•Relay-evaporation cycles for better temperature matching are proposed.•Relay-evaporation cycles can realize 6.8–16.0% higher COP than single-stage cycle.•Proposed cycles can get 6.6% higher COP than ...two-stage compression cycle.
Refrigeration and air conditioning systems consume 20% of the overall global electricity. Developing refrigeration systems using zeotropic refrigerants is an effective approach to save energy in air conditioning systems, because the temperature glide of zeotropic mixtures can improve the temperature matching between the refrigerant and airflow. However, in air conditioning systems, the temperature variation of the indoor airflow is considerably larger than that of the outdoor airflow. Thus, a simple zeotropic vapor compression system cannot simultaneously match the temperatures in the evaporator and condenser. This significantly compromises the benefits of adopting zeotropic refrigerants. In this study, four new relay-evaporation cycles are proposed. They can simultaneously improve the temperature matching in the condenser and evaporator. Thermodynamic models of the proposed cycles using R32/R1234ze(E) are established, and their performances are compared to that of the single-stage and two-stage compression cycles in terms of the coefficient of performance (COP), exergy destruction, and temperature match indicators. Under the basic condition, the relay-evaporation cycles can improve the COP by 6.8%–16.0% compared to the single-stage compression cycle. In particular, the relay-evaporation cycle with a recuperator and an economizer exhibits 3.4%–6.6% higher COPs than that of the two-stage compression cycle.
Data-driven dynamic models typically offer faster execution than their physics-based counterparts described by large systems of nonlinear differential–algebraic equations (DAEs) with quantitatively ...reasonable accuracy. Therefore, development of such models can be extremely useful for design optimization, controls, fault detection and diagnostics of vapor compression based building Heating, ventilation and air conditioning (HVAC) systems. As the complexity and scale of vapor compression systems (VCS) increase rapidly across the industry, a modular approach of generating and interconnecting data-driven component models enables model reuse and efficient adaption to arbitrary system layouts. Despite the flexibility, the modular integration for system model generation can suffer from nonphysical behaviors of violating conservation laws due to inevitable prediction errors associated with each component. This paper presents a data-driven modeling framework that exploits state-of-the-art deep learning techniques for constructing component models, while enforcing physical conservation for system simulations. A general-purpose system solver is developed to handle arbitrary configurations by automatically integrating data-driven or interchangeable physics-based component models into a system model. Results of an air-source heat pump system reveal a significant speedup with good agreement, compared to high-fidelity first-principles models.
•Developed data-driven VCS component models that can respect physical conservation laws.•Exploited gated recurrent unit and feedforward neural network for component model construction.•Proposed a system solver to integrate data-driven component models for arbitrary configurations.•A case study revealed a 3.6 times speedup with negligible errors compared to physics-based models.
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•Investigated the potential of desiccant and VC integrated system.•A significant portion of the total latent heat load is controlled by the dehumidifier.•Humidity ratio greatly ...affects the overall performance of the integrated system.•Trade-off analysis is performed among COP, dehumidifier heat load and humidity ratio.•COP of integrated system is improved by 74.8% relative to standalone VC system.
The present study aims to integrate desiccant dehumidification systems and vapor-compression air conditioning (VC) systems for energy-efficient air conditioning applications. A numerical model is developed for a dehumidifier/regenerator module based on the governing mass, energy, and species balance equations. Subsequently, the desiccant system is integrated with the VC system to investigate the overall system performance. The novelty of the present work is in its exploration of the integration of desiccant systems. The share of dehumidifier load in overall system is studied, and performance of the integrated system is compared with that of a standalone VC system. Trade-off analysis is also performed in consideration of all performance parameters. The comparative results reveal that a significant amount of the total latent heat load is controlled by the dehumidifier. Specifically, it is observed that the integrated system can improve the COP by 40.8–74.8% relative to the standalone VC system under different operating conditions.
•Energy, exergy and economic analysis of compression refrigeration are conducted.•The comparative study of the eight common refrigerants is provided.•The COP of refrigeration systems is maximized ...using four optimization variables.•The comparative study shows that ammonia has maximum COP.
This work focuses on theoretical investigation of the performance of a multistage vapor-compression refrigeration system using energy, exergy and economic analysis. The system was modeled using Engineering Equation Solver (EES) software and the model was validated against published data with maximum error of 1.14%. System optimization was carried out using Conjugate Directions Method. Optimization objective function was maximizing the coefficient of performance (COP) of the multistage vapor-compression refrigeration system by varying four optimization variables. Those variables are sub-cooling, de-superheating parameters, and evaporator and condenser temperatures of the system. Eight refrigerants were used in the investigation. They are: R717, R22, R134a, R1234yf, R1234ze(E), R410A, R404A, and R407C. Results show that COP increases with increasing the sub-cooling parameter. The maximum COP of 6.17 was achieved with ammonia while minimum COP of 4.95 was achieved with R407C. The optimization results give also that, R717 is a best option compared with all refrigerants, while R407C is not recommended to use.
•Low-GWP refrigerant alternatives to synthetics across six applications are reviewed.•Refrigerant replacements are also evaluated with GWP, safety level, and PFAS.•R744 and R290 are the most popular ...replacements employed across all applications.•R1234yf as a substitution in MAC is currently classified as a harmful substance.•Refrigerant mixtures either improve system performance and/or overcome restrictions.
Small-scale vapour-compression systems for refrigeration, air conditioning, and heat pump (RACHP) applications guarantee several essential human needs. The global energy and environmental policies have compelled these solutions to take a new sustainable growth path, i.e., the adoption of low global warming potential (GWP) refrigerants. The working fluids with GWP below 150 as substitutions in vapour-compression systems across six main applications have been exhaustively reviewed, and focus is given to the aspects of system performance, environmental impacts, operating conditions, and cost. Due to the negligible environmental impacts, natural refrigerants are prioritized to be selected as alternatives to high-GWP synthetics. R744 and R290 are the two most popular natural refrigerants employed across all applications. Compared with using R134a, R744 systems can save energy consumption by up to 37% in commercial refrigeration units while R290 systems can provide higher efficiencies for space heating at low ambient temperatures. R600 and R600a are used in domestic appliances to decrease energy consumption by up to 18.6% and to reduce flammability risks while mixing with R290. R1234yf, as a hydrofluoroolefin (HFO) with negligible-GWP, has been widely used as a substitution in mobile air-conditioning (MAC), although it is under pressure as currently classified as per-fluorinated alkylated substance (PFAS).
Extremum seeking controllers have been investigated for multivariable data-driven energy optimization in heat pumps. In particular, proportional–integral extremum seeking control (PI-ESC) has ...demonstrated potential for significant acceleration compared to other ESC variants for nonlinear closed-loop control systems. A barrier to PI-ESC’s utilization in self-optimizing control is the fact that the PI-ESC algorithm is fragile. That is, unless the PI-ESC gains (e.g., controller gains, estimator gains) are carefully tuned, small perturbations to these gains can render the closed-loop unstable. Since arbitrary combinations of PI-ESC gains can result in instabilities, we propose a failure-robust Bayesian optimization (FRBO) algorithm that computes PI-ESC gains that ensure the closed-loop system can be driven rapidly to the optimum, while identifying and avoiding regions in the space of PI-ESC gains that are likely to result in instabilities (i.e., failures). The FRBO-tuned PI-ESC is shown to result in rapid closed-loop convergence to optimal values both on benchmark examples and a production-level model of an air conditioning system.
•We propose failure-robust Bayesian optimization (FRBO) to tune parameters of advanced extremum seeking controllers using data.•The novel FRBO algorithm can automatically identify parameter combinations that cause closed-loop instability.•The FRBO uses data obtained from closed-loop simulations or experiments, without model knowledge.•We demonstrate the effectiveness of the approach on a production-level vapor compression system.
•A novel system for aircraft cabin heating was proposed.•Performances of the new system were verified under cold weather conditions.•The new system shortened heating time and compression ratio.•The ...new system improved coefficient of performance and exergy efficiency.•The new system was proved to be feasible, efficient, and promising.
At present, the cabin heating of aircraft mainly depends on engine bleed air, which results in large power loss from the engine. To address this problem, a new system coupling the vapor compression system and lubricating oil system to recover heat from the lubricating oil to heat the cabin is proposed based on the design concept of the integrated thermal management of aircraft. To investigate the feasibility and efficiency of the new system, an experimental system was established, and performance experiments were conducted under cold ground weather (5, 0 and –5 °C), which was simulated in an environmental chamber. The lubricating oil temperature was kept at approximately 45 °C, and heat was recovered from the lubricating oil at 0 and –5 °C but not at 5 °C. The experimental results and thermodynamic analyses indicate that when the lubricating oil system works rather than being closed, the heating time needed for the cabin temperature to reach 25 °C is shortened from 494 s to 121 and 363 s, the compression ratio of the compressor is reduced from 7.65 to 2.81 and 3.73, the heating coefficient of performance is raised from 3.46 to 6.87 and 5.27, and the system exergy efficiency is improved from 22.9% to 33.2% and 32.8%. Therefore, the proposed system can be treated as feasible, efficient, and promising.
Efficient detection of refrigerant leakage is of utmost importance for industrial refrigeration systems due to its potential to cause substantial impacts on system performance and the environment. ...Existing research on fault detection and diagnosis in refrigeration systems primarily revolves around solutions based on experimental or laboratory data. However, in the industrial use case, achieving accurate and early detection poses significant challenges. This paper reports on the development of a novel refrigerant leak detection method for industrial vapor compression refrigeration systems. Our method leverages real-world data obtained from operational installations, enabling us to assess its reliability and applicability. The proposed data-driven approach involves predicting the fault-free liquid level in the installation receiver and comparing the actual and predicted levels. In this work, we place emphasis on features and model selection. Dedicated metrics combined with a model comparison method are proposed to evaluate and compare the performance of commonly used regression models with two sets of features to determine the most effective one. Furthermore, we provide insights into the results obtained from the deployment of the proposed method in real-world industrial installations.
•Refrigerant leak is detected through the receiver liquid level in the installation.•A regression model can be used to predict the fault-free liquid level.•System performance indicators were proven to be effective in long-term predictions.•Extremely randomized forest emerged as the top performer.
Separate sensible and latent cooling (SSLC) is a promising technology for enhancing the energy efficiency of space cooling systems. In this paper, we propose an SSLC configuration consisting of an ...indirect evaporative cooling (IEC) device and vapor compression cycle (VCC) for handling space sensible load, and a desiccant wheel for handling space latent load. The performance of the proposed configuration is investigated using physics-based or empirical models for each component. A year-round simulation study of the proposed SSLC system is performed for 12 different cities with diverse climatic conditions. This study considers electrical, thermal, or solar energy for desiccant wheel regeneration. Parameters governing the system performance such as the air flowrate ratio in the IEC and the DW, DW regeneration air temperature, and supply temperature are studied. The proposed SSLC configuration is superior to the conventional system in all cities. The enhancement of seasonal coefficient of performance (sCOP) ranges between 50 and 261 %, 67 and 387 %, and 60 and 2072 % with yearly electrical energy savings of 33 and 72 %, 40 and 79 %, and 38 and 95 % for electrical, thermal, and solar energy for regeneration of DW, respectively.
In this study, three new mixed refrigerants with low GWP values,
R1234yf/R134a, R1234yf/R125, and R1234yf/R13I1, were evaluated as
replacements for R134a refrigerant using two vapor compression
...configurations. The experiment revealed that these mixtures can be used as
environmentally friendly alternatives for this configuration.
