Abstract

This paper deals with the experimental assessment of the energy performance of two Advanced Integrated Façade modules (AIF) characterized by two very similar configurations. The two AIF modules were installed on the south-exposed façade of an outdoor test cell facility (a real-scale mockup of an office building) and continuous measurements were carried out for more than one year. Data collected during the experimental campaign were analyzed to evaluate the energy performance and thermo-physical behaviour of the AIF modules. The performances of the two systems were assessed by comparison and by means of conventional and advanced synthetic metrics.

The results of the activity point out the different performances of the two configurations, which only differs on the inner-side glazing (a stratified single clear glass pane vs a stratified low-e double glazed unit). It was demonstrated that just a single additional glass layer can contribute to substantially improve the energy performance of a quite complex façade technology. On average, the façade configuration with the stratified low-e double glazed unit shows the abatement of heat loss and of solar gain of about 30% during the whole year. Moreover, the reliability of some conventional and less conventional metrics in assessing the performance of dynamic façade technologies was also investigated. The results confirm that conventional metrics are not fully reliable when they are used to assess advanced building envelope components with high level of dynamic.

Façades play an important role in architecture, with deep implications in both the quality of the indoor environment and the appearance of the building. R&D in the field of energy conservation is moving toward advanced integrated façades (AIFs): these are innovative and dynamic façades deeply connected with the building equipment. Their dynamic features allow the energy performance of the façade to be optimized, adapting its behavior to different boundary conditions. The substantial lack of synthetic performance parameters to assess and to characterize the energy performance of AIFs is one of the main limitations to the widespread of these technologies. This inconvenience is due to the fact that conventional synthetic metrics (such as U-value and g-value) cannot be fully applied with these technologies. The research activity presented in the paper is an attempt to investigate new synthetic metrics able to characterize the thermal behavior of an AIF. A multiple linear regression (MLR) approach is adopted to identify synthetic parameters able to replicate the energy performance of the façade as a function of the main boundary conditions, e.g., solar irradiance and thermal gradients.

The building enclosure plays a relevant role in the management of the energy flows in buildings and in the exploitation of solar energy at a building scale. An optimized configuration of the façade can contribute to reduce the total energy demand of the building.

Traditionally, the search for the optimal façade configuration is obtained by analyzing the heating demand and/or the cooling demand only, while the implication of the façade configuration on artificial lighting energy demand is often not addressed.

A comprehensive approach (i.e. including heating, cooling and artificial lighting energy demand) is instead necessary to reduce the total energy need of the building and the optimization of the façade configuration becomes no longer straightforward, because non-linear relationships are often disclosed.

The paper presents a methodology and the results of the search for the optimal transparent percentage in a façade module for low energy office buildings. The investigation is carried out in a temperate oceanic climate, on the four main orientations, on three versions of the office building and with different HVAC system’s efficiency. The results show that, regardless of the orientations and of the façade area of the building, the optimal configuration is achieved when the transparent percentage is between 35% and 45% of the total façade module area. The highest difference between the optimal configuration and the worst one occurs in the north-exposed façade, while the south-exposed façade is the one that shows the smallest difference between the optimal and the worst configuration.

The adoption of Phase Change Materials (PCMs) in building components is an up-to-date topic and a relevant number of research activities on this issue are currently on the way. A particular application of PCMs in the building envelope focuses on the integration of such a kind of material into transparent envelope components. A numerical model that describes the thermo-physical behaviour of a PCM layer in combination with other transparent materials (i.e. glass panes) has been developed to perform numerical analyses on various PCM glazing systems configurations. The paper illustrates the structure of the model, the main equations implemented and the hypotheses adopted for the model development. The comparison between numerical simulations and experimental data of a simple PCM glazing configuration is also presented to show the potentials and the limitations of the numerical model. While a good agreement between simulations and experimental data can be shown for the surface temperature of the glazing, the comparison between simulated and measured transmitted irradiances and heat fluxes does not always reach the desired accuracy. However, the numerical tool seems to predict well the thermo-physical behaviour of the system and may therefore represent a good starting point for further simulations on PCM glazing system configurations.

The building enclosure plays a relevant role in the management of the energy flows in buildings and in the exploitation of the solar energy at building scale. An optimized configuration of the façade can contribute to reduce the total energy demand of the building. Traditionally, the search for the optimal façade configuration is obtained by analyzing the heating demand and/or the cooling demand only, while the implication of the façade configuration on the energy demand for artificial lighting is often not considered, especially during the first stage of the design process. A global approach (i.e. including heating, cooling and artificial lighting energy demand) is instead necessary to reduce the total energy need of the building. When considering the total energy use in building, the optimization of a façade configuration becomes not straightforward, because non-linear relationships often occur. The paper presents a methodology and the results of the search of the optimal transparent percentage of a façade module for office buildings. The investigation is carried out for the four main orientations, on three "average" office buildings (with different surface-area-to-volume ratio), and with different HVAC system's efficiency, located in Frankfurt. The results show that the optimal configuration, regardless of the orientations and the surface-area-to-volume ratio, is achieved in an "average" office building when the transparent component of the façade module is between 35% and 45% of the total façade module surface. The north-exposed façade is the one that presents the highest difference between the "optimal configuration" and the worst one, while the south-exposed façade is the one which suffers less in case of the "worst" configuration.

In recent years, Thermal Energy Storage (TES) is becoming more and more important in different engineering applications. As far as the building sector is concerned, TES is considered a crucial feature to reach the net-Zero Energy Building (nZEB) goal. Commonly, TES in building is obtained using the sensible heat property of conventional building materials (building thermal inertia). The drawbacks of this strategy are: the low amount of thermal energy that can be stored; the overheating of the indoor environment that may occur if elevate amount of heat is collected by a conventional building material. On the contrary, the exploitation of the latent heat of dedicate materials (the so-called Phase Change Materials - PCMs) for TES purpose (Latent Heat Thermal Energy Storage - LHTES) presents different advantages: it allows a much higher energy storage density; it allows "temperature selective" thermal energy absorption and release (by choosing the melting temperature range of the PCM). Moreover, since the storage is done at almost isothermal conditions, it is easier to match the energy demand of the building, with the requirements posed by indoor thermal comfort conditions. An exhaustive collection of both laboratory and commercial grade PCMs of different nature and for different purposes can be found in [1]. The exploitation of LHTES is now gaining popularity [2]: different applications of PCMs in construction are currently under investigation [3-5] and, sometimes, commercially available. The most common applications concern the incorporation of PCMs into opaque elements for indoor partitions, structural components and insulation layers. The adoption of PCMs may also occur in combination with active systems (e.g. air-based heating systems, floor heating). Furthermore, because of the ability of certain PCMs to transmit (part of) the visible spectrum of the solar radiation, some transparent/semi-transparent building envelope elements filled with PCMs have been also investigated [6-9]. Some systems based on PCMs integration into transparent components have also appeared on the market and have been adopted in some buildings too. The chemical stability and thermal reliability of PCMs and their live spam are key features to ensure the economic feasibility of exploiting LHTES. The requirements on the stability of the PCMs' thermal properties may depend on the different application of PCMs. However, as a general rule, it is mandatory that no relevant changes occur in the melting temperature and in the latent heat of fusion due to the thermal cycles the PCM undergoes. In common applications, such as in gypsum wallboard, thermal cycles are mostly caused by energy absorption and release by conduction and convection. If radiative transfers occur, only the long-wavelength infrared region is involved. On the contrary, in the case of PCMs contained into transparent elements and directly exposed to the solar radiation, the entire electromagnetic solar spectrum passes through and heats the materials. Thus, it may occur that the high energy radiation contained in the UV-VIS range negatively affects the thermal stability of PCMs. Thermal reliability of PCMs has been investigated since time. Accelerated thermal cycle test is usually adopted to simulate the ageing of the material. This technique consists in multi melt/freeze cycles (up to some thousands of cycles) conducted in laboratory by means of dedicated devices, under controlled and fixed conditions. The evolution of the thermal properties of the materials (i.e. the melting temperature and the latent heat of fusion) is then evaluated by means of Differential Scanning Calorimetry (DSC) on different samples of the material, which correspond to different ageing times. Following this procedure, several studies have been conducted to assess the thermal reliability of different PCMs. The literature survey reveals the lack of dedicated investigations that concern the chemical stability and thermal reliability of PCMs which are supposed to be directly exposed to the solar radiation. Although this application is not one of the main popular and developed, the use of PCMs in transparent components is under evaluation since time [6-9] and some solutions are already commercially available [10]. As mentioned above, this type of application may cause chemical instability and degradation. In fact, PCMs can be damaged by the combined action of the UV-VIS electromagnetic radiation and oxidative processes. The aim of this research is to characterize and analyze the evolution of the thermal properties, and to evaluate the thermal reliability, of a paraffin wax exposed to the solar radiation. The analysis is conducted on four samples of the same PCM collected at regular interval during an on-field experimental campaign on a PCM glazing [9], that was exposed to operative conditions in a test cell equipment for more than one year.