Improving the Passive Energy Performance of the Buildings’ Envelope in the Southern European Area: A Study on the Integration of a Tensile Material

Nowadays, the building sector represents about 40% of the global energy consumption and about one-third of global greenhouse gases (GHG) emissions [1]. The European Union (EU) has set itself targets to progressively reduce its greenhouse gas emissions by 2050 [2]. Moreover, the vast majority of the existing non-residential EU building stock has been built before 1990 and almost the 55% of this stock has yet to be renovated [3, 4]; the problem is especially emphasized in the southern European area, where the combination of a hot summer climate, future increasing trends, and the probability of heatwaves strongly affects the indoor/outdoor comfort and the overall energy consumption [5]. This geographical scenario is characterized by old building stock with poor energy efficiency due to deterioration of the building components during the time or backwardness in the constructions’ standards over the decades (i.e. thermal transmittance of the envelope, the efficiency of the energy systems, etc.) [3, 4, 6]. Therefore, the reduction of the energy consumption in such buildings has been one of the main topics in the policies issued by the EU for the building sector [7, 8], effectively scheduling for the member states a set of goals for the renovation of these assets. In this context, these EU Directives have been developed to introduce several requirements for new and existent buildings. Accordingly, researchers and designers should optimize all possible aspects (heating and cooling system, regulation criteria, building envelope, shadowing components, etc.). Different products and systems have been studies to investigate the improvement of the buildings' energy efficiency in southern European countries. Several parameters are involved in the buildings’ energy consumption, such as external conditions, the characteristics of the envelope and the behavior of the occupants. The analysis of a building’s energy performance requires substantial input data describing detailed constructions, environmental conditions, thermo-physical properties, building geometry, and control strategies [9].

A recent research [10] suggests that the annual energy consumption of an office in Greece could be reduced by 33% as a result of changing envelope components features, like cooling set-point, natural ventilation strategies, glazing g-value and window-to-wall ratio. In France [11], a single-family house (built before 1974) was simulated through retrofit parameters, namely: ventilation, window glazing type, wall insulation, loft insulation, ground slab and infiltration, which lead to significant energy saving as well as cost reduction. An innovative design was considered in Spain [12] for retrofitting approach that makes the façade as an active element to dehumidify the ventilation air before entering the indoor environment and leads to lower energy consumption in both Mediterranean and Subtropical climates. In [13], the authors have presented retrofitting facades by applying solar passive technologies in Portugal that lead to improve the thermal comfort, more energy saving and lower greenhouse gas emissions. Finally, a comprehensive approach to optimizing the energy design of building envelopes in Italy [14] indicated the energy performance increase, economic benefits and thermal comfort simultaneously.

In general, the improvement of the energy performances of old existing buildings could be carried out by means of active and passive refurbishment actions [15]. Active actions include the installation of renewable energy generation technologies (i.e. PV systems), the replacement or redesign of the heating/cooling systems or the installation of different HVAC technologies. Passive actions, conversely, aim either to better managing the thermal gains and the energy losses of the building, or to increase the use of natural heating, cooling and lighting. Intuitively, the building envelope plays a key role in the effectiveness of passive actions. Among the different passive refurbishment actions that are possible to implement on existing buildings, the ventilated façade systems, and, in general, the Second-Skin systems (SS systems), seems to offer a good compromise in terms of ease of installation, performance and cost effectiveness [16]. The SS systems consists of an additional second skin layer hanged on the surface of the external building wall, with an air cavity in-between. Consequently, thanks to their simple structure, the SS systems are well suited adopt new materials as SS layer. Among these, the most interesting ones seem to be tensile and membrane-like materials, due to their lightweight and flexible nature [17]. Based on their characteristics, they are well-suited for both new and existing buildings, providing for thermal comfort through passive cooling/heating and thus reducing the energy consumption and the GHG emissions.

In recent years, different numerical models have been developed and tested by means of dedicated building simulation software as TRNSYS, DOE-2, BLAST, Energy Plus, and SPARK. However, the buildings’ energy performances are commonly evaluated through steady-state models [9].

In this work, the assessment of the energy and environmental impacts of a passive retrofit action on an existing office building, in terms of reduction of primary energy consumption and carbon dioxide equivalent emissions, were carried out using the dynamic simulation software TRNSYS [18], across a whole year. The refurbishment action involves the installation of a SS system, in which the external layer is made of a light and flexible PVC-coated polyester fabric, on the whole building. The study has been carried out considering five cities in five different countries in the Mediterranean biogeographical region (Greece, Spain, Portugal, France and Italy), and varying the orientation of the building. Moreover, different construction characteristics have been considered for each country: the reference buildings’ envelopes were characterized according to each country building stock context, while the refurbishment models were built according to the legal energy requirements of five different EU countries and based on their national standard for the existing building energy performances.

In this section the methodology used to compare the proposed cases has been compared with the reference cases from energy and environmental points of view.

The energy comparison between the proposed case (PC) and the reference case (RC) has been performed considering the primary energy consumption through the index PES (Primary Energy Saving) [16, 33]:

$P E S=\left[\left(E_{p}^{R C}-E_{p}^{P C}\right) / E_{p}^{R C}\right] \cdot 100$     (1)

where, $E_{p}^{R C}$ is the primary energy associated with the reference cases (Cases 0 and 1 see Table 4), while $E_{p}^{P C}$ is the primary energy associated with each of the proposed cases (Cases 0R and 1R, see Table 4). A positive value of the index PES indicates that the proposed refurbishment actions allow for a primary energy reduction in comparison to the reference case.

The values of the $E_{p}^{R C}$ and $E_{p}^{P C}$ are calculated as reported below:

$\begin{aligned}

&E_{p}^{R C}= \\

&\left(\frac{E_{t h}^{R C}}{C O P}+\frac{E_{c o o l}^{R C}}{E E R}+E_{e l, \text { equipment }}+E_{e l, \text { lighting }}\right) / \eta_{P P}

\end{aligned}$     (2)

$\begin{aligned}

&E_{p}^{P C}= \\

&\left(\frac{E_{t h}^{P C}}{C O P}+\frac{E_{c o o l}^{P C}}{E E R}+E_{e l, \text { equipment }}+E_{\text {el,lighting }}\right) / \eta_{P P}

\end{aligned}$     (3)

where, $\eta_{P P}$ is the power plants' average efficiency.

A different value of $\eta_{P P}$ for each location is assumed according to [33, 34].

The environmental comparison between the proposed case (PC) and the reference case (RC) has been performed considering the reduction of carbon dioxide equivalent emission ($\Delta$CO₂) [16, 33]:

$\Delta C O_{2}=m_{C O_{2, e q}}^{R C}-m_{C O_{2, e q}}^{P C}$     (4)

where, $m_{C O 2, e q}^{R C}$ is the mass of the dioxide equivalent emission associated with the reference cases (Cases 0 and 1 see Table 4, while $m_{C O 2, e q}^{P C}$ is the one associated with each of the five proposed cases (Cases 0R and 1R, see Table 4). A positive value of $\Delta$CO₂ indicates that the proposed refurbishment actions reduce the carbon dioxide equivalent emission with respect to the reference case.

The values of the $m_{C O 2, e q}^{R C} \text { and } m_{C O 2, e q}^{P C}$ are calculated as reported below:

$\begin{aligned}

&m_{C O_{2, e q}}^{R C}= \\

&\alpha \cdot\left(\frac{E_{t h}^{R C}}{C O P}+\frac{E_{c o o l}^{R C}}{E E R}+E_{e l, \text { equipment }}+E_{e l, l i g h t i n g}\right)

\end{aligned}$     (5)

$\begin{aligned}

&m_{C O_{2, e q}}^{P C}= \\

&\alpha \cdot\left(\frac{E_{t h}^{P C}}{C O P}+\frac{E_{c o o l}^{P C}}{E E R}+E_{e l, \text { equipment }}+E_{\text {el,lighting }}\right)

\end{aligned}$     (6)

where a is the CO₂ equivalent emission factor for electricity production. Five different values of a are assumed according to [35]. Table 5 reports the values of $\eta_{P P}$ and $\alpha$ for each location [33-35].

Table 5. Values of $\eta_{P P}$ and a considered for each location

This table highlights that the values of $\eta_{P P}$ range from a minimum equal to 0.376 (Marseille) to a maximum value of 0.601 (Marseille), while the values of $\alpha$ vary between 0.03895 kg_CO2,eq/kWh_el and specific 0.54901 kg_CO2,eq/kWh_el for Marseille and Athens, respectively.

In this paper, the impacts of a passive retrofit action, involving the installation, on an office building, of a second-skin system with the external layer made of a PVC-coated polyester fabric, were evaluated in terms of primary energy saving and carbon dioxide equivalent emissions upon varying both weather conditions (Athens, Barcelona, Lisbon, Marseille and Naples were considered) and orientation of the two main façades of the building (north-south and east-west orientation were considered). The simulation results highlight that the best results in terms of PES and were achieved when the building is east-west oriented. In particular, the simulation returned the maximum value of PES (equal to 22.4%) for the Case 1R_NA, while the best results in terms of $\Delta$ CO₂ (equal to 32.0 Mg_CO2,eq) were obtained when the building is located in Athens (Case 1R_AT). In addition, the adoption of the proposed passive lightweight and non-impacting retrofit solution allowed the reduction of both cooling and thermal yearly energy demand up to 57.7% (Case 1R_MA) and 17.8% (Case 1R_BA), respectively.