Modelling, experimental test, and design of an active air permeable wall by utilizing the low-grade exhaust air

Highlights

• A comprehensive heat transfer model of the air permeable wall was developed.

• Thermal insulation mainly lies in the exfiltration process across porous layer.

• Sensitivity analysis was performed to identify the impact of design parameters.

• The thickness of air permeable porous layer between 30 and 50 mm was recommended.

Abstract

The exhaust air insulation (EAI) wall is a novel type of air permeable wall that is characterized by a permeable porous layer. Such a wall design provides a solution to combine the building envelope with exhaust air heat recovery, and allows the exhaust air from a conditioned room to permeate through. The exfiltration process of the exhaust air across the porous layer can partly reduce the inward conductive heat flux through the wall. This makes the temperature at the innermost surface of the EAI wall close to the temperature of the indoor air, and leads to the reduction of cooling and heating loads through the wall. In this study, a comprehensive heat transfer model was developed to analyze the steady state thermal characteristics of the EAI wall. Forty different experiments were performed to verify the proposed model. A quantitative analysis was conducted on the differences when using the unsteady state model and proposed steady state model for the EAI wall with an outdoor thermal disturbance. A sensitivity analysis was carried out to investigate the impact of the design parameters on the thermal characteristics of the EAI wall. The results showed that using a porous material with relatively low thermal conductivity such as phenolic foam is beneficial to improve the heat recovery performance and thermal insulation of the EAI wall. The thermal insulation of the EAI wall is mainly due to the exfiltration process of exhaust air across the porous material. Considering the impact of the design parameters on the thermal characteristic, total thickness and pressure drop, a thickness of 30–50 mm is recommended for the porous material. At an exfiltration velocity of 0.003 m/s, the U-value of the EAI wall was lower than 0.1 W/(m² K) for a porous material thickness of 30–50 mm. These results demonstrate that the EAI wall is applicable to both energy-efficient retrofitting of existing buildings and new buildings, and can potentially contribute to reduce the cooling and heating loads through the wall by the utilization of low-grade thermal energy in exhaust air.

Introduction

Nowadays, the building sectors are responsible for high energy consumption and environmental impact [1]. It is estimated that the buildings account for 40% of the world’s annual energy consumption in the form of lighting as well as heating, ventilation and air-conditioning (HVAC) [2]. This highlights the imperative need for buildings to reduce energy use. Approximately 65% of global building energy consumption globally is due to the HVAC system [3], for which the building envelope wall plays a vital role. Improving the wall insulation properties can reduce the cooling and heating loads, and thus reduce the energy use of HVAC systems [4].

Numerous studies have investigated the high-performance building envelope walls for minimizing the cooling and heating loads. A common approach to achieving a wall with high thermal insulation is to install the foam board, expanded polystyrene (EPS), and extruded polystyrene (XPS) as the insulation layer [5]. In recent years, advanced insulation materials such as aerogels [6], vacuum insulation panels [7], bio-based insulation materials [8], solar insulating material [9], and hygroscopic materials [10], have been extensively investigated to further improve the thermal and energy performance of walls. Technologies are also being developed to increase the thermal inertia of the building envelope through the use of phase change materials (PCM) [11].

The building envelope wall can be integrated with low-grade energy sources such as geothermal energy [12], evaporative cooling water [13], solar energy [14], favorable ambient air [15], and exhaust air [16] to reduce the heat loss or heat gain through the building envelope in the heating or cooling season. A pipe-embedded wall allows low-grade heating or cooling water to circulate for releasing or removing heat inside the structures of the building envelope [12]. Li et al. [17] evaluated the annual energy saving potential of a pipe-embedded wall coupled with a ground heat exchanger in different climate regions. Shen and Li [13] numerically investigated the hourly thermal load of a pipe-embedded wall integrated with the cooling tower in the cooling season. Ibrahim et al. [18] proposed a new concept of transferring the solar energy on the south facade to the north facade by circulating water within the embedded pipes. Buratti et al. [19] investigated a new ventilated brick wall that utilizes the favorable ambient air to increase the thermal insulation of the wall in summer. Integrating the ventilated wall with an earth-to-air heat exchanger (EAHE) [20], photovoltaic (PV) panel [21], PV assisted thermoelectric module [22] was found to further improve the energy saving potential of the wall. Huang et al. [23] experimentally investigated the impact of the key design parameters on the thermal insulation of an exhaust air heat recovery ventilated wall. All of these studies show that utilizing low-grade energy sources can effectively reduce the thermal load of the wall.

Traditionally, the building envelope is impermeable to air. In recent years, the air permeable walls based on light-weight porous materials have been extensively studied. The air permeable walls provide an approach to combine the building envelope with the fresh air supply or exhaust air heat recovery.

The dynamic insulation wall (DIW) is a type of air permeable wall and is mainly composed of air permeable porous materials. It allows fresh outdoor air to permeate through the wall into the room side. The conductive heat loss through the wall in winter can be recovered and utilized to preheat the fresh infiltration air [24]. Dalehaug et al. [25] pointed out that approximately 50% of the conductive heat loss in a residential building can be recovered by applying a DIW system. Condensation may occur inside the DIW in the cooling season, because the fresh air is cooled when the infiltration airflow permeates through the wall [26]. Previous research efforts have mainly focused on the U-value [27], filtration performance [28], indoor thermal comfort [29], and infiltration heat recovery efficiency [30] of the DIW under heating dominant conditions. In recent years, some new research results about the DIW were found. Ascione et al. [31] developed a transient numerical model to analyze the heat transfer and water vapor concentration within an air permeable porous material during warm seasons. Their results indicated that integrating the DIW with nocturnal free cooling can reduce the indoor temperature more quickly. Graig and Grinham [32] proposed an optimal design method for air permeable porous materials to improve the heat recovery efficiency of a DIW. More importantly, they performed an experiment with schlieren imaging to explain the difference between the measurements and model predictions of previous DIW studies. Alongi et al. [33] developed a novel laboratory apparatus called a dual air vented thermal box to evaluate the thermal behavior of the DIW under steady state boundary conditions, and demonstrated that the air velocity distribution at the inlet surface of the DIW is non-uniform according to a computational fluid dynamic (CFD) simulation.

In addition, a new active air permeable wall that utilizes the low-grade exhaust air was proposed in our previous work [34]. The wall is characterized by an air permeable porous material that allows the exhaust air from the conditioned room to permeate through. The exfiltration process of the exhaust air across the air permeable porous material can partly prevent and reduce the inward conductive heat flux through the wall. The main effect is that the temperature difference between the interior surface of the wall and indoor air can be significantly decreased. Such an active air permeable wall is termed as the exhaust air insulation (EAI) wall, which is shown in Fig. 1. Note that, although the EAI wall differs from the DIW with regard to the thermal insulation mechanism, the research method and numerical model of the two air permeable walls are basically the same.

The thermal insulation mechanism of the EAI wall is mainly due to the air permeable porous layer. The temperature distribution within the porous layer varies with the exfiltration of the exhaust air. Fig. 2 explicitly illustrates the influence of the exfiltration airflow on the temperature distribution within the porous layer. The results in the figure were calculated by using the one-dimensional steady state analytical model developed by Taylor et al. [24]. The temperature gradient within a porous material without an exfiltration airflow is linear, and the temperature of the interior surface of the porous material is 27.6 °C in this example. When an exfiltration airflow occurs, the temperature of the area close to indoor side is approximately equal to the indoor temperature (25 °C), because the exfiltration process of the exhaust airflow partly prevents and reduce the inward conductive heat transfer through the porous material. For the horizontal position of the porous material close to outdoor side, the temperature rapidly increases because the enthalpy of exfiltration airflow is limited. A higher airflow velocity can achieve a lower temperature inside the porous material.

The above studies demonstrate that the air permeable wall has promising potential for improving the energy efficiency of buildings and guaranteeing good indoor air quality or thermal comfort. However, current studies on air permeable walls only focus on the heat or water vapor transfer of the single porous layer, and do not consider the additional structure of the complete wall. Therefore, developing a comprehensive heat transfer model of the complete structure of the air permeable wall is important for predicting the thermal characteristic and can provide a guidance for the design and application of the air permeable walls.

The main objectives of this study are to propose a two-dimensional heat transfer model for the whole air permeable wall, identify the impact of the design parameters on the thermal characteristics, and provide a recommended design of the wall. In this study, a two-dimensional steady state heat transfer model was developed to evaluate the thermal characteristics of the EAI wall. An experiment test rig was built, and steady state experiments were conducted to validate the proposed model under 40 different operating conditions. A quantitative analysis was performed on the differences between using the unsteady state model and steady state model for the EAI wall under an outdoor thermal disturbance. A sensitivity analysis was performed to investigate the impact of design parameters, such as the thickness of the porous material, thickness of the external solid wall, and material of the porous layer, on the exfiltration heat recovery (EHR) efficiency and U-value of the EAI wall. The novelty of this work is to provide a solution to combine the building envelope and exhaust air heat recovery for significantly improving the thermal insulation of the exterior wall, utilizing the low-grade thermal energy in the exhaust air, and reducing the total thickness of the wall. The proposed two-dimensional steady state heat transfer model can analyze the steady-state thermal behavior, and also provide accountable and accurate results while considering outdoor thermal disturbance, and particularly, requires less computation time. The results of this study would provide comprehensive understanding, reliable heat transfer model, full experimental data, the recommended design and application guidance for the novel active air permeable wall.