• Energy Research

This paper studies the sensitivity of heat losses in collective heat distribution systems using a simplified calculation method. The approach is applied to a specific type of heat distribution system where the network distributes heat for both space heating and sanitary hot water. The proposed methodology takes design parameters influencing the thermal performance of the heat distribution system into account. Among other parameters, the recirculation control strategy and the length of service pipes are taken into account. The present study assesses the impact of potential variations in the input parameters on the total heat loss and distribution efficiency. To that aim, a simulation study was conducted with a multilevel factorial design, consisting of the combinations of parameters which were varied from the levels at which they were set. Results show how sensitive the solution is to the different parameter values. Furthermore, the potential of the improved calculation method, which allows combining different design conditions within a single heating distribution system, was demonstrated.

Earth-air heat exchangers, also called ground tube heat exchangers, are an interesting technique to reduce energy consumption in a building. They can cool or heat the ventilation air, using cold or heat accumulated in the soil. Several papers have been published in which a design method is described. Most of them are based on a discretisation of the one-dimensional heat transfer problem in the tube. Three-dimensional complex models, solving conduction and moisture transport in the soil are also found. These methods are of high complexity and often not ready for use by designers. In this paper, a one-dimensional analytical method is used to analyse the influence of the design parameters of the heat exchanger on the thermo-hydraulic performance. A relation is derived for the specific pressure drop, linking thermal effectiveness with pressure drop of the air inside the tube. The relation is used to formulate a design method which can be used to determine the characteristic dimensions of the earth-air heat exchanger in such a way that optimal thermal effectiveness is reached with acceptable pressure loss. The choice of the characteristic dimensions, becomes thus independent of the soil and climatological conditions. This allows designers to choose the earth-air heat exchanger configuration with the best performance.

Abstract Recent studies indicated that window opening behaviour is related to environmental and contextual factors, as well as to practices of everyday life, such as morning routines and daily activities. To the authors’ knowledge no studies have been conducted that focus on the habitual window opening behaviour in residential buildings. The aim of this study is to determine the extent of the habitual behaviour and to identify the types of window use habits. Additionally, a window use model is created that includes habitual behaviour. Results of the online survey conducted with Belgian households revealed that almost all occupants habitually interact with at least one window. Some household and building characteristics are related to the presence of certain habits. However, this impact is rather small in comparison to the impact of the habits of other rooms. These insights are utilized for the creation of a window use habit model. The model is explained in detail as well as the validation procedure. While further research is necessary regarding the cultural and seasonal dependency of the window use habits, this model is promising as it links the window use to the everyday life, resulting in the prediction of realistic and justifiable window use.

Abstract This paper presents an investigation carried out for evaluating the dynamic thermal performance of a residential house using a multi-zone building model simulated with the help of TRNSYS 17. The building model integrates systems such as an air–air heat exchanger (AAHE), a water–air heat exchanger (WAHE) coupled with an earth–water heat exchanger (EWHE). The temperature based control strategies for these systems are recommended based on the simulation results. The AAHE and the EWHE have impact of 66% and 7% respectively on reduction of the annual heating consumption of the house. All the integrated systems together contribute significantly (72%) in reducing the annual heating consumption of the house. The annual heating consumption of the house per unit floor area is estimated as 6.9 kWh/(m2 year) which is within the passive house standard. There is significant reduction of the overheating time above 25 °C due to the EWHE installed in the present house with annual ventilation air cooling contribution of 602.6 kWh/year.

This paper describes the coupling of a model for heat and moisture transport in porous materials to a commercial Computational Fluid Dynamics (CFD) package. The combination of CFD and the material model makes it possible to assess the risk of moisture related damage in valuable objects for cases with large temperature or humidity gradients in the air. To couple both models the choice was made to integrate the porous material model into the CFD package. This requires the heat and moisture transport equations in the air and the porous material to be written down in function of the same transported variables. Validation with benchmark experiments proved the good functionality of the coupled model. A simulation study of a microclimate vitrine for paintings shows that phenomena observed in these vitrines are well predicted by the model and that data generated by the model provides additional insights in the physical mechanisms behind these phenomena.

The focus in environmental research is shifting from emission abatement to critical process analysis, including assessment of resource consumption. The exergy theory offers a thermodynamic methodology to account for the consumption of natural resources. However, exergy data on mineral resources available in the literature are inadequate to apply to exergetic life cycle analysis, due to incompleteness, inconsistencies, and a dated thermochemical basis. An uncertainty assessment of the data has to be performed as well. In this work, three recent thermochemical databases were applied to evaluate the chemical exergy of 85 elements and 73 minerals, 21 of which had not yet been quantified in the literature. The process required the choice of a new reference species for aluminum. Muscovite was selected, giving rise to a chemical exergy of 809.4 kJ/mol for aluminum. The theory proved to be robust for the exergy of chemical elements, as exergy values differing by 1.2% on average from most recent literature were found. On the contrary, the exergy values for minerals differed by factors up to 14 from literature values, due to the application of recent thermochemical values and consistently selected reference species. The consistent dataset of this work will enable straightforward resource intake evaluation through an exergetic life cycle assessment.

AbstractIn many countries, the theoretical heating demand within the official energy performance assessment of houses is evaluated using simplified, single-zone quasi-steady state models, based on ISO 13790 and considering one standard, average user profile. Unpredictable variations in user behavior are acknowledged as a major cause of varying prediction errors. However, even considering a single, standard user profile, not heating the bedrooms and switching off the heating at night, requires the use of additional correction formulas, as such behavior appears contradictory to the single zone and quasi-steady state assumptions of the simplified models. This paper compares theoretical heating demands and indoor temperatures using the spatial reduction and intermittency correction formulas from the German (DIN 18599) and Dutch (NEN 7120) standards with results from the simplified, Flemish approach. Results from a multi-zone quasi-steady state model and measured values are added to the comparison. An old neighborhood of uninsulated houses is used as a case-study and renovation scenarios are considered. Results show that, between the German and Dutch approaches, the different predictions are mainly caused by the different standard user profile considered, much more than by the different formulas. Considering the real heating profiles is found to be indispensable, though not sufficient for accurate predictions. The multi-zone approach is proven to be of great value for supporting the selection of specific renovation measures.

AbstractWind-driven rain (WDR) is one of the most important causes for water damage in buildings. Therefore, the first crucial step to assess the hygrothermal performance of the building envelope, is the appropriate estimation of the amount of rainwater striking the building's façade. ISO 15927 offers the annual average index –mainly to assess the moisture content of absorbent surfaces, and the spell index – more related to the likelihood of water penetration through joints. To calculate these indices, assumptions are made concerning the length of the period of ‘no rain’, called ‘spell definition’. Obviously, the choice of this spell definition will characterise the WDR-amount. In this paper, WDR measurements of a 3-storey building in Vancouver, Canada are used to investigate how this spell definition affects the rain load. Different filter criteria are used to exclude errors due to measurement equipment. By means of the catch ratio as a dimensionless parameter, the results of the analysis for different spell definitions are compared to hourly and 5-min data. It is concluded that longer spell definitions result in lower catch ratios and an underestimation of the WDR load. Hourly data turns out to be a more conservative approach for WDR-assessment of this case study, but is able to represent the spread on the catch ratio most closely to the original 5min-dataset.

AbstractThe thermal performance of window frames can easily be calculated using 2D numerical simulations. Several commercial software packages are available, and international standards provide a clear methodology to calculate the thermal transmittance. However, even though these methods are well known in academia and the research community in general, thermal optimization has not reached its full potential in the building industry yet and there is considerable margin for improvement. Specifically for small and medium size enterprises there is a lack of guidelines that are both generic enough to guarantee a wide-spread use, as well as specific enough to allow an easy and straightforward interpretation and implementation. In this research project, generic window sections were developed for vinyl, aluminium and wooden frames in collaboration with the building industry. Based on a market survey, typical approaches for improving the thermal performance for each type of window frame have been identified and described. Subsequently, the impact of separate improvements, as well as combined effects have been studied using both standardized and advanced calculation methods. For this, the heat transfer phenomena and the way these are modelled according to standard calculation procedures are discussed. Next to that, a number of secondary effect originating from standards are discussed, e.g. the thickness of the IGU, the depth of the window rebate, equivalent thermal conductivities and the impact of reduced heat transfer coefficients.