Four different configurations of Multi-Effect-Desalination (MED): backward feed (BF), forward feed (FF), parallel feed (PF) and parallel/cross feed (PCF) are modeled in steady and dynamic operation ...to examine the behavior of these configurations under transient operation. Adding thermal vapor compressor (TVC) unit with the last effect of parallel/cross configuration is also studied and compared to the other configurations to show the advantage of this type of integration. The transient operation is achieved by applying ramp changes (disturbances) in the main input parameters that include steam flow rate at the heat source and cooling seawater temperature and flow rate at the heat sink. Steady state results indicate that parallel/cross feed has a relatively better performance characteristic than the other feed configurations regarding Gain Output Ratio (GOR) and specific heat consumption (SHC). Adding TVC unit to the parallel/cross feed achieves the lowest water production cost. Dynamic results show that MED-TVC with parallel/cross feed has the fastest response while backward feed and forward feed have the slower response to the applied disturbances. In the case of heat source disturbance, MED plants operating in the backward feed arrangement may be exposed to shut down due to the significant increase in brine level for the first effect. The MED-TVC process is susceptible to the heat sink disturbances compared to the PCF and other configurations. Also, changes in input parameters lead to the highest reduction in GOR for the MED-TVC process compared to the different configurations, especially for cooling seawater temperature. The highest reduction in brine level occurs in MED-TVC for a disturbance in the cooling seawater flow rate and may lead to MED-TVC evaporator drying condition. Hence, MED-TVC will require a reliable control system to avoid this type of plant operational disturbance.
•Steady-state and dynamic mathematical models are developed for different MED feed configurations.•Parallel/cross feed has a relatively better steady state performance than the other feed configurations.•MED-TVC with parallel/cross feed has the fastest response to the applied disturbances.•The MED-TVC process is susceptible to the heat sink disturbances compared to the other standing alone of MED systems.
Multi-Effect-Desalination (MED) may be exposed to fluctuations (disturbances) in input parameters during operation. Therefore, there is a requirement to analyze the transient behavior of such MED ...systems. In this work, a dynamic model is developed and used to examine the effect of abrupt and ramp changes in the main operational parameters on the plant behavior and performance. The results show that the disturbance intensity variation has a major role in the desalination plant behavior. For the current MED-TVC configuration, it is recommended to limit the reduction in the seawater cooling flow rate to under 12% of the designed steady-state value to avoid dry out in the evaporators. Here, a reduction in the motive steam flow rate and cooling seawater temperature of more than 20% and 35% of the nominal operating values, respectively, may lead to flooding in the evaporators and a complete plant shutdown. On the other hand, the disturbance period has a minimal effect on plant performance if it avoids the critical values of the disturbance intensity that can cause plant shutdown. Simultaneous combinations of two different disturbances with opposing effects result in a modest effect on the plant operation and they can be used to control and mitigate the flooding/drying effects of the disturbances. For simultaneous combinations of disturbances with similar effect, the plant needs an accurate control system to avoid an operational shutdown.
The increasing penetration of intermittent renewable power requires the coal-fired combined heat and power (CHP) units to be of high flexibility for the purpose of peak load shifting. However, the ...ratio between the heat and power generation of the conventional CHP units is strictly coupled and limited within a narrow region, posing a great challenge in quickly responding to the load requirements. To this end, this paper proposes four improved CHP schemes by incorporating two types of steam ejectors hybrid with or without the thermal storage tank. The thermodynamic model is developed to evaluate the performance of the CHP schemes, whose accuracy is validated in comparison with the field data. The effect of the ejector extraction position on the system performance is studied, in terms of heat-power coupling and the energy-saving characteristics. The results show that the ejector extracting the steam from the reheater shows better peak-shaving capacity and higher energy efficiency than the schemes extracting steam from high-pressure turbine. The inclusion of thermal storage is able to significantly improve the system flexibility. The CHP scheme involving both reheater steam extraction and thermal storage reaches optimal peak shaving capacity of 67.15 MW. The energy efficiency and exergy efficiency can be improved by 20.7 % and 1.21 %, respectively, leading to a daily 584.4 tons of coal conservation compared to the conventional CHP system.
•Four flexibility improvement CHP schemes with steam ejectors and thermal storage are proposed.•The peak shaving capacity, energy efficiency, and exergy efficiency of the various CHP systems are compared.•The heat-power coupling and energy-saving characteristics of the various CHP systems are discussed through a case study.•CHP scheme with reheat steam of ejector and thermal storage has optimal PSC and thermodynamic characteristics.
Given the structural limitations of ejectors, in order to enhance the flexibility of them to suit various industries, several ejectors may be connected to each other serially. Moreover, steam ...ejectors are employed in different applications to draw air. In this study, through the agency of computational fluid dynamics, the geometric parameters of multi-stage steam ejectors for air suction were examined, the result of which was validated against experimental data. In this regard, considering an ejector with two stages, attempts were made to investigate the effects of geometric parameters including the diameters of the nozzle, the mixing chamber, the throat, and nozzle exit position (NXP) for either ejector. Accordingly, having evaluated the effects of the mentioned parameters, the most suitable values were selected to satisfy not only the proper functioning of each stage but the entrainment and compression ratio as well. The compression ratios obtained for the first and the second stages are 4.25 and 2.875, respectively which gives rise to a compression ratio of 12.219 for the multi-stage ejector. Eventually, for the final geometry of the ejectors, the Mach number, as well as pressure contour, were demonstrated and analyzed.
•A multi-stage ejector is investigated using computational uid dynamics method.•Validation of the numerical results against experimental data is performed.•The effects of geometric parameters on the performance of ejectors are presented.•The optimal values for geometric parameters of the ejectors are accomplished.•Match number and shock train of the ejectors are presented and analyzed.
•A numerical model is developed to study the dynamic response of MED-TVC plant under input disturbances.•High disturbance intensity may cause plant shutdown due to drying or flooding of the first ...effect.•For co-effect disturbances combination, the plant needs an accurate control system to avoid operational shutdown.•A ramp type disturbance eliminates the over/under-shoots in the GOR compared to abrupt type.
Multi-Effect-Desalination (MED) may be exposed to fluctuations (disturbances) in input parameters during operation. Therefore, there is a requirement to analyze the transient behavior of such MED systems. In this work, a dynamic model is developed and used to examine the effect of abrupt and ramp changes in the main operational parameters on the plant behavior and performance. The results show that the disturbance intensity variation has a major role in the desalination plant behavior. For the current MED-TVC configuration, it is recommended to limit the reduction in the seawater cooling flow rate to under 12% of the designed steady-state value to avoid dry out in the evaporators. A reduction in the motive steam flow rate and cooling seawater temperature of more than 20% and 35% of the nominal operating values, respectively, may lead to flooding in the evaporators and a complete plant shutdown. On the other hand, the disturbance period has a minimal effect on plant performance if it avoids the critical values of the disturbance intensity that can cause plant shutdown. Simultaneous combinations of two different disturbances with opposing effects result in a modest effect on the plant operation and they can be used to control and mitigate the flooding/drying effects of the disturbances. For simultaneous combinations of disturbances with similar effect, the plant needs an accurate control system to avoid an operational shutdown.
•Develop a wet steam model for steam ejectors with condensing flows.•Dry gas approach overestimates 11.71% higher entrainment ratio than wet steam model.•Wet steam model corrects wrong static ...temperatures predicted by the dry gas assumption.•Dry gas assumption overpredicts 21.95% higher Mach number of under-expansion level.
Supersonic ejectors are of great interest for various industries as they can improve the quality of the low-grade heat source in an eco-friendly and sustainable way. However, the impact of steam condensation on the supersonic ejector performances is not fully understood and is usually neglected by using the dry gas assumption. The non-equilibrium condensation occurs during the expansion and mixing process and is tightly coupled with the high turbulence, oblique and expansion waves in supersonic flows. In this paper, we develop a wet steam model based on the computational fluid dynamics to understand the intricate feature of the steam condensation in the supersonic ejector. The numerical results show that the dry gas model exaggerates the expansion characteristics of the primary nozzle by 21.95%, which predicts a Mach number of 2.00 at the nozzle exit compared to 1.64 for the wet steam model. The dry gas model computes the static temperature lower to 196 K, whereas the wet steam model predicts the static temperature above the triple point due to the phase change process. The liquid fraction can reach 7.2% of the total mass based on the prediction of the wet steam model. The performance analysis indicates that the dry gas model over-estimates a higher entrainment ratio by 11.71% than the wet steam model for the steam ejector.
The single-phase and two-phase flow models are developed and compared for the performance evaluation of a steam ejector for the multi-effect distillation with thermal vapour compression (MED-TVC) ...seawater desalination system. The results show that a single-phase flow model with ignoring the phase change predicts an unphysical temperature of the steam in the supersonic flow with the minimum value of approximately 122 K, which raises the query of the formation of the ice. The two-phase wet steam model corrects the distribution of the flow parameter by predicting the heat and mass transfer during the phase change. The steam achieves the first nonequilibrium condensation process inside the primary nozzle and another four alternating condensation and re-evaporation processes. The single-phase flow model under-predicts the entropy loss coefficient by approximately 15% than the two-phase wet steam model. The performance comparison is achieved against the single-phase model to present the accuracy of the two-phase model for the steam ejector simulation. This demonstrates that the nonequilibrium condensation is essential for the performance analysis of steam ejectors for MED-TVC seawater desalination system.
•Performance analysis on a steam ejector for the MED-TVC desalination system.•Develop and deploy a non-equilibrium condensation model for the steam ejector.•Ignoring phase change process induces an unphysical temperature in a steam ejector.•Alternating condensation-re-evaporation processes in mixing and constant sections.•Single-phase flow model under-predicts the entropy loss by 15% than wet steam model.
•Mathematical model of nonequilibrium condensation processes for a steam ejector.•Flow structure transition from under-expansion to over-expanded flows in steam ejectors.•Droplet formation and ...evaporation in expansion-compression process in an ejector.•Over-expanded flow achieves higher entrainment ratios with more entropy losses.
The multi-effect distillation with thermal vapour compression (MED-TVC) desalination system is efficient to produce freshwater. The steam ejector performance is not fully understood as the phase transition has been ignored in many studies. The present work develops a two-phase condensing flow model to assess the steam ejector performance considering nonequilibrium condensation phenomena. The transition of the flow structure from an under-expanded flow to an over-expanded flow in the steam ejector is investigated in detail. We present that the maximum Mach number can reach 4.02 in the under-expanded flow, which is weakened to 2.88 in the over-expanded flow. The steam undergoes several expansion-compression processes in the steam ejector in the under-expanded flow, which induces the formation and evaporation of massive droplets. In the over-expanded flow, the steam is compressed and then expanded after leaving the primary nozzle and the condensation process is not observed in mixing and constant sections. The increasing suction chamber pressure significantly improves the entrainment ratio while leading to an increasing entropy loss coefficient. The entrainment ratio is improved from 0.25 for the under-expanded flow to 1.69 for the over-expanded flow, while the entropy loss increases from 0.081 for the under-expanded flow to 0.29 for the over-expanded flow. This indicates that the transition of the flow structure from an under-expanded flow to an over-expanded flow can entrain more steam from the last effect while causes more entropy losses in a steam ejector for the MED-TVC desalination system.
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