• Energy Research

Abstract The design of compact, high performance electric swashplate refrigeration compressors demands a clear understanding of different physical phenomena and their interactions taking place inside the compressor. The dynamic characteristics of the compressor are associated with the start-up transients of the swash-plate mechanism and the time variation of suction and discharge pressures. An experimentally validated, easy to implement transient swashplate compressor model has been developed that can capture the essential physics, including inertia of the pistons and swashplate to evaluate the electric motor torque loading during compressor start-up. The effects of moment of inertia, bearing torque, viscous resistance to piston motion, and suction and discharge pressures on the torque and compressor mechanical input power are investigated. For model validation, the start-up behavior is tracked experimentally using a high-speed data logger to monitor the changing phase currents of the brushless DC motor, capturing both the instantaneous power and rotational speed. Rotational mass moment of inertia is found to have only a small effect on the compressor torque and power output and can be made negligible by changing settings in the start-up algorithm for the electric motor controller. Suction and discharge pressures during start-up are found to have the largest influence on the required starting torque. More than 95% of torque is found to be because of the line pressures. Predictions are in good agreement with measurements and show that depending on the starting refrigerant pressures in the supply lines, the starting torque can be lower than the operating torque for the compressor. The original contribution of this work is in deriving a transient swash-plate compressor model that includes the inertia of the swash-plate mechanism and in clarifying the relative importance of inertia, line pressures, viscous losses and bearing resistance on the required start-up torque for this type of compressor.

Abstract The effect of the strength of the self-induced magnetic field around a current carrying wire on thermomagnetic convection cooling in ferrofluid is experimentally and numerically investigated. Temperature-rise characteristics of the hot micro-wire for uniform Joule heating with different electric currents are compared to identify the relative importance of the strength of the axisymmetric magnetic field. Experiments are done with copper and platinum wires with current inputs adjusted to achieve the same Joule heating per unit length of wire. It was found that at high current supply (2A), mixing in ferrofluid is escalated due to thermomagnetic convection that resulted into 28% increase in average Nusselt number value which finally resulted into temperature drop of 8 K in comparison to DIW. Comparison of results for different self-induced magnetic field strengths in ferrofluid and deionized water clearly show that the observed cooling phenomenon is due to the self-induced magnetic field interacting with the magnetic fluid rather than natural-convection or other nanofluid-related mechanisms. For 1.83 W J heating, the temperature of the copper wire is 6 K lower than that of the platinum wire. Temperature and velocity contours obtained from simulations based on a 2-D single-phase model including a temperature-dependent magnetic body force provide flow visualization and further confirmed that the thermomagnetic cooling is responsible for the observed behaviour.

Abstract Vanadium redox flow batteries (VRFBs) are increasingly used in different large-scale stationary applications. In particular, this state-of-the-art energy storage system is used to deal with power management, peak shaving and load leveling and to support a large-scale renewable power grid. VRFBs offer many benefits such as long lifetime, flexibility, and relatively high performance; however, their high capital and operation costs haveremained to be their major drawback compared to other more conventional energy storage systems (ESSs). Thus, significant efforts have been dedicated to modifying VRFB components in order to enhance their economic competitiveness while improving their performance. The present paper comprehensively reviews and discusses various electrode modification approaches, and electrolyte retainment techniques by focusing on their pros and cons and effects on the performance of VRFBs and the research gaps to be addressed. This paper also aims to explore another approach, known as the nanofluidic electrolyte technique, to simultaneously modify electrode and electrolyte. In addition, novel materials with different affecting properties suitable to be utilized in “nanofluidic electrolyte” technique for improving the performance of VRFBs are introduced and discussed.

The ability to convert waste plastic into combustible liquids and gases using solar energy could help transform the problem of disposal of non-recyclable plastic into a valuable and environmentally responsible source of fuel. The purpose of this study is to propose a practical model for a compound parabolic trough solar thermal reactor for pyrolysis of waste plastic. The model integrates predictions of energy available from solar radiation (at a given location, time of day and time of year) with parabolic trough collector orientation and efficiency, a transient energy balance for an evacuated tube reactor and pyrolysis kinetics of waste plastic. The experimental setup used to test the model includes a pyranometer, a commercial solar collector consisting of a 60 cm long evacuated tube with a compound parabolic reflector and multi-channel data loggers to collect temperature, humidity and radiation data. The solar radiation sub-model was found to be in excellent agreement with clear-sky irradiance data collected using the pyranometer. Predictions of reactor temperature and reaction rate were found to be sensitive to the concentrator aperture area, solar irradiance, type of plastic (Arrhenius kinetics) and radiation properties of the evacuated tube reactor but relatively insensitive to humidity, wind velocity and terrestrial irradiance. The model shows that even on a small scale, favourable conditions for pyrolysis of waste plastic can be achieved within a solar reactor. ; Full Text

Abstract A real-gas, restricted-flow valve model is compared with an ideal-gas, ideal-valve model for a 10-cylinder swashplate refrigeration compressor. Real gas properties of R134a are evaluated using the NIST standard reference database. A minor-loss discharge-coefficient approach is used to model the refrigerant flow rate through reed valves while the ideal-valve model requires no pressure difference to open the valve. In contrast with the ideal model, the discharge temperature, refrigerant mass flow rate and volumetric efficiency as a function of rotational speed are predicted well by including real-gas properties and flow restriction on the inlet valve. The ideal-gas model significantly overpredicts the discharge temperature and shows no dependence on rpm. Heat transfer to and from the cylinder wall during compression and expansion is found to have only a small effect on predictions of compressor performance. The valve model for the suction side has the largest influence on compressor performance predictions as a function of rpm.

Abstract The use of electrolyte-based nanofluid is a new approach for enhancing the performance of Vanadium Redox Flow Batteries (VRFBs). This paper, for the first time in the literature, presents an experimental study to comprehensively investigate the rheological behaviour, electrical conductivity, and thermal conductivity of a newly prepared electrochemical graphene oxide (EGO)/ vanadium (IV) electrolyte-based nanofluid in different weight concentration and bulk temperatures. SEM, FT-IR and X-ray Diffraction (XRD) characterizations were performed and different functional groups, smooth 2D layered structures, and low crystallinity were detected in our tailored oxidation EGO which are amongst the enhancing features for VRFBs electrode materials. It was observed that the optimum weight concentration of nanoparticles in the electrolyte-based nanofluid is 0.05 wt% with strong colloidal stability, enhanced electrical and thermal conductivities, and an optimal viscosity. The maximum feasible enhancements in electrical and thermal conductivities that were achieved were 12%, and 4%, respectively, which can positively affect the performance of flow battery in terms of electrochemical activity on the electrode surface and thermal management of the flow battery system. In addition, the rheological evaluation showed that the behaviour of the electrolyte-based nanofluid tends to change from Newtonian to non-Newtonian for weight concentrations higher than 0.05%. The results obtained from rheological behaviour can provide useful insights in choosing an optimum pumping system of the flow batteries working with electrolyte-based nanofluid.

Development of the Vanadium Redox Flow Battery (VRFB) has been widely reported but typically only focuses on one part of the cell (e.g. electrode, electrolyte, or membrane). Improvement to a single part of the cell may cause side effects on other parts during long-term cycling leading to an overall drop in the performance of the battery. To avoid this, the use of nanofluidic electrolyte seems to be a promising approach to enhance the performance of both electrode and electrolyte simultaneously. This paper aims to investigate the electrochemical performance of a newly prepared reduced graphene oxide (rGO) nanofluidic vanadium electrolyte, applicable for Vanadium Redox Flow Batteries (VRFB). Herein, we report for the first time a stable rGO/vanadium nanofluidic electrolyte with improved electrochemical performance. Benefiting from the low degree of oxidation as compared to GO, the rGO can provide high electrical conduction due to the presence of sufficient functional groups, which can facilitate the redox reactions. The effect of various concentrations of rGO on the electrochemical performance is investigated. The current collector (carbon cloth (CC) electrode) was further characterized using different physico-chemical techniques to underpin the stability of rGO nanofluids. The results suggested that the electrochemical performance of vanadium electrolyte increases with the concentration of rGO. Improvements of approx. 15% to 20% were achieved in peak potential separation and current density rates, respectively. In addition, the incorporation of rGO in nanofluidic electrolyte significantly decreases the electrolyte and charge transfer resistance by ∼10% and ∼99%, respectively, and improves the vanadium ion diffusion process by ∼50%.