A High-efficiency Thermoelectric Module with Phase Change Material for IoT Power Supply

A High efficiency Thermoelectric Module with Phase Change Material for IoT Power Supply Procedia Engineering 168 ( 2016 ) 1630 – 1633 1877 7058 © 2016 The Authors Published by Elsevier Ltd This is an[.]

Procedia Engineering 168 ( 2016 ) 1630 – 1633

1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi: 10.1016/j.proeng.2016.11.477

ScienceDirect

30th Eurosensors Conference, EUROSENSORS 2016

A High-Efficiency Thermoelectric Module with Phase Change

Material for IoT Power Supply

K Nakagawa*, T Suzuki

FUJITSU LABORATORIES LTD Atsugi, Japan

Abstract

For a sensing system to detect the early signs of overflows in sewer systems, we fabricate a high-efficiency module with a thermoelectric generator (TEG) to utilize the temperature of a manhole cover as a heat source In order to increase the temperature difference across a thermoelectric generator, a phase change material (PCM) is used as a latent heat storage for storing heat through the TEG from the manhole cover In this paper, the design of the TEG module and the heat storage to enable efficient utilization

of the latent heat, and the results of measurements of electrical energy the City of Koriyama, Japan, are presented

© 2016 The Authors Published by Elsevier Ltd

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

Keywords: TEG, PCM, latent heat, manhole, sewer, water level

1 INTRODUCTION

In recent years, there have been numerous instances of torrential downpours that far exceeded the magnitudes anticipated in the operation of storm sewer systems The damage from overflows stemming from such intense, localized downpours is becoming increasingly extensive With limited human and financial resources, however, there

is a need to utilize Information and Communication Technology, in addition to measures to improve storm sewer system infrastructure To mitigate the damage inflicted on cities from such heavy rainfall, we are developing technology that can detect early signs of sewer-system overflows This technology uses water-level detecting sensors attached to manhole covers to measure water levels for accurately detecting early signs of overflow

* Corresponding author Tel.: 81-46-250-8261; fax: +81-46-250-8844

E-mail address: kanaka@jp.fujitsu.com

© 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

As a power source for a real-time, low-maintenance monitoring sensor inside sewer systems, we select a

thermoelectric generator (TEG) that is attached to a manhole cover as a heat source, because its temperature changes

periodically in one day In order to increase the temperature difference across a TEG, phase change materials (PCM)

are used as a latent heat storage [1]

Our wireless sewer water-level monitoring system with autonomous powering by means of thermoelectric devices,

was practically implemented in the City of Koriyama in Japan’s Fukushima Prefecture[2,3], about 200 km north of

Tokyo The average annual temperature of manhole cover in the City of Koriyama is around 20 °C Fig.1 shows

average temperature change between September and November, 2014, in Koriyama Paraffin is used for the PCM,

because it has a high chemical stability and good repeatability when utilizing latent heat The melting point of used

paraffin is around 20 °C, because it takes advantage of latent heat for as long as possible The design of the heat

storage with the paraffin for utilizing the latent heat was investigated in order to maximize total electrical energy

output from the TEG under the temperature change shown in Fig.1 A power generation module with the TEG (TEG

module) and the heat storage was fabricated and attached to the back of the manhole covers in order to measure

electrical energy output In this paper, the design of the TEG module and the heat storage to enable efficient utilization

of the latent heat, and the results of the measurements of electrical energy are presented

Fig.1 Average temperature change between September and November, 2014, in the City of Koriyama, Japan

2 Design and experiment

Fig.2 is a schematic of the designed high-efficiency TEG module with PCM It consists of one TEG, the heat

storage with the paraffin for the PCM, a heat transfer fin, a tank and a foaming material for a heat insulator, a circuit

board with a booster circuit, an Al part that conducts the heat from the manhole cover to the TEG, a mold resin

protecting the electrical components, and four magnets for good thermal connection between the module and the

manhole cover A DC/DC converter for a booster circuit is designed using a commercial IC (Model LTC3109: Linear

Technology Co, Ltd.) Table 1 shows the electrical characteristics of a TEG (Model TEG254-200-12: Thermalforce

Co, Ltd) Since the thermal conductivity of the paraffin is low, the heat from the manhole cover is not uniformly

transferred to all of the paraffin and it is difficult to effectively utilize its latent heat In order to utilize the heat from

the manhole cover to the greatest extent, the required amount of the latent heat and the gap of the heat transfer fin

were investigated The temperature change at the Al surface of the TEG module, as shown in Fig.1, is reproduced by

means of a programmable water chiller Table 2 shows the physical properties of the used paraffin (Model TS7: JX

Nippon Oil & Energy Co, Ltd) for our TEG module The weight of paraffin for the heat storage is 0.74 kg, which is

calculated from the manhole temperature change, Fig.1 For a time variation of a one-day manhole temperature T(t)

of which average is TAVE, is equal to the melting point of the used paraffin The required amount of the latent heat,

QPCM, is expressed by the following equation:

dt T t T Q

Other TEG

AVE

where Ĭ is the thermal resistance of the TEG, and Ĭ is the sum of thermal resistance of other parts

10 15 20 25 30 35 40 45 50

5:00 9:00 13:00 17:00 21:00 1:00 5:00

Ave Temp.: 20.7 o C

The heat transfer fins with a distance of 4 mm and 2 mm are used Fig.3 shows a schematic of the fin type and the distance of fins Fig.4 shows the measurement result of each TEG output voltage and the paraffin temperature Fig.5 shows the calculated the heat quantity through the TEG

If the full amount of latent heat of the paraffin is utilizable, the paraffin temperature does not increase at all However, the paraffin temperature increases in the time period when the manhole temperature is above TAVE When the temperature of the heat source rapidly increases, it is assumed that only the small amount of latent heat of the paraffin around the fins is utilized because of the low thermal conductivity of the paraffin From the experimental result, a smaller distance between fins is effective in suppressing the rise of the paraffin temperature The heat quantity through the TEG with the fins separated by a distance of 2 mm increases by 18.7% in the time period when the manhole temperature is above TAVE, and increases by 3.5% in the time period when the manhole temperature is below

TAVE, because the efficiency of latent heat utilization is improved by the smaller distance between fins

Table 1 Electrical Characteristics of TEG (Model TEG254-200-12: Thermalforce de Co, Ltd, Th=150 °C, Tc=50 °C)

Dimension (mm) Thickness (mm) Electrical Resistance (ȍ) Thermal Resistance (K/W) Open Circuit Voltage (V)

Table 2 Physical property of paraffin (Model TS7: JX Nippon Oil & Energy Co, Ltd)

Melting Point

(°C)

Latent Heat (kJ/kg)

Specific Heat (15°C) (kJ/kgK)

Thermal Conductivity (W/mK)

Density (15°C) (g/cm 3 )

Density (40°C) (g/cm 3 )

Fig.2 Schematic design of the thermoelectric module with PCM Fig.3 Schematic of the fin type and distance of fins

Fig.4 Output voltage and paraffin temp of TEG module Fig.5 Calculated heat quantity through TEG.

3 Practical implementation

Our wireless sewer water-level monitoring system, autonomously powered by means of the TEG module, was practically implemented in the manhole Fig.6 shows the installation state of the TEG module attached to the manhole cover The TEG module is fixed with four neodymium magnets The gaps between the manhole cover and the Al part

in the TEG module were filled with a thermal conductive grease For the purposes of comparison with the heat storage, the output voltage of the TEG module with the heat sink for natural air-cooling (Model N100, 10 × 10 × 4 cm, 1.4K/W:ALPHA CO, Ltd), shown Fig.7, was measured simultaneously Fig.8 shows the amount of electricity

⋅⋅⋅

generated per day At 50.4 J/day, the average amount of power generated by utilizing the latent heat of the paraffin is more than six times larger than that generated by natural air-cooling, which is 7.8 J/day The amount of power generated is enough to operate the sensor module, which has a calculated power consumption of about 38.0 J/day, operating at both a sensing interval and a transmission interval of once every five minutes

Fig.6 PCM type TEG module installed in a manhole cover Fig.7 Heat sink type with natural air-cooling TEG module

Fig.8 Power generated per day Comparison between heat storage and natural air-cooling.

4 Conclusion

A high-efficiency TEG module with PCM that maximizes total electrical energy output under the temperature change was created Our wireless sewer water-level monitoring system with a TEG module was practically implemented in the City of Koriyama in Fukushima Prefecture, Japan The results of the experiment showed the amount of power generated by the unit is enough to operate the sensor module

References

[1] A Elefsiniotis, et.al J Phys.: Conference Series 476 (2013)

[2] http://www.fujitsu.com/global/about/resources/news/press-releases/2015/0210-03.html

[3] http://www.fujitsu.com/global/about/resources/news/press-releases/2015/1221-01.html