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Subsystem Design Review

Subsystem Design Review. Agenda. Goals of Review Updates from SDR Review of our System Input Desired Functions Engineering Analyses Conducted Next Steps. Goals of Review. Confirm acceptability of mechanical design Get advice on areas of uncertainty: Smart battery procurement

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Subsystem Design Review

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  1. Subsystem Design Review

  2. Agenda • Goals of Review • Updates from SDR • Review of our System • Input Desired • Functions • Engineering Analyses Conducted • Next Steps

  3. Goals of Review • Confirm acceptability of mechanical design • Get advice on areas of uncertainty: • Smart battery procurement • Aluminum vs Stainless enclosure • Pressure sensors for clamping • Isothermal heat spreader

  4. 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.

  5. Updates From SDR • The electrical system will be out of the water. • Mechanical system • Contains only thermoelectrics and a heat source • Waterproof • Tethered to the (out of water) electrical system. • We chose a very simple system design: • Easier to assemble and troubleshoot • Few disadvantages compared to other options. • Like a submarine environment.

  6. Mechanical System

  7. Input Desired • Batteries. • We can’t find the smart batteries we wanted • Enclosure Material. • Aluminum or Stainless Steel? • Heat Spreader • How do we know it will be isothermal? • Clamping Test • Is there a way to determine whether the clamping pressure is uniform? • TEG • Should we purchase them now so we can test them?

  8. Functions We tried to analyze every item in our functional decomposition. Our main functions were: • Protect System • Generate Heat • Transfer Heat • Generate Electricity • Store Electricity • Monitor System

  9. Our Analysis • Thermoelectrics [Generate Electricity] • Heat Sinking [Transfer Heat] • Insulation [Protect System, Transfer Heat] • Clamping Thermoelectrics [Transfer Heat] • Max Power Point Tracking [Store Electricity] • Battery Charging [Store Electricity] • Monitoring [Monitor System] • Heater [Generate Heat] • Heat Spreading [Transfer Heat] • Seals [Protect System]

  10. 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.

  11. Thermoelectrics Analysis: • Determine number of modules required to stay within allowable temperatures and for which Q<480W • Iterate heat balance equations to find power • Taihuaxing Thermoelectric makes some modules that meet our requirements. Need more research to confirm their specs.

  12. Heat Sinking • First calculated a desired heat sink thermal resistance using a basic model

  13. Heat Sinking • Next, set up equations to determine heat sinking needs • Found number of fins needed by varying fin size, spacing and array type (rectangular vs pin)

  14. Heat Sinking • Results show that plate fin array with reasonable dimensions is sufficient

  15. Heat Sinking • Found a fin array in the Thermoelectrics Lab • Plugging in dimensions in the spreadsheet: • Rsink = 0.094 K/W - better than needed! • Could decrease the resistance by modifying fins

  16. Heat Sinking • Increasing fin spacing increases convection coefficient • 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 • Other modifications could lower the resistance

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

  18. Insulation • Using the equation, , we found a few options for insulation that met a compressive strength of ~10 Mpa (1450psi)

  19. Insulation • Top of heat spreader insulation: • 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: • 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 )

  20. Insulation • Created a detailed thermal circuit to determine insulation needs

  21. Insulation • Analyzed heat flow through each node to come up with a system of equations we could solve • Calculated heat that flows through the thermoelectrics out of the total 500W • Used solver in Excel to optimize insulation thicknesses vs cost while maintaining 96% energy delivered through thermoelectrics (480W/500W) • Results: • ½” of primary insulation (Ceramic) • 1 ¼” of secondary insulation (Fiberglass)

  22. Insulation Start of needing insulation between clamp and heat sink

  23. Clamping Thermoelectrics Steel Bolt Analysis 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 lbf Aluminum Housing Bolt Grades Considered: • A307 • A354 • A499 Aluminum Alloys Considered: • 3004 H38 • 5052 O • 5052 H32 • 5083 H112

  24. Clamping Analysis Approach Steel Bolts in Aluminum Bolt Housing • Calculate EL to ensure bolt fails before threads strip. • Calculate Relative Strength, R: • If R > 1: • Fatigue Factor of Safety based on:

  25. Varying Grade of Steel bolts assuming 3004 H38 Aluminum Housing

  26. Varying Aluminum Alloys for Bolt Housing

  27. Clamping - Bolted Connection Design

  28. Pressure Plate Assuming: • 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”

  29. Clamping Conclusions A307 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.

  30. 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

  31. Electrical Systems { TEG MPPT(Perturb and Observe) CHARGING CIRCUIT VARIABLE LOAD 20W 20W LI-ION BATTERY BANK (2) LI-ION BATTERY BANK (1) LI-ION BATTERY BANK (N) ...

  32. 62.5% Power Unused 11.9% Power Unused

  33. Battery Model

  34. Simulink Model

  35. Simulation Results

  36. Battery Charging More parallel batteries = less power lost through internal resistance

  37. Battery Charging Different battery voltages have similar results on power loss

  38. Loading Analysis

  39. Battery Protection Circuit • Short circuit Protection • Overcharge Protection voltage • Overdischarge Protection voltage • Overcurrent detection Protection http://www.freepatentsonline.com/6768289.html

  40. Smart Batteries • Used in laptops,cameras, cell phones,electric wheelchairs,scooters, and military applications • Reports temperature,voltage, currentand SoC

  41. Smart Batteries (GenPort) • SMBus 1.1 compliant • Cell balanced • Protection • Primary and secondary over and under voltage protection • over current protection • short circuit protection • over temperature protection • cell imbalance protection • and more... http://www.genport.it

  42. 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

  43. Step Up/Down Converters

  44. MPPT Algorithm P&O • Fixed output voltage from Batteries • Vary the current via DC/DC duty cycle

  45. Electrical Schematic

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

  47. Monitoring • Moisture Sensor • Need to monitor moisture level inside enclosure to make sure no water ingress has occured • Sparkfun Humidity and Temperature Sensor - SHT15 - $28.95 • Measurement range: 0-100% RH • RH accuracy: +/- 2% RH • Power Consumption: 30 uW https://www.sparkfun.com/products/8227

  48. Heater • Need a 500W electric heater • Cartridge heaters are available and compact. • Size Constraints: • Length - ~80mm (3.15in) • Diameter - <30mm (1.18in) • Options: • 3.5” x 0.370” - $25.63 • 4.5” x 0.370” - $28.72 • 3.0” x 0.495” - $29.85

  49. Heat Spreader • Need to spread the heat (uniformly) from the cartridge heater to the TEGs • Copper: k=400 W/mK • Aluminum: k=210 W/mK • Need detailed model (ANSYS) to do isothermal check

  50. 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-rings are typically designed for applications with oil • Finding an O-ring that is square shaped

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