Abstract

This paper presents the development of a thermodynamic model to determine the extended operating window of an aluminum cell to enable power modulation. The integration of renewable energy to aluminum production needs to overcome the challenge of narrow operating windows of aluminum cells. The proposed model takes an industrial aluminum cell as a case study to demonstrate how to build the operating window, constrained by the cell design technical limitations. The extended operating window can also be used to estimate the heat exchanger heat rate requirements to enable an aluminum cell to operate under a wide power modulation.

Abstract

The gas hydrate is an emerging energy resource, while the blockage of pipelines caused by hydrate is of utmost concern. To investigate the formation properties of hydrate, visual experiments of xenon hydrate formation were carried out on the modified aluminum sheets with contact angles of 61.5°, 90.0°, and 123.6°, which were marked as hydrophilic, untreated and hydrophobic surfaces respectively. The microscopic results show that the hydrate growth behavior and the crystal morphology of the xenon hydrate depend on the wettability. On hydrophilic surfaces, the hydrate grows outside the original water boundary , i.e., halo effect. For the untreated surface, the phenomenon of halo still appeared. The finally formed halo was smaller with a slower growth rate, while the hydrate layer was thicker and rougher. In addition, there was no halo on the surface of hydrophobic. The hydrate shell was firstly smooth, but depressed in 30 minutes. The study is instructive for the wettability treatment of surfaces, and then provide some reference in the efficient application of hydrate.

Abstract

Compared with conventional low-voltage hybrid AC/DC microgrid, a medium-voltage (MV) hybrid AC/DC microgrid is proposed in this paper. In this proposed microgrid, the MV interlinking converter is composed of front-end cascaded H-bridge (CHB) converters and backend dual active bridge (DAB) converters. In order to make system normal operation with grid fault-tolerant ability, a novel power regulation scheme is proposed. First, the idea of virtual faulty-cell is proposed to bypass certain healthy bridge cells and adjust the power consumption between the three-phase clusters under unbalanced grid conditions. Then, this paper presents a modified modulation strategy combining phase-shifted (PS) PWM and sort and select algorithm, where no carrier reconfiguration is necessary with bypassed bridge cells. The proposed method can generate a balanced grid current and require a lower injected zero-sequence voltage under unbalanced grid conditions. Finally, based on an MV hybrid microgrid system (3 kV/150 kW). Verification results verify the effectiveness of the proposed structure and control method of this MV microgrid. By the proposed scheme, AC current balancing of the three-phase grid, DC capacitor voltage balancing of front-end CHB converters and DC bus voltage stability of DC microgrid can be realized simultaneously.

Abstract

District heating is a well-established technology; however, the use of individual heat pumps has been expanding and is now the main competitor to district heating. The prices for both electricity and district heating often vary over time because of the variation of raw material prices in the marketplace. Consequently, for the building owner it would be cost effective if they had the possibility to integrate both district heating and heat pumps. Aiding in the flexibility to switch between the two systems in order to choose the one with the lowest operating cost throughout the year. In the presented work, the modeling and control of a detached house integrated with both district heating and a heat pump are developed. The operating costs of both systems are computed considering the marketplace prices and the coefficient of performance of the heat pump, related to the external temperature. The results show that heat pumps can be well exploited during the spring and fall to cover base loads, and in the summer can be used for ambient cooling.

Abstract

High temperature heat pump has great development potential for industrial heat because it is energy-saving and environmentally friendly. This paper focuses on the optimization analysis and experimental investigation of an air source cascade heat pump (ASCHP) to realize 130℃ heat supply under the ambient temperature of 20℃, with refrigerant R245fa and R410A in the two stages respectively. The high stage is designed with vapor injection (VI). The optimal configuration of the intermediate temperature and VI branch temperature is obtained by optimization analysis. An ASCHP unit is developed and the experimental results match the optimized results well. The coefficient of the VI model is deduced according to the experimental result. The coefficient of performance (COP) of the developed heat pump unit is 1.39.

Abstract

Only 20% of energy consumed by conventional thermal desalination processes is needed for a successful freeze desalination process. Freeze desalination (FD) is a freezing-melting technology where freshwater in form of ice blocks is separated by cooling from salt-water solution. Freezing in directional configuration found to be advantageous for better relocation of impurities away from ice crystals. In this work, desalination of artificial seawater using radial freezing at the side walls of cylindrical crystallizer is assessed experimentally and numerically. A 2-D axisymmetric computational fluid dynamics model of multispecies flow with solidification and melting model is utilized to simulate the experimental apparatus. Results showed a successful radial freezing where salt diffusion towards the center is observed. Removal efficiency and effective partition coefficient were investigated experimentally and numerically with a very good agreement. Nonetheless, higher efficiencies are achievable when optimum freezing temperature and stirring at ice front are present.

Abstract

Thermo-acoustic Engines (TAE) utilize the production of acoustic waves to generate mechanical power when a thermal gradient is applied to a stack placed in the resonator of TAE. Owing to non-existence of moving parts that a conventional engine has, TAEs are typically mechanically more efficient and reliable, hence are an important area of research. The thermos-acoustic phenomenon for TAEs is only driven by temperature gradient that induces fluid flow. However, in the previous works related to numerical study of standing wave TAEs, an initial disturbance in the form of pressure gradient has been imposed to generate fluid flow. In this paper, a 2D numerical analysis of a standing wave TAE is performed using computational fluid dynamics (CFD) modeling to capture the pressure fluctuations (without any initial disturbance) with time in the resonator channel in order to assess its thermo-acoustic performance. The results are obtained for pressure variation at specific points and the development of temperature profiles within the resonator. Using the pressure variations, FFT analysis was performed to identify sound pressure levels and resonant frequencies. Also, a sensitivity study has been carried out. The objective is to analyze the pressure wave development under different fluid properties. In this study, equivalent properties of a certain mixture of gases are prescribed to represent a composite working fluid. Two cases are considered i.e. mixture of air and helium and mixture of air and carbon dioxide. The compositions are varied in each case. It is noticed that in He mixtures the onset of pressure wave is quicker than in only air or CO2 mixtures, this due to the higher thermal conductivities. However, when only He is considered there is no pressure wave unlike only air or only CO2 cases due to low molecular weight. Frequency in He mixtures rises as He composition is increased, and the contrary is seen in CO2 mixtures. This is due to the collective consequence of the Cp, thermal conductivity and molecular weight. The study shows how important the thermal properties of the working fluid are for the pressure wave.

Abstract

The world currently has 117 Liquefied Natural Gas (LNG) regasification facilities, collectively processing 14,400 m3 /s of natural gas. Most of the facilities use processes which waste the cold energy, the most common of which discharges it at sea. Globally, up to 6.1 GW of cold energy can be recovered from LNG regasification terminals, delivering sustainable cooling chain to industries were low-grade waste heat is generated and cooling is required. This paper has proposed a direct heat exchange system, in which cold energy from LNG regasification can be fully recovered and utilised through the use of a brazed plate heat exchanger. Feasibility of implementing the system has been carefully examined through computational fluid dynamics modelling and experimental validation. The results suggested an optimal LNG to water/ethyleneglycol solution mass flow ratio of 1:6 most appropriate with the heat exchanger. This can potentially recover around 37 MJ of cold energy from an average sized LNG regasification terminal.

Abstract

Hydrocarbons (HCs) are eco-friendly and low-cost, and present an attractive prospect to be widely used in organic Rankine cycle (ORC) systems. However, the existing studies are scattered, and most of optimizations and selections only focus on a single performance index. This study carried out an optimization and selection of hydrocarbons for the subcritical ORCs based on multiple evaluation indexes. The heat source was 100-200oC, and five common hydrocarbons were selected as the working fluids. The selection guidelines of optimal working fluids were provided for various evaluation indexes. Results show that R290, R600a, and R600 achieve the largest net power output for heat sources of 100-150oC, 160-180oC, and 190-200oC, respectively. The system efficiency of R601 is the largest. R290, R600a, and R600 obtain the lowest specific investment cost for heat sources of 100- 140oC, 150-160oC, and 170-200oC, respectively. Compared with R245fa, hydrocarbons can achieve larger system efficiency and lower specific investment cost, and the net power output is also larger for heat sources not exceed 180oC.

Abstract

Electricity Supply Limited (SECL) has been the player involved both in generation and transmission of electricity. The study aims at forecasting annual electricity consumption of the company and suggest an optimal combination of renewable options namely wind, small hydro, solar photo voltaic, solar thermal and biomass to meet the RPO targets of subsequent 25 years. A linear regression model has been used for forecasting the consumption and RPO contributions whereas linear programming model has been used for determining the optimal combination of renewable sources under consideration over the segments of generation and short term power purchase agreements for meeting overall REC requirement from non-solar and solar sources. The study recommends for long term PPAs of 1 MW for wind, 30 MW of small hydro and 20MW of biomass instead of 1-year short term PPA and a need for promoting wind farms in the near future.