On the cooling potential of night ventilation techniques in the urban environment
Introduction
The urban microclimate has a strong influence on the performance of the passive cooling techniques in buildings. The heat island effect and the modification of the terrain in the urban environment can strongly affect the local climatic conditions. The climatic characteristics in the centre of a city differ considerably compared with the rural ones. The usage of the appropriate climatic data is essential in order to study the thermal behaviour of buildings in the urban environment, as well as the efficiency of passive cooling techniques.
For night ventilation cooling technique, the key procedure that takes place is related to the introduction of cool ambient air into the building (during the summer nights when ambient temperature is lower than indoor temperature) and the rejection of the heat that is stored in structural elements into the outdoor environment. This procedure can be direct when the ambient air is circulated inside the building and heat is transferred from the exposed opaque elements of the building, or indirect when the ambient air cools a thermal storage medium that is used as a heat sink during the daytime period.
The energy/thermal impact of night ventilation techniques concerns the reduction of the daily cooling load, as well as the shift to the afternoon and the reduction of the peak cooling load, when applied during the cooling season in air-conditioned buildings. For natural ventilated buildings the daily peak indoor temperature is also reduced and shifted to the afternoon, improving the thermal comfort of the occupants in this way.
The efficiency of night cooling depend mainly on the temperature and the flow of the ambient air introduced in the building during the night-time period, the flow patterns of the ambient air when circulated into the building [7] that determine the “quality” of heat transfer between the air and the thermal mass, as well as the thermal capacity of the storage medium.
Various experimental and theoretical analyses have been performed in order to study night ventilation. The study by Cook [5] concerns the indirect application of night ventilation by cooling a thermal storage medium of increased thermal capacity. Stack effect night ventilation has been examined by Van der Mas et al. [23], [24] by introducing a combined model that couples an airflow with a thermal model. Santamouris et al. [18], [19] introduced a method to estimate the energy efficiency of night ventilation in reducing the cooling load of a building. Givoni [8], [9] carried-out experiments in buildings with different thermal masses in order to examine how effectively night ventilation can reduce the maximum indoor daytime temperature and proposed a simple model without considering occupation parameters. A study that concerns the application of passive cooling techniques to Israel has been performed by Hassid [10]. The experimental evaluation of night ventilation in three real-scale buildings has been realized by Geros et al. [6], examining how the airflow rates, the building construction and the climatic conditions affect the efficiency of the technique. Kolokotroni et Aronis [14] examine energy conservation in air-conditioned buildings when night ventilation is applied by natural and mechanical means. Also, Kolokotroni et al. [13] evaluate a pre-design tool for summer cooling with night ventilation for office buildings in moderate climates, by using experimental data. Blondeau et al. [3] carried out full-scale measurements in order to examine both comfort conditions and energy balance. In addition, Blondeau et al. [4] present a multicriteria analysis of ventilation during summertime that includes indoor air quality, thermal comfort and energy consumption, in order to determine the most suitable ventilation strategy in a specific building, under accelerated diurnal or nocturnal ventilation and/or air conditioning. A theoretical investigation of a combined wall-roof solar chimney to improve night time ventilation in buildings is presented AboulNaga [1] and Abdrabboh. Roucoult et al. [17] propose a simplified characterization of thermal inertia of buildings, examining the interactions between thermal inertia and the airflow rate during night ventilation. Tunpenny et al. [21], [22] examine a prototype latent heat storage system by using a phase change material for night-time cooling, presenting a theoretical model and investigating cost- and energy-saving benefits. Herkel et al. [11] evaluate night ventilation efficiency with data from long-term measurements in an office building. Todorovic et al. [20] examine the application of night ventilation techniques to atrium buildings, while Pfafferott et al. [15] carried-out experiment for both mechanical and free night ventilation in an office building, in order to determine the efficiency of the technique and to provide a simple model for integration into a building management system.
For the application of night cooling techniques, the most important climatic parameter that affects their efficiency is ambient temperature. This parameter determines the degree at which the external environment can play the role of a sink for the heat that is stored in the thermal mass of the building. The capacity of this heat-sink is defined by the temperature difference between indoor and outdoor environment. In addition, for a naturally ventilated building the knowledge of the wind profile (velocity and direction) is necessary in order to evaluate the efficiency of the technique. The outdoor wind profile determines the flow of ambient air in indoor spaces and therefore influences the ventilation rates during the application of night ventilation. Therefore, the knowledge of these climatic parameters permits to estimate the efficiency of night ventilation more precisely.
Frequently, designers that examine the application of passive cooling techniques use climatic data measured in locations near the studied building (e.g. in meteorological stations, airports, etc.) as it is not always feasible to realize measurements in-situ. When the building is located in an urban environment in most cases this data does not represent realistic climatic conditions of the location of the study. The modified terrain of urban topography strongly influences the wind field even for adjusted regions. Additionally, the heat island effect increases the outdoor temperature of the urban environment with regard to the rural surroundings of the city.
Since, in a certain way the urban canyons are the units that compose the urban environment, their geometry and orientation, construction materials and anthropogenic heat sources characterize and determine the thermal and airflow conditions in the urban domain. The knowledge of these conditions is important in order to evaluate the application of night ventilation.
The aim of the present study is to examine night cooling from the urban environment point of view and investigate at which degree different types of urban canyons modify the energy performance of the technique. For this purpose experiments have been carried out in 10 urban canyons located in the city of Athens. In order to determine the climatic conditions that affect the application of night ventilation, specific measurements have been performed inside and outside the experimental canyons: outdoor air temperature, wind velocity and wind direction.
To examine at which degree the urban climatic conditions affect the studied technique, a typical single-zone room, which is naturally ventilated during the night period, has been modelled by using a transient simulation tool. Two different operation modes were examined: air-conditioned and free-floating operation. Also, two ventilation strategies were considered: single-sided and cross-ventilation. Moreover, two categories of simulations were carried out. The first category considers that the room is located inside the urban canyon, and thus it is based on the usage of the data of temperature and wind measured in the canyon. The second category is based on the same climatic parameters, but measurements were carried out outside the urban canyon. Consequently, the comparison of the results of the two categories makes it possible to evaluate the impact of the urban canyons on the application of night ventilation.
The following analysis indicates that the existence of the urban canyons reduces, some times strongly, the energy potential of the studied technique, due to the increase in ambient temperature and the reduction in wind velocity that occur inside the canyons.
Section snippets
Experimental procedure
The experiments were carried out in Athens (Greece), in the frame of the Joule programme POLIS [16]. Ten urban canyons were studied, representing different geometries and orientations, anthropogenic heat sources and vegetation. The thermal and airflow conditions were examined by measuring the following parameters between June and September of 1997.
Measurements in the experimental urban canyons
The application of night ventilation was studied by using the outdoor air temperature measurements performed at the same location where the wind field was measured. The following analysis is mainly focused on the night-time period, because this is the period of interest for night cooling. The hourly measurements of the ambient temperature inside and outside “Ippokratous” canyon are presented in Fig. 2. Also, for the same canyon Fig. 3, Fig. 4 illustrate the corresponding wind velocity and
Study of night ventilation in the experimental urban canyons
In order to evaluate the impact of the urban environment on the energy/thermal performance of night ventilation, the application of this technique was studied by using the climatic conditions measured inside and outside the experimental urban canyons. The analysis of the measurements indicates that the dominating climatic conditions inside and outside the urban canyons are frequently different. The local microclimate is strongly related to the form and the geometry of the canyon, its
Synopsis and discussion of the results
The present work examines the application of night ventilation techniques to urban environment and investigates how the efficiency of the technique is affected by the dominating climatic conditions. The carried-out measurements inside and outside 10 urban canyons in Athens wider area indicate that the climatic conditions between these two positions can be very different.
The differences in the thermal and airflow conditions measured inside and outside the 10 urban canyons influence the energy
Conclusions
The present paper investigates efficiency of night cooling techniques, when applied to the urban environment. By using extended measurements that were carried out in 10 urban canyons, the efficiency of night ventilation was examined. The presented analysis clearly shows that the use of the undistributed climatic conditions dominant outside the urban canyons which is not appropriate for the study of night ventilation (and generally for the study of passive or hybrid cooling techniques) when
Acknowledgments
The experimental part of this research was supported by the Commission of the European Community, DG XII, JOULE-THERMIE program under Contract No. JOR3-CT95-0024 in the frame of POLIS project: Urban planning research actions to improve solar access, passive cooling and microclimate.
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