Soobum Lee

Robust segment-type energy harvester and its application to a wireless sensor

This paper presents an innovative design platform of a piezoelectric energy harvester (EH), called a segment-type EH, and its application to a wireless sensor. Energy harvesting technology is motivated to minimize battery replacement cost for wireless sensors, which aims at developing self-powered sensors by utilizing ambient energy sources. Vibration energy is one of the widely available ambient energy sources which can be converted into electrical energy using piezoelectric material. The current state-of-the-art in piezoelectric EH technology mainly utilizes a single natural frequency, which is less effective when utilizing a random ambient vibration with multi-modal frequencies. This research thus proposes a segment-type harvester to generate electric power efficiently which utilizes multiple modes by separating the piezoelectric material. In order to reflect the random nature of ambient vibration energy, a stochastic design optimization is solved to determine the optimal configuration in terms of energy efficiency and durability. A prototype is manufactured and mounted on a heating, ventilation, air conditioning (HVAC) system to operate a temperature wireless sensor. It shows its excellent performance to generate sufficient power for real-time temperature monitoring for building automation.

A new piezoelectric energy harvesting design concept: multimodal energy harvesting skin

This paper presents an advanced design concept for a piezoelectric energy harvesting (EH), referred to as multimodal EH skin. This EH design facilitates the use of multimodal vibration and enhances power harvesting efficiency. The multimodal EH skin is an extension of our previous work, EH skin, which was an innovative design paradigm for a piezoelectric energy harvester: a vibrating skin structure and an additional thin piezoelectric layer in one device. A computational (finite element) model of the multilayered assembly-the vibrating skin structure and piezoelectric layer-is constructed and the optimal topology and/or shape of the piezoelectric layer is found for maximum power generation from multiple vibration modes. A design rationale for the multimodal EH skin was proposed: designing a piezoelectric material distribution and external resistors. In the material design step, the piezoelectric material is segmented by inflection lines from multiple vibration modes of interests to minimize voltage cancellation. The inflection lines are detected using the voltage phase. In the external resistor design step, the resistor values are found for each segment to maximize power output. The presented design concept, which can be applied to any engineering system with multimodal harmonic-vibrating skins, was applied to two case studies: an aircraft skin and a power transformer panel. The excellent performance of multimodal EH skin was demonstrated, showing larger power generation than EH skin without segmentation or unimodal EH skin.

A design and experimental verification methodology for an energy harvester skin structure

This paper presents a design and experimental verification methodology for energy harvesting (EH) skin, which opens up a practical and compact piezoelectric energy harvesting concept. In the past, EH research has primarily focused on the design improvement of a cantilever-type EH device. However, such EH devices require additional space for proof mass and fixture and sometimes result in significant energy loss as the clamping condition becomes loose. Unlike the cantilever-type device, the proposed design is simply implemented by laminating a thin piezoelectric patch onto a vibrating structure. The design methodology proposed, which determines a highly efficient piezoelectric material distribution, is composed of two tasks: (i) topology optimization and (ii) shape optimization of the EH material. An outdoor condensing unit is chosen as a case study among many engineered systems with harmonic vibrating configuration. The proposed design methodology determined an optimal PZT material configuration on the outdoor unit skin structure. The designed EH skin was carefully prototyped to demonstrate that it can generate power up to 3.7 mW, which is sustainable for operating wireless sensor units for structural health monitoring and/or building automation.

PERFORMANCE EVALUATION OF NEW SPACER GRID SHAPES FOR PWRS

A spacer grid, which is one of the most important structural components in a PWR fuel assembly, supports its fuel rods laterally and vertically. Based on in-house design experience, scrutiny of the design features of advanced nuclear fuels and the patents of other spacer grids, KAERI has devised its own spacer grid shapes and acquired patents. In this study, a performance evaluation of KAERIs spacer grid shapes was carried out from mechanical/structural and thermohydraulic view points. A comparative performance evaluation of commercial spacer grid shapes was also carried out. The comparisons addressed the spring characteristics, fuel rod vibration characteristics, fretting wear resistance, impact strength characteristics, CHF enhancement, and the pressure drop level of the spacer grid shapes. The results show that the performances of KAERIs spacer grid shapes are as good as or better than those of the commercial spacer grid shapes.

Smooth Boundary Topology Optimization for Electrostatic Problems Through the Combination of Shape and Topological Design Sensitivities

This paper presents a new methodology for topology optimization in which the topology of the design domain considered may change during the shape optimization process, unlike the case of classical shape or topology optimization. To achieve this, the concept of the topological gradient is introduced to compute the sensitivity of an objective function when a small hole is drilled in the domain, and this is successfully connected to the shape sensitivity. A test problem consisting of an elliptic dielectric material with a circular hole has been selected to illustrate the validity of the proposed method.

New Spacer Grid to Enhance Mechanical/Structural Performance

A spacer grid (SG), which supports nuclear fuel rods (FRs) laterally and vertically with a friction grip, is one of the most important structural components in a PWR fuel. The forms of the grid straps and spring form are known to be closely related with the SG crush strength and the integrity of FR support, respectively. In this study, a new SG form and its manufacturing technology were suggested for enhancing the integrity of the FR support and the SG crush strength by using a systematic optimization technique without significantly increasing the pressure drop across the reactor core. Finite element (FE) analysis and crush strength tests on the new SG form were carried out to check the performance enhancement compared with commercial SGs. According to the results, it is estimated that the actual critical load enhancement of the SG is approximately up to 30%, and the actual contact area, when an FR is inserted into an SG cell, is more than double for the new SG form. And also, some design variables that affect the SG crush strength were classified and their effects on the crush strength were investigated by an FE analysis and a crush strength test.

Smooth boundary topology optimization for eigenvalue performance and its application to the design of a flexural stage

The new topology optimization method known as smooth boundary topology optimization (SBTO) is enhanced to deal with natural frequencies as a design characteristic. SBTO combines boundary representation using computer graphics with a selection criterion which determines the usefulness of generating a hole. Topological changes such as hole generation and hole merging are successfully implemented. SBTO is used to establish a systematic procedure for the design of a flexural stage is established. When designing flexural stages, it is necessary to be compliant in the actuating or moving direction and stiff in the parasitic direction, and the optimization related to maximizing the difference between the first and second natural frequency is formulated. SBTO has been shown to be a promising methodology for topology optimization with smooth boundaries in the design of a flexural stage.

Design under uncertainty for reliable power generation of piezoelectric energy harvester

In recent years, vibration energy harvesters have been widely studied to build self-powered wireless sensor networks for monitoring modern engineered systems. Although there has been significant research effort on different energy harvester configurations, the power output of a vibration energy harvester is known to be sensitive to various sources of uncertainties such as material properties, geometric tolerances, and operating conditions. This article proposes a reliability-based design optimization method to find an optimum design of energy harvester that satisfies the target reliability on power generation. This optimum design of vibration energy harvester demonstrates reliable power generation capability in the presence of the various sources of uncertainties.

Designing Energy Harvesting Skin Structure Utilizing Outdoor Unit Vibration

This paper presents a new energy harvesting (EH) concept design, referred to as EH skin structure. A generic design and experimental verification methodology will be proposed to demonstrate the feasibility of the EH skin for practical applications. In the past, EH researches have primarily focused on designing a device-level energy harvester, such as a cantilever-type EH device. However, such a device-level energy harvester has several drawbacks: (i) need of an extra space for proof masses and fixture, (ii) significant energy loss due to the fixture, and (iii) need of a casing for EH device protection against environmental harms. While this new EH concept design could overcome the drawbacks above, there is no design methodology for EH skin. This paper proposes design and experimental verification methodology for EH skin structure. The design methodology comprises three tasks: (i) construction of a valid computational model, (ii) design optimization of EH skin, and (iii) experimental verification. An outdoor condensing unit of which a fan produces harmonic vibration is chosen for a case study because similar configuration of vibration can be found in many engineered systems (e.g., airplane wing, AC unit). The proposed design methodology determined an optimal EH skin configuration (sizes, locations, etc.) on the vibrating structure. The EH skin was carefully prototyped to demonstrate that it can generate power up to 3.7mW, which is sustainable for operating wireless sensor units for structural health monitoring or building automation.Copyright

Study on the Lateral Dynamic Crush Strength of a Spacer Grid Assembly for a LWR Nuclear Fuel Assembly

A spacer grid assembly is one of the most important structural components in a Light Water Reactor(LWR) nuclear fuel assembly. In the case of the Zircaloy spacer grid assembly, the primary design consideration is to ensure that lateral dynamic crush strength of the spacer grid assembly is sufficient to resist design basis loads and thereby prevent seismic accidents, without a significant increase in the hydraulic head loss for the reactor coolant in the reactor core. In this study, factors affecting the lateral dynamic crush strength of a spacer grid assembly were analyzed by performing lateral dynamic crush tests and finite element analyses. Further, an effective and economical method to enhance the lateral dynamic crush strength of the spacer grid assembly is proposed.