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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 The need for more energy efficient heating and cooling technologies will increase over the coming decades. Climatic concerns associated with greenhouse gas emissions as well as energy costs associated with powering heating and cooling systems necessitate more efficient systems. This work presents an alternative to traditional vapor compression cycles used for heating and cooling. The chemical looping heat pump system is an indirect chemical heat pump that utilizes the possibility of condensation after a chemical reaction to allow the working fluid's compression to take place in the liquid phase. A thermodynamic model of the cycle is developed using electrochemical cells as the reaction drivers of the cycle. Sensitivity analysis is used to identify the most critical components of the system. The chemical looping heat pump may provide a potential COP increase of over 20% when compared to traditional vapor compression systems for representative device efficiencies. [ABSTRACT FROM AUTHOR]

This article introduces a simple empirical model that can be readily implemented in building simulation programs to model the effects of refrigerant charge on air conditioner performance. Under- or over-charged refrigerant faults are relatively common and impact performance of equipment. It is important to understand their overall impacts in the context of whole building energy consumption and comfort in order to prioritize their importance relative to other building issues. This article develops an empirical model using some existing data. The model applies corrections to the rated values of cooling capacity, energy input ratio, and sensible heat ratio of existing air-conditioning equipment modeling approaches. Model predictions are compared with experimental results obtained from three air conditioners that have fixed orifices as expansion devices. The accuracy of the model is compared with a more complex empirical model and is shown to provide better accuracy for predicting cooling capacity and sensible heat ratio. [ABSTRACT FROM PUBLISHER]

In regions with sub-freezing winter temperatures, the performance of conventional air-source vapor compression heat pumps drop drastically during extended periods of low outdoor ambient temperatures. Modifying the basic cycle configuration with liquid flooded compression and regeneration reduces the thermodynamic inefficiencies associated with the heat pump equipment and increases energy efficiency with less degradation in heating capacity especially at low ambient temperatures. Flooding non-volatile liquid along with refrigerant vapor stream into the compressor leads to a near isothermal compression process and thereby reduces desuperheating losses in the condenser. Regeneration improves the system performance by reducing throttling losses in the expansion device. The performance of a heat pump operating with flooded compression and regeneration has been analyzed for three sub-freezing outdoor ambient climatic regions: Boston, Indianapolis and Minneapolis. The results indicate that at −10 °C ambient temperature, COP (heating) of the flooded compression with regeneration cycle was approximately 13% higher than the standard vapor compression cycle. The Heating Seasonal Performance Factor (HSPF) of a flooded compression with regeneration cycle for Boston, Indianapolis and Minneapolis was approximately estimated to be 9%, 10% and 13% higher than the standard vapor compression cycle respectively. [ABSTRACT FROM AUTHOR]

Oil flooding is a technique that can be utilized in compression systems to achieve near-isothermal compression. This can lead to a boost in system efficiency and a reduction in compressor power consumption. In this paper a semi-empirical model for oil flooded compressors using liquid injection was developed. The model is validated with experimental data and integrated into a thermodynamic model of a vapor compression system with oil flooding and regeneration. The performance of the heat pump system is predicted and the semi-empirical model is used to identify and estimate the magnitude of the irreversibilities during the compression process. A method for generalizing the model for different working fluids is also presented. Using this model, design recommendations are made to improve the efficiency of the studied liquid flooded compressors for heat pump applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • First non-isothermal selective membrane-based HVAC system. • Detailed derivation of thermodynamic governing equations and performance metrics. • Thorough analysis of power reduction compared to several state-of-the-art designs. • Case study using EnergyPlus building simulations for hospitals. • Highest efficiency membrane-based HVAC system with up to 66% second law efficiency. As much as 40% of the total load on air conditioning systems can be attributed to condensation dehumidification. However, new water vapor-selective membranes present a unique opportunity to greatly reduce the power requirements for moisture removal by avoiding phase change and have thus been ranked as a top alternative to traditional HVAC systems. To date, however, all such systems have relied on the assumption of constant temperature, even terming the technology "isothermal dehumidification." This work proposes a membrane-based air cooling and dehumidification approach, referred to as the Active Membrane Energy Exchanger (AMX), which is the first to provide simultaneous, yet decoupled, air cooling and dehumidification. The suggested AMX configuration uses two vapor-selective membrane modules with a water vapor compressor in between them, using the second membrane module to reject vapor into the exhaust stream. Cooling and heating coils in each membrane module channel move heat between the air streams using a vapor compression cycle. A detailed steady-state, thermodynamic model is presented for the AMX integrated within a 100% outdoor air conditioning system. The AMX's limiting parameters and design considerations like compressor efficiency are systematically analyzed for a broad range of outdoor air conditions and compared against standard and state-of-the-art dedicated outdoor air systems. This new high efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2–4.7 times the COP over conventional dedicated outdoor air treatment. Lastly, a building simulation case study predicted cooling energy savings as high as 66% in hospital buildings with 100% outdoor systems in hot, humid climates. [ABSTRACT FROM AUTHOR]

The primary goal of this work was to develop, demonstrate, and evaluate a fault detection and diagnostics (FDD) system that incorporates integrated virtual sensors and fault impact models for decision support. FDD systems have the potential for improving energy efficiency along with reducing service costs. To achieve this goal, virtual sensors and fault impact models that only require low cost measurements are implemented to detect the cause of the fault, diagnose the severity of a fault and assess the performance reduction. The integrated virtual sensors can handle multiple simultaneous faults to isolate a specific fault from other faults. The fault impact models can decouple performance impacts of individual faults under multiple fault conditions. A complete diagnostic FDD system was implemented and demonstrated for a rooftop air conditioner. The test conditions included a wide range of fault and operating conditions with multiple simultaneous fault situations. Laboratory tests were performed to define reasonable thresholds for the FDD system with multiple simultaneous faults to assess whether RTU service should be performed. The FDD system provided the following diagnostic outputs for and impacts of: 1) loss of compressor performance, 2) low or high refrigerant charge, 3) fouled condenser, 4) evaporator filter, and 5) faulty expansion device. [ABSTRACT FROM AUTHOR]

Space exploration is gaining new momentum. A near-term goal set by NASA is to return humans to the Moon and establish a sustainable presence on the lunar surface to enable more repeatable and affordable deeper space expeditions. Lunar dust is one of the dangerous environmental hazards causing serious problems for exploration activities. In this paper, a dynamic model is developed and used to study the impact of lunar dust on the performance of a thermal management system for a lunar habitat that is comprised of a vapor-compression cycle heat pump and a radiator loop. A heat pump to lift the heat rejection temperature is required due to high lunar daytime surface temperatures. Parametric studies on radiator size, percentage of dust coverage on the radiator, and lunar habitat location are carried out. The results show that lunar deposits can significantly increase the power consumption of the thermal management system. The lunar dust contamination problem raises the requirement for redundant design of the radiator loop for fault tolerance. A synergetic design of a thermal management system, a power generation system and robotic maintenance system is proposed to achieve maximal mass reduction for a whole lunar habitat system. • A heat pump system coupled with a radiator loop for a lunar habitat. • Performance evaluation of the heat pump system under dust impact. • Synergetic design to minimize system mass. [ABSTRACT FROM AUTHOR]

• Two-phase flow visualizations of evaporator inlet under normal and microgravity. • Consistent increase of condensation temperature at the onset of microgravity captured. • Mild evaporator oscillations observed through five independent measurements. • Categorization of cycle response by refrigerant-side cooling capacity changes to flight maneuvers. Propositions of vapor compression cycles for microgravity applications have a long history. In contrast, experimental efforts and implemented applications are rare in general and in recent years even less than in the 1980s and 1990s. The paper shows experimental results from a vapor compression cycle on parabolic flights. The focus lies on the effects of varying gravity forces on the condenser and evaporator. Flow visualizations clearly showed a change to an annular or slug-annular flow regime at the evaporator inlet shortly after the onset of gravity. The discharge pressure was found to consistently increase sharply by 1–3% across widely varying operating conditions. Oscillations were observed in the evaporator for some parabolas due to microgravity. Overall, the cycle response to varying gravity based on the cooling capacity can be categorized in four different groups, which appear to have a dependence on both the mass flow rate and the evaporator superheat. [ABSTRACT FROM AUTHOR]

• Developed a dynamic model of a triple-evaporator domestic refrigerator/freezer. • Dynamic model validated within a 2 % deviation in average power consumption. • Implemented a two-stage economized cycle and its control scheme. • Achieved 12.9 % and 80 kWh/year reduction in power and energy consumption, respectively. • Estimated 18 TWh/year in global energy savings by 2030 with 10 % market penetration. Domestic refrigerator/freezers account for approximately 6 % of all energy consumption around the globe and mainly rely on vapor compression cycles to operate. Researchers have investigated alternative cycle architectures such as dual-loop cycles and parallel circuit cycles to improve their efficiencies. Despite the demonstrated energy saving potential of these advanced cycles, additional implementation costs are often not justifiable. However, to meet forthcoming stricter energy standards while ensuring flexible multi-temperature operation of domestic refrigerator/freezers, advanced cycle architectures are needed. In this paper, a state-of-the-art bypass circuit cycle triple-evaporator domestic refrigerator freezer with R-600a and a reciprocating compressor has been used as the baseline cycle investigate an alternative cycle configuration. Specifically, this work presents a two-stage vapor-injected cycle with a multi-evaporator system to enable energy savings and cost-effectiveness. The cycle establishes two separate evaporation temperatures to better match the cabinet temperature of fresh food and freezer compartment. The reduced difference between cabinet temperature and its evaporation temperature decreases the irreversibilities in the heat exchanger and improves overall system efficiency. Moreover, the addition of the economization line from the medium temperature evaporator reduces the compressor work. To capture the complex transient behavior of both the baseline system and proposed cycle architecture with their control strategies, a dynamic model has been developed and validated with experimental data. The validated dynamic model with the baseline cycle was modified to consider the two-stage vapor-injected cycle and its control logic, and simulation results yielded up to 13 % energy consumption reduction with respect to the baseline system. [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]

• 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]

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]

• 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]

• 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]