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• Developed a nonlinear model order reduction framework to generate reduced order heat exchanger models. • Applied the proposed model order reduction scheme to a complete VCC system. • Validated and compared with a high-fidelity FV model and experimental measurements. • Reduced order VCC model leads to an 80% computation time reduction compared to a standard FV approach. A computationally efficient but accurate dynamic modeling approach for vapor compression systems is important for many applications. Nonlinear model order reduction techniques which generate reduced order models based on high fidelity vapor compression cycle (VCC) models are attractive for the purposes. In this paper, a number of technical challenges of applying model order reduction methods to VCCs are described and corresponding solution approaches are presented. It starts with a reformulation of a standard finite volume heat exchanger model for matching the baseline model reduction structure. Reduced order models for evaporator and condenser are constructed from numerical snapshots of the high fidelity models using Proper Orthogonal Decomposition (POD). Methodologies for system stability and numerical efficiency of POD reduced order models are described. The reduced order heat exchanger models are then coupled with quasi-static models of other components to form a reduced order cycle model. Transient simulations were conducted over a wide range of operating conditions and results were compared with the full order model as well as measurements. The validation results indicate that the reduced order model can execute much faster than a high-fidelity finite volume model with negligible prediction errors. [ABSTRACT FROM AUTHOR]

Condensation dehumidification in conventional air conditioning technologies is energy-intensive, accounting for up to 50% of building cooling energy used in some climates. Selective vacuum membrane dehumidification (VMD) is one of the leading alternative dehumidification technologies due to its potential for significant energy savings, and the "dual-module humidity pump" is one of the most promising VMD concepts. This work is the first to provide experimental proof-of-concept for the dual-module humidity pump system and provides the first thermodynamic modeling framework that accounts for realistic steady-state operating limitations, both of which are lacking in the current literature. Additionally, this work is the first to provide a system design solution that overcomes practical challenges associated with air accumulation in the vacuum channels. The experimental results in this work show that the current prototype can remove up to 45% of the water vapor in the air stream, and the vapor pressure difference in the vapor rejection module needs to be approximately 2–4 times greater than that of the dehumidification module in order to maintain balanced mass transfer. The thermodynamic model applied to typical air conditioning conditions shows that the ideal dehumidification (latent) COPs can reach up to 40, but practical COPs are limited to approximately 10. Furthermore, the model shows that the overall energy efficiency increases as the membrane air selectivity increases, though this improvement gradually starts to diminish when the membrane selectivity is increased above 10,000. [Display omitted] • First experimental demonstration of the efficient dual-module humidity pump. • Benchtop prototype can remove up to 45% of the humidity in the air flow. • Thermodynamic model that accounts for realistic inefficiencies is presented. • Ideal latent COP can reach up to 40, practical COP limited to approximately 10. • Membrane selectivity beyond 10,000 yields diminishing efficiency enhancements [ABSTRACT FROM AUTHOR]

Selective sensing of trace hydrogen gas is highly desirable in leak detection applications, and it has recently come to the fore to an even larger degree due to the great potential of hydrogen in the current energy transition. One difficulty of room-temperature hydrogen sensing in an open environment is the potential interference from multiple other analytes, including water vapor, especially in high-humidity environments. To develop a high-responsivity, selective hydrogen gas sensor, ultrathin palladium nanosheets (PdNS) were synthesized using a hydrothermal method. Then, the PdNS were utilized as the chemical recognition layers in gravimetric resonant gas sensors for the real-time detection of hydrogen at room temperature. The nanoscale structure of the PdNS provides the material with a high amount of functional surface area; in turn, this allowed for significant surface-analyte interactions and high-performance sensing. Importantly, the PdNS active layer can selectively sense hydrogen gas in the presence of multiple interfering gases (e.g., acetone, alcohols, and water vapor) in the concentration range of 1–5% in both nitrogen and air environments. Therefore, this sensor system may potentially be of interest for future industrial applications regarding the safe handling of hydrogen gas. [ABSTRACT FROM AUTHOR]

Previous mode switching algorithms for heat exchanger moving boundary models in the literature are composed of a set of IF-THEN rules. These representations could lead to numerical challenges due to the inherited discontinuities associated with IF-THEN rules. This paper presents an alternative mode switching methodology which results in a continuous moving boundary heat exchanger model over all possible mode changes. Numerical performance of the proposed method for a heat exchanger has been tested using simulations and sample results are compared with a moving boundary model with switching based on IF-THEN rules and also a finite-volume method. The proposed switching algorithm was implemented within a complete vapor compression cycle model and results were compared with experimental data for a start-up transient. [ABSTRACT FROM AUTHOR]

In comparison with other types of conventional positive displacement compressors for domestic refrigerators, oil-free linear compressors have a number of advantages due to their compactness, low friction, limited number of moving parts, and excellent performance. Limited work was found in the open literature related to comprehensive compressor modeling and model validation of commercially available linear compressors. A comprehensive and generalized simulation model of linear compressors has been recently presented by the same authors. The current paper expands on this work by using experimental data for the simulated compressors to characterize their performance over the operating envelopes and to validate the linear compressor model. The validated model was then used to quantify the major sources of losses and analyze system vibration as well as piston eccentricity, which ultimately could be used to design the next generation of linear compressors. [ABSTRACT FROM AUTHOR]

Building audits are conducted in many commercial buildings to identify opportunities to reduce energy costs and improve building operation. Because audits require significant effort by building engineers, they are usually only affordable for larger commercial buildings. "No-touch" building audit tools have thus been developed to identify potential savings based on a simplified analysis of building energy consumption patterns via high-level energy data such as monthly utility bills. This paper presents a comprehensive and standardized methodology to evaluate the accuracy of no-touch audit tools in detecting and diagnosing building energy problems and quantifying potential energy savings. The test suite is based on output data from a well-characterized set of building energy models, and the methodology is illustrated by applying it to a representative no-touch building audit tool. Results show that the tool estimates building energy end uses with reasonable accuracy but is less accurate in identifying probably causes of high energy. [ABSTRACT FROM AUTHOR]

Linear compressor technology is characterized by the absence of a crank mechanism, which has gained increasing attention in vapor compression systems due to its compactness and lower friction losses in comparison to conventional reciprocating compressors. Limited work was found in the open literature related to the development and in-depth validation of a comprehensive linear compressor dynamic model that couples thermodynamic aspects with both mechanical and electrical sub-models. This paper presents a comprehensive and generalized simulation model that is used to simulate the dynamic performance of a linear compressor. The model is based on mass and energy balance equations applied to open control volumes. The overall model is composed of several sub-models including the piston dynamics, electrical motor, valve dynamics, and leakage flows. The thermodynamic model and the sub-models are integrated into a compressor overall energy balance that describes the different heat flows and losses. The linear compressor model is able to predict both transient and steady-state behaviors of the piston movement, internal pressure and temperatures as well as the overall performance. Comparisons of predicted and measured mass flow rates as a function of operating frequency are also presented within this work. [ABSTRACT FROM AUTHOR]

In recent years, variable electricity pricing has become available to residential consumers to incentivize demand reductions during midday peak hours. Thermal energy storage (TES) systems enable consumers to store cooling energy when demand is low and assist A/C operation during peak demand periods. However, the cost savings achievable using TES are highly dependent on how the system is operated for a given utility rate structure. This study investigates control strategies for a packaged chiller unit integrated with ice storage that leverage available residential utility rate structures in the U.S. to reduce consumer electricity cost. The present work describes the development and evaluation of a generalized rule-based control strategy inspired by the performance of an optimal controller that minimizes monthly electricity cost considering both time-of-use energy and demand charges. The generalized rule-based controller is compared against the optimal controller as well as to heuristic control strategies for TES that were originally developed for commercial buildings for a range of equipment cooling capacities, TES sizes, geographic locations, and residential utility rates. The total electricity cost is determined using a simulation model that includes models for the chiller unit, ice storage tank, and secondary loop components, along with a building load model. Results show that the generalized rule-based controller can approximate the performance of the optimal controller within 20% for all cases tested, and within 10% of the optimal cost in 53% of the cases tested. The controller also performs significantly better than the heuristic strategies for commercial buildings that were evaluated. [ABSTRACT FROM AUTHOR]

Abstract The energy savings potential of passive chilled beams in various climatic zones is analyzed in this study. A passive chilled beam model, developed based on full-scale experiments, is used as a system module in a whole building simulation tool to account for the convective and radiative effects of passive chilled beams. The model was validated with measurements from a field study in an open plan office equipped with multiple passive chilled beams. In addition, a parallel field study in an adjacent identical office space equipped with an air (VAV) system was conducted to compare the resulting energy consumption with the two systems. To further study the energy savings potential and indoor thermal conditions with passive chilled beams, four different chilled beam system configurations, combined with a parallel variable air volume system, were studied using the whole building model in different US climatic zones. The results showed that using a dedicated outdoor air system, or a separate chiller for the passive chilled beams, or a desiccant wheel after the cooling coil may result in significant relative energy savings depending on the location. The last option proved to be the most efficient, providing up to 12% for hot and humid climates and up to 20% for hot and dry climates –compared to a conventional variable air volume system. Finally, the radiative-to-total cooling effect of passive chilled beams varied between 7% and 8% and has only a small impact on energy savings and indoor thermal conditions. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The author reflects on the trend and development of heating, ventilation, air-conditioning & refrigeration (HVAC&R) equipment in the U.S. The author cites the trend strated in 1980s wherein intelligent buildings are developed promising effective operational cost savings. The author explains that the said trend has encouraged manufacturing companies to design innovative and environmental friendly HVAC&R equipment systems for intelligent buildings.

It is important to have practical methods for constructing and learning a good mathematical model for a building’s thermal system in the presence of unmeasured disturbances and using data from closed loop operation. With this goal in mind, this paper presents a mathematical framework that explains the asymptotic behavior of an estimated model under those conditions and that can aid in learning an accurate model. Some analytic results from the literature of system identification are extended and interpreted for building systems. A new identification approach for determining an accurate thermal network (RC) model for a multi-zone building is developed based on the analytic result, and its superior performance over a conventional grey-box modeling approach is demonstrated experimentally. [ABSTRACT FROM AUTHOR]

Organic Rankine cycle with liquid-flooded expansion and internal regeneration (ORCLFE) has been demonstrated leading to improvements in system efficiency with respect to conventional ORC with internal regeneration in the heat input temperature range from 80 ∘ C to 200 ∘ C. From a previous study [1], it was concluded that the selection of the positive displacement expander, the working fluid and its interaction with the flooding medium, and the two separate heat inputs heavily impacted the exergetic conversion efficiency of the cycle, but they were not analyzed in depth. In this paper, a more detailed thermodynamic cycle model has been developed to include a physical-based expander model to evaluate the impact of built-in volume ratio, flooding ratio, mechanical losses and the solubility of the working fluid into the lubricant oil. As waste heat recovery from 100 ∘ C to 150 ∘ C is the main application of the ORCLFE under investigation, low-GWP refrigerants such as R1234ze(Z), R1233zd(E) and R1336mzz(Z) have been considered as potential replacements of R245fa. A single-screw expander, which has been previously characterized, has been employed to optimize the ORCLFE at three heat source temperatures (100 ∘ C, 120 ∘ C and 150 ∘ C) for each refrigerant, in order to obtain more realistic performance predictions. [ABSTRACT FROM AUTHOR]

Frost accumulation on evaporator coil surfaces can significantly degrade the performance of air-source heat pump (ASHP) systems in winter operations. The continued buildup of frost eventually necessitates a defrosting mode to remove the accumulated frost and return the system to its normal operating characteristics. A simulation tool capable of capturing the system dynamics with continuous mode-switching between heating and defrosting operation is extremely useful in the development and evaluation of improved control designs. However, first-principles simulation of a reverse-cycle defrost imposes a variety of numerical challenges including mode switching and flow reversal. This paper presents a comprehensive dynamic modeling framework that could overcome the challenges and experimental validations for ASHPs under cycling of frosting and defrosting operations. A complete first-principles ASHP cycle model is described where frost formation and melting models are integrated into a finite volume evaporator model to characterize the cycle behavior with non-uniform frost formation and melting. Comparisons of simulation results against experimental data collected from a residential heat pump unit indicate that the developed model is able to accurately capture the cycling behaviors of the ASHP system between heating and defrosting operations. In addition, no state event occurred during the simulation despite the presence of mode switching and flow reversal, strongly supporting the reliability of the proposed modeling approaches. • Developed dynamic heat pump models incorporating frost formation and melting. • Proposed robust formulations across component models. • Validated the developed heat pump model under cycling of frosting and defrosting. • The cycling dynamics are well captured with favorable numerical performance. [ABSTRACT FROM AUTHOR]

• Microgravity two-phase flow fundamentals directly compared with system level research from parabolic flights. • Condensation heat transfer was found to be reduced due to microgravity by fundamental studies and system level testing. • Effects of microgravity on friction and evaporation heat transfer coefficient very small for system level testing. • Decreased gravity dependence for higher flow velocities partially confirmed on system level through inclination testing. • Fundamental and system level research for microgravity two-phase flow should be closer aligned to increase mutual utility. Research on two-phase channel flow in microgravity has extensively investigated flow regimes, pressure drop, and heat transfer during evaporation and condensation processes over several decades. This literature has been primarily motivated by the goal of enabling future space applications for two-phase thermal management systems, among them vapor compression refrigeration cycles. However, relative to the number of two-phase channel flow experiments, research on vapor compression cycles in microgravity has been almost non-existent. This paper reviews two-phase flow research and compares key outcomes with those from system-level measurements obtained from recent testing of a vapor compression cycle at varying inclination angles and on parabolic flights. Previous two-phase channel flow experiments have resulted in several microgravity-specific flow regime maps and have also generally found reduced condensation heat transfer coefficients in microgravity as compared to normal gravity. The flow pattern map of Jayawardena et al. (1997) and generally reduced condensation heat transfer are confirmed by the vapor compression cycle experiments. Regarding evaporation heat transfer coefficients and friction factors, there is not a consensus in the literature on the effects of microgravity in two-phase channel flows. Similarly, the system-level experiments do not show repeatable increases or decreases for those parameters. Lastly, the dependence of system-level behavior on orientation with respect to gravity was tested by performing dynamic inclination experiments, where the inclination angle was changed every 2 min by a 90-degree increment. These tests reveal an increased relative dependence of the vapor compression cycle to orientation with decreasing mass fluxes, in line with two-phase channel flow research that has generally shown that increasing the flow inertia acts to mitigate various flow instabilities and dependence on orientation. However, inclination testing with longer locking times per angle showed a constant relative dependence of the vapor compression cycle to orientation changes. [ABSTRACT FROM AUTHOR]

• Systematic investigation of liquid flooding upon compressor start-up. • Characteristic flooding curve shown for 12 combinations of heat exchangers and refrigerants. • Correlation proposed capturing influence of heat exchanger size and refrigerant properties. Refrigeration cycles with fixed speed compressors cycle on and off to modulate their time averaged cooling capacity. While the cycles are turned off, refrigerant can migrate from the condenser into the colder evaporator. The next start-up process can then pull liquid refrigerant out of the evaporator and into the compressor causing liquid flooding. This phenomenon results in a loss of latent heat for cooling, can undercool the lubricant, and may cause damage to the compression mechanism. The geometric and thermophysical conditions that lead to liquid flooding are poorly understood. Experimental data collected for four refrigerants and three heat exchangers was analyzed for flooding conditions. A correlation was fitted to the experimental data, predicting the experimentally determined charge threshold for flooding with an average deviation of 6.6% and a maximum deviation of 16.7%. Repeating the tests with other heat exchanger types and smaller or larger compressors is suggested as important future work. [ABSTRACT FROM AUTHOR]

[Display omitted] • First experiments for simultaneous cooling and vacuum membrane dehumidification. • Cooling provides up to a 6% increase in prototype dehumidification performance. • Crossflow cooling tubes provide a 4–8% increase in dehumidification performance. • Straightforward framework for understanding practical latent COP limitations. • Development of a CFD model for air channel pressure drop estimates. Vacuum membrane air dehumidification has gained significant interest in recent years as a highly efficient means of air dehumidification. Prior theoretical modeling work by the authors introduced the Active Membrane Energy Exchanger (AMX) concept, which combines active heat exchange and vacuum membrane dehumidification into one non-isothermal system, and found that it could outperform other air conditioning technologies under many conditions. However, no experimental literature exists on combining active heat exchangers and vacuum membrane dehumidification. The goal of the present study is to evaluate the dehumidification (mass transfer) performance of the AMX concept relative to isothermal membrane dehumidification through three main methodologies: (1) membrane material-level characterization, (2) experimental prototype development and testing, and (3) computational fluid dynamics (CFD) simulations. The dense membranes used in this work showed up to a 40% increase in water vapor permeance at cooler temperatures, and the prototype system showed up to a 6% increase in humidity removal when the air was simultaneously cooled. Furthermore, the membrane module-integrated heat exchange tubes provided additional mixing and turbulence, leading to a 4–8% increase in humidity removal. The upper limit coefficient of performance was equal to approximately 2.2, necessitating advanced system designs to improve efficiency. This study is the first to highlight that combining the cooling and vacuum dehumidification processes can improve dehumidification performance. [ABSTRACT FROM AUTHOR]

Improving the energy efficiency of air conditioners and heat pumps is crucial for reducing the effects of continuously rising energy demands for space conditioning. One of the effective and tested approaches for achieving this goal has been to set energy efficiency benchmarks based on minimum energy performance standards which drive technological innovation and implementation in the market. For heat pumps, a testing and rating procedure that estimates equipment seasonal performance from laboratory measurements forms the technical basis for these standards. However, with current residential heat pumps rating standards, significant differences have been observed between efficiency measured in the lab and actual operational performance in field applications. One of the reasons for this is that with current rating approaches, the test unit native controls are overridden when measuring performance in the laboratory. As an alternative, a load-based testing approach has been developed that captures dynamic interactions of the integrated controls, equipment and building with the goal of better-representing field application conditions. In this paper, a load-based testing approach previously developed for air-conditioners is extended for heat–pump heating-mode and demonstrated for a variable-speed system. Climate-specific heating seasonal performance ratings were determined by propagating the test results through a temperature-bin method. [ABSTRACT FROM AUTHOR]

This paper introduces the differences of in-system testing of a dual-port vapor injected compressor as compared to test stand testing and presents the performance results. Single-port vapor injected compression has been studied previously, typically using scroll compressors. Single-port vapor injection employs one port with a single intermediate injection pressure, while dual-port vapor injection uses two ports at different angular locations, leading to two intermediate pressure levels for the injected vapor. The main differences between in-system testing and test stand testing can be found in suction and injection superheat values, discharge temperature, and range of injection flow rates. Dimensionless PI-type mappings are introduced that can be used for system simulation. [ABSTRACT FROM AUTHOR]

Formaldehyde, a compound commonly employed in many construction materials, paints, and plastics, has been linked to deleterious health effects. Thus, monitoring the presence of formaldehyde in interior locations is increasingly important when it comes to public health. Currently, there is a crucial need for a low-cost, small-scale, selective, and sensitive indoor sensor capable of real-time formaldehyde detection. To meet these performance metrics, materials need to be incorporated onto existing gas sensor platforms to act as chemically selective recognition layers. A main challenge when addressing this issue is creating a material that can remain easily processable, can be easily synthesized, and can operate in practical environments (i.e., at common temperatures, humidity values, and in the presence of distractant analytes). Here, we show the unique properties of poly(5-carboxyindole) (P5C), an easily synthesized polymer, for practical indoor air monitoring of formaldehyde gas at concentrations as low as 25 ppm with rapid response and recovery times characterized by time constants of 27 s and 16 s, respectively. Importantly, we demonstrate that β-cyclodextrin (BCD), when blended into P5C to create a poly(5-carboxyindole) with β-cyclodextrin composite (P5C–BCD), offers distinct properties that enhance the response to formaldehyde gas in common operational conditions. Specifically, BCD adds features into the P5C such as its ability to form strong host–guest interactions with formaldehyde, its ability to buffer P5C protonation states to allow for more protonated carboxylic acid moieties on P5C which can hydrogen bond more effectively with formaldehyde, as well as creating a cylindrical morphology with the polymer film to assist the diffusion of formaldehyde into the polymer matrix. Additionally, these materials provide for chemically selective adsorption to formaldehyde gas in environments where interfering analytes exist. Due to the practical advantages these materials offer, they have the potential to unlock new avenues for future formaldehyde sensor materials. [ABSTRACT FROM AUTHOR]

Approaching quasi-isothermal compression is possible by flooding the refrigerant vapor stream with a liquid that has high specific heat to absorb some heat of compression. To evaluate the benefits of liquid flooded compression, an R410A scroll compressor was tested over a wide range of operating conditions while flooding POE oil into the compression chamber using a hot-gas bypass test stand. The experimental results indicate that oil flooded compression leads to an increase in refrigerant mass flow rate and a decrease in compressor discharge temperature. Also, every operating condition has a maximum limit of possible oil injection, which depends on the system pressure ratio and the dimensions of the injection and discharge ports. By fitting the experimental data, a performance map for the compressor with oil flooding is developed using a semi-empirical model. This map can be used to assess the benefits of the oil flooding technology for various conditions employed in other applications. [ABSTRACT FROM AUTHOR]

In conventional vapor compression systems, electronic expansion valves (EXVs) are used for refrigerant flow control. In addition, the EXV control signal can be used along with a model and measurements of boundary conditions to provide a virtual sensor for refrigerant mass flow rate as outlined by Kim and Braun (2012). For typical single-phase inlet conditions, models for characterizing the relationship between the inlet and outlet conditions, the opening, and the mass flow rate have been extensively studied, e.g. by Park et al. (2007), and semi-empirical as well as empirical correlations have been developed. However, two-phase inlet conditions can exist at low refrigerant charge, or for certain applications such as when balancing valves are used in a hybrid control scheme as proposed by Kim et al. (2008). This paper presents a continuous correlation that can be used for both two-phase and subcooled valve inlet conditions. [ABSTRACT FROM AUTHOR]

In this work, a comprehensive framework and geometric solutions are presented for the most common type of scroll machine, the symmetric constant-wall-thickness scroll compressor based on the involute of a circle. This analytic treatment comprises solutions for the volume, derivative of volume, centroid, normalized force and moment parameters, and leakage areas for all chambers in the machine. The representations for the centroid and area are based on Green's Theorem analysis which allows for general solutions that can then also be applied to other machines. Analytic solutions are also presented for the discharge geometry corresponding to arc-line-arc, perfect-meshing-profile, one-arc, and two-arc discharge geometries. Each of the derived parameters is validated against numerical solutions based on polygons. Source code in the Python programming language is provided for the analytic derivations as well as the implementation of each of the analytic outputs. [ABSTRACT FROM AUTHOR]

Abstract: An empirical frictional correction factor to the isentropic nozzle model has been developed for application to refrigerant leakage modeling in scroll, rotary and other similar compressors and expanders. This correction factor is derived by calculating the leakage mass flow rate with a compressible, variable area, real gas properties model and referencing the results to an isentropic nozzle model. The ratio of flows is correlated to the Reynolds number, a characteristic length and the leakage gap width. A representative selection of fluids and geometries are employed. For all the correlations, at least 93% of the points are predicted within an absolute error band of 20%. [Copyright &y& Elsevier]

Abstract: In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of −10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased. [Copyright &y& Elsevier]

[Display omitted] • First system design with membrane dehumidification and indoor air recirculation. • Consideration of energy consumption for both indoor and outdoor air conditioning. • Energy savings potential up to 50% compared to conventional systems. • Enables efficient supply of 3x outdoor air flowrates compared to current standards. • Simulation of buildings in 114 cities implemented into a contour energy savings map. It is widely known that increasing outdoor air ventilation rates in buildings can help improve indoor air quality and mitigate the spread of diseases. However, cooling and dehumidifying outdoor air requires significantly more energy than treating indoor recirculated air. Recent developments in water vapor-selective membranes, which use a vapor partial pressure gradient to remove water vapor from air, offer a unique opportunity to provide highly efficient dehumidification in HVAC systems that can justify high outdoor air ventilation rates without drastic increases in energy consumption. This work presents an analysis of a novel membrane HVAC system referred to as the Active Membrane Energy Exchanger (AMX), which couples a refrigeration cycle with selective membranes to simultaneously cool and dehumidify air. While past selective membrane dehumidification systems have been isothermal, the AMX is the first to deliberately target non-isothermal operation by combining membrane dehumidification and active heat exchange. A thermodynamic model is developed for the AMX in a system that uses both outdoor air ventilation and indoor air recirculation (AMX-R) to elucidate the limitations around increasing ventilation rates. This study is among the first to consider recirculation for selective membrane processes, rather than only 100% outdoor air. The AMX-R can achieve up to 50% cooling and dehumidification electricity savings in warm and humid climates under current ventilation standards. Furthermore, ventilation rates can be nearly tripled with the AMX-R while consuming similar amounts of energy as conventional HVAC systems. Lastly, the incorporation of EnergyPlus building simulations for 114 US cities shows that the AMX-R has the greatest potential in the southern regions of the US. [ABSTRACT FROM AUTHOR]

• Metal hydrides show potential for year-round energy storage with residential HVAC. • Hydride storage can shift both heating and cooling loads away from on-peak hours. • Using hydride storage results in higher cost savings in winter than in summer. • Hydride storage has a long payback period even for favorable TOU rates. • Long payback periods are due to high initial cost and hydrogen compression costs. Thermal energy storage (TES), such as ice storage, is often used with air conditioning systems to shift cooling loads away from peak hours, reducing operating costs if time-of-use electricity rates are available. Phase change materials (PCMs) have been investigated for energy storage with heating systems; however, a single PCM cannot be used for storage with both cooling and heating since PCMs have fixed phase-change temperatures. Thermochemical energy storage with metal hydrides can be used for this purpose since the reaction used to store energy depends on both temperature and pressure, allowing for variable-temperature storage. In this paper, we propose a novel TES system design using metal hydrides coupled to a heat pump for residential heating and cooling and present a system model. The model is used to analyze system performance and predict cost savings for year-round use in a residential building. This system is shown to be capable of shifting heating and cooling loads, but only achieves cost savings of $13/year at existing utility rates. The payback period of the system exceeds 35 years across a range of rates more favorable to energy storage, primarily due to the high cost of metal hydrides. Potential improvements to the system are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel radiant heating system with a wall-embedded micro heat pump (WEMHP) for residential buildings. The interior surface temperature of the exterior wall-section empowered by the micro heat pump is independently controlled, enabling distributed thermal comfort delivery. The micro heat pump consists of a miniature compressor along with an aluminum roll-bond condenser and evaporator. In contrast to conventional hydronic radiant systems and heat pumps with separate indoor and outdoor units (i.e., split systems), the WEMHP is a "packaged" system that would not require on-site installation and charging of a secondary water loop and circulation pump or refrigerant piping. This could ultimately lead to reduced installation and maintenance costs. In this study, a detailed system model is presented and integrated with a single-zone building model to assess the energy performance of the WEMHP in heating mode. For a designed living room scenario with an external wall WEMHP-to-wall ratio (HPWR) of 0.6 and window-to-wall ratio (WWR) of 0.2, the simulation results show that up to 23 % energy savings compared with a baseline air-to-air heat pump are possible. The results also indicate the HPWR needs to be higher than 0.4 to achieve performance comparable to the baseline. Additionally, experiments were conducted in a laboratory environment in which surface temperatures were controlled to emulate different scenarios for integration of WEMHP technology and to evaluate thermal comfort conditions. The experimental results show that the median operative temperature differences in the indoor environment were within 0.5 °C for various locations while the largest radiant asymmetry was 2.2 °C for the designed room configuration. Moreover, simulations were conducted to evaluate thermal comfort conditions in applications having a smaller HPWR of 0.4 with various values of WWR. Overall, the combined simulation and experimental study shows that the WEMHP has potential to reduce energy use while maintaining thermal comfort at acceptable levels where the HPWR is higher than 0.4. [ABSTRACT FROM AUTHOR]

• First vapor compression refrigeration cycle experiment on parabolic flights. • Simple stability indicators show on average less than 15% changes for different measurements across range of operating conditions. • Loss of superheat due to parabolas observed for initial condition of less than 3 Kelvin superheat at onset of parabola maneuvers. • Comparison of operational changes due to parabolas with effects of varying inclination angle in a laboratory. Long duration manned space travel is projected to bring a need for large cooling capacities in microgravity for typical freezing and refrigeration temperatures. Among possible cooling technology alternatives, the vapor compression cycle has the highest coefficient of performance but the confidence for microgravity applications is very low. This research effort experimentally investigated the effects of hyper and microgravity on a vapor compression cycle during parabolic flights. A total of 122 parabolas were flown over four days with a repeating pattern of 5 sequential parabolas. Transient data of the evaporation temperature and cooling capacity for selected parabolas are presented as the test stand experienced alternating hyper and microgravity levels. Most measurements and performance indicators showed mild effects on the cycle operation during the varying g-forces. For example, the refrigerant side cooling capacity fluctuated on average within a band of 15% (+/-7.5%) through all sets of parabolas. A loss of superheat due to gravity changes was observed for one set of parabolas but the set was started with an unusually low superheat of only 3 K. For all other sets of parabolas, superheat and subcooling were maintained. Flight results were compared with inclination testing in a laboratory using the same test stand. Inclination changes from -90° to + 90° impacted both the liquid and suction line mass flow rate while varying the gravity level between 0 and 1.8 g affected the suction line mass flow rate but not the liquid line mass flow rate. [ABSTRACT FROM AUTHOR]

Abstract: In two companion papers, simulation models for the working processes of liquid-flooded scroll compressors and expanders have been developed and validated against experimental data. In this study, analytic models are presented for the modes of irreversibility generation in the liquid-flooded scroll compressor including built-in volume ratio maladjustment, pressure drop and leakage. A thermodynamic model is used to derive the ideal volume ratio for a liquid-flooded compressor, which is higher than that of dry compression. An optimum set of built-in volume ratio and scroll base circle radius is found which maximizes the overall isentropic efficiency by minimizing the leakage and pressure drop irreversibilities. The irreversibility generation models are used to optimize a scroll compressor for the Liquid-Flooded Ericsson cycle application. The model predicts scroll compressor overall isentropic efficiency of over 80% (based on the shaft power) at an oil mass fraction of 88%. [Copyright &y& Elsevier]

Carbon dioxide (CO2) has been linked to many deleterious health effects, and it has also been used as a proxy for building occupancy measurements. These applications have created a need for low-cost and low-power CO2 sensors that can be seamlessly incorporated into existing buildings. We report a resonant mass sensor coated with a solution-processable polymer blend of poly(ethylene oxide) (PEO) and poly(ethyleneimine) (PEI) for the detection of CO2 across multiple use conditions. Controlling the polymer blend composition and nanostructure enabled better transport of the analyte gas into the sensing layer, which allowed for significantly enhanced CO2 sensing relative to the state of the art. Moreover, the hydrophilic nature of PEO resulted in water uptake, which provided for higher sensing sensitivity at elevated humidity conditions. Therefore, this key integration of materials and resonant sensor platform could be a potential solution in the future for CO2 monitoring in smart infrastructure. [ABSTRACT FROM AUTHOR]

[Display omitted] • A vapor-selective membrane-integrated heat pump dryer is proposed. • Integration of hollow fiber membranes with partial pressure-driven ε-NTU method. • Membrane geometries are the dominant factors compared to physical properties. • Optimized COP increases by an average of 460% under a wide range of conditions. • Multiple smaller units in parallel perform better than a single large unit. Dehumidification and drying processes significantly contribute to energy consumption in air conditioning and dry material manufacturing. Conventionally, water is removed by cooling the airstream below its dew point, demanding substantial energy for phase change and reheating. Vapor-selective membranes mitigate these energy penalties by selectively separating water vapor without condensing it, offering notable energy savings for transitioning to sustainable drying processes with heat pumps. However, efficient membrane dehumidification requires a large surface area, necessitating innovative designs for high efficiency and compactness. This study proposes a new drying cycle combined with vapor-selective membranes, focusing on the design optimization of hollow fiber membrane-based energy recovery ventilators. Hollow fiber membrane properties, geometry, and pressure drops are integrated into a model of the partial pressure-driven ε-NTU method. The simulation is validated within a 10% error range against experimental data. Global sensitivity analysis emphasizes that membrane geometries significantly influence dehumidification performance more than physical properties. Design optimization is then conducted with four competing objectives: water vapor removal, fan power consumption, sensible heat loss, and compactness. The optimized design demonstrates a compactness of 13.05 cm2·cm−3 and an exceptional equivalent coefficient of performance of 16.8 under nominal conditions. Compared to the baseline, this optimized design showcases a 20% increase in compactness and an impressive 460% increase in coefficient of performance, even under varying operating conditions. Scale-up design guidance suggests that employing multiple small units in parallel is more efficient than a single large unit when dealing with dehumidification at high air flow rates. [ABSTRACT FROM AUTHOR]

• A load-based testing and performance rating approach for residential cooling equipment is developed. • Test equipment with its native controls behaves as if it were installed in the field. • Test is automated by implementing standardized procedures and convergence criteria. • Climate-specific seasonal performance ratings are generated from test results. • The proposed approach was implemented to a variable-speed heat pump. In the U.S., with about 64% of primary occupied homes having unitary air conditioners, a small improvement in the energy efficiency of residential cooling equipment can lead to significant energy savings. Currently, standardized equipment energy efficiency rating is based on standard AHRI 210/240, that provides a metric for comparing the performance of different equipment. However, it is generally recognized that this approach fails to appropriately rate and credit equipment with advanced controls and variable-speed components. Contrarily, a load-based testing and rating approach is presented in this paper that can capture equipment performance with its integrated controls that is more representative of the field. In this approach, representative building sensible and latent loads are emulated in a psychrometric test facility at different indoor and outdoor test conditions utilizing a virtual building model. The indoor test room conditions are continuously adjusted to emulate the dynamic response of the virtual building to the test equipment sensible and latent cooling rates and the equipment dynamic response is measured. Moreover, an automated testing procedure is presented along with convergence criteria for measuring equipment steady-periodic performance. Climate-specific cooling seasonal performance ratings can be determined by propagating the coefficient of performance (COP) from load-based testing results through a temperature-bin method to estimate a seasonal COP (SCOP). In this work, the proposed approach was implemented to test and rate a variable-speed residential heat pump. Furthermore, the sensitivity of load-based testing results to virtual building parameters was investigated. Finally, repeatability results from the load-based testing approach are presented. [ABSTRACT FROM AUTHOR]

• First review of vapor compression refrigeration for space applications. • Summarizing more than 40 years of development. • Covering gas-liquid two-phase flow, thermal gravitational scaling and system design considerations. • Suggesting a general evaluation criterion to select cooling technologies for spacecraft. Using a vapor compression cycle for cooling in microgravity environments was already suggested in the 1970s to leverage the high coefficient of performance. Since then, only a few systems have operated in microgravity with scarce documentation of these flights. The lack of measured data and detailed documentation makes identifying the necessary next steps difficult for researchers entering the field of refrigeration in space. This paper provides a review of available literature for vapor compression systems flown in microgravity by outlining the history of vapor compression devices in space and presenting performance data. Moreover, gaps in the literature are highlighted and open questions are posed based on the reviewed material. Next steps of research are suggested to support and ultimately achieve reliable vapor compression refrigeration in space. Calculating equivalent masses for a fair comparison of different microgravity cooling technologies is proposed by capturing both energy consumption and used volume. [ABSTRACT FROM AUTHOR]