Development and Validation of a Mechanistic Vapor-Compression Cycle Model

Detailed models are crucial tools for engineers in designing and optimizing systems. In particular, mechanistic modeling of vapor compression systems for accurate performance predictions at both full- and part-load conditions have been improved significantly in the past decades. Yet, fully deterministic models present still challenges in estimating charge inventory in order to optimize the performance. In this work, a generalized framework for simulating vapor compression cycles (VCC) has been develvoped with emphasis on a charge-sensitive model. In order to illustrate the capabilities of the tool, a direct–expansion (DX) cycle has been considered. In the cycle model, the compressor was mapped by employing the ANSI/AHRI 540 10-coefficient correlation, the evaporator and the condenser were constructed based on the ACHP models (Bell, 2010). Furthermore, a TXV model was implemented based on Li and Braun (2008) formulation. With respect to the charge inventory estimation, the two-point regression model proposed by Shen et al. (2009) was used to account for inaccurate estimation of refrigerant volumes, ambiguous flow patterns for two-phase flow, and amount of refrigerant dissolved in the oil. The solution scheme required manufacturer input data for each component as well as the amount of refrigerant charge. Hence, the degree of superheating at the evaporator outlet, the subcooling at the condenser outlet and the perfromance parameters of the VCC system can be predicted. The model was validated with available experimental and numerical data available in literature. The simulation results demonstrated that the proposed model is more accurate and more generic than other methods presented in the literature

Validation of a Charge-Sensitive Vapor-Injected Compression Cycle Model with Economization

In recent years, research on economized vapor injected (EVI) compression systems showed potential improvements to both cooling capacity and coefficient of performance (COP). In addition, the operating range of compressors can be extended by reducing the discharge temperature. However, the optimum operation of such systems is directly related to the amount of refrigerant charge, which often is not optimized. Therefore, an accurate charge estimation methodology is required to further improve the operation of EVI compression systems. In this paper, a detailed cycle model has been developed for the economized vapor injected (EVI) compression system. The model aims to predict the performance of EVI systems by imposing the amount of required refrigerant charge as an input. In the cycle model, the EVI compressor was mapped based on the correlation of Tello-Oquendo et al. (2017), whereas evaporator, condenser and economizer heat exchanger models were constructed based on the available ACHP models (Bell, 2010). With respect to charge inventory, the 2-point regression model from Shen et al. (2009) was used to account for inaccurate estimation of refrigerant volumes, ambiguity in slip flow model, solubility of refrigerant in the lubricating oil, among others. The cycle model has been validated with experimental performance data taken with a 5-ton Environmental Control Unit (ECU) that utilizes EVI technology. The developed cycle model showed very good agreement with the data with a MAE in COP of less than 4%. Furthermore, the estimated charge inventory has been compared to the one-point regression model. Results showed that the former method allowed to predict the charge inventory with an MAE of less than 0.5%

Enhancing the Performance of a Transportable Environmental Control Unit (ECU) Operated in High-Temperature Climates

Numerous people live or venture into environments, such as the Middle East, where the temperatures can skyrocket as high as 54°C. This leaves many air conditioners unable to operate efficiently. Although much research has been conducted for incorporating vapor injection processes into refrigeration systems, such as ones used in supermarkets, little has been researched regarding the application in extreme heat environments. While most basic air conditioning units do not require the addition of a vapor injection process along with an economizer, this project requires the air conditioning (AC) unit to operate under extreme high temperature conditions. The project investigates the addition of a vapor injection process and an economizer on the performance of a transportable Environmental Control Unit (ECU), operating with refrigerant R-407C, running under ambient temperatures as high as 51.7°C (125°F) and an indoor temperature of 32.2°C (90°F), and using a single injection port scroll compressor. The retrofitted AC unit was compared to a baseline cycle without vapor injection. Results for fixed injection vapor superheat at 7°C show that the cooling capacity increased for all test conditions from 6.7% to 15%. Due to the high compression ratio, the compressor discharge temperature relatively increased for the extreme test conditions. In addition, the compressor power consumption increased by up to 18%, while the COP of the system increased by 1%. Furthermore, the second law of thermodynamics analysis showed that the compressor had the largest irreversibilities, with the condenser and evaporator showing significant irreversibilities as well. Future work includes optimizing each component to improve the system’s COP

Performance Testing of a Vapor Injection Scroll Compressor with R407C

Current studies indicate that the method of economized vapor injection (EVI) increases both cooling capacity and coefficient of performance (COP) of vapor compression systems and enlarges the operating range of compressors by reducing the discharge temperature. The design and analysis of EVI systems require comprehensive and comparable performance data of the compressor. In this work, a thermodynamic model was developed to simulate the potential benefit of EVI systems. Furthermore, the performance of a vapor injection (VI) scroll compressor has been experimentally investigated using a modified compressor calorimeter and the refrigerant mixture R407C. During the experiments, the injection flow was regulated by controlling the injection superheat. The experimental results confirm the predicted tendencies of the EVI model. The investigation also reveals that the injection pressure affects the VI compressor performance and needs to be included in the compressor performance evaluation

Technologies to Improve the Performance of A/C Systems in Hot Climate Regions

Air conditioning contributes significantly to building energy consumption in hot climate regions. In addition to greater cooling requirements in hot climates, cooling equipment efficiency decreases with increasing outdoor temperature. Therefore, it is advantageous to develop improved technologies that can achieve higher efficiency at high ambient conditions. In this paper, two novel compression technologies are investigated for application in high ambient temperature air conditioning via simulations. These technologies are liquid flooded compression with regeneration and vapor injected compression with economizing. The systems are modeled using the EES software and compared with a baseline conventional vapor-compression cycle that utilizes R410A as the refrigerant. The cycle enhancements are considered for a number of refrigerant alternatives, including R410A, propane (R290), R32, and R1234yf. Parametric studies are conducted for air conditioning design conditions to predict the improvements in coefficient of performance (COP) for both system configurations with the various refrigerants. The simulation results show that the two novel technologies provide improvements in air conditioner performance and lower compressor discharge temperatures at high ambient temperatures. With respect to compressor discharge temperature, the vapor injection technology is superior to the oil flooding concept for the investigated working fluids. The COP comparisons indicate that oil flooding only improves the system performance when using the refrigerant R1234yf with a 14% increase in COP, whereas the vapor injection leads to significant improvements for all refrigerants with a maximum improvement of 21.5% for the refrigerant R410A

Machine Learning Applied to Positive Displacement Compressors and Expanders Performance Mapping

Positive displacement compressors are critical components in today’s vapor compression refrigeration, air conditioning, and heat pumping applications and can also be applied as expanders in power generation systems, such as organic Rankine cycles (ORC). The simulation of such systems is essential to predict and optimize the performance behavior at full- and part-load conditions. To this end, comprehensive system models are built by including different sub-models corresponding to each cycle component (e.g., heat exchangers, compressor, linesets). In general, the higher the complexity of each sub-models utilized to capture the physics, the higher the computational time required to solve a simulation run. In this work, deep learning is utilized to obtain high-accuracy performance predictions of positive displacement machines. A fixed-speed two-phase injected and vapor injected scroll compressor for air-conditioning applications and an oil-free scroll expander for low-grade waste heat recovery by means of an ORC are considered as test cases. In particular, Artificial Neural Network (ANN)-based models have been developed for each of the machines and trained using experimental data collected at the Ray W. Herrick Laboratories. The results of the training and testing of the models are presented as well as a discussion of the reliability of such models for extrapolating performance. In addition, the ANN models are compared with conventional empirical and semi-empirical modeling approaches. The models have been implemented in the Python programming language by using the open-source Keras package

Nano-refrigerants and nano-lubricants in refrigeration : synthesis, mechanisms, applications, and challenges

Addressing global energy security and environmental concerns, the utilization of nano-refrigerants and nano-lubricants has emerged as an innovative path for enhancing heat transfer. This research focuses on enhancing the thermophysical properties, heat transfer efficiency, and tribological characteristics of nanofluids—nanoparticles dispersed in refrigerants or lubricants. These nanofluids have demonstrated significant potential in applications such as cooling, air conditioning systems, and heat transfer equipment including pumps and pipes. A comprehensive understanding of parameters like thermal conductivity, viscosity, pressure drop, pumping power, and energy performance is delivered, with the aim of enhancing the overall efficiency of refrigeration systems, particularly the coefficient of performance (COP). Additionally, the review covers existing research on flow and pool boiling heat transfer, nano-lubricant tribological enhancement, and nano-refrigerant condensation. The study also addresses the challenges associated with the use of nano-refrigerants and nano-lubricants and offers a prospective outlook for their usage. These novel nanofluids are anticipated to emerge as effective solutions for increasing the COP and reducing energy consumption in the industrial sector, thus extending beyond the scope of previous efforts in this field. This review could serve as a valuable resource for a broad audience interested in this novel approach to energy efficiency

Carbon-based nanofluids and their advances towards heat transfer applications—a review

Nanofluids have opened the doors towards the enhancement of many of today’s existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-base were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic trough solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions

Second-Law Analysis to Improve the Energy Efficiency of Environmental Control Unit

This paper presents a second-law of thermodynamics analysis to quantify the exergy destruction in each component of an Environmental Control Unit (ECU) for military applications. The analysis is also used to identify the potential con- tribution from each component to improve the overall energy efficiency of the system. Three ECUs were investigated experimentally at high ambient temperature conditions to demonstrate the feasibility of the model presented herein. The investigated ECUs have capacities of 1.5 (5.3 kW), 3 (10.6 kW), and 5 (17.6 kW) tons of refrigeration (RT). The results indicate that the largest potential to improve exergetic efficiency of each unit resides in the compressor. This is followed in order by the evaporator and the condenser in the case of 1.5 RT and 3 RT units, whereas for the 5 RT unit, relatively high irreversibility is associated with the evaporator when compared to the compressor. The second law analysis may help to focus on the components with higher exergy destruction and quantify the extent to which modifying such components can increase the exergetic efficiency of any ECU operating in high ambient temperature environments.

Analysis of Packaged Air Conditioning System for High Temperature Climates

Packaged air conditioning (AC) units, called Environmental Control Units (ECUs), are being increasingly used by the U.S. military, especially in hot ambient temperature climates. The compact packaging of ECUs resembles unitary-type rooftop or room AC systems, and they are used to cool personnel and equipment in enclosed spaces such as shelters, vehicles, and containers. Despite these similarities, ECUs have distinctive features that aren’t found in commercial packaged AC units. An ECU is designed to sustain harsh and extreme weather conditions up to 51.7 °C (125 °F) which is a design set-point by the military. As the outdoor temperature increases, both the cooling capacity and coefficient of performance (COP) of ECUs drop dramatically. In addition, the compact design degrades airflow uniformity due to air maldistribution across evaporator coil, which results in further performance degradation. Therefore, the goal of this study is to identify ways to improve the component as well as the system performance of the ECUs in the field at high ambient temperatures. A passive solution was evaluated to compensate for the degradation in performance of ECU evaporators, known as the interleaved circuitry method. The interleaved circuitry method, where the refrigerant from a circuit with high air flow is routed to a circuit with low air flow and vice-versa, has been investigated to determine its effectiveness in reducing the air maldistribution effect. Air velocity measurements in front of the ECU’s evaporator have been conducted in psychrometric chambers and the measurement locations have been defined by the log-Tchebycheff rule. The velocity profile was obtained by the Lagrange Interpolation method as percentage values. The system performance after interleaved circuitry implementation was compared to the baseline system at different operating conditions up to 51.7 °C (125 °F). The results showed that the interleaved circuitry method increased the superheat uniformity of the individual circuits and improved the cooling capacity and COP up to 16.6% and 12.4%, respectively. Furthermore, the tuned model predicted the evaporator cooling capacity within a mean absolute error of approximately ±10%. Moreover, vapor injection (VI) with economization, where cool gas is injected to the compressor at an intermediate stage to absorb the heat generated during the compression process, has been experimentally and numerically assessed to significantly improve system performance. The ECU has been retrofitted with an economized vapor injection (EVI) system and experimentally characterized in side-by-side psychrometric chambers. The performance of the EVI system for superheated and saturated injection conditions were compared to the case of without injection at different operating conditions. The results showed that the EVI system reduced the compressor discharge temperature by up to 5 °C, and improved the cooling capacity and COP by up to 12.7% and 3.1%, respectively. The experimental data have been used to develop, tune, and validate a detailed steady-state cycle model. The predictions of suction and injection mass flow rates, compressor power consumption, and system COP were within a mean absolute error of approximately ±5%. At last, the model has been employed to optimize the economizer geometry in order to maximize the system COP at designed ambient condition of 51.7 °C (125 °F). The optimization process resulted in maximum improvements in compressor discharge temperature, cooling capacity, and COP of 8.5 °C, 22.3%, and 17.3%, respectively