John M. Heinzel

Evaluation of a Hybrid Energy Storage Module for Pulsed Power Applications

Before pulsed power systems can be fielded in either mobile or small footprint stationary applications, the prime power source must be optimized for both size and operational efficiency. In large footprint laboratories, prime power supplies are connected to a local utility grid to charge intermediate storage systems. In mobile platforms, alternative energy sources, such as electrochemical batteries or supercapacitors, must be used to backup smaller fossil fuel generators. The prime power source used in a pulsed power system must store high energy, to maximize the number of shots stored, and be able to source high power to recharge the intermediate store as fast as possible. Finding a single electrochemical energy storage device that has the right energy and power density for most applications is nearly impossible. Therefore, usage of batteries, which possess high energy density, along with electrochemical capacitors, which offer high power density, in a hybrid energy storage module (HESM) configuration is a promising way of combining both of these features into a single supply. Usage of this topology reduces the stress on the batteries, thereby prolonging their life, and also increases the instantaneous power capabilities of the system. This paper presents the design and validation of an actively controlled HESM built using commercial off the shelf power electronics and simple control strategies.

Capacity Fade of 26650 Lithium-Ion Phosphate Batteries Considered for Use Within a Pulsed-Power System’s Prime Power Supply

There is considerable need for a mobile, reliable, efficient, and compact prime power supply for use in a host of directed energy applications. Recent improvements in the energy and power density of electrochemical lithium-ion batteries have made them a very viable option for these types of applications where fast and rep-rate operation is of interest. Despite the proven ability of lithium-ion batteries to source high currents, it is still unclear how they age when they are used to repeatedly source high-rate currents in a pulsed manner, as they must when used in a repetitive rate prime power supply. Similarly, it is unclear how elevated rate recharge affects the life of the battery. Research has been performed at University of Texas at Arlington in which high-power, 2.6 Ah lithium-ion batteries have been repeatedly discharged and recharged at high pulsed rates. This paper will discuss the potential of lithium-ion batteries for use in these applications and will present experimental results performed when two 2.6 Ah cells were both discharged at 28 A (10.8C) and recharged at 9 A (3.5C) to 2.5 and 2.0 V, respectively.

Evaluation of an actively contolled battery-capacitor hybrid energy storage module (HESM) for use in driving pulsed power applications

As the U.S. Navy moves towards the fielding of all electric ships (AES), there will be a unique shift in the electrical load profile seen by its generation sources [1,2]. In addition to the electrical propulsion system, unique high pulsed power loads, which have high peak to average power requirements, will be deployed. Traditionally, fossil fuel generators have been used to power ships, but the transient power demands induced by pulsed power loads can impart extreme stress on generators, resulting in a loss in both fuel efficiency and power quality. One solution to this problem involves buffering the generators power demand with an energy storage device (ESD). Normally the go-to solution for implementing an ESD might be to utilize lead acid batteries however with recent developments in lithium-ion batteries (LIB), it is desirable to use them instead due to their high combined power and energy density [3-6]. In order to further preserve the lifetime of the LIBs and maintain their safe operation, many have proposed to augment them with electric double layer capacitors (EDLCs). Such a configuration is known as a hybrid energy storage module (HESM) since it incorporates both energy dense LIBs and power dense EDLCs into a single power supply topology [7-9]. A HESM is used to augment the generator in providing power to pulsed loads when they are energized while also acting as a sink to the generator when the pulsed loads are in an idle state. This enables the generator to maintain a nearly constant output, increasing its efficiency and power quality considerably. A few different HESM topologies have been previously presented [8-9]. In the work presented here, an actively controlled LIB as well as an actively controlled EDLC are deployed. This unique configuration is aimed at maximizing the energy density of the LIB while also maximizing the power and energy density of the EDLC. The HESM has been designed, constructed, and validated using only commercial off-the-shelf (COTS) technologies. This paper will present the advantages and capabilities that this type of HESM topology brings to a pulsed power system along with insights into the current abilities of COTS products to facilitate this application.

Electrochemical Energy Storage Devices in Pulsed Power

Numerous research efforts have been undertaken to improve the feasibility of fielding compact pulsed power systems for a wide variety of applications. Each of these research efforts focuses on specific areas of pulsed power, but few papers have focused on the performance of a grid independent prime power source. This critical component is often overlooked in mobile pulsed power systems. Though not within the pulsed power community, a great deal of work has been performed by those in the electrochemical energy storage community to improve the energy density, power density, working voltage, and cycle life of electrochemical cells. This paper has helped to develop new energy storage technologies for applications such as consumer electronics, electric vehicles, and electrical grid energy storage. However, these electrochemical cells were not developed with pulsed power applications in mind. For that reason, a large amount of research is needed to understand how to effectively use electrochemical cells in pulsed power applications. In this paper, a number of different energy storage chemistries, including lithium-ion batteries, lead acid batteries, and bi-polar nickel metal hydride batteries, have been evaluated for use in pulsed power applications as a result of their high power and energy densities.

Monitoring pulsed power on ship electrical systems

In this paper, forthcoming distributed generation and energy storage systems for ships are monitored by Sampled State Estimation (SSE). The paper demonstrates how a high data rate from transducers on the electrical system can be used in a discrete version of State Estimation that is effective in transient conditions as well as in steady operation. Various energy storage devices such as batteries are distributed around the ship and employed in conjunction with rotating machine generation in order to deliver pulses of power that far exceed the capability of the rotating machines. The distributed energy storage is coordinated to serve high pulse loads and employed to service local loads via dc/dc or dc/ac converters for fail-safe local power operation. The advantages of State Estimation on ship power systems are demonstrated for sampled data from current and voltage transducers.

A Dynamic Model of a Shipboard PEM Fuel Cell Reformer System With an Integrated Gas Turbine

A dynamic model is presented that predicts the transient characteristics of an integrated autothermal fuel reformer and gas turbine engine for extracting a high purity stream of hydrogen from logistical fuels such as F76 marine diesel or JP-5 for use in a shipboard proton exchange membrane (PEM) fuel cell power plant. The model incorporates a water-gas shift reactor for increasing hydrogen yield, and a separation membrane for extracting a pure stream of hydrogen gas from the reformer output. Compressed air supply and energy recovery is achieved by an integrated gas turbine generator. The dynamic model serves as a testing ground for the development of control methodologies and to predict limitations in transient response during electrical load variations.Copyright

Current sharing in parallel cell batteries cycled at high C rates

The high combined power and energy density of lithium-ion batteries give them great potential for being used as the prime power source for several future naval shipboard pulsed power applications. To meet the power and energy requirements, multiple lithium-ion cells will need to be connected in series and parallel. The number of cells needed will depend on the voltage, capacity, and current limit of the individual cells used. The simplest way of connecting multiple cells in parallel is to tie all of the positive terminals together and all of the negative terminals together, creating a new array of cells that can be treated as a single cell within the battery. In this configuration, a single battery management system (BMS) can be used and though this configuration is simpler and more cost effective, it makes it impossible to monitor the individual cells within each parallel array. Any impedance mismatches among a parallel array of cells introduces the possibility for current imbalances to occur. Significant current imbalance could negatively impact the batterys lifetime as well as its safety. The study performed here aimed at measuring the current imbalance present in a battery that has thirty cells connected in parallel (1S/30P) when it is cycled in a pulsed manner at high discharge rates. The current balance observed across two different rates is being characterized and the possible impact it has on battery performance and lifetime is being evaluated. A novel test bed with individual cell diagnostics has been developed that will be discussed along with the methodology behind the research, and the results collected to date.

Integration and study of hardware in the loop diesel generator with a hybrid energy storage module for naval applications

The US Navy is presently developing a number of new advanced electrical loads for deployment upon future vessels. Many of these loads will require high power supplied in a transient manner for successful operation. The transient nature brings about unique challenges to the shipboard power system that have not been previously faced. Traditional power generation sources alone, such as diesel and gas powered engine/generators, will likely not be sufficient to meet these demands and still maintain the power quality requirements required for deployment. To overcome these challenges, distributed generation sources, such as electrochemical energy storage systems for example, may be required to augment the traditional generation. Active control and monitoring will be critical in maximizing the power and energy capabilities of each distributed source. An energy storage system that utilizes multiple storage technologies is typically referred to as a Hybrid Energy Storage Module (HESM). It has been previously shown that both active and passive HESM topologies, utilizing power dense ultra-capacitors and energy dense lithium-ion batteries, can be very beneficial in meeting the power and energy requirements of transiently operated loads. It has also been shown that integration of a HESM with a gasoline generator can improve the generators power quality, keeping it within the bounds of MIL-STD-1399B when high power transient loads are sourced. In order to evaluate the integration of a HESM with a more representative Navy power source, hardware in the loop (HIL) modeling is being integrated with an existing HESM to replace the gasoline generator with a diesel engine — generator. This paper will discuss the HESM constructed, the HIL engine — generator model, and the results obtained when the two are integrated to source high power transient loads.

Model validation of multi-pulse rectifiers for charging capacitors

It is conceivable that the future fleet of US Navy vessels will deploy multiple high power AC and DC loads that must be reliably energized. Integration and operation of AC and DC loads will likely be achieved using an intelligently controlled microgrid architecture that is able to actively regulate and distribute power from both AC generation sources and DC energy storage devices. Some future loads may utilize intermediate energy storage that is highly capacitive, 10s of mF, and they may operate in a highly transient manner that is very stressful on the shipboard power system. Silicon controlled rectifier (SCR) based, multi-pulse rectifiers that are intelligently controlled, may be an attractive option for regulating AC power to charge capacitive DC loads in a repetitive manner. Though SCR multi-pulse rectifiers are well documented in the literature, there has been no published work found that studies their power quality when they are used to power repetitively operated capacitive loads. The work discussed here is aimed at developing experimentally validated Simulink® models of small scale, ∼ 1 kW, 6, 12, 18, and 24 pulse rectifiers, respectively, when they are used charge repetitive capacitive loads. The simulation models will be discussed along with results collected to date.

Investigation of harmonic distortion in multi-pulse rectifiers for large capacitive charging applications

The United States Navys future fleet of vessels will deploy an increasing number of high power electrical loads that will operate in a transient nature. As the electrical requirements increase, the efficiency of power conversion will become more important than ever before. Converting AC power to DC power can be very inefficient due to the generation of harmonics that are injected into the power system during conversion. Previous work has shown that the magnitude of the harmonics generated can be reduced significantly when multipulse rectifiers are implemented. These types of rectifiers work through splitting the three-phase input into additional phases so that the output ripple and distortion can be reduced. Higher numbers of phases result in improvement in power quality but come with added costs and complexity. Previous work in the literature has evaluated these types of converters into purely resistive or slightly inductive loads but not into highly capacitive loads, similar to those the Navy has plans to utilize. The work presented here investigates the harmonic content of multi-pulse rectifiers when used to charge large capacitive loads. Six, twelve, eighteen, and twenty-four pulse rectifiers have been simulated using Simulink® to predict how higher pulse numbers reduce the injection of harmonic distortion. Model validation will be carried out using a hardware implementation of each rectifier at the 1 kW level using a three-phase motor-generator source. Preliminary results from the simulations will be presented here.