Development, Characterization, and Optimization of a Thermoelectric Generator SystemLindsey Bunte, Jonny Hoskins, Tori Johnson, Shane McCauleySchool of Chemical, Biological and Environmental Engineering

Sponsors: Perpetua Power Source Technologies & ONAMI

Background

Heat Transfer Fundamentals Experimental

Thermoelectric generators (TEGs) work using the Seebeck effect, which converts temperature differences across dissimilar metals into an electrical potential, or voltage.

The TEG will use the temperature difference between the water and the solar absorber to create renewable energy.

Battery life is currently the biggest limitation to wireless sensors. Thermoelectric power can harvest renewable energy from virtually any source of temperature difference.

Key benefits of incorporating self-powered wireless sensors: • Reduced battery replacement labor costs• Ability to take more measurements and collect more data• Maintenance-free solutions• Network autonomy• Environmentally-conscious choice

Design of an outdoor, wireless monitoring system that is powered by a thermoelectric generator (TEG). The design of the generator will consist of a solar absorber and a reservoir in the soil. The absorber will capture the sunlight’s energy during the day and the reservoir will provide a heat sink. In the evening, the reservoir will act as the heat source and the solar absorber will act as the heat sink.

Future Plans1. Optimizing Design

1. Stake length for optimized heat transfer 2. Perforation to increase surface area and convective mixing3. Improve insulation of reservoir for decreased heat loss

2. Outdoor Tests Questions1. What is the sunlight exposure for energy harvesting within the

reservoir?2. How will rain/wind/weather effect the convective heat loss to the

system?3. How suitable is the system for extended field use?

3. End User Application Considerations1. Seasonal demands of agriculture in relation to energy gathering

capabilities2. Voltage requirements and sample rate of sensors3. Sensor types and placement

Objective

Heat Source

Absorber

Rubber Stopper

Thermocouple

Thermocouple

Thermocouple

Thermocouple

Data Logger

Acknowledgements:Dennis BowersMarshall Field

Dr. Philip H. HardingAndy BrickmanSpencer Bishop

The graph above shows a 48 hour day/night cycle of a thermos. During this test the bottom TEG thermocouple failed. The bottom TEG should follow the temperature of the water closely as seen in the 24 hour test. If this were the case this would have produced a workable temperature difference capable of creating a large voltage.

Solar Absorber

Reservoir

Earth

ENERGY INRadiation

ENERGY OUTConduction

Conductionthroughplates

Convectionin water

Conduction through stake

ENERGY OUTTEG

Energy is always conserved. The energy into the system from radiation from the sun leaves the system through the TEG and the energy lost to the ground.

System Boundary

Radiative heat transfer in the TEG system from sunlight can be modeled with the Stefan-Boltzmann Law for non-ideal, or gray bodies, where ε is emissivity, σ is the Stefan-Boltzmann constant, TC is the temperature of the colder surroundings.

Conduction down the stake can be calculated using Fourier’s Equation, where k is thermal conductivity, L is the stake length, T1-T2 is the difference between the inner and outer wall, R2 is the external radius, and R1 is the internal radius.

Heat loss due to convection is represented by Newton’s Law of Cooling where h is the heat transfer coefficient, Tsurf is the temperature of the exposed surface, Tsurr is the temperature of the surroundings, and A is the exposed surface area.

The energy stored in the water can be found using sensible heat change, the amount of energy it takes to change the temperature of the material. The energy is shown in terms of the mass of the material m, heat capacity Cp, temperature difference dT , and time difference dt.

Water was chosen to be our reservoir liquid material because it can store a large amount of energy before changing temperature in comparison to other liquids

because of its high volumetric heat capacity value.

4 4radC

QT T

A

1 2

2 1

2

ln lncond

k L T TQ

R R

convsurf surr

Qh T T

A

Hot side

Cool SidePower Output TEG

p

dTQ mC

dt

The greater the temperature difference between the top and bottom of the TEG the more voltage can be produced.

0 6 12 18 240

30

60

90

Absorber

Top TEG

Bottom TEG

Water

Time (hr)

Tem

pera

ture

(°C)

Low ∆T

0 12 24 36 480

30

60

90

Absorber

Top TEG

Bottom TEG

Water

Time (hr)

Tem

pera

ture

(°C)

The graph above is a 24 hour day/night cycle of our current reservoir design. The current design needs modification because the bottom and top TEG should have a larger temperature difference during the night cycle. A possible solution is to insulate the reservoir better.

Lea ClaytonManfred DittrichStephen Etringer

Qradiation=QTEG+QConduction

High ∆T