ON THE EFFICIENCY OF ENERGY STORAGE SYSTEMS – THE INFLUENCE OF THE EXCHANGED POWER AND THE PENALTY OF THE AUXILIARIES

Storage is an important domain of the energy sector, with its traditional, classical solutions for smaller and larger amounts of energy. Energy storage has become of higher importance in relation with the development of alternative energy sources, leading to the development of new technologies. The energy efficiency of the storage means is an important parameter, being often not considered in the conception and design of the applications. For the evaluation of the energetic performance of a storage device, a well-adapted tool has been proposed, namely “The Theory of Ragone Plots”. This tool sets in evidence in what way the effectively recoverable energy amount of a device is depending on the power level of the charge/discharge process. Further, the taking into account of the power needed for the auxiliary equipment of a storage system like the circulation pumps of a flow battery, the vacuum pumps of a flywheel or the forced cooling of a battery can lead to a globally negative value of the efficiency

A HIGH EFFICIENCY PNEUMATIC DRIVE SYSTEM USING VANE-TYPE SEMI-ROTARY ACTUATORS

A compressed air driven generator is proposed, where the pneumatic energy is converted into mechanical energy using two vane-type rotational actuators. The use of a second actuator with a higher displacement in order to produce a thermodynamic expansion allows to reach a better energetic efficiency in comparison to the classical operation of such actuators. The alternating movement of the angular actuators is transformed into a unidirectional rotational motion with the help of a mechanical motion rectifier. The paper analyses the enhancement of the energetic performance of the system. An experimental set-up is also described. The performance of the new system is described, and the limits of its realization is commented on the base of experimental recordings of the evolution of the pressure in the chambers

Understanding the Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRB) are large stationary electricity storage systems with many potential applications in a deregulated and decentralized network. Flow batteries (FB) store chemical energy and generate electricity by a redox reaction between vanadium ions dissolved in the electrolytes. The most significant feature of the FB is maybe the modularity of their power (kW) and energy (kWh) ratings which are independent of each other. In fact, the power is defined by the size and number of cells whereas the energetic capacity is set by the amount of electrolyte stored in the reservoirs. Hence, FB can be optimized for either energy and/or power delivery. Over the past 30 years, several redox couples have been investigated (Bartolozzi, 1989): zinc bromine, polysulfide bromide, cerium zinc, all vanadium, etc. Among them, VRB has the best chance to be widely adopted, thanks to its very competitive cost, its simplicity and because it contains no toxic materials. In order to enhance the VRB performance, the system behaviour along with its interactions with the different subsystems, typically between the stack and its auxiliaries (i.e. electrolyte circulation and electrolyte state of charge), and the electrical system it is being connected to, have to be understood and appropriately modeled. Obviously, modeling a VRB is a strongly multidisciplinary task based on electrochemistry and fluid mechanics. New control strategies, based on the knowledge of the VRB operating principles provided by the model, are proposed to enhance the overall performance of the battery

System Architectures for Multiports, Bidirectional and Buffered Charging Units for EV’s

Bidirectional buffered units for Multiports charging of EV’s are presented, allowing to charge with high power even if the line current capability is limited. The systems are also dedicated to operate as reactive power compensators, or to provide grid system services as V2G operation or other power smoothing functions

A Bidirectional Buffered Charging Unit for EV’s (BBCU)

A bidirectional buffered charging unit for EV’s is presented, allowing the charge with high power even if the line current capability is limited. The bidirectional buffer is connected to the AC line and is mainly dedicated to vehicles with on-board AC-DC chargers. Extensions are described for the integration of RES as well as for DC charging. The system is also dedicated to operate as a reactive power compensator, or to provide grid system services similar to V2G operation or other power smoothing functions. At the side of the buffer battery, the system presented uses a bidirectional DC-DC converter which can be realized in different technologies. A conventional solution with three interleaved channels is compared with a fast switching converter using SiC components

On The Strategies Towards Isothermal Gas Compression And Expansion

Isothermal compression/expansion is regarded as the most promising process in many applications and many researchers and inventors have tried different methods to achieve this goal. The current article first studies the gradual roadmap from adiabatic towards isothermal process from thermodynamics and heat transfer point of view. Different strategies are investigated to achieve this goal by evaluating different possibilities; the bottleneck of the problem is then identified and then increment of heat exchange surface is being focused on as the most promising solution. Finally a global map is illustrated to show the starting point, proposed solution standing point and the ideal situation and its requirement

Modelling and simulation of a Three-Stage Air Compressor Based on Dry Piston Technology

The core of this modelling is to study heat transfer and fluid dynamics processes for a compression expansion system, and the main particularity is that heat transfer and air movement are due to the movement of the piston. We have implemented a "moving mesh" solver to compute the volume changes of the compression chamber followed by a "Fluid dynamics" type solver. It allows correct computation of the fluid behavior in the system and enable us to identify the pressure change of the fluid, and heat generation impacts. The last solver is "heat transfer" solver, to identify the temperature gradient. It is then a strongly non-linear modelling approach. Finally we have plotted corresponding diagrams as they enable us to analyze directly the performance

Multilevel modular DC/DC converter for regenerative braking using supercapacitors

Regenerative braking is presented in many electric traction applications such as electric and hybrid vehicles, lifts and railway. The regenerated energy can be stored for future use, increasing the efficiency of the system. This paper outlines the benefits of the MMC (modular multilevel converter) in front of the cascaded or series connection of converters to achieve high voltage from low voltage storage elements such as supercapacitors. The paper compares three different solutions and shows that the MMC can benefit from weight and volume reduction of the output inductance when shifted switching modulation strategy is used. Using this modulation strategy, not only the output frequency is increased, but also the magnitude of the inductor applied voltage is reduced, reducing inductor size and volume.Postprint (published version

An Ultrafast EV Charging Station Demonstrator

This paper deals with the design of a grid-friendly ultrafast electric vehicle charging demonstrator. High charging power and short charging times impose peaks to an electricity distribution system, which necessitate over-dimensioning of the grid connection. A mitigation option lies in partial decoupling the load from the grid, achieved with the application of energy storage elements. A calculation methodology for energy storage elements is proposed and their interconnection possibilities to an ultrafast EV charging spot discussed

Toward Ultrafast Charging of Electric Vehicles

The paper is devoted to the problems arising from the ultrafast (≤ 10 min) charging of an electric vehicle (EV). An ultrafast charging station (UFCS) must provide high power output with minimal influence on the electricity transmission system, which can only be achieved by the application of energy storage acting as an additional buffer between the vehicle and the grid. Besides storage, interfaces between a fast charging station and the outside environment (vehicle, utility grid) must be designed to fulfil a set of requirements. The main challenge is to be found within the specification of parameters for the design of future energy supply systems, providing for fast charging of the vehicle batteries while avoiding solicitations of the local distribution system which exceed its instantaneous power capabilities. The possible impact of an UFCS on the power distribution system is analysed with the stochastic approach, based on the utilisation of such a station. The general aspects of highly variable load profile clearly include the use of energy storage means that must be specified regarding both the energy storage and the instant power capabilities. Different technologies are analysed in terms of performances and costs. In conclusion, one of the important problems facing electric vehicles is the possibility of short-time charging, both as seen from the EV battery itself as well as from the local supply system. In this context, large load variations as seen from the local power system, at multiple levels, must be carefully assessed with special attention to feasible load changes at the coupling points. By addressing these technical issues as well as the financial constraints, the research aims at providing viable solution for broad ultrafast charging systems integration into the power distribution infrastructure