P17280: Hot Wheelz Thermal Management Systemp. 1 of 8

P17280: Hot Wheelz Thermal Management Systemp. 1 of 8

Multidisciplinary Senior Design
RIT Hot WheelzFormula SAE Electric
Thermal Management System
P17280
Caitlin Babul, Missy Miller, Jordan Short and Kristin Zatwarnicki
May 11, 2017

Abstract

An important aspect of today’s new hybrid and electric vehicles is the thermal management system. This system ensures proper vehicle function and helps avoid decreased drivetrain efficiency, battery wear, and safety hazards. This not only holds true for commercial electric vehicles, but it is also important for Formula SAE electric vehicles. On the 2016 Hot Wheelz vehicle, which took home third place at competition, the system consisted of oversized air vents and bulky heat sinks to cool the electrical components.Due to the undertaking of designing and building a custom battery pack for the 2017 vehicle, the Hot Wheelz team was looking to outsource the design of an effective cooling system. The expected end result of this project is a data-driven lightweight system that optimizes the vehicle’s performance through effective cooling methods for the electrical powertrain components. The system should comply with all Formula Hybrid regulations as well as the constraints of the team’s vehicle. The designed cooling systems relied on air cooling, consisting of programmable fans and vents for the batteries and a heat sink for the motor controller.

Background

The RIT Hot Wheelz Formula SAE Electric vehicle team is an all-female undergraduate engineering team that designs and builds a fully electric formula-style racecar. The team placed third in the electric-only category in their first Formula Hybrid competition in May 2016. They competed with a new vehicle in May 2017. One of the most significant design changes in the new vehicle is the location of the batteries. In the 2016 design, the lithium-ion batteries were housed in large pods attached to the sides of the car on opposite sides of the driver. Although they were effective in supplying the car with power, the pods were heavy and caused aerodynamic drag during competition. In an effort to combat these challenges, the pods were replaced with a new battery pack located behind the driver's seat. The system eliminates the air resistance and weight problems but introduces a new problem in that there is much less space for the batteries to operate. Concerns were raised that the decreased airflow would cause the batteries to overheat and shut down the vehicle.

Another area of concern on the vehicle was the heat sink for the motor controller. This component converts the current of the batteries from direct current (DC) to alternating current (AC) and regulates and controls the amount of current provided to the vehicle’s motor. During the 2016 competition, the heat sink was noted to have been extremely hot. Although the controller functioned properly, the team wanted to verify that it would perform under higher loading conditions.

Since the 2016 vehicle was designed to pass the vigorous inspection process in order to get on track, the cooling of electrical components was not a critical design. This senior design team was formed to optimize the heat transfer for these electrical components. The purpose of the team was to quantify and optimize heat flow through the vehicle's electrical system, specifically for the battery structures and motor controller subsystems. There were several main requirements for the project:

●The system must be fully functional by the May 2017 competition.

●The system must be lightweight, durable, fully waterproof, and comply withall Formula Hybrid rules.

●The system must have a limited impact on the Hot Wheelz team’s design.

●The project must provide sufficient documentation and testing to verify the effectiveness of the system.

Process

Battery Cooling

Our process for the first phase of senior design was to identify and quantify the extent of the problem given by the Hot Wheelz team and to design a system that solved these issues. As the new battery system design was finalized by the Hot Wheelz team near the end of MSD I, models for the heat transfer through the system were developed. Since there were no physical components for this new battery system built at the time, initial testing was delayed to MSD II. Due to this, the battery cooling design was purely based off of initial models.

There were two main ideas for cooling the battery box when the subsystem was first being developed: cooling with liquid or cooling with air. Using Pugh analysis, several designs with liquid and air cooling were compared with each other to determine the best design. There were several important engineering requirements that helped make this decision. Waterproofing the system and protecting the components were critical both to the vehicle being safe to drive and for it to pass inspection for competition. In every iteration of the Pugh process, liquid cooling scored lower than air cooling because of the increased difficulty of isolating electrical components from the liquid.This difficulty brought up another engineering requirement: the system’s budget. Liquid cooling was much more complex and expensive than the air coolingoptions. The complexity of liquid cooling would also make installation and maintenance difficult for the Hot Wheelz team. The requirement of various pumps, motors, and other system controls for a liquid cooling system would exceed the specifications set by the Hot Wheelz team for this design. Despite being very effective in function, liquid cooling would add unnecessary weight and volume to the system. Based on the limited data from the previous competition and the initial modeling for the new battery system, it was decided that the amount of cooling needed did not justify using a liquid cooled system.

After air cooling was chosen as a design foundation, heat transfer analysis was performed to help create the system. The design of the battery enclosure and the battery configuration was owned by the Hot Wheelz team and could not be changed by the senior design team. A system needed to be built around their defined structure. This proved to be challenging as the design of the battery enclosure was not optimized for heat transfer or air flow. There was very little data known about these batteries, as the Hot Wheelzteam had never used them before. A heat transfer analysis was critical to predict the heat generation within the box and help influence the design of the system. After receiving guidance from Professor Rob Stevens, the heat transfer analysis used was idealized flow across banks of tubes. This method seemed appropriate since the batteries are cylinders, and the idea of the system was to flow air over the arrangement of the banks of batteries. There are several important equations related to the theories surrounding the heat transfer flow from the Fundamental of Heat Transfer textbook. They are listed in Figure 1.

There were also several key assumptions used in this analysis:

1. Continuous current through a battery cell is 30A during the endurance event.
2. The battery cell’s internal resistance is 8mΩ.
3. Each battery structure will generate 33% of the total heat generated by the entire battery pack (3 structures total).
4. A battery structure will be represented by a staggered bank of tubes consisting of 11 rows of 4 cells each.
5. Inlet air temperature into the battery structure is 18℃.
6. Inlet air velocity is equivalent to the fan blade velocity.

The basic idea behind this theory was to calculate the heat generated from each battery at a specific discharge rate. After assuming several air properties and air velocity, various flow parameters were calculated such as the Reynolds number, Nusselt number (Nus), and the convective heat transfer coefficient. The convective heat transfer coefficient was calculated using
where K is the thermal conductivity of the battery and D is the diameter of the battery. After this value was calculated, the outlet temperature of the air coming out of the battery bank was computed. Using this temperature, a more accurate set of air properties were found and the process was repeated for a more accurate outlet temperature. The log-mean temperature difference was then computed and used to find the overall heat transfer rate. Using this value, the pressure difference between the inside and outside the battery box was calculated. This pressure drop allowed the team to pick appropriate fans for the design.

After the design concept was finalized, preliminary testing was done to see if the idea for the system would be effective. The testing was mostly to see if the airflow would move through the actual structures. Since the model made a major assumption about the arrangement of the batteries, which was quite different from the actual battery pack, it was important to validate the model through actual testing. A mock of the front battery packs was made using plywood and galvanized steel piping to simulate the structures and the batteries. The structure was sealed with duct tape, and a fan was installed in the same location as the full-scale system. A plastic panel was also installed on one side so that the air flow could be observed. Using a fog machine, smoke was pumped into the test box in order to be able to visualize the airflow through the box.

The test revealed that airflow was not getting to the batteries near the bottom left of the enclosure. The solution was to insert a barrier between the bottom row and the second to bottom row of the battery banks. Re-testing in the mock-up showed that airflow was now reaching the batteries in question, and the additional design feature was integrated into the overall system.

Controller Cooling

At the 2016 Formula Hybrid Competition, the Hot Wheelz team noticed that the motor controller’s heat sink was extremely hot following the endurance race. The car completed 19 kilometers in 35 minutes. Once the motor controller reaches a certain temperature, it begins to reduce the current the motor can pull and, therefore, reduces the performance of the vehicle. One of the goals for this project was to increase airflow around the controller so that it would be less likely to overheat and reduce vehicle performance.

The 2016 vehicle had the motor controller placed inside of a polycarbonate box. The box was meant to protect the controller from water and protect team members from accidentally touching the high voltage connections. The controller was also mounted to a half inch thick aluminum plate. The plate acted as both a heat sink and the mounting method. Although the box was able to protect the controller, it did not allow forairflow. Drive testing was performed in October that confirmed the temperature inside the box was much hotter than the temperature of the heat sink. The senior design team needed to find a way to get as much airflow to the controller as possible while still complying with all Formula Hybrid rules.

Based on conversations with a representative from Curtis Instruments Inc., the manufacturer of the controller, it was determined that the controller temperature needed to remain below 85ºC in order for the controller to be optimized for performance. The controller heat sink was designed to withstand 400 W of heat, which was based on a worst-case scenario endurance run. Assumptions were made based on the speed of the vehicle, which would affect the air flow running over the controller. Thermal simulation of 400 W being applied at the center of the plate resulted in temperatures of 49ºC at the edges of the plate and 67ºC at the center. The analysis also confirmed that additional cooling methods such as fins and fans would not be required for effective heat transfer.

The finalized design consisted of four main components: an aluminum heat sink, a polycarbonate lid, four aluminum posts to hold up the lid, and four polycarbonate side panels to protect people from accidentally touching the high voltage connections. The side panels extended to just below the top surface of the controller. However, during a pre-inspection, it was discovered that this design would not pass the “Finger Probe” test. A design change was required to extend these panels further to the base of the heat sink. A series of holes were also drilled in all of the side panels to facilitate airflow into the enclosure. The holes were covered with a hydrophobic mesh to prevent water from getting in the enclosure or on the controller connections.Lastly, cavity plugs were sourced for the controller low voltage wiring housing to ensure that it would be waterproof. Cavity plugs made specifically for the controller were recommended by Patrick Cody, a customer support engineer at Curtis Instruments Inc., who also volunteers as a mentor for the Hot Wheelz team. These plugs were used in any port on the harness that was not populated with a pin and wire.

Results and Discussion

Battery Cooling

Testing of the battery cooling system occurred during drive testing of the vehicle before competition, as the batteries were delayed several weeks and could not be tested beforehand. This meant not being able to test and validate the design until late April, instead of late January, which was originally planned.

The final battery cooling system, which can be seen in Figure 2, includes three fans mounted in the battery box, two locations on the battery box with vents, hydrophobic mesh to prevent water from entering the vents, segmentation material to limit airflow to only the middle of the battery packs, barriers between select batteries to control airflow, and fan control through the Battery Management System (BMS). Each fan was mounted so that it pulled air from inside the battery box and then blows it out. Vents were cut into the battery box in two locations: one set on the bottom back panel of the box, where air can enter at the base of the battery structures, and one set on the top panel of the box, where air can enter at the top of the battery structures. Whenever a side of the battery pack was not mounted against the battery box (excluding the tops of the packs), a piece of G-7 Garolite material was mounted to the structure. These pieces ensured that airflow stayed within the battery structures to cool the middle of the battery cells rather than the ends of the cells. Rubber foam was also used between the bottom two rows of battery cells in the two triangle packs to control airflow to the far corner of the pack, which was an area of concern to keep cool. The fans were wired to a relay which was controlled by the BMS. The BMS was programmed to turn on the fans when a battery cell temperature reached 30°C and turn off the car if temperatures exceeded 60°C.

The success of the battery cooling system was evaluated by observing the performance of the batteries during drive testing and at competition. At competition, the weather was fairly sunny and in the mid-50s during the dynamic events. The batteries saw a maximum temperature of approximately 30°C, which is the point at which the fans turned on to facilitate more airflow within the box. The operating temperature stayed well below the degradation temperature of the batteries and the cooling system was considered a success.

Overall, the battery cooling system met all critical engineering requirements. The number of electrical connections was kept to a minimum to ensure easy installation and maintenance of the system. The fans used in the cooling system operate at 12V and together pull less than 2A to meet our voltage and current requirements set by the Hot Wheelz team. The resistance of the fans to the grounded low voltage complied with the Formula Hybrid rules. The low voltage wires of the fans were not mounted far enough from high voltage components within the battery box, therefore, insulating material was needed to separate the fan wires from the high voltage components. All of the vent holes were cut smaller than the diameter of the probe used at competition to ensure the team would pass the Finger Probe test. The hydrophobic mesh also kept the probe out of the battery box. The total weight of the battery cooling system was less than 1lb (0.45kg), which is approximately 7% of the total weight of the cooling systems on the vehicle. This satisfied our requirement of keeping the system lightweight. The total volume of the battery cooling system was approximately 16 cubic inches (260 cubic centimeters), or approximately 10% of the total volume of the cooling systems on the vehicle. The battery cooling system impeded very little on the other systems assembled on the vehicle, but the entire cooling system did not meet our volume requirements. The total cost of the battery cooling system was $655.25, which was approximately 67% of the total funds spent. This cost covered the materials used for the final product and the materials needed for testing.