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Abstract The optimized performance of two advanced CO2 capture processes is compared to that of a monoethanolamine (MEA) baseline for a gas-powered CO2 capture retrofit of an existing coal-fired facility. The advanced temperature-swing processes utilize piperazine and mixed-salt solvents. The mixed-salt treatment involves the use of ammonia for CO2 absorption and potassium carbonate primarily to control ammonia slip. The processes are represented in terms of energy duty requirements within a modular heat integration code developed for CO2 capture modeling and optimization. The model includes a baseload coal plant, a gas-fired subsystem containing gas turbines and a heat recovery steam generator (HRSG), and a CO2 capture facility. A formal bi-objective optimization procedure is applied to determine the design (e.g., detailed HRSG components and pressure levels, gas turbine capacity, CO2 capture capacity) and time-varying operations of the facility to simultaneously maximize net present value (NPV) and minimize total capital requirement (TCR), while meeting a maximum CO2 emission intensity constraint. For a realistic scenario constructed using historical data, optimization results indicate that both advanced processes outperform MEA in both objectives, and the mixed-salt process in turn outperforms the piperazine process. Specifically, for the scenario considered, the base case mixed-salt process achieves 16% greater NPV and 14% lower TCR than the MEA process, and 10% greater NPV and 5% lower TCR than the piperazine process. A five-case sensitivity study of the mixed-salt process indicates that it is competitive with the piperazine process and consistently outperforms the MEA process.

With increasing air traffic, rising fuel costs and tighter environmental targets, efficient airport ground operations are one of the key aspects towards sustainable air transportation. This complex system includes elements such as ground movement, runway scheduling and ground services. Previously, these problems were treated in isolation since information, such as landing time, pushback time and aircraft ground position, are held by different stakeholders with sometimes conflicting interests and, normally, are not shared. However, as these problems are interconnected, solutions as a result of isolated optimisation may achieve the objective of one problem but fail in the objective of the other one, missing the global optimum eventually. Potentially more energy and economic costs are thus required. In order to apply a more systematic and holistic view, this paper introduces a multi-objective integrated optimisation problem incorporating the newly proposed Active Routing concept. Built with systematic perspectives, this new model combines several elements: scheduling and routing of aircraft, 4-Dimensional Trajectory (4DT) optimisation, runway scheduling and airport bus scheduling. A holistic economic optimisation framework is also included to support the decision maker to select the economically optimal solution from a Pareto front of technically optimal solutions. To solve this problem, a multi-objective genetic algorithm is adopted and tested on real data from an international hub airport. Preliminary results show that the proposed approach is able to provide a systematic framework so that airport efficiency, environmental assessment and economic analysis could all be explicitly optimised.

Abstract An enhanced TRNSYS simulation model, NEM, of the behaviour of a domestic hot-water (DHW) store, with an immersed heat-exchanger (HX), has been developed and validated. This model simulates the dynamic heat-depletion and recovery processes in the immersed HX and predicts the transient temperature-patterns for various DHW draw-off versus time profiles. Realistic daily profiles (RDPs), based on field studies, were developed to provide representative draw-off patterns for the testing of thermal stores and simulation studies. The effects of these RDPs and five other existing profiles on the store’s performance are analysed using the enhanced model. The simulation results indicate the importance of the HX’s recovery, as well as the number, type and time of occurrence of the draw-offs in the profile, on the thermal store’s performance. It is concluded that RDP profiles should be used in the performance testing of thermal stores to obtain results that reflect conditions experienced in the field.

Abstract A coiled-pipe exchanger was employed to extract heat rapidly at relatively high temperatures from a 90-litre hot-water charged tank, the water being initially at a temperature of approximately 80°C. The free-convective movements of the water around the outside of the coiled pipe (immersed in the store) were due to buoyancy forces induced by colder water being forced through the heat-exchanger's pipe. For the heat-exchanger orientations tested, the maximum effectiveness, with respect to the quality of the heat extracted was achieved (i) by having the axis of the coiled heat-exchanger arranged horizontally with its inlet at the lowest level; and (ii) with the lower rate tested (=6·6 litre/min) of water being passed through the heat-exchanger's pipe, partly because this led to a lower rate of disruption of the stratification of the water within the store.

A performance analysis and optimization of a open-cycle regenerator gas-turbine power-plant is performed in this paper. The analytical formulae about the relation between power output and cycle overall pressure-ratio are derived taking into account the eight pressure-drop losses in the intake, compression, regeneration, combustion, expansion and discharge processes and flow process in the piping, the heat-transfer loss to the ambient environment, the irreversible compression and expansion losses in the compressor and the turbine, and the irreversible combustion loss in the combustion chamber. The power output is optimized by adjusting the mass-flow rate and the distribution of pressure losses along the flow path. Also, it is shown that the power output has a maximum with respect to the fuel-flow rate or any of the overall pressure-drops and the maximized power output has an additional maximum with respect to the overall pressure-ratio. The numerical example shows the effects of design parameters on the power output and heat-conversion efficiency.

Abstract This paper examines methods to simultaneously improve efficiency and reduce emissions from a 34 cc air-cooled two-stroke engine configured to operate on natural gas, using tuned intake plus exhaust resonators designed from Helmholtz resonance theory to promote effective scavenging. The engine was developed for novel application in a small, free piston engine for decentralized power generation. Operation occurred at wide-open-throttle at a speed of 5400 RPM for both port injection (PI) and low-pressure direct injection (LPDI). In both cases, the electronic ignition timing was adjusted for maximum brake torque, while the fuel was adjusted such that both rich and lean combustion were examined. The LPDI engine was then operated over a variety of engine speeds. An energy balance was completed for both PI and LPDI operation, which quantified all energy pathways, as engine operating regimes changed. Results showed that exhaust chemical energy (ECE) for LPDI was reduced significantly when compared to PI, mostly due to less fuel slip. The fuel slip rate ranged from 35 to 40% for PI operation while it was in the range of 6–20% for LPDI operation. However, mixture stratification with LPDI operation increased carbon monoxide emissions. The start-of-injection (SOI) was also examined, targeting a SOI to provide the highest trapping efficiency and most stable combustion. For LPDI operation, trapping efficiency, which is function of engine speed, showed significant effects on overall efficiency and fuel slip. Air volumetric efficiency increased due to the absence of gaseous fuel within the intake manifold. It was shown that, relative to PI, the overall engine indicated efficiency increased by 90% to a maximum indicated efficiency of 30% for LPDI operation with a SOI of 180 CAD BTDC.

A stand-alone analytical model using the NTU (number of transfer units) effectiveness approach was developed for a residential desuperheater integrated to a thermal-energy storage (TES) system. This paper describes the basic heat-transfer analysis and presents the characteristics of the model. The output prediction of the desuperheater model was the desuperheater rate. Error analyses of the desuperheater rate indicated that the model predictions were within 12% of the experimental values. A factorial-design sensitivity analysis was conducted to show that the variables Tri and Twi as well as interactions Twi∗hrefrig, hrefrig∗F, Tri∗hrefrig, and hwtr∗hrefrig∗F had significant effects on the desuperheater rate. Although the model involved various assumptions, the results were considered helpful for the design and understanding of the behaviour of the desuperheater.