Energy and exergy analysis of an experimental ventilated façade

Energy and exergy analysis of an experimental ventilated façade Ana Picallo-Perez ⇑ , José María Sala-Lizarraga University of the Basque Country, ENEDI Research Group, Energy Engineering Department, Pl. Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain article info Article history: Received 19 October 2022 Revised 8 December 2022 Accepted 19 December 2022 Available online 21 December 2022 Keywords: Ventilated façade Experimental test Exergy analysis Performance indexes abstract This work, analyzes ventilated façades through the first and second law of thermodynamics. In addition to the energy-balances, it presents the exergy-balances on the interior and exterior surface of a façade, taking into account the different mechanisms of heat exchange. It proposes two new indexes (EQC and ExQC) to characterize the behavior of ventilated façades, by comparing their behavior with a reference façade and considering the energy balance in one case and the exergy balance in the other. An experimental test of a forced ventilated façade serves as the case study, using the test-methodology based on Paslink cells. The test data serve to characterize the behavior of the façade, both from an energy and exergy point of view. Overall, 53.05 kWh of heat is lost to the outside through the façade during 6 days of November, which corresponds to 0.31 kWh of exergy-loss. The internal energy change of the façade is decomposed according to its layers, showing that in terms of energy the sandwich insulation layer influences the most (99.45 % of the total change) but in terms of exergy, on the contrary, the metal sheet affects the most (83.66 %). The values obtained for the two indexes show that, under the test conditions, although the ventilated façade and the reference façade present similar values from the energy point of view, when the exergy is used, it is clearly seen that the behavior of the ventilated façade is 44 % better. Ó2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/). 1. Introduction The population does not value comfort homogeneously when responding about thermal and air quality aspects. Therefore, balance parameters and strategies are established to supply a common comfort level-range within architectural design. As a rule, comfort is guaranteed when the body temperature is within a certain range, the skin has a low humidity and the physiological effort of regulation is minimum. The active and passive systems of buildings are responsible for maintaining thermal comfort by consuming natural resources, so their role is essential to sustaining society itself. Consuming natural resources involves using renewable or fossil fuels, although these last emit greenhouse gases and contribute to climate change. Currently, buildings account for about 40 % of the total energy consumption and 36 % of the CO 2 emissions in the European Union [1]. To reduce these consumptions, passive solutions can be used by involving the building’s architecture. Ventilated Façades (VF) [2] are constructive solutions, which can be incorporated in refurbishment or new buildings, and improve the energy efficiency of the buildings to help ensure indoor comfort. They are based on a double skin with an air chamber which slows down the rate of heat transfer. On the one hand, heat is stored in a thermal mass-wall and is conducted, radiated and transmitted to the interior space that is being acclimatized. On the other hand, VF extracts the thermal energy of the heated air directly to the outside, or introduces it into the interior through the openings at the ends of the façade. This air moves between the dampers by natural convection or can be forced and controlled by fans. More and more prefabricated slabs and façade modules are becoming available due to advances in production, which offer the opportunity to implement VF modules more easily, as is the case of Ref. [3], which studies the renovation of a high school building with prefabricated VF elements. The work in Ref. [4] studies and discusses the most recent and cutting-edge research into double-skin façades for building retrofit. VFs provide a thermal buffer zone, energy savings and other benefits. Ref. [5] theoretically compares the energy performance of an Opaque Ventilated Façade against a conventional one through a CFD analysis, considering two specific days, different orientations and two wind-velocities; the work concludes that the VF saves between 20 and 55 % of energy compared to the conventional one. Ref. [6] also uses CFD simulations and optimizes a novel opaque dynamic façade, with an integrated ventilation module, phase change materials and an adjustable insulation system. As an outcome, the airflow can increase or decrease the thermal resistance of the façade to control the heat loss along the year. Maciel and https://doi.org/10.1016/j.enbuild.2022.112737 0378-7788/Ó2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). ⇑ Corresponding author. E-mail address: [email protected] (A. Picallo-Perez). Energy & Buildings 280 (2023) 112737 Contents lists available at ScienceDirect Energy & Buildings journal homepage: www.elsevier.com/locate/enb Carvalho [7] develop a methodology to compare opaque ventilated façades to cladding façades based on several simulations and statistical processing data. Ref. [8] also studies opaque ventilated façades, through a sensitivity analysis, to evaluate the performance and influence of the outdoor boundary conditions. The state-ofthe-art overviewed in the work highlights that, after 2011, the experimental energy-analysis of naturally ventilated façades is the most widespread approach in the literature. Ref. [9] studies different wall layer properties and their effect on the thermal performance of ventilated façades by measuring heat flow and ventilation. As a result, the presence of an internal mass increases the heat transfer towards the indoor environment. The study of Ref. [10] evaluates the design, construction and thermal performance of an experimental VF. As it aims to keep the principles of the circular economy, all the materials are bio-based. As a result, this façade is a promising option for warm climates. An opaque ventilated façade made up of recycled materials is tested during a winter period in Ref. [11]. This work does a hygro-thermal analysis to account for heat losses as well as to control the moisture. As mentioned above, throughout the published literature review, a large number of works analyze experimentally the behavior of VFs; the most recent works design VFs with innovative materials and special characteristics, and many of the works perform the analysis dynamically. However, all the literature focuses on energy studies, based on the first law of thermodynamics, but no work has been found that applies the second law introducing the exergy property in ventilated façades solutions. Indeed, the exergy method is a well-established thermodynamic method that helps to improve the efficiency of processes. However, its application in buildings is not as popular as energy analyses. This exergy analysis can be applied either at facility level, building level or even for urban planning. One of the most detailed publications applying exergy in buildings appears in Ref. [12], which details the application of exergy, both in the building envelope and in its facilities. As with energy analysis, exergy analysis aims to search for techniques that reduce fossil fuel consumption, increase the use of renewable energy and use energy more efficiently. In addition, it provides additional information to conventional energy analysis, thus uncovering energy and environmental problems and therefore bringing about better design solutions. Although VF exergy analyses rarely appear in the literature, there are a few papers that refer to the second law of thermodynamics applied to buildings envelope. Ref. [13] simulates a triangular-roof enclosure to obtain the numerical solution of the entropy production due to natural convection. Ref. [14] also deals with mathematical approaches and remarks the importance of second law analyses to evaluate the quality of energy sources in order to get better strategies for building designs. Therefore, the work analyzes thermal exergy flows through the building and defines the human body exergy balance. Human body exergy balances are also deeply developed in Ref. [15]. Low-exergy buildings are defined and simulated in Ref. [16] and the detailed analysis of a building envelope appears in Ref. [17]; this work simulates four different building envelops and compares the exergy analysis results. Ref. [18] analyzes the envelope and the heating system of a nZEB in Spain under the exergy point of view; the results show the possibilities for energy efficiency improvement that cannot be appreciated with pure energy analysis. The entire heating process in buildings is investigated in Ref. [19], from the exergy of the building envelope, room air, heat energy emission system, distribution, storage, generation system, until the primary energy transformation, dealing with the simulation results. Something similar is done in Ref. [20], which compares the energy and exergy performances of an old and a retrofitted building in Spain, from the envelope until the primary energy, or in Ref. [21] which studies a low exergy heating system from the ground-source heat pump to the building envelope. Ref. [22] defines the mathematical model for applying dynamic exergy and exergoeconomic analyses to the building envelope, to calculate the heating and cooling demands. Unsteady-state exergy analysis, based on a finite difference software, are also applied in Ref. [23] on externally and internally insulated envelopes to have new insights for the buildings design. The review of heat transfer and energy flow characteristics of the building envelopes in Ref. [24] concludes that exergy analysis becomes more significant and helpful with active envelops, i.e., in buildings with integrated PV (BIPV) or thermal (BIPV-T) solutions. In a similar way, Ref. [25] reviews the present day application of BIPV and BIPV-T technologies under the energy and exergy perspectives. The performance of semi-transparent hybrid photovoltaic thermal double pass façades (HPVT-DPF) are energetically and exergetically analyzed in Ref. [26] by results obtained from simulations. In addition, Ref. [27] evaluates the building integrated semitransparent photovoltaic (BISPV) modules for roof and façade, to determine the energy and exergy performance of the building. A building with BIPV-T is also simulated in Ref. [28] where the dynamic exergy analyses of all the system components is done and is compared to a reference building model, i.e., a building without BIPV-T. The work points out that usually exergy studies are focused only on the BIPV-T collector itself, disregarding the whole building-plant. 2. Objective and methodology As justified in the previous introduction section, some works (1) analyze the VFs from an energy point of view and (2) several studies relate the envelope thermal losses to the exergy analysis; however, we have not found works linking these two fields together. The following points summarize the reviewed literature in order to get a generic picture of the state of the art: Referring to (1) VFs analysis: o As described in Refs.[4–11], the recent works experimentally characterize ventilated façades only from the energy point of view; all these papers are published during the last two years, 2020–2022. Referring to (2) exergy analyses in buildings envelope: o These works are scarcer so the literature review has been extended from 2009 to 2020, and no work has been found after that 2020 year. o As marked in Refs.[22,23], the majority of studies in the literature follow a steady-state approach when conducting an exergy analysis by fixing a constant reference temperature. Nevertheless, there is a necessity of making a dynamic analysis because of the environmental conditions change. o Most of the works [13,17,19,21–23,26,28] are associated with simulation results, since it is often not possible to validate the energy and exergy performance with experimental data. o The works of Refs.[13–16,18–21,24–28] develop the mathematical formulae to define the exergy losses through façades, focusing exclusively on the losses from the interior to the exterior, without considering the type of façade under analysis nor the composition of its specific layers. Therefore, this work aims to fill the research gap of VFs analysis with the following objectives and novelties: To define the mathematical formulae for applying dynamic energy and exergy analyses in VFs. To propose new indexes, EQC and ExQC, to characterize the energy and exergy behaviour of a VF versus a conventional one. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 2 To carry out an experimental dynamic essay of a forced VF in a Paslink test-cell, to obtain the corresponding dynamic data at each layer of the façade. To apply the proposed balances and indexes to the forced VF tested. Therefore, this work explains how to apply the exergy methodology to characterize the thermal behavior of a VF, after doing the corresponding energy analysis. The structure of the work appears in Fig. 1. After the introduction of Section 1 and the work justification in Section 2, Section 3 develops the energy and exergy balances in the internal and external surfaces of a conventional façade, taking into account all the different heat transfer mechanisms. Based on these expressions, the work formulates the energy and exergy balances for a VF in Section 4 and defines some indexes that characterize its behavior versus a conventional façade. Section 5 describes the characteristics of the test performed on a VF in a PASLINK test-cell, together with their corresponding data obtained. Section 6 presents the numerical results of the energy and exergy behavior and shows the values of the performance coefficients and finally, Section 7 highlights the conclusions of the work and Section 8 contains the discussion. 3. Energy and exergy balances in the surfaces of a conventional façade The following section contains the mathematical approach to characterize a façade through its energy and exergy balances. According to the adopted sign convention, the heat flux that reaches the surface is considered positive and the flux that comes out is negative. 3.1. Energy balance in the internal surface If an energy balance is applied to the internal surface of the façade in winter conditions, when the outdoor temperature T 0 is lower than the indoor temperature T i (T 0 <T i ), heat goes from the inside through the façade, see Fig. 2 (a): _ Q r;lw;is þ _ Q r;sw;is þ _ Q cv;is ¼ _ Q cd;is ð1Þ where:  _ Q r;lw;is : corresponds to the exchanged longwave radiation (by absorption - emission). This term includes the exchanges with the other interior surfaces at different temperatures, as well as the radiant exchange with the internal heat sources, without considering lighting.  _ Q r;sw;is : represents the redistributed and absorbed shortwave radiation; it considers contributions from the sun (through the openings) and from internal sources, such as lighting.  _ Q c v ;is : represents the heat exchanged by convection with the indoor air.  _ Q cd;is : refers to the heat transfer by conduction through the façade. The analysis of the radiant exchange of interior surfaces is complex due to the different surfaces and natures of this radiation. Therefore, the _ Q c v ;is convection and ( _ Q r;lw;is þ _ Q r;sw;is ) radiation fluxes are considered to be parallel, so a combined convectionradiation coefficient h c v r;is is used to simplify the calculation and to consider the heat flux directly through Newton’s cooling equation. h cvr;is ¼h cv þh r ð2Þ _ Q is ¼Ah cvr;is ðT i T si Þð3Þ where A is the heat transfer area and h r is an equivalent radiation coefficient, being h r ¼4 r e is T 3 m ;T m ¼ðT si þT sj  Þ=2 and T sj  is the average of the other internal surface temperatures that exchange radiation with the surface considered, r is the Boltzmann constant, and e is is the emissivity of the internal surface considered. In Spain, the Technical Building Code (CTE) [29] limits the maximum thermal transmittance values K lim W=m 2 K  that buildings can have through standardized convection-radiation coefficients h c v r;is , whose values take into account whether the flow is horizontal or vertical (ascending or descending), see Table 1. In order to calculate K lim , the characteristics of each element that make up the thermal envelope need to be included, taking into account the compactness and its heat exchange surface with the exterior. Therefore, the calculation is on the basis of each element, which in turn must comply with a limiting thermal transmittance (U lim W=m 2 K  )[30]. Knowing the h c v r;is coefficient and the internal surface temperature T is , the energy balance results as: Ah cvr;is ðT i T is Þ¼ _ Q cd;is ð4Þ 3.2. Exergy balance in the internal surface Referring now to the exergy balance in the interior surface: _ B r;lw;is þ _ B r;sw;is þ _ Q cv;is 1T 0 T i  ¼ _ Q cd;is 1T 0 T is  þ _ D is ð5Þ Fig. 1. Summary of the work structure. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 3 where _ B r;lw;is and _ B r;sw;is designate the exergy of longwave and shortwave radiation exchanged by the internal surface respectively, T 0 is the dead state temperature, and _ D is is the rate of exergy destruction at the internal surface. This destruction is due to (1) absorption of longwave radiation from the rest of the interior surfaces, (2) emission from the surface itself, (3) redistributed shortwave absorption from the sun, (4) from lights and (5) exergy destruction associated with convection in the boundary layer between the air and the surface. Annex 49 [31] recommends using the properties of the ‘‘air around the building” to define the dead state temperature, because when the indoor air (in imbalance with the outdoor air) passes through the façade, it destroys all its exergy until it reaches equilibrium with the outdoor environment. Therefore, as said, the surrounding ambient-air can be considered as the source or sink for the energy processes of buildings and their facilities, always keeping in mind the dynamic behavior of the building and of the outdoor temperature. Although the radiation and convection exchanged have different thermodynamic qualities, in a first approximation, the convection-radiation coefficient h c v r;is is considered for a combined _ Q is term. Then, as the exergy flux that arrives on one side of the surface is greater than the exergy flux that leaves on the other side, the difference is the rate of exergy destruction that takes place on the surface: Ah cvr;is T i T is ðÞ1T 0 T is  ¼ _ Q is 1T 0 T is  ¼ _ Q cd;is 1T 0 T is  þ _ D is ð6Þ In summer, when T 0 >T i , the same exergy balance equation applies, but now the exergy associated with the energy exchanged has the opposite sense and this can be seen algebraically in eq. (6), as the coefficient 1  T 0 T is  has a negative sign. 3.3. Energy balance in the external surface If the formulae focus on the external surface of the façade, under winter conditions, there will be a net heat flow by conduction from inside the façade to the external surface, see Fig. 2 (b), and as the heat flux is the same on one side of the surface as on the other: _ Q cd;es ¼ _ Q r;lw;sky þ _ Q r;lw;sur þ _ Q cv;es  _ Q r;sun;ab ð7Þ where:  _ Q cd;es corresponds to the rate of heat by conduction from the interior of the wall to the external surface.  _ Q c v ;es is the rate of heat exchanged by convection with the outside air.  _ Q r;sun;ab is the rate of shortwave radiation absorbed from the sun (inward (-)).  _ Q r;lw;sky is the rate of longwave radiation exchanged with the sky.  _ Q r;lw;sur is the rate of longwave radiation exchanged with the surroundings, such as the ground, other buildings, etc. In the same way as in the interior surface, a h c v r;es coefficient can be defined to combine _ Q c v ;es convection and ( _ Q r;lw;sky þ _ Q r;sur ) longwave radiation, and introducing the equivalent temperature T eq [12]: h cvr;es ¼h cv þ4 re es T 3 m ð8Þ T eq ¼T 0 þ e es r T 4 0 F es;sky e sky þF es;sur 1  h cvr;es ð9Þ _ Q r;lw;sky þ _ Q r;lw;sur þ _ Q cv;es ¼h cvr;es T eq T es  ð10Þ Fig. 2. (a) Energy balance in the internal surface and (b) in the external surface of the façade. Table 1 Maximum thermal transmittance values according to CTE. K lim W=m 2 K  Compactness Winter climate zone V/A [m 3 /m 2 ] a ABCDE New buildings and expansions V/A 1 0.67 0.6 0.58 0.53 0.48 0.43 V/A 4 0.86 0.8 0.77 0.72 0.67 0.62 Changes in use V/A 1 1 0.87 0.83 0.73 0.63 0.54 Renovations of more than 25 % of the total envelope area. V/A 4 1.07 0.94 0.9 0.81 0.7 0.62 A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 4 where:  e es is the emissivity of the external surface,  r is the Stefan-Boltzmann coefficient, T m is the arithmetic mean of T es and T 0 to linearize the T 4 es T 4 0 expression,  e sky is the emissivity of the celestial vault calculated as e sky ¼ e 0 þ0:81 e 0 ðÞC cloud ,C cloud being the cloudiness factor and e 0 the equivalent emissivity corresponding to a clear sky [32]. F es;sky and F es;sur are the vision factors surface/sky and surface/ surroundings respectively. Then eq. (7) becomes: _ Q cd;es ¼h cvr;es T eq T es   _ Q r;sun;ab ð11Þ The shortwave radiation from the sun absorbed by the exterior surface _ Q r;sun;ab is: _ Q r;sun;ab ¼ a es G T Að12Þ where a es is the absorptivity for shortwave radiation of the exterior surface, G T is the solar irradiation W m 2  ;and A is the surface. It is composed of direct and diffuse radiation (with different associated exergies) and its value depends on location, orientation, day and time. 3.4. Exergy balance in the external surface In the same way as eq.(7), an exergy balance on the external surface gives: _ Q cd;es 1T 0 T es  ¼ _ B r;lw;sky þ _ B r;lw;sur þ _ Q cv;es 1T 0 T es   _ B r;sun;ab þ _ D es ð13Þ The exergy fluxes referring to the exchange of longwave radiation between the sky _ B r;lw;sky and the exterior surface-surroundings _ B r;lw;sur can be combined through the fictitious temperature T f;sky as follows: T f;sky ¼1F es;sky  T sur þF es;sky T sky ð14Þ and, using the Petela expression to calculate the exergy of radiation: _ B r;lw;sky þ _ B r;lw;sur ¼A e es r T 4 es T 4 f;sky  14 3T 0 T 3 es T 3 f;sky  T 4 es T 4 f;sky  2 43 5ð15Þ Many energy analyses also combine the short and longwave radiation mechanisms with convection into a single term, to apply Newton’s cooling law by the convection-radiation coefficient, and the sun-air temperature [12]. However, although using the h c v r;is coefficient in the exergy balance of the internal surface (eq. (6)), encompassing shortand longwave mechanisms in the external surface is not adequate for the exergy calculation, due to the various thermodynamic qualities of these energy transfers. Nevertheless, longwave radiation exergy flows can be joined with the external convection mechanism through the h c v r;es convectionradiation coefficient in the _ B es term, _ B es ¼ _ B r;lw;sky þ _ B r;lw;sur þ _ Q cv;es 1T 0 T es  ¼Ah cvr;es T eq T es  1T 0 T es  ð16Þ Thus, summarizing the exergy balance stays as follows: _ Q cd;es 1T 0 T es  ¼ _ B es  _ B r;sun;ab þ _ D es ð17Þ where, following Petela _ B r;sun;ab ¼ a es AG T 1þ1 3 T 0 T sun  4 4 3 T 0 T sun () ð18Þ andT sun = 5.700 K is the solar radiation temperature approximated to a black body. As mentioned above, for a more accurate analysis, G T should be decomposed into direct and diffuse irradiance [12]. The net heat-loss through the exterior surface is finally exchanged with the environment by convection and longwave radiation until it reaches the ambient temperature T 0 (the dead state). Thus, the exergy of this heat-loss is completely destroyed in the environment, resulting in an exergy loss: _ Q cd;es 1T 0 T es  þ _ B r;sun;ab ¼ _ L es ð19Þ In winter, the conduction heat flux in the external surface and its exergy have the same sense, both are from the interior to the exterior, while in summer the conduction heat flux in the external surface is inward, whereas its associated exergy flow, on the contrary, is outward. 4. Thermodynamic analysis of a ventilated façade Ventilated façades incorporate insulation together with a ventilated air chamber, with an outer sheet joined to the interior by means of a substructure, see the scheme of Fig. 3. The air chamber is the primary component of the system, performing various functions: it prevents the dynamic forces of the wind from reaching the interior components, it acts as a drainage system against eventual infiltrations, it allows the evacuation of water vapor coming from the perspiration of the enclosure of the building and, likewise, the circulation of air cools the excess of solar radiation incident on the skin of the cladding. 4.1. Energy and exergy balances in a ventilated façade According to the above nomenclature, from an energy balance in a non-stationary state, considering the whole façade as a control volume and that heat goes from the exterior to the interior, the following equation is written: dU VF dt ¼ _ Q r;sun;ab þ _ Q cvr;es  _ Q cvr;is þ _ H in  _ H out ð20Þ dU VF dt ¼A a es G T þAh cvr;es T eq T es  Ah cvr;is T is T i ðÞ þ _ H in  _ H out ð21Þ where U VF is the internal energy accumulated in the materials that make up the façade, which can be calculated knowing the temperature T j , the thermal capacity c P j and the mass m j of each jlayer; and _ H in and _ H out are the rate of air enthalpy at the entrance and exit of the ventilated façade. U VF ¼Xðm j c P j ðT j T 0 ÞÞ ð22Þ dU VF dt  D U VF D t¼Xm j c P j T j t T j tDt D tT 0t T 0t D t D t  ð23Þ _ H in  _ H out ¼_ m air c P T in T out ðÞ ð24Þ A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 5 If an exergy balance is performed then: dB VF dt ¼ _ B r;sun;ab þ _ Q cvr;es 1T 0 T es   _ Q cvr;is 1T 0 T is  þ _ B in  _ B out  _ D T ð25Þ where _ B in  _ B out is the exergy change of the air entering in the chamber ( _ B in  _ B out ¼_ m air c P air T in T out ðÞ T 0 ln T in T out  ); _ D T is the rate of total exergy destruction in the façade, including all the heat exchange mechanisms and the irreversibilities associated with the air flow in the chamber, and B VF is the exergy accumulated in the materials: B VF ¼Xm j uu 0 ðÞjT 0 ðss 0 ÞðÞj  ¼Xm j c P j T j T 0  T 0 ln T j T 0  ð26Þ dB VF dt  D B VF D t¼ Xm j c P j  D T j D t D T 0 D t   D T 0 D tln T j T 0  T 0  D T j D t T 0 T j D T 0 D t T j T 0 ! !"# ð27Þ 4.2. Performance coefficients of a ventilated façade As said in the literature review, no exergy index is found to characterize the thermal behavior of ventilated façades or roofs. The most common exergy indicators focus on industrial processes or energy generation systems. Among them, the following are found: the exergy efficiency used by Boelman & Sakupipatsin [33], and Cornelissen & Hirs [34], or the functional efficiency of Kotas [35] and Tsatsaronis [36]. Therefore, a coefficient to characterize the behavior of building envelopes needs to be defined in terms of exergy, particularly for VFs. Furthermore, this coefficient has to consider the envelope as a dynamic system and must be easy to interpret and not distant from the energy coefficients. Considering the requirements, the Doctoral Thesis of I. Flores [37] defines five possible parameters. After analyzing the results for different walls, climates, etc., the most interesting is the one based on the ISO 9869-1 standard expression [38], for the ‘‘in situ” determination of the thermal resistance of a wall, and is called ‘‘dynamic exergy transmittance”. In this work, a different point of view is adopted. In order to assess the interest of incorporating or not a VF, its thermal behavior is compared with that of a conventional façade that meets the minimum requirements required by the regulations, and therefore depending on the geographical area and climatic conditions. Under the point of view of the first law, this work defines a coefficient that relates the energy lost through the conventional (reference) façade versus that lost in the VF. For winter conditions, which are the most relevant in our zone, the heat flow _ Q is ¼Ah c v r;is T i T is ðÞgoes from the indoor air at T i to the internal surface at T is , being T ref is the internal surface temperature in the reference façade. The heat exchanged by the reference façade internal surface through convection and longwave radiation is _ Q ref is ¼Ah c v r;is T i T ref is  ,Fig. 4 (b). This work proposes naming this ratio the Energy Quality Coefficient (EQC) so, Fig. 3. Scheme of a simple ventilated façade (summer). A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 6 EQC ¼P n j¼1 Q ref is j P n j¼1 Q is j ð28Þ where n¼1jcorresponds to the heat transferred during the n periods of time considered. Applying the energy balance, the above definition can be expressed with reference to the external surface ‘‘es”, by considering in the term Q es j the solar absorption (Q r;sun;ab j ) and the convection-radiation (Q c v r;es j ), then, EQC ¼P n j¼1 Q ref es j þ D U ref j hi P n j¼1 Q es j þH out H in ðÞ j þ D U j hi ð29Þ If EQC is greater than one, the VF is thermally preferable to the reference-façade defined by the application of the Spanish Technical Code (CTE) limiting values. In winter conditions, if the preheated air of the chamber is used in the building, H out H in should appear in the denominator of eq. (28), so that P n j¼1 Q is ðH out H in Þis now the heat-loss. Comparing the exergy lost by such a reference-façade with the exergy lost by the VF, we can similarly define an exergy quality coefficient ExQC, as follows ExQC ¼P n j¼1 Q ref is j 1 T 0j T ref isj ! P n j¼1 Q is j 1 T 0j T isj  ð30Þ Referring this coefficient to the external surface ‘‘es” of the FV, through the exergy balance equation: ExQC ¼P n j¼1 B ref es j þ D B ref j þD ref j hi P n j¼1 B es j þB out B in ðÞ j þ D B VF j þD j hi ð31Þ In summer, when T 0 >T is , the heat flux-gain _ Q is supposes an exergy output from indoor equal to _ Q is 1 T 0 T is  , so the heat flux and its corresponding exergy have opposite senses. The objective now is to reduce the heat flow _ Q is so that the exergy associated with it and that goes out from the indoor air is as low as possible. In these conditions, the hot air in the chamber is sent outdoors. 5. Case study 5.1. Characteristics of the paslink cell-tests PASLINK dynamic cell-tests are highly standardized tests that allow construction solutions to be thermally characterized under real dynamic conditions thanks to their numerous and strategic sensors. The development of PASLINK tests started more than 20 years ago, through international work coordinated and funded by EU research projects [39]. PASLINK cells consist of a test room where the sample to be tested is placed, and an adjacent service room where the heat exchange through the sample is measured with high precision, see Fig. 5. 5.2. Description of the forced ventilated façade tested The construction-solution tested as the case study in the PASLINK cell is a light and industrialized ventilated façade with dry assembly and prefabricated materials, see Fig. 6. It is a simple façade consisting of a 2 mm galvanized steel sheet, a 3 cm air chamber and a sandwich panel with a 6 cm polyurethane core. The air inside the ventilated chamber moves in a forced way by means of three extraction hoppers located in the upper part of the sample. Fig. 7 shows the location of the temperature sensors (Pt 100) in the various layers of the ventilated façade; as it can be seen, the air chamber has a calibrated thermopile with numerous sensors to measure the increase in the air temperature across it. In addition to the sensors in the sample there are: 12 Pt100 sensors for ambient temperature, 3 Kypp and Zonnen CMP11 Solarimeters to measure the total vertical and horizontal solar radiation and 1 shadow ring to measure diffuse radiation, 1 sensor for the outside wind direction and another one for its speed, 1 differential pressure meter, 1 air flow meter, and 1 power transducer. Fig. 4. (a) Energy loss in ventilated façade vs (b) reference façade. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 7 5.3. Data obtained from the test The test was carried out at the Building Quality Control Laboratory of the Basque Government (LCCE) in Vitoria/Gasteiz, northern Spain, over 6 days (from November 8th to 13th) and data were collected every 10 min. Fig. 8 shows the data collected directly from the sensors in each layer. All temperatures show a similar trend, which allows the values to be averaged to a single surface-temperature value (except for the air temperature inside the chamber, which increases its value vertically from T in at the entrance to T out at the exit). The average standard deviations r a v g ;when averaging the temperatures for each surface at a single surface temperature T s;j ;are gathered in Table 2. Fig. 9 shows the outdoor and indoor temperatures, and the average internal and external surface temperatures of the VF, as well as the solar irradiation (vertical global, horizontal global, and horizontal diffuse) and the airflow rate through the air chamber. 6. Results This section shows the results of the analysis performed with the data obtained in the test. The results are presented in three sections: 6.1 Energy Analysis, 6.2 Exergy Analysis and 6.3 Performance Coefficients. 6.1. Energy analysis This section contains the dynamic and global results of energy analysis. 6.1.1. Dynamic energy analysis of the VF Calculating the components of the energy balance equation in the façade, eq. (20), the values obtained are shown in Fig. 10. The following trends are observed: When there is solar irradiation G T , the external surface temperature T es rises markedly, making _ Q c v r;es negative (i.e., loss) as T eq <T es ; On the other hand, the air-flow temperature in the chamber rises from inlet T in to outlet T out , participating in the heat transfer, which makes _ H in < _ H out and therefore preheating the air to be after inserted inside the house. Furthermore, as the coefficient h c v r;is is low, there is hardly any temperature difference between the indoor temperature T i and the internal surface temperature T is . In addition, as T i >T is , _ Q c v r;is flows outwards (it has a negative sign). When the solar radiation absorbed _ Q r;sun;ab rises and the external surface temperature T es increases, the VF loses heat to the outside, so its internal energy decreases dU VF dt <0:Fig. 11 justifies this fact by depicting the solar irradiation vs the change in interFig. 5. Structure of a Paslink cell. Fig. 6. (a) Layers of the ventilated façade, (b) PASLINK cell, (c) materials. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 8 nal energy. It can be seen that the solar irradiation absorbed noticeably stimulates the internal energy increase and decrease of the VF. 6.1.2. Energy analysis of the VF over the test period As a summary, Table 3 shows the accumulated energy during the 6-day test period for each of the energy balance terms. Fig. 7. a) Location of sensors in the different layers of the ventilated façade. b) Zoom in the inside of the air chamber. Fig. 8. Temperatures of each layer according to sensor location. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 9 [22] S. Sayadi, G. Tsatsaronisb, T. Morosuk. (2016, June). A new approach for applying dynamic exergy analysis and exergoeconomics to a building envelope. In ECOS 2016: Proceedings of the 29th International Conference on Efficiency, Cost, Optimization, Simulation and environmental impact of energy systems (pp. 1-17). [23] W. Choi, R. Ooka, M. Shukuya, Unsteady-state exergetic performance comparison of externally and internally insulated building envelopes, Int. J. Heat Mass Transf. 163 (2020). [24] Y. Wang, A. Shukla, S. Liu, A state of art review on methodologies for heat transfer and energy flow characteristics of the active building envelopes, Renew. Sustain. Energy Rev. 78 (2017) 1102–1116. [25] M. Debbarma, K. Sudhakar, P. Baredar, Thermal modeling, exergy analysis, performance of BIPV and BIPVT: a review, Renew. Sustain. Energy Rev. 73 (2017) 1276–1288. [26] D. Kamthania, G.N. Tiwari, Energy and exergy analysis of semi transparent hybrid photovoltaic thermal double pass façade, Int. J. Renew. Energy Tech. 6 (2) (2015) 119–141. [27] A.K. Shukla, K. Sudhakar, P. Baredar, Exergetic analysis of building integrated semitransparent photovoltaic module in clear sky condition at Bhopal India, Case Stud. Therm. Eng. 8 (2016) 142–151. [28] A. Buonomano, F. Calise, A. Palombo, M. Vicidomini, Transient analysis, exergy and thermo-economic modelling of façade integrated photovoltaic/thermal solar collectors, Renew. Energy 137 (2019) 109–126. [29] M. Borrallo-Jiménez, M. LopezDeAsiain, P.M. Esquivias, D. Delgado-Trujillo, Comparative study between the Passive House Standard in warm climates and Nearly Zero Energy Buildings under Spanish Technical Building Code in a dwelling design in Seville, Spain, Energy Build. 254 (2022). [30] https://www.codigotecnico.org/pdf/Documentos/HE/DcmHE.pdf. [31] D. Schmidt, H. Torio. ECBCS Annex 49. Low Exergy Systems for HighPerformance Buildings and Communities, 182. 2011. [32] H.A. Ahmed, T. Yu-xin, Y. Qi-chang, I.M. Al-Helal, M.R. Shady, A.M. AbdelGhany, Estimation of sky thermal irradiance in arid climate under clear sky conditions, Int. J. Thermophys. 41 (6) (2020) 1–18. [33] E.C. Boelman, S. Popping. Critical analysis of exergy efficiency definitions applicable to buildings and building services. Tc, 10, 20. 2004. [34] R.L. Cornelissen, G.G. Hirs, Exergetic optimisation of a heat exchanger, Energ. Conver. Manage. 38 (15–17) (1997) 1567–1576. [35] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, Paragon Publishing, 2012. [36] G. Tsatsaronis, Definitions and nomenclature in exergy analysis and exergoeconomics, Energy 32 (4) (2007) 249–253. [37] I. Flores Abascal. El método de análisis exergético en los edificios. Su aplicación en la caracterización en régimen dinámico de los cerramientos. 2016. [38] D.S. Choi, M.J. Ko, Analysis of convergence characteristics of average method regulated by ISO 9869–1 for evaluating in situ thermal resistance and thermal transmittance of opaque exterior walls, Energies 12 (10) (2019) 1989. [39] C. García-Gáfaro, C. Escudero-Revilla, I. Flores-Abascal, A. Erkoreka-González, K. Martín-Escudero, Dynamical edge effect factor determination for building components thermal characterization under outdoor test conditions in a PASLINK Test Cell: a methodological proposal, Energ. Build. 210 (2020). [40] R. Petela, Exergy of undiluted thermal radiation, Sol. Energy 74 (6) (2003) 469– 488. [41] http://www.anape.es/pdf/Catalogo%20de%20Elementos%20Constructivos% 20CAT-EC-v06.3_marzo_10.pdf. A. Picallo-Perez and José María Sala-Lizarraga Energy & Buildings 280 (2023) 112737 16