PROPOSALS ACCEPTED: Phase I Only
TECHNOLOGY AREA(S): Materials/Processes, Space Platforms
OBJECTIVE: Demonstrate a piezo energy harvesting system (PEH) for a cubesat or similar small satellite platform that complements existing photovoltaic elements, trickle-charging the spacecraft’s batteries in periods of eclipse when photovoltaic output is low, thereby reducing the required spacecraft battery capacity.
DESCRIPTION: The Department of Defense has a critical need for greater availability of timely services provided by space-based assets. These services range from communications to navigation and overhead imagery. New service offerings depend upon more capable and resilient small satellites that may be launched more frequently and at lower cost relative to traditional space systems. Battery size (capacity) requirements for satellites are determined in part by system energy needs. Significant stored energy is necessary to power subsystems while the spacecraft is in the nighttime portion of the orbit when solar panels are in eclipse and receive little sunlight.
To realize the next generation of highly capable small satellites, the mass fraction of batteries must be reduced. Adapting energy harvesting systems originally developed for terrestrial utility and wearable electronics applications have been proposed to complement improvements in battery energy density and solar cell efficiency.
Piezo energy harvesters (PEHs) convert the kinetic energy of structural vibrations or oscillatory motion to useful electricity. When PEHs are subjected to a harmonic mode, piezoelectric composite material segments embedded in the structure periodically deform, thereby generating electricity. An initial perturbation is typically delivered from the thermally induced release of strain energy in a bistable mechanism with spring- and/or shape-memory-alloy storage member. Following the perturbation, multifunctional structures attached to the spacecraft that incorporate photovoltaics, sensors, and PEH structures may freely vibrate, thereby charging the batteries.
PEH systems for satellites must overcome several technical challenges to be practical. First, new piezo composite transducers must be engineered that exhibit greater current density under large strain, on the order of mA/g, resulting in a measurable reduction in the required battery size. Second, the mechanism that applies the perturbation and the PEH smart structure dynamic properties (mass, effective stiffness, and damping) should be engineered for an oscillation that decays over a period of time comparable to the nighttime portion of the orbit. It should also be capable of being passively reset during the daytime portion of the orbit. Third, the multifunctional smart structure containing the PEH system must not add more than 2 percent to the spacecraft total mass budget. Lastly, the system must operate in a manner that does not adversely affect spacecraft attitude control or introduce excessive jitter in imaging sensors.
Innovative solutions are sought to these problems, leading to the first operational small satellite PEH system. The PEH prototype is envisioned to be demonstrated on the ground in an evacuated neutral gravity environment, and in low Earth orbit (LEO) on board a 6U cubesat or similar small (less than 50 kg) satellite.
PHASE I: Design a small satellite with a technically feasible multifunctional smart structure that supports photovoltaics and the PEH system with supporting sensors and rectification electronics. Design a passive perturbation mechanism that can be reset upon emerging from eclipse. Verify the PEH system is energy-positive and calculate the system output in watts and the corresponding reduction in the required spacecraft bus battery capacity. Power budget calculations should be based on a notional small satellite with visible spectrum imaging payload. Analyze the free-vibration transmissibility to the payload, assuming nominal damping properties, and estimate the impact on sensor jitter and platform accelerations in six axes. Phase I deliverables include a preliminary design report that contains the mechanism design, system output and predicted savings in battery capacity.
PHASE II: Prototype the proposed PEH smart structure with perturbation mechanism and demonstrate on the ground in an evacuated neutral-gravity environment. In addition to basic functionality, verify dynamic properties (mass, stiffness, damping ratio, logarithmic decrement, natural frequency), PEH system power output, and total energy conversion efficiency (i.e., thermal-to-kinetic, plus kinetic-to-electrical). Measure vibration transmissibility to a representative sensor mass mock-up. Phase II deliverables include the working system prototype and critical design report with experimental characterization results.
PHASE III DUAL USE APPLICATIONS: The mature PEH system may be incorporated in the next generation of small military or commercial overhead imaging satellites that deliver near-real-time imagery to the warfighter. The reduced battery capacity enabled by the PEH system means more power available for on-board image processing or more mass available for payloads. The technology may also be applied in strategic naval applications, where PEH components attached to a sea bed deliver power to an unmanned undersea vehicle-charging station. Integrate the PEH system smart structure into a 6U cubesat or similar commercial off-the-shelf (COTS) small satellite (<50 kg) bus. Launch the satellite as a ride share to LEO and verify functionality, including sensor jitter and platform accelerations while the system is in operation.
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KEYWORDS: Vibration, energy harvesting, satellite