Transcritical Rankine Cycle Research Articles

Thermal and economic analyses of a compact waste heat recovering system for the marine diesel engine using transcritical Rankine cycle

The aim of this study is to investigate the economic performance of a novel compact waste heat recovering system for the marine diesel engine. The transcritical Rankine cycle is employed to convert the waste heat resources to useful work with R1234yf. To evaluate the utilizing efficiency and economic performance of waste heat resources, which are exhaust gas, cylinder cooling water and scavenge air cooling water, three operating models of the system are investigated and compared. The levelized energy cost, which represents the total cost per kilo-watt power, is employed to evaluate the economic performance of the system.The economic optimization and its corresponding optimal parameters of each operating model in the compact waste heat recovering system are obtained theoretically. The results show that the minimal levelized energy cost of the proposed system operated in Model I is the lowest of the three models, and then are Model II and Model III, which are 2.96% and 9.36% lower for, respectively. Similarly, the CO2 emission reduction is the highest for Model I of the three models, and 21.6% and 30.1% lower are obtained for Model II and Model III, respectively. The compact waste heat recovering system operated in Model I has superiority on the payback periods and heavy diesel oil saving over the others. Finally, the correlations using specific work of working fluid and condensation temperature as parameters are proposed to assess the optimal conditions in economic performance analysis of the system.

Recuperated power cycle analysis model: Investigation and optimisation of low-to-moderate resource temperature Organic Rankine Cycles

A numerical model for recuperated power cycles for renewable power applications is described in the present paper. The original code was written in Python and results for a wide range of working fluids and operating point conditions are presented. Here, the model is applied to subcritical and transcritical Rankine cycles. It comprises a brute-force search algorithm that covers a wide parametric study combining working fluid, resource and cooling temperatures as well as high-side pressures in order to ascertain the best working fluid for a given resource temperature and operating point. The present study determined the fluids that maximise the specific energy production from a hot stream for a range of low-to-medium temperature (100–250 °C) resources. This study shows that for the following resource temperatures: 100 °C, 120 °C, 150 °C, 180 °C and 210 °C, R125, R143a, RC318, R236ea and R152a were found to maximise specific energy production, respectively. In general, the inclusion of a recuperator within the power cycle results in greater specific energy production for a given operating temperature. However, it was found that for all fluids there was a threshold pressure above which the inclusion of a recuperator did not enhance system performance. This may have design and economic ramifications when designing next-generation transcritical and supercritical power cycles.

Thermo-economic optimization of low-grade waste heat recovery in Yazd combined-cycle power plant (Iran) by a CO2 transcritical Rankine cycle

A transcritical CO2 Rankine cycle is proposed for recovering low-grade waste heat of Yazd combined-cycle power plant in Iran. Each power generation module of this plant consists of two 159 MW Siemens SGT-5-2000E gas turbines and one 132 MW steam turbine. Reducing exhaust gas temperature from 150 °C to 70 °C, the plant can generate excessive power. From thermodynamics approach, it is demonstrated that by fixing the maximum temperature at 145 °C and varying the maximum pressure, the efficiency and the net power output are maximized at Pmax = 185 bar. In the aforementioned operating point, about 6.3 MW is retained for the selected power plant with a nominal 450 MW of power generation. A more actual case considering thermodynamic losses and economic considerations is then investigated. Genetic algorithm is implanted to conduct a parametric optimization to maximize the benefit-cost ratio which is defined on the basis of total bare module cost and net power output. The results indicate that the cycle costs are more influenced by the maximum pressure rather than the maximum temperature. Through this parametric optimization, the CO2 cycle can produce about 4.04 MW. This is about 0.9% of the plant capacity and increases the total efficiency about 0.4%.

Economic performances optimization of the transcritical Rankine cycle systems in geothermal application

The aim of this study is to investigate the economic optimization of a TRC system for the application of geothermal energy. An economic parameter of net power output index, which is the ratio of net power output to the total cost, is applied to optimize the TRC system using CO2, R41 and R125 as working fluids. The maximum net power output index and the corresponding optimal operating pressures are obtained and evaluated for the TRC system. Furthermore, the analyses of the corresponding averaged temperature differences in the heat exchangers on the optimal economic performances of the TRC system are carried out. The effects of geothermal temperatures on the thermodynamic and economic optimizations are also revealed. In both optimal economic and thermodynamic evaluations, R125 performs the most satisfactorily, followed by R41 and CO2 in the TRC system. In addition, the TRC system operated with CO2 has the largest averaged temperature difference in the heat exchangers and thus has potential in future application for lower-temperature heat resources. The highest working pressures obtained from economic optimization are always lower than those from thermodynamic optimization for CO2, R41, and R125 in the TRC system.

Normalized performance optimization of supercritical, CO2-based power cycles

This study considers the multivariable thermodynamic analysis and optimization of transcritical Rankine cycles operating with carbon dioxide as working fluid. Three dependent variables were used as figures of merit: the net power produced by the cycle, and its 1st and 2nd Law efficiencies, all calculated in absolute terms and per unit of global conductance (UA)Total, where (UA)Total accounts for the conductance of all heat exchangers used in the cycle. The key variables were the high pressure of the CO2 within the cycle and the temperature of the heat source, along with four different cycle configurations: (i) a basic power cycle, (ii) a cycle with a recuperator, (iii) a cycle with re-heating and (iv) a cycle with a recuperator and re-heating, namely, combined cycle. The optimization process relied on optimization routines and considered latent and sensible heat sources. This procedure was able to show that while the individually defined figures of merit mostly presented established trends, the normalized figures of merit (i.e., those defined per unit of UA) are highly dependent on the parameters considered and clearly show the existence of optimum values, which are a function of the cycle's configuration, figures of merit considered and operation parameters.

A comparative study on thermo-economic performance between subcritical and transcritical Organic Rankine Cycles under different heat source temperatures

In this paper, the net power output, exergy efficiency and levelized energy cost of system were selected as performance indicators for assessing Organic Rankine Cycle (ORC). Firstly, the turbine inlet temperature and pressure meeting the requirement of pinch point temperature difference of evaporator in transcritical ORC (trans-ORC) were determined based on performance optimization. Subsequently, the thermo-economic performance of a subcritical ORC (sub-ORC) using R601 as working fluid and a trans-ORC using R134a as working fluid were compared under different heat source temperatures and a fixed outlet temperature of flue gas. Results show that for trans-ORC, when the pinch point temperature difference of evaporator lies between the inlet and outlet of evaporator, a lower inlet pressure of turbine is favorable; when the pinch point temperature difference of evaporator is located at the outlet of evaporator, there exists an optimal inlet pressure of turbine. Either for sub-ORC or trans-ORC, the net power output increases and levelized energy cost decreases with the increase in heat source temperature. For sub-ORC, exergy efficiency of system increases monotonously with heat source temperature, while for trans-ORC, exergy efficiency of system grows up firstly and then reduces (or keeps constant) with the increasing of heat source temperature. Moreover, for net power output and exergy efficiency of system, there exist a range of heat source temperatures making trans-ORC better than sub-ORC, and the heat source temperature region extends with the increase in pinch point temperature difference of evaporator. For levelized energy cost of system, the sub-ORC is always superior to trans-ORC.

Comparison of CO2 and Steam in Transcritical Rankine Cycles for Concentrated Solar Power

High temperature, high pressure transcritical condensing CO2 cycle (TC-CO2) is compared with transcritical steam (TC-steam) cycle. Performance indicators such as thermal efficiency, volumetric flow rates and entropy generation are used to analyze the power cycle wherein, irreversibilities in turbo-machinery and heat exchangers are taken into account. Although, both cycles yield comparable thermal efficiencies under identical operating conditions, TC-CO2 plant is significantly compact compared to a TC- steam plant. Large specific volume of steam is responsible for a bulky system. It is also found that the performance of a TC-CO2 cycle is less sensitive to source temperature variations, which is an important requirement of a solar thermal system. In addition, issues like wet expansion in turbine and vacuum in condenser are absent in case of a TC-CO2 cycle. External heat addition to working fluid is assumed to take place through a heat transfer fluid (HTF) which receives heat from a solar receiver. A TC-CO2 system receives heat though a single HTF loop, whereas, for TC-steam cycle two HTF loops in series are proposed to avoid high temperature differential between the steam and HTF.

A supercritical or transcritical Rankine cycle with ejector using low-grade heat

A supercritical or transcritical Rankine cycle with ejector (ESRC, ETRC) based on the basic supercritical or transcritical Rankine cycle (SRC, TRC) are proposed for the conversion of low-grade heat to power in this paper. The thermodynamic comparative analyses on the SRC, TRC and ESRC, ETRC are conducted on the power output and thermal efficiency of the cycles. The carbon dioxide is chosen as the working fluid for the cycles. Water is chosen as the fluid of the low-grade heat. The water temperature is selected in a range of 60–90°C, a typical water temperature is 80°C, and the mass flow rate is 1kg/s. The same temperature and mass flow rate of the water is the standard condition for the comparative analysis of the thermodynamic performance. Results show that the net power output of the cycles could be ranked from high to low: ESRC>ETRC>SRC>TRC, and the thermal efficiency could be ranked from high to low: TRC>SRC>ESRC>ETRC.

Thermodynamic analysis of carbon dioxide blends with low GWP (global warming potential) working fluids-based transcritical Rankine cycles for low-grade heat energy recovery

Carbon dioxide is a promising natural working fluid that can be used in transcritical Rankine cycles due to environmental and safety concerns. However, the high operation pressure has to be reduced and the relatively low efficiency of the system has to be increased. Traditional working fluids have been widely investigated to reclaim low-grade heat energy, and most of them have high GWPs (global warming potentials) or are flammable or even toxic. Consequently, to mitigate the above disadvantages, we studied zeotropic mixtures of carbon dioxide blends with 7 low GWP working fluids for use in a TRC (transcritical Rankine cycle) for low-grade heat conversion. The results revealed that these zeotropic mixtures can help improve the thermal efficiency of the TRC and decrease the operation pressure compared to that of pure CO2. Owing to the perfect thermal match in the heat transfer process, higher exergy efficiencies were achieved for the entire system when zeotropic mixtures were used than pure CO2. Maximum exergy efficiencies exist for the TRC at the corresponding optimal pressures for each mixture. Finally, the mixture CO2/R161 is recommended for small capacity instruments for its high efficiency, in spite of its high flammability; the mixtures CO2/R1234yf and CO2/R1234ze can be used in TRCs with larger capacities due to their lower flammability.

Exergy Analysis of Transcritical Regenerative Organic Rankine Cycle Using Isobutane

Exergy analysis is performed for transcritical Organic Rankine Cycle (ORC) with internal heat exchanger based on the second law of thermodynamics. Effects of source temperature as well as turbine inlet pressure (TIP) are investigated on the exergy destructions (or anergies) of the system as well as exergy efficiency. Results show that irreversibility of the system decreases with increasing TIP or decreasing source temperature. Exergy efficiency decreases with increasing source temperature; however has a maximum value with respect to TIP.

On the coupled system performance of transcritical CO2 heat pump and rankine cycle

As one of the natural refrigerants, CO2 is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of rankine cycle (RankC), the coupled system cycle (CSC) was designed and the performance was analyzed in this paper, which the CSC is combined by the RankC and the transcritical CO2 heat pump cycle with an expander. Based on thermodynamic principles, the performance analysis platform was designed and the performance analysis was employed. The results show that the average efficiency of the RankC is about 30 %, and the extraction cycle is about 32 %, while the CSC is about 39 %, and the last one is better than the others at the same parameters. With increasing of the boiler feed water temperature, the efficiencies of the three kinds of cycles show increasing trend. With increasing of pressure in conderser–evaporator or outlet temperature of gas cooler, the efficiency of the CSC shows a downward trend. Some fundamental data were obtained for increasing the RankC efficiency by waste heat recovery, and play an active role in improvement the efficiency of power plants.

Off-design analysis of ORC and CO2 power production cycles for low-temperature surplus heat recovery

In process industry large amounts of energy are rejected to the ambient. Recovery of this surplus energy is a wide topic. Among the strategies for energy recovery, production of electricity is very interesting, due to the versatility of this form of energy. For the relatively low temperature heat, which is most commonly encountered for industrial surplus heat, the Organic Rankine Cycle is a well established technology. The transcritical Rankine Cycle recently received special attention due to its performances for energy recovery from low temperature sources. CO2 is a natural candidate as working fluid for this technology. It combines high performance, low cost, low toxicity, is non-flammable and has no environmental impact. This paper focuses on the off-design operation of Rankine cycles and compares the behaviour of transcritical CO2 cycles and an ORC cycle with R-123 as working fluid. The main observation is that the ORC is very sensitive to reduction in available heat, and will with only small changes get droplets in the inlet of the expander. This shows that it is reasonable to operate the ORC with some degrees superheat, to have a buffer. Superheating the outlet 5 to 10 K has only a small effect on the cycle performance. However the gained robustness is also relatively small. With small increments in the available heat source, the CO2 cycle also seem to have a marginally better response without control of the process, which indicates that it is more robust and less in need of detailed control.

Performance Analysis of Coupled System of CO<sub>2</sub> Transcritical Heat Pump and Rankine Cycle

Based on thermodynamic principles, the coupled systems performance of the Rankine cycle and the transcritical CO2compression cycles with a throttle valve and the cycle with an expander were analyzed, respectively. The results show that the corresponding maximum performance in transcritical CO2heat pump, there is an optimal efficiency of the coupled system. With the temperature increasing of boiler feed water, the efficiency of the two coupled system showed an increasing trend; with increasing of discharge pressure of condenser or outlet temperature of gas cooler, both of the two kinds coupled system efficiency showed a downward trend. Under the same conditions, the efficiency of the coupled system with an expander is better than the system with a throttle valve. This paper can provide the fundamental datum for the power plant waste heat recovery.

Comparative analysis of natural and conventional working fluids for use in transcritical Rankine cycle using low-temperature geothermal source

Theoretical analyses of natural and conventional working fluids-based transcritical Rankine power cycles driven by low-temperature geothermal sources have been carried out with the methodology of pinch point analysis using computer models. The regenerator has been introduced and analyzed with a modified methodology considering the considerable variation of specific heat with temperature near the critical state. The evaluations of transcritical Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature and optimized gas heater pressures at various geothermal source temperature levels ranging from 80 to 120°C. The performances of CO2, a natural working fluid most commonly used in a transcritical power cycle, have been indicated as baselines. The results obtained show: optimum thermodynamic mean heat injection temperatures of transcritical Rankine cycles are distributed in the range of 60 to 70% of given geothermal source temperature level; optimum gas heater pressures of working fluids considered are lower than baselines; thermal efficiencies and expansion ratios (Expr) are higher than baselines while net power output, volume flow rate at turbine inlet (V1) and heat transfer capacity curves are distributed at both sides of baselines. From thermodynamic and techno-economic point of view, R125 presents the best performances. It shows 10% higher net power output, 3% lower V1, 1.0 time higher Expr, and 22% reduction of total heat transfer areas compared with baselines given geothermal source temperature of 90°C. With the geothermal source temperature above 100°C, R32 and R143a also show better performances. R170 shows nearly the same performances with baselines except for the higher V1 value. It also shows that better temperature gliding match between fluids in the gas heater can lead to more net power output. Copyright © 2010 John Wiley & Sons, Ltd.

Energetic and exergetic analysis of CO2- and R32-based transcritical Rankine cycles for low-grade heat conversion

Transcritical Rankine cycles using refrigerant R32 (CH2F2) and carbon dioxide (CO2) as the working fluids are studied for the conversion of low-grade heat into mechanical power. Compared to CO2, R32 has higher thermal conductivity and condenses easily. The energy and exergy analyses of the cycle with these two fluids shows that the R32-based transcritical Rankine cycle can achieve 12.6–18.7% higher thermal efficiency and works at much lower pressures. An analysis of the exergy destruction and losses as well as the exergy efficiency optimization of the transcritical Rankine cycle is conducted. Based on the analysis, an “ideal” working fluid for the transcritical Rankine cycle is conceived, and ideas are proposed to design working fluids that can approach the properties of an “ideal” working fluid.

Comparative analysis of CO2-based transcritical Rankine cycle and HFC245fa-based subcritical organic Rankine cycle using low-temperature geothermal source

A detailed thermodynamic and techno-economic comparison is presented for a CO2-based transcritical Rankine cycle and a subcritical organic Rankine cycle (ORC) using HFC245fa (1,1,1,3,3-pentafluoro-propane) as the working fluid driven by the low-temperature geothermal source, in order to determine the configuration that presents the maximum net power output with a minimum investment. The evaluations of both Rankine cycles have been performed based on equal thermodynamic mean heat rejection temperature by varying certain system operating parameters to achieve each Rankine cycle’s optimum design at various geothermal source temperature levels ranging from 80°C to 120°C. The results obtained show that the optimum thermodynamic mean heat injection temperatures of both Rankine cycles are distributed in the scope of 55% to 65% of a given geothermal source temperature level, and that the CO2-based transcritical Rankine cycle presents 3% to 7% higher net power output, 84% reduction of turbine inlet volume flow rate, 47% reduction of expansion ratio and 1.68 times higher total heat transfer capacity compared with the HFC245fa-based subcritical ORC. It is also indicated that using the CO2-based transcritical system can reduce the dimension of turbine design. However, it requires larger heat transfer areas with higher strength heat exchanger materials because of the higher system pressure.

A Transcritical CO2 Rankine Cycle With LNG Cold Energy Utilization and Liquefaction of CO2 in Gas Turbine Exhaust

A novel transcritical Rankine cycle is presented in this paper. This cycle adopts CO2 as its working fluid with exhaust from a gas turbine as its heat source and liquefied natural gas (LNG) as its cold sink. With CO2 working transcritically, large temperature difference for the Rankine cycle is realized. Moreover, the CO2 in the gas turbine exhaust is further cooled and liquefied by LNG after transferring heat to the Rankine cycle. In this way, not only is the cold energy utilized but also a large part of the CO2 is recovered from burning of the vaporized LNG. In this paper, the system performance of this transcritical cycle is calculated. The influences of the highest cycle temperature and pressure to system specific work, exergy efficiency, and liquefied CO2 mass flow rate are analyzed. The exergy loss in each of the heat exchangers is also discussed. It turns out that this kind of CO2 cycle is energy-conservative and environment-friendly.

96/05421 HCFC22-based absorption cooling systems. Part III: Effects of different absorber and condenser temperatures

The aim of this study is to investigate the economic performance of the waste heat recovery (WHR) system for a marine diesel engine. Four waste heat sources, which are exhaust gas, cylinder cooling water, scavenge air cooling water and lubricating oil of a marine diesel engine, are first applied to drive the transcritical Rankine cycle (TRC). R1234yf, R1234ze, R134a, R152a, R236fa and R290 are employed in the system as working fluids. The effects of expander inlet pressure and temperature on net power output, thermal efficiency, total cost, mass flow rate, and available efficiency of the WHR system are analyzed. The levelized energy cost is used to evaluate the economic optimizations and their corresponding optimal parameters in the WHR system. The results show that the optimal levelized energy cost of R236fa is the most excellent and is lower than that of R1234ze, R134a, R152a, R1234yf or R290 by 5.07%, 6.25%, 7.42%, 9.77% or 12.11%, respectively. The payback period, fuel oil saving, and CO2 emission reduction are applied to assess the suitability of these working fluids. Furthermore, the economic optimization correlations in terms of dimensionless optimal pressures and temperature difference ratios are proposed for the system design of the optimal operating conditions.