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The design of integrated and highly efficient solutions for thermal management is a key capability for different aerospace products, ranging from civil aircraft using hydrogen on board to miniaturized satellites. In particular, this paper discloses a novel numerical tool for the design and thermal performance assessment of heat pipes. To achieve this goal, a numerical Ansys Parametric Design Language code is set up to verify the effective subtractive heat flux guaranteed by the selected heat pipe arrangement. The methodology and related tool show their ability to provide good thermal performance estimates for different heat pipe designs and operating conditions. Specifically, the paper reports two very different test cases: (1) solid metal heat pipes to cool down the crotch leading-edge area of the air intake of a Mach 8 civil passenger aircraft, and (2) a copper-water heat pipe to cool down a Printed Circuit Board of a generic small LEO satellite. The successful application of the methodology and numerical code confirms the achievement of the ambitious goal of developing in-house tools to support heat pipe thermal performance prediction for the entire aerospace domain. [ABSTRACT FROM AUTHOR]

Given the fact that the operation of electrical machines generates various loss components that finally become heat, developing advanced thermal management technologies is essential to control temperature increases and to guarantee safe operations. Meanwhile, the armature winding can stand larger currents when the machines are equipped with advanced cooling systems, which directly improves torque/power densities. This paper aims to provide a systematic review of the latest developments of advanced thermal management technologies of electrical machines. According to different heat dissipation mechanisms, the cooling systems studied in this paper are categorized into five major types: enclosed housing cooling, enhanced conductive cooling, embedded heat pipe cooling, direct oil cooling, and enhanced rotor cooling. The advantages and disadvantages of these cooling systems are researched and compared comprehensively. This study contributes to the revelation of insights on the thermal management of electrical machines and offers good guidance for the thermal management of electrical machines. [ABSTRACT FROM AUTHOR]

Achieving the global electricity demand and meeting the United Nations sustainable development target on reliable and sustainable energy supply by 2050 are crucial. Portable energy storage (PES) units, powered by solid-state battery cells, can offer a sustainable and cost-effective solution for regions with limited power-grid access. However, operating in high-dust and high-temperature environments presents challenges that require effective thermal management solutions. This paper is a comprehensive review of thermal management systems for PES units, with a specific focus on addressing the challenge of overheating in airtight designs. The review of various active and passive cooling systems is conducted through extensive study of the relevant literature, which is significant in providing insights into the operation, performance parameters, and design options for different cooling system technologies. The findings from this review show heat pipe (HP) technologies as key cooling-system solutions for airtight PES units. Specifically, loop and oscillating HPs, as well as the vapour chamber, offer desirable features such as compactness, low cost, and high thermal conductivity that make them superior to other alternatives for the cooling systems in PES. The insights and knowledge generated via this review will help facilitate the design and development of innovative, efficient, and reliable PES units, thereby contributing to the advancement of off-grid renewable energy applications and enabling sustainable power solutions worldwide. Furthermore, an appropriate design of PES units can help in reducing capital and maintenance costs. [ABSTRACT FROM AUTHOR]

This paper presents a novel technology to improve the energy efficiency of refrigeration and air-conditioning (RAC) systems by applying a condenser cooling approach. The approach is based on the integration of an innovative radiation-enhanced thermal diode tank (RTDT) with a RAC system. The thermal diode tank (TDT), consisting of heat pipes and an insulated water tank, is a passive device to generate cooling water at a minimum night ambient temperature. When the radiation-enhanced heat pipe (RHP) is equipped with the TDT, it becomes an RTDT, which could theoretically lower the water temperature below the ambient temperature. In this study, a radiation-enhanced thermal diode tank (RTDT) is proposed to supply cooling water to the RAC system. Simulation models for the proposed RTDT-assisted RAC (RTDT-RAC) system are developed in order to investigate the impacts of the tank size to cooling capacity (TS/Qc) ratio, day/night ambient temperature fluctuations on the system's coefficient of performance (COP) and the energy saving percentage (ESP). The results show that a greater day/night ambient temperature difference and a larger TS/Qc value can both enhance the COP and ESP of the RTDT-RAC system. The optimal and threshold TS/Qc values were 1 m3/kW and 0.18 m3/kW, respectively. These findings demonstrate the potential of the RTDT-RAC system to achieve significant energy savings and provide valuable insights for the design and optimization of an RTDT-RAC system. [ABSTRACT FROM AUTHOR]

This study concerns the numerical simulation and the experimental implementation of a low-scale Phase Change Material-based (PCM-based) condenser, to be included in a PCM-based portable cooling systems. In this category of cooling systems, the PCM can be integrated either in the condenser or in the evaporator. In the present study, the PCM is integrated in the condenser of the vapor compression cycle to absorb the heat power released from the refrigerant fluid (R134a) during condensation, thus eliminating the need to transfer heat to the external environment. The main objective of the present study is to realize and validate a numerical model capable of simulating both the refrigerant fluid and the PCM thermofluid dynamics. For this purpose, a commercial solver was used for the implementation of the developed numerical model, and experimental tests were performed to validate the numerical simulations results. The paper reports the details and test results of both the numerical model and the experimental apparatus. The simulation results indicate a good accordance between the numerical and experimental data. [ABSTRACT FROM AUTHOR]

This paper presents an adsorption cooler (AC) driven by the surplus heat of a solar thermal domestic hot water system to provide cooling to residential buildings. A cylindrical tube adsorber using granular silica gel as adsorbent and water as adsorbate was considered. The AC was modelled using a two-dimensional distributed parameter model implemented in previous adsorption heating and cooling studies. The performance coefficients of the resultant thermally driven cooling system were obtained for a broad range of working conditions. The thermally driven AC was found to have coefficient of performance (COP) of 0.5 and a specific cooling power (SCP) of 44 W·kg−1 when considering condenser, evaporator, and regeneration temperatures of 30 °C, 15 °C, and 70 °C, respectively. Moreover, the results showed that the AC could be used for refrigeration purposes at temperatures as low as 2 °C and that it could also operate during hotter days under temperatures of 42 °C. [ABSTRACT FROM AUTHOR]

In the age of the digital economy, the data center is the most crucial piece of infrastructure. The issue of the excessive power consumption of a data center's cooling system needs to be addressed as the national objective of "peak carbon and carbon neutrality" is increasingly promoted. In this study, a microchannel-separated heat pipe-cooling system with evaporative condensation is introduced. The system may switch between three modes of operation in response to changes in outdoor air quality parameters, thereby maximizing the utilization of natural cooling sources while lowering data centers' cooling costs. The purpose of this paper is to analyze the energy-saving potential of the hybrid system through experimental tests. The results show that 114.4% is the ideal liquid-loading rate for the heat pipe system. Under working conditions in Xi'an, the annual operating hours of the three modes accounted for 47.2%, 6.1%, and 46.7%. The hybrid cooling system may save 62.04% of the energy used annually compared to the standard cooling system and the cooling system in the server room thanks to its yearly average COP of 9.43. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized cooling system combined with a traditional computer room air conditioning unit and server-level heat pipe exchangers for thermal environment optimization in a data center. Two cooling strategies, with heat exchangers installed above and below the servers respectively, are proposed and compared with the original CRACs system in terms of thermal environment. The simulation results of the original data center model are in good agreement with the on-site measurement results, and thus its reliability can be validated. The results show that a decentralized cooling system can effectively improve the thermal environment in data centers. To obtain the highest cooling efficiency, altogether 18 cases, where heat pipe exchangers were installed at different locations and heights, are analyzed and compared. The results show that the thermal environment is optimal when heat pipe exchangers are installed 0.01 m below each server. The local hotspot temperature is reduced by 6.8 °C, and the temperature distribution of the rack is the most uniform, which can effectively reduce the heat accumulation in data centers. [ABSTRACT FROM AUTHOR]

This paper presents a new type of split natural cooling system that maximizes the use of natural cold energy to significantly reduce the power consumption of the air conditioning system in data centers. A split natural cooling system module, which consisted of indoor and outdoor heat exchanger based on micro heat pipe arrays connected by liquid circulation system, was selected for experimental research. The heat transfer process and flow characteristics were analyzed under different outdoor environment temperatures, air and water flow rates, and different ratios of heat transfer components (N) of indoor and outdoor heat exchangers. To improve the utilization of natural cold energy, two kinds of heat dissipation conditions, namely room and heat channel-based, were proposed. The indoor temperature of two conditions at 28 °C and 38 °C were simulated in the laboratory at constant temperature-humidity, respectively. Results indicated that the air flow rate had a greater influence on the heat transfer performance than the water flow rate. The pressure drop of the air and water sides was at a lower level, and the fitting curve of the pressure drop was obtained to provide a reference for the heat exchanger design and equipment selection. When the ratio of heat transfer components (N) of the indoor and outdoor heat exchanger was approximately 0.75, the split natural cooling system showed optimal comprehensive performance. Under heat channel-based conditions, the maximum heat transfer rate reached 12.4 kW, and the maximum energy efficiency ratio was 17.15; the maximum heat transfer rate and the maximum energy efficiency ratio increased by 42.5% and 22.64% compared with the room-based condition, respectively. The fitting curve of the energy efficiency ratio was calculated under different outdoor temperatures at two heat dissipation conditions. [ABSTRACT FROM AUTHOR]

This paper deals with the experimental research and verification of a passive cooling system operating on the principle of a loop gravity heat pipe designed for cooling electrical cabinets. This type of cooling works automatically by changing the state of the working substance and thus saves energy consumption. Since the designed cooling system ensures heat transfer from the interior cabinet to the outdoor space, where the heat can naturally dissipate to the surroundings, it is dustproof. The heat pipe consists of an innovative evaporator concept designed to minimize liquid and vapour phase interference in the refrigeration circuit. The aim of the research was to experimentally determine the limit performance parameters of the refrigeration system for different volumes of working medium in the evaporator and decrease heat loss in the cabinet interior. The designed device was verified experimentally and by mathematical calculations as well. The greatest benefit of the work is that the cooling device was able to ensure temperature conditions inside the electrical enclosure at a heat load of 2000 W under 60 °C, 1500 W under 55 °C, 1000 W under 50 °C, 750 W under 45 °C and 500 W under 40 °C. [ABSTRACT FROM AUTHOR]

Heat switches are devices for controlling heat flow in various applications, such as electronic devices, cryogenic cooling systems, spacecraft, and rockets. These devices require non-linear transient thermal simulations, in which there is a lack of information. In this study, we introduce an innovative 1D thermo-hydraulic lumped parameter model to simulate loop heat pipes as heat switches by regulating the temperature difference between the evaporator and the compensation chamber. The developed thermo-hydraulic model uses the continuity, energy, and momentum equations to represent the behaviour of loop heat pipes as heat switches. The model also highlights the importance of some thermal conductance parameters and correction coefficients for accurately simulating the different operational states of a loop heat pipe. The simulations are conducted using the proposed 1D model, solved through the application of the Mathcad block function. The numerical model presented is successfully validated by comparing the temperatures of the evaporator and condenser inlet nodes with those of a referenced loop heat pipe from the literature. In conclusion, in this research, the mathematical modelling of loop heat pipes as heat switches is presented. This is achieved by incorporating correction coefficients with Boolean logic that results in non-linear transient simulations. The presented 1D thermo-hydraulic lumped parameter model serves as a valuable tool for thermal system design, particularly for systems with non-linear operational modes like sorption compressors. The graphical and nodal representation of this proposed 1D thermo-hydraulic model further enhances its utility in understanding and optimising loop heat pipes as heat switches across various thermal management scenarios. [ABSTRACT FROM AUTHOR]

This paper presents the concept of a hybrid thermal management system (TMS) including natural convection, heat pipe, and air cooling assisted heat pipe (ACAH) for electric vehicles. Experimental and numerical tests are described to predict the thermal behavior of a lithium titanate oxide (LTO) battery cell in a fast discharging process (8C rate). Specifications of different cooling techniques are deliberated and compared. The mathematical models are solved by COMSOL Multiphysics® (Stockholm, Sweden), the commercial computational fluid dynamics (CFD) software. The simulation results are validated against experimental data with an acceptable error range. The results specify that the maximum cell temperatures for the cooling systems of natural convection, heat pipe, and ACAH reach 56, 46.3, and 38.3 °C, respectively. We found that the maximum cell temperature experiences a 17.3% and 31% reduction with the heat pipe and ACAH, respectively, compared with natural convection. [ABSTRACT FROM AUTHOR]

The expansion device is the critical component of micro-to-small scale organic Rankine cycle (ORC) systems, substantially affecting system efficiency and cost. Low isentropic efficiency and lubrication requirements are the main issues associated with using volumetric expanders in ORC systems. Despite lubrication contributing to reducing internal leakages in an expander, it may compromise the performance of the ORC system by adversely affecting the evaporator's thermal capacity. This study tests a recently developed and modified revolving vane expander (M-RVE) in a micro-scale ORC test rig by implementing an adjustable oil mass flow rate. The impact of the lubricant oil on the performance of the M-RVE prototype is investigated within a wide range of oil circulation rates (OCR). The results demonstrate a negligible improvement in the filling factor for OCRs higher than 1%. Moreover, the shaft power is not considerably sensitive to OCR, while the calculated isentropic efficiency of the expander improves with OCR. Furthermore, the impact of the lubricant oil on the performance of the evaporator is studied, assuming the exact OCR as the expander and measured temperature and pressure similar to the pure refrigerant for the lubricant-refrigerant mixture in the evaporator. The study shows that the evaporator capacity is penalized with OCR, especially for values higher than 1%. Hence, an OCR of about 1% is a good compromise, and it can be used as a guideline for designing revolving vane expanders for micro-scale ORC systems without a dedicated lubricant oil circuit. [ABSTRACT FROM AUTHOR]

New guidelines set by international organizations for refrigeration companies cause that natural working fluids such as carbon dioxide are increasingly used in new refrigeration systems. Carbon dioxide (R-744) is used in freezing, cooling, or air conditioning installations, in which the cooling load fluctuates hourly. To adapt the cooling capacity of the evaporator to the current cooling load of the cooled space, a number of control elements are used. The paper proposes a new method of regulating the cooling capacity for a one-stage refrigeration cycle with the R-744 refrigerant and an internal heat exchanger (IHX). The proposed method involves using an additional evaporator and combines the possibility of regulating the cooling capacity with the possibility of energy efficiency ratio (EER) improvement. The energy analysis of the proposed method of regulating the cooling capacity was performed and the results were compared with the control method. The control method was using the compressor hot gas bypass valve which allows the flow of hot vapor refrigerant to the suction side. The energy analysis was carried out for both subcritical and supercritical cycles using the energy equations. For each of the considered methods, the characteristics of the change in the EER as a function of the reduction of the cooling capacity in both supercritical and subcritical cycles were determined. It was found that when the cooling capacity decreased by 50%, the hot gas bypass regulating method was around 30% less efficient compared to the proposed additional evaporator regulating method. [ABSTRACT FROM AUTHOR]

The heat generation of lithium ion batteries in electric vehicles (EVs) leads to a degradation of energy capacity and lifetime. To solve this problem, a new cooling concept using an oscillating heat pipe (OHP) is proposed. In the present study, an OHP has been adopted for Li-ion battery cooling. Due to the limited space in EVs, the cooling channel is installed on the bottom of the battery module. In the bottom cooling method with an OHP, generated heat can be dissipated easily and conveniently. However, most studies on heat pipes have used bottom heating and top or side cooling methods, so we investigate the various effects of parameters with a top heating/bottom cooling mode with the OHP, i.e., the inclination angle of the system, amount of working fluid charged, the heating amount, and the cold plate temperature with ethanol as a working fluid. The experimental results show that the thermal resistance (0.6 °C/W) and uneven pulsating features influence the heat transfer performance. A heater used as a simulated battery was sustained under 60 °C under 10 W and 14 W heating conditions. This indicates that the proposed cooling system with the bottom cooling is feasible for use as an EV's battery cooling system. [ABSTRACT FROM AUTHOR]

Automatic proportional integral derivative control techniques are applied in a single-stage solar absorption cooling system, showing 3.8 kW (~1 ton) cooling capacity, with a coefficient of performance of 0.6 and −4.1 °C evaporator cooling temperature. It is built with plate heat exchangers as main components, using ammonia–water as the working mixture fluid and solar collectors as the main source of hot water. Control tuning was verified with a dynamical simulation model for a solution regarding mass flow stability and temperature control in the solar absorption cooling system. The controller improved steady thermodynamic state and time response. According to experimental cooling temperatures, the system could work in ranges of refrigeration or air-conditioning end-uses, whose operation makes this control technique an attractive option to be implemented in the solar absorption cooling system. [ABSTRACT FROM AUTHOR]

The operating limits of oscillating heat pipes (OHP) are crucial for the optimal design of cooling systems. In particular, the dryout limit is a key factor in optimizing the functionality of an OHP. As shown in previous studies, experimental approaches to determine the dryout limit lead to contradictory results. This work proposes a compact theory to predict a dryout threshold that unifies the experimental and analytical data. The theory is based on the influence of vapor quality on the flow pattern. When the vapor quality exceeds a certain limit (x = 0.006), the flow pattern changes from slug flow to annular flow and the heat transfer decreases abruptly. The results indicate a uniform threshold value, which has been validated experimentally and by the literature. With that approach, it becomes possible to design an OHP with an optimized filling ratio and, hence, substantially improve its cooling abilities. [ABSTRACT FROM AUTHOR]