Specification of a Miniature Natural Gas Power Plant for an Unmanned Surface Vehicle

Photo of James Novogoratz, Jake Ralph, Rishi Rebello, Austin Taliaferro Students: James Novogoratz, Jake Ralph, Rishi Rebello, Austin Taliaferro

Sponsor: Trident Research

Date: Fall 2012

Requirements:
The main requirement of the power plant is to power the USV's sensors and trolling motors. Combined, these components require 2.5-3.2 kW, depending on which USV components Trident selects, of 12 VDC electrical power. Additionally, the power plant should function for a minimum of 24 hours and have an electric start such that it can be deployed and started remotely. Also, the power plant should withstand the impact of deployment, a five meter drop into water. The main constraints for the device are to use compressed natural gas (CNG) - a storage method that achieves high energy density via high pressures - or adsorbed natural gas (ANG) - a relatively new technology for storing natural gas at lower pressures by allowing the fuel to bond onto a porous, high surface area filler material - as fuel. Geometrically, the engine is constrained to fit within a two-foot cube. The power plant's enclosure must protect against water seepage and overheating. Primarily the weight, and secondarily the cost, of the power plant should be better than that of the all-battery alternative.

Problem:
Trident Research produces equipment for land-based and offshore weapons testing. Trident wants to expand their offshore capabilities by creating an unmanned surface vehicle (USV) to operate radar, telemetry, and optics. Our team was tasked with selecting a power plant that runs on natural gas, designing the fuel storage system for either compressed or adsorbed natural gas, and providing design recommendations for an enclosure to protect the power plant from the ocean environment.

Solution:
Our final recommendation includes an ensemble of commercial off-the-shelf (COTS) components comprising a Yamaha EF3000iSEB generator affixed with an after-market tri-fuel carburetor, a 200 amp DC power supply from TRC Electronics, supplemental 75 A-hr batteries from Power-Sonic, and a fuel storage system. The generator and power supply provide 1932 watts after a 31% reduction of the rated continuous power from the supplier (due to the shift from gasoline to natural gas and the 86% efficiency of the DC power supply). This provides energy for the majority of the USV's 24-hour mission, primarily during the root mission wherein only communications, acoustics, navigation, and motors operate. Supplemental batteries are required for the comparatively brief periods of time when the USV's radar and telemetry operate. Due to CNG tanks being lighter (only 93-124 lbs as compared to ANG's 780-1300 lbs) and more compact (only 4.7-6.3 ft3 as compared to ANG's 19.0-31.7 ft3), we recommend the CNG solution. Although our solution may cost more initially, due to fewer recurring expenses our recommendation is more cost viable than the all-battery alternative in 4-10 years. Additionally, based on a risk analysis of our proposed solution, we provide generic recommendations for mitigating such risks, some of which directly relate to an enclosure design. Potential risks which were considered include choking, overheating, water seepage, and impact damage. Our enclosure design guidelines account for these failures.

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