the surrounding rural environment. Different studies showed that thermophysical properties of urban surfaces and buildings’ envelopes affect the microclimate and can lead to a rise in air temperature and
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mean radiant temperature values [7,8] and to a variation in thermal-energy performances [9]. This is why outdoor urban areas must be planned by considering, where necessary, the right mitigation
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strategies. Some of these strategies, partially affecting one another, are: the use of high albedo surfaces [10,11], evaporation from porous surfaces [12,13], evaporation from ground-level water surfaces [14] and
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roof ponds [15], vegetated surfaces [16], rooftop gardens [17], and trees [11,18].
This paper, through the analysis of a case study concerning the Cloister of San Peter in Chains (inside the
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Faculty of Engineering of the Sapienza University of Rome), examines how those mitigation strategies affect the microclimate and the outdoor thermal comfort. In particular, through the analysis of different configura-
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tions with different solutions for the vertical and horizontal boundaries of the site, the influence of innovative high albedo materials and vegetation is examined.
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The innovative materials studied were developed to optimize the energy needs of the buildings and it is important to analyze how they affect the outdoor thermal comfort of the pedestrians. The solutions chosen are
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geometric configuration.
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also determined by the historical/architectonic importance of the urban context, characterized by an enclosed
Finally the selected performance metric is the comfort sensation of a pedestrian standing in the Cloister and this sensation is estimated through the PMV (Predicted Mean Vote) [19]. 1.1. MITIGATION STRATEGIES AND MICROCLIMATE Before applying the microclimate mitigation strategies through different solutions for the vertical and horizontal boundaries of the site, what reported in the previous studies was taken into consideration. This paragraph summarizes the results of some studies concerning the exertion of high albedo materials and the presence of vegetation and how they affect the microclimate. In particular high albedo materials lead to lower temperatures of the surfaces exposed to solar radiation than those characterized by more traditional materials, thus having a lower temperature characterizing the environment [20]. Page 3 of 45
-5From this point of view it is important to consider that different studies [21,22] showed how the most commonly used materials for urban spaces, such as asphalt, brick and stone pavements (with a low albedo: respectively 0.05, 0.20 and 0.40) intensify the urban heat island phenomenon. Other studies [23,24] showed how the use of low albedo materials with high heat capacity determine an increase of the temperature differ-
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ence between the city and the surrounding areas, during the night in particular. Another study [25] compared then 93 materials used for outdoor flooring noticing that the albedo is mainly affected by the color of the el-
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ement examined, the surface texture (roughness) and the type of material. The conclusions reached by this study show that flat and smooth tiles made of marble or stone present a higher albedo respect to similar tiles
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made of concrete or granite.
A high albedo leads to lower surface temperatures characterizing the exterior finish of the buildings leading
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to lower temperatures in indoor environments [26], but on the other hand it can affect in a negative way the psychophysical wellbeing of the pedestrians [27-29]. Hence the goal of this study is to consider the influence
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of the albedo on the outdoor thermal comfort too. The existing bibliography shows how a study [30] carried out in Shanghai revealed that if the albedo of the exterior surfaces increases of 0.4, the conditions of the out-
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door comfort get worse with a rise of 5-7 °C of the PET index. On the other hand, another study [31] showed how, in Portland, the use of a white material (with an albedo of 0.91) determines an increase of the globe
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temperature and mean radiant temperature of respectively 0.9 K and 2.9 K and a drop in temperature of in-
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door environments of 1.3 K. It should not be forgotten, as the study [32] points out, that the use of high albedo materials on exterior and opposite walls determines an increase of the cooling load due to the reflected solar radiation inside the environments through the windows. For what concerns the effect of the vegetation on the microclimate, the existing bibliography shows some studies which focused on this issue: the first studies [33] were carried out in the early nineteenth century. Then, more than one study [34-37] reported how, contrary to the urban heat island, the park cool island can lead to a drop in air temperature of 3-4 °C during the summer. In fact vegetation makes the environment cooler through evapotranspiration phenomena which are the result of evaporation (from the earth’s surface) and transpiration (of the vegetation) phenomena [38-40]; moreover a surface with some vegetation reflects a higher amount of solar radiation than one characterized by asphalt [41] and collects a lower amount of heat [36,42]. Another study [43] quantified the decrease of the air temperature determined by the evaporation
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-6processes connected to the grassy and humid soil compared to a similar surface with exposed soil in 6 °C; this study also revealed that in a sunny day a 1 m2 of grass absorbs 12 MJ. A further benefit of having green surfaces (green roofs for example) is the effect that they have on the energy needs for the comfort in indoor environments [44]. In accordance with some studies [45-47] trees and shrubs placed next to a building de-
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termine a decrease of 15-35% of the costs of the air conditioning (reducing the annual cooling load of 10%). Moreover a roof characterized by a black surface exposed to solar radiation during the summer can be as
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much as 50 °C hotter than a roof covered with vegetation with a similar position [48], leading to a rise in air temperature.
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Such rise in temperature has an influence on energy requirements as well [49]. A study [50] showed that during the summer the cooling load of a building, characterized by the presence of offices, placed in the metro-
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politan area of Hangzhou, China, would increase of the 10.8% due to a rise in air temperature of 0.5 K. Other studies [10] reported that a rise in air temperature of 1 K leads to an increase of 2-4% of electric con-
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sumption in the cities of the United States and that in urban areas the 5-10% of electric energy is consumed to cool the buildings thus counterbalancing an increase of the air temperature of 0.5-3.0 K.
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Finally a study carried out in the Mediterranean area [51] showed that during the last forty years, in a classic building for offices in Greece, occurred a decrease of the heating load of about 1 kWh/m2 per decade,
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whereas the cooling load increased of about 5 kWh/m2 per decade.
2. CASE STUDY
The case study examined was the Cloister (Fig. 1) placed next to San Peter in Chains church, in the historic centre of Rome. The city is characterized by a typical Mediterranean climate, that is mild and warm temperatures during the spring and fall. Most rainfall is seen in spring and fall, during the months of November and April in particular. Summer season is usually hot, humid and characterized by low precipitation, whereas winter tend to be mild and wet with isolated phenomena of low temperatures and snowfall. According to Köppen’s classification such weather belongs to the Csa category [52]. The Cloister is part of the complex which nowadays represents the Faculty of Engineering of “Sapienza” University of Rome; its location is right in the historic centre, near the Colosseum. Its origins go back to the Renaissance and the credit for such work goes to Giuliano da Sangallo: this is why it is important both hisPage 5 of 45
-7torically and artistically. The ground floor is characterized by a rectangular portico where on the sides there are seven/eight arches supported by columns. In the middle of the yard is present an octagonal well attributed to Simone Mosca. On the top of the structure there is a plain tripod formed by two pairs of columns supporting an architrave whose father is supposed to be Michelangelo Buonarroti.
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The yard, made of Lombard cobblestones, presents a fountain and an orange tree whereas the portico floor is made of ceramic tiles.
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This configuration is the authentic one and recently the Cloister has received a restoration which chose to re-
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turn to this original structure.
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Fig. 1: Present configuration of the Cloister of the Faculty of Engineering of “Sapienza” University.
In order to make an evaluation of how the urban context affects the thermal comfort in the Mediterranean
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area, the present configuration of the Cloister was then compared to other four configurations and different solutions for the materials of the boundary surfaces were suggested. One of them was the typical Italian gar-
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den and it was not a random choice since the Cloister was characterized by this configuration before the last
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restoration (Fig. 2).
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Fig. 2: Cloister: on the left Configuration 1 with the garden (picture taken in the 90s), on the right Configuration 5 with cobblestones (present days).
Instead, in other configurations the idea was to use a particular covering [53] generated to optimize the albedo coefficient of roofs characterizing buildings placed in historical centers without altering their configuration. This optimization was meant to control the thermal energy transferred inside the building to improve its energy performances, during the summer in particular. For this type of covering, thermal and optical properties of pigments and clay binders were examined because they had, in the visible region, the same characteristics of the traditional tiles whereas in the other regions of the solar spectrum presented a high reflection coefficient (reflectance) (Tab. 1).
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7.2 0.6 100.0 7.9 58.5 79.6 66.0 0.89
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Composition [g] Yellow iron oxideY Red iron oxide R Brown iron oxide B Potassium silicate binder with titanium dioxide Reflectance [%] 300-380 nm (UV) 380.5-780 nm (VISIBLE) 781-2500 nm (NIR) 300-2500 nm (SOLAR) Emissivity [%] IR
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Tab. 1 - Characteristics of the N1C covering used.
values in the region of visible. So in the present study the examined configurations were (Tab. 2):
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It is possible to notice how the reflectance is higher in the region of infrared but it is characterized by lower
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Configuration 1: characterized by a typical Italian garden with twelve orange trees, two palm trees and six bushes. The intervention is restricted to the Cloister courtyard.
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Configuration 2: this configuration was an intervention restricted to the flooring. The covering made of potassium silicate, titanium dioxide and iron oxides previously described was added to the present cob-
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blestone;
Configuration 3: the third configuration was represented by the garden (previously mentioned). The cov-
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ering was not applied on the horizontal boundary but on the inside of the walls. So a simultaneous inter-
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vention occurred on the walls and the courtyard; Configuration 4: in this case the hypothesis was to intervene on both vertical and horizontal surfaces of the structure applying everywhere the covering previously described; Configuration 5: this is the present configuration of the examined Cloister.
Configuration
Walls
Courtyard Portico floor
1
2
3
4
5
Lime plaster
X
X
-
-
X
High albedo material
-
-
X
X
-
Cobblestone
-
-
-
-
X
High albedo material
-
X
-
X
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Lawn and vegetation
X
-
X
-
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Ceramic tiles
X
X
X
X
X
Tab. 2 – Materials of the boundary surfaces of the five configurations examined.
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3. MODEL VALIDATION To validate the model, the analysis focused on a typical cold day and on a typical hot day characterizing Rome’s weather (respectively February 13th and May 21st, 2014) and the experimentally measured values of 4 microclimatic variables (air temperature, mean radiant temperature, relative humidity and global radiation)
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were compared with the results provided by the simulations performed thanks to ENVI-met [54-56]. This software is a 3D prognostic microclimate model based on computational fluid dynamics and thermody-
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namics, with a typical resolution of 0.5–10 m in space and 1–10 s in time. The model is capable of simulating: flow around and between buildings, exchange processes of heat and vapor at urban surfaces, turbulence
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and exchanges of energy and mass between vegetation and its surroundings. Its input parameters include then weather conditions, initial soil wetness and temperature profiles, structures and physical properties of
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urban surfaces, and plants and its reliability is certified by different scientific studies [57-66]. For what concerns the simulations, ENVI-met has allowed the reconstruction of a 3D model of the Cloister
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which is a grid of calculation points based on a matrix (27 x 30) cells which grows in height for 29 cells. The horizontal spacing among the calculation points is then of 2 m, while the vertical spacing is of 0.5 m for the
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first 3 m; once this value was exceeded a telescopic grid with an extension factor of 1.2 was used. The time step for the simulation was then set to 2 seconds while the materials of the boundary surfaces of the Cloister
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presented the following thermo-physical properties:
courtyard (cobblestones): reflectance: 0.40 + emissivity: 0.90;
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walls (lime plaster): reflectance: 0.27 + heat transmission coefficient: 1.25 W/m2K.
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These values derive from the fact that the model validation was carried out by considering the present configuration of the Cloister (Configuration 5). It was also taken into consideration that the structures surrounding the Cloister are massive and able to store much heat in the walls and Cloister ground. For these reasons the site has been monitored through numerical simulation for 3 days and only the results of last simulated day have been considered valid to eliminate the errors due to the transitory period.
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- 10 On the other hand, for what concerns the experimental measurements, the equipment (Tab. 3) was formed by a Delta OHM HD 2102.2 datalogger to which a LP PYRA 03 pyranometer was connected and by an 11 inputs LSI Babuc/A portable microclimate control unit with the following probes: -
a Pt 100 Class B platinum heat-resistant probe to measure the air temperature in the examined environment; a forced ventilation psychrometer probe with distilled water tank to measure the relative humidity;
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a hot-wire anemometric probe to measure wind speed;
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a globothermometric probe to measure the globe temperature with a black opaque copper globe (re-
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-
Hot-wire anemometer
Measuring Range
Air temperature
-50 ÷ +80 °C
0.01 °C
Wind speed
0 ÷ 45 ms-1
0.01 ms-1
Relative humidity
Globothermometer Pyranometer
0 ÷ 100 %
0.1 %
Globe temperature
-40 ÷ +80 °C
0.01 °C
Global radiation
0 ÷ 2000 W/m2
10-3 W/m2
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Forced ventilation psychrometer
Resolution
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Platinum heat-resistant PT100
Measured Variable
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Type of probes
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flection less than 2%) inside it, whereas the centre is characterized by a thermometric sensor.
Accuracy
±0.1 °C at 0 °C DIN 43760 - Class B ± 0.05 ms-1 ISO 7726 ± 0.2 °C over the total range ± 0.1 °C within any scale section of 10 °C DIN 58661 ≤ 0.1 °C at 0 °C DIN 43760 1/3 Second Class ISO 9060 + WMO Classification
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Tab. 3 – Metrological properties of the measuring instruments used.
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The value of the mean radiant temperature (already studied with different software in an enclosed urban area [67]) was then calculated through the following relation [68]:
(1)
where:
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is the globe temperature, measured through the globothermometric probe [°C];
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is the air temperature [°C];
-
is the wind speed [m/s];
-
are respectively the emissivity of the globothermometer and its diameter [mm].
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and
It is also necessary to consider that the air temperature values, expressed in absolute temperature, were con-
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nected to the potential temperature (furnished in output by the software ENVI-met) thanks to the following
is the potential temperature [K];
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is the absolute air temperature [K];
will be determined according to .
[hPa];
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is the pressure.
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-
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where:
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(2)
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relation:
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In order to compare the measured data and the software results, a five points measurement circuit was then set inside the Cloister (Fig. 3).
Fig. 3: Location of the 5 measurement points in the Cloister. Point 1 to point 5 were used for the validation of the model, point 1 to point 3 were used to examine the different configurations.
Finally, it was reported (Figs. 4-7) only the comparison for one of the five points examined, since the variations had the same order of magnitude for every measurement point. For what concerns wind speed, it was not taken into consideration during the model validation phase only because ENVI-met simulates this variable with constant values deriving permanently from the same direction.
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Fig. 4: Trend of the air temperature furnished by ENVI-met and experimentally measured values (for measurement point 1 in Fig. 3).
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Fig. 5: Trend of the mean radiant temperature furnished by ENVI-met and experimentally measured values (for
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measurement point 1 in Fig. 3).
Fig. 6: Trend of the global radiation furnished by ENVI-met and experimentally measured values (for measurement
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point 1 in Fig. 3).
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Fig. 7: Trend of the relative humidity furnished by ENVI-met and experimentally measured values (for measurement
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point 1 in Fig. 3).
It can be noticed how the results obtained through the simulations were close, in a really satisfying way, to
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the experimental data and there was an uniformity between the variables in output from the software and the values of the same variables measured experimentally. The variations reached are represented by Tab. 4.
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This table, with reference to Figs. 4-7, reports for the measurement point 1 the average variance between the measured values and those provided by ENVI-met (absolute values difference averaged based on the 12
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daily sampling performed) and the maximum variance according to those measures.
Summer
Winter
(Δ Value)MED
(Δ Value)MAX
(Δ Value)MED
(Δ Value)MAX
Air temperature [°C]
3,3
4,3
1,5
4,5
Mean radiant temperature [°C]
3,9
7,0
3,5
11,0
Relative humidity [%]
1,3
2,5
1,2
3,0
Global radiation [W/m2]
26,9
130,0
46,6
205,0
Tab. 4 – Average and maximum differences between experimental measurements and ENVI-met output data with reference to point 1 reported in Fig. 3.
Such variations and all the considerations deriving from that, can be extended to the variables sampled in the other four measurement points used for the model calibration.
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4. RESULTS FOR THE MICROCLIMATIC PARAMETERS In this section the values of the microclimatic variables in the five configurations were compared. The microclimatic variables taken into consideration were: mean radiant temperature, air temperature, rela-
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tive humidity and wind speed. Their evaluation was carried out on August, 7th 2012 and February 6th, 2012 representing respectively a heat wave during the summer and a cold one during the winter. The positions of
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the points are represented in Fig. 3. For what concerns point 1, the values of the quantities examined were graphed; whereas for points 2 and 3 the quantities values are showed in Tab. 5 and Tab. 6. The graphs show-
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ing the global radiation were not reported because they had very similar values in all configurations. Finally, the variations of the microclimatic parameters are comparable to each other because they refer to
4.1 THE CASE STUDY DURING SUMMER
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simulations with the same input data, but with different properties of the materials of buildings.
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The following graphs concern point 1 (Fig. 3). The first variable examined was the air temperature. Fig. 8
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shows the trend of this quantity on August 7th, 2012.
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Fig. 8: Graph of the air temperature in the case study during summer.
While analyzing the trend it is possible to notice how the most advantageous configuration, when speaking of air temperature, is the one presenting the typical Italian garden characterizing the courtyard (configuration 1).
Such environment caused a drop in temperature, during the hottest hours of the day, of about 0.7 °C respect to the present configuration (configuration 5). It is also possible to notice a decreasing of the value of this variable by using, in terms of flooring, the covering previously described. Such solution (configuration 2) determined a reduction of the air temperature respect to the present configuration (configuration 5) of about 0.3 °C.
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- 14 The situation got worse in the remaining two configurations: the rise in air temperature estimated was of about 1.4 °C in the configuration with the garden and with the covering applied to the opaque and vertical surfaces of the structure (configuration 3) and of 1.5 °C in the case where the covering had to be applied to the courtyard and to the vertical surfaces (configuration 4). The second variable examined was the mean radiant temperature. Fig. 9 shows the graph according to an
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evaluation of 24 hours on August 7th, 2012.
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Fig. 9: Graph of the mean radiant temperature in the case study during summer.
It can be noticed how the configuration with the lowest values of mean radiant temperature is the one with
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the typical Italian garden without the addition of the covering on the walls (configuration 1). Such change would imply an increasing of the values: the covering on walls and flooring (configuration 4) would deter-
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mine a rise of the mean radiant temperature of about 20 °C respect to configuration 5, which is the present one. This is due to the structure of the Cloister where most of the emitted radiation was reabsorbed by the
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vertical and horizontal boundaries of the examined environment.
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Fig. 10 shows the trend of the relative humidity.
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Fig. 10: Graph of the relative humidity in the case study during summer.
The trend of this variable is strictly connected to the one of the air temperature and the configuration presenting a mean radiant temperature with the lowest values was that characterized by higher values of relative humidity (configuration 1). This configuration is characterized by high relative humidity values due to the resulting soil evaporation and plants transpiration phenomena too. Instead those presenting lower relative humidity values were the configurations with higher air temperature values, that are those with the covered vertical boundaries (configurations 3 and 4). These differences can reach a maximum of 2.5%. Similarly to what was done for other variables examined, while analyzing Fig. 11, it can be noticed how the different configurations affect the wind speed.
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Fig. 11: Graph of the wind speed in the case study during summer.
Hence configurations 1 and 3, both having plants, report the lowest values. As a matter of fact trees can modify, sometimes considerably, the direction and speed of the wind in a certain place; in this study the differ-
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ence between the configurations is of 0.01 m/s.
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4.2 THE CASE STUDY DURING WINTER
This section reports the trend of the variables during different hours of a specific day (February 6th, 2012).
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Fig. 12 shows the air temperature.
Fig. 12: Graph of the air temperature in the case study during winter.
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The case study shows how the lowest values were those of the configuration with the vertical boundaries of the structure unaltered with the covering on the base surface (configuration 2). If this configuration was
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compared to both the present configuration (configuration 5) and the one characterized by the Italian garden (configuration 1) the air temperature would rise respectively of 0.2 °C and 0.1 °C in the middle of the day. In
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fact, by applying the covering on the courtyard, part of the incident radiative flux was reflected without hit-
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ting, in a significant way, the walls and this is due to the high values of sky view factor characterizing the courtyard (the opposite situation is the one with the covering on the vertical boundaries of the structure as well).
Hence the use of the covering on the base surface determined a drop of the parterre temperature and consequently a decrease of the air temperature. Whereas, if the covering is applied on the vertical boundaries as well (configurations 3 and 4), there is a rise in temperature of the opaque surfaces, hence of the air. This happened because the air gets hotter or colder when in contact with bodies which get warm or cool. During the nighttime, the configuration characterized by lower air temperature values was the one with plants only (configuration 1), without the presence of the covering on any surface. For what concerns the mean radiant temperature (Fig. 13), values were lower than those reported by the case study during summer.
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Fig. 13: Graph of the mean radiant temperature in the case study during winter.
The presence of the covering made of potassium silicate and titanium dioxide pigments determined an increase of this variable, in particular when the covering was applied on the vertical boundaries presenting a
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low sky view factor (configurations 3 and 4). Then it is possible to notice how, in the present configuration (configuration 5), mean radiant temperature reached values slightly higher than 42 °C during the hottest
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hours of the day and how those values diminished, of about 2-3 °C, in the configuration with the garden
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Fig. 14 shows the trend of the curves of the relative humidity.
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(configuration 1).
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Fig. 14: Graph of the relative humidity in the case study during winter.
A decrease of the variable during the night is reported: in fact bodies, during the night, tend to cool and
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therefore the vapor present in the air condenses. Whereas in the middle of the day, due to solar radiation, bodies warm and the vapor ,that before condensed, tend to evaporate thus increasing the value of the relative
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humidity.
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The fourth variable examined is the wind speed (Fig. 15).
Fig. 15: Graph of the wind speed in the case study during winter.
Even in this case the presence of trees or plants in general led to the lower values in wind speed (configuration 3). All configurations did not reach differences higher than 0.01 m/s. In the winter velocities are about 0.08 m/s higher than the case study during summer. This is because in this case study the wind speed at 10 m above the ground level, given as input to the software during winter, was higher than the one in summertime (this is a consequence of the meteorological configurations examined). What follows are the tables summing up the quantities examined for points 2 and 3 (Fig. 3) during both seasons. For point 2 it is possible to notice that wind speed values were higher because of their proximity to the
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- 17 side, which is the most aired section of the Cloister, and it is positioned right in the middle of the access points. Case study during summer
Case study during winter Time
Global Radiation [W/m2]
18:00 3.8 3.8 5.7 5.6 3.8 0.4 0.4 1.8 1.8 0.6 58.4 59.4 58.9 59.7 59.1 0.0 0.0 0.0 0.0 0.0 0.7 0.8 0.7 0.8 0.8
ip t
14:00 9.1 9.1 11.8 11.6 9.2 13.5 12.0 24.7 23.0 12.8 57.6 59.1 58.4 60.3 58.4 42.1 42.0 41.6 41.5 42.0 0.7 0.7 0.7 0.7 0.7
cr
10:00 2.3 2.2 4.3 4.1 2.3 9.1 8.8 19.6 18.9 9.2 55.5 56.3 55.6 56.7 56.0 37.5 37.5 37.3 37.3 37.5 0.7 0.8 0.7 0.7 0.8
ed
Wind Speed [m/s]
18:00 33.3 33.5 35.2 35.2 33.7 21.1 23.0 27.7 27.6 24.9 69.1 69.5 67.4 67.8 68.8 11.0 10.7 9.5 9.3 10.6 0.6 0.7 0.6 0.7 0.7
us
Relative Humidity [%]
14:00 39.3 39.8 41.4 41.6 40.0 51.6 64.6 72.2 76.9 57.9 63.4 63.2 61.4 61.6 62.2 968.5 964.6 955.2 952.0 964.5 0.6 0.6 0.6 0.6 0.6
an
Mean Radiant Temperature [K]
10:00 30.1 30.4 32.8 32.8 30.6 25.1 26.0 39.8 37.7 27.2 64.4 64.5 63.3 63.8 63.7 58.2 58.1 57.5 57.4 58.1 0.6 0.6 0.6 0.6 0.6
M
Air Temperature [K]
Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5
Tab. 5 - Summarizing data for point 2.
pt
Case study during summer
Ac ce
Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4
10:00 30.6 30.8 33.3 33.3 31.1 50.4 66.7 70.6 72.6 59.9 63,1 63,2 61,8 62,4 61,9 1004,7 1003,0 993,5 992,0 1002,8 0,3 0,3 0,3 0,3
Air Temperature [K]
Mean Radiant Temperature [K]
Relative Humidity [%]
Global Radiation [W/m2] Wind Speed [m/s]
Case study during winter Time
14:00 39.8 40.3 41.9 42.1 40.5 52.0 68.3 72.8 74.1 63.9 62,1 61,9 59,9 60,1 60,7 975,8 971,8 962,4 959,2 971,8 0,3 0,3 0,3 0,3
18:00 33.6 33.8 35.4 35.5 33.9 20.9 23.7 27.3 27.8 25.6 68,3 68,7 66,5 66,9 68,0 12,4 12,0 10,7 10,4 12,0 0,3 0,3 0,3 0,3
10:00 2.6 2.5 4.7 4.4 2.6 7.6 7.3 17.0 16.3 7.7 53,3 54,1 53,4 54,5 53,7 42,3 42,3 42,0 42,0 42,3 0,4 0,4 0,4 0,4
14:00 9.5 9.5 12.2 12.0 9.6 12.1 10.6 22.4 20.4 11.6 55,3 56,8 56,0 57,9 56,0 47,4 47,4 46,8 46,8 47,4 0,4 0,4 0,4 0,4
18:00 4.0 4.0 5.9 5.9 4.1 -0.6 -0.6 0.9 0.9 -0.2 56,7 57,6 57,0 57,9 57,3 0,0 0,0 0,0 0,0 0,0 0,4 0,4 0,4 0,4
Page 16 of 45
- 18 -
Conf. 5
0,3
0,3
0,3
0,4
0,4
0,4
Tab. 6 - Summarizing data for point 3.
5. ESTIMATING OUTDOOR THERMAL COMFORT
ip t
5.1 PMV COMFORT INDEX The PMV (Predicted Mean Vote) [19] was used to make an estimation of how the five configurations exam-
cr
ined affect the outdoor thermal comfort: it is a thermal comfort index developed thanks to some tests performed in an indoor environment on a valid sample of people and later adapted to outdoor climate through
us
the addition of the shortwave radiation [69].
It combines the energy balance of the human body and the personal sensation of the people exposed to a cer-
an
tain thermal stress and it is able to predict the mean value of the thermal sensation vote of a large group of people.
M
Usually the range of the PMV is between -4 (very cold) and +4 (very hot) where 0 is the thermal neutral (comfort) value. Since the PMV value is a function of the local climate, it can reach higher or lower values
ed
than its range (-4) ÷ (+4) [54]. The higher the distance from the thermal neutral value is, the more it will take the human thermoregulatory system to keep under control the thermal balance by modifying the skin tem-
pt
perature and the secretion of sweat.
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In the Eq. 3, for the estimation of the PMV, M is the metabolic rate while L is the thermal load of the human body, defined as the difference between the inner production of heat and the heat exchanged with the environment. The thermal load takes into consideration: metabolic rate, mechanical factor, partial water vapor pressure, air temperature, mean radiant temperature, surface temperature of clothing, thermal resistance of clothing, convective heat transfer coefficient and relative air velocity.
(3)
Finally, for what concerns L, it is defined as follows:
(4)
Page 17 of 45
- 19 -
where W is the physical work output; Q is the radiation budget, a function of mean radiant temperature TMR and air velocity VA; QH is the turbulent heat flux of sensible heat, a function of air temperature TA and air velocity VA; QL is the latent heat flow due to evaporation of moisture diffused through the skin, a function of relative humidity RH and air velocity VA; QSW is the latent heat flow from sweat evaporation; QRE is the res-
ip t
piratory heat flux (sensible and latent). All the terms in Eq. 3 are expressed in [W/m2] and they have a posi-
cr
tive sign if they represent energy gains for the body: for this reason M is always positive while W, QL e QSW are always negative.
us
5.2 THE PMV IN THE CLOISTER
an
PMV values are provided by ENVI-met and the use of this index is suggested by the German engineering guidelines VDI 3787 [70], developed for outdoor environments.
This paper examines the thermal sensations of a person standing in the Cloister. Thus the metabolic rate will
M
be 70 W/m2, while the mechanical factor 0.0 [71].
ed
For what concerns the insulation factor of the clothes worn by the people usually staying in the Cloister, it was assumed to be 0.4 clo (light summer clothes) in the summer case and 0.95 clo (typical winter clothes) in the winter case (1 clo = 0.155 m2K/W).
pt
Fig. 16 shows the results of each of the three points considered both for the summer and winter case together
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with the estimation of seven different hours representing the usual time interval when people stay in the site.
Fig. 16: PMV during summer and winter for the three points of analysis for every configurations of the case study.
6. CONSIDERATIONS
6.1 CASE STUDY DURING SUMMER Mean radiant temperature: analyzing the data derived from the simulations (Fig. 9) it can be noticed how the configurations characterized by the presence of a garden were subjected to a decrease of the value of this variable respect to those without green surfaces and vegetation. Bushes, with a low green foliage which is closer to the ground and whose growth in height is limited, function at the
Page 18 of 45
- 20 same time as a surface shield and as a barrier to the reflected radiation. On the other hand trees are mostly shields protecting from the radiation and only in part function as surface shields preventing the solar radiation from reaching the surfaces on the side. As a matter of fact shields represented by trees, bushes, etc… are the most successful when there is the necessity to control solar radiation.
ip t
This is why in the configuration characterized by the garden and unaltered surfaces there is a reduction of the mean radiant temperature with the following advantages: radiative exchanges, in relation
cr
to longwave radiations, were reduced, null or reversed contributing to the refreshing of the environment.
us
Instead, in those simulations characterized by the presence of the covering (with a high albedo) made of potassium silicate, titanium dioxide and iron oxides on the vertical surfaces of the structure, the
an
reflected radiation increased facilitating the storage of sensible heat inside the Cloister in the daylight, with a rise of the mean radiant temperature. During this process, just a small part of the radia-
M
tion, hitting the surfaces of the buildings, was reflected back to the sky due to the reduced sky view factor. In fact, if the buildings are positioned in a particularly urbanized context, the visible sky fac-
ed
tor is reduced because of the obstructions produced by other structures. Then the fountain gives a marginal contribution to the wellbeing of the people in the considered site:
pt
this is due to the small dimensions of its water mirror and the view factor value between a pedestrian
Ac ce
and the fountain is high only for those who are in the nearby areas (they can perceive a decrease in mean radiant temperature thanks to the lower surface temperature of water). Air temperature: it is possible to notice (Fig. 8) how the configuration with the potassium silicate and titanium dioxide applied on the courtyard determined a decrease of the value of such variable respect to the present configuration. This was due to the high reflectance of the covering which prevented the temperature of the courtyard from reaching high values, thus keeping the air temperature at lower values than those characterizing the present configuration of the Cloister. The most advantageous configuration was the one with the garden (configuration 1). Since the lawn has a high thermal capacity, it can be considered the perfect solution in terms of thermal comfort, even though its albedo is not very high.
Page 19 of 45
- 21 Configuration 1 can also be considered the ideal solution because of the evapotranspirative effect of the grass and the lawn is one of the main factors for the decrease of the air temperature and mean radiant temperature. The low level of reflection and a low overheating make the garden the perfect choice, in particular if we are dealing with places characterized by sedentary activities. Then the gar-
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den can absorb about 80% of the incident radiation and 70% of the absorbed part is used to activate transpiration phenomena; in this way a rise in temperature will not occur. Finally, for what concerns
cr
the reflected radiation, it is about 15% and it does not represent a big problem for the surrounding zones.
us
Wind speed: another issue taken into consideration is the effect of natural elements (plants, trees, etc…) on the ventilation (Fig. 11). Plants are one of the most commonly used and the most effective
an
windbreak elements, since they can modify both the direction and speed of the wind. Generally, the influence of the plant is directly proportional to the density of its foliage. In the summertime the
M
presence of wind, which favors the cooling process thanks to a higher convective coefficient, is ad-
of these latitudes).
ed
vantageous; hence it should be preferred a vegetation which is less dense such as palm trees (typical
PMV: it can be noticed (Fig. 16) how, in the summer case, the differences among PMV values tend
pt
to be higher when there is a direct shortwave radiation, whereas they decrease in the late after-
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noon/night. Configuration 1 is the best solution for what concerns the thermal comfort. As a matter of fact in the summer such configuration has lower PMV values. The main thermal stresses characterize configurations 3 and 4 presenting wall covering; in particular configuration 4, also with a covering of the basic surface of the yard, has the highest PMV values. While configuration 2, with a direct shortwave radiation, leads to median PMV values: microclimatic conditions get worse respect to configurations 1 and 5, but there is an improvement respect to configurations 3 and 4. The difference among PMV values decreases with diffuse solar radiation. 6.2 CASE STUDY DURING WINTER In a Mediterranean climate the main problems concerning thermal comfort arise during the summer. For this reason, the thermohygrometrics factors must be optimized in this season while during winter the behavior of the site examined will be a direct consequence of the choices previously made. Page 20 of 45
- 22 Mean radiant temperature: it is the variable which is affected the most by such evaluations (Fig. 13). The problems characterizing the summer are completely different to those of the winter. In fact in winter the most advantageous configurations are those with the covering applied on the vertical boundaries of the structure; though such solution would lead to mean radiant temperature values too
ip t
high during the summer followed by a pronounced thermal stress. Air temperature: the city of Rome does not have cold winters: by increasing the level of heat insu-
cr
lation of our clothing, cold temperatures can be absolutely tolerable (Fig. 12).
Wind speed: ventilation must be controlled during winter since it can increase the amount of thermal
us
stress. Obviously plants with deciduous leaves are effective during summer, but not in wintertime. The arboreal choices should be oriented towards evergreen elements. Those configurations with the
an
typical Italian garden present shrubs: they have the same function of trees but on a different level. This is why, to have a reduction of the wind speed, in an area thought to host sitting people, they are
M
more effective than trees because they obstruct and represent a filtration to the wind. PMV: during winter PMV values tend to be similar without reporting high thermal stresses as in the
ed
summer. The closest values to 0, that is the thermal neutral value, are those characterizing configurations 3 and 4 with higher levels of thermal stresses in the summer case. It should not be forgotten
pt
that the city of Rome belongs to the Csa category (Köppen climates classification), hence it has hot
Ac ce
summers with an average temperature during the hottest month which is higher than or equal to 22 °C [52]; this is why the microclimate must be mitigated, in order to reduce the thermal stress, especially in the summer. When dealing with enclosed urban contexts, as the one here examined (surfaces with a low sky view factor), configuration 1 seems the best solution.
7. CONCLUSIONS This study takes into consideration the Cloister of San Peter in Chains and the effects of different mitigation strategies on both the microclimate and the outdoor thermal comfort. Particular attention is paid to the examination of how the presence of some vegetation and high albedo materials (formed by potassium silicate, titanium dioxide and iron oxides) affect the microclimate. This is why the paper analyzes five different con-
Page 21 of 45
- 23 figurations, with different solutions for the vertical and horizontal boundaries of the structure: configuration 1 presents a garden; configuration 2 has high albedo materials applied on the basic surface of the yard; configuration 3 is characterized by both the garden and high albedo materials applied on walls; configuration 4 presents high albedo materials applied on both walls and the basic surface of the yard; configuration 5, the
ip t
one of the present, has cobblestones covering the basic surface of the yard and lime plaster on the walls. The comparison of all configurations was made through numerical simulations performed thanks to the ENVI-
cr
met software (a predictive model where the furnished outputs were verified through experimental measurements) and considering, as outdoor thermal comfort index, the PMV (Predicted Mean Vote).
us
For each of the five configurations it was carried out an analysis of the microclimate and outdoor thermal comfort during a hot summer day and a cold winter day. The main conclusions to the summer case are (with
an
reference to the middle hours of the day and configuration 5, that is the one of the present): air temperature: configuration 1 reports a decrease of about 0.7 °C; configuration 2 a decrease of
M
about 0.3 °C; configuration 3 an increase of about 1.4 °C and configuration 4 an increase of about 1.5 °C.
ed
mean radiant temperature: configuration 1 reports a decrease of about 6 °C; configuration 2 an in-
about 20 °C.
pt
crease of about 8 °C; configuration 3 an increase of about 13 °C and configuration 4 an increase of
Ac ce
relative humidity: values are similar. The maximum variance registered is about 2.5%. wind speed: configurations 2 e 4 report similar values; configurations 1 e 3 present a decrease of about 0.01 m/s.
PMV: when there is direct shortwave radiation configuration 1 reports a decrease of about 1.2; configuration 2 an increase of about 1; configuration 3 an increase of about 1.5 and configuration 4 an increase of about 2. Differences decrease when the direct shortwave radiation is not present. On the other hand the conclusions to the winter case are (with reference to the middle hours of the day and configuration 5): air temperature: configuration 1 reports a decrease of about 0.1 °C; configuration 2 a decrease of about 0.2 °C; configuration 3 an increase of about 2.3 °C and configuration 4 an increase of about 2.1 °C.
Page 22 of 45
- 24 mean radiant temperature: configuration 1 reports a decrease of about 5 °C; configuration 2 an increase of about 7 °C; configuration 3 an increase of about 10 °C and configuration 4 an increase of about 20 °C. relative humidity: values are very similar. The maximum variance registered is about 3%.
ip t
wind speed: configurations 1, 3 and 4 show a decrease (the maximum variance registered is about 0.01 m/s for configuration 3) while 2 a slight increase.
cr
PMV: when there is direct shortwave radiation the difference among the configurations are similar to those of the summer case. Without a direct shortwave radiation configurations 1 and 2 do not show
us
significant variances; configuration 3 presents an increase of about 0.8 and configuration 4 of about 0.6.
an
Hence, according to what was previously reported and the outputs provided by the simulations: The application of high albedo materials on vertical and horizontal boundaries of urban contexts, as
M
the one here examined, determines a worsening of the thermal comfort which leads to increases in the PMV that can reach 2.3 in summer. In particular the extent of this worsening is inversely propor-
ed
tional to the sky view factor of the high albedo surfaces. The exertion of high albedo materials usually causes a microclimate improvement during winter. In
pt
these type of climates, as the one here considered, such improvement is not equal to the worsening
Ac ce
that the use of these materials determines during the summer. The improvement can be quantified, in terms of PMV, with a difference of 1.3. Moreover, in the Mediterranean climate, thermal stresses in the winter are not that high and people can control it by wearing clothes with a higher thermal insulation.
The presence of some vegetation in particularly enclosed urban contexts and with hot climates guarantees the best conditions for what concerns thermal comfort. Especially evapotranspiration phenomena and the capacity of plants of controlling both the incident and reflected direct shortwave radiation determine a decrease in air and mean radiant temperature. This causes decreases, during the middle hours of a hot summer day, of the PMV values until 1.5.
Page 23 of 45
- 25 -
8. ACKNOWLEDGEMENTS This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. A special thanks to Mrs. Flavia Franco for the help she provided in the preparation of this paper.
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[57] X. Yang, L. Zhao, M. Bruse, Q. Meng, Evaluation of a microclimate model for predicting the thermal behavior of different ground surfaces. Building and Environment 60 (2013) 93-104. [58] F. Ali-Toudert, H. Mayer, Numerical study on the effects of aspect ratio and orientation of an urban street canyon on outdoor thermal comfort in hot and dry climate. Build Environ 41 (2006) 94-108. [59] R. Emmanuel, H. Rosenlund, E. Johansson, Urban shading-a design option for the tropics? A study in Colombo, Sri Lanka. Int J Climatol 27 (2007) 1995-2004. [60] M. Fahmy, S. Sharples, On the development of an urban passive thermal comfort system in Cairo, Egypt. Build Environ 44 (2009) 1907-1916. [61] E.L. Krüger, F.O. Minella, F. Rasia, Impact of urban geometry on outdoor thermal comfort and air quality from field measurements in Curitiba, Brazil. Build Environ 46 (2011) 621-634.
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- 30 [62] A. Wania, M. Bruse, N. Blond, C. Weber, Analysing the influence of different street vegetation on traffic-induced particle dispersion using microscale simulations. J Environ Manage 94: (2012) 91101. [63] M. Samaali, D. Courault, M. Bruse, A. Olioso, R. Occelli, Analysis of a 3D boundary layer model at
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local scale: validation on soybean surface radiative measurements. Atmos Res 85 (2007) 183-98. [64] W.T.L. Chow, R.L. Pope, C.A. Martin, A.J. Brazel. Observing and modeling the nocturnal park cool
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island of an arid city: horizontal and vertical impacts. Theor Appl Climatol, 103 (2011) 197-211. [65] E. Ng, L. Chen, Y. Wang, C. Yuan, A study on the cooling effects of greening in a high-density city:
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an experience from Hong Kong. Build Environ 47 (2012) 256-271.
[66] E. Lahme, M. Bruse, Microclimatic effects of a small urban park in densely built-up areas: meas-
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urements and model simulations, in: Fifth International Conference on Urban Climate, ICUC-5, Lodz, Poland, 2003.
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[67] de Lieto Vollaro, R., Vallati, A., Bottillo, S. Differents methods to estimate the mean radiant temperature in an urban canyon 2013 Advanced Materials Research 650, pp. 647-651.
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[68] ISO. International Standard 7726, thermal environment-instruments and method for measuring physical quantities. Geneva: International Standard Organization; 1998.
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[69] Jendritzky, G., Nubler, W.: A model analyzing the urban thermal environment in physiologically
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significant terms. In: Arch. Met. Geoph. Biokl., Ser. B, 29 (1981) 313-326. [70] VDI 3787, 2008. Part 2, Environmental Meteorology – Methods for the Human Biometeorological Evaluation of Climate and Air Quality for Urban and Regional Planning at Regional Level; Part I: Climate. Beuth Verlag, Berlin. [71] ISO 7730, 2005. Ergonomics of the Thermal Environment – Analytical Determination and Interpretation of Thermal Comfort using Calculation of the PMV and PPD Indices and Local Thermal Comfort Criteria. International Organization for Standardization, Geneva.
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S0378-7788(15)00309-6 http://dx.doi.org/doi:10.1016/j.enbuild.2015.04.010 ENB 5802
To appear in:
ENB
Received date: Revised date: Accepted date:
3-12-2014 8-4-2015 9-4-2015
Please cite this article as: F. Salata, I. Golasi, A.L. Vollaro, R.L. Vollaro, HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY., Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
-2-
Title: HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY.
Author names and affiliations:
DIAEE - Area Fisica Tecnica, Università degli Studi di Roma “Sapienza”.
2
DIMI - Università degli Studi “Roma TRE”.
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1
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Ferdinando Salata1,*, Iacopo Golasi1,@, Andrea de Lieto Vollaro1,#, Roberto de Lieto Vollaro2,§
* Corresponding author: Ph.D. Ferdinando Salata - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy;
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Phone: +390644585661; Fax. 064880120; Email: [email protected]
@ Ing. Iacopo Golasi - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy; Phone: +390644585661; Fax.
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064880120;Email: [email protected]
# Prof. Andrea de Lieto Vollaro - Postal address: Via Eudossiana, 18 - 00184 Rome, Italy; Phone:
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+390644585720; Fax. 064880120; Email: [email protected] § Ph.D. Roberto de Lieto Vollaro - Postal address: Via Vito Volterra, 62 Rome 00146, Italy, Phone:
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Highlights:
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+390657333505; Email: [email protected]
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Microclimate analysis of enclosed urban context in the Mediterranean area; Experimental calibration of simulation in ENVI-met; Influence of urban green and high albedo materials on outdoor thermal comfort; High albedo materials should be used considering the sky view factor; Urban green could lead to a decrease of 1.5 of the Predicted Mean Vote.
Keywords: urban heat island; outdoor thermal comfort; microclimate mitigation strategies; urban requalification; Predicted Mean Vote; ENVI-met.
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HOW HIGH ALBEDO AND TRADITIONAL BUILDINGS’ MATERIALS AND VEGETATION AFFECT THE QUALITY OF URBAN MICROCLIMATE. A CASE STUDY
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ABSTRACT The wellbeing and life quality also depend on the climatic conditions of the surrounding environment. In this
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case study the focus is on those interventions that can be performed, especially on enclosed urban contexts, to control the thermal environment. It pays attention on the effect of the vegetation and high albedo materials
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characterizing horizontal and vertical boundaries of the site and the Cloister by Giuliano da Sangallo, a historical site in Rome, is taken as case study. The model of the site was simulated with the software ENVI-met
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and it was verified thanks to a measurement campaign in situ. Five scenarios with different vertical and horizontal materials of the present buildings were simulated together with an analysis of the variations of physi-
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cal quantities (air temperature, mean radiant temperature, relative humidity, wind speed) affecting the perception of environmental comfort (calculated through the Predicted Mean Vote).
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The result is that in those areas characterized by a Mediterranean climate, where the summer months with high temperatures must be mitigated, the vegetation can be a significant benefit to the environment, and high
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albedo materials can ease the thermal load of the buildings with a higher thermal stress for the pedestrians.
1. INTRODUCTION
Examining the urban microclimate is really important in order to establish the life quality of an urban context, which in turn affects the image of a city. Since the ancient times suitable public spaces were designed to attract as many people as possible and make them more livable spaces [1]. During the years the techniques used for these purposes improved, but only today we have the sophisticated means for an optimized designing of outdoor spaces. In other words it can be possible to improve the quality of an outdoor public space by taking care of those factors influencing the thermohygrometric comfort [2-6]: actually, spaces characterized by a discomfort tend to be avoided or poorly exploited.
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-4It should be kept in mind that recently the chaotic development of cities led to different and unwanted microclimatic changes which in turn determined that temperatures, in the urban setting, were higher than those characterizing the surrounding rural environment. Different studies showed that thermophysical properties of urban surfaces and buildings’ envelopes affect the microclimate and can lead to a rise in air temperature and
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mean radiant temperature values [7,8] and to a variation in thermal-energy performances [9]. This is why outdoor urban areas must be planned by considering, where necessary, the right mitigation
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strategies. Some of these strategies, partially affecting one another, are: the use of high albedo surfaces [10,11], evaporation from porous surfaces [12,13], evaporation from ground-level water surfaces [14] and
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roof ponds [15], vegetated surfaces [16], rooftop gardens [17], and trees [11,18].
This paper, through the analysis of a case study concerning the Cloister of San Peter in Chains (inside the
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Faculty of Engineering of the Sapienza University of Rome), examines how those mitigation strategies affect the microclimate and the outdoor thermal comfort. In particular, through the analysis of different configura-
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tions with different solutions for the vertical and horizontal boundaries of the site, the influence of innovative high albedo materials and vegetation is examined.
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The innovative materials studied were developed to optimize the energy needs of the buildings and it is important to analyze how they affect the outdoor thermal comfort of the pedestrians. The solutions chosen are
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geometric configuration.
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also determined by the historical/architectonic importance of the urban context, characterized by an enclosed
Finally the selected performance metric is the comfort sensation of a pedestrian standing in the Cloister and this sensation is estimated through the PMV (Predicted Mean Vote) [19]. 1.1. MITIGATION STRATEGIES AND MICROCLIMATE Before applying the microclimate mitigation strategies through different solutions for the vertical and horizontal boundaries of the site, what reported in the previous studies was taken into consideration. This paragraph summarizes the results of some studies concerning the exertion of high albedo materials and the presence of vegetation and how they affect the microclimate. In particular high albedo materials lead to lower temperatures of the surfaces exposed to solar radiation than those characterized by more traditional materials, thus having a lower temperature characterizing the environment [20]. Page 3 of 45
-5From this point of view it is important to consider that different studies [21,22] showed how the most commonly used materials for urban spaces, such as asphalt, brick and stone pavements (with a low albedo: respectively 0.05, 0.20 and 0.40) intensify the urban heat island phenomenon. Other studies [23,24] showed how the use of low albedo materials with high heat capacity determine an increase of the temperature differ-
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ence between the city and the surrounding areas, during the night in particular. Another study [25] compared then 93 materials used for outdoor flooring noticing that the albedo is mainly affected by the color of the el-
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ement examined, the surface texture (roughness) and the type of material. The conclusions reached by this study show that flat and smooth tiles made of marble or stone present a higher albedo respect to similar tiles
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made of concrete or granite.
A high albedo leads to lower surface temperatures characterizing the exterior finish of the buildings leading
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to lower temperatures in indoor environments [26], but on the other hand it can affect in a negative way the psychophysical wellbeing of the pedestrians [27-29]. Hence the goal of this study is to consider the influence
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of the albedo on the outdoor thermal comfort too. The existing bibliography shows how a study [30] carried out in Shanghai revealed that if the albedo of the exterior surfaces increases of 0.4, the conditions of the out-
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door comfort get worse with a rise of 5-7 °C of the PET index. On the other hand, another study [31] showed how, in Portland, the use of a white material (with an albedo of 0.91) determines an increase of the globe
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temperature and mean radiant temperature of respectively 0.9 K and 2.9 K and a drop in temperature of in-
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door environments of 1.3 K. It should not be forgotten, as the study [32] points out, that the use of high albedo materials on exterior and opposite walls determines an increase of the cooling load due to the reflected solar radiation inside the environments through the windows. For what concerns the effect of the vegetation on the microclimate, the existing bibliography shows some studies which focused on this issue: the first studies [33] were carried out in the early nineteenth century. Then, more than one study [34-37] reported how, contrary to the urban heat island, the park cool island can lead to a drop in air temperature of 3-4 °C during the summer. In fact vegetation makes the environment cooler through evapotranspiration phenomena which are the result of evaporation (from the earth’s surface) and transpiration (of the vegetation) phenomena [38-40]; moreover a surface with some vegetation reflects a higher amount of solar radiation than one characterized by asphalt [41] and collects a lower amount of heat [36,42]. Another study [43] quantified the decrease of the air temperature determined by the evaporation
Page 4 of 45
-6processes connected to the grassy and humid soil compared to a similar surface with exposed soil in 6 °C; this study also revealed that in a sunny day a 1 m2 of grass absorbs 12 MJ. A further benefit of having green surfaces (green roofs for example) is the effect that they have on the energy needs for the comfort in indoor environments [44]. In accordance with some studies [45-47] trees and shrubs placed next to a building de-
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termine a decrease of 15-35% of the costs of the air conditioning (reducing the annual cooling load of 10%). Moreover a roof characterized by a black surface exposed to solar radiation during the summer can be as
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much as 50 °C hotter than a roof covered with vegetation with a similar position [48], leading to a rise in air temperature.
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Such rise in temperature has an influence on energy requirements as well [49]. A study [50] showed that during the summer the cooling load of a building, characterized by the presence of offices, placed in the metro-
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politan area of Hangzhou, China, would increase of the 10.8% due to a rise in air temperature of 0.5 K. Other studies [10] reported that a rise in air temperature of 1 K leads to an increase of 2-4% of electric con-
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sumption in the cities of the United States and that in urban areas the 5-10% of electric energy is consumed to cool the buildings thus counterbalancing an increase of the air temperature of 0.5-3.0 K.
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Finally a study carried out in the Mediterranean area [51] showed that during the last forty years, in a classic building for offices in Greece, occurred a decrease of the heating load of about 1 kWh/m2 per decade,
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whereas the cooling load increased of about 5 kWh/m2 per decade.
2. CASE STUDY
The case study examined was the Cloister (Fig. 1) placed next to San Peter in Chains church, in the historic centre of Rome. The city is characterized by a typical Mediterranean climate, that is mild and warm temperatures during the spring and fall. Most rainfall is seen in spring and fall, during the months of November and April in particular. Summer season is usually hot, humid and characterized by low precipitation, whereas winter tend to be mild and wet with isolated phenomena of low temperatures and snowfall. According to Köppen’s classification such weather belongs to the Csa category [52]. The Cloister is part of the complex which nowadays represents the Faculty of Engineering of “Sapienza” University of Rome; its location is right in the historic centre, near the Colosseum. Its origins go back to the Renaissance and the credit for such work goes to Giuliano da Sangallo: this is why it is important both hisPage 5 of 45
-7torically and artistically. The ground floor is characterized by a rectangular portico where on the sides there are seven/eight arches supported by columns. In the middle of the yard is present an octagonal well attributed to Simone Mosca. On the top of the structure there is a plain tripod formed by two pairs of columns supporting an architrave whose father is supposed to be Michelangelo Buonarroti.
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The yard, made of Lombard cobblestones, presents a fountain and an orange tree whereas the portico floor is made of ceramic tiles.
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This configuration is the authentic one and recently the Cloister has received a restoration which chose to re-
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turn to this original structure.
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Fig. 1: Present configuration of the Cloister of the Faculty of Engineering of “Sapienza” University.
In order to make an evaluation of how the urban context affects the thermal comfort in the Mediterranean
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area, the present configuration of the Cloister was then compared to other four configurations and different solutions for the materials of the boundary surfaces were suggested. One of them was the typical Italian gar-
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den and it was not a random choice since the Cloister was characterized by this configuration before the last
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restoration (Fig. 2).
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Fig. 2: Cloister: on the left Configuration 1 with the garden (picture taken in the 90s), on the right Configuration 5 with cobblestones (present days).
Instead, in other configurations the idea was to use a particular covering [53] generated to optimize the albedo coefficient of roofs characterizing buildings placed in historical centers without altering their configuration. This optimization was meant to control the thermal energy transferred inside the building to improve its energy performances, during the summer in particular. For this type of covering, thermal and optical properties of pigments and clay binders were examined because they had, in the visible region, the same characteristics of the traditional tiles whereas in the other regions of the solar spectrum presented a high reflection coefficient (reflectance) (Tab. 1).
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7.2 0.6 100.0 7.9 58.5 79.6 66.0 0.89
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Composition [g] Yellow iron oxideY Red iron oxide R Brown iron oxide B Potassium silicate binder with titanium dioxide Reflectance [%] 300-380 nm (UV) 380.5-780 nm (VISIBLE) 781-2500 nm (NIR) 300-2500 nm (SOLAR) Emissivity [%] IR
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Tab. 1 - Characteristics of the N1C covering used.
values in the region of visible. So in the present study the examined configurations were (Tab. 2):
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It is possible to notice how the reflectance is higher in the region of infrared but it is characterized by lower
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Configuration 1: characterized by a typical Italian garden with twelve orange trees, two palm trees and six bushes. The intervention is restricted to the Cloister courtyard.
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Configuration 2: this configuration was an intervention restricted to the flooring. The covering made of potassium silicate, titanium dioxide and iron oxides previously described was added to the present cob-
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blestone;
Configuration 3: the third configuration was represented by the garden (previously mentioned). The cov-
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ering was not applied on the horizontal boundary but on the inside of the walls. So a simultaneous inter-
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vention occurred on the walls and the courtyard; Configuration 4: in this case the hypothesis was to intervene on both vertical and horizontal surfaces of the structure applying everywhere the covering previously described; Configuration 5: this is the present configuration of the examined Cloister.
Configuration
Walls
Courtyard Portico floor
1
2
3
4
5
Lime plaster
X
X
-
-
X
High albedo material
-
-
X
X
-
Cobblestone
-
-
-
-
X
High albedo material
-
X
-
X
-
Lawn and vegetation
X
-
X
-
-
Ceramic tiles
X
X
X
X
X
Tab. 2 – Materials of the boundary surfaces of the five configurations examined.
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3. MODEL VALIDATION To validate the model, the analysis focused on a typical cold day and on a typical hot day characterizing Rome’s weather (respectively February 13th and May 21st, 2014) and the experimentally measured values of 4 microclimatic variables (air temperature, mean radiant temperature, relative humidity and global radiation)
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were compared with the results provided by the simulations performed thanks to ENVI-met [54-56]. This software is a 3D prognostic microclimate model based on computational fluid dynamics and thermody-
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namics, with a typical resolution of 0.5–10 m in space and 1–10 s in time. The model is capable of simulating: flow around and between buildings, exchange processes of heat and vapor at urban surfaces, turbulence
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and exchanges of energy and mass between vegetation and its surroundings. Its input parameters include then weather conditions, initial soil wetness and temperature profiles, structures and physical properties of
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urban surfaces, and plants and its reliability is certified by different scientific studies [57-66]. For what concerns the simulations, ENVI-met has allowed the reconstruction of a 3D model of the Cloister
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which is a grid of calculation points based on a matrix (27 x 30) cells which grows in height for 29 cells. The horizontal spacing among the calculation points is then of 2 m, while the vertical spacing is of 0.5 m for the
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first 3 m; once this value was exceeded a telescopic grid with an extension factor of 1.2 was used. The time step for the simulation was then set to 2 seconds while the materials of the boundary surfaces of the Cloister
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presented the following thermo-physical properties:
courtyard (cobblestones): reflectance: 0.40 + emissivity: 0.90;
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walls (lime plaster): reflectance: 0.27 + heat transmission coefficient: 1.25 W/m2K.
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These values derive from the fact that the model validation was carried out by considering the present configuration of the Cloister (Configuration 5). It was also taken into consideration that the structures surrounding the Cloister are massive and able to store much heat in the walls and Cloister ground. For these reasons the site has been monitored through numerical simulation for 3 days and only the results of last simulated day have been considered valid to eliminate the errors due to the transitory period.
Page 8 of 45
- 10 On the other hand, for what concerns the experimental measurements, the equipment (Tab. 3) was formed by a Delta OHM HD 2102.2 datalogger to which a LP PYRA 03 pyranometer was connected and by an 11 inputs LSI Babuc/A portable microclimate control unit with the following probes: -
a Pt 100 Class B platinum heat-resistant probe to measure the air temperature in the examined environment; a forced ventilation psychrometer probe with distilled water tank to measure the relative humidity;
-
a hot-wire anemometric probe to measure wind speed;
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a globothermometric probe to measure the globe temperature with a black opaque copper globe (re-
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-
Hot-wire anemometer
Measuring Range
Air temperature
-50 ÷ +80 °C
0.01 °C
Wind speed
0 ÷ 45 ms-1
0.01 ms-1
Relative humidity
Globothermometer Pyranometer
0 ÷ 100 %
0.1 %
Globe temperature
-40 ÷ +80 °C
0.01 °C
Global radiation
0 ÷ 2000 W/m2
10-3 W/m2
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Forced ventilation psychrometer
Resolution
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Platinum heat-resistant PT100
Measured Variable
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Type of probes
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flection less than 2%) inside it, whereas the centre is characterized by a thermometric sensor.
Accuracy
±0.1 °C at 0 °C DIN 43760 - Class B ± 0.05 ms-1 ISO 7726 ± 0.2 °C over the total range ± 0.1 °C within any scale section of 10 °C DIN 58661 ≤ 0.1 °C at 0 °C DIN 43760 1/3 Second Class ISO 9060 + WMO Classification
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Tab. 3 – Metrological properties of the measuring instruments used.
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The value of the mean radiant temperature (already studied with different software in an enclosed urban area [67]) was then calculated through the following relation [68]:
(1)
where:
Page 9 of 45
- 11 -
is the globe temperature, measured through the globothermometric probe [°C];
-
is the air temperature [°C];
-
is the wind speed [m/s];
-
are respectively the emissivity of the globothermometer and its diameter [mm].
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and
It is also necessary to consider that the air temperature values, expressed in absolute temperature, were con-
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nected to the potential temperature (furnished in output by the software ENVI-met) thanks to the following
is the potential temperature [K];
-
is the absolute air temperature [K];
will be determined according to .
[hPa];
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-
is the pressure.
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-
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where:
-
(2)
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relation:
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In order to compare the measured data and the software results, a five points measurement circuit was then set inside the Cloister (Fig. 3).
Fig. 3: Location of the 5 measurement points in the Cloister. Point 1 to point 5 were used for the validation of the model, point 1 to point 3 were used to examine the different configurations.
Finally, it was reported (Figs. 4-7) only the comparison for one of the five points examined, since the variations had the same order of magnitude for every measurement point. For what concerns wind speed, it was not taken into consideration during the model validation phase only because ENVI-met simulates this variable with constant values deriving permanently from the same direction.
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Fig. 4: Trend of the air temperature furnished by ENVI-met and experimentally measured values (for measurement point 1 in Fig. 3).
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Fig. 5: Trend of the mean radiant temperature furnished by ENVI-met and experimentally measured values (for
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measurement point 1 in Fig. 3).
Fig. 6: Trend of the global radiation furnished by ENVI-met and experimentally measured values (for measurement
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point 1 in Fig. 3).
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Fig. 7: Trend of the relative humidity furnished by ENVI-met and experimentally measured values (for measurement
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point 1 in Fig. 3).
It can be noticed how the results obtained through the simulations were close, in a really satisfying way, to
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the experimental data and there was an uniformity between the variables in output from the software and the values of the same variables measured experimentally. The variations reached are represented by Tab. 4.
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This table, with reference to Figs. 4-7, reports for the measurement point 1 the average variance between the measured values and those provided by ENVI-met (absolute values difference averaged based on the 12
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daily sampling performed) and the maximum variance according to those measures.
Summer
Winter
(Δ Value)MED
(Δ Value)MAX
(Δ Value)MED
(Δ Value)MAX
Air temperature [°C]
3,3
4,3
1,5
4,5
Mean radiant temperature [°C]
3,9
7,0
3,5
11,0
Relative humidity [%]
1,3
2,5
1,2
3,0
Global radiation [W/m2]
26,9
130,0
46,6
205,0
Tab. 4 – Average and maximum differences between experimental measurements and ENVI-met output data with reference to point 1 reported in Fig. 3.
Such variations and all the considerations deriving from that, can be extended to the variables sampled in the other four measurement points used for the model calibration.
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4. RESULTS FOR THE MICROCLIMATIC PARAMETERS In this section the values of the microclimatic variables in the five configurations were compared. The microclimatic variables taken into consideration were: mean radiant temperature, air temperature, rela-
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tive humidity and wind speed. Their evaluation was carried out on August, 7th 2012 and February 6th, 2012 representing respectively a heat wave during the summer and a cold one during the winter. The positions of
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the points are represented in Fig. 3. For what concerns point 1, the values of the quantities examined were graphed; whereas for points 2 and 3 the quantities values are showed in Tab. 5 and Tab. 6. The graphs show-
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ing the global radiation were not reported because they had very similar values in all configurations. Finally, the variations of the microclimatic parameters are comparable to each other because they refer to
4.1 THE CASE STUDY DURING SUMMER
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simulations with the same input data, but with different properties of the materials of buildings.
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The following graphs concern point 1 (Fig. 3). The first variable examined was the air temperature. Fig. 8
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shows the trend of this quantity on August 7th, 2012.
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Fig. 8: Graph of the air temperature in the case study during summer.
While analyzing the trend it is possible to notice how the most advantageous configuration, when speaking of air temperature, is the one presenting the typical Italian garden characterizing the courtyard (configuration 1).
Such environment caused a drop in temperature, during the hottest hours of the day, of about 0.7 °C respect to the present configuration (configuration 5). It is also possible to notice a decreasing of the value of this variable by using, in terms of flooring, the covering previously described. Such solution (configuration 2) determined a reduction of the air temperature respect to the present configuration (configuration 5) of about 0.3 °C.
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- 14 The situation got worse in the remaining two configurations: the rise in air temperature estimated was of about 1.4 °C in the configuration with the garden and with the covering applied to the opaque and vertical surfaces of the structure (configuration 3) and of 1.5 °C in the case where the covering had to be applied to the courtyard and to the vertical surfaces (configuration 4). The second variable examined was the mean radiant temperature. Fig. 9 shows the graph according to an
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evaluation of 24 hours on August 7th, 2012.
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Fig. 9: Graph of the mean radiant temperature in the case study during summer.
It can be noticed how the configuration with the lowest values of mean radiant temperature is the one with
an
the typical Italian garden without the addition of the covering on the walls (configuration 1). Such change would imply an increasing of the values: the covering on walls and flooring (configuration 4) would deter-
M
mine a rise of the mean radiant temperature of about 20 °C respect to configuration 5, which is the present one. This is due to the structure of the Cloister where most of the emitted radiation was reabsorbed by the
ed
vertical and horizontal boundaries of the examined environment.
pt
Fig. 10 shows the trend of the relative humidity.
Ac ce
Fig. 10: Graph of the relative humidity in the case study during summer.
The trend of this variable is strictly connected to the one of the air temperature and the configuration presenting a mean radiant temperature with the lowest values was that characterized by higher values of relative humidity (configuration 1). This configuration is characterized by high relative humidity values due to the resulting soil evaporation and plants transpiration phenomena too. Instead those presenting lower relative humidity values were the configurations with higher air temperature values, that are those with the covered vertical boundaries (configurations 3 and 4). These differences can reach a maximum of 2.5%. Similarly to what was done for other variables examined, while analyzing Fig. 11, it can be noticed how the different configurations affect the wind speed.
Page 13 of 45
- 15 -
Fig. 11: Graph of the wind speed in the case study during summer.
Hence configurations 1 and 3, both having plants, report the lowest values. As a matter of fact trees can modify, sometimes considerably, the direction and speed of the wind in a certain place; in this study the differ-
ip t
ence between the configurations is of 0.01 m/s.
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4.2 THE CASE STUDY DURING WINTER
This section reports the trend of the variables during different hours of a specific day (February 6th, 2012).
an
us
Fig. 12 shows the air temperature.
Fig. 12: Graph of the air temperature in the case study during winter.
M
The case study shows how the lowest values were those of the configuration with the vertical boundaries of the structure unaltered with the covering on the base surface (configuration 2). If this configuration was
ed
compared to both the present configuration (configuration 5) and the one characterized by the Italian garden (configuration 1) the air temperature would rise respectively of 0.2 °C and 0.1 °C in the middle of the day. In
pt
fact, by applying the covering on the courtyard, part of the incident radiative flux was reflected without hit-
Ac ce
ting, in a significant way, the walls and this is due to the high values of sky view factor characterizing the courtyard (the opposite situation is the one with the covering on the vertical boundaries of the structure as well).
Hence the use of the covering on the base surface determined a drop of the parterre temperature and consequently a decrease of the air temperature. Whereas, if the covering is applied on the vertical boundaries as well (configurations 3 and 4), there is a rise in temperature of the opaque surfaces, hence of the air. This happened because the air gets hotter or colder when in contact with bodies which get warm or cool. During the nighttime, the configuration characterized by lower air temperature values was the one with plants only (configuration 1), without the presence of the covering on any surface. For what concerns the mean radiant temperature (Fig. 13), values were lower than those reported by the case study during summer.
Page 14 of 45
- 16 -
Fig. 13: Graph of the mean radiant temperature in the case study during winter.
The presence of the covering made of potassium silicate and titanium dioxide pigments determined an increase of this variable, in particular when the covering was applied on the vertical boundaries presenting a
ip t
low sky view factor (configurations 3 and 4). Then it is possible to notice how, in the present configuration (configuration 5), mean radiant temperature reached values slightly higher than 42 °C during the hottest
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hours of the day and how those values diminished, of about 2-3 °C, in the configuration with the garden
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Fig. 14 shows the trend of the curves of the relative humidity.
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(configuration 1).
M
Fig. 14: Graph of the relative humidity in the case study during winter.
A decrease of the variable during the night is reported: in fact bodies, during the night, tend to cool and
ed
therefore the vapor present in the air condenses. Whereas in the middle of the day, due to solar radiation, bodies warm and the vapor ,that before condensed, tend to evaporate thus increasing the value of the relative
pt
humidity.
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The fourth variable examined is the wind speed (Fig. 15).
Fig. 15: Graph of the wind speed in the case study during winter.
Even in this case the presence of trees or plants in general led to the lower values in wind speed (configuration 3). All configurations did not reach differences higher than 0.01 m/s. In the winter velocities are about 0.08 m/s higher than the case study during summer. This is because in this case study the wind speed at 10 m above the ground level, given as input to the software during winter, was higher than the one in summertime (this is a consequence of the meteorological configurations examined). What follows are the tables summing up the quantities examined for points 2 and 3 (Fig. 3) during both seasons. For point 2 it is possible to notice that wind speed values were higher because of their proximity to the
Page 15 of 45
- 17 side, which is the most aired section of the Cloister, and it is positioned right in the middle of the access points. Case study during summer
Case study during winter Time
Global Radiation [W/m2]
18:00 3.8 3.8 5.7 5.6 3.8 0.4 0.4 1.8 1.8 0.6 58.4 59.4 58.9 59.7 59.1 0.0 0.0 0.0 0.0 0.0 0.7 0.8 0.7 0.8 0.8
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14:00 9.1 9.1 11.8 11.6 9.2 13.5 12.0 24.7 23.0 12.8 57.6 59.1 58.4 60.3 58.4 42.1 42.0 41.6 41.5 42.0 0.7 0.7 0.7 0.7 0.7
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10:00 2.3 2.2 4.3 4.1 2.3 9.1 8.8 19.6 18.9 9.2 55.5 56.3 55.6 56.7 56.0 37.5 37.5 37.3 37.3 37.5 0.7 0.8 0.7 0.7 0.8
ed
Wind Speed [m/s]
18:00 33.3 33.5 35.2 35.2 33.7 21.1 23.0 27.7 27.6 24.9 69.1 69.5 67.4 67.8 68.8 11.0 10.7 9.5 9.3 10.6 0.6 0.7 0.6 0.7 0.7
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Relative Humidity [%]
14:00 39.3 39.8 41.4 41.6 40.0 51.6 64.6 72.2 76.9 57.9 63.4 63.2 61.4 61.6 62.2 968.5 964.6 955.2 952.0 964.5 0.6 0.6 0.6 0.6 0.6
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Mean Radiant Temperature [K]
10:00 30.1 30.4 32.8 32.8 30.6 25.1 26.0 39.8 37.7 27.2 64.4 64.5 63.3 63.8 63.7 58.2 58.1 57.5 57.4 58.1 0.6 0.6 0.6 0.6 0.6
M
Air Temperature [K]
Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5
Tab. 5 - Summarizing data for point 2.
pt
Case study during summer
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Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4 Conf. 5 Conf. 1 Conf. 2 Conf. 3 Conf. 4
10:00 30.6 30.8 33.3 33.3 31.1 50.4 66.7 70.6 72.6 59.9 63,1 63,2 61,8 62,4 61,9 1004,7 1003,0 993,5 992,0 1002,8 0,3 0,3 0,3 0,3
Air Temperature [K]
Mean Radiant Temperature [K]
Relative Humidity [%]
Global Radiation [W/m2] Wind Speed [m/s]
Case study during winter Time
14:00 39.8 40.3 41.9 42.1 40.5 52.0 68.3 72.8 74.1 63.9 62,1 61,9 59,9 60,1 60,7 975,8 971,8 962,4 959,2 971,8 0,3 0,3 0,3 0,3
18:00 33.6 33.8 35.4 35.5 33.9 20.9 23.7 27.3 27.8 25.6 68,3 68,7 66,5 66,9 68,0 12,4 12,0 10,7 10,4 12,0 0,3 0,3 0,3 0,3
10:00 2.6 2.5 4.7 4.4 2.6 7.6 7.3 17.0 16.3 7.7 53,3 54,1 53,4 54,5 53,7 42,3 42,3 42,0 42,0 42,3 0,4 0,4 0,4 0,4
14:00 9.5 9.5 12.2 12.0 9.6 12.1 10.6 22.4 20.4 11.6 55,3 56,8 56,0 57,9 56,0 47,4 47,4 46,8 46,8 47,4 0,4 0,4 0,4 0,4
18:00 4.0 4.0 5.9 5.9 4.1 -0.6 -0.6 0.9 0.9 -0.2 56,7 57,6 57,0 57,9 57,3 0,0 0,0 0,0 0,0 0,0 0,4 0,4 0,4 0,4
Page 16 of 45
- 18 -
Conf. 5
0,3
0,3
0,3
0,4
0,4
0,4
Tab. 6 - Summarizing data for point 3.
5. ESTIMATING OUTDOOR THERMAL COMFORT
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5.1 PMV COMFORT INDEX The PMV (Predicted Mean Vote) [19] was used to make an estimation of how the five configurations exam-
cr
ined affect the outdoor thermal comfort: it is a thermal comfort index developed thanks to some tests performed in an indoor environment on a valid sample of people and later adapted to outdoor climate through
us
the addition of the shortwave radiation [69].
It combines the energy balance of the human body and the personal sensation of the people exposed to a cer-
an
tain thermal stress and it is able to predict the mean value of the thermal sensation vote of a large group of people.
M
Usually the range of the PMV is between -4 (very cold) and +4 (very hot) where 0 is the thermal neutral (comfort) value. Since the PMV value is a function of the local climate, it can reach higher or lower values
ed
than its range (-4) ÷ (+4) [54]. The higher the distance from the thermal neutral value is, the more it will take the human thermoregulatory system to keep under control the thermal balance by modifying the skin tem-
pt
perature and the secretion of sweat.
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In the Eq. 3, for the estimation of the PMV, M is the metabolic rate while L is the thermal load of the human body, defined as the difference between the inner production of heat and the heat exchanged with the environment. The thermal load takes into consideration: metabolic rate, mechanical factor, partial water vapor pressure, air temperature, mean radiant temperature, surface temperature of clothing, thermal resistance of clothing, convective heat transfer coefficient and relative air velocity.
(3)
Finally, for what concerns L, it is defined as follows:
(4)
Page 17 of 45
- 19 -
where W is the physical work output; Q is the radiation budget, a function of mean radiant temperature TMR and air velocity VA; QH is the turbulent heat flux of sensible heat, a function of air temperature TA and air velocity VA; QL is the latent heat flow due to evaporation of moisture diffused through the skin, a function of relative humidity RH and air velocity VA; QSW is the latent heat flow from sweat evaporation; QRE is the res-
ip t
piratory heat flux (sensible and latent). All the terms in Eq. 3 are expressed in [W/m2] and they have a posi-
cr
tive sign if they represent energy gains for the body: for this reason M is always positive while W, QL e QSW are always negative.
us
5.2 THE PMV IN THE CLOISTER
an
PMV values are provided by ENVI-met and the use of this index is suggested by the German engineering guidelines VDI 3787 [70], developed for outdoor environments.
This paper examines the thermal sensations of a person standing in the Cloister. Thus the metabolic rate will
M
be 70 W/m2, while the mechanical factor 0.0 [71].
ed
For what concerns the insulation factor of the clothes worn by the people usually staying in the Cloister, it was assumed to be 0.4 clo (light summer clothes) in the summer case and 0.95 clo (typical winter clothes) in the winter case (1 clo = 0.155 m2K/W).
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Fig. 16 shows the results of each of the three points considered both for the summer and winter case together
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with the estimation of seven different hours representing the usual time interval when people stay in the site.
Fig. 16: PMV during summer and winter for the three points of analysis for every configurations of the case study.
6. CONSIDERATIONS
6.1 CASE STUDY DURING SUMMER Mean radiant temperature: analyzing the data derived from the simulations (Fig. 9) it can be noticed how the configurations characterized by the presence of a garden were subjected to a decrease of the value of this variable respect to those without green surfaces and vegetation. Bushes, with a low green foliage which is closer to the ground and whose growth in height is limited, function at the
Page 18 of 45
- 20 same time as a surface shield and as a barrier to the reflected radiation. On the other hand trees are mostly shields protecting from the radiation and only in part function as surface shields preventing the solar radiation from reaching the surfaces on the side. As a matter of fact shields represented by trees, bushes, etc… are the most successful when there is the necessity to control solar radiation.
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This is why in the configuration characterized by the garden and unaltered surfaces there is a reduction of the mean radiant temperature with the following advantages: radiative exchanges, in relation
cr
to longwave radiations, were reduced, null or reversed contributing to the refreshing of the environment.
us
Instead, in those simulations characterized by the presence of the covering (with a high albedo) made of potassium silicate, titanium dioxide and iron oxides on the vertical surfaces of the structure, the
an
reflected radiation increased facilitating the storage of sensible heat inside the Cloister in the daylight, with a rise of the mean radiant temperature. During this process, just a small part of the radia-
M
tion, hitting the surfaces of the buildings, was reflected back to the sky due to the reduced sky view factor. In fact, if the buildings are positioned in a particularly urbanized context, the visible sky fac-
ed
tor is reduced because of the obstructions produced by other structures. Then the fountain gives a marginal contribution to the wellbeing of the people in the considered site:
pt
this is due to the small dimensions of its water mirror and the view factor value between a pedestrian
Ac ce
and the fountain is high only for those who are in the nearby areas (they can perceive a decrease in mean radiant temperature thanks to the lower surface temperature of water). Air temperature: it is possible to notice (Fig. 8) how the configuration with the potassium silicate and titanium dioxide applied on the courtyard determined a decrease of the value of such variable respect to the present configuration. This was due to the high reflectance of the covering which prevented the temperature of the courtyard from reaching high values, thus keeping the air temperature at lower values than those characterizing the present configuration of the Cloister. The most advantageous configuration was the one with the garden (configuration 1). Since the lawn has a high thermal capacity, it can be considered the perfect solution in terms of thermal comfort, even though its albedo is not very high.
Page 19 of 45
- 21 Configuration 1 can also be considered the ideal solution because of the evapotranspirative effect of the grass and the lawn is one of the main factors for the decrease of the air temperature and mean radiant temperature. The low level of reflection and a low overheating make the garden the perfect choice, in particular if we are dealing with places characterized by sedentary activities. Then the gar-
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den can absorb about 80% of the incident radiation and 70% of the absorbed part is used to activate transpiration phenomena; in this way a rise in temperature will not occur. Finally, for what concerns
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the reflected radiation, it is about 15% and it does not represent a big problem for the surrounding zones.
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Wind speed: another issue taken into consideration is the effect of natural elements (plants, trees, etc…) on the ventilation (Fig. 11). Plants are one of the most commonly used and the most effective
an
windbreak elements, since they can modify both the direction and speed of the wind. Generally, the influence of the plant is directly proportional to the density of its foliage. In the summertime the
M
presence of wind, which favors the cooling process thanks to a higher convective coefficient, is ad-
of these latitudes).
ed
vantageous; hence it should be preferred a vegetation which is less dense such as palm trees (typical
PMV: it can be noticed (Fig. 16) how, in the summer case, the differences among PMV values tend
pt
to be higher when there is a direct shortwave radiation, whereas they decrease in the late after-
Ac ce
noon/night. Configuration 1 is the best solution for what concerns the thermal comfort. As a matter of fact in the summer such configuration has lower PMV values. The main thermal stresses characterize configurations 3 and 4 presenting wall covering; in particular configuration 4, also with a covering of the basic surface of the yard, has the highest PMV values. While configuration 2, with a direct shortwave radiation, leads to median PMV values: microclimatic conditions get worse respect to configurations 1 and 5, but there is an improvement respect to configurations 3 and 4. The difference among PMV values decreases with diffuse solar radiation. 6.2 CASE STUDY DURING WINTER In a Mediterranean climate the main problems concerning thermal comfort arise during the summer. For this reason, the thermohygrometrics factors must be optimized in this season while during winter the behavior of the site examined will be a direct consequence of the choices previously made. Page 20 of 45
- 22 Mean radiant temperature: it is the variable which is affected the most by such evaluations (Fig. 13). The problems characterizing the summer are completely different to those of the winter. In fact in winter the most advantageous configurations are those with the covering applied on the vertical boundaries of the structure; though such solution would lead to mean radiant temperature values too
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high during the summer followed by a pronounced thermal stress. Air temperature: the city of Rome does not have cold winters: by increasing the level of heat insu-
cr
lation of our clothing, cold temperatures can be absolutely tolerable (Fig. 12).
Wind speed: ventilation must be controlled during winter since it can increase the amount of thermal
us
stress. Obviously plants with deciduous leaves are effective during summer, but not in wintertime. The arboreal choices should be oriented towards evergreen elements. Those configurations with the
an
typical Italian garden present shrubs: they have the same function of trees but on a different level. This is why, to have a reduction of the wind speed, in an area thought to host sitting people, they are
M
more effective than trees because they obstruct and represent a filtration to the wind. PMV: during winter PMV values tend to be similar without reporting high thermal stresses as in the
ed
summer. The closest values to 0, that is the thermal neutral value, are those characterizing configurations 3 and 4 with higher levels of thermal stresses in the summer case. It should not be forgotten
pt
that the city of Rome belongs to the Csa category (Köppen climates classification), hence it has hot
Ac ce
summers with an average temperature during the hottest month which is higher than or equal to 22 °C [52]; this is why the microclimate must be mitigated, in order to reduce the thermal stress, especially in the summer. When dealing with enclosed urban contexts, as the one here examined (surfaces with a low sky view factor), configuration 1 seems the best solution.
7. CONCLUSIONS This study takes into consideration the Cloister of San Peter in Chains and the effects of different mitigation strategies on both the microclimate and the outdoor thermal comfort. Particular attention is paid to the examination of how the presence of some vegetation and high albedo materials (formed by potassium silicate, titanium dioxide and iron oxides) affect the microclimate. This is why the paper analyzes five different con-
Page 21 of 45
- 23 figurations, with different solutions for the vertical and horizontal boundaries of the structure: configuration 1 presents a garden; configuration 2 has high albedo materials applied on the basic surface of the yard; configuration 3 is characterized by both the garden and high albedo materials applied on walls; configuration 4 presents high albedo materials applied on both walls and the basic surface of the yard; configuration 5, the
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one of the present, has cobblestones covering the basic surface of the yard and lime plaster on the walls. The comparison of all configurations was made through numerical simulations performed thanks to the ENVI-
cr
met software (a predictive model where the furnished outputs were verified through experimental measurements) and considering, as outdoor thermal comfort index, the PMV (Predicted Mean Vote).
us
For each of the five configurations it was carried out an analysis of the microclimate and outdoor thermal comfort during a hot summer day and a cold winter day. The main conclusions to the summer case are (with
an
reference to the middle hours of the day and configuration 5, that is the one of the present): air temperature: configuration 1 reports a decrease of about 0.7 °C; configuration 2 a decrease of
M
about 0.3 °C; configuration 3 an increase of about 1.4 °C and configuration 4 an increase of about 1.5 °C.
ed
mean radiant temperature: configuration 1 reports a decrease of about 6 °C; configuration 2 an in-
about 20 °C.
pt
crease of about 8 °C; configuration 3 an increase of about 13 °C and configuration 4 an increase of
Ac ce
relative humidity: values are similar. The maximum variance registered is about 2.5%. wind speed: configurations 2 e 4 report similar values; configurations 1 e 3 present a decrease of about 0.01 m/s.
PMV: when there is direct shortwave radiation configuration 1 reports a decrease of about 1.2; configuration 2 an increase of about 1; configuration 3 an increase of about 1.5 and configuration 4 an increase of about 2. Differences decrease when the direct shortwave radiation is not present. On the other hand the conclusions to the winter case are (with reference to the middle hours of the day and configuration 5): air temperature: configuration 1 reports a decrease of about 0.1 °C; configuration 2 a decrease of about 0.2 °C; configuration 3 an increase of about 2.3 °C and configuration 4 an increase of about 2.1 °C.
Page 22 of 45
- 24 mean radiant temperature: configuration 1 reports a decrease of about 5 °C; configuration 2 an increase of about 7 °C; configuration 3 an increase of about 10 °C and configuration 4 an increase of about 20 °C. relative humidity: values are very similar. The maximum variance registered is about 3%.
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wind speed: configurations 1, 3 and 4 show a decrease (the maximum variance registered is about 0.01 m/s for configuration 3) while 2 a slight increase.
cr
PMV: when there is direct shortwave radiation the difference among the configurations are similar to those of the summer case. Without a direct shortwave radiation configurations 1 and 2 do not show
us
significant variances; configuration 3 presents an increase of about 0.8 and configuration 4 of about 0.6.
an
Hence, according to what was previously reported and the outputs provided by the simulations: The application of high albedo materials on vertical and horizontal boundaries of urban contexts, as
M
the one here examined, determines a worsening of the thermal comfort which leads to increases in the PMV that can reach 2.3 in summer. In particular the extent of this worsening is inversely propor-
ed
tional to the sky view factor of the high albedo surfaces. The exertion of high albedo materials usually causes a microclimate improvement during winter. In
pt
these type of climates, as the one here considered, such improvement is not equal to the worsening
Ac ce
that the use of these materials determines during the summer. The improvement can be quantified, in terms of PMV, with a difference of 1.3. Moreover, in the Mediterranean climate, thermal stresses in the winter are not that high and people can control it by wearing clothes with a higher thermal insulation.
The presence of some vegetation in particularly enclosed urban contexts and with hot climates guarantees the best conditions for what concerns thermal comfort. Especially evapotranspiration phenomena and the capacity of plants of controlling both the incident and reflected direct shortwave radiation determine a decrease in air and mean radiant temperature. This causes decreases, during the middle hours of a hot summer day, of the PMV values until 1.5.
Page 23 of 45
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8. ACKNOWLEDGEMENTS This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. A special thanks to Mrs. Flavia Franco for the help she provided in the preparation of this paper.
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