Pinch Temperature Difference Research Articles

Optimization of binary zeotropic mixture working fluids for an organic Rankine cycle for waste heat recovery between centrifugal compressor stages

AbstractThe organic Rankine cycle is an effective way to recover low‐temperature waste heat, and a mixture working fluid can effectively improve the performance of the organic Rankine cycle. In order to find the best pure and mixture working fluid quickly and accurately, thirteen pure working fluids and zeotropic mixtures of two of those fluids are selected to study the performance of the organic Rankine cycle, and the net power output is selected as the objective function. Through calculation and analysis, the best pinch temperature difference and evaporation temperature are determined. By calculating the approximate net power output, we find that the parameter (k·Δs5‐1)−1, which is determined by the physical properties of a working fluid, plays an important role in the pure working fluid optimization. In addition, the condensation temperature glide and (k·Δs5‐1)−1 together determine the optimized binary zeotropic mixture. As a result, R227ea and R227ea/hexane (0.98/0.02) are shown to be the optimal pure working fluid and binary zeotropic mixture. Using this mixture resulted in a net power output of 424.63 kW, which is 29.3% higher than that of the pure working fluid R227ea.

Thermodynamic Performance of Molten Salt Heat Storage System Used for Regulating Load and Supplying High Temperature Steam in Coal-Fired Cogeneration Power Plants

In order to accept more electricity from renewable energy, cogeneration power plants are considering to reduce electricity production, which affects the heat supply. Here we present a molten salt heat storage system for coal-fired cogeneration power plants, which can supply high temperature steam to users and decouple the heat and electricity production. The first and second law-based analytical models for the cycle and a real device are built. Two water input methods are taken into account. The results show that the high and low temperatures in the two molten salt tanks influence the design of the components and the entropy generation distribution significantly. The pinch temperature difference in the discharge duration limits the lowest molten salt temperature. The device with real heat exchangers produces higher entropy generation and lower second law efficiency. Environmental water input requires more heat and entropy generation for the same steam supply. Recommendations are provided for practical designs.

A study on heat storage sizing and flow control for a domestic scale solar-powered organic Rankine cycle-vapour compression refrigeration system

This paper presents the off-design modelling of a domestic scale solar organic Rankine cycle (ORC) and vapour compression cycle (VCC) in a coupled operation in different operating modes by using evacuated flat plate (EFP) collectors. Thermodynamic and parametric studies of such coupled system in literature usually assume that the isentropic efficiencies of expander and compressor and the heat exchanger pinch temperature differences are constant. Moreover, studies for directly coupling the ORC-VCC system with solar collectors are somewhat rare. Transient performance of the solar ORC-VCC considering the off-design behaviour of the system components needs to be investigated. A simulation for a period of 24 hrs is conducted by considering the electricity and cooling demand of a 60 m2 office building during a typical day in July for Istanbul. The effect of heat storage capacity on meeting the demand is investigated for a given area of EFP collectors. Moreover, the water flow rate to the boiler is controlled periodically to provide the demanded electricity and cooling. The required EFP collector area and heat storage unit volume have been determined to be 80 m2 and 9.4 m3, respectively. For these design parameters, 25.6 kWh cooling and 18.76 kWh electricity can be generated during a typical day in July. Finally, a simulation is given for such design on a sunny winter day in February, 5.5 kWh electricity and 104 kWh heat can be produced.

Energetic, entropic and economic analysis of a heat pump coupled humidification dehumidification desalination system using a packed bed dehumidifier

In this paper, the humidification dehumidification (HD) desalination system, driven by a mechanical vapor compression heat pump (MVCHP), is proposed. The direct contact dehumidifier, with packings filled, is applied to condense the humid air for water producing. On the basis of the thermodynamic laws, the energetic and entropic analysis, with the modified effectiveness to describe the heat and mass transfer characteristics of humidification, are first achieved to assess the desalination performance of the combined system. The influence laws from the terminal temperature difference (TTD) of the evaporator, the pressure ratio (PR) during compression and pinch temperature difference (PTD) of the condenser on the desalination performance are also explored and presented. The research results presented that the top thermodynamic performance of the MVCHP coupled HD desalination system arises as 71.56 kg h−1 for the water production, 5.28 for the gained-output-ratio (GOR) and 1.73 kJ kg−1 K−1 for the specific entropy generation rate, while two fully coupled cases at 1.21 and 1.96 for the mass flow rate ratio will emerge. It is found that water production can be elevated effectively through increasing the pressure ratio, while the peak value of GOR keeps at the design conditions. Furthermore, the positive effects from lower values both for the TTDe and PTDcon on the performance of the MVCHP coupled HD desalination system are proved. After the economic analysis, it is calculated that the cost of fresh water is about 0.018 $ L−1 with a lifetime of 10 years. According to the performance comparison, the great potential of energy conservation for the MVCHP coupled HD desalination system are validated, although the water production will be reduced after the integration of the MVCHP.

Optimal heat rejection pressure of CO2 heat pump water heaters based on pinch point analysis

The control of heat rejection pressure is of great importance for transcritical CO2 heat pump water heaters. However, almost all the existing correlations of optimal heat rejection pressure are based on an assumption of constant CO2 exit temperature from gas cooler when changing the high-side pressure. Actually, pinch effect occurring in CO2 gas cooler would have great impacts on CO2 exit temperature as the high-side pressure varies, which in turn affects overall system performance. A cycle simulation model was developed in the present study incorporating pinch point analysis for gas cooling process. The model is independent on heat exchanger geometry parameters, which guarantees the generality of the model. Sensitivity analyses of main impact factors were conducted in detail including inlet and outlet water temperature, refrigerant evaporation temperature and superheat degree, pinch temperature difference and isentropic efficiency of compressor. Furthermore, a new correlation of optimal heat rejection pressure was proposed.

Thermodynamic, economic analysis and optimization of a heat pump driven desalination system with open-air humidification dehumidification configurations

Abstract In this paper, the heat pump is coupled to a humidification dehumidification desalination system, with open-air configurations, to enhance the energy conversion efficiency. After establishing the energetic and entropic equations for all the thermal processes, the correlations between the desalination performance and the critical parameters, including the compression pressure ratio, pinch temperature difference of the condenser, terminal temperature difference of the evaporator, are revealed. Afterwards, the mass flow rate ratio and effectiveness during humidification and dehumidification are treated as the decision variables to optimize the energy conversion efficiency of the heat pump driven desalination system. The simulation results show that the actual top water production and gained-output-ratio of the desalination system reach 88.34 kgh−1 and 3.72 at the balance condition of the humidifier. It is also obtained that raising the compression pressure ratio and reducing the pinch temperature difference of the condenser and terminal temperature difference of the evaporator, can promote the desalination performance. Furthermore, based on the particle swarm optimization algorithm, the best desalination performance, with 151.03 kgh−1 for the water production, and 5.95 for the gained-output-ratio, is optimized within the prescribed range of the decision variables, while the corresponding cost of produced water arrives at 0.015$L−1 through the economic analysis.

Thermodynamic performance analysis of a R245fa organic Rankine cycle (ORC) with different kinds of heat sources at evaporator

The first and the second law analyses of a 20 kWeR245fa organic Rankine cycle (ORC) with low grade temperature heat sources were numerically studied. There were three different heat sources such as hot water, saturated steam, and combined hot water/saturated steam used to supply heat at the ORC evaporator. The heat source temperature was varied in a range of 80–110 °C and the pinch temperature difference between the heat source temperature and the evaporating temperature was in a range of 1–10 °C. The ORC condensing temperature was kept constant at 40 °C. The results revealed that the combined hot water/saturated steam heat source provided a highest second law or exergy efficiency due to less irreversibility generated at the evaporator followed by the saturated steam and the hot water heat sources, respectively. The increase of heat source temperature at a specific pinch could increase the second law efficiency of the cycle. However, the increase of pinch value resulted in lower second law efficiency. In addition, for the below pinch zone, with the hot water/saturated steam heat source, the reduction of the hot water flow rate slightly enhanced the ORC second law efficiency as the exergy destruction below pinch was reduced. However, there was a limit that the flow rate should not be reduced below a specific value otherwise a very high heat exchanger area was needed.

Advanced gas turbine performance modelling using response surface methods

ABSTRACTSurrogate models are widely used for dataset correlation. A popular application very frequently shown in public literature is in the field of engineering design where a large number of design parameters are correlated with performance indices of a complex system based on existing numerical or experimental information. Such an approach allows the identification of the key design parameters and their impact on the system’s performance. The generated surrogate model can become part of wider computational platforms and enable optimisation of the complex system without the need to run expensive simulations.In this paper, a number of design point simulations for a combined gas-steam cycle are used to generate a response surface. The generated response surface correlates a range of cycle’s key design parameters with its thermal efficiency while it also enables identification of the optimum overall pressure ratio and the high pressure level of the raised steam across a range of recuperator effectiveness, pinch temperature difference across the heat recovery steam generator and the pressure at the condenser. The accuracy of a range of surrogate models to capture the design space is evaluated using root mean square statistical metrics.

Investigation on humidification dehumidification desalination system coupled with heat pump

In this paper, a mechanical compression heat pump was coupled into the water-heated humidification dehumidification (HDH) desalination system, in which the condenser is applied to heat the seawater while the evaporator is considered to recover the carried heat from the discharged brine. In view of the integration mechanisms between the desalination unit and heat pump, mathematical models based on mass and energy equilibrium are built. Performance of the combined system, including the water production and gained-output-ratio (GOR), at the design conditions are first obtained. Moreover, the relevant influences from the prescribed critical parameters on the performance of the desalination system are simulated and analyzed. The simulation results show that the best performance of the coupled HDH desalination system, appearing as 82.12 kgh−1 for the water production and 5.14 for the GOR, is acquired at the balance condition of the dehumidifier. Two fully coupled cases, in which the heater or cooler is no longer required, when the air mass flow rate is located at 0.17 kgs−1 and 0.35 kgs−1, are found. During the influence analysis, a best value of pressure ratio, PR = 4, is found due to a highest value of GOR as 5.14 although the relevant value of water production is attained as 106.53 kgh−1 when the pressure ratio is fixed at 5. Furthermore, it is also observed that a lower value of the pinch temperature difference for the condenser, higher values of effectiveness both for the humidification and dehumidification, are effective to elevate the water production and the corresponding thermal efficiency.

Thermodynamic analysis on a modified Kalina cycle with parallel cogeneration of power and refrigeration

A modified Kalina cycle with parallel cogeneration of power and refrigeration abbreviated as PPR-KC was proposed and investigated. The cycle cogenerates refrigeration by further reclaiming heat resource from the boiler. A parallel branch of work solution is split at the outlet of the mid-p-absorber, and it undergoes pumping, generation, rectification and condensation before evaporation for refrigeration. Then the vapor from the evaporator goes to a mixer at the low-p-absorber inlet via a subcooler, while the dilute solution from the generator heats the basic solution in the second recuperator before going to the mixer via a throttle valve. The split fraction which determines the relative percentage of work solution flow rate for power generation or refrigeration can be altered arbitrary at user’s demand. The parameter optimization was conducted and the optimum work concentration is 0.5 and the optimum basic concentration shall be adjusted with different split fraction. Under temperatures of heat resource/cooling water are 400 °C/30 °C and minimum pinch temperature differences in heat exchangers with/without flue gas are 20/5 K respectively, the comprehensive power recovery efficiency of the PPR-KC reaches 27.2%, which is 9.85% and 20.24% respectively higher than those of the cogeneration cycle proposed by Liu et al. and the Kalina cycle.

Evaluation and optimization of organic Rankine cycle (ORC) with algorithms NSGA-II, MOPSO, and MOEA for eight coolant fluids

In this study, a simple organic cycle for eight subcritical coolant fluids has been studied thermodynamically and economically. For all the coolants, the present cycle was optimized for the best thermal and exergy efficiencies and the best cost of energy production. In a multi-purpose procedure, using the three methods NSGA-II, MOPSO, and MOEA/D, design variables in the optimization are the inlet turbine pressure and temperature, the pinch temperature difference, the proximity temperature difference in regenerator exchanger, and condenser temperature difference. The optimization results show that, in all three methods, the impact of the parameters’ inlet turbine temperature and pressure on the three objective functions is much more than other design parameters. Coolant with positive temperature gradients shows a better performance but lower produced power. In optimization methods, among all the coolants, the MOPSO method showed higher thermal and energy efficiency, and the MOEA/D showed lower production power costs. In terms of the rate of convergence, also both the MOPSO and NSGA-II methods showed better performance. The fluid R11 with the 25.7% thermal efficiency, 57.3% exergy efficiency, and 0.054 USD cost per kWh showed the best performance among all of the coolants.

A supercritical CO2 Brayton cycle with a bleeding anabranch used in coal-fired power plants

This study investigates a novel supercritical carbon dioxide Brayton cycle with a bleeding anabranch used in coal-fired power plants. For the proper utilization of the medium-low temperature heat in the boiler, the coupling between the air-preheater and the cycle is concentrated. After analyzing the distribution of heat duty in the boiler, the flow diagram of the Brayton cycle is design. Subsequently, the influences of maximal temperature/pressure, pinch temperature difference, component pressure drop, and preheated air temperature on the cycle efficiency are studied. Results show that the proposed cycle coupled with a coal-fired power plant can achieve 52.33% gross cycle efficiency and 49.5% net low heat value efficiency with 296 bar/650 °C. A further increment of 1% pt. in gross cycle efficiency can be achieved with the decrement of 5 K pinch temperature difference or 0.7 bar pressure loss in the recuperators. The simulation indicates that an increment of 2.14% pt. in gross cycle efficiency can be further achieved with 370 bar/700 °C. Besides, the parameter of the bleeding anabranch do not affect the cycle performance. In conclusion, this study proposes a promising novel supercritical carbon dioxide cycle coupled with coal-fired power plants.

Thermodynamic optimization and thermoeconomic analysis of four double pressure Kalina cycles driven from Kalina cycle system 11

In this paper four configurations of double pressure Kalina cycle system are presented and optimized all of which are modifications of Kalina cycle system 11. In order to set the exact pinch temperature difference an iterative method is applied. Prior to the optimization, the base cycle is validated by comparing the result with a reference. The heat transfer fluid of the inlet stream is supposed to be the product of combustion at 3 different temperatures, 383.15K, 413.15K and 443.15 and the results are compared at the base case and the optimum conditions. In order to present a thorough evaluation, thermoeconomic analysis is also presented in which levelized cost of electricity is selected as the criterion. Different decision variables can be defined for the cycles based on the cycle’s degrees of freedom. Pressure levels, mass flow rate and ammonia concentration of the base stream and split ratio are the decision variables. Exergy efficiency is considered as the objective function and the innovated double pressure Kalina cycles as well as the base Kalina cycle are compared. Results show that the Kalina cycle system named 112b is the most efficient cycle at the base condition. It is also shown that by increasing the heat source temperature the exergy efficiency and the purchased equipment cost at the optimum condition rises while the levelized cost of electricity lowers. Thermoeconomic evaluation indicates that at both base and the optimum conditions, the levelized cost of electricity of the base cycle is less.

Evaluation of the ejector refrigeration system with environmentally friendly working fluids from energy, conventional exergy and advanced exergy perspectives

This paper presents the evaluation of the ejector refrigeration system from three levels: energy analysis, conventional exergy analysis and advanced exergy analysis, in light of introducing the thermal conductance and the objective function, the exergy efficiencies, the exergetic rehabilitation ratio and the exergetic improvement potential ratio, respectively. Five environmentally friendly working fluids, namely R600, R600a, R601a, R1233zd(E) and R1234ze(E), are used to compare their performance and working characteristics in the system. At the normal condition, it is found that these candidates have different orders of highest performance by different analysis, but agree that the ejector has the highest priority of improvement. The parametric study shows that these candidates have the same optimum pinch temperature differences in the condenser (7°C and 3°C) and in the evaporator (3°C and 2°C) to minimize the thermal conductance and the objective function, while they are different in the generator. The ejector efficiencies have considerable influence on the system performance, and a 0.1 increase in ejector efficiency could lead to the increase of system exergy efficiency from 1.38% to 10.33%. The pump efficiency has insignificant influence the system performance, but a 0.1 increase in the pump efficiency results in a 7.37% decrease in exergetic improvement potential ratio of the pump. By comprehensive comparisons, R1233zd(E) has generally higher system performance than the other four candidates, it is therefore recommended as the good working fluid for the ejector refrigeration system.

Performance simulation of a power-water combined plant driven by low grade waste heat

Both power and freshwater are obligatory for life and industry production. The present paper suggested a novel power-water combined plant (PWCP) driven by low grade waste heat to supply power and water simultaneously, in which the organic Rankine cycle (ORC) and air-heated humidification dehumidification (HDH) desalination subsystem are coupled. Through the connection effect of the condenser for the ORC power subsystem, mathematical models of the combined plant are presented. The relevant performance of the combined plant at the designed conditions is simulated iteratively, and the influences from the condensing temperature and pinch temperature difference (PTD) are investigated. The simulated results show that the peak value of the output power arrives at Wnet=14.90kW when the evaporation temperature of R123 is T1=401.15K. Based on the conditions at the maximum power case of the ORC, the maximum freshwater production reaches mfw=381.66kgh−1 at the balance condition of the dehumidifier, HCRd=1 at msw=2.9kgs−1. Accordingly, the peak value of the gained output ratio (GOR), GOR=1.93, also indicates the best energy utilization situation within the HDH desalination system at HCRd=1. After the regulation of the condensing temperature as well as pinch temperature difference of the ORC subsystem, a peak value of power for the ORC system, Wnet=16.56kW at T3=338.15K, and maximum value of the freshwater production, mfw=381.66kgh−1 at T3=343.15K are obtained.

Optimisation of a small solar organic Rankine cycle based on the exergetic analysis

A mathematical model is written for a solar organic Rankine cycle (ORC) taking into account the Pinch temperature differences in the system heat exchangers. The (exergetic factor)-(variation in enthalpy) diagram of the hot and cold composite curves of the heater-evaporator and the superheater of the ORC gives information about the strategy to be followed in order to improve the structure of the system. The introduction of an internal recuperative heat exchanger to preheat the ORC fluid and the splitting of the solar fluid to balance the heat capacities in the heater and the superheater are also considered. An exergetic analysis reveals the magnitude of the exergy destruction in the key pieces of the system. A comparative simulation of the way, the ORC operates with different organic fluids points out that R245fa gives the highest exergetic efficiency.

Optimization and energy integration of heat recovery and power generation system

A large quantity of waste gas from industrial processes can be used for steam and power generation. Thus, it is of great interest to define a strategy for these power generation systems to get improved performances and efficiency. Three detailed thermodynamic models of heat recovery and power generation from industrial waste gas specified as single-pressure, dual-pressure, and energy integration systems are presented; Meanwhile, impact factors such as steam parameters, pinch temperature difference, and fluctuation of waste gas source on power generating capacity and total site efficiency are comparable analyzed by adopting a thermodynamic analysis combined with pinch method. Also, a case study of energy integration which doubles the improving effect of power generation accompanied with considerable energy saving is performed on basis of fuel efficiency and exergy calculation. In particular, the hierarchical strategy of energy integration of the total site is proposed and exampled.

Thermoeconomic and environmental analyses of a low water consumption combined steam power plant and refrigeration chillers – Part 1: Energy and economic modelling and analysis

Two novel configurations are proposed to replace the conventional Rankine cycle based steam power plants (SPP) with natural draft wet cooling towers (NDWCT) as cooling units. Closed feedwater heater unit of the base SPP-NDWCT system is eliminated in order to combine a vapor compression refrigeration (VCR) and an absorption heat pump (ABHP) with the base SPP-NDWCT system. Both VCR-SPP-NDWCT and ABHP-SPP-NDWCT systems are integrated to decrease the NDWCT load which could result in water losses decrease. In part one of the presented two-part paper, model-based energy and economic analyses are performed to compare systems performance and applicability. The temperature difference at pinch point and temperature difference between the hot and cold sides of the heat exchangers which were used for systems integration in VCR-SPP-NDWCT, and the absorber pressure and temperature in ABHP-SPP-NDWCT system are studied using parametric analysis procedure. A water-fuel management strategy is also introduced for the ABHP-SPP-NDWCT system according to the influence of the absorber pressure changes on system water and fuel consumption. In part 2, environmental and thermoeconomic analyses are performed to complete a comprehensive study on designing steam power plants. The results of part 1 showed that water losses and total annual costs decreased by 1–18% and 0–4.7% for the ABHP-SPP-NDWCT system but increased by 11% and 60% for the VCR-SPP-NDWCT system, respectively.

Thermodynamic and Economic Analysis and Multi-objective Optimization of Supercritical CO2 Brayton Cycles

Supercritical CO2 Brayton cycles (SCO2BC) including the SCO2 single-recuperated Brayton cycle (RBC) and recompression recuperated Brayton cycle (RRBC) are considered, and flexible thermodynamic and economic modeling methodologies are presented. The influences of the key cycle parameters on thermodynamic performance of SCO2BC are studied, and the comparative analyses on RBC and RRBC are conducted. Nondominated Sorting Genetic Algorithm II (NSGA-II) is selected for the Pareto-based multi-objective optimization of the RRBC, with the maximum exergy efficiency and the lowest cost per power (k$/kW) as its objectives. Artificial neural network (ANN) is chosen to accelerate the parameters query process. It is shown that the cycle parameters such as heat source temperature, turbine inlet temperature, cycle pressure ratio, and pinch temperature difference of heat exchangers have significant effects on the cycle exergy efficiency. The exergy destruction of heat exchanger is the main reason why the exergy efficiency of RRBC is higher than that of the RBC under the same cycle conditions. RBC has a cost advantage from economic perspective, while RRBC has a much better thermodynamic performance, and could rectify the temperature pinching problem that exists in RBC. It is also shown that there is a conflicting relationship between the cycle cost/cycle power (CWR) and the cycle exergy efficiency. The optimization results could provide an optimum tradeoff curve enabling cycle designers to choose their desired combination between the efficiency and cost. ANN could help the users to find the SCO2BC parameters fast and accurately.

Multi objective optimisation of a Kalina power cycle integrated with parabolic trough solar collectors based on exergy and exergoeconomic concept

The objective of this research is to assess the variation of exergoeconomic parameters and optimisation of a solar driven Kalina cycle. A thermal storage system and an auxiliary boiler are used to provide continuous power output when solar radiation is not sufficient. Ammonia basic concentration, turbine inlet pressure, turbine outlet pressure, evaporator inlet temperature, evaporator outlet temperature, and evaporator pinch temperature difference are selected as the decision variables owing to their significant effects on the energy and exergy efficiencies and unit cost of electricity. The results indicate that the collectors exhibit low exergy performance and do not play an important role on exergoeconomic performance. Then, the genetic algorithm-based optimisation is carried out to maximise the efficiencies and minimise the electricity production cost. The results demonstrate that the multi objective optimisation causes reduction of electricity production cost around 14% and enhancement the energy and exergy efficiencies within 8.5% and 6.7%, respectively.