The power requirements for the next generation carriers are complex and have many variables and possible solutions. The complexity of the power distribution system is immense and requires much higher power densities than a standard utility distribution system. Today’s conventional power electronic systems and design practices result in systems that are 10X too large and heavy for application on the next generation carrier. The EMPF is working closely with the Office of Naval Research (ONR), DARPA, PEO Carriers, Penn State, and the Navy ManTech Electro-Optics Center of Excellence to develope viable alternatives for power distribution and pulsed power generation and usage in the next generation carriers (CVN-21, Figures 1-1 & 1-2) and electric warships (Figure 1-3).

The first of the next generation aircraft carriers is scheduled to undergo construction in 2007 and be placed in commission in 2014. The CVN-78, of the CVN-21 class carriers, will replace the USS Enterprise (CVN-65), which by then will be 53 years old. The next generation carriers and electric warships will be able to generate up to 104MVA (mega volt amps) of power. That is equivalent to a small electric utility power generation facility, or the energy required for approximately 80,000 average US households (907 kWh/month), per DOE statistics.

This power is required for new systems such as the Electromagnetic Aircraft Launch System (EMALS), Electromag-
netic Aircraft Recovery System (EARS), electromagnetic (EM) weapons, pulsed energy and laser weapons. All of these systems have at least one thing in common. They all require high energy electric pulses. How these pulses are generated and distributed is a new and novel problem that must be solved. In addition, conventional power electronic systems and design practices result in systems that are 10X too large and heavy for application on the next generation carrier.

The generation and distribution of pulsed energy is a critical component in the power distribution system. The current idea for generating pulsed power is through the use of advanced flywheel technologies. Each flywheel system is heavy and bulky. If each system that requires pulsed power has its own local flywheel system, this will place heavy equipment in areas of the carrier that are not optimal. Both weight and center of gravity are critical concerns of the next generation design. Placement near upper decks of the carrier are not desirable, so lower decks are preferred. Sharing some of the components of the pulse power supply, mounting them lower in the ship for better weight distribution, and distributing the lighter, weapon specific components closer to the guns and launchers are possible solutions.

How can the weight and size of power conversion equipment be reduced? The weight of a transformer made from iron and copper can potentially be reduced by using a solid-state transformer. This combines power electronics with a transformer that is reduced in size due to the operating frequency of the power electronics. Another possible advantage of this technology would allow the output voltage to be better regulated with fluctuations of the input. The development of wide band gap (WBG) semiconductors is presenting a possible paradigm shift in semiconductor power density. WBG devices operate at higher temperatures and require less cooling. They also have higher blocking voltages than conventional silicon devices and operate at higher switching frequencies, thus allowing for the use of smaller transformers and inductors (magnetics).

The EMPF has worked with Virginia Tech and Sandia Labs to develop the Emitter Turn-off Thyristor (ETO), an enhanced Gate Turn-off Thyristor (GTO) design with an integrated gate drive that operates at higher switching frequencies and allows for snubberless operation. Improved thermal management of semiconductors and passive components through upgraded packaging techniques would allow more current to be handled by a given device and lead to improved power density designs. Changes in cabinet designs have resulted in devices being mounted directly to advanced technology heat exchanger plates, providing a 25% increase in power density. The new design improves maintainability and reduces the number of cabinets required at unit cost parity. Advancements in power electronics such as these will allow for higher power densities, which translates to lower weight and size for equivalent power ratings. A significant change in power density would change the space and perhaps weight required to convert power on the ship.

The increased usage of motors and power electronics raises the concern of poor power factor loads along with possible high harmonic loads. Both of these issues are a burden on the power generation equipment and affect the overall efficiency of the power distribution system. Conventional distribution system designs and power conversion equipment would be a cause of degradation in the power factor that the shipboard generators would see. The power requirements may be better managed to enable the generator to achieve a net power factor of 1.0 by 1) replacing wire-wound power conversion systems with solid state transformers, 2)adding active harmonic filters, and/or 3) utilizing innovative rectifier configurations 12-pulse, 18-pulse, or active pulse width modulations (PWM).

There are many ways to distribute 104MVA. Is AC or DC distribution the best solution or is there a combination of both that makes better sense? Each has its own distinctive advantages and disadvantages. In a standard utility power distribution system, AC distribution is the only viable option in almost all cases. That is not the case on the next generation carrier. Variable Frequency Drives (VFD) and power electronics used to generate AC power all use DC voltage as their input. Because of these types of systems, DC distribution is probably a better solution.

Lead acid batteries used for energy storage for uninterruptible power supplies (UPSs) are another major concern aboard the carriers. Alternates for lead-acid batteries could be fly wheels, fuel cells, super-caps or lithium ion batteries. Will any of these technologies make a better solution than lead acid batteries? The total lifetime cost must be evaluated along with the concern of logistics and replacement.

The Navy has plans for advanced and integrated power electronics in future platforms. To fully exploit the electric power available on these new platforms, a fundamental change in how electric power is converted, delivered and managed will be required. These next generation warships will depend highly on the ability to rapidly shift power to major loads to support tactical needs. The EMPF is working to address the science and technology issues that must be addressed at the material and component levels to ensure this capability can occur in a timely and affordable manner.

References
1) Empfasis February 2004/ October 2002.

2)http://www.onr.navy.mil/media/extra/fncs_fact_sheets/electric_ships.pdf.

3) http://www.navsource.org/archives/02/78.htm.

4) http://www.onr.navy.mil/sci_tech/engineering/334_shiphull/by_thrust/aeps.htm.

5) IEEE Industrial Applications Society Annual Meeting Rome, Italy, October 8-12, 2000, "Power Electronic Transformers for Utility Applications".

6)http://www.eia.doe.gov/neic/quickfacts/quickelectric.htm.

7) http://www.globalsecurity.org/military/systems/ship/cvx-gallery.htm