System Thermal Resistance Research Articles

Experimental investigation on a flat loop heat pipe coupled with thermoelectric cooler

The flat Loop Heat Pipe (LHP) has the advantages of small size, light weight, and direct contact with the heat source without additional thermal resistance, which has gradually drawn attention and importance in the aerospace field. However, for the vertically arranged flat LHP, there is a significant amount of reverse heat leakage from the evaporator to the compensation chamber, resulting in a low operating efficiency of the heat pipe. This paper proposes a thermal control structure coupling the flat LHP with thermoelectric cooler (TEC), which investigates the effects of TEC on the start-up speed, working temperature, system thermal resistance, temperature fluctuation, and reverse temperature control of the flat LHP through experimental studies. The results show that the TEC can help improve the start-up speed of the heat pipe by approximately 50 % under low heat load. The heat resistance of the LHP is greatly reduced to approximately 0.13–0.15 °C/W, and the working temperature curve changes from “V” type to a linear line with increasing heat load power. Moreover, by using the TEC to provide reverse power to the compensation chamber for temperature control, the temperature control efficiency is extremely high, and a temperature rise of approximately 10 °C can be achieved with only 0.1 W TEC power. The above research indicates that the TEC can significantly weaken the disadvantage of the flat LHP and improve its engineering practicality.

Experimental investigation on the thermal performance of a novel loop heat pipe (LHP) with micro-channel structure

Loop heat pipe (LHP) is widely used to solve the problem of high heat flux in electronic chips due to its excellent heat transfer efficiency, good thermal equilibrium, and large heat transfer limit. To further improve the heat transfer efficiency of the LHP, a novel LHP was designed with a parallel arrangement of 13 vapor channels (2 mm × 2 mm) capillary wick structure. Experimental investigations were conducted to evaluate the start-up and steady-state performance of the LHP under both regular and random variable loads ranging from 30 W to 180 W. The findings indicate that the LHP exhibits rapid start-up and operates smoothly within the heat load range of 30 W–180 W, maintaining the heat source temperature below 75 °C, thereby meeting the temperature requirements for chip operation.Under a regular variable heat load, the LHP experiences minimal temperature fluctuations, but the system operated with good stability. Meanwhile, under random variable heat loads, the LHP demonstrates quick responsiveness within 5 s and reaches a stable state within 8 min. The temperature fluctuations remain within 2 °C, effectively fulfilling the temperature control demands of electronic devices. Furthermore, the evaporator thermal resistance, condenser thermal resistance, loop thermal resistance and system thermal resistance of the LHP gradually decrease with increasing of heat load. When reaching a heat load of 180 W, the evaporator thermal resistance (Re), condenser thermal resistance (Rc), loop thermal resistance (RLHP), and system thermal resistance (Rsys) reach their minimum, with values of 0.0168 °C/W, 0.121 °C/W,0.137 °C/W and 0.229 °C/W, respectively. In comparison with other LHPs, the evaporator temperature was only 59.56 °C with the highest heat load of 210 W (52.5 W/cm2), which expanded the application range of the LHP for electronic devices cooling.

Numerical analysis of pumped two-phase loop: Characterization of steady-state performance

With growing heat dissipation from advanced electronics, two-phase cooling promises effective and unmatched cooling. A pumped two-phase loop (P2PL) can acquire high power and heat flux from the electronics (heat source) and transport and dissipate it to a heat sink with small total thermal resistance at negligible pumping power consumption because of highly effective phase change (boiling and condensation) heat transfer. Owing to many advantages, in recent years, extensive research efforts have been dedicated to understanding fundamental mechanisms of heat and mass transfer in phase change heat exchangers (evaporators or condensers) at a component level. However, there is little work done for an entire two-phase cooling system as a closed loop which is essential to study the system performance characteristics and the contributions/interactions of each component to the system performance. A comprehensive thermal–hydraulic flow network model was developed to study the system performance characteristics. The pressure variation of the P2PL was calculated by considering the compressibility of the vapor volume in the P2PL to account for the effect of the amount of vapor in the system. The numerical results on the thermal and hydraulic performance characteristics of the P2PL for various operating and design parameters – evaporator heat load, mass flow rate, heat sink temperature, and microchannel dimensions – are discussed. It was found that the numerical results match well with the measured steady-state temperature and pressure profiles of the P2PL. The study also highlights the trade-off between the total system thermal resistance reduction and the increased pressure drop (pumping power) with an increase in mass flow rate. It was also found that the microchannels with smaller hydraulic diameters had significant heat transfer enhancement in the evaporator but poor system performance.

Thermal performance of an ammonia loop heat pipe using a rectangular evaporator with liquid-guiding channels

In order to enhance the heat transfer capacity of a stainless steel-ammonia loop heat pipe using a rectangular evaporator, liquid-guiding channels were adopted to significantly reduce the liquid flow resistacne in the evaporator wick. An experimental system was established, and extensive experimental studies have been implemented to investigate its startup characteristics, heat transfer capability and thermal resistance. To be specific, the effects of heat load, evaporator attitude and antigravity on the startup performance were evaluated. According to the experimental results, the loop heat pipe showed excellent startup performance, which could realize the startup at small heat loads for different attitudes of the evaporator. Both adverse evaporator attitude and anti-gravity can cause an increase of the operating temperature of the loop heat pipe, due to poor cooling effect of return liquid and increased heat leak from the evaporator respectively. The loop heat pipe could achieve a heat transfer capability up to 300 W, corresponding to a heat flux of 20 W/cm2. The minimum system thermal resistance reached as low as 0.048 K/W at the head load of 140 W. The present study contributes greatly to the future loop heat pipe applications in high heat flux electronics cooling.

Study of the heat transfer deterioration of supercritical CO2 in vertical pipes using a hybrid RANS/LES method

Heat transfer deterioration (HTD) is one of the core factors affecting the efficiency of the supercritical carbon dioxide (S–CO2) Brayton cycle. When HTD occurs in a heat exchanger, the local wall temperature increases sharply, which may lead to pinch points in the heat exchanger of the circulation system and excessive thermal resistance, resulting in safety problems during the operation of Brayton cycle power generation systems. In studies concerning the HTD of S–CO2, most simulations currently use Reynolds-averaged equations (RANS) to predict and analyze the S–CO2 flowing upward in vertical tube transfer deterioration problems. In this paper, a hybrid RANS/LES method is used for simulations. The RANS model is used to describe the HTD in the near-wall region, and the LES model is used to calculate the flow details in the bulk region and accurately describe the flow regime and vortex structure of a compressible fluid with variable properties. Firstly, the growth and evolution process of HTD is analyzed from the transient-state, and then the corresponding relationship between HTD temperature and velocity field is expounded from the steady-state. Finally, combined with the buoyancy effect, acceleration effect and relaminarization caused by the change of S–CO2 physical properties, the mechanism of the occurrence and recovery of HTD flowing in vertical tubes and the second peak of wall temperature is expounded.

Trombe wall's thermal and energy performance—A retrofitting approach for residential buildings in arid climate of Yazd, Iran

A Trombe wall is a passive solar technology attached to the building envelope to reduce energy demands. In warm climates, due to overheating problems in the cooling season, its efficiency is limited and proper operation is required. In this study, the thermal behavior of a bedroom of a house equipped with a Trombe wall in Yazd with a hot and arid climate under different design configurations and various masonry materials were investigated using the dynamic simulation software DesignBuilder. Monthly ventilation strategies and a schedule of blinds for external glass cover throughout the year were proposed to optimize its energy efficiency. The blinds are applied for shading solar irradiance during summer. They also increase the system's thermal resistance during winter nights. According to the results, a concrete Trombe wall with 2/3 of the façade area is capable of reducing the heating load by 86%. However, its function for summertime is negative, and even in the insulation mode, it can increase the cooling load by 5%. Natural ventilation with the Trombe wall is applicable during moderate seasons; however, its cooling efficiency is limited compared to cross ventilation. The results also highlight that retrofitting a room with a Trombe wall can reduce the annual energy demand by 63%, which is equal to a reduction of 124 kg CO2 emission.

Low-profile heat pipe consisting of wick-lined and non-adiabatic wickless wettability-patterned surfaces

Heat pipes are metal-wick equipped systems that circulate a phase-changing liquid and transfer heat more effectively than solid-metal compact heat spreaders. Metal wicks are effective fluid transporters, but they can face capillary limits due to significant pressure drops and pore clogging. Wettability-patterned (WP) surfaces, on the other hand, are not limited by these constraints and, as demonstrated in this work, may offer a viable supplement for metal wicks in heat pipes, which come in many shapes and sizes. The use of WP surfaces facilitates the collection and efficient transport of condensate, thus improving heat transfer from the hot (evaporator) to the cold (condenser) side of the device. Most commonly, flat heat pipes consisting of a fully wick-lined plate opposed to a wickless surface, are evaluated considering a uniform adiabatic condition on the wickless plate, which does not promote condensation. However, in a practical scenario, the heat pipe portions in contact with an external heat sink could induce condensation on the wickless side, thus affecting the heat pipe’s operation and thermal resistance. To explore this scenario, a flat heat pipe of 10 cm operating length with axially graded copper-wick evaporator and non-adiabatic wickless WP surface is designed, fabricated and tested. A wettability pattern is imparted on the wickless surface to control the maximum droplet size of the condensate and condensation mode via spatially designed superhydrophilic/hydrophobic juxtaposed areas to regulate both Dropwise Condensation (DwC) and Filmwise Condensation (FwC) towards enhanced heat transfer with reduced overall system thermal resistance. Low capillary pressure domains are allocated in this design to collect condensate on the wickless plate and return it to the evaporator’s wick. On the evaporator side of the system, variable-porosity copper wicks are being used to improve liquid conveyance from the cold to the hot domains. Four heat-transfer fluid charging ratios are examined with the best thermal resistance 0.27 K/W achieved at 105 W. The effect of wettability patterning on the wickless surface and the wick characteristics of the wick-lined evaporator are investigated. • Low-profile hybrid heat pipe comprised of wick-lined and wickless components. • Three distinct wick structures and wettability patterns investigated and compared. • Axially-graded wick structure outperforms uniform wicks. • Wettability-patterned wickless plate lowers thermal resistance. • Highest-performing system’s thermal resistance below 0.3 K/W.

Numerical simulation of heat transfer and flow of Al2O3-water nanofluid in microchannel heat sink with cantor fractal structure based on genetic algorithm

The heat generated during operation of microelectronic devices often adversely affects product performance. A remedy for this problem can be through the incorporation of nanofluidic microchannel heat sinks. Advanced working fluids and channel structures can be used to improve the heat transfer performance of the microchannel heat sink. First, thermal resistance is treated as a single objective function, the geometry of the microchannel was optimized using a genetic algorithm. The flow and heat transfer characteristics of the optimized microchannel are analyzed. The effect of different nanofluid volume fractions and geometric parameters on the inlet and outlet pressure drop, flow resistance coefficient, substrate temperature, Nusselt number (Nu), and system thermal resistance in the fractal microchannel are investigated. The thermal resistance of Al2O3 nanofluid with a volume fraction of 5% is 12.5–14.7% lower than that of deionized water, and the microchannel substrate temperature is 6.26 °C lower than that of deionized water.

Analysis of the transient performance of coaxial and u-tube borehole heat exchangers

While coaxial borehole heat exchangers often have lower borehole thermal resistances than u-tubes, studies in the literature have mixed results as to whether the coaxial design always outperforms the u-tube. This study contributes a systematic comparison between the two designs, accomplished using a custom numerical model in OpenFOAM that provides detailed predictions of the heat transfer within the ground heat exchangers. The modelling and subsequent analysis of system thermal resistances demonstrated that the u-tube and coaxial heat exchanger performances differed the most during the early, transient phase of operation. Furthermore, modelling showed that the coaxial design does not necessarily exceed the u-tube performance long term; testing of coaxial heat exchangers with polyethylene piping showed small differences in outlet temperature compared to the u-tube after 72 h. Additional testing showed that using a steel coaxial outer tube could provide a 22% improvement to performance over the u-tube design. These findings are used to compare predictions of thermal resistance with available analytical models. The impact of the results on the potential for length reductions is also discussed.

Design, fabrication, investigation and analysis of a novel flat evaporator loop heat pipe for cooling high heat flux server chips

• A novel LHP with a flat evaporator has been proposed and tested. • SEM, porosity, and capillary rise of different sintered powder size are measured. • The heat transfer performance of the novel LHP is studied at heat loads from 50 to 750 W. • The performance comparison of the novel LHP is presented. A novel loop heat pipe (LHP) with a flat evaporator has been proposed to meet the heat dissipation requirement of high heat flux server CPU. The sintered porous copper wick was provided with a double arrayed vapor channels to increase the evaporation area. And a 1 mm thick smaller particle size copper powder embedded with two 0.5 mm thick stainless steel rings was sintered on the inner surface of the compensation chamber to enhance the strength, and increase the working fluid into the porous wick such that the liquid saturation was enhanced and the dry zone was reduced at high heat flux. Additionally, a dual layer structure condenser was presented to increase the cooling area of the condenser. In this study, the systematic experimental studies were carried out mainly focusing on the optimization of powder size, heat transfer performance of the novel LHP and performance comparison of the novel LHP. The results demonstrated that the sintered copper powder of the 106–150 μm particle size shown the best capillary performance in comparison with other powder sizes. More importantly, in the heat load range of 500–750 W, the novel LHP of filling ratio of 46.1% exhibited better operating thermal characteristics. The minimum values of the LHP thermal resistance and the total system thermal resistance at the heat load of 750 W were 0.040 °C/W and 0.090 °C/W, respectively. Compared with the conventional LHP and the simple water block, the novel flat evaporator LHP was a more promising option when the heat load was higher than 250 W.

Startup characteristics of an ammonia loop heat pipe with a rectangular evaporator

Flat-plate loop heat pipe (FLHP) is a passive two-phase heat transfer device. Comparing with traditional LHP with a cylindrical evaporator, it can be directly connected to a flat heat source without the employment of a saddle, which can effectively reduce the system thermal resistance and enhance the temperature uniformity. In this work, a stainless steel-ammonia FLHP was developed, and extensive experiments have been conducted to investigate its startup characteristics with the evaporator in the horizontal and vertical positions. Experimental results show that the FLHP exhibits excellent startup performance. It can successfully start up at a small heat load as low as 2 W with no obvious temperature overshoot. In a wide power range of 5–35 W, the FLHP generally starts up in only one situation, much simpler than the startup of a LHP with a cylindrical evaporator. For this rectangular evaporator, the heat leak from the evaporator to the compensation chamber (CC) becomes very small. As a result, the vapor can easily exit the condenser in most cases in the power range of 5–35 W, leading to a 100% utilization efficiency of the condenser and the resultant satisfactory thermal performance of the FLHP. In addition, the startup performance and the system thermal resistance of the FLHP are insensitive to the evaporator orientation, promising great application potential in future electronics cooling.

Effect of servers’ arrangement on the performance of a loop thermosyphon system used in data center

• A loop thermosyphon system was studied under four typical servers’ arrangements. • The servers’ arrangement had little influence on the steady-state operating performance. • Clumped the servers at the bottom of the rack can shorten the start-up time under low rack utilization. In applications, servers’ arrangements are often chaotic owing to a lack of regulation, which may affect the performance of loop thermosyphon systems. In this study, the optimal filling ratio, thermal resistance and operating performance of a loop thermosyphon system were experimentally investigated under four typical servers’ arrangements. Compared with other servers’ arrangements, the optimal filling ratio range changed from 30.3–37.3% to 28.0–35.0% when clumped the servers at the bottom of the rack. However, the system thermal resistances were approximately equal (0.0034–0.0036 °C/W) at optimal filling ratios under the four servers’ arrangements, indicating that a filling ratio within 30.3–35.0% was suitable for all servers’ arrangements. At 32.7% filling ratio, the operating performances under different servers’ arrangements and heating powers were compared and analyzed. Although the maximum difference in the evaporator inlet air temperatures exceeded 10 °C, the servers’ arrangement did not affect the steady-state operating performance. Moreover, the servers’ arrangement had little influence on start-up times under 4 kW and 6 kW heating powers but had a significant influence under 2 kW heating power. Clumped the servers at the bottom of the rack can shorten the start-up time under low rack utilization (2 kW heating power).

High-performance energy-saving miniature loop heat pipe for cooling compact power semiconductors

A miniature loop heat pipe (mLHP) is developed to cool compact wide bandgap power semiconductors with a high heat flux (≥30 W/cm2) and high allowable operating temperature (≤250 °C) for the first time. Since the mLHP consists of an evaporator (15 × 16 × 3 mm), a condenser (12 × 12.5 × 2 mm), and transport tubes (100 mm), it can be mounted on a small product equivalent to the size of a gaming smartphone. To achieve high heat flux performance, the mLHP uses Shirasu porous glass for the wick and ethanol for the working fluid. Moreover, for the condenser, heat dissipation is enhanced using a thin copper plate (60 × 60 × 0.5 mm), a thin aluminum cold plate (35 × 20 × 3 mm), or a small cooling fan (17 × 17 × 3 mm). When the condenser dissipates heat using the thin aluminum cold plate, the mLHP achieves a maximum heat-transport performance of 15 W (60 W/cm2) in the horizontal attitude. At the maximum heat load, the heat-source temperature, evaporator temperature, and system thermal resistance at 15 W are 249.8 °C, 142.2 °C, and 0.72 °C/W, respectively. Additionally, the heat transfer coefficient of evaporation at the evaporator is approximately 24,000 W/m2/K. The proposed steady-state numerical model is consistent with the experimental results, excluding the condenser part, with a relative error of 7%.

Experimental investigation on the thermal performance of a water-cooled loop thermosyphon system under fan failure conditions

Fan failure of water-cooled loop thermosyphon system (LTS) is a major factor for thermal safety of data center, thus it is necessary to investigate the effects of fan failures on the thermal performance. In this paper, different operating conditions including normal and eight possible fan failure modes of a LTS are tested at various refrigerant filling ratios. The optimal refrigerant filling ratio is ranged from 55.8% to 74.4% under normal operating condition. Compared with the conditions of leakage (Filling ratio = 46.5%) or overcharge (Filling ratio=83.7%), the LTS has better anti-failure capability with an optimal filling ratio. The cooling capacity at 55.8% filling ratio decreases by 8.2–10.1% and 20.8–23.8% under mild and serious fan failure modes, respectively, and the decrements increase with the filling ratio increasing. This is because the additional refrigerant mass flow rate increases with the filling ratio increasing, which increases the total system thermal resistance. However, the cooling capacity at the minimum optimal filling ratio (55.8%) is only 9.6% larger than that at the maximum optimal filling ratio (74.4%) under the worst fan failure condition. Considering the universality of the slight refrigerant leakage in refrigeration systems, it is better to choose a high optimal filling ratio to ensure the good operating performance and anti-failure capability of LTS.

Generalizing thermal resistance and a general thermal engine

This article outlines a generalization of the thermal resistance concept used to model and quantify the rates of spontaneous heat transfer. This leads to the natural corollary that a Carnot cycle possesses zero thermal resistance and provides restatements of the Clausius and Kelvin statements of the second law of thermodynamics. Subsequently, basic aspects of a general thermal engine are modeled by the series combination of two non-zero thermal resistances with a Carnot cycle. With constant thermal resistances, the efficiency at maximum power is found to be in agreement with previous literature also concluding that engine power is zero at the Carnot efficiency. An explicit form limiting maximum power output to system thermal resistance and reservoir temperatures is given. Minimizing the thermal resistance maximizes power output, congruent with maximizing heat flow and entropy production. Implications of this model towards practical power generation are presented.

Flat miniature heat pipe with sintered porous wick structure: experimental and mathematical studies

Miniature heat pipes are considered as an innovative solution able to dissipate high heat fluxes with low working fluid charge, temperature control, and operating with minimum energy consumption and less noise. In this paper, results show that tilt ranging [10°:60°] has not an impact on heat transfer meanwhile the system thermal resistance is the lowest at horizontal position. A theoretical analysis on heat pipe thermal performance has been carried out based on experimental data by predicting heat pipe evaporation and condensation heat transfer coefficient. The measured heat transfer coefficients are close to the predicted results with a mean deviation less than 10%. A hydraulic mathematical model is developed and shows a good agreement with Kaya's study using water as working fluid. This work comprises a parametric study that investigates the effect of particles size, porosity, permeability, heat pipe length on heat transfer and capillary pressure. The obtained results show that the heat pipe operating temperature rises when particles size increases from 1μm to 90 μm and the optimum porosity range is between [10%–40%].

A prediction model of surface heat transfer coefficient in insulating packaging with phase change materials

Abstract Surface heat transfer coefficient is one of the crucial parameters for determining the system thermal resistance of insulating packaging. However, the surface heat transfer coefficient which couples with the temperature field in insulating packaging is difficult to experimentally measure and independently calculate. In this study an iterative method based on a semi-infinite phase change model was proposed to solve the coupling parameters of surface heat transfer coefficient and temperature field. Then, the semi-infinite phase change model curves were compared with classic Neumann model and Mehling model. Furthermore, the affecting parameters of surface heat transfer coefficient were discussed through this semi-infinite phase change model, and a concise prediction model of surface heat transfer coefficient was also developed for engineering application. Finally, this concise prediction model was verified by “Ice Melting Method”. The results indicate that the proposed semi-infinite phase change model and iterative method can simultaneously calculate the coupling parameters of surface heat transfer coefficient and the temperature field in insulating packaging, and show a consistent with the classic Neumann model and Mehling model. The key parameters of surface heat transfer coefficient include the thickness and thermal conductivity coefficient of insulating wall, and the excess temperature also significantly influences the surface heat transfer coefficient. Furthermore, the system thermal resistance calculated by the novel concise prediction model has a good agreement with the experimental data of “Ice Melting Method” presented by Burgess.

An Integrated Liquid Metal Thermal Switch for Active Thermal Management of Electronics

Heat dissipation is a key obstacle to achieving reliable, high-power-density electronic systems. Thermal devices capable of actively managing heat transfer are desired to enable heat dissipation optimization and enhanced reliability through device isothermalization. Here, we develop a millimeter-scale liquid metal droplet thermal switch capable of controlling heat transfer spatially and temporally. We demonstrate the thermal switch by integrating it with gallium nitride (GaN) devices mounted on a printed circuit board (PCB) and measure heat transfer and temperature of each device for a variety of switch positions and heat dissipation levels. When integrated with a single GaN device (2.6 mm $\times4.6$ mm face area) dissipating 1.8 W, the thermal switch shows the ability to actively control heat transfer by conducting 1.3 W in the ON mode with the GaN device at 51 °C ± 1 °C, and 0.5 W in the OFF mode with the GaN device at 95 °C ± 1 °C. To elucidate the heat transfer physics, we developed a 1-D system thermal resistance model in conjunction with an independent 3-D finite-element method (FEM) simulation, showing excellent agreement with our experimental data. Finally, we demonstrated that when the switch is integrated with two GaN devices, the switch can balance the device heat transfer rate and enhance junction temperature uniformity and system reliability by lowering the device-to-device temperature difference from > 10 °C (no switch) to 0 °C.

Experimental study and steady-state model of a novel plate loop heat pipe without compensation chamber for CPU cooling

In this study, a new type of loop heat pipe (LHP) without compensation chamber is designed and manufactured, meanwhile the liquid line is fitted with a sintered wick to enhance startup and steady operation capacity. Specifically, the thermal characteristics of the LHP are studied experimentally. The results show that the LHP can start up smoothly at any heat load and the highest temperature of LHP does not exceed 90 °C. The fluid at the liquid line is subcooled, which is beneficial for the startup and steady operation of LHP. When the heat load is in range of 10 W to 50 W, the LHP works at variable conductance mode, and the system thermal resistance and the loop thermal resistance both decrease sharply with the increasing heat load. When the heat load is in range of 50 W to 150 W, the LHP enters a constant conductance mode, and the system thermal resistance and the loop thermal resistance remain basically constant. Besides, the steady-state model of LHP used to predict the operating temperature of LHP is established. The simulation results were compared with the experimental data, and the maximum relative error of evaporator, vapor, condenser outlet and liquid line wick inlet temperature are less than 15%.

Thermal performances study on a façade-built-in two-phase thermosyphon loop for passive thermo-activated building system

Pipe-embedded building envelopes, as a burgeoning active insulation technique that has its pipe circuits inside the building envelope, are of particular interest to architects and engineers due to their excellent performances and invisibility characteristic. However, performances of this pump-driven technique highly rely on long-term electricity consumption, resulting in degradation of system efficiency in building load reduction and supplementary heating. In the present study, a facade-built-in two-phase thermosyphon loop (TPTL) for passive thermo-activated building system (i.e. TPTL-embedded building envelope) is proposed to turn the pump-driven heat injection process of active insulation technique into non-pump-driven. Experiments on a prototype are conducted to explore operational characteristics of the TPTL whose condenser is operated in the thermal boundary of Dirichlet and dynamic thermal response characteristics of the composite building envelope after coupling TPTL at different heat loads and filling ratios. Results show that the filling ratio has a critical impact on thermal resistances and startup behavior of TPTL, and the optimal filling ratio (around 116%) is not a fixed value at different operating conditions. Besides, condenser thermal resistance occupies the largest proportion among the system thermal resistance due to the heat accumulation phenomenon, therefore, the interlayer material also has a great impact on heat injection and release of TPTL-embedded building envelope. In addition, reverse startup and operation under specific test condition are observed and discussed accordingly. Overall, these results and conclusions are useful for the system design, operation control, and further investigations on the façade-built-in TPTL and the TPTL-embedded building envelope.