Impact of buildings on the urban climate: modeling and experimental approach

Invited talk to the Smart Living Lunch, Fribourg, Switzerlan

Multi-scale modelling to improve climate data for building energy models

The recent AR5 report from the Intergovernmental Panel on Climate Change has again stressed on the need for mitigation and adaptation measures to tackle issues related to climate change. Tackling future urban planning and energy efficiency in the building sector is crucial as they account for almost 40% of energy use in developed countries. A one-dimensional canopy interface module (CIM) was recently developed to improve the surface representation in meteorological models and to enhance boundary conditions for building energy models. In the present study, we explain the methodology to couple CIM to the CitySim software. We show that CIM can be used to provide high-resolution vertical profiles to improve the calculation of the energy balance

The energy hub concept applied to a case study of mixed residential and administrative buildings in Switzerland

The concept of Energy Hub (EH) is getting popular as a method to integrate non-dispatchable energy sources at building and neighbourhood scale with the support of energy storage and grid. It is interesting to study the effectiveness of EH concept to integrate solar energy and wind energy at both building and neighbourhood scale considering the real-time price and curtailments in the grid. This paper presents a case study conducted to evaluate the effectiveness of EH in integrating solar energy in the SwissTech Convention Centre (STCC) and Quartier Nord on the EPFL campus in Lausanne considering both building and neighbourhood scale. The results depict that EH is more effective when both compared to standalone operation and grid integrated mode (considering grid curtailments and RTP) in the process of integration of renewable energy sources. Interacting with the grid seems to be more economical when compared to storage. Grid curtailments cause the storage to operate more frequently in both charging and discharging cycles

Development of a new 1D urban canopy model: coherences between surface parameterizations

A 1-D Canopy Interface Model (CIM) was developed in order to better simulate the effect of urban obstacles on the atmosphere in the boundary layer. The model solves the Navier-Stokes equations on a high-resolved gridded vertical column. The effect of the surface is simulated testing a set of theories and urban parameterizations. The final proposition guarantees its coherence with past theories in any atmospheric stability and terrain configuration. Obstacle characteristics are computed using surface and volume porosities in each cell of the model domain. These porosities are used to weight several terms in the Navier-Stokes equations. A 1.5-order turbulence closure is used in order to compute the turbulent coefficients with the TKE. The mixing length takes into account the density of the obstacles and their height. The turbulent scheme is designed in order to keep CIM coherent with the Prandtl theory in neutral atmospheric conditions and with the MOST in stratified atmospheric stability when CIM is used over plane surfaces. The modifications brought to the main governing equations are discussed following theoretical analysis and experiences with CIM, simulating the averaged meteorological variables (wind speed, turbulent kinetic energy (TKE), temperature and humidity). Simulations are compared with analytical solutions, when possible, and also simulations issued from a computational fluid dynamics (CFD) model. The results show how constant values, usually prescribed, can be theoretically estimated and how the buoyancy term of the turbulent kinetic energy balance equation should be adjusted accordingly. After modifications, it is shown that CIM is coherent with past propositions in any case of atmospheric stabilities over plane surfaces. The use of CIM in presence of obstacles is based on the extension of the 1.5 order turbulence closure to compute the turbulent coefficients with the TKE. CIM shows simulations in good agreement with the CFD simulations in the presence of obstacles. It is able to reproduce an inertial sub-layer as described by the Prandlt and constant-flux layer theory above a displacement height over a homogeneous canopy

Fisher–Shannon Complexity Analysis of High-Frequency Urban Wind Speed Time Series

One-hertz wind time series recorded at different levels (from 1.5–25.5 m) in an urban area are investigated by using the Fisher–Shannon (FS) analysis. FS analysis is a well-known method to gain insight into the complex behavior of nonlinear systems, by quantifying the order/disorder properties of time series. Our findings reveal that the FS complexity, defined as the product between the Fisher information measure and the Shannon entropy power, decreases with the height of the anemometer from the ground, suggesting a height-dependent variability in the order/disorder features of the high-frequency wind speed measured in urban layouts. Furthermore, the correlation between the FS complexity of wind speed and the daily variance of the ambient temperature shows a similar decrease with the height of the wind sensor. Such correlation is larger for the lower anemometers, indicating that ambient temperature is an important forcing of the wind speed variability in the vicinity of the ground

Evaluating the need for energy storage to enhance autonomy of neighborhoods

Energy storage is generally considered as a means to bridge a period between when/where energy is available and when/where it is in demand. Storage plays an important role by providing flexibility to energy systems, increasing the potential to accommodate variable renewables generation and improving management of electricity networks. However, currently it remains unclear when and under which conditions energy storage can be profitably operated at a district level. The present study aims to quantify the level of integration of solar energy and storage in the Junction district of Geneva. A simulation tool is developed to investigate the techno-economical and environmental assessment under different scenarios. For a given investment over 20 years, the model calculates the levelized cost of electricity (LCOE), the autonomy level as well as the CO2 emissions. Given the assumptions of the model, four scenarios are analysed based on the combination of solar PV, storage, solar thermal and heat pump to find out an economically optimal configuration in terms of system size. A comparison with the Homer software is performed to test the robustness of the solar PV and battery model. The economic profitability of solar PV and battery system is in very good agreement with Homer and the autonomy level is validated by using a simulation tool created by SI-REN (Services Industriels des Energies Renouvelables de Lausanne). However, combining solar PV with battery system doesn’t bring additional autonomy to the model for Geneva study case. Under the assumptions of the model, to foster investments in solar PV and battery installations, falling investments costs seem necessary for the future. A reduction gap between buying and selling price in grid for solar panel is recommended to increase solar installations. A validated simulation tool has been developed in this work and provide a reliable based that will be extended in the future to include the thermal demand and production. The availability of thermal storage at a large scale as well as the production over a district should further increase the autonomy of the district

Multi-scale modelling to evaluate building energy consumption at the neighbourhood scale

A new methodology is proposed to couple a meteorological model with a building energy use model. The aim of such a coupling is to improve the boundary conditions of both models with no significant increase in computational time. In the present case, the Canopy Interface Model (CIM) is coupled with CitySim. CitySim provides the geometrical characteristics to CIM, which then calculates a high resolution profile of the meteorological variables. These are in turn used by CitySim to calculate the energy flows in an urban district. We have conducted a series of experiments on the EPFL campus in Lausanne, Switzerland, to show the effectiveness of the coupling strategy. First, measured data from the campus for the year 2015 are used to force CIM and to evaluate its aptitude to reproduce high resolution vertical profiles. Second, we compare the use of local climatic data and data from a meteorological station located outside the urban area, in an evaluation of energy use. In both experiments, we demonstrate the importance of using in building energy software, meteorological variables that account for the urban microclimate. Furthermore, we also show that some building and urban forms are more sensitive to the local environment

Multi-scale modelling to assess human comfort in urban canyons

As the impact of climate change progresses, heat waves are expected to increase significantly in the future. Coupled with the urban heat island effect, this will tend to have a major impact on the comfort of the inhabitants in urban areas. It is thus crucial to adopt the necessary sustainable measures and development scenarios to improve city liveability and human health. The main physical parameters that affect the outdoor human comfort are the air temperature, the relative humidity and the wind speed. Various tools, such as CFD or LES models, have been used in the past to evaluate these variables for the calculation of human comfort indices. These tools however are computationally too expensive and require extensive resources and data. Moreover, in our previous studies on the outdoor human comfort realized with the CitySim software, the meteorological variables were not linked to the urban form, geometry and roughness. To overcome these barriers, the CIM (Canopy Interface Model) was developed to calculate high-resolution vertical profiles of meteorological variables. The CitySim software to perform energy and temperature simulations then used these outputs. In this study, virtual pedestrians were located in two different areas of the EPFL campus, in Lausanne (Switzerland): a natural environment - characterized by clay soil and cherry trees - and an artificial environment, the new asphalt square near the SwissTech Convention Centre. The analysis carried out with the CitySim software compares the outdoor human comfort of pedestrian with the wind data from the traditional Meteonorm dataset, and the new CIM wind simulations. A sensitivity analysis of the results shows the difference between both simulations, quantifying the impact of the new wind model in the calculation of the indices