The Status and Future of Flywheel Energy Storage

Flywheels, one of the earliest forms of energy storage, could play a significant role in the transformation of the electrical power system into one which is fully sustainable yet low cost. This article describes the major components that make up a flywheel configured for electrical storage and why current commercially available designs of steel and composite rotor families coexist. In the process, design drivers, based on fundamentals are explained in a clear and simple manner inclusive of approaches to safety. The robust characteristics of flywheels deem them highly suitable for applications requiring fast response and high daily cycles, a need that is growing as grid inertia reduces. Lithium Ion batteries are currently the technology of choice for fast response but suffer from limited cycle and calendar life. This can be mitigated by having sufficient energy capacity to limit depth of discharge during short duration cycles whilst using this capacity to earn revenue for provision of other services. Now, as other mechanical, thermal to electric and renewable fuel based storage technologies develop, these will provide storage at a lower cost, greater duration and in a more sustainable way than Lithium Ion. However, the need for fast response storage will remain and steel flywheels are well placed to provide this given potential for low power cost and their sustainability credentials. In order to obtain cost estimates for flywheels in volume production, the cost of the power and storage elements were separated out with costs for each based on similar technologies in volume production. These indicate significantly lower costs than given for current commercially available flywheels, none of which are in volume production relative to Lithium Ion. Finally, some areas of research with potential to improve performance are described but, to be worthwhile, these developments must not lead to increased costs

Radioactive nondestructive test method

Various radioisotope techniques were used as diagnostic tools for determining the performance of spacecraft propulsion feed system elements. Applications were studied in four tasks. The first two required experimental testing involving the propellant liquid oxygen difluoride (OF2): the neutron activation analysis of dissolved or suspended metals, and the use of radioactive tracers to evaluate the probability of constrictions in passive components (orifices and filters) becoming clogged by matter dissolved or suspended in the OF2. The other tasks were an appraisal of the applicability of radioisotope techniques to problems arising from the exposure of components to liquid/gas combinations, and an assessment of the applicability of the techniques to other propellants

Vehicles for rural transport services in sub-Saharan Africa

There is a critical lack of affordable transport services linking villages to markets, healthcare and other facilities in rural areas of sub-Saharan Africa, which significantly restrains rural development. A key factor is the severe constraint on profitability of services due to the high operating costs of conventional vehicles operating at relatively low speeds on rural roads. This paper argues that there is a need to consider lower-speed alternatives based on motorcycles. It shows that motorcycle-based vehicles such as trailers and three-wheelers can carry loads up to 1 t on rural roads when geared down to an appropriate speed. The operating costs of these vehicles are shown to be around half those of conventional vehicles, greatly increasing the potential for setting up profitable transport services. Further development is needed to show that motorcycle-based vehicles can be used safely and profitably to help rural people improve their livelihoods and facilitate rural development

Characterization of Flywheel Energy Storage System for Hybrid Vehicles

Flywheels are excellent secondary energy storage devices and several applications in road vehicles are under development. They can be used in hybrid vehicles with an internal combustion engine (ICE) as the prime mover or can be used in hybrid energy storage (HES) to complement the battery. When used in HES, they are utilized to load level the battery so as to protect it from peak loads and enhance its capacity and life. This paper deals with defining the main characteristics of the flywheel for an application as a secondary energy storage device for an electric vehicle. Various strategies for defining flywheels are explained. A real world customer usage data is also presented. This data is analyzed and its results are used to support the selection of the flywheel characteristics. The results show that the chosen flywheel is sufficiently sized to perform its intended tasks for a c-segment passenger car electric vehicle

The Effect of Pre-swirl on the Discharge Coefficient of Rotating Axial Orifices

The characteristic of the flow through a set of rotating orifices for particular conditions is quantified in terms of a discharge coefficient which is the ratio of the actual flow divided by the flow for the ideal case. In the present study the pre-swirl was created by having Inlet Guide Vanes (IGVs) at the inlet of the rotating orifices. Pre-swirl changes the angle of attack to the orifice, and thus affects the incidence angle and the discharge coefficient. Pre-swirl improves the discharge coefficient in inclined orifices since it allows the incidence angle to reach zero at a much lower rotational speed than when there is no pre-swirl

Analysis of Dual Mode Continuously Variable Transmission for Flywheel Energy Storage Systems

There are different types of energy storage devices which are used in today’s hybrid and electric vehicles. Batteries, ultra capacitors and high speed flywheels are the most commonly used ones. While batteries and supercapacitors store energy in the form of electric energy, the flywheel (FW) is the only device that keeps the energy stored in the original form of mechanical energy the same as the moving vehicle. The flywheel needs to be coupled to the driveshaft of the vehicle in a manner which allows it to vary its speed independently of the moving vehicle in order to vary its energy content. In other words a continuously variable transmission (CVT) is needed. The common mechanical variators used in automotive applications, namely the rolling traction drives and the belt drives, have the disadvantage that their speed ratio range defined as the maximum to minimum speed ratio is generally not sufficient for flywheel energy storage system (FESS). One of the ways to improve the ratio range is by using a dual mode transmission, where the ratio coverage of the variator is exploited twice. This paper presents the fundamental kinematics of such a transmission including its variants. The equations of speed ratio, power flow and efficiency are derived for a variator only transmission and a power split CVT (PSCVT) used in dual mode and the results compared

Simulation based study of battery electric vehicle performance in real world cycles

The development of battery electric vehicles (BEV) must continue since this offers the leading route towards a zero emission transport system. The fuel flexibility of the BEV offers the greatest potential to utilize power from renewable or low emission sources to be used in the transport system. However the limited range and high cost of the BEV remain important issues to be addressed. The battery is the element which strongly affects the cost and range of the BEV. The batteries offer either high specific power or high specific energy, but not both. This paper presents the modelling of a BEV which is used to study the potential for improvement in its energy efficiency. The battery model types have been discussed. The vehicle and other component models have been described. The choice of model parameters and the control strategy has been explained. The simulations have been performed on homologation and real world cycles for different scenarios. Results show significant potential for improvement in the energy efficiency of the BEV in real world usage by the utilization of a secondary energy storage device

Assessment of the Carbon and Cost Savings of a Combined Diesel Generator, Solar Photovoltaic, and Flywheel Energy Storage Islanded Grid System

The use of diesel generators to provide power for islanded grids has been the technology of choice but they generate substantial carbon emissions unless the part or all the fuel comes from a renewable source. Notwithstanding this, the engine must be sized to meet maximum demand and will operate inefficiently at part load most of the time, which is particularly bad for a synchronous constant speed engine. Given the availability of low cost solar photovoltaic (PV) systems, it is very enticing to fit a diesel generator and allow the engine to be turned off during PV generation. However, this combination will not work without some form of energy storage since it takes time for the engine to start, leading to gaps in supply and instability of the system. Lithium-ion batteries are typically considered to be the best solution to this problem because they have a high response rate, costs are lower, and they are available as products. However, they will suffer from the limited cycle and calendar life due to high cycling requirements in the application described. It is, therefore, proposed that a flywheel system could offer a lower lifetime cost alternative since only short duration bridging power storage is needed and flywheels of appropriate design can offer lower power cost than Lithium-ion battery systems. Flywheels are particularly attractive since they have a very high calendar with almost an infinite cycle life and are fully recyclable at the end of life. This research, therefore, presents an assessment of the flywheel energy storage system (FESS) as an alternative to electrochemical batteries to supplement solar PV systems backed up by diesel generators. The model of an islanded PV system combined with a diesel generator and a FESS supplying power to a residential load is implemented in MATLAB/Simulink. The results of the analysis for the cases with and without storage based on a number of different charge-discharge strategies provide evidence to support this hypothesis

Cooking and lighting habits in rural Nepal and Uganda

Nearly a quarter of the world’s population has no access to electricity and nearly half cook with biomass or coal. Over the past 30 years there have been several initiatives to design and introduce appropriate, renewable energy technologies for household energy supply. However, few of these have been successful in terms of significant penetration and long term sustainability. A new initiative, the SCORE project, was launched in 2007 with the objective of producing a clean biomass cook stove which also incorporates a thermo-acoustic engine which converts some of the heat into electricity. It was vital to determine the technical requirements of this new device as a pre-requisite to engineering development particularly in the two candidate countries where this new technology was to be trialled: Nepal and Uganda. Accurate data could not be sourced in the open literature and therefore studies were commissioned in order to obtain the relevant data. The data obtained is of significant value for any development projects involving the provision of small-scale off-grid electrical power and heat for cooking

Optimisation of flywheel energy storage systems with geared transmission for hybrid vehicles

Flywheel energy storage devices may be coupled to mechanical transmissions for braking energy recovery and the provision of additional power for acceleration in hybrid vehicles. Power transmission across a continuous range of speed ratios is necessary. The flywheel size and depth-of-discharge must be chosen for a particular application, and this has a direct effect on transmission efficiency, required gearing ratios and mass of components. Optimisation of these parameters requires a fundamental understanding of this interaction, which has not previously been investigated and reported. To address this, the current paper presents a new method of analysing mechanical flywheel systems. A simple algebraic analysis can be used to specify flywheel system parameters for any regenerative braking application where the flywheel is used to provide initial acceleration of the vehicle from stationary. This has been applied to systems using geared transmissions with continuous speed variation achieved through sliding contact in clutch and brake components. The results of the analysis highlight how the optimum selection of flywheel depth-of-discharge must achieve a balance between high transmission efficiency and low system mass. This is illustrated for a passenger car application, allowing a full assessment of system performance and the specification of appropriate design parameters