Hot Box Investigations of a Ventilated Bioclimatic Wall for NZEB Building Façade

The building sector is the largest consumer of energy and there are still major scientific challenges in this field. The façade, being the interface between the exterior and interior space, plays a key role in the energy efficiency of a building. In this context, this paper focuses on a ventilated bioclimatic wall for NZEB zero energy buildings. The objective of this study is to investigate an experimental set-up based on a Hot Box allowing characterizing the thermal performances of the ventilated wall. A specific ventilated prototype and an original thermal metrology has been developed. This paper presents the ventilated prototype, the experimental set-up and experimental results on the thermal performances of the ventilated wall. The influence of the air space thickness and the air flow rate on the thermal performances of the ventilated wall is studied.


Introduction
The building sector is the industry mainly responsible for energy consumption in Europe [1].Reducing the energy consumption and the emission of gases from buildings has become a recent priority in Europe.In this context, France is placing the construction sector at the center of its strategy to meet this challenge [2].Nowadays, buildings must ensure low energy consumption and comfortable environment quality.However, the building envelope has one of the largest impacts on building energy consumption.These aspects generate innovative façade design [3,4].In particular, a ventilated façade can be also a response to this approach [5].
Ventilated façades are used often by architects.In addition to the aesthetic aspect, the primary purpose of this type of façade is to protect the insulation materials by dissipating moisture.A ventilated façade can also provide energy savings.Nowadays, in Europe, the construction of zero-energy buildings (ZEB) and the transformation of existing buildings into nearly zero-energy buildings (NZEB) are expected.For these reasons, a ventilated façade can be an interesting solution for both existing and new buildings.
A ventilated façade is composed of two panes separated by a ventilated cavity.One possible definition of ventilated façade is "A ventilated double facade can be defined as a traditional single façade doubled inside or outside by a second, essentially glazed facade.Each of these two facades is commonly called a skin.A ventilated cavity-having a width which can range from several centimetres at the narrowest to several metres for the widest accessible cavities-is located between these two skins" [6].
There are several different ventilated façade concepts and several classifications [5,6].Generally, classifications are based on façade geometry, the type of ventilation of the cavity, and the airflow concept [7][8][9].The ventilated façade can be continuous or divided into modules.The air layer forms a thermal buffer zone.The façade geometry can be box window type, shaft box type, a corridor façade, or a multi-story double skin façade.The cavity ventilation that occurs through openings in the external and internal skin can be Energies 2021, 14, 1327 2 of 16 natural, mechanical, or hybrid.Possible airflow concepts are as follows: supply air, exhaust air, static air buffer, external air curtain, and internal air curtain [10].The presence of integrated systems (photovoltaic, solar collector) can also be classification criteria.For example, Ibanez-Puy [11] classifies ventilated façades as double skin façades, building integrated photovoltaic, solar chimneys, Trombe wall, solar wall, façade solar collectors, and opaque ventilated façades.
For several years, publications on ventilated walls increased significantly [12][13][14][15].Despite the high number of publications on ventilated walls, the influence of the air space thickness cavity on the thermal performances of the ventilated wall was rarely studied.Table 1 provides an overview of different studies on the ventilated façade from the point of view of air space thickness.

Years Sample Air Space Thickness
A. Alaidroos [16] 2016 Ventilated wall 0.1 m O. Aleksandrowicz [17] 2018 Double skin façade 0.2 m J. Wang [18] 2017 Triple glazed windows 0.03 m, 0.01 m L. C. O. Souza [19] 2018 Ventilated double-skin façade 0.1 m J. Parra [20] 2015 Ventilated double-skin façade 0.2 m, 2 m G. Michaux [21] 2019 Triple glazed windows 0.01 m F. Gloriant [22] 2021 Triple glazed windows 0.013 m A hot box apparatus is used to measure the thermal behavior of large specimens of building materials.Two standard methods are given in the literature: guarded hot box (GHB) and calibrated hot box (CHB) [23,24].The guarded hot box (GHB) is composed of three cells: a hot cell, a cold cell, and a guarded chamber used to minimize lateral heat losses.A calibrated hot box (CHB) is composed only of two cells: a hot cell and a cold cell [24].Different standard hot box experiments can be carried out: the European EN ISO 8990, the American ASTM C1363-05, and the Russian GOST 26602.1-99[23,24].Many publications deal with the thermal characterization of building materials using a hot box apparatus.However, few papers studied the thermal properties of air cavity wall using a hot box.Aviram [25] studied the thermal behavior of a variable ratio cavity wall using a guarded hot box and CFD analysis.Lucchi [24] introduces a hot box procedure to study the thermal performance and the air flux in a wall with an internal air cavity.More recently, a new hot box was developed to study the hydrothermal behavior of historic walls [26].
This paper describes an experimental study on the thermal performance of a ventilated bioclimatic wall.The objective of this study was to develop an experimental setup able to characterize the influence of the air space thickness and the air flow rate on the thermal performances of the ventilated wall.A specific ventilated prototype and an original thermal metrology based on a hot box was developed in this study.
The overall structure of this paper takes the form of four parts, including this introduction.The second part, Experimental Setup, presents the ventilated bioclimatic prototype, the hot box, and the instrumentation involved in the test specimen.The third part presents the experimental results for ambient temperature, surface temperature, heat transfer coefficient, and pre-heating efficiency, which allows for the evaluation of the thermal performance of the ventilated wall.Finally, a conclusion gives a summary of the findings.

Bioclimatic Ventilated Wall and Hot Box
The ventilated wall prototype is presented in Figure 1.This prototype consists of two walls and a ventilated air chamber (a cavity between the two walls).The first wall is fixed, while the second one is mobile, allowing the thickness of the ventilated cavity to be varied.These two walls are made of an aluminum alloy and polyethylene.The dimensions are Energies 2021, 14, 1327 3 of 16 1 m high, 1 m wide, and 3 mm thick.Based on the manufacturer's data, the aluminum composite has a thermal resistance of 0.008 m 2 •K•W −1 and has low roughness, with the surfaces being smooth.The ventilated air chamber is a non-hermetically sealed cavity that participates in the supply of fresh air to the building.A convective exchange takes place in the cavity along the axis of the wall, which disturbs the radial heat flow between the interior and exterior environments (Figure 1).

Bioclimatic Ventilated Wall and Hot Box
The ventilated wall prototype is presented in Figure 1.This prototype consists of two walls and a ventilated air chamber (a cavity between the two walls).The first wall is fixed, while the second one is mobile, allowing the thickness of the ventilated cavity to be varied.These two walls are made of an aluminum alloy and polyethylene.The dimensions are 1 m high, 1 m wide, and 3 mm thick.Based on the manufacturer's data, the aluminum composite has a thermal resistance of 0.008 m 2 •K•W −1 and has low roughness, with the surfaces being smooth.The ventilated air chamber is a non-hermetically sealed cavity that participates in the supply of fresh air to the building.A convective exchange takes place in the cavity along the axis of the wall, which disturbs the radial heat flow between the interior and exterior environments (Figure 1).The experimental study was carried out in a hot box (Figure 2).This hot box is a device composed of two climatic chambers whose ambiances are controlled.The first climatic chamber reproduced indoor conditions, and the second reproduced outdoor conditions (Figure 2).The hot box is characterized by a range of temperature from −30°C to +60°C and a range of humidity from 10% to 98%.Each climatic chamber has its own refrigeration unit that allows for the production of cold with a temperature accuracy of ± 2 °C.Each climatic chamber also has 3 electric resistors of 2.5 kW that allow for the production of heat with a temperature accuracy of ± 3 °C.The hot box was connected to a computer to control the temperature and humidity of each climatic chamber.These two climatic chambers were separated by a sample holder (size of 1 × 1 × 0.4 m).One of the two chambers was mounted on slides, allowing access to the sample holder as well as to the inside of the two climatic chambers.The ventilated bioclimatic prototype was fixed in the specimen holder.The experimental study was carried out in a hot box (Figure 2).This hot box is a device composed of two climatic chambers whose ambiances are controlled.The first climatic chamber reproduced indoor conditions, and the second reproduced outdoor conditions (Figure 2).The hot box is characterized by a range of temperature from −30 • C to +60 • C and a range of humidity from 10% to 98%.Each climatic chamber has its own refrigeration unit that allows for the production of cold with a temperature accuracy of ±2 • C. Each climatic chamber also has 3 electric resistors of 2.5 kW that allow for the production of heat with a temperature accuracy of ±3 • C. The hot box was connected to a computer to control the temperature and humidity of each climatic chamber.These two climatic chambers were separated by a sample holder (size of 1 × 1 × 0.4 m).One of the two chambers was mounted on slides, allowing access to the sample holder as well as to the inside of the two climatic chambers.The ventilated bioclimatic prototype was fixed in the specimen holder.A ventilation system was used to impose air flow in the cavity.Moving the mobile wall can vary the air space thickness.Experiments were carried out for different configurations to provide variable air space thickness and variable air flow rate.The air space thickness varied from 5 mm to 85 mm.The air flow rate varied from 10 to 30 m 3 •h −1 .Fifty experiments were carried out.An overview of the fifty experiments is given in Table 2.

Thermal Metrology
The instrumentation was composed of K-type thermocouples and PT-100 probes.The hot box ambient temperature was measured in the hot cell and the cold cell with K-type thermocouples and PT-100 sensors (hot box sensors).The hot box instrumentation is presented in Figure 3.The ventilated prototype consisted of two walls and an air cavity.The first wall was fixed while the second one was mobile, allowing the thickness of the ventilated cavity to be varied.The wall surface temperature was measured by 24 K-type thermocouples (12 thermocouples on the fixed wall and 12 thermocouples on the mobile wall).The position of the thermocouples is given in Figures 4-6.Thermocouple measurements were compared to those obtained using an infrared camera (FLIR T650 SC).The infrared camera was installed in the front of the wall.The area covered by the infrared camera had a surface area of 10 cm 2 .The thermal flux was measured with 8 Captec flux meters (4 flux meters on the fixed wall and 4 flux meters on the mobile wall), allowing for the estimation of heat flux on both sides of the ventilated cavity (Figure 4).The air cavity temperature was measured by 10 K-type thermocouples (4 thermocouples in the cavity, 3 thermocouples at the entrance of the cavity, and 3 thermocouples at the exit of the cavity).To control the airflow velocity inside the air cavity, a convergent plenum was installed (Figure 4).A fan was used to vary the airflow velocity inside the cavity.A CTV 210 hot wire anemometer from the manufacturer KIMO measured the airflow rate.All the Energies 2021, 14, 1327 5 of 16 sensors were connected to a Keithley 2700 data acquisition system.Data acquisition and processing were then carried out via the LabVIEW software (Figure 7).The characteristics of the sensors used are given in Table 3.

Type Metrologic Means Range Uncertainty
Air

Experimental Results
Specific experiments were undertaken to test the hot box and the ventilated wall prototype (Figure 8).Experiments were carried out in steady state under the following conditions: -Hot cell: ambient temperature conditions of 20 • C and relative humidity (55%); -Cold cell: temperature conditions of 0 • C and relative humidity (75%); -Variable airflow rate: 10, 20, and 30 m 3 •h −1 ; -Variable air space thickness: from 5 to 85 mm.
These specific experiments were carried out to investigate the ventilated wall prototype in terms of its capability to pre-heat the air in the cavity.Our experiments are distinguished from standard hot box methods used for the estimation of the thermal resistance of a wall in steady-state regime by using a calibrated hot box or guarded hot box such as EN ISO 8990.

Ambient Temperature
The ambient temperature was measured in both the hot cell and cold cell with K-type thermocouples and PT-100 sensors (hot box sensors).The temperature difference between the hot cell and the cold cell was 20 • C.This temperature difference allowed us to obtain a heat flux that could be measured correctly by the flux meters [27].Under the specified conditions (20 • C in the hot cell and 0 • C in the cold cell), a significant difference (2.6 • C for the cold cell and 4.3 • C for the hot cell) was observed between the ambient temperature measured with the hot box sensor and the ambient temperature measured by our thermocouples.This difference can be explained by the fact that the cells are not watertight, since an artificial air flow was created between them.However, for standard use of the hot box, there should not be any air exchange between the cells; the regulation system of the device was thus disturbed.
Figure 9 presents the air temperature in the cold cell and the hot cell.The temperature was measured by six K-type thermocouples (three thermocouples (T42, T43, and T44) in the cold cell and three thermocouples (T39, T40, and T41) in the hot cell).It can be seen in Figure 9 that the air temperature inside the hot box was homogenous in both the cold cell and the hot cell.The mean air temperature was −4.6 ± 0.05 • C for the cold cell and 15.7 ± 0.18 • C for the hot cell.

Surface Temperatures
Measurement of surface temperatures was performed by thermocouples.Thermocouple measurements (T8 and T5 thermocouple measurements) were compared to data obtained using an FLIR 650 TS infrared camera (Figure 10).The area covered by the infrared camera had a surface area of 10 cm 2 .The T8 and T5 thermocouples were located on the upper and lower part of this surface.The mean value measured by thermocouples T5 and T8 and the infrared camera were 12.14 • C, 12.11 • C, and 12.18 • C, respectively.The measurements of the surface temperature performed by the thermocouples are in accordance with the measurements made by the infrared camera and confirm the reliability of the instrumentation of the walls (Figure 11).

The Convection Heat Transfer Coefficient
The convection heat transfer coefficient was estimated from Newton's law, involving the heat flux, the ambient temperature, and the wall temperature: The heat flux, the ambient temperature, and the wall temperature were measured using thermal metrology describes before.We used two cavity thicknesses (0.005 and 0.01 m) and three airflow rates (10, 20, and 30 m 3 •h −1 ).Each sample wall was equipped with four flux meters and four thermocouples at different heights, making it possible to obtain four local estimates of the h-factor per cell.For the ambient temperature in each of the cells, a single air temperature was considered, corresponding to the average of the measurements of the three thermocouples installed in each cell.Figures 12 and 13 show the local convection heat transfer coefficient h in the cold cell and in the hot cell.The values of the heat transfer coefficient varied between 15 and 25 W.m −2 •K −1 .The mean value of the heat transfer coefficient was 19.9 ± 1.3 W.m −2 •K −1 for the cold cell and 18.7 ± 2.3 W.m −2 •K −1 for the hot cell.This order of magnitude is consistent with the correlations that can be found in the literature [28].From Figures 12 and 13, we also noted that the thickness of the cavity and the airflow rate had no significant influence on the convection heat transfer coefficient value.This means that these two parameters had no influence on the ambiances of the two climate cells.On the other hand, the coefficient h depends on the position from which it was calculated, and it can be seen that for a given height, the estimates of the coefficient h are relatively homogeneous.It can be thought that, in each of the cells, the air movements caused by the regulation system were not uniform near the walls of the sample, causing local variations in the coefficient h.As depicted in Figure 12, two values are apart for 10 m 3 /h_5 mm.This can be explained by a bad contact between the flux meters and the sample during this test.Finally, the uncertainties for the local convection heat transfer coefficient h were estimated (Table 4).An uncertainty of 5% for the flux meters and an uncertainty of 0.02 • C for the thermocouples were considered.As can be observed from Table 4, the uncertainties are smaller in the hot cell, as the fluxes and temperature differences involved are larger.It is in the cold cell, at the lower part of the wall, that the uncertainties are the greatest; at this point, the temperatures between the wall and the environment of the cold cell are very close, often of the order of one-tenth of a degree.

Thermal Performance of the Ventilated Wall
The objective of this study was to characterize the influence of the air space thickness and the air flow rate on the thermal performances of the ventilated wall.The thermal performance of the ventilated wall was evaluated through the pre-heating efficiency according to Equation (2) "which represent the potentiality of the ventilatd wall in terms of its capability to pre-heat the air in the cavity" [29].
where -T inlet : the supply air temperature measured in the top of the air cavity; -T out : the outside air temperature (cold cell); -T in : the inside air temperature (hot cell).
Energies 2021, 14, 1327 14 of 16 Figure 16 shows the pre-heating efficiency of the ventilated wall.Experiments were carried out for different air space thicknesses (5, 20, 50, and 85 mm) and different air flow rates (10, 20, and 30 m 3 •h −1 ).It can be seen that the pre-heating efficiency of the ventilated wall increased with the air space thickness.For an air flow of 10 m 3 •h −1 , the pre-heating efficiency of the ventilated wall increased from 57% (for an air space thickness of 5 mm) to 65% (for an air space thickness of 85 mm).It can be also seen that the pre-heating efficiency of the ventilated wall decreased with the air flow rate.For an air space thickness of 85 mm, the pre-heating efficiency of the ventilated wall decreased from 65% (for an air flow rate 10 m 3 •h −1 ) to 54% (for an air flow rate 30 m 3 •h −1 ).

Conclusions
This work describes an experimental study on the thermal performance of a ventilated bioclimatic wall.A ventilated wall prototype and an original hot box thermal metrology were developed.This paper presents the ventilated wall prototype, the experimental setup, and the experimental results.The ambient temperature, the surface temperature of the ventilated prototype, and the local heat transfer coefficients of the ventilated wall were measured.In addition, an uncertainty analysis on the local heat transfer coefficient is presented.Experimental results show that vertical position had a significant influence on the local heat transfer coefficient value.Finally, the influence of the air space thickness and the air flow rate on the thermal performances of the ventilated wall was studied.The results show the pre-heating efficiency of the ventilated wall increased with the air space thickness and decreased with the air flow rate.It is suggested that the increase in air space thickness of the ventilated wall can be used to obtain an interesting ventilated façade performance.
A ventilation system was used to impose air flow in the cavity.Moving the mobile wall can vary the air space thickness.flux partly recovered by the air direction of air flow between the two cells

Figure 3 .
Figure 3. Hot box instrumentation.Two climatic chambers are separated by a sample holder.The ambient temperature is measured by 6 K-type thermocouples (3 thermocouples, T42, T43, and T44 in the cold cell and 3 thermocouples, T39, T40, T41 in the hot cell).

Figure 4 .of 16 Figure 5 .
Figure 4. Ventilated wall instrumentation: wall surface temperature is measured by K-type thermocouples, air temperature is measured by K-type thermocouples, thermal flux is measured by flux meters, and airflow rate is measured by a hot wire anemometer.

Figure 9 .
Figure 9. Ambient temperature in the cold cell and in the hot cell.

Figure 10 .
Figure 10.Surface temperature measurement by infrared camera.

Figure 12 .
Figure 12.Local convection heat transfer coefficient in the cold cell.

Figure 13 .
Figure 13.Local convection heat transfer coefficient in the hot cell.

Figures 14 and 15 17 Figure 14 .
Figures 14 and 15  show the global heat transfer coefficient in both the hot cell and the cold cell.The global heat transfer coefficient was estimated from temperature measurements (thermocouples) and heat fluxes (flux meters measurements).As we can see in Figures14 and 15, there are small differences between the measurements, which can be explained by the uncertainties of the convection heat transfer coefficient.Energies 2021, 14, x FOR PEER REVIEW 13 of 17

Table 1 .
Overview of different studies on ventilated façades.

Table 4 .
Uncertainties for the local convection heat transfer coefficient.