1,720,999 research outputs found
Extensive techno-economic assessment of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery
No full textThis work is a techno-economic study of the combination of inverted Brayton cycle, and organic Rankine cycle (combined IBC-ORC) applied for high-temperature waste heat recovery (WHR) of the engine exhaust energy. In IBC, exhaust gases expand to subatmospheric pressure in the turbine, transmit heat residuals to ORC, and restore pressure to 1 atm in the compressor. The system is analysed in the Aspen Hysys software in design conditions at the case study of 1.4 MW gas-fueled internal combustion engine as a high-temperature waste heat source (470–570 °C). Firstly, the paper shows the performance of the system optimised for different ambient temperatures. The role of water condensation contained in flue gas is emphasised for these bounds. Then, the paper presents a multi-objective optimisation illustrated by Pareto fronts for the objective functions of system electric efficiency and levelized cost of energy (LCOE) in the mentioned range of exhaust temperatures. TOPSIS-based Pareto-front analysis results in recommendations of the best sets of cycle parameters in this trade-off. For exhaust temperatures 470 °C, 520 °C, and 570 °C, optimal configurations identified via TOPSIS methodology demonstrate 10.8%, 12.1%, 13.3% efficiencies with LCOE equal to 185.5 /MWh and 146.1 $/MWh correspondingly
Techno-economic analysis of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery
No full textMany practical cases with waste heat recovery potential such as exhaust gases of reciprocating engines, cement kilns or heat-treating furnaces, are nowadays often integrated with organic Rankine cycle to convert waste heat to the mechanical power. However, when dealing with high-temperature waste heat, organic Rankine cycle faces efficiency limit due to the physical properties of the working and thermal fluids. That gives room for further enhancement of the waste heat recovery technologies via the investigation of different non-conventional schemes as one of the possible ways. In the present work, a system introducing the combined inverted Brayton plus organic Rankine cycle is under investigation. Aspen Hysys models of both conventional organic Rankine cycle and combined cycle were designed, orienting on waste heat recovery from the heavy-load gas-fueled reciprocating engine exhaust. In this way, the performance of the combined scheme was benchmarked versus the conventional organic Rankine cycle. An assessment of the organic Rankine cycle working fluids was provided, and pentane has shown the best thermodynamic performance. The study on inverted Brayton cycle defined the remarkable effect of the water condensation in the gas duct on the inverted Brayton cycle performance. Finally, both thermodynamic and economic optimizations of the models were conducted, setting the stage for the comparison of solutions. Results have shown the 10% advantage of the combined scheme over organic Rankine cycle in generated power and system efficiency. The levelized-cost-of-energy-based optimization for variable capacity factors has highlighted above 6% advantage of the investigated solution. The analysis of the sensitivity from machines’ efficiencies and heat exchangers’ pinches has shown that with some sets of parameters, the studied scheme may concede to the organic Rankine cycle
Control variables and strategies for the optimization of a WHR ORC system
No full textIn this paper, the dynamic behavior of a WHR (Waste heat Recovery) ORC system with positive displacement rotary expander has been analyzed and an optimal control strategy was defined to increase the system efficiency and flexibility. Input heat flow was varied in time by varying the heat source mass flow and inlet temperature, according to two different load cycles. Three different control strategy were implemented and compared. The first strategy was sliding pressure, where expander speed was kept constant and system power output was controlled by evaporator pressure variations. The second control strategy was sliding velocity, where expander speed was varied to keep the evaporating temperature to a constant set point value. The third control strategy was a combination of sliding-pressure and sliding velocity: the set point of evaporating pressure varied according to a suitable function of easily measurable variables, with the objective of maximizing system efficiency. A function of the heat source admission temperature and of the product of the volume flow rate by the admission pressure was used to define the evaporating temperature set point. This function was evaluated in steady-state conditions from the model of the plant. Results showed that the last control strategy, maximized system efficiency and flexibility, and that the control parameter chosen were suitable to drive the set point variation
Organic Flash Cycles: Off-design behavior and control strategies of two different cycle architectures for Waste Heat Recovery applications
Full text linkOff-design characterization of energy systems has become interesting, especially for waste heat recovery application, where the heat source temperature and mass flow rate can vary over time. Low-grade heat is generally converted into power through ORC modules: the problem of the constant temperature evaporation lead to the definition of alternative architectures, among which organic flash cycles. In this work, the off-design behavior of two different architectures of single-stage Organic Flash Cycles has been analyzed in steady-state condition, for small scale waste heat recovery (WHR) purposes. The main difference between the two architecture is the regeneration: in the first architecture (Single-Stage Organic Flash Cycle SS-OFC), the liquid of the flash evaporator, after lamination is mixed with the vapor from the expander and then sent to the condenser; in the second architecture Single-Stage Organic Flash Regenerative Cycle, SS-OFRC, the liquid from the flash evaporator is mixed with the liquid from the condenser, to regenerate the cycle. The most appropriate fluid for the two cycles was selected from a list of sixteen fluids with the objective of minimizing volume flow rates and maximizing the system efficiency and i-Pentane was chosen. For the off-design behavior, a rotary volumetric expander derived from a Wankel engine was considered, taking into account the performance variation of the device at various rotating speed and pressure ratios. Three different control strategies were considered and compared in off-design analysis for both the cycle architectures: sliding-pressure, in which the expander speed was constant and flash pressure varied with the load; sliding-velocity, in which the load was controlled by the speed variation of the expander and flash pressure was retained constant; combined strategy in which the expander speed was varied to drive the flash pressure according to a function which maximized the system efficiency. Results showed that the efficiency of the two cycles was similar in all the operating field whatever was the control strategy considered: SS-OFRC demonstrated a better behavior at low temperatures of the heat source (<170 °C), while SS-OFC had a better efficiency at higher temperature. The maximum absolute efficiency difference in off-design conditions between the two cycles was lower than 0.3%. SS-OFRC however had a wider field of operation than SS-OFC, due to the better flexibility of this type of cycle. As for the control strategy, with both the architectures, combined strategy maximized the system efficiency and flexibility for every temperature and mass flow rate of the heat source considered
Impact of ambient temperature on the effectiveness of inlet air cooling in a co-digestion biogas plant equipped with a mGT
No full textBiogas plants are an interesting solution for production of clean energy. Biogas produced in an anaerobic digester can be used locally to produce electric energy in an internal combustion engine or in a micro-gas turbine. A portion of the waste heat is used to keep the digester at a constant temperature which is a necessary condition for a correct operation of the digester. Generally, waste heat overcomes that necessary to keep the digester at the desired temperature and a big portion of this thermal energy is dissipated. Many solutions exist to increase the amount of heat recovered. In this study, the use of an absorption chiller to decrease the inlet temperature of a micro-gas turbine operating with biogas was considered. The advantage of this solution relies in a more stable operation of the micro gas turbine and an increase in the power output. The integration of the absorber chiller in the biogas plant and the effect of the ambient conditions were investigated in detail. The study was based on an existing plant operating near Pisa (Italy). A model of the system has been developed in AMESim and a numerical simulation has been performed. The effects of different temperature profiles, corresponding to different climate conditions, have been investigated both from the energy and economic points of view. The results showed that local climate conditions strongly influence the effectiveness and the profitability of the inlet turbine air cooling technique. In those climates where the temperatures are constantly high over the year, this technique may lead to interesting benefits and profitability
Modeling and optimization of an ocean thermal energy conversion system for remote islands electrification
No full textElectrification of remote zones is often characterized by high energy costs as a result of the fossil fuels supply and cost associated to its logistic. This study performs a theoretical model and optimization of an ocean thermal energy conversion (OTEC) system coupled to an organic Rankine cycle (ORC) generator for small scale applications in the San Blas archipelago (Panama). The gross electric power has been previously set at 125 kW for the eleven working fluids selected: ammonia, R152a, R1234yf, R1234ze, R125, R134a, R161, propane, isobutene, RE143a and decafluorobutane. Results show R1234yf gets the maximum thermodynamic and net electric efficiency (3.60% and 2.57%, respectively). Ammonia reaches the maximum net electric power (99.3 kW) and, thus, the lowest pumping losses (20.59% of the gross). Besides, despite decafluorobutane shows slightly lower electric power (98 kW) and efficiencies, this fluid does not present environmental hazardous features. Sensitivity analyses show that all performance parameters of the plant are strongly affected by deep and surface seawater temperature variation. Finally, for a surface seawater temperature of 30 °C and deep 5 °C, the net electric power reached is 94.6 kW for R1234yf, 99.3 kW for ammonia and 98.0 kW for decafluorobutane. The net electric efficiency is 2.57%, 2.53% and 2.42%, and the total area required by the heat exchangers is 890 m2, 940 m2 and 986 m2, respectively
Biogas upgrading and liquefaction in an anaerobic digester plant
Full text linkThanks to the high energy density and to the extended range it can ensure, LNG is an attractive energy vector especially in heavy-duty transportation. This fuel is even more interesting if produced starting from biogas because of the reduced carbon footprint. Biogas production occurs in anaerobic digestion plants and various production strategies can be pursued to transform biogas into bio-LNG. In this study the anaerobic digester of the municipal wastewater treatment plant of Viareggio has been analyzed. The plant is equipped with a Capstone C600s co-generative micro gas turbine and with a boiler for the sludge heating. Three different bio-LNG production strategies have been considered. In the first strategy (the baseline case), the biogas fueled boiler provides the heat necessary for the sludge heating. The electric energy required for wastewater treatment and for upgrading and liquefaction processes is bought from the electric grid. In the second strategy, the micro gas turbine is operated in thermal following mode and the electric energy required by the plant processes is partly self-produced and partly acquired from the grid. In the third strategy, the micro-turbine is operated in electric following mode, attempting to cover all the electric load necessary to the process and part of the heat necessary to the sludge heating. The plant was analyzed in steady-state conditions by considering the heat requested by the anaerobic digesters, the efficiency curves of the micro gas turbine and the boiler, and the heat exchangers off-design behavior. Results showed that, according to the economic reference scenario, the solution with the high profitability can be that with the boiler of that with the micro gas turbine operated in thermal following mode
Thermo-economic analysis of a novel trigeneration cycle enabled by two-phase machines
No full textA novel trigeneration cycle with two-phase fluid devices (compressors and expanders) and a wet working fluid is proposed. The steady-state mathematical model of the cycle is developed and implemented in Matlab environment. The sensitivity analysis is performed by changing simultaneously five process parameters to determine the best operating conditions serving two typical trigenerative consumers with various energy requirements (i.e. tertiary and industrial). The cycle exploits a generic typology of thermal source. The energy performances and the economic profitability of the cycle are compared with the counterparts of the first cycle with two-phase machines that the Authors have investigated in previous publications (TC2M) and with the counterparts of several commercialized CCHP systems. For both consumers, the cycle allows increasing the energy performances compared to TC2M with varying of the two-phase machines isentropic efficiencies. Moreover, the highest energy performances of the cycle are larger or similar to those of the best energy performing marketed CCHP plants for tertiary or industrial consumers, respectively. The cycle shows higher economic profitability than TC2M and several commercialized CCHP plants except for tertiary consumer at the minimum number of annual operating hours
Hybridization of an internal combustion engine with a molten carbonate fuel cell for marine applications
No full textThis study presents a proposed hybrid ship propulsion system combining an internal combustion engine and a molten carbonate fuel cell both powered by liquefied natural gas (LNG). Exhaust from the internal combustion engine is used as a CO2 source for cell operation, reducing CO2 emissions. Use of fuel stored at very low temperature requires heat for evaporation purposes. The fuel is used to condense water vapor from the fuel cell exhaust gases, returning the remainder to the fuel cell with the right amount of water. This solution increases the electricity generation efficiency of the fuel cell. We analyzed two different system configurations that differ in the way the anode off-gas is recirculated. In the first, all the unoxidized fuel is recirculated to the anode inlet; in the second, off-gas is joined with engine flue gas, and residual fuel burned in a combustion chamber before being sent to the cathode of the fuel cell, allowing to maintain an optimal CO2:O2 ratio in the cathode flow of the fuel cell. A detailed numerical model of the system including cell operation was created in Aspen Hysys and optimized to maximize the system efficiency. Results showed that in configuration I the efficiency gain is about 4.9% with respect to the traditional engine. In configuration II the efficiency gain was only about 0.8%. We also analyzed the sensitivity of the systems from the point of view of the limitations occurring here (e.g., steam-to-carbon ratio or operating temperature). Finally, we discussed the size of such a fuel cell in relation to the internal combustion engine, the entire ship, as well as the impact of the increase in efficiency on the range of the vessel
Small-scale desalination plant driven by solar energy for isolated communities
Full text linkIn the last years, an increasing number of countries has been affected by water shortage. Seawater desalination driven by solar energy, which is usually available in arid regions, might be a solution to satisfy the freshwater demand. In this study, the feasibility of a stand-alone multi-effect desalination (MED) plant driven by solar energy for an isolated community was studied. The system was made up of a solar field, a MED unit, and a thermal storage that mitigated solar energy fluctuations. Simulations with different top brine temperature and inclination and number of the solar panels were carried out in Matlab and Aspen Plus on an hourly basis by considering one typical meteorological year for ambient temperature and solar radiation. Two different sources of electrical energy were considered: A photovoltaic (PV) field and a diesel generator. The results were compared from an energetic and economic point of view, by considering the adoption of plastic as a material for MED heat exchangers. The maximum water production was obtained with December as the design month. Polytetrafluoroethylene heat exchangers allowed the cost of water to be reduced up to 9.5% compared to conventional exchangers. The lowest cost of water (7.09 $/m3) was obtained with September as the design month and a tilt angle of 45◦ with the PV field as the electrical power source
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