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The present study aims to conduct multiobjective optimization investigations by using the genetic algorithm on the small scale and combined LNG cold utilized micro cogeneration system. The system combines the Stirling engine and the microturbine plant with the related heat exchanger for the hot water production. Before the optimization, thermodynamic and environmental analyses are performed in ranges of 288.15-313.15K with the constant relative humidity. Four different multiobjective optimization cases are considered which are: maximization of the generated net work/minimization of the CO2 emission rate, maximization of the exergetic efficiency/ minimization of the CO2 emission rate, maximization of the generated net work/ maximization of the thermal efficiency, and minimization of the overall exergy destruction/ minimization of the CO2 emission rate, respectively. Results show that the net generated power and exergetic efficiency decrease by 15% and 32%, respectively while the thermal efficiency and overall exergy destruction increase by 3.3% and 25% with 40K temperature increment. The CO2 emissions find their peak value at 298.15K. Due to different trends of the investigated parameters, multiobjective function studies infer that the best trade-off region is between 308.15 and 313.15K for the investigated 4 optimization cases.

Hybrid AC/DC microgrid (MG) is a promising electrical power system architecture that enables to harmonize AC and DC energy conversions. Due to the complicated configuration, comprehensive stability analyses for a complete hybrid MG still stand as an unsurmountable challenge. To address this issue, this paper decomposes the MG system into several electrical modules, i.e., an AC source, a DC source, multi-paralleled interlinking converters (ICs) and line impedances. These modules are first individually modeled in the small signal sense. They are then combined to establish a full autonomous mathematical expression for characterizing dynamics of the hybrid MG. To investigate MG system stability, the influences of diversified operating points, line impedances and the selected key control parameters are examined by evaluating eigenvalue loci of the derived mathematical model. Time-domain simulations reveal the perfect matching between the results given by Matlab/Simulink and eigenvalue loci plots.

Liquefied natural gas (LNG) cold utilization systems are feasible options for the inland and stranded regions where the conventional pipeline technology is challenging. For the best operation conditions of the systems in the real engineering applications, the optimization is a significant step after the design and analysis steps. This study aims to optimize an LNG cold utilized micro-cogeneration system by using the multiobjective optimization strategy with the genetic algorithm. Thermodynamic and thermoeconomic investigations are performed, and then the multiobjective optimization study is carried out according to the net generated power rate, exergetic efficiency, and the levelized product cost as multiobjectives. The study is conducted between 288.15 K and 313.15 K, and the optimum operation conditions are detected between 301.0 K and 302.5 K with respect to the different relative humidities. NRF (Natl Research Foundation, S’pore) Accepted version

This paper presents the establishment and operation control strategy of a DC microgrid incorporated with bidirectional inductive power transfer (BIPT) system for Electric Vehicles (EVs) charging applications. The primary side of IPT system is powered by a DC microgrid which is established from PV as renewable energy source and battery for energy storage. The proposed concept minimizes power electronic components requirement while maintaining high power efficiency due to the reduction of power conversion stages and the proposed control strategy. The proposed concept is applicable for low to medium power level EV charging applications.

This study proposes a compensation strategy to deal with both current and voltage measurement errors in the three-phase PWM rectifier system. The DC offset and scaling errors in the voltage and current measurements cause the injection of undesired DC and unbalanced currents into the three-phase input current and subsequently lead to voltage ripple at the DC output voltage. This paper proposes a compensation scheme for current measurement error where the DC offset and scaling errors in the current measurement are estimated from the characteristic of the DC output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, an advanced current controller designed with a proportional integral plus two resonant controllers tuned at the fundamental grid frequency (ω s ) and 2ω s in the synchronous (d-q) reference frame is suggested to reject the impact of the DC offset and scaling errors in the voltage measurement. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost effective and robust solution. The effectiveness of the proposed solution is verified by experimental results.

Grid-connected inverters with LCL passive filters have been wildly utilized in renewable energy community. The high order filter helps to suppress harmonic current in high frequency range and inject the power of high quality into the utility grid. However, due to long distance transmission line and penetration of transformers, the point of common coupling (PCC) voltage may present variant grid impedances which could induce filter system instability. In addition, the harmonic current caused by nonlinear load flowing through grid impedance possibly distorts the voltage of PCC and even results in unbalanced grid voltage. These salient defects of weak grid challenge the regulation of LCL output current. A robust current control method by integrating backstepping with global output regulation approach (BGOR) is proposed in this paper to realize the enhanced performances of LCL filter. The proposed method ensures internal stability by choosing a proper Lyapunov function for the entire system. Inverter system robustness is also ensured against uncertainties of grid impedance, PCC voltage distortion and unbalanced grid voltage. Exhaustive derivation of BGOR is presented in this paper and simulation results verify the effectiveness of the proposed method.

This study proposes an enhanced current control strategy to eliminate DC and harmonic current at the output current of three-phase grid-connected inverters caused by DC offset and scaling error in grid voltage measurement. The proposed current controller designed in the stationary (α-β) reference frame is developed with a proportional integral (PI) plus three resonant controllers where the resonant controller tuned at the fundamental grid frequency (ω s ) is used to regulate the fundamental component; whereas the PI and resonant controllers tuned at 5ω s and 7ω s are employed to eliminate the DC and harmonic components in the grid current. The proposed control scheme is developed without the need of phase-locked loop, DC or harmonic detection scheme, and extra hardware circuit so that it can be integrated into the existing grid-connected inverter systems without extra cost. The effectiveness of the proposed solution is verified by experimental results.

Vaporization of liquefied natural gas (LNG) can be used as heat sinks in the external thermal cycles, and these processes are known as LNG cold utilization systems. Inlet air cooling application is one of the LNG cold utilization methods in the power generation sector, and it provides lower compressor inlet temperatures to increase the thermal efficiency and the net generated work. This study performs the simulations of energetic, environmental and economic approaches for the 30 kW and 65 kW microturbine systems for various relative humidity values under different ambient air temperatures. It is found that the relative humidity did not have a crucial impact on the net generate work rate and thermal efficiency although it causes a slight decrement from 60% RH to 90% RH. The environmental analyses show that the emission rate increases by the rising of relative humidity. Moreover, the environmental payback period increases by the rising of the relative humidity value. Lastly, the payback periods are investigated and it is seen that the relative humidity and the ambient air temperature do not have any crucial impact on the payback periods. The payback periods are found as 4.34 and 3.27 years for the 30 kW and 65 kW models, respectively.

We have developed an integrated solar-driven desalination system that uses membrane distillation process to produce potable water. The system encompasses the sources of both water and energy. The system is an integrated (self-contained) system that utilizes solar energy for its operation by combining solar photovoltaic (PV) and solar thermal collectors. The system is intended for autonomous operation in arid remote areas of Saudi Arabia where electricity and potable water are not readily available. Due to its portability, the system can be used in emergency situations in which potable water is essential for survival, such as natural disasters. The system has three major components: the solar-thermal system, solar-PV system, and membrane distillation system. A well-characterized memsys Vacuum Multi-Effect Membrane Distillation (V-MEMD) module was used as the core of the system. In addition, a heat pump was integrated into the system to improve the performance of the system. The novelty of the system is the engineered design of a portable and efficient integrated system that is reliable and easy to maintain. The system could be referred to as an environmentally friendly and sustainable desalination technology. The paper describes the design, configuration, and performance of the solar-driven desalination system.

Exergy analyses provide to investigate the available work performance of thermal cycles. In this study, exergy analyses have been conducted for the conventional micro-cogeneration system and the Liquefied Natural Gas (LNG) cold utilized micro cogeneration system, respectively to detect exergy destruction and loss ratios which are related to second law analyses. In the LNG cold utilized micro-cogeneration system, the exhausted gas of micro turbine is supplied to the LNG vaporizer to change the phase of LNG from liquid to gas. Due to fact that exergy destruction and loss rates, exergetic efficiency decreases with the ambient air temperature increment by nearly 28%. Owing to the application of exhausted gas in the LNG vaporizer in the LNG cold utilized micro-cogeneration system, the exergy loss ratio decreases by approximately 81% when it is compared to the conventional micro-cogeneration system. At the same time, the exhausted gas temperature decreases nearly 2.93 K. Exergy destruction ratio is another evaluation criterion for exergy analyses and it is seen that the combustion chamber had a significant impact on the overall exergetic performance of both systems. The minimum exergy destruction rate belongs to the LNG pump and also the LNG vaporizer does not have a significant exergy destruction ratio. NRF (Natl Research Foundation, S’pore) Published version

Power sharing performance is a critical issue of hybrid energy storage system (HESS) in autonomous DC microgrid (MG). In this paper, a novel integral droop (ID) is proposed to mimic dynamic characteristics of the capacitor by using energy storages (ESs) with quick response. The main advantage of the proposed controller is that dynamic power sharing among ESs is automatically realized in the decentralized level. For a given HESS, ESs with the proposed ID enables to compensate fast power change while ESs with conventional voltage-power (V-P) droop provide low frequency components of power demand. With coordinated control between ID and V-P droop, high/low pass filters (LPF/HPF) are intrinsically formulated in HESS in order to obtain reasonable dynamic power allocations among ESs. Matlab/Simulink model of HESS is established for the verification of ID controller, in which the impacts of different ID coefficients on transient performances of the system are analyzed in detail. Finally, the effectiveness of proposed ID is experimentally validated on a HIL (hardware in loop) HESS platform. Accepted version

The present paper investigates the dynamic energetic, exergetic and environmental performances of two different small-scale LNG cold utilized micro-cogeneration systems under the different climate conditions. The hourly weather data are collected for Singapore, and the dynamic simulations are performed according to the hourly data for each month. Three sessions are determined for the investigations which are the morning, midday and evening sessions. Both designs have their lowest power generation rates at the midday session. The power generation and the exergetic efficiency of the Stirling engine have opposite trend against to the overall power rate of system. The maximum energetic efficiency values are at the midday session while the exergetic efficiencies reach their lowest values at the same session. The environmental analyses show that the emission reductions range between 6.8 and 7.7% when two designs are compared, and the highest emission reduction is obtained at the midday session. NRF (Natl Research Foundation, S’pore) Published version

This paper presents a coordination secondary control strategy for autonomous hybrid AC/DC microgrids with global power sharing operation. In the previous work, primary control with droop control method was applied to distributed generators to ensure local power sharing without communication links in either ac or dc microgrid. An extension of local power sharing for individual microgrid is the global power sharing for hybrid microgrids. With global power sharing control, all distributed generators throughout the hybrid system are capable of sharing the entire ac and dc loads without any information exchange. The coexistence of local power sharing and global power sharing in hybrid system is thereafter defined as generalized primary control, which is usually employed for fully decentralized power management throughout the whole hybrid system. To eliminate the inherent voltage/frequency deviations caused by the generalized primary control, the secondary control is usually applied to each distribution generator for voltage/frequency restoration. This however will degrade the performance of global power sharing operation because of its characteristics of voltage/frequency deviation dependence. To achieve voltage/frequency restoration while maintaining the global power sharing operation, a coordination secondary control was proposed and the effectiveness of the proposed method has been verified by the simulation results.

Membrane based liquid desiccant dehumidifier and regenerator system have recently gained much attention in liquid desiccant air-conditioning (LDAC) system due to their merit of zero contamination or carryover and resistance to corrosion. A typical system for the LDAC application will consist of a number of stages depending on the capacity of the LDAC system. The multi-effect design allows energy recovery to be achieved to reduce the overall thermal energy consumption. The objective of this study was to test and evaluate the effectiveness and performance of solar powered multi-effect distillation system with desiccant salt solution. Design Expert was used to characterize the output of regenerator system and optimize overall design using sets of experiment test data. The optimized working zone for performance ratio for different feed flow rate and hot water temperature has been derived. It was found that the optimized zone for LiCl inlet concentration 31.71% was representing performance ratio of 0.5 if operated at feed flow rate range of 32 L/min to 60 L/min and hot water temperature of 54°C to 67°C and optimized zone for LiCl inlet concentration 37.74% was representing performance ratio of 0.5 if operated at feed flow rate range of 40 L/min to 50 L/min and hot water temperature of 59°C to 67°C.