| 1. |
- Haque, Mohammad Mazharul, 1984, et al.
(författare)
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Thermal influence on the electrochemical behavior of a supercapacitor containing an ionic liquid electrolyte
- 2018
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686. ; 263, s. 249-260
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Tidskriftsartikel (refereegranskat)abstract
- Emerging demands on heat-durable electronics have accelerated the need for high temperature supercapacitors as well as for understanding the influence of elevated temperatures on the capacitive behavior. In this work, we present a comprehensive study of the thermal influence on a supercapacitor containing 1-ethyl-3-methylimidazolium acetate (EMIM Ac) electrolyte and activated carbon (AC) electrodes. The performance variation as a function of temperature in a range from 21 °C to 150 °C reveals that a high specific capacitance of 142 F g−1 can be achieved at 150 °C at a current density of 2 A g−1 with a rate capability of 87% at 15 A g−1 (relative to 2 A g−1). At 150 °C, equivalent series resistance (ESR) is only 0.37 Ω cm2, which is a result of improved ionic conductivity of the electrolyte at elevated temperature. The ESR value of 2.5 Ω cm2 at room temperature reflects a good compatibility between EMIM Ac and AC. In addition, a capacitance retention of more than 95% (in the end of 1000 cycles) is maintained up to120 °C followed by 85% at 150 °C. These results confirm EMIM Ac as a suitable candidate for carbon-based high temperature supercapacitors, and the observations regarding the thermal influence on performance metrics e.g. usable operation voltage could be applicable to other energy storage devices.
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| 2. |
- Joshi, N., et al.
(författare)
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MEMS Based Micro Aerial Vehicles
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 757:1
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Konferensbidrag (refereegranskat)abstract
- Designing a flapping wing insect robot requires understanding of insect flight mechanisms, wing kinematics and aerodynamic forces. These subsystems are interconnected and their dependence on one another affects the overall performance. Additionally it requires an artificial muscle like actuator and transmission to power the wings. Several kinds of actuators and mechanisms are candidates for this application with their own strengths and weaknesses. This article provides an overview of the insect scaled flight mechanism along with discussion of various methods to achieve the Micro Aerial Vehicle (MAV) flight. Ongoing projects in Chalmers is aimed at developing a low cost and low manufacturing time MAV. The MAV design considerations and design specifications are mentioned. The wings are manufactured using 3D printed carbon fiber and are under experimental study. © Published under licence by IOP Publishing Ltd.
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| 3. |
- Köhler, Elof, 1980, et al.
(författare)
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Analytic modeling of a high temperature thermoelectric module for wireless sensors
- 2014
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 557:1
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Konferensbidrag (refereegranskat)abstract
- A novel high temperature thermoelectric module with thermoelectric materials never before combined in a module is currently researched. The module placement in the cooling channels of a jet engine where the cold side will be cooled by high flow cooling air (550 degrees C) and the hot side will be at the wall (800 degrees C). The aim of the project is to drastically reduce the length of the wires by replacing wired sensors with wireless sensors and power these (3-10mW) with thermoelectric harvesters. To optimize the design for the temperature range and the environment an analytic model was constructed. Using known models for this purpose was not possible for this project, as many of the models have too many assumptions, e.g. that the temperature gradient is relatively low, that thick electrodes with very low resistance can be used, that the heat transfer through the base plates are perfect or that the aim of the design is to maximize the efficiency. The analytical model in this paper is a combination of several known models with the aim to examine what materials to use in this specific environment to achieve the highest possible specific power (mW/g).
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| 4. |
- Köhler, Elof, 1980
(författare)
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Energy Harvesting and Energy Storage for Wireless and Less-Wired Sensors in Harsh Environments
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Engineering requires sensors to control and understand the environment. This is particularly important in harsh environments. The drawbacks, especially in gas turbines is the complexity of installing a wired sensor and the weight of the wires. This makes wireless sensors attractive. A wireless sensor requires a power source for transmission of data. Batteries have previously taken the role of power source for most wireless sensors, but is unfortunately not suitable for all applications. Lately, with energy harvesting and supercapacitors in the picture, sensor applications in high temperature environments, with high power requirements or with long life requirements have the possibility of wireless interface. A supercapacitor can handle higher temperatures, higher power output and can have a cycle life that exceeds batteries by a factor of 10000. The lower energy density and high self-discharge makes it unsuitable to power a wireless sensor without power source. However, connected to an energy harvester converting waste energy into electricity makes this a powerful combination. Energy harvesters thrives in environments where waste energy is plentiful and low conversion efficiencies can be enough to power both the sensor and the transmitter. A thermoelectric harvester is designed and fabricated for the middle to rear part of a gas turbine. The temperature in this region can reach 1600 ◦ C and require extensive cooling. In the cooling channels the wall temperature reach 800-950 ◦ C when the cooling air is 450-600 ◦ C. In this location a thermoelectric harvester will have access to high thermal gradients and active cooling. To harvest the vibrations a piezoelectric energy harvester was built. To harvest enough energy the resonance frequency of the energy harvester is frequency-matched with the high energy vibrations. In many applications these frequencies drift and thus require a broad bandwidth harvester. Simulation and assembly of a broadband coupled piezoelectric energy harvester is presented in the thesis. A piezoelectric harvester require electronics and energy storage to gather enough energy to power up and run a wireless sensor. The thesis covers the fabrication of a high temperature supercapacitor capable of temperatures up to 181 ◦ C.
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| 5. |
- Köhler, Elof, 1980
(författare)
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Energy Harvesting for Wireless and Less-Wired Sensors in Gas Turbines
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Four types of energy harvesters aimed for gas turbine applications were developed during this thesis. The unique gas turbine environment shaped the design- and material choices. A semiconductor thermoelectric harvester was built for a location in the gas turbine with active cooling at 600°C, with 800°C wall temperature. The thesis covers the material synthesis, design, assembly and proof-of-concept tests of this harvester at 800°C. A metal thermoelectric harvester was also built, but instead for locations without active cooling. The harvester design is long metal strips, capable of reaching active cooling far away. This harvester was successfully used to power wireless sensors and reached 290 μW power output after power management electronics. Two different types of piezoelectric harvesters were developed, both consisting of coupled off-the-shelf cantilevers. The development included simulations, analytic models and assembly/measurements on harvesters. The first design was a 2-degree-of-freedom folded coupled harvester which after optimizations achieved a minimum of 2.75 V in the frequency range 92-162 Hz with peak power output of 1.80 mW. The second design was a 4-degree-of-freedom self-tuning harvester, showing increased 3 dB-bandwidth from 8 Hz to 12 Hz with the use of a sliding weight.
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| 6. |
- Köhler, Elof, 1980, et al.
(författare)
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Evaluation of 3D printed materials used to print WR10 horn antennas
- 2016
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 757:1
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Konferensbidrag (refereegranskat)abstract
- A WR10 waveguide horn antenna is 3D printed with three different materials. The antennas are printed on a fusion deposition modeling delta 3D printer built in house at Chalmers University of Technology. The different plastic materials used are an electrically conductive Acrylonitrile butadiene styrene (ABS), a thermally conductive polylactic acid containing 35% copper, and a tough Amphora polymer containing at least 20% carbon fiber. The antennas are all printed with a 0.25 mm nozzle and 100 μm layer thickness and the software settings are tuned to give maximum quality for each material. The three 3D printed horn antennas are compared when it comes to cost, time and material properties.
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| 7. |
- Köhler, Elof, 1980, et al.
(författare)
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Fabrication of High Temperature Thermoelectric Energy Harvesters for Wireless Sensors
- 2013
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 476:1, s. Art. no. 012036-
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Konferensbidrag (refereegranskat)abstract
- Implementing energy harvesters and wireless sensors in jet engines could simplify development and decrease costs. A thermoelectric energy harvester could be placed in the cooling channels where the temperature is between 500–900°C. This paper covers the synthesis of suitable materials and the design and fabrication of a thermoelectric module. The material choices and other design variables were done from an analytic model by numerical analysis. The module was optimized for 600–800°C with the materials Ba8Ga16Ge30 and La-doped Yb14MnSb11, both having the highest measured zT value in this region. The design goal was to be able to maintain a temperature gradient of at least 200°C with high power output. The La-doped Yb14MnSb11 was synthesized and its structure confirmed by x-ray diffraction. Measurement of properties of this material was not possible due to insufficient size of the crystals. Ba8Ga16Ge30 was synthesized and resulted in an approximated zT value of 0.83 at 700°C. Calculations based on a module with 17 couples gave a power output of 1100mW/g or 600mW/cm2 with a temperature gradient of 200K.
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| 8. |
- Köhler, Elof, 1980, et al.
(författare)
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High temperature energy harvester for wireless sensors
- 2014
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Ingår i: Smart Materials and Structures. - : IOP Publishing. - 0964-1726 .- 1361-665X. ; 23:9, s. Art. no. 095042-
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Tidskriftsartikel (refereegranskat)abstract
- Implementing energy harvesters and wireless sensors in jet engines will simplify development and decrease costs by reducing the need for cables. Such a device could include a small thermoelectric generator placed in the cooling channels of the jet engine where the temperature is between 500-900 degrees C. This paper covers the synthesis of suitable thermoelectric materials, design of module and proof of concept tests of a thermoelectric module. The materials and other design variables were chosen based on an analytic model and numerical analysis. The module was optimized for 600-800 degrees C with the thermoelectric materials n-type Ba8Ga16Ge30 and p-type La-doped Yb14MnSb11, both with among the highest reported figure-of-merit values, zT, for bulk materials in this region. The materials were synthesized and their structures confirmed by x-ray diffraction. Proof of concept modules containing only two thermoelectric legs were built and tested at high temperatures and under high temperature gradients. The modules were designed to survive an ambient temperature gradient of up to 200 degrees C. The first measurements at low temperature showed that the thermoelectric legs could withstand a temperature gradient of 123 degrees C and still be functional. The high temperature measurement with 800 degrees C on the hot side showed that the module remained functional at this temperature.
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| 9. |
- Köhler, Elof, 1980, et al.
(författare)
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Impact of designed asymmetries on the effective bandwidth of a backfolded piezoelectric energy harvester
- 2019
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Ingår i: Sensors and Actuators, A: Physical. - : Elsevier BV. - 0924-4247. ; 292, s. 77-89
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Tidskriftsartikel (refereegranskat)abstract
- For the successful realization of autonomous wireless sensors, they will have to be able to harvest energy from their surroundings. Vibrational energy harvesting is one possible power source for wireless sensors, since vibrations are abundant in many environments. In order to make vibrational energy harvesters more useful, a broad bandwidth is desirable since many vibrations are stochastic in nature. In this paper we implement asymmetry to a backfolded piezoelectric energy harvester to achieve broader effective bandwidth with maintained power output. The optimized harvester achieves a minimum of 2.75 V in the frequency range 92–162 Hz with peak power output of 1.80 mW. Asymmetry based on different lengths of the conjoined cantilevers is experimentally and numerically shown to have the largest impact on the bandwidth, compared to the impact of modified mass loadings.
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| 10. |
- Köhler, Elof, 1980, et al.
(författare)
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MEMS meander harvester with tungsten proof-mass
- 2019
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Ingår i: Journal of Physics. - : Institute of Physics Publishing.
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Konferensbidrag (refereegranskat)abstract
- Using current battery technology the life-time of a leadless pacemaker is approximately 6-10 years, with a large portion of the pacemaker occupied by the battery. This paper investigates the possibility to use a MEMS piezoelectric harvester as a complementary energy source in leadless pacemakers. The challenge is to combine the low resonance frequency required to harvest energy from a heartbeat with the small volume of 20×4×3 mm3 available, with the corresponding harvester displacement restricted to 2 mm. Due to the displacement restriction the selected structure was a double clamped bridge in order to reduce the mass displacement, with various meander-type designs simulated to reduce resonance frequency. To further reduce resonance frequency large proof-masses of tungsten were attached by gluing. Two types of tungsten proof-masses were added to four different harvesters, 16.4 mg and 16.6 mg on sample 1 and 2 and 502 mg and 492 mg proof-mass on sample 3 and 4. The structures have 2 μm patterned PZT (deposited by sol-gel technique) and Pt metal electrodes for d31 mode harvesting. The power output measured from one of the two PZT/electrodes was 0.13 nW with 50 μm deflection at 100 k Ω optimal load resistance and 9.1 mVpp at 232 Hz.
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