There is a growing interest in the supercritical CO2 Brayton cycle (SCO2BC) as a high-efficiency power generation technology across various energy sectors, including renewable energy. SCO2BC dynamic modelling is key to grasp its performance, especially under off-design conditions such as fluctuating operating load. Numerous studies that compare the steady-state behavior of different SCO2BC layouts are present. However, existing literature lacks in-depth analyses of transient responses of different SCO2BC layouts with accuracy-improved turbomachinery performance-map. This research fills this gap by comparing four layouts: recuperated (RCU), recompression (RCOMP), reheat (RHT), intercool (INC), through dynamic simulations with accuracy-improved turbomachinery models. The transient responses of the key SCO2BC cycle performance parameters are deeply investigated, offering novel insights into their transient behavior. An additional novelty of this work is the usage of corrected turbomachinery performance maps with improved prediction accuracy for the dynamic comparison. The study reveals that mass flow rate and compressor inlet conditions are most affected by a cooling water temperature disturbance (ColdT), while turbine inlet state is more sensitive to a heater temperature change (HotT). For all the studied layouts, net power responses are primarily driven by mass flow rate and turbine inlet temperature (TIT) for ColdT and HotT disturbances, respectively. Layout configurations (number and arrangement of turbomachines) influence transient response sensitivity. RCU and RHT layouts exhibit larger net power fluctuations for ColdT, while RCU and INC layouts show larger responses for HotT. The RCOMP is generally the most stable layout, while RHT and INC demonstrate conditional stability depending on the disturbance.

The domestic hot water air-source heat pump has gained great attention recently due to global efforts to decarbonize the energy sector. Its coefficient of performance (COP) falls within the range of 3 to 4, which contributes to significant energy consumption and increased carbon emissions. Addressing these issues, the present study proposes an effective integration between a dual-condenser vapor compression cycle and an ejector to overcome these shortcomings. In the proposed vapor compression-ejector heat pump (VCE-HP), the compressor's discharge pressure is further exploited to lift extra thermal energy from the ambient and bring it to the low-condenser temperature for water heating. A flow control valve is installed to regulate the amount of the refrigerant flow rate used to power the ejector and the condensers' heating capacity to be able to achieve the targeted hot water temperature and flow rate over a wide range of operations. An exergoeconomic model estimating the COP, exergetic efficiency (eta II), specific cost of hot water (SCHW), and product cost (Cproduct) is built to assess the techno-economic feasibility of the proposed VCE-HP. In addition, the performance of the VCE-HP using low global warming potential (GWP) refrigerants (R1234yf, R290, R1234ze(E), and R717) is assessed and compared with that using R134a. It has been determined that an increase in the ejector entrainment ratio does not inherently translate into an improvement in the COP or a reduction in system cost. Results indicate that the performance of the VCE-HP is always higher than that of traditional heat pumps by 20-50% based on the type of refrigerant. Results conclude that R1234ze(E) can achieve a COP of 5.38, making it a potential alternative to R134a due to its low GWP and zero ozone depletion potential.

Ramy H. Mohammed, Rohit Bhagwat, Michael Schmid, Paul Glanville, and Saeed Moghaddam

The present study proposes an effective integration between a dual-condenser vapor compression cycle and an ejector to deliver hot water at high performance and with less carbon emissions. In the proposed vapor compression- ejector heat pump (VCE-HP), the compressor's discharge pressure is further exploited to lift extra thermal energy from the ambient and bring it to the low-condenser temperature for water heating. A flow control valve is installed to regulate the amount of the refrigerant flow rate used to power the ejector and the condensers' heating capacity to achieve the targeted hot water temperature and flow rate over a wide range of operations. Thermodynamics model estimating the coefficient of performance (COP) and exergetic efficiency (.II). In addition, the performance of the VCE-HP using low Global Warming Potential (GWP) refrigerants (R1234yf, R290, R1234ze(E), and R717) is assessed and compared with that using R134a. It is observed that achieving a higher ejector entrainment ratio does not guarantee a higher COP and lower cost.

Wicking structures have been widely used within passive heat transfer devices with high heat fluxes, such as heat pipes, to enhance their thermal performance. While wicking structures promote capillary pumping of the working fluid and thin film evaporation, they can result in capillary evaporation and further enhance the evaporation heat transfer. In this study, a 0.5 mm thick layer of 105 mu m sintered copper particles was added to the inner wall of a copper tube, aiming to form an "annular flow" and enhance the heat transfer characteristics by taking advantage of thin film and capillary evaporation. Acetone was chosen as the working fluid, and the performance of an evaporation tube was tested for power inputs of 10, 30, 50, and 70 W. For each power input, trials were run at inclination angles varying from -90 deg to 90 deg to investigate the capillary effects. The temperature measurements showed that the temperature distribution along the evaporation tube is always downward sloping, meaning the temperature at the fluid inlet is larger than the outlet. Results show that an "annular flow" formed by a thin layer of sintered particles can promote thin film and capillary evaporation and, therefore, boost the evaporation heat transfer coefficient.

Oscillating heat pipes are heat transfer devices with the potential of addressing some of the most pressing current thermal management problems, from the miniaturization of microchips to the development of hypersonic vehicles. Since their invention in the 1990s, numerous studies have attempted to develop predictive and inverse design models for oscillating heat pipe function. However, the field still lacks robust and flexible models that can be used to prescribe design specifications based on a target performance. The fundamental difficulty lies in the fact that, despite the simplicity of their design, the mechanisms behind the operation of oscillating heat pipes are complex and only partially understood. To circumvent this limitation, over the last several years, there has been increasing interest in the application of machine learning techniques to oscillating heat pipe modeling. Our survey of the literature has revealed that machine learning techniques have successfully been used to predict different aspects of the operation of these devices. However, many fundamental questions such as which machine learning models are better suited for this task or whether their results can extrapolate to different experimental setups remain unanswered. Moreover, the wealth of knowledge that the field has produced regarding the physical phenomena behind oscillating heat pipes is still to be leveraged by machine learning techniques. Herein, we discuss these applications in detail, emphasizing their advantages, limitations, as well as potential paths forward.

The supercritical CO2 Brayton cycle (SCO2BC) has emerged as a promising next-generation power generation technology due to its potential for high thermal efficiency and compact components design. Dynamic modeling of SCO2BC is crucial for understanding and analyzing its performance under design and off-design conditions. Turbomachines are critical components in SCO2BC dynamic models. While turbomachinery components are often modeled using their design-point turbomachinery performance maps (TPM), these maps become less accurate during off-design operations. Despite the existence of various TPM correction methods that ensure model validity, the implementation of the accurate ones within dynamic SCO2BC models remains scarce in the literature. This is crucial as it potentially compromises the accuracy of the obtained results. Therefore, there is a need to investigate the errors in the existing literature TPM correction models (in dynamic SCO2BC simulation) by comparing them against dynamic SCO2BC models employing a highly accurate correction method. Thus, in the current work, the common TPM correction methods from the literature are implemented in SCO2BC dynamic models and compared against a baseline mode, which uses the highly-accurate Pham method (at both the component and cycle levels). To the authors' knowledge, this is the first work to address this research gap for the SCO2BC dynamic simulations and on both component and cycle levels. Additionally, another novelty is the usage of Simcenter Amesim software to dynamically model the SCO2BC aided with Pham model. Multible dynamic models for both the turbomachines of SCO2BC and the whole cycle are constructed and equipped with the different TPM correction methods found in literature to compare their dynamic behaviour that of Pham model. The Ideal Gas Compressibility factor (IGZ) method demonstrates a better performance than the other tested methods, as it exhibits the lowest discrepancy compared to the Pham model. Many of the other tested methods show significant errors. Furthermore, a popular hybrid correction combining IGZ and Pham (IGZ-Ph) is also investigated. Surprisingly, while seemingly beneficial, this hybrid approach negatively impacts accuracy, even resulting in predictions as if no correction was applied.

Water heating is a major source of energy consumption in the U.S. residential sector. Heat pumps can significantly increase the energy efficiency of water heating. An ejector heat pump (EHP) is a novel, thermally driven heat pump that uses an ejector as a thermocompressor. Choosing suitable working fluids is critical in developing high-performance EHPs. Therefore, this research screens binary fluid pairs (BFPs) for EHPs to produce domestic hot water at a high coefficient of performance (COP). The criteria for screening BFP candidates for EHP water heaters (EHPWHs) are established, and BFP candidates are shortlisted. This study identifies HFE7000, Novec649, HFE7100, HFE7200, and HFE7500 for the primary fluids and RE170, R600a, R600, and R1234ze(Z) for the secondary fluids. The thermodynamic model is employed to investigate the performance of EHPWHs using the shortlisted BFPs under various operating parameters, including the evaporation pressure of the primary working fluid in the high-temperature evaporator and the condensation temperature. The highest heating-cycle COP of 1.328 is achieved by an EHPWH operating with HFE7000/R600 at a condenser temperature of 50(degrees)C and a pressure of 1.69 MPa in the high-temperature evaporator.

Decarbonization and electrification require advancing air source heat pumps for space cooling. Conventional air conditioning systems have a cooling coil and a reheater, which simultaneously handle the latent and sensible loads. This approach results in low energy efficiency, particularly for spaces with small sensible heat ratios where supply air should be reheated. Among several developed systems, separate sensible and latent cooling technology has been proposed to solve these issues by decoupling the latent and sensible loads. The present work proposes novel SSLC systems in which a compressor-ejector operating heat pump with two evaporators dissociates sensible and latent load. Moreover, two psychrometric air treatment arrangements are proposed and evaluated. A thermodynamic model is developed for the proposed compressor-ejector operating heat pump and air treatment arrangements to evaluate the energy efficiency of the proposed separate sensible and latent cooling systems compared to the conventional air conditioning system under the same operating conditions. Results indicate that the proposed separate sensible and latent cooling systems outperform the conventional system in terms of coefficient of performance when the room sensible heat ratio is less than 0.74. A 72% improvement in the coefficient of performance is estimated when the room sensible heat ratio is 0.6. Therefore, the proposed separate sensible and latent cooling systems would bring a significant reduction in operating costs, making them a good alternative to the conventional air conditioning system.

An oscillating heat pipe (OHP) is a special kind of heat pipe in which the working fluid experiences an oscillatory motion without the need for wick structures or external electrical power input beyond a driving temperature difference. In contrast to traditional heat pipes and thermosyphons, which rely on capillarity or gravitation, OHPs operate based on pressure difference which causes oscillating motion. This oscillation is very important since it is the main reason behind the higher heat flux acquisition capability that OHPs exhibit with respect to other types of heat pipes. However, this oscillation is nondeterministic and thus difficult to model, which hinders the ability to control and design OHPs. Since the invention of OHPs in the early 1990s, many researchers have tried to analyze and predict the oscillating motions in OHPs under different working conditions to enhance their performance and reliability to make them suitable for industrial applications. This review presents the evolution of OHP modeling, as well as mathematical approaches to the analysis of experimental data obtained from OHPs. Furthermore, the machine learning (ML) models applied on OHPs are reviewed.

Due to its numerous advantages, including the capability to achieve high thermal efficiencies and compactness, the supercritical carbon dioxide Brayton cycle (SCO2BC) has recently been considered as an attractive next-generation heat-to-power conversion cycle. Dynamic modelling of the SCO2BC is essential to simply grasp and analyze its performance in either design or off-design conditions. Turbomachines are key components in the SCO2BC, which are typically modelled by adopting their design-point performance map. However, during off-design operations, the performance maps become inaccurate, and correction relations must be integrated into these models to enhance their validity. Despite the development of multiple performance map correction, reduction, and normalization methods, there is a scarce literature that implemented highly accurate correction relations in SCO2BC transient models, which impacts the validity of the obtained results. Therefore, there is a need to assess the accuracy of these results compared to the results of a SCO2BC model adopting an accurate correction method. However, some information needs to be provided first, which include how turbomachinery correction impacts the dynamic behavior of SCO2BC compared to using uncorrected turbomachinery models, and if the correction of one turbomachine (i.e., the compressor) is more significant than the other. This work aims to provide this data and more by developing dynamic SCO2BC models (CPP models), comprehensively evaluating their transient performance, and comparing them to SCO2BCs with uncorrected turbomachinery models (FPP models). This work possesses novelties of utilizing the most accurate turbomachinery correction model found for SCO2BC and building the model in Simcenter Amesim. It is found that FPP models may be viable when the dynamic responses of the components near the fluctuation source are desired. Otherwise, at least a compressor model correction is required to obtain reliable results. Moreover, all turbomachinery models should be corrected for accurate mass flow rate and power responses.

This research work discusses how the thermodynamic properties of refrigerant constrain the upper limit of the coefficient of performance (COP) of adsorption cooling (AC) cycle. A mathematical model is developed based on the mass and energy balance across the adsorption bed and used to estimate the maximum COP. Langmuir model is used to derive the ideal COP. Besides, Carnot COP and exergy efficiency are derived to evaluate the adsorption cooling cycle further. These new parameters are applied to the available cycles in the open literature to assess how much room for improvement is still left in the adsorption technology based on the refrigerant type. Water, ammonia, methanol, ethanol, Butane, R134a, R32, CO2 and R1234ze are used in this study. The maximum cooling COP cannot exceed 1.0 for cooling applications and is always less than 2.0 for heat pumps. Results show that AC cycles using water or ethanol as refrigerants achieve the highest exergy efficiency. The AC cycle using water as an adsorbate and silica gel as an adsorbent has the highest relative COP, about 80 % at 85 degrees C regeneration temperature. In turn, CO2 exhibits the lowest one. This work is merely an attempt to save experimental effort and time by identifying the most suitable refrigerant based on its physical and thermal properties and the cycle operating conditions.

Cooling and desalination cycles are energy-intensive with low thermal efficiency due to waste heat. Integrating these cycles and recovering waste heat can achieve energy saving and increase their efficiency. This work proposes an effective integration between a water-heated humidification-dehumidification desalination cycle and an open-loop ejector cycle to produce freshwater and cooling energy simultaneously to serve a certain number of inhabitants. After heating the seawater, a part of it is used for the humidification process, while the other portion is further heated to power the open-loop ejector cycle. The proposed hybrid plant is equipped with a three-way valve to match the cooling energy with the amount of freshwater required for a certain number of persons. A mathematical model is built based on energy, exergy, and cost balance across each component to assess the plant's performance at different operating conditions. Energy utilization factor (EUF), amount of water production rate (WPR), total water price (TWP), and product cost rate are used as evaluation parameters. Additionally, a sensitivity analysis is carried out to estimate the variation in the system performance due to the uncertainty of operating conditions. At the optimal seawater-to-air mass flow rate ratio, the proposed plant could produce freshwater of 1800 Gal/day at an EUF of 1.75 and a TWP of 1.13 $/m3 when the maximum seawater temperature in the HDH cycle is 60 degrees C and the evaporator temperature is 8 degrees C. This performance is much better than previously published works, and could be further improved by increasing the maximum seawater temperature.

Ejector heat pump water heaters (EHPWHs) could significantly increase the thermal efficiency of domestic water heating and reduce greenhouse gas emissions. This study addresses two major technical barriers to using EHPWHs in domestic water heating-low heating cycle coefficient of performance (COP) and low condensation temperatures-using binary fluid pairs as a working fluid in a two-stage ejector system. A comprehensive, geometry-free model of binary-fluid ejectors was built and validated to predict the entrainment ratios of binaryfluid ejectors. A thermodynamic model of a two-stage binary-fluid ejector EHPWH was built to predict the heating cycle COP of EHPWHs. The performance of the EHPWH was theoretically evaluated using HFE7000 and Novec649 as primary fluids and R600 and R1234ze(Z) as secondary fluids. HFE7000/R600 gave the highest heating cycle COP (i.e., 1.356) in producing domestic hot water at 60.0 ?. An optimum evaporation temperature of the primary fluid was identified for the maximum effective entrainment ratio and effective pressure lift ratio of two-stage ejectors. The effective entrainment ratio of the two-stage ejector dominated the heating cycle COP of the EHPWH. Primary fluids with lower latent heats of evaporation and/or secondary fluids with higher latent heats of evaporation yielded higher heating cycle COPs of EHPWHs.

The scarcity of clean water and the lack of sustainable cooling systems are continuously pressing. Although many technologies are well-established, such as vapour compression for refrigeration and reverse osmosis for desalination, they are energy-intensive and conventional refrigeration technology utilising working fluids of long-lasting ozone-depleting and greenhouse effects. Alternatively, adsorption and absorption technologies can meet such demands, and they are the most feasible to utilise the waste and renewable heat abundant in many locations. Therefore, this paper computationally studies the emerging integrated adsorption-absorption system for cooling cum desalination employing transient waste heat sources of various waveform characteristics. A previously validated computational model for the adsorption subsystem was coupled with a thermodynamic model for the absorption subsystem and experimental heat profiles obtained from an internal combustion engine. The energy and exergy analysis of the integrated system utilising the actual heat source from an internal combustion engine and predefined waveforms were undertaken and benchmarked against that operated under steady heat sources. The integrated system operated with a relatively low exergy efficiency in the absorption cycle of up to 15.33%. The adsorption bottoming cycle successfully utilised the heat from the absorption subsystem at a relatively higher exergy efficiency of up to 42.69%. The execution of a transient heat source of sinusoidal waveform enhanced the water production by up to 30% and the cooling of absorption and adsorption subsystems by 24% and 15%, respectively. However, admitting realistic waveforms of an internal combustion engine showed marginal differences compared to the steady heat sources owing to their high frequencies and small amplitudes.

Producing cooling energy and freshwater using environmentally friendly systems is inevitable to achieve the ambitious goals of the federal sustainability plan, which aims to reach zero emissions in buildings by 2050. Among several cooling and desalination technologies, ejector refrigeration cycle (ERC) and humidification-dehumidification (HDH) desalination unit have good potential to deliver cooling energy and freshwater, respectively, at low cost using low-grade thermal energy. In this paper, we propose an integration between a water-heated humidification-dehumidification (HDH) desalination cycle and an ejector cooling cycle to produce freshwater and cooling energy simultaneously. The seawater is heated to drive the HDH cycle, while a portion of it is evaporated and used as motive steam for the ejector cycle. The ejector cycle is designed to produce cooling power at a temperature range from 5 to 15 degrees C for cooling building applications and freshwater as well. Therefore, both cycles can produce freshwater. The proposed cycle is equipped with a three-way valve to match the cooling power and freshwater to serve a certain number of persons. A thermodynamic model is built and validated to study the performance of the proposed cycle at different operating parameters. It is found that identifying the optimal seawater-to-air mass flow rate ratio is critical to achieve maximum energy efficiency. This value is found to be dependent on the operating conditions. The results provide guidelines for designing and building an efficient HDH/ejector cycle that could be powered by solar energy or waste heat.

Ejector heat pump (EHP) is an efficient-energy technology with a promising potential to replace the vapor compression cycle in heating and cooling applications. It can be powered by low-grade waste heat and uses environmentally friendly working fluids. EHP has also become a viable solution in research seeking cooling applications. However, little attention has been paid to using EHP for heating purposes. In order to investigate this, a steam EHP for domestic water heating was designed and built. The coefficient of performance (COP) is evaluated at various operating and design conditions in sub-critical operational modes to achieve higher condensation temperatures. Two primary nozzles with a throat diameter of 1.5 mm and 2.0 mm were investi-gated. The primary nozzle is movable along the ejector's axis, allowing investigation of its positional effects on the EHP's COP. Experimental measurements revealed that using a smaller throat diameter results in a high COP and low back pressure. The EHP COP and back pressure increase when the LTE temperature increases. Using a throat diameter of 1.5 mm, the EHP COP increases as the nozzle exit position (NXP) becomes closer to the constant area section. A COP of 2.42 and a back pressure of 4.28 kPa are achieved at a high-temperature evaporator (HTE) temperature of 130 degrees C and a low-temperature evaporator (LTE) temperature of 30 degrees C using a primary nozzle with 1.5 mm.

Buildings account for about 39% of the total energy consumption in the United States. Developing highly energy-efficient and environmentally friendly systems that are either actively or passively integrated into buildings plays a crucial role in decarbonizing the building sector. Among various technologies, desiccant-based energy systems have received particular attention in recent years due to their unique advantages, such as being thermally driven using low-grade waste or solar energies and being reliable over a wide range of operating ranges. Extensive research efforts have been exerted on desiccant-enabled technologies at both material and system levels aiming to increase their performance and achieve high technological readiness levels. The present review paper comprehensively discusses research works made at the system level. It overviews desiccant-based air conditioning systems, desiccant-based humidity pumps, desiccant-based thermal energy storage systems, and desiccant-based appliances. The study identifies challenges and opportunities to accelerate the commercialization of desiccant-enabled technologies. It is found that desiccant materials offer great promise to improve energy efficiency and functionality of future buildings through decoupling the latent and sensible cooling loads in air conditioning systems, humidity pumps integrated into building facades, and next-generation appliances. However, commercial viability and widespread acceptance of desiccant-based systems have been hampered by several major obstacles, including liquid and air flow mal-distribution and inferior thermo-physical properties of desiccant materials resulting in low ab/adsorption and regeneration rates and bulky/costly systems.

The adsorption desalination cycle is a promising technique to address water scarcity. However, its production rate is low due to the low capacity of adsorbent materials. In this study, four metal organic frameworks namely aluminum fumarate, MOF-801, MIL-100(Fe), and CAU-10 H are synthesized and characterized and are tested for an application in adsorption desalination. After synthesizing the adsorbents, powder x-ray diffraction patterns for the samples are obtained to ensure a successful synthesis. The samples then undergo thermogravimetric water adsorption measurements at three different temperatures of 30 degrees C, 50 degrees C, and 70 degrees C at varying evaporator pressures. The water adsorption isotherms are also correlated with the Sun-Chakraborty adsorption isotherm model. It is found that the maximum water uptakes of aluminum fumarate, MOF-801, MIL-100(Fe), and CAU-10 H are 0.47, 0.35, 0.72, and 0.31 kg of water/kg of adsorbent, respectively. The highest water uptake of MIL-100(Fe) is due to its largest BET surface area and pore volume of 1634 m2/g and 0.47 m3/g, respectively. Consequently, MIL-100(Fe) provides the highest specific daily water production for adsorption desalination. Adsorption desalination cycles using MIL-100(Fe) could produce 2.2 and 2.3 times the freshwater obtained employing silica gel and CAU-10 H, respectively, with a heat recovery loop between the evaporator and condenser sections.