In-Situ Power Generation for AUVs

Technology Roadmap Sections and Deliverables

This is a technology roadmap for:

• 4GEN - In-Situ Power Generation

This is a “Level 4” roadmap, indicating that it addresses a technology at the subsystem component level. Higher level roadmaps related to this subject would address technology progression at the market level (Ocean Exploration), product level (AUV), and subsystem level (Power System).

Roadmap Overview

The working principle and architecture of a generic Autonomous Underwater Vehicle (AUV) is depicted below.

Solar-electric aircraft are built from light-weight materials such as wood or carbon-fiber reinforced polymers (CFRP) and harvest solar energy through the photoelectric effect by bonding thin film solar cells to the surface of the main wings, and potentially the fuselage and empennage as well. The electrical energy harvested during the day is then stored in on-board chemical batteries (e.g. Lithium-Ion, Lithium-Sulfur etc…) and used for propelling the aircraft at all times, including at night. For the system to work there needs to be an overproduction of energy during the day, so that the aircraft can use the stored energy to stay aloft at night. The flight altitude of about 60,000-70,000 feet is critical to stay above the clouds and not to interfere with commercial air traffic. Depending on the length of day, i.e. the diurnal cycle which determines the number of sunshine hours per day, which itself depends on the latitude and time-of-year (seasonality) the problem is easier or harder. The reference case in the technology roadmap is an equatorial mission (latitude = zero) with 12 hours of day and 12 hours of night.

Design Structure Matrix (DSM) Allocation

The 4GEN tree shown above reveals that in-situ power generation (4GEN) is an enabling technology for the power (3POW) and propulsion (3PRO) subsystems that, together with communication (3COM), navigation (3NAV), control (3CON), and payload (3PAY) subsystems, form the persistent AUV (2PAO) product. Enabling technologies for in-situ power generation are energy conversion technologies for wave (5WAV), thermal (5THE), solar (5SOL), and wind (5WIN) energy. 4GEN is a direct enabling technology for the propulsion subsystem because some forms of wave energy conversion allow direct conversion to propulsive force without the need for conversion to electrical energy.

Roadmap Model using OPM

The Object-Process Diagram (OPD) in the figure below shows how the 4GEN technology is related to the broader persistent AUV system. The diagram shows the AUV and its constituent subsystems including the power subsystem, which consists of an energy conversion device that is the physical embodiment of 4GEN technology. As is shown in the diagram, 4GEN technology acts as the instrument that converts environmental thermal, wave, wind, and solar energy into usable energy that is used by the AUV to power its subsystems. The figure also depicts the maritime data collection function, which is the primary purpose of the AUV. Figures of merit (FOM) characterizing the energy generation technology are also shown.

An Object-Process Language (OPL) associated with the OPD is auto-generated and is shown below. It reflects the same content as the previous figure in narrative form.

Figures of Merit

The following table shows figures of merit (FOM) that can be used to characterize in-situ AUV power generation technology. Each FOM provides unique information: specific power offers insight into likely power output when integrated into a host system, conversion efficiency provides information about how much the technology can still be improved before the theoretical maximum is reached, maximum operating duration reveals how readily the technology can support persistent AUV operations, and unit cost provides insight into economic feasibility.

The following table shows important relationships and governing equations related to this technology for each of the in-situ energy sources. They are used to understand the richness of a source of environmental power, as well as calculate how much of that energy can be converted to usable AUV energy.

The following plots show specific power of various in-situ energy sources chronologically, as well as a comparison of reported power output to total vehicle mass for various autonomous maritime system that utilize at least one in-situ energy source.