Embedded Energy Storage Research Articles

A Hybrid Modular Multilevel Converter with Partial Embedded Energy Storage

Modular and cascaded multilevel converters provide a functional solution for the integration of energy storage systems (ESSs). This paper develops a hybrid multilevel converter based on the modular multilevel converter (MMC) that can be functionally extended with partial embedded ESS as a fraction of the overall converter power rating. The configuration, which can operate as a typical DC-AC converter, enables multi-directional power flow between the DC- and AC-side of the converter, as well as the embedded energy storage elements. The use of a three-phase flying-capacitor submodule eliminates the second-order harmonic oscillations present in modular cascaded multilevel converters. Current, voltage and power control are discussed in the paper while simulation results illustrate the operation of the hybrid MMC as a DC-AC converter in a typical inverter application and the additional functions and control of the embedded ESS.

An Actuator Control Unit for Safety-Critical Mechatronic Applications with Embedded Energy Storage Backup

This paper presents an actuator control unit (ACU) with a 450-J embedded energy storage backup to face safety critical mechatronic applications. The idea is to ensure full operation of electric actuators, even in the case of battery failure, by using supercapacitors as a local energy tank. Thanks to integrated switching converter circuitry, the supercapacitors provide the required voltage and current levels for the required time to guarantee actuator operation until the system enters into safety mode. Experimental results are presented for a target application related to the control of servomotors for a robotized prosthetic arm. Mechatronic devices for rehabilitation or assisted living of injured and/or elderly people are available today. In most cases, they are battery powered with lithium-based cells, providing high energy density and low weight, but at the expense of a reduced robustness compared to lead-acid- or nickel-based battery cells. The ACU of this work ensures full operation of the wearable robotized arm, controlled through acceleration and electromyography (EMG) sensor signals, even in the case of battery failure, thanks to the embedded energy backup unit. To prove the configurability and scalability of the proposed solution, experimental results related to the electric actuation of the car door latch and of a robotized gearbox in vehicles are also shown. The reliability of the energy backup device has been assessed in a wide temperature range, from −40 to 130 °C, and in a durability test campaign of more than 10,000 cycles. Achieved results prove the suitability of the proposed approach for ACUs requiring a burst of power of hundreds of watts for only a few seconds in safety-critical applications. Alternatively, the aging and temperature characterizations of energy backup units is limited to supercapacitors of thousands of farads for high power applications (e.g., electric/hybrid propulsion) and with a temperature range limited to 70 °C.

A Storage Integrated Modular Power Electronic Interface for Higher Power Distribution Availability

This paper proposes active power distribution nodes (APDNs) as an effective interface that can improve availability of power distribution networks. APDNs can be used as a modular interface block for critical power networks, such as secure facilities and vehicular platforms. By having bidirectional power flow capability and embedded energy storage, APDNs are able to increase both operational flexibility and availability of power networks. The embedded energy storage can minimize potential power interruptions experienced by the load. In addition, modularity is also inherent in the configuration of APDNs. The advantages of using APDNs as a connection interface inside a power network are discussed from an availability point of view in a quantitative manner by performing a comparison using Markov-based availability models. The configuration, characteristics, and operation states of the APDN are introduced, and a two-level hierarchical control approach that regulates the voltage/current level and performs energy management is discussed. The design is verified with simulation and experiments.

Decentralized Hierarchical Control of Active Power Distribution Nodes

This paper explores control strategies for active power distribution nodes (APDNs) using a hierarchical approach. The proposed APDN consists of multiple bidirectional interface modules and an embedded energy storage that is commonly connected to these modules. Such configuration properties make the APDN attractive as a major interface unit for active power flow control in microgrids and power distribution networks. A two-level hierarchy is considered so that the primary level performs load sharing using droop control, and the secondary level performs embedded energy storage management. The stability of the proposed control framework is studied and the proposed design is verified by an experimental setup.

Smart' Buoy Could Turn Waves into Platform and Subsea Power

Technology Update The use of moored, oceangoing “smart” buoys that can harvest energy from waves could be an efficient and economic means of supplying electric power for various offshore oil and gas operations, including well trees, monitoring systems, and autonomous underwater vehicles (AUVs). Potentially, such technology could reduce, or in some cases eliminate, the use of diesel-powered generators on offshore facilities. The PowerBuoy (Fig. 1), developed by Ocean Power Technologies (OPT), is new to the oil industry but has been used for a number of years in the defense and utility sectors. The autonomous buoy system consists of a surface float, a spar containing a power takeoff (PTO), a battery system, and a heave plate that constrains the spar’s motions. The system is capable of delivering energy from a few kilowatts to several hundred kilowatts, with future evolution planned to deliver even more power. The process begins with the rising and falling of waves, and the resultant mechanical stroking is converted by a specially designed PTO to drive an electric generator. This power is transmitted to external equipment by means of an underwater power cable or directly to payloads integrated into the structure itself. Continuous power is then available with the option of larger, timed power bursts. Advanced internal control systems continuously monitor the various subsystems and the surrounding environment and optimize performance of those systems with data transmitted to shore in real time, providing health and status updates on itself and its attached payloads. With this information, the operator gains a high level of control. Depending on the model of the buoy, the system is designed to need no maintenance for up to 3 years, providing potential savings in operating and life cycle expenses compared with existing power generation alternatives. Advanced control algorithms have been developed that actively assess oncoming waves to tune the internal PTO dynamically to ensure that it extracts maximum power. In the event of especially large oncoming waves, the buoy will automatically protect itself by locking the float and PTO subsystems and continuing to supply electric power to its payloads by means of the embedded energy storage system (i.e., batteries). The power management system of the buoy manages the state of charge of the battery to ensure efficient overall system operation and to preserve battery life.

Coordinated DC Voltage Control of Wind Turbine With Embedded Energy Storage System

This paper investigates dc voltage control strategies for output power smoothing of a fully rated converter-based wind turbine with energy storage device connected to the common dc link via a bidirectional dc/dc converter. Since the dc link voltage ripple reflects power oscillation, coordinated dc voltage control schemes are used for the ac network side converter and energy storage system to ensure that generated high-frequency power fluctuation is absorbed by the energy storage system, whereas the low-frequency components are transmitted to the connected ac network. Two methods have been proposed: one is based on proportional-integral (PI) voltage controller and low-pass filter with large time constant, and the other is based on PI with low natural frequency and droop controller with large droop gain. The detailed controller designs are described and system stability is assessed and shown to be stable. Both the MATLAB/Simulink simulations and experimental results are presented to demonstrate the effectiveness of the proposed methods for power smoothing.

Energy Management Systems for Energy Harvesting in Structural Health Monitoring Applications

Autonomous structural health monitoring systems with independent power sources and wireless sensor nodes are increasingly seen as the best solution for monitoring a diverse range of machines and structures including pumps, bridges and aircraft. Powering these systems using harvested energy from ambient sources provides an attractive alternative to the use of batteries which may be either inaccessible for routine maintenance or unsuitable (for example in aerospace applications). A number of techniques are currently being considered including harvesting energy from vibration and thermal gradients. Harvesting energy can however lead to a highly variable power supply in opposition to the requirements of a wireless sensor node which requires continuous standby power with an additional capacity for power peaks during transmission of data. A power management system with embedded energy storage is therefore necessary in order to match supply and demand. Due to the low levels of power harvested in a number of applications, an important factor in the design of such a system is its efficiency to ensure sufficient power reaches the sensor node. Based on the requirements for a simple power management system for thermoelectric power harvesting consisting of a rectifier, a DC/DC convertors and a battery, this paper first examines the possibilities in terms of basic components with a number of commercially available units tested and characterised. Potential designs for a management system incorporating these components are then discussed and a blueprint for an optimal system is suggested.

Characterization of multifunctional structural capacitors for embedded energy storage

Multifunctional composites are a class of materials that combine structural and other functionalities such as sensing, actuation, energy harvesting, and vibration control in order to maximize structural performance while minimizing weight and complexity. Among all the multifunctional composites developed so far, piezoelectric composites have been widely studied due to the high coupling of energy between the electrical and mechanical domains and the inherently high dielectric constant. Several piezoelectric fiber composites have been developed for sensing and actuation applications; however, none of the previously studied composites fully embed all components of an energy storage device as load bearing members of the structure. A multifunctional fiber that can be embedded in a composite material to perform sensing and actuation has been recently developed [Y. Lin and H. A. Sodano, Adv. Funct. Mater. 18, 592 (2008)], in addition to providing load bearing functionality. The design was achieved by coating a common structural fiber, silicon carbide, with a barium titanate piezoelectric shell, and poling the active material radically by employing the structural fiber as one of the electrodes. The silicon carbide core fiber also carries external mechanical loading to protect the brittle barium titanate shell from fracture. The excellent piezoelectric and dielectric properties of the barium titanate material make the active structural fiber an outstanding candidate for converting and storing ambient mechanical energy into electrical energy to power other electric devices in the system. This paper focuses on the characterization of energy storage capability of the multifunctional fiber provided by the dielectric properties of the barium titanate shell. The capacitances of the multifunctional fibers with four different aspect ratios are tested and compared with the theoretical expressions for the cylindrical capacitor, while the breakdown voltages of the multifunctional fibers are tested according to American Society for Testing and Materials standards (ASTM D 149–97a). The stored energy is calculated from the testing results and the best aspect ratio for energy storage application can be determined. The resulting capacitive fiber is shown to have an energy density approximately two orders of magnitude higher than structural capacitors in the literature.

Lifetime prediction and sizing of lead–acid batteries for microgeneration storage applications

Existing models of microgeneration systems with integrated lead–acid battery storage are combined with a battery lifetime algorithm to evaluate and predict suitable sized lead–acid battery storage for onsite energy capture. Three onsite generation portfolios are considered: rooftop photovoltaic (2.5 kW), micro-wind turbine (1.5 kW) and micro combined heat and power (1 kW). With no embedded energy storage, the dwelling exports energy when the microgeneration system generates excess power leading to a high level of generated export throughout the year. The impact that the size of installed battery has on the proportion of the generated export that is reserved onsite, along with the annual energy discharged per year by the energy store is assessed. In addition, the lifetime algorithm is utilised to predict corresponding lifetimes for the different scenarios of onsite generation and storage size, with design tables developed for expected cost and weight of batteries given a predicted generated export and lifetime specification. The results can be used to indicate optimum size batteries for using storage with onsite generation for domestic applications. The model facilitates the choice of battery size to meet a particular criteria, whether that be optimising size, cost and lifetime, reducing grid export or attempting to be self-sufficient. Suitable battery sizes are found to have lifetimes of 2–4 years for high production microgeneration scenarios. However, this is also found to be highly variable, depending on chosen microgeneration scenario and battery size.

On Embedded Energy Storage for High Penetration of Wind Power

It is recognised that to enable high penetration of wind power it is essential for modern wind farms to meet some technical requirements. These requirements are specified, or planned to be included, in grid codes by some utilities around the world. In this paper it is shown that having an Embedded Energy Storage (EES) unit, a battery bank, in a wind turbine can help to meet these requirements and to reduce the overall wind farm development and system integration costs. After a review of wind power integration and power system load frequency control, two cases are presented. These are based on (a) the statistics of the wind speeds measured at Dunstaffnage, Scotland, (b) the manufacturers' data for two turbine generators, and (c) some mild assumptions used for the calculations. In the first case, for the same amount of wind energy annual output and a specified frequency control requirement: the delta production constraint, a wind farm of 50 wind turbines can be replaced by 10 wind turbines with an EES in each of them plus 33 ordinary wind turbines. In the second case, again for the same amount of wind energy annual output, wind turbines with EESs can meet the absolute power constraint and at the same time the ratings of transformers, transmission lines and other equipments associated with a wind farm development can be reduced, leading to a significant reduction in costs. Wind farms with such wind turbines can also provide some vital support in an emergency, for example, the event in Denmark on the 8th January 2005, when the total wind power output dropped suddenly from the scheduled 1843MW to 126MW, due to the shut down of most wind turbines under strong winds.