• Energy Research

Abstract There is pressing need to explore the fundamental link between the mixing efficiency and the entrainment performance for ensuring an efficient working of ejector in the associated energy systems. This study details where, how and to what extent the primary and entrained flows mix inside the steam ejector through a novel ejector model with species transport. The evolution laws of the mixing layer growth, the entrainment performance and the mass transfer of two streams are systematically discussed under various operational conditions. Further, their fundament links are clearly presented in a form of functional relation. The key findings emerged from this study: The mixing begins over a fluctuating mixing layer since the mixing chamber entrance. In the situation of critical operational models, the mixing layer experiences a fluctuating and exponential growth successively, but its development is severely obstructed under the subcritical models. Additionally, the entrainment ratio and the non-mixing length ratio are highly linear correlation with the mass transfer ratio. The variations of operational parameters essentially change the relative mass transfer capacity of the entrained flow. The more easily the entrained flow mixes into the primary jet flow, a faster mixing layer growth and higher entrainment performance are to be gained.

Abstract There is pressing need to explore the fundamental link between the mixing efficiency and the entrainment performance for ensuring an efficient working of ejector in the associated energy systems. This study details where, how and to what extent the primary and entrained flows mix inside the steam ejector through a novel ejector model with species transport. The evolution laws of the mixing layer growth, the entrainment performance and the mass transfer of two streams are systematically discussed under various operational conditions. Further, their fundament links are clearly presented in a form of functional relation. The key findings emerged from this study: The mixing begins over a fluctuating mixing layer since the mixing chamber entrance. In the situation of critical operational models, the mixing layer experiences a fluctuating and exponential growth successively, but its development is severely obstructed under the subcritical models. Additionally, the entrainment ratio and the non-mixing length ratio are highly linear correlation with the mass transfer ratio. The variations of operational parameters essentially change the relative mass transfer capacity of the entrained flow. The more easily the entrained flow mixes into the primary jet flow, a faster mixing layer growth and higher entrainment performance are to be gained.

Abstract Gas turbine combined cycle (GTCC) based combined cooling, heating and power (CCHP) or combined heating and power (CHP) system driven by natural gas is encouraged to set up for district heating/cooling demand in China due to the clean and efficient energy conversion. This paper presents a GTCC based CCHP system, which consists of a heavy-duty gas turbine, triple-pressure heat recovery steam generator (HRSG), steam turbines, heat exchanger and absorption chiller. Turbine inlet temperature (TIT) strategy and inlet guide vanes (IGV) strategy for the gas turbine are adopted to access the part load performance. The energy distribution and exergy destruction of the CCHP system are investigated under different gas turbine loads. The primary energy saving rate (PESR) and carbon dioxide emission rate (CDER) are set up to evaluate the system at various gas turbine loads and steam extraction ratios. The ability of shaving peak power for the system is investigated. The results show that IGV plays a role in increasing the steam turbine power output and reducing the exhaust heat in HRSG but causes more exergy destruction in the steam turbine expansion process. The PESR and CDER have been enhanced as the steam extraction ratio increases for the same gas turbine load. The IGV strategy reinforces the part-load performance of the CCHP system. For instances, the PESR has been enhanced from 0.2409 to 0.3108, and CDER has been strengthened from 0.8274 to 0.8465 by the IGV strategy at half of gas turbine load and without steam extraction. For the same heating load, both PESR and CDER are enhanced by the IGV strategy. The ability of supplying heating is deteriorating as the decrease in TIT. The ability of shaving peaking power is going to be deteriorated as heating load increases. For the small heating load, like 50 MW, the advantage of the IGV strategy is prominent, the PESR and CDER is advanced 5.81% and 1.48% respectively by the IGV strategy.

Abstract Gas turbine combined cycle (GTCC) based combined cooling, heating and power (CCHP) or combined heating and power (CHP) system driven by natural gas is encouraged to set up for district heating/cooling demand in China due to the clean and efficient energy conversion. This paper presents a GTCC based CCHP system, which consists of a heavy-duty gas turbine, triple-pressure heat recovery steam generator (HRSG), steam turbines, heat exchanger and absorption chiller. Turbine inlet temperature (TIT) strategy and inlet guide vanes (IGV) strategy for the gas turbine are adopted to access the part load performance. The energy distribution and exergy destruction of the CCHP system are investigated under different gas turbine loads. The primary energy saving rate (PESR) and carbon dioxide emission rate (CDER) are set up to evaluate the system at various gas turbine loads and steam extraction ratios. The ability of shaving peak power for the system is investigated. The results show that IGV plays a role in increasing the steam turbine power output and reducing the exhaust heat in HRSG but causes more exergy destruction in the steam turbine expansion process. The PESR and CDER have been enhanced as the steam extraction ratio increases for the same gas turbine load. The IGV strategy reinforces the part-load performance of the CCHP system. For instances, the PESR has been enhanced from 0.2409 to 0.3108, and CDER has been strengthened from 0.8274 to 0.8465 by the IGV strategy at half of gas turbine load and without steam extraction. For the same heating load, both PESR and CDER are enhanced by the IGV strategy. The ability of supplying heating is deteriorating as the decrease in TIT. The ability of shaving peaking power is going to be deteriorated as heating load increases. For the small heating load, like 50 MW, the advantage of the IGV strategy is prominent, the PESR and CDER is advanced 5.81% and 1.48% respectively by the IGV strategy.

Abstract Regasification of LNG for combustion in power plants typically employ seawater as a heat carrier in Open-Rack Vaporizers (ORV), causing much of the cold energy to be lost to the ambient. A comprehensive literature review shows that, thus far, no studies have been conducted to simultaneously consider the impacts of the exergy, economy and environment in the optimal design of a hybrid LNG recovery system. This paper aims to address this knowledge gap by establishing a multi-objective optimization model for a novel cascading quad-generation cold energy LNG recovery system. Single- and multi-objective optimizations based on Fuzzy method and Pareto optimal method are carried out on the proposed system to obtain the optimal operating parameters and component sizing, as well as the corresponding performances for each condition. The optimal sizing for each stage is computed for the maximizing of exergy efficiency and CO2 savings rate, and the minimizing of capital cost. The exergy efficiency obtained from the triple-objective optimization yields 12.3% improvement compared to the best result from the single-objective optimization with a 5 kg/s LNG mass flow rate. In addition, when the LNG mass flow is larger than 1 kg/s, the maximized exergy efficiency remains constant (around 0.13) with increasing LNG mass flow rate while the maximized CO2 emission reduction rate and minimized total cost per year increase linearly with the LNG mass flow rate. It has been demonstrated in this work that the system is able to maintain consistency in performance for the optimal design conditions over a wide range of LNG demands and hence good scalability for possible industrial and commercial settings.

Abstract Regasification of LNG for combustion in power plants typically employ seawater as a heat carrier in Open-Rack Vaporizers (ORV), causing much of the cold energy to be lost to the ambient. A comprehensive literature review shows that, thus far, no studies have been conducted to simultaneously consider the impacts of the exergy, economy and environment in the optimal design of a hybrid LNG recovery system. This paper aims to address this knowledge gap by establishing a multi-objective optimization model for a novel cascading quad-generation cold energy LNG recovery system. Single- and multi-objective optimizations based on Fuzzy method and Pareto optimal method are carried out on the proposed system to obtain the optimal operating parameters and component sizing, as well as the corresponding performances for each condition. The optimal sizing for each stage is computed for the maximizing of exergy efficiency and CO2 savings rate, and the minimizing of capital cost. The exergy efficiency obtained from the triple-objective optimization yields 12.3% improvement compared to the best result from the single-objective optimization with a 5 kg/s LNG mass flow rate. In addition, when the LNG mass flow is larger than 1 kg/s, the maximized exergy efficiency remains constant (around 0.13) with increasing LNG mass flow rate while the maximized CO2 emission reduction rate and minimized total cost per year increase linearly with the LNG mass flow rate. It has been demonstrated in this work that the system is able to maintain consistency in performance for the optimal design conditions over a wide range of LNG demands and hence good scalability for possible industrial and commercial settings.