P17280: Hot Wheelz Thermal Management System
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Preliminary Detailed Design

Table of Contents

Project Objective Statement

Deliver a thermal management system for the batteries and motor controller to maintain optimum temperature for competitive performance.

Engineering Requirements (Specifications)

Customer Requirements

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The full revision history can be viewed here.

Engineering Requirements

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The full revision history can be viewed here.

Major Changes: Removed liquid-cooling specs, provided ideal value for max current draw, and added safety specification.

Note: For full mapping of specs and customer requirements, please see the House of Quality document.

Requirements Efficiency
Time Spent Projected Time Efficiency
25 mins 15 mins 60%

Finalized Concept Selection

Controller

Possible Mounting Location

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Note: Heat sink is exposed to air on all sides except for area covered by controller.

Potential Heat Sink Design

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Important Design Parameters

Batteries

Potential Duct Design Concept

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Important Design Parameters

Preliminary Battery Box Design

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Important Design Parameters

Fan Circuitry

Wiring Schematic from the Orion BMS Wiring Manual - Full document here

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Important Design Parameters

Proof of Concept

Controller

Justification of Design Parameters

Theoretical Calculations The PDF for the current motor controller heat sink analysis can be found here.

Note: The following analysis was reviewed by Dr. Rob Stevens, a professor in Mechanical Engineering.

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The largest assumption made in this analysis was the power dissipated. This parameter is still unknown for the actual system, and the power is based off a proprietary spec sheet from Curtis. Further testing is needed to determine whether this number is accurate. Another number that changed from the last phase is the convection coefficient. An error was found in the previous calculations and has been fixed here.

The worst case scenario is based off the endurance event during competition. In this event, the car is traveling around a track for as long as it can go. There is a significant hill on this course which we determined would cause the biggest load on the controller. The car will be traveling about 9 m/s on this track and the convective coefficient was based off of this speed.

Based on the current calculations, fins will not be required to keep the controller below 85 degrees C, the point at which performance degradation begins. We will validate this assumption in later phases.

Heat Transfer Analysis Efficiency
Time Spent Projected Time Efficiency
600 mins 120 mins 20%

Simulation

Note: The following analysis was reviewed by Dr. Rob Stevens, a professor in Mechanical Engineering.

Thermal simulation was done in Solidworks Simulation using the results of the above calculations. Simulation was done to confirm the surface temperature results from above, and to get a better idea of the temperature gradient over the plate's surface.

Parameters:

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The temperature will be highest in the center of the plate, where the controller sits. The lowest temperature is 120F at the edges farthest away from the controller, and 152F in the center.

The steady state temperature range shown by this model implies that the controller’s temperature will never be within 20 degrees of the temperature at which performance begins to degrade (185F).

These results also imply that we don’t need to utilize fins in our design. Although fins will make heat transfer more efficient, they are not necessary to keep our controller within our desired temperature range.

Testing so Far

Wiring Testbench - full pdf here.

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Testbench on Dyno

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The test plan for running the Hot Wheelz test bench on the RIT FSAE shop dyno can be found here

Several problems were encountered when trying to execute the test plan. The Hot Wheelz program designed specifically for the test bench did not work. To work around this, our team decided to run the dyno manually. During testing, the batteries were measured and found to be gaining voltage, indicating that the electric motor might be acting as a generator. The dyno might have been over-torquing the motor to cause this to happen. Since we were unable to find a SME on the manual mode of the dyno, we could not determine if this was the problem or not. With the phase coming to an end, and the dyno being requested for other teams' projects, testing could not be completed on time.

Efficiency of Test Bench Testing
Sub Team Time Spent Projected Time Efficiency
Electrical 4320 mins 320 mins 7.4%
Mechanical 1200 mins 120 mins 10%

Testing to be Done

Testing to determine whether the heat sink and airflow speed are effective at removing the heat needed will be important to justifying the system. Designing off of the team specs is good enough for now, but how the heat sink actually performs is what really matters. As dyno testing did not work for this phase, to test the heat sink an external heater will be mounted on it and air will be blown across it, with temperature sensors at various locations. This will be a good intermediary test for the sub system before the entire system is tested on the car for MSD II.

Theoretical Models

The heat sink motor controller resistance model (shown above) developed for this phase will be used for testing the heat sink. The power will change based on how much heat the external heater is producing and the h value if we decide to run different speeds of air across it. We will calculate surface temperature based on these parameters to validate that the heat sink works like we predict it will.

If it is found during testing that the heat sink alone is inadequate, theoretical models for the resistance needed to remove the heat will be needed. This in turn would be used to purchase fins that would add the right amount of resistance. Another option would be to use this data to design fins, and a model would need to be developed to explore this option. However, this can not be determined until more testing is complete.

Efficiency of Modeling
Time Spent Projected Time Efficiency
180 mins 60 mins 33%

Benchmarking

Decided to not pursue further research on the topic until further analysis was done on heat sink / fins.

Efficiency of Benchmarking
Time Spent Projected Time Efficiency
60 mins 45 mins 75%

Batteries

Justification of Design Parameters

Preliminary Airflow Design
Preliminary Airflow Design Justification - File

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There are two possible options to duct air into the battery box. The first option is the place an air intake/duct near the top of the main roll hoop and direct the air in through the top of the battery box. The second option is to place air intakes/ducts within the body side panels to direct the air in through the sides of the battery box.

One concern with the main roll hoop concept was that there would not be enough available space to mount a duct since it cannot come into contact with the driver's head and there is a warning light mounted on the top of the roll hoop. However, after checking with the solidworks model, there would be approximately 4-7 inches of available space.

The other concern with the main roll hoop concept was how effective the airflow would be by entering through the top of the box. The first components the airflow would encounter would not be the battery cells. Also, due to Formula Hybrid Rules EV2.3.4 (listed below) would require the BMS to be sectioned off from the rest of the battery box. Any directed air into the top of the box near the BMS would get trapped in this section and never reach battery cells.

EV2.3.4: The accumulator container may not contain circuitry or components other than the accumulator itself and necessary supporting circuitry such as the AIRs, BMS, and pre-charge circuitry. Note 1: The purpose of this requirement is to allow work on other parts of the tractive system without opening the accumulator container and exposing (always-live) high voltage. Note 2: It is possible to meet this requirement by dividing a large box into an accumulator section and a non-accumulator section, with an insulating barrier between them. In this case, it must be possible to open the non-accumulator section while keeping the accumulator section closed, meeting the requirements of the “finger probe” test.

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Due to the preliminary duct design, fans will most likely be required to induce airflow for the battery cells near the top and rear of the battery box. If fans are located here, it would be reasonable to assume that vent openings should be located on the front face of the battery box to allow the air to exhaust out. Vent openings would need to be required on the sides of the battery box as well to allow the air from the side ducts to enter.

Preliminary Fan Design
Preliminary Fan Design Justification - File

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A full speed fan system would be simpler than a variable speed system which would help ensure that the team meets CR7: System is easy to install and service. A simpler system would also aid the team in meeting CR12: System is built and tested in time for competition. A MOSFET device has never been used by the team before and will add complexity to the thermal management system which could require more troubleshooting and testing.

The team is also aiming to only utilize the thermal management system when the batteries reach a critical temperature before they start to degrade. At this point in time, it may be necessary to have a fan blowing at full speed to ensure the batteries stay under the degradation temperature.

Testing so Far

Testing to be Done

Theoretical Models

Battery Cell Heat Generation/Temperature Rise

A detailed working document can be viewed here.

The MATLAB script can be viewed here.

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Airflow in Battery Box
A theoretical model has not been developed as of yet to model the airflow to be expected in the battery box based on the preliminary design. Working towards a theoretical model for the airflow is a task for the next phase.

Benchmarking

The full working document can be viewed here.
Relays
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600 px

Fans
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Efficiency for Battery Box Proof of Concept

Time Spent Projected Time Efficiency
600 mins 240 mins 40%

System Design Layout

Functional Decomposition

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Concepts/specs have not been finalized so this was not changed.

System Architecture

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Full revision history here.

Detailed Design of System, Subsystem, and Parts

Controller

Heat Sink Design

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Electrical Schematics

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Full PDF here.

Batteries

Airflow through Box

Electrical Schematics

PDF Electrical Schematic - File

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Efficiency of Detailed Design
Time Spent Projected Time Efficiency
30 mins 10 mins 33%

Procurement

Controller

Manufacturability

If fins need to be added
We determined through analysis that fins are not necessary. If testing proves that we do need fins, they can be added in the following way:

Assembly

The updated heat sink will be mounted to the frame the same as the previous version. The following will need to be completed again

Batteries

Manufacturability

Assembly

Efficiency for Procurement
Time Spent Projected Time Efficiency
120 mins 45 mins 38%

Bill of Materials

Controller

A full revision history can be found here

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The estimated total cost of all components in the controller assembly is $83. Although we have not sourced all of the components, we are confident that we will fall well within our $2000 budget.

We are unsure of what the fins will actually cost. Various suppliers have fins that meet our size and material requirements ranging from $35 to $100. Many websites also require a quote to get a more accurate price. We can also obtain fins through one of out GM mentors for a discounted cost.

Efficiency for BOM motor controller
Time Spent Projected Time Efficiency
45 mins 20 mins 44%

Batteries

A full revision history can be found here.

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Yellow cells are estimations and assumptions and will be finalized during the next phase.

Efficiency for BOM Battery Box
Time Spent Projected Time Efficiency
60 mins 20 mins 33%

Test Plans

Controller

Heat Sink Validation

The link to the motor controller heat sink test plan is here

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This test will be able to gather temperature data of the motor controller heat sink sub system under an induced load independently of any other components. This will be verification that the design works as predicted.

Efficiency of motor controller test plan
Time Spent Projected Time Efficiency
120 mins 60 mins 50%

Batteries

Test plans will be created during the next phase when a more concrete design for the battery box cooling has been determined.

Risk Management

Risk Management Update

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A working document can be found here.

Plot of Total Risk Importance

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Note: Since previous phase focused mostly on dyno testing, not many of the importance values changed since the last review.

Addressing High Risks

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The full mitigation plan can be viewed here.

Project Plan & Budget

Project Schedule

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Full PDF here.

Work Breakdown Structure

Actual from Phase 3 - full PDF here.

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Projected for Phase 4 - full PDF here.

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Projected Costs

Action Items from Review

See full document of notes and action items here.


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