Elsevier

Energy and Buildings

Volume 231, 15 January 2021, 110638
Energy and Buildings

Numerical modelling and experimental validation of the microclimatic impacts of water mist cooling in urban areas

https://doi.org/10.1016/j.enbuild.2020.110638Get rights and content

Highlights

  • A 3D microclimatic model in ENVI-met is used to simulate a misted urban area.

  • The model predicts accurately the horizontal and vertical air temperature distribution.

  • Wind speed plays a major role both in terms of cooling capacity and influence area.

  • Water flow rate and height of injection can be tuned to regulate the cooling capacity.

Abstract

Among water-based mitigation strategies against urban overheating, dry mist systems are especially promising, given their local impact, cost-effectiveness and controllability. Intense cooling capacity has been reported under a variety of climates, however, there is a growing need to define specific design guidelines towards an informed and optimized use of the technology. Parametric analysis on validated models would assist in determining type and degree of correlation between key parameters, as well as magnitude and predictability of the cooling capacity. In this paper, for the first time, a 3D microclimatic model in ENVI-met is used to simulate a misting system installed in Rome, Italy, with high prediction accuracy for the air temperature (R2≃0.87, RMSE≃0.84 °C). The calibrated ENVI-met model is used then to perform parameterizations on the water mist system, focused on the role of three key design variables: i) water flow rate, ii) injection height and iii) local wind speed. Results show that the most significant thermal drops tend to occur close but out of the misted perimeter following the wind direction, with cooling effects further stretched for tens of meters. The cooling capacity increases with the total water flow rate (+0.2 °C per 10 l/h increment) and in presence of calm air (+35–40% per 0.8 m/s deceleration). Lower injections intensify the cooling a pedestrian height, which could be especially beneficial under windy conditions. Further research would target climate dependencies to extend the applicability of the above results and build up cohesive guidelines at the hands of urban planners and practitioners.

Introduction

Urban overheating is a well-known environmental hazard at both global and local scale, as documented by several scientific studies and observations of air temperature trends. Main consequences are the thermal deterioration of both indoor and outdoor thermal environments, serious thermal comfort concerns for human beings, and enhanced cooling energy uses in buildings [1], [2]. Measured temperatures' rise and evolution scenarios are the object of many studies. The most significant are those produced under the UN framework by the Intergovernmental Panel on Climate Change [3], dedicated to global strategies and countermeasures to limit the temperature rise to 1.5 °C. Beyond global warming, the “Urban Heat Island” (UHI) phenomenon is a major cause of overheating in the urban built environment, investigated for decades. Its intensity depends on several factors as: climate, size of the city, urban texture, etc. Peak values up to 12 °C are reported in literature, whereas the average maximum orbits around 2 °C [4]. Moreover, UHI intensity is typically higher in cooling dominated climates [5].

Urban overheating has a strong impact on the energy use in buildings especially for cooling, as documented worldwide in [4]. It further enhances the risks related to energy poverty for an impressive share of the global population: 1 billion people living in poorer countries [7] and more than 60 million living in Europe [8]. The risk of skyrocketing energy uses for cooling and the unfeasibility of ensuring decent outdoor thermal conditions for many citizens call for diversified and effective thermal mitigation/adaptation plans and architectural rehabilitation of urban spaces, in the attempt of improving health, comfort and liveability in the outdoor realm [6], [7]. Many studies have been carried out to size the impact of different UHI mitigation technologies and strategies: green infrastructures, innovative cool materials, blue technologies, as reviewed in [5], [8], [9].

Main findings prove that passive solutions offer moderate mitigation potentials (generally below 2 °C) and are effective if applied at large scale in the city. Urban overheating can be combated, at local scale, through the inclusion of blue features, as evaporative processes largely influence both the natural hydrological balance and the human thermoregulation, thus strongly impacting on comfort and liveability [10]. Especially on hot and dry days or during heat waves, evaporation is a fairly effective way to draw the excessive heat from the air through a thermodynamically spontaneous phase change process, that naturally magnifies as temperature rises (at equal specific humidity) [11] and that is way more intense than a purely sensible heat transfer between air and water [12].

Beyond natural water bodies, a whole spectrum of artificial installations can be considered (ranging from fountains, to sprinklers, to evaporative towers, to ponds). Among them, an especially promising blue mitigator is represented by dry mist systems, namely high-pressure water injectors able to pulverize the water into fine droplets of few tens of microns. The high surface-to-volume ratio promotes flash evaporation, hence i) the water consumption can be minimized compared to alternative technologies and ii) the risk of wettedness, even close to the nozzles, can be neutralized as long as a proper layout is configured [11]. Further, it was experimentally demonstrated that dry misters are beneficial in terms of building energy use minimization: Ishii et al. [13] projected a 10% air conditioning energy reduction at the expense of 5% humidity gain per degree of air cooling, whereas Narumi et al. monitored three misting techniques (rooftop spraying, veranda spraying, and outdoor air-conditioning spraying) in an apartment house in Osaka and reported on cooling consumptions decremented by more than 80% [14]. Other secondary benefits are related to air quality and erythema reduction since finely pulverized droplets help at scavenging dust, removing pollutants [15] and attenuating solar attenuation [16].

These systems could be deployed throughout the cityscape in strategic hot or vulnerable spots to alleviate the risk of heat-related mortality and morbidity with expected higher impact than single massive water bodies [17]. Furthermore, compared to water bodies, these systems can be controlled in capacity and can be triggered by specific events (temperature, humidity, wind speed exceedances or rain occurrences) not to provide evaporative cooling when unneeded or potentially counterproductive [18]. This is especially attractive for heating-dominated countries, where overheating concerns are sporadic yet extreme events.

On the other side, the efficacy of a misting system is largely a function of the environmental conditions. In a recent review by Ulpiani dedicated to mist cooling [11], the author collected data from 12 countries and 7 climatic zones. Air temperature and relative humidity were identified as driving factors, as they dictate the wet bulb depression (difference between dry-bulb and wet-bulb temperature) which represents the theoretical limit for evaporative cooling [19]. Additionally, wind speed (and gusts) proved to be pivotal as directly related to the level of particle dilution [20]. Demonstration of the governing role of local wet bulb depression as both instantaneous and short-term trend is reported in [21], where the cooling capacity of an overhead dry mist system was found to be negatively and positively correlated with solar irradiation and wind speed, respectively. This implies that not just the microclimate of the installation site deeply affects the results, but also the very local urbanscape (e.g. space enclosure, canyon effect, greenery and other competing evaporative sources, wind-breaking features and provisions, etc.). In such a complex scenario, modelling and parameterization, based on adequate experimental validation, are especially useful to determine type and degree of correlation between key parameters, as well as magnitude and predictability of the cooling capacity. This is pre-condition to define substantiated design guidelines towards an informed and optimized use of the technology [11].

Almost all numerical models of mist cooling are platformed in ANSYS Fluent [11]. In 2008, Yamada et al. reproduced a 50 m × 15 m × 4 m semi-enclosed outdoor space in Tokyo (Japan, Cfa climate) fitted with an overhead misting system, using a mesh grid of 11,200 cells [22]. On a typical summer day (Ta of 33.4 °C, RH of 58%, ws of 0.1 m/s), the cooling reached −1.5 °C at the spray location and −0.5 °C about 2.5 m away, at the expense of a + 0.8 and + 0.3 g/kg gain in absolute humidity, respectively. In 2010, Wang et al. [23] investigated the role of airflow rate and site of installation (open outdoors, semi-enclosed area) when a misting fan was introduced in a 15 m × 10 m × 4 m domain represented by 570,000 hexahedral cells, under initial Ta of 35 °C, RH of 60% and solar radiation of 600 W/m2. The change in airflow simply moved the region of higher cooling closer or farther from the injection. On the other hand, the maximum temperature drop more than doubled (from 2 °C to 5 °C) when shifting from the open to the semi-enclosed scenario, bringing out the role of wind. The humidity gain was found to be negligible (1–6%) in any case. In 2015, Farnham et al. [12] ran an experimental and numerical study in Osaka (Japan, Cfa climate). They used Fluent to investigate the role of i) water temperature in almost the whole liquid range (8–92 °C), ii) the distance from the injection and iii) the initial T, RH conditions (7 cases) by looking at a single injection with mean droplet diameter of 23.4 – 25.5 µm. A 400 cm × 45 cm × 45 cm air volume was discretized into 1-cm tetrahedral mesh near the nozzle, ranging up to 4-cm mesh at the air inlet and outlet, for a total of 1.2 million elements. The cooling ranged between 0.5 and 2.5 °C, with negligible loss due to the different water temperature (0.26 °C when water was heated up from 20 °C to 60 °C). In the same year, Montazeri et al. systematized the CFD analysis of the impact of physical parameters on evaporative cooling by a mist spray system [24]. The authors modelled a 0.585 m × 0.585 m × 1.9 m wind tunnel, meshed into 1,018,725 hexahedral cells, to parameterize i) the inlet air temperature, ii) the inlet air humidity ratio, iii) the inlet airflow rate, iv) the inlet water temperature, v) the inlet droplet mean diameter and vi) the droplet spread. Five different cases were considered for each parameterization. It was found that the maximum temperature drop incremented from 3.3 to 9.3 °C by increasing the air temperature, from 3 to 9 °C by decreasing the humidity ratio, from 0.8 to 7 °C by reducing the relative velocity, from 2 to 9 °C by cooling down the water (in contrast with [12]), from 4 to 10 °C by reducing the mean droplet size and remained fairly unchanged at 5.8 °C when tweaking the spread. The same authors, in [25], extended the simulation to an outdoor location, by modelling a courtyard in the Bergpolder Zuid region of Rotterdam (The Netherlands, Cfb climate), equipped with 15 nozzles featuring mean droplet diameter of 20 µm. A circular inner subdomain of diameter equal to 1200 m was populated with explicitly modeled buildings, while the surrounding outer hexagonal subdomain (side 1200 m) was implicitly modeled in terms of roughness, for a total of 6,610,456 hexahedral and prismatic cells. Three different values for i) total mass flow (2–9 l/min) and ii) height above the ground (3–5 m) were considered. The maximum temperature drop (underneath the injection in the middle of the spray line, at a pedestrian height of 1.75 m) increased from 1 to 7 °C as higher flow rates were processed. Spatially, a 2 °C maximum drop was reported to stretch up to a distance of 8 m from the spray line. By decreasing the height of the injections, the cooling capacity incremented from 5 to 7 °C. Results are to be trusted for relatively low pedestrian ws (0.5–1.5 m/s).

Other than Fluent, Q-basic was used at first [26], then also Fire Dynamics Simulator [27] and Simplified Analysis System for Housing Air Conditioning Energy [14]. In some instances [28], [29], [30] the software was left unspecified. These models typically apply an Eulerian-Lagrangian approach to follow the discrete phase (water) in its kinetics and thermodynamics inside the continuous medium (air) and thus provide a punctual representation of the two-phase interaction. Yet, as mist cooling gains ground into the world of urban heat island mitigation, there is a need to incorporate and validate its effects in urban-scale simulations, contemplating the complex interplays with other microclimatic phenomena, distinctive of the urban environment (e.g. canyon effects, evapotranspiration, reduced soil permeability, anthropogenic heat).

ENVI-met is a three-dimensional microclimate modelling system designed to simulate the surface-plant-air interactions in urban environment, based on the fundamental laws of fluid- and thermodynamics. Recently, it has been widely adopted to estimate the thermal benefits of urban heat mitigation strategies [31], [32], [33], [34], [35], [36]. This software includes a water droplet dispersion and evaporation model to simulate the cooling effect of water spray. However, to the best of our knowledge, no study reports on experimentally-validated water mist models corroborated by multi-point in-situ measurements using ENVI-met. This study contributes to the current body of knowledge on urban overheating countermeasures based on evaporative cooling in two ways: i) by assessing the effectiveness of the software ENVI-met in predicting the microclimate perturbation in the misted area and ii) by determining substantiated design criteria to be applied at both manufacturing and urban planning level. The calibration of the ENVI-met model for water misting poses some challenges, essentially due to the non-trivial interlacement between droplets’ behaviour and the other climate-impinging urban elements: a suite of solutions is proposed and discussed, leading to accurate simulations that capture real trends and evolutions. Recommended settings are presented to be used in future scenario analysis for UHI mitigation.

Section snippets

Materials and method

In this paper, ENVI-met version 4.4.3 is used to model a grid of overhead dry misters installed in a green playground in Rome (Italy, Csa climate). The simulation results for both the “misted condition” (MC) and the “undisturbed condition” (UC) of the site are validated on the basis of the thermal and hygrometric mapping obtained through a bespoke sensor network. The calibrated ENVI-met model is then used to perform parameterizations on the water mist system, focused on the role of three key

ENVI-met model validation

The accuracy of ENVI-met simulations at different heights and locations, under the misted and undisturbed conditions was examined with reference to the observed data. Concerning the UC, Fig. 6 shows the comparison between the values measured by the climatic station and those simulated (Ta, RH, GSR). Owing to the vertical grid resolution, values are averaged between the cells centres at 0.5 m and 1.5 m, to be closest to the weather station. It can be observed how the simulated values of Ta and

Discussion

In this study, a water spray model was developed in ENVI-met. Compared to other CFD platforms, ENVI-met’s approach to water spray tracking relies on two simplifying assumptions: i) all droplets have the same diameter (no statistical distribution) and ii) the amount of droplets in a given air volume changes due to evaporation, but not their size. By not considering any statistical distribution, ENVI-met’s water spray model might be prone to inaccuracies both in terms of cooling and

Conclusion

Dry mist systems may play a crucial role in combating the phenomenon of urban overheating at local scale. Compared to alternative evaporative technologies, the water consumption is very modest and the risk of wettedness very marginal, especially when the cooling action is smartly controlled. Their beneficial action includes air conditioning energy use minimization, solar radiation attenuation and pollution removal. As mist cooling gains popularity as urban overheating countermeasure and gets

CRediT authorship contribution statement

Elisa Di Giuseppe: Conceptualization, Methodology, Investigation, Writing - original draft. Giulia Ulpiani: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. Claudia Cancellieri: Software, Validation, Visualization. Costanzo Di Perna: Resources, Supervision. Marco D'Orazio: Resources, Supervision. Michele Zinzi: Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors warmly thank the Italian national agency for new technologies, energy and sustainable economic development (ENEA) – under the funding PAR 2017 sub-project D.6 Sviluppo di un modello integrato di smart district urbano – for providing the resources to build the prototype.

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