Thermodynamic Limitations and Exergy Analysis of Brackish Water Reverse Osmosis Desalination Process

The reverse osmosis (RO) process is one of the most popular membrane technologies for the generation of freshwater from seawater and brackish water resources. An industrial scale RO desalination consumes a considerable amount of energy due to the exergy destruction in several units of the process. To mitigate these limitations, several colleagues focused on delivering feasible options to resolve these issues. Most importantly, the intention was to specify the most units responsible for dissipating energy. However, in the literature, no research has been done on the analysis of exergy losses and thermodynamic limitations of the RO system of the Arab Potash Company (APC). Specifically, the RO system of the APC is designed as a medium-sized, multistage, multi pass spiral wound brackish water RO desalination plant with a capacity of 1200 m3/day. Therefore, this paper intends to fill this gap and critically investigate the distribution of exergy destruction by incorporating both physical and chemical exergies of several units and compartments of the RO system. To carry out this study, a sub-model of exergy analysis was collected from the open literature and embedded into the original RO model developed by the authors of this study. The simulation results explored the most sections that cause the highest energy destruction. Specifically, it is confirmed that the major exergy destruction happens in the product stream with 95.8% of the total exergy input. However, the lowest exergy destruction happens in the mixing location of permeate of the first pass of RO desalination system with 62.28% of the total exergy input.

Experimental Analysis of the Vapour Compression Refrigeration System with Microchannel Condenser using R134a and R1234yf Refrigerant

Vapour Compression Refrigeration (VCR) system with high COP, low input power consumption and minimal exergy losses are of great research hotspot. The current research focuses on analysis of exergy and performance of vapour compression refrigeration (VCR) cycle is in a real-world application. Experiments were carried out in the VCR system using round tube condenser and microchannel condenser along with different refrigerants R134a, and R1234yf. Temperatures were varied from +20°C to 5°C for the evaporator and 40°C to 50°C for the condenser. Among the two refrigerants, experimental results show that refrigerants operating in micro-channel condensers have COP of 6% and 4%greater higher with5% and 3% lower exergy loss compared to the round tube condenser for R134a and R1234yf respectively.R1234yf appears to be a better equivalent for R134a, according to the findings. The efficiency defect in the condenser is the greatest and least in the evaporator for the coolants examined.

Efficiency Enhancement of an Ammonia-Based Solar Thermochemical Energy Storage System Implemented with Hydrogen Permeation Membrane

The ammonia-based solar thermochemical energy storage (TCES) is one of the most promising solar TCESs. However, the solar-to-electric efficiency is still not high enough for further commercialization. The efficiency is limited by the high ammonia decomposition reaction temperature, which does not only increase the exergy loss through the heat recuperation but also causes a large re-radiation loss. Nonetheless, lowering the reaction temperature would impact the conversion and the energy storage capacity. Thanks to the recent development of the membrane technology, the hydrogen permeation membrane has the potential to enhance the conversion of ammonia decomposition under the moderate operating temperature. In this paper, an ammonia-based solar thermochemical energy storage system implemented with hydrogen permeation membrane is proposed for the first time. The system model has been developed using the Aspen Plus software implemented with user-defined Fortran subroutines. The model is validated by comparing model-generated reactor temperatures and conversions profiles with data from references. With the validated model, an exergy analysis is performed to investigate the main exergy losses of the system. Furthermore, the effects of the membrane on system efficiency improvement are studied. The results show that exergy loss in the charging loop is dominant, among which the exergy losses of Heat Exchanger Eh,A, together with that of the re-radiation Er, play important roles. Compared with the conventional system, i.e., the system without the membrane, the Eh,A and Er of the proposed system are more than 30% lower because the hydrogen permeation membrane can improve ammonia conversion at a lower endothermic reaction outlet temperature. Consequently, the proposed system, presumably realized by the parabolic trough collector at ~400 °C, has a theoretical solar-to-electric efficiency of ηste, which is 4.4% higher than the conventional ammonia-based solar thermochemical energy storage system. Last but not least, the efficiency is 3.7% higher than that of a typical parabolic trough solar power plant, which verifies the thermodynamic feasibility of further commercialization.

Exergy Analysis of Coal-Based Series Polygeneration Systems for Methanol and Electricity Co-Production

This paper quantifies the exergy losses of coal-based series polygeneration systems and evaluates the potential efficiency improvements that can be realized by applying advanced technologies for gasification, methanol synthesis, and combined cycle power generation. Exergy analysis identified exergy losses and their associated causes from chemical and physical processes. A new indicator was defined to evaluate the potential gain from minimizing exergy losses caused by physical processes—the degree of perfection of the system’s thermodynamic performance. The influences of a variety of advanced technical solutions on exergy improvement were analyzed and compared. It was found that the overall exergy loss of a series polygeneration system can be reduced significantly, from 57.4% to 48.9%, by applying all the advanced technologies selected. For gasification, four advanced technologies were evaluated, and the largest reduction in exergy loss (about 2.5 percentage points) was contributed by hot gas cleaning, followed by ion transport membrane technology (1.5 percentage points), slurry pre-heating (0.91 percentage points), and syngas heat recovery (0.6 percentage points). For methanol synthesis, partial shift technology reduced the overall exergy loss by about 1.4 percentage points. For power generation, using a G-class gas turbine decreased the overall exergy loss by about 1.6 percentage points.

Analysis of Local Exergy Losses in Combustion Systems Using a Hybrid Filtered Eulerian Stochastic Field Coupled with Detailed Chemistry Tabulation: Cases of Flames D and E

A second law analysis in combustion systems is performed along with an exergy loss study by quantifying the entropy generation sources using, for the first time, three different approaches: a classical-thermodynamics-based approach, a novel turbulence-based method and a look-up-table-based approach, respectively. The numerical computation is based on a hybrid filtered Eulerian stochastic field (ESF) method coupled with tabulated detailed chemistry according to a Famelet-Generated Manifold (FGM)-based combustion model. In this work, the capability of the three approaches to capture the effect of the Re number on local exergy losses is especially appraised. For this purpose, Sandia flames D and E are selected as application cases. First, the validation of the computed flow and scalar fields is achieved by comparison to available experimental data. For both flames, the flow field results for eight stochastic fields and the associated scalar fields show an excellent agreement. The ESF method reproduces all major features of the flames at a lower numerical cost. Next, the second law analysis carried out with the different approaches for the entropy generation computation provides comparable quantitative results. Using flame D as a reference, for which some results with the thermodynamic-based approach exist in the literature, it turns out that, among the sources of exergy loss, the heat transfer and the chemical reaction emerge notably as the main culprits for entropy production, causing 50% and 35% of it, respectively. This fact-finding increases in Sandia flame E, which features a high Re number compared to Sandia flame D. The computational cost is less once the entropy generation analysis is carried out by using the Large Eddy Simulation (LES) hybrid ESF/FGM approach together with the look-up-table-based or turbulence-based approach.

A Case study of Exergy Losses of a Ground Heat Pump and Photovoltaic Cells System and Their Optimization

The aim of this scientific research is to experimentally determine the exergy losses of a ground heat pump and further optimization for more efficient use of operating modes and improvement of individual structural elements. In addition, it is proposed to use photovoltaic panels as a backup power source for the experimental installation under study. The exergetic losses are calculated, not only for the ground heat pump itself, with R407C refrigerant. The research methodology consists in a comprehensive assessment of exergetic flows, their optimization using new methods of approximation of piecewise linear functions, and the development of prerequisites for the use of anergy as one of the components of a new type of analysis of the efficiency of low-potential energy sources. As a result of processing the experimental data, the values of Coefficient of performance (COP) 4.136, exergetic temperature for the lower heat source 0.0253 and for the upper heat source 0.155, exergetic efficiency of the installation 0.62, and total loss of specific exergy of the heat pump 24.029 kJ/kg were obtained. Controllers with the Modbus protocol were used for data collection. Matlab Simulink was used to process the experimental data. When carrying out the procedure for optimizing the operating modes and selecting several modes with minimal exergetic losses, an important role is given to mathematical methods of processing statistical data. The method of increasing the efficiency of the heat pump is shown, first of all, based on the use of photovoltaic panels as a backup power source and optimization of exergetic losses due to exergo-anergetic evaluation of operating modes. The authors present the measurement errors of the heat pump plant parameters in the form of a 3D Gaussian curve, which becomes possible only when applying new approximation methods in the processing of measurements.

Energy and exergy analyses of okra drying process in a forced convection cabinet dryer

A forced convection automatic cabinet dryer integrated with a data logger was designed and fabricated. The okra samples were dried in the dryer at drying temperatures of 50, 60, and 70 °C and at three different load densities of 200, 300, and 400 g at a continuous air velocity of 0.7 m·s^–1. Energy and exergy analyses of the drying process were performed. The obtained results showed that the energy efficiency, energy utilisation, and utilisation ratio increased from 26.59 to 68.24%, 5.47 to 114.36 W, and 0.36 to 0.71 as the temperature increased to 70 °C, respectively. The inflow, outflow, and exergy losses were in the range of 7.02 to 26.14 W, 4.43 to 14.16 W, and 2.59 to 11.98 W, respectively, while exergy efficiency varied from 49.15 to 63.47%. The findings show that exergy efficiencies decrease with an increase in the drying temperature, but increase with a lower load rate. The index of sustainability varies from 2.14 to 2.77, the value increases as the load density decreases while it decreases with a temperature increment.

Thermodynamic Analysis of Alternating Steam and Flue Gas Injection Process in Application to Heavy Oil Fields in Colombia

This paper presents the thermodynamic analysis of the cyclic steam and flue gas injection process in application to heavy oil production for Colombian oilfields in order to improve oil recovery as well as reduce the environmental impact. The process comprises two subsystems: the steam generation subsystem and flue gas compression process. Working fluid parameters were selected based on the depth of the producing wells and the experimental data provided for Colombian oilfields. As part of the thermodynamic analysis, exergy losses were calculated for the subsystems operating separately as well as together in the cyclic flue gas-steam alternating injection process. The analysis was conducted for varying ratio between the duration and steam and flue gas injection over a five-day cycle. Is was determined that the efficiency of the subsystems operating together in the process (which is achieved by minimizing the total exergy losses) is drastically different depending on whether centralized power or local power generation is used for energy supply. It was concluded that an economic analysis is required in addition to the thermodynamic analysis. The varying part of the relative costs for the cyclic steam-flue gas injection process was assessed and it was shown that the optimal solution would be steam-flue gas injection with an injection ratio of 4.5:0.5 (for a five-day cycle) that uses a centralized power source.

Assessing the Exergetic and Inherent Safety Performance of a Shrimp-Based Biorefinery via Computer-Aided Tools

Although shrimp processing wastes have been studied as source of high-value products at lab-scale, no contributions are found in the literature regarding the energetic and safety performance of shrimp-based biorefineries at pilot or large-scale. This works is focused on the inherent safety assessment and exergy analysis of a pilot-scale biorefinery designed to produce shrimp meat and four by-products: chitin, chitosan, nitrogenous extract, and astaxanthin. Total irreversibilities, exergy losses, exergy of wastes and utilities were calculated for stages and the overall process using mass and energy balances of the biorefinery. The hazards associated with chemicals and process conditions were analyzed through substance properties and process data. A Numerical Descriptive Inherent Safety Technique (NuDIST) score of 557.23 suggested a moderate level of risk for the biorefinery compared to other processes, reaching chemical and process safety scores of 185.88 and 371.35, respectively. Sections (b) and (c) were identified as major hotspots from a safety point of view. The overall exergy efficiency was quantified at 25.61%, which is higher than the chitosan-from-exoskeleton linear production chain (4.58%). The highest exergy losses were found on stages as deacetylation, fresh shrimp washing and deproteinization stages. The beheading stage most contributed to irreversibilities, with 98.315%, followed by sorting, with 1.653%. These results could identify opportunities for improvement from an exergy and safety point of view by mapping less efficient and hazardous stages.