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ORC is a good solution to recover the cold exergy of LNG (liquefied natural gas) and the thermal exergy of low grade heat simultaneously. The optimal working fluids and parameters change greatly with different configurations and heat source temperatures. In this paper, three different ORC configurations: SORC (single-stage ORC), PTORC (parallel two-stage ORC) and CTORC (cascade two-stage ORC), combined with DEC (direct expansion cycle) are analyzed and compared. Up to seven key parameters of sixty four fluid combinations under four heat source temperatures are optimized by the particle swarm optimization. The results show that PTORC is more suitable for lower heat source temperatures and CTORC performs better for higher heat source temperatures. PTORC+DEC with ammonia/ethane as the fluid combination yields the maximum efficiency of 17.36% for the heat source temperature of 50 °C. CTORCs without DEC, also with ammonia/ethane as the fluid combination, achieve the maximum efficiencies of 19.49%, 21.69% and 24.37% for the heat source temperatures of 100, 150 and 200 °C, respectively. The optimal turbine inlet temperatures are usually their upper bounds and the optimal condensation temperatures are the normal boiling temperatures of the fluids. There are optimum evaporation temperatures for all configurations, which increase with the heat source temperature. [ABSTRACT FROM AUTHOR]

NG (Natural gas) distribution pressure varies in a wide range in different applications and has significant effect on system exergy efficiency. RC (Rankine cycle) and DEC (direct expansion cycle) can utilize the cold and pressure exergies of LNG (liquefied natural gas), respectively. In this paper, SRC (single-stage RC), PTRC (parallel two-stage RC) and CTRC (cascade two-stage RC) with and without DEC are optimized by Particle Swarm Optimization and compared to obtain the optimal configurations for different NG distribution pressures. Propane, propylene, ethane and ethylene are adopted as the working fluids. The results show that the addition of DEC could increase the exergy efficiency by lifting the evaporation temperature of LNG. CTRC + DEC with propane/ethylene and CTRC + DEC with propane/ethane as the working fluid achieve the largest exergy efficiencies of 23.89% and 18.25% at NG distribution pressures of 1 MPa and 2 MPa, respectively. PTRC + DEC with propane/ethane is the optimal system with maximum exergy efficiencies of 15.18%, 13.24%, 11.7% and 10.4% at NG distribution pressures of 3, 4, 5 and 6 MPa, respectively. PTRC + DEC with ethane as the working fluid in both cycles is a generally suitable system for all NG distribution pressures with high efficiency and only one type of working fluid. [ABSTRACT FROM AUTHOR]

Decreasing the condensation pressure of organic Rankine cycle (ORC), which is usually limited by ambient pressure, can greatly increase the system efficiency. In this paper, the condensation pressure is extended from traditional ambient condensation to vacuum condensation. Liquefied natural gas (LNG) is used as the cold sink to generate vacuum condensation pressures between 0.001 MPa and 0.1 MPa. Nine substances with no or low flammability are chosen as the working fluids. The system main parameters are optimized with the objective of maximum exergy efficiency. The results show that the exergy efficiency of R227ea increases from 18.87% to 26.03% as the condensation pressure decreases from 0.1 MPa to 0.001 MPa. For some substances, however, their efficiencies turn to decrease as the condensation pressure is approaching 0.001 MPa and their optimal condensation pressures are different. The working fluids share the same optimal condensation temperature of around −90 °C under the studied conditions. R227ea is a generally good substance because of its higher critical temperature to match with heat source and lower triple point pressure (temperature) to reach lower condensation pressure (temperature). The condensation pressure always has opposite effect on exergy destructions of condenser and steam generator, as well as on direct expansion turbine and ORC turbine. • Improvement of ORC is studied by breaking the barrier of ambient pressure. • Vacuum condensation can greatly improve the exergy efficiency of ORC system. • Optimal parameters under different vacuum condensation pressures are provided. • R227ea is a generally good substance for most conditions. • Vacuum pressure has opposite effect on exergy destructions of condenser and SG. [ABSTRACT FROM AUTHOR]

• 5 Rankine cycles, 8 fluids, 4–7 parameters are optimized under different conditions. • Modified single-stage RCs are better than two-stage RCs. • Regenerative-reheat RC is the optimal configuration for most conditions. • The regenerator improves the thermal match for fluid with low critical temperature. • DEC improves efficiency for fluid with high NBPT under low pipeline pressure. In this study, we present the evaluation of two modified single-stage Rankine cycles (RC) and two two-stage RCs to provide a guidance for configuration selection under different working conditions. Moreover, a regenerative-reheat RC, which is the combination of the modified single-stage RCs, is proposed to utilize the cold energy of liquefied natural gas (LNG) and the thermal energy of low grade heat. The key parameters of these five configurations using eight working fluids are optimized with exergy efficiency as the optimization objective. The results show that the modified single-stage RCs are usually better than the two-stage RCs since they have simpler configurations with similar or higher efficiencies. The regenerative-reheat RC performs even better under most working conditions and it achieves the maximum efficiency of 24.57% by using ethane as the working fluid under the heat source temperature of 180 °C and the pipeline pressure of 3 MPa. The addition of direct expansion cycle can improve the exergy efficiencies of most fluids and configurations under lower pipeline pressures. However, the improvements decrease greatly for fluids with low normal boiling point temperatures (NBPTs) and especially under higher pipeline pressures. As to the fluid selection, high critical temperature and low NBPT are both important for reheat RC and parallel two-stage RC. NBPT weights more than critical temperature for regenerative RC and regenerative-reheat RC. Besides, wet working fluids are more suitable for reheat RC while dry and isentropic fluids are more suitable for regenerative RC and regenerative-reheat RC. The optimal regenerative ratio and reheat pressure ratio vary with different working fluids and working conditions. Lower reheat pressure ratio usually leads to higher regenerative ratio. [ABSTRACT FROM AUTHOR]

• Three two-stage Rankine cycles with direct expansion are compared. • Fluids with high critical and low normal boiling temperatures are better. • A maximum efficiency of 30.54% is achieved by ammonia. • Two-condensation ORC is both the best thermodynamic and economic systems. • The systems could greatly reduce CO 2 emission. Two-stage Rankine cycles draw more and more attention with the increasing trade of LNG (liquefied natural gas) and the recovery of both the cold exergy of LNG and the thermal exergy of low temperature heat source. To figure out the best configuration of two-stage Rankine cycles, three mostly studied configuration, cascade two-stage ORC (organic Rankine cycle), parallel two-stage ORC and two-condensation ORC are optimized and compared under different heat source temperatures and different NG distribution pressures. Ten substances are selected as the working fluids. The evaporation, condensation and expander inlet temperatures of both ORCs and the LNG regasification pressure are optimized to obtain the maximum exergy efficiency. The results show that both the increase of heat source temperature and the decrease of NG distribution pressure can improve the exergy efficiencies greatly. The increase of heat source temperature not only increases the optimal key parameters but also affects the ranking of optimal fluids, while the increase of NG distribution pressure only affects the optimal parameters. Different from conventional Rankine cycles, there is also an optimal condensation temperature to match the heat absorption line of LNG and it increases with increase of the NG distribution pressure. The LNG regasification pressure is a key parameter which determines the proportion of power outputs between ORCs and direct expansion cycles. The thermodynamic performances of parallel two-stage ORC and two-condensation ORC are almost the same, which are obviously better than that of cascade two-stage ORC. Both parallel two-stage ORC and two-condensation ORC with ammonia as the working fluid achieve the maximum efficiency of 30.54% under the heat source temperature of 200 ℃ and the NG distribution pressure of 3 MPa. Two-condensation ORC is the optimum configuration with both good thermodynamic and economic performances. The lowest electricity production cost of two-condensation ORC achieved in this paper is 0.033$/kW·h. The environmental side effect caused by working fluid leakage is negligible compared with the saved CO 2 emission via the electricity generation. [ABSTRACT FROM AUTHOR]

Abstract: A new combined power and refrigeration cycle is proposed for the cogeneration, which combines the Rankine cycle and the ejector refrigeration cycle by adding an extraction turbine between heat recovery vapor generator (HRVG) and ejector. This combined cycle could produce both power output and refrigeration output simultaneously, and could be driven by the flue gas from gas turbine or engine, solar energy, geothermal energy and industrial waste heats. Parametric analysis and exergy analysis are conducted to examine the effects of thermodynamic parameters on the performance and exergy destruction in each component for the combined cycle. The results show that the condenser temperature, the evaporator temperature, the turbine inlet pressure, the turbine extraction pressure and extraction ratio have significant effects on the turbine power output, refrigeration output, exergy efficiency and exergy destruction in each component in the combined cycle. It is also shown that the biggest exergy destruction occurs in the heat recovery vapor generator, followed by the ejector and turbine. [Copyright &y& Elsevier]