Jan Hensen

Application of computational fluid dynamics in building performance simulation for the outdoor environment: an overview

This article provides an overview of the application of computational fluid dynamics (CFD) in building performance simulation for the outdoor environment, focused on four topics: (1) pedestrian wind environment around buildings, (2) wind-driven rain on building facades, (3) convective heat transfer coefficients at exterior building surfaces and (4) air pollutant dispersion around buildings. For each topic, its background, the need for CFD, an overview of some past CFD studies, a discussion about accuracy and some perspectives for practical application are provided. This article indicates that for all four topics, CFD offers considerable advantages compared with wind tunnel modelling or (semi-)empirical formulae because it can provide detailed whole-flow field data under fully controlled conditions and without similarity constraints. The main limitations are the deficiencies of steady Reynolds-averaged Navier–Stokes modelling, the increased complexity and computational expense of large eddy simulation and the requirement of systematic and time-consuming CFD solution verification and validation studies.

Electrically switchable polymer stabilised broadband infrared reflectors and their potential as smart windows for energy saving in buildings

Electrically switchable broadband infrared reflectors that are relatively transparent in the visible region have been fabricated using polymer stabilised cholesteric liquid crystals. The IR reflectors can change their reflection/transmission properties by applying a voltage in response to changes in environmental conditions. Simulations predict that a significant amount of energy can be saved on heating, cooling and lighting of buildings in places such as Madrid by using this switchable IR reflector. We have also fabricated a switchable IR reflector which can also generate electricity. These polymer based switchable IR reflectors are of high potential as windows of automobiles and buildings to control interior temperatures and save energy.

An investigation of the option space in conceptual building design for advanced building simulation

This article describes research conducted to gather empirical evidence on size, character and content of the option space in building design projects. This option space is the key starting point for the work of any climate engineer using building performance simulation who is supporting the design process. The underlying goal is to strengthen the role of advanced computing in building design, especially in the early conceptual stage, through a better integration of building performance simulation tools augmented with uncertainty analysis and sensitivity analysis. Better integration will need to assist design rather than automate design, allowing a spontaneous, creative and flexible process that acknowledges the expertise of the design team members. This research investigates and contrasts emergent option spaces and their inherent uncertainties in an artificial setting (student design studios) and in real-life scenarios (commercial design project case studies). The findings provide empirical evidence of the high variability of the option space that can be subjected to uncertainty analysis and sensitivity analysis.

Application of broadband infrared reflector based on cholesteric liquid crystal polymer bilayer film to windows and its impact on reducing the energy consumption in buildings

An infrared (IR) polymer reflector based on chiral nematic (cholesteric) liquid crystals has been fabricated which can reflect more than 60% of solar IR energy without interfering with the visible solar radiation. Simulations show that the polymer bilayer film applied to a window of a typical building can have a significant impact on the interior temperature in living and working spaces.

Mesoporous Germanium Formation by Electrochemical Etching

Weight reduction of multi-junction III-V semiconductor solar cells is an important budget issue for space applications. Typically, space solar cells are epitaxially formed on a Ge or GaAs substrate wafer. The substrate material determines the lattice constant of the stack, provides mechanical stability during the cell process, and serves as bottom cell. The substrate wafer is typically more than 100 µm thick for reasons of mechanical stability during cell processing, whereas a few µm thickness are sufficient for the bottom cell to match the photogenerated currents in the top and middle cells and not to be current limiting. Unnecessarily heavy substrate wafers hence reduce the available payload for satellite missions. There are several techniques that permit the production of very-thin lightweight highly-efficient space solar cells. Ge or GaAs substrates are commonly removed by chemical wet etching, which reduces weight but has the disadvantage that the substrate wafer is lost for further use. Separating the electrically active solar cells from their substrates by a lift-off process, could save the substrate and reduce costs. The application of a layer transfer process for multi-junction III-V semiconductor space solar cells is hence of main interest for all space agencies. Lift-off processes based on epitaxial growth of the absorber layer onto a porous etched substrate already exist for the fabrication of monocrystalline silicon solar cells. Brendel demonstrated the so-called Porous Silicon (PSI) process for the production of monocrystalline thin-film Si solar cells. This method uses a double layer of mesoporous Si formed by means of electrochemical etching: A mesoporous layer with low porosity at the surface of the substrate is used as a seed layer for the Si epitaxy, while a buried high porosity layer is used as a pre-determined breaking-point. The formation of porous germanium (PGe) has been not intensively studied. This doctoral work focuses on the fabrication and characterization of porous germanium layers by means of electrochemical etching. This thesis evaluates the potential applications of porous Ge layers for the fabrication of very-thin space solar cells. Additionally, the formation of mesoporous GaAs and mesoporous Si layers with miscut orientations is investigated.

Review of current status, requirements and opportunities for building performance simulation of adaptive facades†

Adaptive building envelope systems have the potential of reducing greenhouse gas emissions and improving the energy flexibility of buildings, while maintaining high levels of indoor environmental quality. The development of such innovative materials and technologies, as well as their real-world implementation, can be enhanced with the use of building performance simulation (BPS). Performance prediction of adaptive facades can, however, be a challenging task and the information on this topic is scarce and fragmented. The main contribution of this review article is to bring together and analyse the existing information in this field. In the first part, the unique requirements for successful modelling and simulation of adaptive facades are discussed. In the second part, the capabilities of five widely used BPS tools are reviewed, in terms of their ability to model energy and occupant comfort performance of adaptive facades. Finally, it discusses various ongoing trends and research needs in this field.

A view of energy and building performance simulation at the start of the third millennium

The use of computer-based models for performance predictions has become almost ubiquitous in the design, operation and management of buildings and the systems that service them. This special issue bears witness to the fact that the building performance simulation field is rapidly evolving. The techniques and applications of building performance simulation are undergoing rapid change. Dramatic improvements in computing power, algorithms, and physical data make it possible to simulate physical processes at levels of detail and time scales that were not feasible only a few years ago. Applications that were not attainable or practicable some years ago are now commonplace.

Analysis and improvement of the representativeness of EN ISO 15927-4 reference years for building energy simulation

Representativeness of weather inputs is crucial to limit the global uncertainty of building energy simulation results. The length of the multi-year weather data series and the methodology used for the typical month selection largely influence the results of the reference year development process. In this work, we investigate two possible modifications to the EN ISO 15927-4:2005 procedure aimed at improving the representativeness of reference year heating and cooling needs. The first modification maintains the reference years independent of their final use while the second one leads to the development of specific weather files for heating or cooling analyses by introducing weighting coefficients for the different weather parameters. The study is performed for five North Italy localities with 10 or less years in the data-set and for a sample of 48 simplified buildings. Both proposed modifications brought improvements to the representativeness of the reference year results.

Directional Heating and Cooling for Controlled Spalling

The fabrication of thin solar cells by kerfless wafering techniques offers a high potential for the reduction of photovoltaic costs. We present an experimental setup for the exfoliation of thin crystalline silicon foils from a silicon substrate induced by the difference in thermal expansion coefficient of the silicon and an aluminum stressor layer at moderate temperatures. A moving temperature gradient across the substrate controls the crack propagation parallel to the silicon surface. We measure and simulate the spatial temperature distribution during thermal treatment and find that the direction of crack propagation is controlled by the temperature distribution. We detach foils with an area of 19.6 cm2 with thickness values ranging from 50 to 80 μm within one layer. The foils have a smooth surface with some irregularities near the edge.

4.5 ms Effective Carrier Lifetime in Kerfless Epitaxial Silicon Wafers From the Porous Silicon Process

Kerfless silicon wafers epitaxially grown on porous silicon (PSI) and subsequently detached from the growth substrate are a promising candidate for reducing the cost of the silicon wafer, which is particularly important for silicon photovoltaics. However, the carrier lifetime of these epitaxial wafers has to be at least as high as that of todays standard Czochralski (Cz)-grown wafers in order to become competitive. Here, we compare the measured lifetimes of n-type epitaxial silicon wafers that grow on PSI and epitaxial silicon wafers that grow on nonporous surfaces of epi-ready wafers. The latter are subsequently ground to have free-standing epitaxial wafers. Gettering improves the carrier lifetime of the ground wafers up to 4.2 ms. In contrast, PSI wafers show regions with effective lifetimes of 4.5 ms, even without gettering. This lifetime value is a factor of four larger than lifetimes of Cz wafers which are typically employed in todays PERC solar cells. We model the lifetime measurements with three Shockley–Read–Hall (SRH) defects: two defects that exist in the PSI and in the epi-ready wafer and a third defect that is only present in the epi-ready wafer.