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Selecting appropriate working fluids is important for improving the technology readiness level of electrochemical heat pumps (EHP), which is a promising emerging technology for space conditioning and thermal management applications. The major contribution of this paper is to develop and evaluate key metrics to identify electrochemically active working fluids based on heat pump cycle requirements. The key metrics include the possibility of phase change after a chemical reaction, reversible cell voltage and power consumption, cooling and heating capacity, and environmental and safety aspects. Such metrics are applied to conduct fluid screening to assess their suitability. Isopropanol/acetone showed the highest potential to deliver efficient cooling and heating when applied to an EHP system; the use of ethanol/acetaldehyde and methanol/formaldehyde would easily enable electrochemical fluid condensation over a wide range of operating temperatures. Although the approach is demonstrated for selected organic fluids, the screening framework can be readily employed for other chemical compounds expanding the design space of electrochemical heat pumps. • Developed a tool for thermodynamic evaluation of electro-active working fluids. • Evaluation metrics include thermophysical, reactions, GWP, and safety properties. • In cooling mode, system efficiency is improved by 15%–25% using isopropanol/acetone. • Ethanol/acetaldehyde system can operate down to −27.4 °C in heating mode. • Flammability and toxicity of the fluids should be addressed using secondary loops. [ABSTRACT FROM AUTHOR]

The current testing and rating procedure for residential air conditioners and heat pumps is based on a steady-state performance measurement approach with a degradation coefficient to account for cycling losses at part-load conditions. Test equipment performance is measured under various ambient conditions with varying compressor and fan speeds, and the results are propagated through a temperature-bin method to estimate seasonal performance. Although the current rating approach offers a standardized performance metric for comparing the relative performances of different equipment, it involves disabling the native controls and, as a result, does not consider the impact of integrated controls for test units and their dynamic interactions with representative building loads. As an alternative, a load-based testing methodology (CSA EXP07) has been developed in which the dynamic performance of equipment is measured in a test facility by allowing it to respond to a simulated virtual building model. This study compares the steady-state and dynamic load-based performance measurement methodologies for use on a 5-ton residential heat pump system in order to understand the differences and their significance for the next-generation rating procedure. The differences in the two test methodologies' performance evaluation results are discussed with a causal analysis of the observed differences. [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]

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]

• Extensive experiments for R454C through an EEV were carried out and studied. • Two semi-empirical correlations were developed for mass flow rate prediction. • The proposed correlations can predict 95% of the experimental results within 10%. • A potential relation between molar mass and mass flow coefficient was observed. The use of electronic expansion valves (EEVs) in heat pump systems has seen a rapid increase in recent years due to their advantages such as fast response, accurate control, and ability to improve seasonal performance. Modeling of the refrigerant flow characteristics through EEVs is needed during design for optimizing system performance and operation. Previous studies have not led to any generalized correlations for EEVs that accurately predict performance for different refrigerants. When working with a new refrigerant and/or EEV, it is necessary to perform mass flow measurements for the EEV over a wide range of operating conditions that are then used to develop and validate a refrigerant and EEV specific correlation. R454C, a low-GWP refrigerant mixture, is gaining attention as a potential replacement for R404A and R410A systems. However, there is currently no available EEV experimental data or correlation specifically for R454C in the literature. The objective of this paper is to present experimental results and characterization of R454C flowing through an EEV in a heat pump split system across a wide range of operational conditions. The impacts of valve opening, subcooling degree and inlet pressure on mass flow rate though the EEV were investigated. Two semi-empirical correlations were developed that map the experimental data based on the Buckingham π theorem and a polynomial function. Both mapping methods predicted mass flow rate for test data within 10%. The Buckingham π correlation was slightly more accurate. Both correlations were developed to enable mass flow rate predictions for R454C passing through EEVs with different valve sizes. Finally, a relationship between refrigerant molar mass and mass flow coefficient was observed that could be useful in developing a general EEV correlation that can be applied for different refrigerants. However, further investigation is needed to validate this relationship. [ABSTRACT FROM AUTHOR]

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]

Metal hydrides have been investigated for use in environmentally friendly heat pumps. These systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator). Some challenges faced by compressor driven metal hydride heat pumps are poor heat transfer in the metal hydride beds and high compressor discharge temperatures. To overcome these challenges, this article investigates the use of a metal-hydride slurry in conjunction with various isothermal compression techniques. Liquid-flooded, electrochemical, and liquid piston compressors were modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system. [ABSTRACT FROM PUBLISHER]

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]

Residential variable-speed ductless heat pump (DHP) systems are claimed to operate more efficiently than conventional single-speed ducted systems because of fewer on-off cycles and smaller fan power consumption. However, few methods exist for quantifying the energy-saving benefits of DHP technology because of complicated and proprietary control strategies employed in the DHP system. In this paper, a model in the literature is used to assess the energy savings relative to a baseline ducted system with the same heat exchangers and rated capacities. Process efficiencies are introduced to highlight the sources for differences in performance between systems. Analyses of process efficiencies at different building loads and ambient temperatures show that DHP systems perform better than conventional systems primarily because of lower fan power consumption associated with the use of variable-speed fans. Greater benefits exist at lower heating loads because of lower air flow requirements and the cubic dependence of fan power on flow. In contrast, conventional heat pumps typically operate with on/off fans. Heating seasonal performance factors (HSPF) were determined for different climatic regions and the DHP system was found to perform 22%–107% better than a conventional system, depending on the climate. [ABSTRACT FROM AUTHOR]

The market for variable-speed ductless heat pump (DHP) systems has grown in North America in recent years. However, the tools available for modeling their performance within building simulation programs have not kept pace. In general, simple empirical models are typically used for characterizing equipment performance for use in system simulation. However, unlike more conventional single-speed systems, DHPs have complicated control algorithms for managing electronic expansion valve opening, compressor speed, fan speed, and defrost operation. Very little work has been done in the development of empirical models that explain the impact of these characteristics on the performance of DHP systems. In this article, an empirical approach is introduced to model the heating and defrost performance of DHP systems under a wide range of conditions. The model incorporates separate relationships for performance associated with maximum, minimum, and intermediate (part-load) heating capacities. The defrost operation of the systems is also modeled empirically. The approach was tested on data from two DHP systems and the agreement is very good. The performance of the systems under different building loads was simulated and it is shown that the performance is highly dependent on the system control characteristics. [ABSTRACT FROM AUTHOR]

Abstract: Multi-split ductless heat pumps are touted as an energy efficient option for air conditioning and heating in residential and small commercial applications. However, their use of multiple indoor units with variable speed compressor and fans and complicated controls leads to difficulties in understanding their characteristics by experiments only. A method to model the system using gray-box component models and an empirical control algorithm is described in this paper. The model was constructed from experimental results of an 8 kW R410A dual-unit ductless heat pump system having an accumulator. Strategies for modeling systems with accumulators and saturated control inputs are presented. Parametric studies with varying indoor air temperature, individual indoor unit load ratio, indoor fan speed and total indoor load were conducted. The results show that the model predicts system performance accurately and can help to understand the effect of saturated control inputs and changes of accumulator status on system performance. [Copyright &y& Elsevier]

Abstract: Models for unitary air conditioners and heat pumps typically have difficulties in providing accurate predictions for the following situations: 1) charge modeling over a large range of charge levels, 2) two-phase refrigerant entering fixed-orifice expansion devices, 3) two-phase refrigerant entering compressors, and 4) non-uniform refrigerant mass flow allocation among evaporator circuits at low indoor air flow rates. This paper gives descriptions and assessments of improved modeling methodologies for all four issues. The study utilizes data obtained from extensive testing at a wide range of operating conditions for two R-410A units and one R-407C unit. [Copyright &y& Elsevier]

Abstract: A hybrid method for optimizing refrigerant distribution in evaporators is presented that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. The flow balancing valves could be located upstream or downstream of the evaporator. This paper presents the results of a study to investigate the benefits of this hybrid scheme for both upstream and downstream flow balancing for the case of air flow mal-distribution. In order to perform this investigation, a simulation model was developed to consider evaporator flow mal-distributions for a 10.55kW residential R410A heat pump and then validated through comparisons of predicted results with measurements. Simulation results show that there are significant benefits in controlling the superheat of each circuit of evaporators through the hybrid–individual superheat control method. Furthermore, the upstream refrigerant flow control consistently outperforms the downstream refrigerant flow control, and recovers most of the loss in cooling capacity and COP due to non-uniform air flow distribution. [Copyright &y& Elsevier]

Refrigerant pressures are critical measurements for monitoring, control, diagnostics, and optimization of vapor compression cycle equipment. Direct refrigerant pressure measuring practices are expensive and more than often problematic. This paper describes a method, termed virtual pressure sensing, wherein refrigerant pressure values are indirectly derived from low-cost temperature sensors that can be surface-mounted. In this manner, physical pressure sensors are eliminated and pressure sensing can be achieved at a much lower cost and in a non-invasive way. Five virtual pressure sensors are developed to obtain the five most important pressures in vapor compression cycle equipment: compressor discharge line pressure, condensing pressure, liquid line pressure, evaporating pressure, and suction line pressure. The performance of the proposed virtual pressure sensors, in terms of accuracy in estimating pressures and inferring liquid line subcooling, suction superheat, compressor power consumption, and refrigerant flow rate, is evaluated extensively using laboratory data collected from four systems. These systems include air conditioners and heat pumps, split and packaged systems, refrigerants R-22 and R-410α, fixed-orifices and thermal expansion valves, and reciprocating compressors and scroll compressors. Ultimately, the virtual sensors are used as a part of a decoupling-based fault detection and diagnosis (FDD) technique to diagnose multiple simultaneous faults. The impact of the virtual pressure sensors on the FDD performance is evaluated extensively using the laboratory data collected from the four various systems. [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]

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]

• 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]

[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]

Metal hydrides have been studied for use in energy storage, hydrogen storage, and air-conditioning (A/C) systems. A common architecture for A/C and energy storage systems is two metal hydride reactors connected to each other so that hydrogen can flow between them, allowing for cyclic use of the hydrogen. This paper presents a nonlinear dynamic model and multivariate control strategy of such a system. Each reactor is modeled as a shell-and-tube heat exchanger connected to a circulating fluid, and a compressor drives hydrogen flow between the reactors. We further develop a linear state–space version of this model integrated with a model predictive controller to determine the fluid mass flow rates and compressor pressure difference required to achieve desired heat transfer rates between the metal hydride and the fluid. A series of case studies demonstrates that this controller can track desired heat transfer rates in each reactor, even in the presence of time-varying circulating fluid inlet temperatures, thereby enabling the use of a two-reactor system for energy storage or integration with a heat pump. • A nonlinear dynamic model of a two-reactor metal hydride system is derived. • The nonlinear model is linearized analytically and validated. • A model predictive controller is designed to track desired heat transfer rates. • Successive linearization is used to track setpoints across a large operating region. • The controller enables use of two-reactor systems for continuous heating or cooling. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this study was to measure the effects of improper airflow and refrigerant charge on the seasonal performance of a typical 10.6kW, R-410A residential heat pump with a thermostatic expansion valve. Heating and cooling tests were performed in combinations of three refrigerant charges of 75%, 100%, and 125% of nominal value and two airflows of 75% and 100% of rated airflow. In addition, cyclic tests were performed to estimate the heating and cooling seasonal coefficient of performance (COP) at six climate zones specified by Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Standard 210/240-2008. Results showed that, in each climate zone, increases in refrigerant charge at the rated airflow could improve the unit''s heating seasonal COP by as much as 5%. However, combined decreases in airflow and refrigerant charge could penalize the unit''s heating seasonal COP by as much as 10%. [Copyright &y& Elsevier]

• Experiments were conducted to optimize the control of TES defrosting for CASHPs. • To regulate heat distribution, the LSC and HSC refrigerant flow rates were adjusted. • Optimization included changing the LSC compressor frequency and HSC EEV opening. • LSC compressor frequency of 90 Hz is recommended. • Reducing the HSC EEV opening is recommended. Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively. [ABSTRACT FROM AUTHOR]