Abstract

This paper deals with the search for the optimal window-to-wall ratio (WWR) in different European climates in relation to an office building characterized by best-available technologies for building envelope components and installations. The optimal WWR value is the one that minimizes, on an annual basis, the sum of the energy use for heating, cooling and lighting.

By means of integrated thermal and lighting simulations, the optimal WWR for each of the main orientations was found in four different locations, covering the mid-latitude region (35° to 60° N), from temperate to continental climates. Moreover, the robustness of the results was also tested by means of sensitivity analyses against the efficiency of the building equipment, the efficacy of the artificial lighting and the compactness of the building.

The results indicate that although there is an optimal WWR in each climate and orientation, most of the ideal values can be found in a relatively narrow range (0.30 < WWR < 0.45). Only south-oriented façades in very cold or very warm climates require WWR values outside this range. The total energy use may increase in the range of 5–25% when the worst WWR configuration is adopted, compared to when the optimal WWR is used.

Abstract

Introduction of more dynamic building envelope components have been done throughout the last decades in order to try to increase indoor thermal comfort and reduce energy need in buildings for both temperature and light control. One of these promising technologies is phase change materials (PCM), where, the latent heat storage potential of the transition between solid and liquid state of a material is utilized as thermal mass. A PCM layer incorporated in a transparent component can increase the possibilities to harvest energy from solar radiation by reducing the heating/cooling demand and still allowing the utilization of daylight. The introduction of dynamic components in the building envelope makes the characterization of conventional static performance indices insufficient in giving a clear picture of the performance of the component in question.
Measurements have been performed on a state-of-the-art window that integrates PCM using a large scale climate simulator. The glazing unit consists of a four-pane glazing with an integrated layer that dynamically controls the solar transmittance (prismatic glass) in the outer glazing cavity. The innermost cavity is filled with a phase change material.
This article presents and assesses the series of measurements and the related methodologies with the aim of investigating the thermal behavior and thermal mass activation of the PCM-filled window. The experiments have been carried out using several static and dynamic test cycles comprised of temperature and solar radiation cycling. A conventional double-pane window has also been experimental investigated using the same test cycles for reference purpose.
It was found that even for temperatures similar to a warm day in Nordic climate, the potential latent heat storage capacity of the PCM was fully activated, but relatively long periods of sun combined with high exterior temperatures are needed.

Summary

At the Research Centre on Zero Emission Buildings of NTNU, a new test facility (Living Laboratory) is currently in the final stage of construction and will start its operation in summer 2015. The Living Laboratory was designed to carry out experimental investigations at different levels, ranging from envelope to building equipment components, from ventilation strategies to action research on lifestyles and technologies, where interactions between users and low (zero) energy buildings are studied.
The test facility is a single family house with a gross volume of approximately 500 m3 and a heated surface (floor area) of approximately 100 m2. It is realized with state-of-the-art technologies for energy conservation measurements and renewable energy source exploitation. In this paper, the test facility is described and its architectural features and technological aspects highlighted. The focus is then placed on the detail description of the proposed measurement and control system.

Abstract

The building envelope plays a crucial role in reducing operational energy demand. In particular, the two main properties of the building envelope to look at in this perspective are thermal transmittance (U, W/m2K1) and thermal inertia, which is often expressed by a metric called periodic thermal transmittance (Yie, W/m2K1). These two properties are also traditionally connected to two different energy demands: while thermal transmittance is crucial to reduce heating energy demand, thermal inertia has an impact on energy demand for cooling. However, a question may rise about the impact of each property on the other demand – i.e. the impact of thermal insulation on the cooling energy demand and the impact of thermal inertia on the heating demand.
A parametric analysis on the influence of the thermal inertia on the energy performance of a single family house in a Nordic climate has been carried out to find an answer to this question. “Ideal envelopes” have been modelled and simulated, meaning that used thermophysical properties do not represent any configuration, but the entire spectrum of technological configurations.
The results show that the influence of the thermal inertia on the heating energy need is very limited. Even a relatively high value of Yie, which means no or little thermal inertia, does not determine a significant increase in energy need. Parallel to this, solutions characterized by very high thermal inertia do not allow heating energy demand to be sensibly decreased. Periodic thermal transmittance has instead an impact on the heating load. The impact of the thermal inertia is also assessed in the warmer season, and the results show that this parameter does not significantly contribute to a better behavior (especially when the upper limit of the indoor air temperature is controlled). Limitations to value of thermal transmittance are also pointed out to avoid non-energy effective conditions when the total (heating plus cooling) annual performance is considered.

Abstract

The building envelope plays a crucial role in reducing operational energy demand. In particular, the two main properties of the building envelope to look at in this perspective are thermal transmittance (U, W/m2K1) and thermal inertia, which is often expressed by a metric called periodic thermal transmittance (Yie, W/m2K1). These two properties are also traditionally connected to two different energy demands: while thermal transmittance is crucial to reduce heating energy demand, thermal inertia has an impact on energy demand for cooling. However, a question may rise about the impact of each property on the other demand – i.e. the impact of thermal insulation on the cooling energy demand and the impact of thermal inertia on the heating demand.
A parametric analysis on the influence of the thermal inertia on the energy performance of a single family house in a Nordic climate has been carried out to find an answer to this question. “Ideal envelopes” have been modelled and simulated, meaning that used thermophysical properties do not represent any configuration, but the entire spectrum of technological configurations.
The results show that the influence of the thermal inertia on the heating energy need is very limited. Even a relatively high value of Yie, which means no or little thermal inertia, does not determine a significant increase in energy need. Parallel to this, solutions characterized by very high thermal inertia do not allow heating energy demand to be sensibly decreased. Periodic thermal transmittance has instead an impact on the heating load. The impact of the thermal inertia is also assessed in the warmer season, and the results show that this parameter does not significantly contribute to a better behavior (especially when the upper limit of the indoor air temperature is controlled). Limitations to value of thermal transmittance are also pointed out to avoid non-energy effective conditions when the total (heating plus cooling) annual performance is considered.

Abstract

Background

Phase change materials (PCMs) have been proposed as a means to increase the thermal inertia of glazing systems. These materials have optical features that need to be investigated and characterised in order to better understand the potential of these systems and to provide reliable data for numerical simulations.

Methods

The spectral and angular behaviour of different PCM glazing samples, characterised by different thicknesses of PCMs, were investigated by means of commercial spectrophotometer and by means of a dedicated optical test bed that includes a large integrating sphere with a diameter of 0.75 m. Such equipment was necessary because of the highly diffusive behaviour of the PCM layer when in the solid state of aggregation.

Results

The paper provides a data set of luminous and solar properties of glazing units with PCMs in gaps; the data set uses results from an advanced optical facility that overcomes the intrinsic limitations of commercial spectrophotometers in measuring the optical properties of the advanced transparent materials. In detail, transmittance, reflectance and absorptance spectra of double glazing units characterised by different PCM layer thicknesses in the gap, measured at different incident beam angles, are reported. Integrated values calculated according to relevant international standards are thus provided. Optical features of PCM glazing systems are also highlighted and issues related to the integration of these systems in buildings are discussed.

Abstract

This paper shows the results of a research activity aimed at assessing the advantages of an ideal adaptive building skin over conventional building envelope systems.

The basic idea underlying the research consists in imagining an ideal building envelope system characterised by the capability of continuously changing (within a certain range) some of its thermo-physical and optical properties. The reason for the continuous tuning of thermo-physical and optical properties lies in the assumption that an optimised (fixed) configuration, where the properties do not change over time, is not able to minimise the total energy demand of the building at each moment.

For the sake of this purpose, an ideal dynamic WWR (Window-to-Wall Ratio) building envelope system for low energy office buildings was modelled and simulated. An integrated, commercial thermal-lighting building simulation tool (EnergyPlus) was used to perform the calculation. The energy performance of such a system was then analysed and compared against the performance of a conventional façade realised with best-available technologies.

The results of the investigation demonstrated the advantages of a dynamic WWR configuration over a static one. However, the improvements achieved in energy demand were lower than expected. This behaviour is strictly related to the configuration of the building used as a reference, which already showed a very high energy performance.

Limitations presented by the research method are also briefly pointed out and discussed.

Abstract:

Background

Phase change materials (PCMs) have been proposed as a means to increase the thermal inertia of glazing systems. These materials have optical features that need to be investigated and characterised in order to better understand the potential of these systems and to provide reliable data for numerical simulations.

Methods

The spectral and angular behaviour of different PCM glazing samples, characterised by different thicknesses of PCMs, were investigated by means of commercial spectrophotometer and by means of a dedicated optical test bed that includes a large integrating sphere with a diameter of 0.75 m. Such equipment was necessary because of the highly diffusive behaviour of the PCM layer when in the solid state of aggregation.

Results

The paper provides a data set of luminous and solar properties of glazing units with PCMs in gaps; the data set uses results from an advanced optical facility that overcomes the intrinsic limitations of commercial spectrophotometers in measuring the optical properties of the advanced transparent materials. In detail, transmittance, reflectance and absorptance spectra of double glazing units characterised by different PCM layer thicknesses in the gap, measured at different incident beam angles, are reported. Integrated values calculated according to relevant international standards are thus provided. Optical features of PCM glazing systems are also highlighted and issues related to the integration of these systems in buildings are discussed.