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
Autonomous underwater vehicles (AUV) are widely used to make observations and make sense of the ocean without the limitation of requiring a human-in-the-loop to function. Despite advancements in battery and other energy storage technologies, power availability remains a key limiting factor for many AUVs. Power constraints limit payload capabilities, reduce design trade space for key functions like communication and navigation, and restrict the reach of AUVs by reducing mission durations and preventing access to remote areas of the ocean. Despite its austerity, the ocean offers abundant sources of environmental energy in the form of wind, solar, waves, and thermal gradients that can be harvested with the right technology. The figure below shows a notional autonomous maritime system that utilizes each of these energy sources to provide an in-situ power generation capability.
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.
Alignment with Company Strategic Drivers
| Number | Strategic driver | Alignment and targets |
|---|---|---|
| 1 | Deliver a portable offshore wave energy conversion system with enough capacity to sustain large-class (>500kg) AUV operations. | Target: Generate an average of 1kW of electrical power using a device with a maximum characteristic width of 5m.
Increasing wave energy conversion efficiency at a deployable size is the focus of this roadmap, therefore this strategic driver is aligned with the technology roadmap. therefore this strategic driver is aligned with the technology roadmap. |
| 2 | To meaningfully reduce the latency experienced by our subscribers, thereby enabling our service to support new low-latency use cases and opening new selling points to consumers such as voice over IP, gaming, virtual reality, live captioning etc. | The current technology roadmap considers low-orbit satellite systems that will significant reduce the propagation delay and latency between ground and satellite when compared with current GEO satellite architectures. This driver is currently aligned with the technology roadmap. |
| 3 | To improve the quality and reliability of our service indoors, which is important for potential subscribers in high-wealth, highly-urbanized areas, and take market share from our competitors in the cable and DSL sectors. | While our roadmap targets higher receiver gain, the resulting signal is generally not strong enough to penetrate building walls. Improving service indoors will require installation of more antennas on roofs by working closely with developers and building administrators, which is not part of the current technology roadmap. This driver is not currently aligned with the technology roadmap. |
Positioning of Company vs. Competition
Technical Model
There are various ways to convert the wave energy, and the amount of energy significantly changes depending on the wave's situation. For simplicity, we divide the model into three parts: 1)the wave energy converter (WEC) part, which converts the wave energy as mechanical, pneumatic, or potential energy, 2)the power take-off (PTO) part, which converts absorbed energy into useful mechanical energy, and 3) the generator, which transforms useful mechanical energy into electrical energy.
Some important variables:
- B = Characteristic Dimension [m]
- A = Maximum Horizontal Cross-sectional Area of the WEC device [m^2]
- H = Significant Wave Hight [m]
- T = Wave Energy (mean) Period [seconds]
- η = Hydrodynamic Efficiency [%]
- ρ = Water Density [kg/m^3] (≈1000 kg/m^3 )
- g = Gravitational Constant [m/s^2] (≈ 9.81 m/s^2)
Key equations:
Assuming that we use the heaving device, which has a spherical floating device, the power converted by the WEC is given by:
Also, based on Budal’s limit, the upper theoretical limit for power generation is given by: