• Energy Research

In a changing outdoor climate, new buildings as well as the existing building stock need to adapt in order to keep providing their inhabitants and users a comfortable and healthy indoor environment, with a minimum or – preferably – no increase in energy consumption. In this paper, the effectiveness of six passive climate change adaptation measures applied at the level of building components is assessed using building energy simulations for three generic residential buildings as commonly built in – among others – the Netherlands: (1) detached house; (2) terraced house; (3) apartment. The study involves both residential buildings that are built according to the regulations and common practice in 2012, and residential buildings that were constructed in the 1970s, with a lower thermal resistance of the opaque and transparent parts of the building envelope. The climate change adaptation measures investigated are: (i) increased thermal resistance; (ii) changed thermal capacity; (iii) increased short-wave reflectivity (albedo); (iv) vegetation roofs; (v) solar shading; and (vi) additional natural ventilation. This paper quantifies the effectiveness of these climate change adaptation measures for new residential buildings as well as for renovation of the current building stock. The performance indicator is the number of overheating hours during a year. It is shown that exterior solar shading and additional natural ventilation are most effective for this performance indicator. Furthermore, increasing thermal insulation to reduce energy use for heating demands additional measures to prevent overheating.

Air curtains (ACs) are of interest in building applications to support energy efficiency and air quality by restricting heat and mass transport across continuously open entrances. Environmental conditions to which ACs are generally subjected, such as differences in temperature and pressure, induce loads on ACs that alter their flow pattern, hence can considerably influence their separation efficiency. Therefore, proper design and implementation of ACs demands detailed knowledge of the impact of these parameters both individually and combined. This is addressed here by a systematic evaluation of the AC separation efficiency under moderate environmental temperature (5 °C ≤ ΔT ≤ 25 °C) and pressure (1 Pa ≤ ΔP ≤ 8 Pa) difference conditions. RANS CFD simulations of an AC are conducted on verified computational grids, validated with experimental data and calibrated with field measurements. Results show a strong yet non-linear dependency of AC performance on environmental parameters and indicate there is an optimal separation efficiency (based on total mass transfer) in which competing effects between a given jet momentum flux on the one hand and cross-jet loads caused by temperature or pressure differences on the other hand are counterbalanced. Findings suggest to dynamically control the supplied AC jet momentum as a function of variable environmental conditions in order to continually deliver an optimal performance.

Natural ventilation can be used to improve indoor air quality, remove contaminants from spaces and to remove heat from a building during the day, or during the night. In some cases, openings for natural ventilation are equipped with shading devices – such as louvers – to reduce solar heat gains while allowing natural ventilation. This study presents wind-tunnel experiments and computational fluid dynamics (CFD) simulations of a cross-ventilated building equipped with louvers. Four opening positions are studied: (i) openings in the center, (ii) upper or (iii) lower part of the windward and leeward facades or (iv) one opening in the upper part of the windward facade and one opening in the lower part of the leeward facade. The 3D steady Reynolds-averaged Navier-Stokes (RANS) simulations are performed with three turbulence models (RNG k-ε, SST k-ω, RSM) and validated with the wind-tunnel experiments. The experimental results show that the largest velocities occur in a building with openings in the upper part of the facade. The best agreement with experimental data is provided by RSM. In addition, CFD simulations for buildings without louvers are conducted for the same opening positions to evaluate the effect of louvers on the dimensionless volume flow rate, age of air and air exchange efficiency. The highest dimensionless volume flow rate at reduced scale (0.69) is obtained in the building with louvered openings in the upper part of the facade and the highest air exchange efficiency is achieved for a building with louvered openings in the center of the facade (45%).

Urban wind energy can provide a decentralized local source of energy for residential areas and reduce the cost of energy by avoiding the losses/costs of long-distance energy transmission. In this perspective, a preliminary assessment of urban wind energy is highly desired by turbine developers, investors and policy makers. However, given the large number of parameters involved, predictions of the wind energy potential in urban areas are very challenging. The present paper, therefore, intends to present a straightforward framework to provide a preliminary and large-scale assessment of the urban wind energy potential, i.e. at city or country scales, for roof-mounted turbines. The framework is based on four main steps: (i) collecting the building data, i.e. the number of potential candidate high-rise buildings and their height and rooftop surface area; (ii) obtaining the annual mean wind speed statistics at the height of these buildings and sorting the building data based on these statistics; (iii) obtaining the turbine characteristics and determining the average number of turbines per building roof; (iv) calculating the annual energy production (AEP). The application of the framework is then illustrated at the country scale for the Netherlands. In this case, the urban wind energy potential is assessed by considering the installation of 18,156 small wind turbines on the roofs of 1513 existing high-rise buildings in 12 major cities in the Netherlands, yielding an annual energy production of 150.1 GWh.

Accurate CFD simulation of coupled outdoor wind flow and indoor air flow is essential for the design and evaluation of natural cross-ventilation strategies for buildings. It is widely recognized that CFD simulations can be very sensitive to the large number of computational parameters that have to be set by the user. Therefore, detailed and generic sensitivity analyses of the impact of these parameters on the simulation results are important to provide guidance for the execution and evaluation of future CFD studies. A detailed review of the literature indicates that there is a lack of extensive generic sensitivity studies for CFD simulation of natural cross-ventilation. In order to provide such a study, this paper presents a series of coupled 3D steady RANS simulations for a generic isolated building. The CFD simulations are validated based on detailed wind tunnel experiments with Particle Image Velocimetry. The impact of a wide range of computational parameters is investigated, including the size of the computational domain, the resolution of the computational grid, the inlet turbulent kinetic energy profile of the atmospheric boundary layer, the turbulence model, the order of the discretization schemes and the iterative convergence criteria. Specific attention is given to the problem of oscillatory convergence that was observed during some of these coupled CFD simulations. Based on this analysis, the paper identifies the most important parameters. The intention is to contribute to improved accuracy, reliability and evaluation of coupled CFD simulations for cross-ventilation assessment.

The Scale-Adaptive Simulation (SAS) approach has emerged as an improved unsteady Reynolds-Averaged Navier-Stokes (URANS) formulation to bridge the gap between the less accurate commonly used URANS and the computationally expensive hybrid RANS/LES for highly separated unsteady flows, e.g. dynamic stall. However, while the SAS has been successfully used at several occasions, it has not yet been tested for the complex case of dynamic stall. Therefore, the present study analyzes the SAS predictions of dynamic stall on a vertical axis wind turbine at a chord Reynolds number of 5 × 104 and a reduced frequency of 0.125. The analysis is based on comparison of the SAS predictions of the blade aerodynamics and the turbine power performance against the corresponding URANS and hybrid RANS/LES predictions. The results show that the SAS predictions are closer to hybrid RANS/LES than URANS with respect to: (i) the instant of the bursting of the laminar separation bubble (LSB), the leading-edge suction collapse, the formation of the dynamic stall vortex (DSV) and the trailing-edge vortex (TEV) and the shedding of the TEV; (ii) the size and strength of the TEV; (iii) the DSV-TEV interaction; (iv) the drag prediction during the downstroke. On the other hand, both URANS and SAS fail to corroborate with hybrid RANS/LES with respect to: (i) the instant of the formation of the LSB and the shedding of the DSV (the stall angle); (ii) the drag jump at the stall angle; (iii) the lift values during the downstroke; and (iv) the chordwise extent of the LSB.

Ventilation is of primary importance for the creation of healthy and comfortable indoor environments and it has a significant impact on the building energy heating and cooling demand. The aim of this study is to assess the application of time-periodic supply velocities to enhance mixing in mixing ventilation cases to reduce heating and cooling energy demands. This paper presents computational fluid dynamics (CFD) simulations of a generic mixing ventilation case, in which the time-averaged velocities and pollutant concentrations from a reference case with constant supply velocities were compared with those obtained from a case with time-periodic supply velocities (sine function). The unsteady Reynolds-averaged Navier-Stokes (URANS) CFD simulations indicate that the use of time-periodic supply velocities can reduce high pollutant concentrations in stagnant regions, reduces the overall time-averaged pollutant concentrations and increases contaminant removal effectiveness with about 20%. The influence of the period of the sine function was assessed and the results showed that for the periods tested, the differences are negligible. Finally, the URANS approach was compared with the large eddy simulations (LES) approach, indicating that URANS leads to very similar results (NMSE < 3.2%) as LES and can thus be regarded as a suitable approach for this study.