1. Introduction
According to the International Energy Agency (IEA), the building sector ranks second place in the overall electricity consumption in Mexico with 5441 ktoe in 2018, representing 22% of the country’s total consumption. In the same year, this sector was responsible for the emission of 17 Mt
[
1]. In addition, according to Mexico’s population growth projections, a considerable increase in energy consumption for cooling and heating is forecast. The average potential consumption today is 4.5 times the average consumption of households between 1990 and 2015, and the urban residential sector exhibits a higher consumption [
2]. It is also worth mentioning that according to IEA, under the Efficient World scenario, the increase in energy consumption could be limited to just 10% between now and 2040, which would save 1 EJ of additional energy use compared to expected trends. The savings would mainly come from the transport (45%) and industry (30%) sectors, followed by the construction sector. Therefore, it is evident that there is a need to reduce the energy consumption of buildings as much as possible, specially the energy destined for thermal comfort, and to find sustainable and effective technologies that guarantee energy savings. Thermal comfort is a complex phenomenon influenced by a variety of factors, including air temperature, humidity, individual metabolic rates, and even clothing, playing a crucialrole [
3,
4].
Passive solar techniques are an excellent option to reduce the electricity consumption used for space conditioning. These techniques have been reported to reduce the annual heating demand by up to 25% [
5]. These techniques include the Trombe wall (TW) system, which stores solar energy that directly affects a building facade to later transfer such energy to the building’s interior space, even in the absence of solar radiation. A classic TW comprises an energy-absorbing storage wall and a glass cover that creates the greenhouse effect; both elements make up a vertical channel connected to a room [
6]. This channel has a lower vent that allows air to enter the room through the channel, where the wall heats the air. Then, the heated air returns to the room through an upper vent because of natural convection. In classic TW, the storage wall is generally painted black to improve absorption and it is made of a material with high-energy storage capacity. It is worth mentioning that TW is a widely recommended technique due to its simple configuration and high efficiency. Previous research demonstrates that when a TW is used in buildings it provides heating energy savings between 16% and 30% [
7,
8]. Further, to improve the thermal performance of the classic TW, some authors have modified its materials or configuration. There are configurations with storage walls integrated with phase change materials (PCMs), water, or nanofluids. Some researchers have also added fins to the wall or solar cells to increase the energy efficiency of TW.
There are several experimental studies available in the literature. In 2020, Wang et al. [
9] divided the experimental methods for TW evaluation into four types: reduced-scale thermal box, full-scale thermal box, stand-alone Trombe wall module, and actual testing house. Among these methods, the reduced-scale thermal box stands out because it has no limitations for its application and has the advantages of space-saving, flexible arrangement, and cost savings. Such a method involves constructing a reduced-scale thermal box with a TW to simulate the thermal behavior of a building under specific weather conditions or in a climatic chamber.
For this reason, some authors have used this method to analyze the thermal performance of the classical TW using different construction materials for the storage wall, such as Zalewski et al. [
10], who used a 15 cm thick concrete slab and found that it causes a time lag of 6 h under climatic conditions in Cadarache, France. Later, Zalewski et al. [
11] analyzed the composite solar wall with PCM bricks and concluded that its time delay (2 h 40 min) is almost 2.5 times longer than the results of the concrete wall. A couple of years later, Abbassi et al. [
12] investigated the thermal performance of a Trombe wall with a storage wall composed of 0.1 m thick concrete bricks on the Mediterranean coast of Africa (Tunis, Tunisia). The experiment measured a maximum room temperature between 22 and
C for a maximum ambient temperature of
C during the day. At night, their temperature falls close to the ambient temperature between 10 and
C. In 2016, Dimassi and Dehmani [
13] carried out another experiment on the Mediterranean coast of Africa. They built a classic TW with a concrete wall of 10 mm in thickness coupled to a test cell. They painted the south surface of the storage wall using black matt, the air gap between the wall and glazing was 0.12 m in width, and the upper and lower vents had dimensions of 0.25 m/0.15 m. Their results showed that thermal radiation was greater than the convection in both the interior of the test cell and the air gap. The same authors [
14] proposed adding a thin black copper panel instead of the painted black wall of the previous TW. They reported a mean maximum thermal efficiency of 0.671 and 0.441 for the improved TW and the classic TW, respectively; this result was because the copper panel increased both conductive and convective flux.
On the other hand, it has been observed that the integration of fins into the storage wall significantly improves the thermal behavior of the TW; therefore, some research has focused on improving the efficiency of the fins. Regarding the study of fins in the TW, Rabani and Rabani [
15] analyzed three fin materials, brass, aluminum, and copper under climatic conditions in Yazd, Iran. The authors concluded that the copper fin showed the maximum heating efficiency of the TW because it promotes a higher heat transfer rate by natural convection inside the channel. In the same year, Baïri et al. [
16] experimentally evaluated the heat transfer by natural convection in a TW enhanced by the interposition of transparent vertical partitions in the active enclosure of the TW. Their study was performed in a steady state under a controlled thermal environment to quantify the natural convective heat flux in the active cavity. The authors found that the proposed TW achieves an improvement in average convective heat transfer between 10.0 and 14.4% with respect to a conventional TW. For its part, Abbassi and Dehmani [
17] studied an unvented Trombe wall with internal fins to maximize the heat transfer to the room. Their experiment reveals that the fins increased the room temperature around 3–
C on sunny days. It did not exceed
C during cloudy days in a Mediterranean region. They used the experimental results to validate simulations in TRNSYS. Later, Qi et al. [
18] developed a three-dimensional numerical model to evaluate the thermal performance of a finned/unfinned Trombe wall attached to a room under cold climate conditions. In their study, the authors used ANSYS-Fluent 2022R1 software. The authors optimized the height, transverse spacing, and longitudinal spacing of the vertical fins for a finned Trombe wall. The results showed that the thermal performance of the Trombe wall improved when the height of the fins was 20 mm, the transverse spacing was 0.20 m, the longitudinal spacing was 0.533 m, and in-line fins with a top angle of 90°, i.e., isosceles right triangle fins were used.
Some authors have proposed modifications to the traditional TW design to improve its energy efficiency. In 2020, Hu et al. [
19] studied and proposed a TW with blinds (WBTW) that were made of micro-channel pipes, through which water can flow. They concluded that the WBTW reduces room overheating in the non-heating season. Recently, Islam et al. [
20] proposed to replace the glass of the TW with a PV panel (PVTW) and located a Venetian blind installed in the air gap between the PV panel and the TW (PVTW_Ven). Their results showed that the proposed system reduces the average interior surface temperature by
C compared to a classic TW with Venetian blinds in semi-arid conditions of the Abha Asir province of Saudi Arabia. Later, Zelazna et al. [
21] studied the energy and environmental impact of a Trombe wall with the integration of a phase change material (PCM) under temperate climate conditions in Rzeszów, Poland. The PCM used was RT25HC. The authors performed laboratory tests to determine the aging characteristics of the PCM, in which they established its temperature and latent heat of fusion/solidification. The results showed that the Trombe wall with PCM reduced heating loads by up to 11.3% compared to the Trombe wall without PCM.
Based on the reviewed literature, it was observed that the reduced-scale test box has been effectively used to study the thermal behavior of the classical TW and its various modifications. The research conducted has provided valuable information about the thermal performance of TW under diverse climatic conditions, and a few have validated numerical models. However, most of them did not provide detailed experimental data and just monitored a data point measure on the wall and room temperature, which does not allow us to observe the variation temperature along the room cavity or the channel of the TW. To address this gap, an experimental study was carried out using two reduced-scale test boxes with different façade designs. The temperatures of the absorber wall, the air in the bottom and top vents, the glass cover, and the air at the cross-section plane of the test boxes were monitored during the experiments. Furthermore, the literature review showed that a conventional TW with a concrete wall improves the indoor environment of buildings in cold and Mediterranean climates in countries such as France and Tunisia. However, no studies have examined the thermal performance of Trombe walls in the northwestern region of Mexico, characterized by arid and semi-arid conditions and low winter temperatures. Therefore, this research work presents the thermal evaluation of a TW in winter conditions of Ensenada, B.C., Mexico. This work also shows the benefits of a classic TW built with inexpensive materials in terms of thermal efficiency, thermal load leveling, and temperature distribution. In addition, the thermal behavior of the TW is compared to that of a traditional concrete facade (CF). The CF is used as a reference facade because it is present in many Mexican buildings [
2] and it can be used as a basis for the construction of TW in buildings.
2. Description of the Experimental Model
The thermal performance of the TW and CF systems was evaluated using two test boxes that represent scaled-down building rooms (
Figure 1).
Figure 1a illustrates the test box with the TW system, while
Figure 1c shows the one with the CF system. Both test boxes were designed to isolate and analyze the effects of the TW and CF systems on indoor air temperature. The TW system consists of a concrete storage wall measuring 1 m in length, 0.7 m in height, and 50 mm in thickness, paired with a glazing cover measuring 1 m in length, 1 m in height, and 0.006 m in thickness, with a transmittance of 0.82. An air gap, 1 m long, 1 m high, and 0.125 m wide, separates the glazing cover from the concrete storage wall (see the TW cross-section in
Figure 1b). Furthermore, the vents connecting the TW system to the test box measure 0.125 m in height and 1 m in length. Because Mezhrab and Rabhi [
22] found that the TW system achieves greater heat transfer efficiency when the ventilation path is unobstructed, all tests were carried out with the vents open. In contrast, the CF test box working as a reference case includes a concrete facade wall with dimensions of 1 m in length, 1 m in height, and 50 mm in thickness. Concrete was selected to construct TW and CF because it is a common building material in Mexico. The thermal conductivity and specific heat of the concrete are 0.18 W/m·K and 1.08 kJ/kg·K, respectively [
23]. To simulate the solar radiation absorbed by the facades, flexible electrical resistances covered with silicon were installed: 15.5 m in the CF and 11.5 m in the TW.
The facade of a building absorbs ≈ 70–80% () of the solar radiation it receives. Part of this absorbed energy is stored within the facade, while the remainder is transferred to the indoor air through conduction. In the case of the CF, it exchanges heat with the outdoor and indoor environments via convection and radiation. For the TW, heat is transferred to the indoor space through convection and radiation. Additionally, the TW transfers heat by convection and radiation to the air gap between the storage concrete wall and the glazing cover. This process causes the air in the channel to return to the indoor space of the test box at a higher temperature. The air transfers energy to the glazing cover within the TW channel, which also conducts heat. Finally, some energy is either lost or gained through convection and radiation at the external surface of the glazing cover, depending on the outdoor environmental conditions.
The top, bottom, right, left, and bottom walls of the test boxes were designed as composite walls, consisting of a medium-density fiberboard layer (0.0127 m), an extruded polystyrene layer (0.0254 m), and another medium-density fiberboard layer (0.0127 m). This configuration aimed to reduce heat transfer by conduction because the extruded polystyrene layer has a thermal conductivity of 0.0288 W/m·K. Further, to minimize radiative heat exchange in the indoor space a low-emissivity foil ( = 0.1) was applied to the inner surfaces of the composite walls. Each test box has an internal volume of 1 m × 1 m × 1 m.
Instrumentation
All composite walls were equipped with evenly distributed differential thermopiles to measure the average temperature difference between the internal and external surfaces. This temperature difference was used to determine the heat flux through the walls. Additionally, thermocouples were installed at the center of the inner surface of each wall (
,
,
,
,
,
,
) to serve as temperature references. All thermocouples and thermopiles used in the experiment were “T” type with a
C uncertainty, PFA (perfluoroalkoxy) insulation, and special limit error (SLE). All thermocouples and anemometers were calibrated to set up the experiment. Moreover, the accuracy of the sensors was according to the intended measurements.
Figure 2 shows the thermocouple distribution on a cross-section of the test boxes (
,
,
,
,
). Here,
is positioned at the midpoint of the test box, while
and
are 25 cm vertically above and below
, respectively. Similarly,
and
are located 25 cm horizontally to the east and west of
, respectively. For the TW, thermocouples were installed on the glazing cover (
,
) and at the middle of both the inlet and outlet vents (
,
). In addition, a thermocouple (
) and an anemometer (
) were placed in the air-gap channel, 2 cm away from the CSW (
Figure 2a). All sensors used in the experiment were connected to three 34901A acquisition cards, which were integrated into a 34972A data acquisition system. This system facilitated real-time monitoring and data recording on a connected PC, as shown in
Figure 3, which depicts a schematic diagram of the experimental setup.
A programmable Power Supply N5769A (1500 W) was used to power the electrical resistance of the concrete wall (CF), with an uncertainty of 100 mV and 45 mA. For the thermal storage wall (TW), a programmable Power Supply TP10015 (1500 W) was used to supply energy to its electrical resistance, offering a reading accuracy of 500 mV and 450 mA. The power supplies were controlled based on the hourly average solar radiation calculated for a typical year.
Figure 4 shows the test boxes: (a) TW and (b) CF. The experimental setup was installed at the Center for Scientific Research and Higher Education at Ensenada (CICESE), B.C., Mexico. Ensenada has a Mediterranean climate (Köppen classification: Csa), influenced by its coastal location along the Pacific Ocean. The Mediterranean conditions make it somewhat unique in Mexico, resembling climates more commonly found in southern California or parts of the Mediterranean basin. Climatic variables were monitored using a meteorological station equipped with a 16-bit data acquisition system, located
N latitude and
W longitude within the CICESE campus.
Figure 5 shows a diagram that summarizes the procedure or methodology followed by the authors to develop the experimental tests using the test boxes.
4. Conclusions
This research presented the construction and experimental thermal evaluation of two test boxes designed to represent a room space, one coupled to a Trombe wall (TW) and the other one coupled to a concrete facade (CF) facade. Experiments were conducted under winter conditions in Ensenada, Baja California, characterized by a semi-arid Mediterranean climate. This study evaluated the thermal performance of the TW during a sunny week in winter, as well as on individual sunny and cloudy days. Additionally, a comparative analysis was performed to assess the stored energy and indoor air temperature behavior associated with each façade type. The main conclusions of this research are as follows:
The glazing cover in the TW presents a nonuniform thermal behavior, with the temperature on the top region being up to C higher than in the bottom region on a sunny winter day. The results also indicate that the most significant oscillations of glazing temperature occur on windy days. Thus, it can be concluded that the thermal performance of the TW is mainly affected by the wind velocity.
The TW can increase the air temperature through its channel up to C, which yields a maximum thermal efficiency of 84% during the sunny winter day. On the other hand, the maximum thermal efficiency during a windy day with solar radiation similar to a sunny day is up to 64%.
The comparison between the results from the sunny day and the cloudy day revealed a difference up to C in the maximum indoor air temperature, which corresponds to values of C for the sunny day and C for the cloudy day. Additionally, the thermal load leveling (TLL) was 0.55 for the sunny day and 0.31 for the cloudy day. These findings indicate that higher solar radiation contributes to improved thermal stability of the indoor air.
It was found that the TW test module shows the highest indoor air temperature values (9.4 ≤ C) regarding the CF-test module (8.1 ≤ C) because the cover glazing of the TW reduces heat losses to the outdoor environment. We also observed that the indoor air temperature on the CF-test module is very close to the ambient temperature at night and even below the ambient temperature during the morning.
Finally, it is concluded that TW can improve the thermal behavior of buildings under winter conditions in Ensenada, B.C., Mexico. The thermal behavior observed in the TW might increase the cooling load in the summer. To prevent overheating in the summer, passive cooling strategies, such as natural ventilation or shading devices, should be incorporated. Moreover, TW should face south to have a good thermal performance, which may not be feasible in all buildings. Therefore, the authors are studying further experiments to observe the TW thermal performance in summer conditions of Ensenada. Further, the authors are examining the possibility of isolated configurations to avoid an adverse effect of the TW on summer conditions and propose a modified TW coupled to a solar chimney configuration in future works.
