Material and Structural Optimization of Novel Phase-Change Thermal Diode for Dynamic Building Envelope

Hengxin Zhao; Yifan Wu; Guochen Jiang; Minlin Zhong; Hongli Sun; Borong Lin

doi:10.1016/j.eng.2025.07.008

Engineering ›› 2026, Vol. 63 ›› Issue (8) :287 -299. DOI: 10.1016/j.eng.2025.07.008

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Material and Structural Optimization of Novel Phase-Change Thermal Diode for Dynamic Building Envelope

Hengxin Zhao ^a^,^b
, Yifan Wu ^a^,^b
, Guochen Jiang ^c
, Minlin Zhong ^c
, Hongli Sun ^b^,^d^,^*
, Borong Lin ^a^,^b^,^*

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Abstract

Passive energy savings through the building envelope represent a critical strategy for reducing both energy consumption and carbon emissions. However, traditional technologies are limited by cumbersome control mechanisms and narrow adjustment scopes. To overcome these limitations, we propose a novel phase-change thermal diode with an enhanced unidirectional heat-transfer capacity. This thermal diode utilizes hydrophobic and hydrophilic materials, presenting dual benefits in material fabrication and structural applications. To verify the potential of this approach for building applications, the effect of different hydrophobic materials and vacuum on the performance of the thermal diode were compared experimentally. The optimally formulated material, which is readily manufacturable over large areas, demonstrated thermal rectification ranging from 8.72 to 23.62, thus offering an extensive adjustment range. For structural applications, the thermal diode could be combined with the building envelope to create a dynamic envelope for passive heat dissipation and insulation. Simulation studies confirmed that this novel dynamic adjustment method provides superior adjustment capabilities and achieves greater energy conservation than conventional dynamic methods. Specifically, cooling energy savings between 11.83% and 21.36% were attainable across various climate zones in China. This research fosters cross-innovation in the fields of buildings and materials, serving as a foundational reference for developing dynamic building envelopes.

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Keywords

Dynamic building envelope / Energy consumption simulation / Phase-change thermal diode / Material optimization / Passive energy saving

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Hengxin Zhao, Yifan Wu, Guochen Jiang, Minlin Zhong, Hongli Sun, Borong Lin. Material and Structural Optimization of Novel Phase-Change Thermal Diode for Dynamic Building Envelope. Engineering, 2026, 63 (8) : 287-299 DOI:10.1016/j.eng.2025.07.008

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1. Introduction

The acceleration of global urbanization in recent years has led to increased construction and rapid growth in building energy consumption [1]. Globally, the building sector accounts for approximately 36% of total energy use. With projections indicating a 50% increase in carbon emissions by 2050 [2], energy conservation becomes imperative for promoting low-carbon environmental protection. Notably, the total energy consumption of public buildings is growing rapidly, at a rate of 10% per year [3]. In future, the energy consumption of China’s public buildings is expected to increase at a higher rate than that of residential buildings, from 580 million tonnes of standard coal in 2020 to between 1.44 billion and 1.7 billion tonnes of standard coal by 2060 [4]. Public buildings consume high amounts of energy due to their large size and complex operational needs [5,6], thus, energy savings in this sector are critically important.

In public buildings, energy is mostly consumed by air conditioning, lighting, and equipment, with cooling and heating accounting for up to 60% [7]. In this context, energy-saving methods can be divided into two main approaches: active and passive [8]. Active methods include using efficient equipment [9], optimizing unit control strategies [10], using renewable energy sources such as photovoltaics [11], and developing decarbonization policies [12]. Passive methods include natural ventilation [13] and efficient, switchable building envelopes [14]. Although the potential energy-saving effects of passive technologies are lower than those of active technologies, they do not require other energy sources and have a greater application potential. The building envelope is an important part of the building and accounts for 30% of the primary energy consumption [15]. Upgrading the capacity of the building envelope is thus a direct and effective method for improving the energy efficiency of public buildings.

As an alternative to traditional building envelopes, where performance improvement depends on increasing thermal insulation and using new materials [16,17], performance-adjustable building envelopes have received considerable research attention [18], [19], [20]. Dynamic building envelopes can be adapted to the needs of people and energy efficiency, with the advantages of adaptability and energy saving. However, existing dynamic adjustment methods are cumbersome to control, require complex designs, and only exhibit small adjustment margins [21]. Therefore, new materials and methods for dynamic building envelopes should be explored, with flat-plate construction and passive phase-change heat transfer as potential solutions. A passive planar thermal diode incorporating phase-change heat transfer exhibits unidirectional heat transfer and can be effectively combined with the building envelope, thereby providing a new dynamic adjustment method. Some scholars have recently conducted cutting-edge research on applying thermal diodes in buildings. These studies included a convective thermal diode with wick [22], a five-layer thermal diode composite wall [23], and a thermal diode combined with phase change materials [24], among others. These progressive studies demonstrate the potential of thermal diodes for building applications.

Considering the above advantages of thermal diode for building applications, a novel jumping-droplet thermal diode [25], [26], [27] was noted in this study. The thermal diode typically has three sections: a superhydrophilic layer, a superhydrophobic layer, and a thermal spacer. Furthermore, these diodes have two working modes: forward and reverse. The forward mode utilizes the phase change heat transfer process of water, which has high thermal conductivity. The reverse mode involves ineffective thermal conductivity through the insulating gasket with a low heat transfer coefficient. When combined with the building envelope, jumping-droplet thermal diodes can exploit passive phase change, potentially solving the problems associated with existing dynamic building envelopes [21].

However, jumping-droplet thermal diodes are currently mostly applied for heat dissipation in electronics [28,29]. Consequently, these diodes are relatively small, and research efforts have focused on enhancing their forward thermal conductivity for heat removal. To maximize the heat transfer effect of thermal diodes, their inner surface and the heat transfer medium have been optimized [30], [31], [32], [33]. Although significant improvements in the unidirectional thermal rectification and forward thermal conductivity have been achieved, the fabrication methods remain relatively complex. In particular, preparing the hydrophobic surface requires various chemical reagents, which hinders large-scale production. In addition to focusing on the forward thermal conductivity, the reverse thermal conductivity must be considered to ensure heat insulation for building applications. The focus of the research on jumping-droplet thermal diodes for building applications, where manufacturing aspects and large-area applications are critical, differs from that for electronics.

Therefore, we have explored the jumping-droplet thermal diode further for building applications. In our previous research [21], we introduced a new method of combining jumping-droplet thermal diode with building envelope from the perspective of buildings, which promotes cross-innovation in different fields. Although this previous study demonstrated the potential and feasibility of using jumping-droplet thermal diode, there are three aspects that need to be further explored: ① The previous study did not include a theoretical analysis of the thermal diode’s heat transfer mechanisms; ② while the significance of vacuum conditions was confirmed experimentally, the study lacked a detailed and comprehensive investigation, particularly at the chemical level for optimization; ③ in terms of energy saving effect verification, there is a lack of quantification of existing dynamic adjustment methods, and the relationship between experiment and simulation is relatively weak. Building upon these findings [21], this paper explores these three areas, expanding on the groundwork laid by our earlier research.

Herein, a new phase-change thermal diode with a simple structure and convenient internal surface treatment was fabricated based on the concept of the jumping-droplet thermal diode (Fig. S1 in Appendix A). The effect of different hydrophobic materials and vacuum on the performance of the thermal diode was investigated, and the optimized structure was used to evaluate the impact of its application in the building sector. This achieved a double optimization of the thermal diode on physical and chemical levels. Furthermore, the influence of different dynamic structural adjustment capacity on building energy savings was quantitatively analyzed. In addition, the advantages of building envelopes combined with this novel phase-change thermal diodes over previous dynamic building envelopes were demonstrated. Finally, the potential energy savings for buildings using this novel thermal diode were determined in different climate zones in China. Leveraging the cross-innovation in buildings and materials sectors, it enables the development of energy-saving technologies for public buildings and the realization of the dual-carbon goal.

2. Methods

2.1. Composition and application of new phase-change thermal diode

Figs. 1(a) and (b) show that the novel phase-change thermal diode consisted of two metal plates and a thermal insulating gasket [26]. Hydrophilic and hydrophobic surfaces were fabricated on the two metal plates for heat-transfer circulation of the internal medium. The hydrophilic surface was machined and then sintered using metal powders to achieve superhydrophilic effect. Because the properties of hydrophobic surfaces influence the condensation heat transfer of droplets, three different fabrication methods were explored in this study, as detailed in Section 3.1. The gasket material, which is used for thermal insulation upon reverse heat transfer, typically consists of polytetrafluoroethylene (PTFE) or acrylic, both of which have relatively high thermal resistance.

This study chose a public building with high energy consumption as an example. This building had the following dynamic adjustment needs: ① During daytime in summer, the air conditioner was turned on inside owing to the high temperature outside the room; the temperature of the hydrophobic layer on the outer layer of the thermal diode was higher than that of the hydrophilic layer inside. In this case, the dynamic building envelope was in the reverse mode, effectively reducing the entry of heat from outside to realize passive thermal insulation. ② During summer nights, since the air conditioning was off and the heat gain delayed, the temperature of the inner hydrophilic layer was higher than that of the outer layer. In this case, water evaporation inside moved heat to the outside through phase-change heat transfer, thereby achieving passive heat removal. Based on the above application requirements, the building’s need for heat removal at night and insulation during the day is compatible with the forward and reverse modes of the thermal diode, which can enhance the energy-saving potential of public buildings when applied as a building envelope material.

Considering the bearing capacity of a building, aluminum was chosen to fabricate the thermal diode [21]. For the metal plate, the initial experimental dimensions were 100 mm with a thickness of 10 mm. A hollow of 80 mm × 80 mm × 3 mm was cut into the hydrophilic plate to enclose the liquid-absorbing core. A 5 mm high tab was manufactured on the hydrophobic plate to process the hydrophobic surface and fix the thermal insulation gasket. PTFE was used to manufacture the 6 mm high thermal insulation gasket for reverse heat insulation. The height of the phase-change space inside the thermal diode was 2 mm.

The thermal conductivity and insulation performance of the new phase-change thermal diode in forward and reverse modes were suitable for the heat dissipation and insulation demands of a building envelope, respectively. Therefore, we envision combining the thermal diode with the building envelope of public buildings for passive heat removal and insulation. To stabilize the circulation of the internal working medium, the thermal diode was first placed above the roof of the building. At this time, the liquid evaporated from the hydrophilic layer on the lower side, after condensing in the upper layer, could be effectively returned under the action of gravity to complete the evaporation-condensation heat-transfer cycle. A schematic of this application is shown in Fig. 1(c).

2.2. Heat transfer model of new phase-change thermal diode

In the forward mode, the internal heat transfer mainly depends on phase-change heat transfer from the heat transfer medium (deionized water). In contrast, in the reverse mode, an internal phase change is precluded because of the temperature and pressure differences. The heat transfer thus depends on thermal conductivity through the sidewalls of the thermal diode. A complete heat transfer model was constructed and analyzed computationally to analyze the main factors influencing the operation of the new phase-change thermal diode.

The thermal resistance network diagram in Fig. 2 shows that the heat transfer model integrates the conductive thermal resistance of the hot plate (R_C-H, m²·K·W⁻¹) and cold plate (R_C-C, m²·K·W⁻¹). Because the internal convective and long-wave radiative heat transfers are relatively small, the long-wave radiative thermal resistance (R_R, m²·K·W⁻¹) and natural convection thermal resistance of the vapor (R_V, m²·K·W⁻¹) and liquid (R_L, m²·K·W⁻¹) phases can be neglected. Among them, the forward mode is dominated by the phase-change heat transfer, which mainly depends on the conductive thermal resistance of the absorbing core (R_w, m²·K·W⁻¹), evaporation thermal resistance (R_e, m²·K·W⁻¹), mass-transfer resistance of the vapor (R_ec, m²·K·W⁻¹), and condensation thermal resistance (R_c, m²·K·W⁻¹), whereas, the conductive thermal resistance through the sidewalls (R_s, m²·K·W⁻¹) can be neglected. Because the new phase-change heat diode has a small length, large cross-section, and high temperature, the smaller mass-transfer resistance of the vapor can also be omitted. In summary, the total thermal resistance of heat transfer in the forward mode (R_f, m²·K·W⁻¹) is expressed by Eq. (1):

(1)$ R_{\mathrm{f}}=R_{\mathrm{C}-\mathrm{H}}+R_{\mathrm{w}}+R_{\mathrm{e}}+R_{\mathrm{c}}+R_{\mathrm{C}-\mathrm{C}}$

In contrast, the reverse mode of the thermal diode mainly depends on the conductive thermal resistance through the sidewall insulation, so the total thermal resistance of heat transfer in the reverse mode (R_r) is expressed by Eq. (2):

(2)$ R_{\mathrm{r}}=R_{\mathrm{C}-\mathrm{H}}+R_{\mathrm{s}}+R_{\mathrm{C}-\mathrm{C}}$

In each of these equations, the thermal conductivity and thermal resistance (R) through the hydrophilic plate, hydrophobic plate, liquid-absorbing core, and sidewalls can be calculated using Eq. (3).

(3)$ R=\frac{H}{k}$

where H is the thickness, m, and k is the thermal conductivity, W·m⁻¹·K⁻¹, of the corresponding materials, respectively.

The calculation of thermal resistances R_e and R_c, which are related to the phase change, is more complicated owing to the evaporation and condensation of the working medium. These parameters are usually calculated using empirical equations and experimental back-fitting methods [25,30,33]. The heat transfer coefficient related to the evaporation heat resistance can be expressed as follows [33]:

(4)$ h_{\mathrm{e}}=\frac{2 \alpha}{2-\alpha} \frac{\rho_{\mathrm{v}} i_{\mathrm{lv}}^{2}}{T_{\mathrm{e}}} \sqrt{\frac{M}{2 \pi N T_{\mathrm{e}}}}$

where h_e is the heat transfer coefficient on the evaporation side, W·m⁻²·K⁻¹; α is the adjustment coefficient; i_lv is the enthalpy of evaporation of the working medium, J·kg⁻¹; T_e is the temperature of evaporation of the working medium at each moment, K; ρ_v is the vapor density, kg·m⁻³; M is the molar mass of the working medium, kg·mol⁻¹; and N is the universal gas constant (8.314 J·mol⁻¹·K⁻¹).

For R_c on the condensation side, the rate of steam flow to the cold plate and the nucleation rate of the condensate, which are typically related to the supercooling degree, droplet size distribution, and surface properties of the cold plate, must be considered. Currently, two main calculation methods exist: the Nusselt equation correction [30] and integral calculation of the droplet size [33]. These condensation heat transfer equations are complicated and mainly rely on empirical calculations, which are not effective or accurate. Considering that phase change dominates the evaporation and condensation thermal resistances, the condensation heat transfer model can be simplified using geometrical parameters [25]:

(5)$ h_{\mathrm{c}}=\chi \frac{2 \alpha}{2-\alpha} \frac{\rho_{\mathrm{v}} i_{\mathrm{lv}}^{2}}{T_{\mathrm{e}}} \sqrt{\frac{M}{2 \pi N T_{\mathrm{e}}}}$

where h_c is the heat transfer coefficient on the condensation side, W·m⁻²·K⁻¹; χ is the proportion of partial droplet condensate coverage on the condenser surface. The internal heat transfer of the thermal diode can be simplified as follows:

(6)$\frac{1}{h_{\mathrm{in}}}=\frac{H_{\mathrm{w}}}{k_{\mathrm{w}}}+\frac{1}{h_{\mathrm{pc}}}=\frac{H_{\mathrm{w}}}{k_{\mathrm{w}}}+\frac{1}{h_{\mathrm{e}}}+\frac{1}{h_{\mathrm{c}}}$

(7)$h_{\mathrm{pc}}=\frac{\chi}{1+\chi} \frac{2 \alpha}{2-\alpha} \frac{\rho_{\mathrm{v}} i_{\mathrm{lv}}^{2}}{T_{\mathrm{e}}} \sqrt{\frac{M}{2 \pi N T_{\mathrm{e}}}}$

where h_in is the heat transfer coefficient inside the thermal diode, W·m⁻²·K⁻¹; h_pc is the phase-change heat transfer coefficient considering evaporation and condensation, W·m⁻²·K⁻¹; H_w is the thickness of the absorbing core, m; k_w is the thermal conductivity of the absorbing core, W·m⁻¹·K⁻¹; and χ and α are affected by various factors, including the experimental and manufacturing conditions. The values of these adjustment coefficients are typically determined by backfitting from experimental calculations to theoretical equations.

2.3. Experimental system and devices

To verify the unidirectional heat transfer effect of the new phase-change thermal diode, different temperatures were applied on both sides of the thermal diode to calculate the forward and reverse thermal conductivities at various temperature differences. The forward and reverse thermal conductivities were calculated using Eq. (8):

(8)$k=\frac{q H}{\Delta T}$

According to the above equation, the corresponding thermal conductivity between the two sides can be calculated from the heat flow density (q, W·m⁻²) and the temperature difference (ΔT, K).

Subsequently, to calculate the unidirectional heat transfer effect of the thermal diode, thermal rectification η, which is defined as the ratio of the difference between the effective thermal conductivities in the forward (k_f, W·m⁻¹·K⁻¹) and reverse modes (k_r, W·m⁻¹·K⁻¹) to the effective thermal conductivity in the reverse mode [27], was introduced.

(9)$\eta=\frac{k_{\mathrm{f}}-k_{\mathrm{r}}}{k_{\mathrm{r}}}$

A larger η value indicates a higher unidirectional heat transfer capacity and thus better performance of the novel thermal diode.

Thermocouples were employed to experimentally measure the average temperature of the cold and hot plates, and a heat flux density sensor was used to record the heat-flow density through the thermal diode. Besides, to verify the vacuum effect, a dry vacuum pump and vacuum gauge were used to vary the internal pressure of the thermal diode. The instruments used in these experiments are summarized in Table 1.

Considering the typical range of temperatures maintained in buildings, experiments were performed at ten different temperature settings divided into two groups of five experiments, as shown in Table 2. In the first group, the temperature of the cold source was fixed at 25 °C, and the temperature of the heat source was gradually increased to investigate the effect of increasing the temperature difference between the cold and hot sources. In the second group, the temperature difference between the cold and hot sources was fixed at 2 °C, and the average temperature of the thermal diode was gradually increased to investigate the effect of the average temperature on the heat transfer performance. In subsequent experiments, the average temperature of the two plates was used as the primary parameter. The experimental configuration used for these experiments is illustrated in Fig. S2 in Appendix A.

3. Results

3.1. Effect of hydrophobic material on thermal diode performance

3.1.1. Parameter comparison for hydrophobic materials

According to the heat transfer model of the new phase-change thermal diode described in Section 2, the phase-change heat transfer is an important factor for diode performance. During phase-change heat transfer in thermal diodes, the hydrophobic surface structure affects droplet condensation, whereas the vacuum (amount of non-condensable gases) affects the degree of phase change and condensation heat transfer. Therefore, the effect of different hydrophobic materials and vacuum on the heat transfer characteristics was investigated to optimize the performance of the thermal diode.

Three methods were considered to conveniently fabricate a stable hydrophobic surface for building applications: laser fabrication, fluorosilane coating, and PTFE coating. For the fabrication of superhydrophobic surfaces, laser direct writing can be programmed for large-scale manufacturing. Moreover, this technique produces multiscale micro- and nanostructures that are more robust [34], [35], [36] than the hydrophobic particles prepared by chemical etching. Fluorosilane coatings are primarily based on fluorosilane polymers dissolved in butyl acetate, with the fluoroalkyl groups providing hydrophobicity and silicon enhancing the coating adhesion and stability. The PTFE coating was used to compare the performance of superhydrophobic and hydrophobic surfaces. A low-surface-energy PTFE solution was directly applied to the metal substrate, and this simple method generated a hydrophobic surface.

Confocal laser scanning microscopy images of the different hydrophobic materials are shown in Fig. 3. The laser-fabricated coating had a uniform distribution of micro- and nanostructures on the surface, with a depth of 50-60 μm. Furthermore, the contact angle (CA) exceeded 160°, indicating strong superhydrophobicity. Because the fluorosilane and PTFE coatings were directly sprayed on the substrate using chemical reagents, the surface structures were flatter, and the CAs were less than 160°, which indicates that these coatings were hydrophobic. Therefore, in terms of overall hydrophobicity, the laser-fabricated coating was superior to the fluorosilane and PTFE coatings.

3.1.2. Experimental performance of hydrophobic coatings

To verify the influence of the surface hydrophobicity on the heat-transfer effect of the new phase-change thermal diode, each coating was incorporated into a thermal diode structure, and measurements were carried out at an absolute pressure of 3 kPa. The forward and reverse thermal conductivities calculated using Eq. (8) are shown in Fig. 4.

These results indicate that enhanced hydrophobic performance leads to a greater condensation heat transfer coefficient. Therefore, for the forward mode, the thermal conductivity of the laser-fabricated coating was greater than that of the fluorosilane and PTFE coatings. In contrast, no significant differences were observed for the reverse thermal conductivity, with all the values being approximately 0.2 W·m⁻¹·K⁻¹. In the reverse mode, the internal deionized water does not undergo a phase change; therefore, this process mainly depends on the thermal conductivity through the surrounding insulation gaskets. Consequently, the properties of hydrophobic surfaces had minimal influence on the reverse thermal conductivity.

Furthermore, the thermal rectification of the thermal diodes with different hydrophobic materials was compared. Notably, the thermal diode with the laser-fabricated thermal coating exhibited thermal rectification of 7.18-11.31 and a unidirectional multiplier of 8-12, which resulted in an excellent unidirectional heat transfer. In contrast, the thermal rectification of the thermal diodes with fluorosilane (3.70-6.58) and PTFE (1.72-2.85) coatings were significantly lower, evidencing that the properties of the hydrophobic surface influenced unidirectional heat transfer in the thermal diode. Owing to increased CA and optimization of the surface micro- nanostructures, the thermal rectification of the thermal diode with the laser-fabricated coating was enhanced. Therefore, subsequent experiments mainly focused on laser-fabricated coatings.

3.2. Effect of vacuum on thermal diode performance

3.2.1. Dependence of forward thermal conductivity on vacuum

In addition to the effect of the hydrophobic surface properties on condensation heat transfer, the internal non-condensable gas can influence the thermal diode performance because heat transfer within the diode mainly depends on the evaporative phase change of deionized water. A vacuum pump was used to control the absolute pressure to explore the dependence of the forward and reverse thermal conductivities on the vacuum.

As shown in Fig. 5, the absolute pressure varies between 2.1 and 12.5 kPa. With the optimization of the vacuum, the forward thermal conductivity of the thermal diode increased continuously. Specifically, at 26 °C, the forward thermal conductivity increased by a factor of 6.06, from 0.17 to 1.20 W·m⁻¹·K⁻¹. Owing to the reduction in internal non-condensable gases, the capability of the thermal diode was effective when the absolute pressure was 5.5 kPa or less. At 7.3 and 12.5 kPa, the forward and reverse thermal conductivities of the thermal diode were similar, resulting in a loss of the unidirectional heat-transfer effect. Therefore, the vacuum is an important factor in determining the unidirectional effect of the thermal diode. In subsequent experiments, the thermal diode was operated at absolute pressures of 2.1-3.0 kPa or less.

3.2.2. Comparison of reverse thermal conductivity with conventional thermal insulation materials

The effect of vacuum on the forward thermal conductivity is mainly attributable to the condensation of non-condensable gases. The new phase-change thermal diode not only exhibits large forward thermal conductivity, but the reverse thermal conductivity is also reduced because of the lack of an internal phase change, which provides a certain degree of thermal insulation. To verify the effect of reverse thermal insulation, the variation in the reverse thermal conductivity at different absolute pressures was compared with that of a commonly used insulating material for buildings.

As shown in Fig. 6, the reverse thermal conductivity of the thermal diode remained in the range of 0.05-0.19 W·m⁻¹·K⁻¹ at absolute pressure of 2.1-4.3 kPa. Generally, for buildings, materials with a thermal conductivity lower than 0.23 W·m⁻¹·K⁻¹ are considered conventional insulation materials, whereas those with thermal conductivity lower than 0.14 W·m⁻¹·K⁻¹ and lower than 0.05 W ·m⁻¹·K⁻¹ are considered thermal insulation and high-efficiency thermal insulation materials, respectively. Accordingly, the reverse thermal-insulation effect of the thermal diode meets the requirements of conventional insulation materials over a wide range of vacuum. Furthermore, at an absolute pressure of 2.1 kPa, the reverse thermal conductivity of the new phase-change thermal diode was in the range of 0.09-0.11 W·m⁻¹·K⁻¹, which meets the requirements of thermal insulation materials. Advantageously, the new phase-change thermal diode exhibited a large forward thermal conductivity and a reverse thermal insulation performance comparable to that of commonly used thermal insulating materials in buildings. In the future, the reverse thermal insulation performance of the phase-change thermal diode can be further improved by using thermal insulating gaskets that are more efficient than the current PTFE gaskets.

3.3. Unidirectional heat transfer in doubly optimized thermal diodes

As shown in 3.1 Effect of hydrophobic material on thermal diode performance, 3.2 Effect of vacuum on thermal diode performance, a more hydrophobic surface and higher vacuum both improved unidirectional heat transfer in the new phase-change thermal diode. Therefore, in subsequent analyses of the thermal diode integrated with a building envelope, data for the superhydrophobic laser-fabricated surface at 2.1 kPa were used for analytical verification. The forward and reverse thermal conductivities of this thermal diode are shown in Fig. 7.

The thermal conductivity increased with the average temperature in the forward mode (Fig. 7(b)). In the first set of experiments, the forward thermal conductivity of the thermal diode increased slowly because the increase in the average temperature was slight. In the second set of experiments starting at 35 °C, the forward thermal conductivity increased rapidly with the increase in average internal temperature. Overall, the forward thermal conductivity was in the range of 1.20-2.60 W·m⁻¹·K⁻¹ (the average value). In contrast, in the reverse mode (Fig. 7(c)), the thermal conductivity did not vary significantly with the average temperature, remaining in the range of 0.09-0.13 W·m⁻¹·K⁻¹ (the average value). This results from the absence of a phase-change heat transfer inside the thermal diode, which mainly relied on the ineffective thermal conductivity through the surrounding PTFE gasket. The value of η was 8.72-23.62 (Fig. 7(a)), and the overall unidirectional heat-transfer multiplier was maximized to nearly 18 times in the range of 25-40 °C for building applications. This behavior suggests that the new phase-change thermal diode has considerable passive-energy saving potential for building applications.

We also compared the results of the optimized experiments with those of the previous study [21] under the same conditions. The specific results are shown in the Table 3 [21].

As illustrated in Table 3 [21], following optimization, the thermal rectification in the current study shows a substantial improvement, ranging from 31.16% to 86.71% over the prior study, indicating a more effective enhancement. Additionally, the maximum value of forward thermal conductivity at the same temperature has increased by 64.08% from 1.42 to 2.33 W·m⁻¹·K⁻¹. This enhancement shows the significance of optimization in this research.

4. Discussion

4.1. Parameter settings for building simulation

In Section 3, the factors affecting the performance of the new phase-change thermal diode were investigated experimentally, and the thermal diode performance was optimized. In order to verify its energy efficiency when integrated with a building envelope, the relationship between the forward and reverse adjustment capacities of the dynamic building envelope and energy savings was explored using simulations. The dependence of the building energy savings on the magnitude of the dynamic building envelope adjustment capability was thus obtained. Furthermore, the advantages of integrating building envelope with the new phase-change thermal diode were identified by comparing this integrated system with existing switchable building envelopes. Finally, the simulations were refined to determine the energy conservation and climate suitability of the building envelope incorporating the new phase-change thermal diode in different climate zones.

Initially, the deployment of the new phase-change thermal diode on the building roof was considered. The building structure was modeled in DesignBuilder (DesignBuilder Software Ltd., UK), and the building features are summarized in Table 4. To simulate a dynamic building envelope, the heat transfer coefficients of the building envelope were varied for different application requirements. Therefore, the programming and validation of the control logic were performed in the energy management system (EMS) section of EnergyPlus (US Department of Energy). In EMS, the corresponding control procedures are mainly carried out by monitoring indoor and outdoor temperatures and the temperature of the thermal diode. The specific operation methods can be referred to in our previous research [21].

4.2. Relationship between adjustment capacity of dynamic envelopes and building energy conservation

For a dynamic building envelope, the dynamic adjustment capacity can be expressed as the ratio of the maximum heat transfer coefficient to the minimum heat transfer coefficient of the dynamic components. Theoretically, a larger ratio indicates an enhanced ability of the building for heat removal and absorption, leading to higher energy savings. Therefore, to investigate the relationship between the adjustment multipliers of the dynamic building envelope and the energy-saving effect, we conducted simulation using the standard heat transfer coefficient of the building envelope in each climate zone as the baseline. For example, in Harbin of China, the standard heat transfer coefficient of the roof is 0.25 W·m⁻²·K⁻¹. In the simulation, we set the heat transfer coefficients of 0.25, 0.50, 1.00, and 1.50 W·m⁻²·K⁻¹, corresponding to dynamic adjustment capacities of 1, 2, 4, and 6, respectively. Based on the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) climate zone and a cooling degree day with a base temperature of 10 °C (COD 10), seven cities in China (Kunming, Haikou, Guangzhou, Chongqing, Beijing, Shenyang, and Harbin) were selected for the simulation analysis (Section 4.4). The simulation results obtained for Harbin are shown in Fig. 8.

As the multiplier of the maximum and baseline heat transfer coefficients increased, the cooling energy consumption in Harbin gradually decreased, achieving a maximum energy-saving ratio of 22.24%, demonstrating a strong energy-saving effect. Besides, the upper curve of the total energy saving ratio and the lower curve of the energy saving growth revealed that upon an initial increase of the adjustment multiplier, the growth rate of energy saving was larger, and for 12 adjustment times, it reached 84.21% of the total growth amount. As the adjustment multiplier increased further, the growth in energy savings gradually slowed, indicating that the gains obtained by expanding the switching effect of the dynamic building envelope at a later stage decreased gradually.

Therefore, for the existing research, the common active methods for creating dynamic switching effects for building envelopes, such as mechanical adjustment or embedded pipes [18,19], can achieve switching multipliers of 2-10, resulting in an overall energy-saving effect of 35.47%-79.49%. In the later stages of dynamic adjustment, if the adjustment capacity is further increased, the cost would exceed the benefits resulting from the energy savings of the building. Consequently, active adjustment methods typically lack high dynamic regulation capacities. For dynamic adjustment using passive methods, such as phase change heat transfer, larger adjustment capacities of 12 times or more can typically be achieved at low cost. The resulting increase in energy savings is a key advantage of combining new passive phase-change thermal diodes with building envelopes.

Furthermore, the simulation was expanded to seven climate zones, as shown in Fig. 9. The relationship between the adjustment capacity and energy savings in these climate zones was similar to the effect observed in Harbin. The energy-saving potential was enhanced as the dynamic adjustment capacity increased, but the growth trend slowed gradually.

4.3. Comparison of new dynamic building envelope with other building envelopes

Preliminary analyses presented in Section 4.2 revealed that different adjustment multipliers of the dynamic building envelope yielded varying energy-saving effects when applied in buildings. To further quantify these effects, the new phase-change thermal diode was benchmarked against various dynamic building envelope components Table 5 [18],[19],[40], [41], [42], [43], [44], [45], [46], [47]. To compare the heat transfer coefficients of switchable building envelopes, we divided the existing studies into two types. In the first type of the studies, the heat transfer coefficients (K values) encompass the convective heat transfer between indoor and outdoor environments, which represent comprehensive heat transfer coefficients and are directly summarized in Table 5. In contrast, the second type of studies only report the corresponding forward and reverse thermal conductivities. Therefore, the comprehensive heat transfer coefficient for these studies was calculated considering an indoor and outdoor convective heat transfer resistance of 0.14 m²·K·W⁻¹ [37], [38], [39], with a material thickness of 100 mm. For the results in Table 5, a standard benchmark was established for the reverse coefficient (0.5 W·m⁻²·K⁻¹ for Haikou), whereas the forward heat transfer coefficient (i.e., maximum adjustment capacity) was calculated from the reported values for different components.

Based on the results of these uniform calculations, the forward K value achieved in combination with the building envelope is relatively minor for most dynamic adjustment methods, making it difficult to achieve the advantages of forward heat dissipation. In addition, for most previously reported systems, the overall multiplier compared to the heat transfer coefficient of the building envelope in the standard was between 1.97 and 5.40 times. Thus, according to the simulation results in Section 4.2, the energy savings are not fully exploited, remaining in the range of 2.87%-17.73%. For the new phase-change thermal diode described in this study, the comprehensive heat transfer coefficient obtained via the forward thermal conductivity calculation reached 5.60 W·m⁻²·K⁻¹, which is 11.2 times the benchmark value of 0.5 W·m⁻²·K⁻¹. Thus, the heat absorption and dissipation of the dynamic building envelope can be maximized, contributing to achieving the dual-carbon goal in the building sector.

4.4. Energy saving effect of new phase-change thermal diode for building applications

Based on the analyses in 4.2 Relationship between adjustment capacity of dynamic envelopes and building energy conservation, 4.3 Comparison of new dynamic building envelope with other building envelopes, further simulations were performed to clarify the advantages of the new phase-change thermal diode for building applications. To quantify the energy-saving effect of this thermal diode when applied in buildings, the experimental forward thermal conductivity values from Section 3 were substituted into the simulation at temperature intervals of 5 °C, and the dynamic building envelope was simulated in sections. This approach strengthened the correlation between simulations and experiments, providing closer correspondence with the experimental results, and allowed further quantification of the energy conservation of this new building envelope.

Haikou, a hot summer and warm winter region, was selected as the primary location for the EMS simulation. As shown in Fig. 10, the building envelope integration with the new phase-change heat diode fully exploited heat removal at night. The energy saving percentage for the annual cooling energy consumption was 11.97%, whereas the energy saving percentage for the total energy consumption was 11.85%, which confirmed the energy-saving effect of this passive building envelope. Further, Table 6 summarizes the energy savings achieved for the cities in the seven climate zones.

As shown in Table 6, dynamic building envelopes incorporating the new phase-change thermal diode are applicable in different climate zones. Because this system mainly uses passive heat removal by thermal diodes, passive energy savings in cooling energy consumption were predominant, ranging between 11.83% and 21.36%. For the selected cold regions, the relative energy savings were higher because smaller absolute amounts of cooling energy were consumed; however, because the energy consumption for heating is substantial, the energy savings for the total annual energy consumption were lower than in hot regions. This energy-saving potential is slightly lower than our previous study [21] due to differences in building models and parameters. In this study, the selected building had a small internal heat (20 W·m⁻²), which was turned on to 20% at night. Increasing the building’s internal heat is conducive to further improving the energy savings of this building envelope. Therefore, the dynamic adjustment method incorporating the novel phase-change thermal diode is more suitable in large-roof public buildings with high internal heats and small heights.

In addition to the simulation analysis of energy consumption, we further compared the temperature variation of the dynamic building envelope with that of the static building envelope after applying the thermal diode. As depicted in Fig. S3 in Appendix A, the blue, green, and red curves represent the outdoor temperature, average indoor temperature of the dynamic building envelope, and average indoor temperature of the static building envelope, respectively. At night, when the air conditioner was turned off, the indoor temperature of the dynamic building envelope was 3-4 °C lower than that of the static building envelope in Fig. S3(a). This evidenced that the thermal diode had a passive heat removal effect at this time, which is conducive to building energy conservation. It can also be seen in Fig. S3(b) that the green curve is always lower than the red curve at night, which illustrates the effect of passive heat removal in summer. Overall, the maximum heat removal temperature difference reached 4.5 °C and could be increased by enhancing the internal heat source.

4.5. Limitations and future work

The new phase-change thermal diode proposed in this study was optimized by assessing various hydrophobic materials and adjusting the vacuum levels. The effectiveness of applying the material on the building envelope was verified. Nonetheless, the study encountered certain limitations. Due to constraints in material selection, only a finite number of hydrophobic materials were tested, which limited potential enhancements to the novel phase-change thermal diode. Additionally, the fixed configuration of hydrophilic and hydrophobic panels restricts the structure’s suitability primarily to regions with hot summers and warm winters.

In future research, we plan to further optimize the hydrophobic surface through chemical etching techniques and the integration of additional hydrophilic point structures. In addition, we will combine the new phase-change thermal diode with building chambers to verify the energy-saving effects under practical conditions. Another focal point of our upcoming studies will be incorporating a suitable chemical solution into the thermal diode’s water. This solution will enable the internal working medium to solidify at lower temperatures, minimizing ineffective heat dissipation during winter.

5. Conclusions

In this study, a new phase-change thermal diode with unidirectional heat-transfer capability was proposed. This thermal diode was easy to manufacture and could be combined with a building envelope to form a switchable building envelope with a large dynamic adjustment capacity. The feasibility of this approach and potential applications of the advanced performance-adjustable building envelope were investigated through experimental analysis and simulations. The key findings of the study are summarized as follows.

(1) The effects of hydrophobic materials fabricated using three different methods on the heat-transfer performance of the thermal diode were compared using experimental methods. The hydrophobic surface created by laser machining proved to be the most durable and mechanically robust, offering convenience for large-scale applications in the building sector. Furthermore, the heat-transfer capability of the laser-fabricated surface outperformed the other two methods.

(2) The effect of the vacuum on the forward and reverse thermal conductivities of the thermal diode was explored. The forward thermal conductivity was increased by a factor of 6.06 by optimizing the vacuum. In addition, the reverse thermal conductivity ranged from 0.09 to 0.11 W·m⁻¹·K⁻¹, aligning with the standards for thermal insulation materials. After a comprehensive optimization, the η of the thermal diode ranged from 8.72 to 23.62, and the overall unidirectional heat-transfer multiplier was maximized to nearly 18 times in the range of 25-40 °C suitable for building applications.

(3) Comparisons with traditional dynamic building envelopes demonstrated that while the dynamic adjustment capacity of conventional systems ranged from 1.97 to 5.40, it reached 11.21 for the building envelope incorporating the new phase-change thermal diode. Furthermore, simulation results across various climate zones indicated cooling energy savings between 11.83% and 21.36%, highlighting the energy-saving efficacy and application potential of the new phase-change thermal diode for building applications.

In conclusion, this research successfully adapted jumping-droplet thermal diodes, commonly used for heat dissipation in electronics, to the building sector by integrating hydrophobic materials suitable for this new application. This enhancement significantly improved the dynamic adjustment capacity of traditional switchable building envelopes. The study promotes cross-innovation in the fields of buildings and materials, offering valuable guidelines for developing dynamic building envelopes.

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