FSMA-based Energy Harvesting using a Magnetic Heat Source

Taking into account the knowledge based on previous experiments, described in the chapters before, a modified FSMA-based energy harvesting demonstrator is designed.

The FSMA-based energy harvesting device using a magnetic heat source comprises a brass cantilever, a FSMA film attached to its tip, a microcoil, and a magnetic heat source, as shown in Figure 5.23. The FSMA film, fixed on the tip of a non-magnetic cantilever, allows for the independent tuning of magnetic and mechanical properties.

The actuation therefore differs from the basic FSMA-based actuation principle, as there is no active SME based reset force, but only a passive elastic reset force, counteracting the magnetic attraction force of the FSMA film.

The magnetic heat source is optimized in order to increase and improve the magnetic field gradient and its distribution. By combining the magnet and the heat source, the movement of the microcoil happens in the highest magnetic field gradient of the exter-nal magnet. The overall magnetic field is increased additioexter-nally by the longish magnet geometry, which provides a more favorable distribution of the magnetic field gradient.

The goal of this setup is to increase the amplitude of oscillation of the electromagnetic conversion, as the microcoil moves in a large magnetic field gradient during its whole actuation period.

Fmag

T < TC

Coil Heat source/

Magnet

Brass cantilever

T > TC N

S

a) b)

N S

Freset

FSMA film

Figure 5.23: FSMA-based Energy Harvesting Concept using a magnetic heat source and a FSMA film at-tached to a non-magnetic brass cantilever.

5.4.2 Fabrication

Similar to the FSMA-based energy harvesting device using an integrated microcoil, also for this energy harvesting demonstrator a microcoil is fabricated according to the method described in Chapter 2.5.5. The size of the coil is slightly larger with an overall dimension of 2 x 3 mm²and a thickness of approximately 400 µm. The number of turns is 200. The microcoil is fixed on the bottom side of a brass cantilever tip, cut from a 10 µm thick brass foil. The cantilever size is 3 x 5 mm². On the top of the cantilever tip a 2 x 2 mm² Ni51.4Mn28.3Ga20.3 film of 5 µm thickness is attached. In Figure 5.24 the set-up of the demonstrator is shown. For the magnetic heat source, a 3 x 3 x 8 mm³CoSm

FSMA-based Energy Harvesting using a Magnetic Heat Source 5: FSMA-based Energy Harvesting

magnet with a maximum operation temperature of 350 °C is used. It is equipped with a small resistance heater and a temperature sensor. A good heat transfer coefficient is ensured by polishing the front side, which is in contact with the FSMA film on the cantilever tip.

Figure 5.24: Fabrication of the FSMA harvesting demonstrator, based on a brass cantilever and a mag-netic heat source.

5.4.3 Mechanical Characterization

First, the free oscillation of the cantilever is investigated, as shown in Figure 5.25. The negative displacement at the beginning of the oscillation is not complete, due to a mis-alignment of the laser beam and the large deflections of the cantilever tip. Therefore, the measurable maximum negative displacement is limited to -2.5 mm. For the decay-ing oscillation, a strong dampdecay-ing at high amplitudes, and a much lower dampdecay-ing at am-plitudes below 1 mm can be identified. The quality factor is determined by

Q= 2⋅π ⋅E_Cycle

E_Loss , (10)

whereas ECycle is the energy stored at a certain amplitude, and ELoss is the energy loss, calculated from the amplitude decrease.

0 0.1 0.2 0.3 0.4 0.5

−3

−2

−1 0 1 2 3

Time / s

Displacement / mm

f_Res = 91.67 Hz

Detection limit Q₁ = 430

Q₂ = 2340

Figure 5.25: Decrease of the free oscillation of the brass cantilever with a microcoil at the tip. The resolu-tion is limited at the lower maximum displacement due to a misalignment of the laser beam.

5: FSMA-based Energy Harvesting FSMA-based Energy Harvesting using a Magnetic Heat Source

The quality factor Q1for the fast decay is 430, while the oscillation at about 1 mm am-plitude has a quality factor Q2of 2340. The eigenfrequency is measured by Fast Fourier Transformation to be 91.67 Hz.

When actuated with the magnetic heat source near the optimum position, one millime-ter above the neutral position of the cantilever, the oscillation of the cantilever tip be-comes almost harmonic, as can be seen in Figure 5.26. The positive and negative dis-placement are almost the same and the frequency of 84 Hz is close to the eigenfrequency of the cantilever. At the heat source temperature of approximately 150 °C, no distinct heating time can be identified. It seems that a short impact on the heat source, not visible in the displacement measurement, is enough to transfer a suffi-cient amount of heat in order to keep the oscillation going.

0.06 0.08 0.1 0.12 0.14

−1

−0.5 0 0.5 1

Time / s

Displacement / mm

Figure 5.26: Mechanical displacement, measured with the laser distance sensor.

5.4.4 Thermal Characterization

Thermal characterization of high frequency temperature changes with thermography becomes very challenging and partly impossible with the available measurement setup, described in Chapter 2.4.8. Therefore, a new technique, using a thin thermoelectric ele-ment of 4.4 x 4.4 x 0.5 mm³size, is implemented to determine the heat flux through the sensor element. It is mounted directly on the magnetic heat source. The open circuit voltage of the thermoelectric sensor is measured with a high definition measurement unit, being capable of measuring very small changes in voltage and current. The sens-ing mechanism behind this principle is explained in more detail in Chapter 3.2.1. With the measurement setup it is possible to determine the heat flux through the heat source during actuation with a rate of 5000 Hz.

Figure 5.27 shows the combination of the measured heat flux, calculated from the ther-moelectric sensor data, the open circuit voltage of the harvesting demonstrator, and the displacement, measured with the laser distance sensor. It can be seen that the heat flux increases abruptly at the maximum displacement of the cantilever (closest to the heat source), until the cantilever moves away. The heat flux then decreases to a base level until the cantilever touches the heat source again.

FSMA-based Energy Harvesting using a Magnetic Heat Source 5: FSMA-based Energy Harvesting

The change of heat flux is about 1.5 mW. Integrating the signal without the base level leads to the result that a net-heat of about 15 µJ per cycle is transferred. This value is a lower limit of heat transferred to the energy harvesting system and would account for a temperature change of the FSMA tip of only 0.19 K. The calculation does not take into account the parasitic heat loss, which is released during the whole time to the sur-rounding. This heat flux is about 90 mW, corresponding to the base level of the measured voltage on the thermoelectric sensor. Additionally, the time constant of the heat flux sensor my be to slow for an accurate measurement of the highly dynamic heat exchange at the magnetic heat source.

The electrical output and the mechanical oscillation amplitude (stroke) are not as high as in the setup without thermoelectric sensor because the sensor has a finite thickness of 0.5 mm and thus limits the actuation range of the cantilever. Only half of the ampli-tude is possible. In addition, the heat transfer coefficient is affected. It is therefore not possible to measure the heat flux during optimal actuation, limiting the interpretation of the results to a qualitative description of the harvesting system.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Figure 5.27: Heat flux measurement combined with the open circuit voltage of the harvester and its displacement.

5.4.5 Electrical Output

The electrical output of the FSMA-based energy harvesting demonstrator using a mag-netic heat source shows even higher duty cycles than the FSMA-based energy harvest-ing demonstrator usharvest-ing an integrated microcoil and an individual heat source. This is because the amplitude is not reduced as strongly, when the microcoil is at its turning point away from the magnet due to the larger magnetic field. The current signal for ac-tuation at a heat source temperature of 150 °C is shown in Figure 5.28.

Besides the very high current amplitude of above 150 µA at 220 Ω load resistance, the

5: FSMA-based Energy Harvesting FSMA-based Energy Harvesting using a Magnetic Heat Source

induced current signal is very smooth and no break at the turning points of the cantilever can be identified. Only a slight decline in slope at the turning point away from the heat source is visible. This improvement is due to the integrated magnetic heat source, which facilitates a much more homogeneous magnetic field gradient during the complete actuation of the cantilever tip. The frequency of the induced current signal is 84.7 Hz. The average power, which can be calculated from the current signal, is found to be 2.38 µW and the power density, derived from the used 2 x 2 x 0.005 mm³ Ni51.4Mn28.3Ga20.3 film, is 118.5 mW·cm^-3. This is a further significant increase by two or-ders of magnitude in average power and power density compared to the previous FSMA -based energy harvesting principles. A comparison to thermoelectric state-of-the-art energy harvesting devices is made in Chapter 8.3 to underline the importance of this result.

0.06 0.08 0.1 0.12 0.14

−150

−100

−50 0 50 100 150

Time / s

Current / A

Figure 5.28: Electrical output of the FSMA harvesting device based on a brass cantilever and a magnetic heat source.

5.4.6 Performance at Different Ambient Temperatures

The applicable temperature range of the FSMA-based energy harvesting device using a magnetic heat source is investigated by measuring the performance at different tem-perature differences ∆T between ambient (Tair) and heat source temperature (Tsource). As the heat source temperature has to remain above TC, the temperature difference is reduced by increasing the ambient temperature. In order to do so, the whole setup is placed in an oven, where a temperature sensor close to the FSMA-based energy har-vesting device measures the air temperature, while a second sensor monitors the heat source temperature. The ambient temperature is increased from 25 to a maximum of 70 °C, while the heat source temperature is adjusted between 135 and 100 °C. At high ambient temperatures, the heat source temperature had to be decreased to allow ac-tuation, as otherwise the heat transfer and cooling time would not match sufficiently and actuation becomes non-periodic. Figure 5.29 shows the results of the measure-ment series. The electrical power decreases strongly by two orders of magnitude when the temperature difference ∆T is below 30 K. For ∆T below 50 K, no continuous

actua-FSMA-based Energy Harvesting using a Magnetic Heat Source 5: FSMA-based Energy Harvesting

tion is observed, as the heating and cooling times are increased strongly compared to larger temperature differences. For ∆T > 50 K, however, a continuous periodic oscilla-tion can be measured. The frequencies are determined with Fast Fourier Transforma-tion. From a temperature difference of 50 K on, frequencies of above 60 Hz are found in a wide temperature range. The system also shows the effect of self-tuning, allowing for a broad operation range with much larger temperature differences. The power out-put remains almost constant at about 1 µW at ∆T > 70 K. It seems that larger tempera-ture differences only have a minor effect on the oscillation frequency and heat transfer times. Therefore, it seems that for optimal actuation a temperature difference of about 70 K is sufficient and no direct correlation of temperature difference and power output is existent.

0.01 0.1 1

Power / W

20 30 40 50 60 70 80 90 100 110 120

0 20 40 60 80

T / K

Frequency

Continuous Actuation Frequency

up-conversion

Figure 5.29: Electrical power output at different temperature differences.

5.4.7 Conclusion

By adjusting the magnet geometry and individually tuning the mechanical properties of the cantilever and the active material, significant improvements in the overall power out-put can be achieved. Thereby, the tuning of the magnetic field had the largest influence on the enhanced electrical power output. The results of the demonstrator device of the FSMA-based energy harvesting device using a magnetic heat source show the impor-tance of the optimization of magnetic and mechanical design in order to get an efficient conversion of ferromagnetic transitions to electrical energy, competitive to state-of-the-art thermal energy harvesting devices.

Further improvements of the material, especially the ratio of ∆M/∆T would allow even more efficient FSMA-based energy harvesting devices. Besides the ferromagnetic tran-sitions, also first order phase transformations show large abrupt changes in magnetiza-tion on heating and cooling. If hysteresis-widths decrease to a minimum, it is expected that similar actuation and energy harvesting can be realized, as will be discussed in the next chapter.

5: FSMA-based Energy Harvesting FSMA-based Energy Harvesting using a Magnetic Heat Source