Subsystem Design Review

Agenda

1. Goals of Review2. Updates from SDR3. Review of our System4. Input Desired5. Functions6. Engineering Analyses Conducted7. Next Steps

Goals of Review

• Confirm acceptability of mechanical design• Get advice on areas of uncertainty:

o Smart battery procuremento Aluminum vs Stainless enclosureo Pressure sensors for clampingo Isothermal heat spreader

Review: System Goals

Our task is to create an underwater thermoelectric generator that generates 20W of electrical power from 500W of heat.

The main driver of this project is efficiency.

Updates From SDR

1. The electrical system will be out of the water.2. Mechanical system

a. Contains only thermoelectrics and a heat sourceb. Waterproofc. Tethered to the (out of water) electrical system.

3. We chose a very simple system design:a. Easier to assemble and troubleshootb. Few disadvantages compared to other options.c. Like a submarine environment.

Mechanical System

Input Desired

1. Batteries.a. We can’t find the smart batteries we wanted

2. Enclosure Material.a. Aluminum or Stainless Steel?

3. Heat Spreadera. How do we know it will be isothermal?

4. Clamping Testa. Is there a way to determine whether the clamping

pressure is uniform?

5. TEGa. Should we purchase them now so we can test them?

FunctionsWe tried to analyze every item in our functional decomposition. Our main functions were:

1. Protect System2. Generate Heat3. Transfer Heat4. Generate Electricity5. Store Electricity6. Monitor System

Our Analysis

1. Thermoelectrics [Generate Electricity]

2. Heat Sinking [Transfer Heat]

3. Insulation [Protect System, Transfer Heat]

4. Clamping Thermoelectrics [Transfer Heat]

5. Max Power Point Tracking [Store Electricity]

6. Battery Charging [Store Electricity]

7. Monitoring [Monitor System]

8. Heater [Generate Heat]

9. Heat Spreading [Transfer Heat]

10.Seals [Protect System]

Thermoelectrics: Last Time

• We planned to use the Taihuaxing TEP1-1264-1.5 modules from the Sustainable Energy Lab

• We thought we could generate 20W, but Dr. Stevens had some concern about our numbers.

Thermoelectrics

Analysis:1. Determine number of modules required to stay

within allowable temperatures and for which Q<480W

2. Iterate heat balance equations to find power3. Taihuaxing Thermoelectric makes some modules

that meet our requirements. Need more research to confirm their specs.

Heat Sinking

• First calculated a desired heat sink thermal resistance using a basic model

Heat Sinking

• Next, set up equations to determine heat sinking needso Found number of fins needed by varying fin size,

spacing and array type (rectangular vs pin)

Heat Sinking

• Results show that plate fin array with reasonable dimensions is sufficient

Heat Sinking

• Found a fin array in the Thermoelectrics Lab• Plugging in dimensions in the spreadsheet:

o Rsink = 0.094 K/W - better than needed! Could decrease the resistance by modifying fins

Heat Sinking

• Increasing fin spacing increases convection coefficiento Removing 8 of the 16 fins increases the convection

coefficient from 58.6 W/m^2K to 142 W/m^2K, but resistance is unchanged

o Other modificationscould lower theresistance

Insulation

• Created a basic thermal circuit to determine the thermal conductance needed for the insulation

Insulation• Using the equation, , we found a few

options for insulation that met a compressive strength of ~10 Mpa (1450psi)

Insulation

• Top of heat spreader insulation:o 310M Silica Ceramic from Cotronics Corp

Compressive Strength = 1200 psi Thermal Conductivity = 0.187 W/mK Trial Kit contains 2 pieces of 4.5” x 3” x 3” at

$84.65( http://www.cotronics.com/catalog/58%20%20310M%20%20311.pdf )

• Sides of heat spreader insulation:o 2600F Ultra Temperature Tape from Cotronics Corp

Thermal Conductivity = 0.06 W/mK at 500F 391W-1 Tape, Size = 0.02” x 1” x 20’ at $124.50

( http://www.cotronics.com/vo/cotr/pdf/391.pdf )

Insulation

• Created a detailed thermal circuit to determine insulation needs

Insulation

• Analyzed heat flow through each node to come up with a system of equations we could solveo Calculated heat that flows through the

thermoelectrics out of the total 500Wo Used solver in Excel to optimize insulation

thicknesses vs cost while maintaining 96% energy delivered through thermoelectrics (480W/500W)

• Results:o ½” of primary insulation (Ceramic)o 1 ¼” of secondary insulation (Fiberglass)

Insulation

Start of needing insulation between

clamp and heat sink

Clamping ThermoelectricsAnalysis Assumptions:

• Compression assembly achieved by 4 bolts threaded into aluminum bolt housing

• 2 56 x 56 mm TEMs clamped at 200 psi

• Evenly distributed clamping pressure

• Max Thermal Expansion load of 500 lbfBolt Grades Considered:● A307● A354● A499

Aluminum Alloys Considered:● 3004 H38● 5052 O● 5052 H32● 5083 H112

Alu

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sing

Stee

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Clamping Analysis Approach

• Calculate EL to ensure bolt fails before threads strip.

• Calculate Relative Strength, R:

• If R > 1:

• Fatigue Factor of Safety based on:

Steel Bolts in Aluminum Bolt Housing

Varying Grade of Steel bolts assuming 3004 H38 Aluminum Housing

Varying Aluminum Alloys for Bolt Housing

Clamping - Bolted Connection Design

Pressure PlateAssuming:• Plate as 1” wide beam

bolted on each end

• Low carbon steel (E = 29 Msi)

⅜” thick beam:y at center = 0.015”

½” thick beam:y at center = 0.0065”

Clamping ConclusionsA307 ¼ - 20 bolt in 3004 H38 Aluminum best meets design requirements. EL = 0.362” and Fatigue Safety Factor = 6.7

Low grade ¼ - 20 bolts are easily obtained, and the Thermoelectric Lab has a supply of them

3004 H38 Aluminum bolt housing requires an OD of 0.5” for n = 12 (not fatigue safety factor) McMaster sells 0.5” by 1’ rods of 6061 T6511 (similar properties) for $6.29

Pressure Plate bending appears manageable with low carbon steel, but more analysis is required.

Alternative Materials

• What if we used Stainless Steel instead of Aluminum Alloy?

• Easier to weld Stainless Steel, minimal increase in prices, increased strength, minimal power loss, less corrosion

• Potential problem with bending since it must be thinner than aluminum to get required thermal resistance

• Increased weight

Electrical Systems

TEG 20WMPPT

(Perturb and Observe)

VARIABLE LOAD

CHARGING CIRCUIT

LI-ION BATTERY

BANK

(1)

LI-ION BATTERY

BANK

(2)

...LI-ION

BATTERYBANK

(N)

20W {

62.5% Power Unused

11.9% Power Unused

Battery Model

Simulink Model

Simulation Results

Battery Charging

More parallel batteries = less power lost through internal resistance

Battery Charging

Different battery voltages have similar results on power loss

Loading Analysis

• Short circuit Protection• Overcharge Protection voltage• Overdischarge Protection voltage• Overcurrent detection Protection

Battery Protection Circuit

http://www.freepatentsonline.com/6768289.html

• Used in laptops,cameras, cell phones,electric wheelchairs,scooters, and military applications

• Reports temperature,voltage, currentand SoC

Smart Batteries

Smart Batteries (GenPort)

• SMBus 1.1 compliant

• Cell balanced

• Protectiono Primary and secondary

over and under voltage protection

o over current protectiono short circuit protectiono over temperature protectiono cell imbalance protectiono and more...

http://www.genport.it

Battery Test Plan

• Use TI EV2300 to communicate from battery to the computer using SMBus communication protocol.

• Test the standard charging algorithm (CC/CV), slow charge current condition, constant power charging condition

• Compare to Simulink model, adjust Simulink model (if required), and repeat

http://www.batteryspace.com

Step Up/Down Converters BUCK-BOOST NON-INVERTING BUCK

BOOSTCUK SEPIC ZETA

# Switches 2 4 2 2 2

# Capacitors 1 1 2 2 2

# Inductors 1 1 2 2 2

Inverted Output + x + x x

Continuous Iin x x + x +

Continuous Iout x x + + x

MPPT AlgorithmP&O

● Fixed output voltage from Batteries

● Vary the current via DC/DC duty cycle

Electrical Schematic

Monitoring

• Thermocouples & DAQ system - from lab• “Smart Batteries” with State-Of-Charge

sensors

Monitoring

• Moisture Sensoro Need to monitor moisture level inside enclosure to

make sure no water ingress has occured

• Sparkfun Humidity and Temperature Sensor - SHT15 - $28.95o Measurement range: 0-100% RHo RH accuracy: +/- 2% RHo Power Consumption: 30 uW

https://www.sparkfun.com/products/8227

Heater

• Need a 500W electric heater• Cartridge heaters are available and

compact.• Size Constraints:

o Length - ~80mm (3.15in)o Diameter - <30mm (1.18in)

• Options:o 3.5” x 0.370” - $25.63o 4.5” x 0.370” - $28.72o 3.0” x 0.495” - $29.85

Heat Spreader

• Need to spread the heat (uniformly) from the cartridge heater to the TEGso Copper: k=400 W/mKo Aluminum: k=210 W/mK

o Need detailed model (ANSYS) to do isothermal check

Seals• To seal the cover plate and enclosure,

different methods of static sealing were investigated.

• Face seal gland is the best choice with flange design

• Areas of concern: o O-rings are typically designed for applications with oilo Finding an O-ring that is square shaped

Seals

• O-Ring Cord Stock - Material: EPDM (~$0.60/foot)

- Can customize shape

- Good resistance to water- Can withstand temperatures up to 212F- Would need to purchase Silicone Adhesive ($5.39)

Seals

• Sealed enclosure connectionso Glands

Cheap Limited waterproofing

o Underwater Connectors More expensive Good waterproofing Easy to repair

• Underwater connectors better fit our needs

Next Steps

• Full analysis of pressure plate including thermal loads

• Weld design, and final geometries of clamping components

• Choose O-Ring dimensions and design groove for seal

• Design enclosure flange once sealing design is finished

• Preliminary Testing / Model Validationo TEM powero Battery Testingo Clamping Force

Questions?

ThermoelectricsTaihuaxing Thermoelectric makes TEGs and supplies the efficiency information. Two TEP1-12656-0.8 deliver 18W of power from 480W of heat input.

Model width alpha R K n Power

TEP1-1263-3.4 30 0.0489 7.0 0.33 8 18.0

TEP1-12635-3.4 35 0.0489 6.5 0.36 8 17.1

TEP1-1264-3.4 40 0.0489 7.0 0.33 8 18.0

TEP1-1264-1.5 40 0.0478 3.0 0.78 4 15.1

TEP1-12656-0.8 56 0.0483 1.7 1.33 2 18.0

TEP1-12656-0.6 56 0.0478 1.2 1.94 2 12.4

Thermoelectrics

Functional Decomposition