Qualitative modelling of the natural ventilation potential in urban context

The two objectives of natural ventilation in buildings are the improvement of indoor air quality and thermal comfort. The methodology proposed hereafter provides an assessment of the natural ventilation potential for indoor air quality, while taking into consideration the other objective. More precisely, requirements on comfort impose the appropriate ventilation strategies to be applied for each given meteorological situation. On the basis of these strategies, the methodology evaluates the appropriateness of a given location to natural ventilation. After a historical introduction (Chapter 1), the statement of the problem (Chapter 2) and a review of the state of the art (Chapter 3), Chapter 4 presents a novel methodology to assess the natural ventilation potential starting from its driving forces (wind-induced and buoyancy-induced pressures) and from its constraints (noise pollution and atmospheric pollution), which applies multicriteria analysis. The method uses the results of the atmospheric simulation model Lokal-Modell (LM, described in section 3.4). The related software tool, developed within the European project Urbvent, and tested against real cases, is presented as well. Chapter 5 gives a view of the results of an atmospheric mesoscale model, referred to as Finite Volume Model (FVM), that includes an urban module (described in section 3.6) allowing to compute the effects of the urban tissue on meteorological parameters. This model has been adapted as part of this work to use the outputs of the above-mentioned Lokal-Modell (LM) as boundary conditions. (To this end, previous works utilized NCEP/NCAR Reanalysis data, with an original spatial resolution 40 times lower than LM in latitude, 25 times lower in longitude and 6 times lower in time.) After a verification of the urban model results against on site measurements carried out in the Basel region (Switzerland) over a summertime period, Chapter 6 presents the utilization of these results within the Urbvent method of Chapter 4. The ensuing potential for natural ventilation is the object of a verification resorting to an air pollution exposure study (Expolis) in section 6.4. Attention is drawn to the fact that it has been decided to assess the natural ventilation potential of the site itself, in the most building-independent way. In so doing, an analysis of the results can tell what type of ventilation a given site is suited for and what kind of building should be built there (or how to refurbish an existing one). Put another way, the model examines whether external conditions required for applying natural ventilation coexist. How to properly design a building taking advantage of theses conditions is not addressed here. This is left to the reader or to the designer. Incidentally, the bibliography dealing with this very topic is quite abundant.

Assessing the natural ventilation potential of the Basel region

Mario Germano

A method is proposed to assess the natural ventilation potential (NVP) of the Basel Region, Switzerland, by taking into account the most comprehensive set of factors involved in natural ventilation. These factors are either driving forces, such as wind pressure and stack effect, or constraints, like noise pollution and atmospheric pollution. The process considers these factors in an ordinal qualitative scale and gives its result in this same scale. Climatic data are extracted from a mesoscale atmospheric simulation model referred to as FVM (Finite Volume Model), accounting for the effect of the urban tissue and taking as boundary conditions data coming from a model of the Swiss national weather service (MeteoSwiss). FVM provides an urban turbulence module which specifically simulates the effects of urban areas on the meteorology. Qualitative scales allow to bypass the problem of the inaccuracy of some parameters, which can be very high, especially in urban environment and in the pre-design phase of a construction project. Actually, the method is particularly suitable for designers intending to take early-stage decisions.

2007

Development and application of a numerical simulation system to evaluate the impact of anthropogenic heat fluxes on urban boundary layer climate

Andrea Krpo

Increasing economic development, and growing population, generated during the last decades a very important growth of cities. Urban regions include nowadays more than half of the global population and, by 2030, this proportion is forecasted to increase to three quarters. A consequent more and more extensive use of natural resources, together with increasing anthropogenic activities such as emissions from traffic and factories, or heating from air-conditioning facilities, modify local climate in many different ways, leading to a progressive degradation of life quality in urban areas. Taking these facts into account, there is nowadays a real need for efficient urban planning guidelines and sustainability policies in order to improve life quality in urban areas. Different points should be considered including urban warming, air pollution, human health, economic, and cooling energy needs. The present work goes in this direction, aiming at developing a simulation system for the study of the complex interactions between buildings and the atmosphere. The system of equations describing atmospheric flows is highly non linear and it is common to employ numerical techniques in order to solve them. Moreover, the representation of urban canopy climate, and related air pollution problems, requiree taking the interaction between urban scale (tens of kilometers), and mesoscale (hundreds of kilometers) processes into account. At first, starting from previous studies, a mesoscale meteorological model has been developed as part of this work. Urban induced processes have been considered by implementing inside the model a detailed urban parameterization scheme developed in a previous work. The scheme is able to capture different urban processes, and reproduces the effects of cities in a more accurate way than traditional methods usually used in mesoscale models. However, the heat generated in buildings, and the way this heat is exchanged with the exterior, was not explicitly resolved. In particular, recent studies indicated that anthropogenic heat from air-conditioning facilities can play an important role and should be taken into account for more complete urban climate studies. To this purpose, a Building Energy Model (BEM) has been developed, and coupled to the Urban Canopy Parameterisation (UCP). This building model takes into account the diffusion of heat through walls, roofs, and floors, the natural ventilation, the generation of heat from occupants and equipments, and the consumption of energy through air conditioning systems. Comparisons with other programs indicate that BEM is able to accurately simulate the basic heat transfer phenomena, and to reproduce the heat fluxes exchanged between buildings and the atmosphere. In a second part of the work, the simulation system composed by the Mesoscale Model (MM), UCP, and BEM is tested with respect to one, and two-dimensional configurations. In particular, the impact of BEM on the meteorological variables is analyzed, as well as the efficiency of different urban warming countermeasures, and cooling energy demands control strategies. Two-dimensional results are then utilized as guidance for the application of MM-UCP-BEM over the realistic configuration including the city of Basel and the surrounding areas. At first, comparisons of urban and rural simulated temperatures with measures provided by the BUBBLE project (Basel Urban Boundary Layer Experiment), have shown that the model could reasonably well reproduce the variations in outdoor air temperature. In a second time, the simulations considered for the two-dimensional configuration are applied to the case of Basel. Numerical results confirm that anthropogenic fluxes from air-conditioning facilities can have a non negligent impact on the urban meteorology. Typically, they modify the outdoor temperature and increase the Urban Heat Island (UHI) phenomenon. In the last part of the study, the applicability of the model in performing urban warming countermeasures, and cooling energy demands control strategies, is evaluated. In this purpose in mind, a sensitivity analysis is carried out indicating that appropriate physical properties of built materials, efficient air-conditioning systems, and the application of simple energy saving policies, can lead to very important cooling energy savings. In general, the application of BEM inside the UCP allows computing the heat released into the atmosphere by air-conditioning facilities, as well as the corresponding feedbacks produced on the different meteorological variables. It also increases the capability of the urban parameterisation to provide more detailed studies of urban warming countermeasures, and cooling energy demands in real cities.

EPFL2009

Life cycle efficiency ratio: a new performance indicator for a life cycle driven approach to evaluate the potential of ventilative cooling and thermal inertia

Jérôme Philippe Bonvin, Arianna Brambilla, Flourentzos Flourentzou, Thomas Bernard Paul Jusselme

Building envelope design has gained importance as a means to reduce heating and cooling demand related to a building’s operational phase. However, in high internal load buildings, such as offices, internal gains can easily lead to overheating. Thermal inertia (TI) and night ventilation have a great potential for reducing heat loads and temperature. However, their influence is difficult to predict due to the complex nature of the TI phenomenon, which is related to the interactions of multiple factors such as architecture, building physics and external conditions. Moreover, TI efficacy has often been studied in relation to energy savings or temperature analysis, overlooking other aspects implicated in buildings’ efficiency, such as the embodied energy involved. This paper presents a multidimensional approach to evaluate ventilative cooling and thermal inertia as a sustainable strategy to improve building performances. To that end, several scenarios of night ventilation strategies have been applied to the case study of an experimental double-office room placed in Fribourg (Switzerland). Based on this monitoring, a dynamic software simulation tool has been calibrated and used to analyze the energy savings potential and the life-cycle performance of TI. A new ratio index has been introduced to easily evaluate the life cycle efficiency. The results show the importance of balancing operational and embodied impacts when evaluating a design choice. Although high TI levels have great benefits on reducing the cooling loads, the results are completely different when a life cycle assessment is applied. Natural ventilation coupled with middle levels of TI have been identified as the best strategy to optimize the building’s energy and environmental performances, without compromising indoor temperatures.