Liquid Cooling for Li-Ion Battery

The Hyster-Yale group needs a custom cooling solution for a diesel to electric forklift conversion. The company is using a battery pack consisting of seven lithium ion batteries and that produce more heat than the forklift can remove by itself. The goal is to solve this problem by using a two fluid refrigeration system that fits between the batteries. Our solution will cool the batteries to a stable temperature autonomously and will cycle on and off as needed to reduce the energy usage of the system.
=Problem Definition=
The capstone team must find a liquid cooling method to run in between the batteries and keep them under a specific temperature at all times, even when charging.
==COVID-19 Update==
Due to the COVID-19 Pandemic during the second half of Spring 2020, the scope of this project was changed to be modeled and completed virtually.
==Background==
 
In an attempt to help reduce carbon emissions, the Hyster-Yale Group is converting the above internal combustion 9-ton forklift into an electric one. The battery pack they use, however, requires additional cooling to operate in hot environments. 
After the work of last year's capstone team to attempt to cool these lithium ion batteries using air, Hyster Yale Group decided in order to get the cooling capacity they need, a liquid cooling method will be required. 
==Deliverables==
* Proof of Concept Prototype
** Prototype to cost less than $750 per unit to produce.
* CAD Models & Drawings
** The prototype model is to be modeled in SolidWorks.
** The design is to be validated using SolidWorks Simulation
==Specifications==
===Functional===
The system must:
*Remove 180W of heat from each battery
*Cool batteries below 45 °C in 45 °C ambient environments
*Begin and end operation after batteries pass an internal temperature of no more than 40°C
*Minimize temperature gradient across the battery system
===Spatial===
The cooling system needs to fit within the space defined above
In addition, it must have these dimensions:
*Height < 502mm
*Width < 652mm
*Depth < 272mm
The interface between batteries must be no more than a total of 10mm wide
=Project Learning=
Since they summarize the project learning for the project, please see the Fall Design Review Presentation and Spring Engineering Release Presentation
==Design Alternatives==
===System Alternatives===
Image	Description
 	This vapor absorption cycle fortunately has a very high coefficient of performance. Similarly to our vapor compression cycle, it relies on a condenser and evaporator, but it also uses a separate vapor generator and absorber to create vapor of the working fluid, usually ammonia. Unfortunately, these systems often require larger scale operations, since you need to insulate the heat added to the ammonia (often with a flame) from the part of the cycle you wish to keep cold. Because we have such a small space for our design, this option was removed.
 	Another consideration was to use a thermoelectric diode, in this case an IceProbe, to try and cool our fluid. This cooling method has a very low coefficient of performance, and the number of diodes we would need to cool our fluid would have broken our project budget and drawn too much power. Although this option is the most space-efficient, it is not the right choice for this system.
 	Our chosen option for the refrigeration cycle, a vapor compression cycle offers a reasonably high coefficient of performance for the amount of space it requires. Luckily, last year's project already used a vapor compression cycle, so our group was able to salvage the condenser, metering device, and compressor, saving money. Originally we also considered using only the refrigeration cycle -- that is, running the refrigerant directly between the batteries. Because vapor compression cycles don't respond well to constantly being cycled on and off, we decided against it.
 	The final design for our cycle, featuring two separate loops integrated via a heat exchanger. The left side runs a water and ethylene glycol solution to cool the batteries, and the right side runs a vapor compression cycle, for when the glycol needs additional cooling. One advantage of this setup is the glycol side can constantly run without draining too much power, and refrigeration system can turn on during max charge/discharge, when the temperatures will be highest. The addition of a radiator to the glycol side is necessary since the addition of our liquid cooling system removes the possibility for natural convection to cool the batteries during normal operation.
===Battery Interface Alternatives===
Image	Description
 	Although we initially looked at other concepts such as heat pipes, the first design we believed to be manufacturable within the UofI's machine shop was a simple bent copper tube design. We decided to get more contact area by flattening a slightly larger tube into our 10mm gap, rather than relying on the line of contact we might get with a circular pipe. This design would have one pipe between each pair of batteries, trying to remove heat for both of them.
 	Our second main idea for the interface was a large, brazed reservoir through which fluid could flow. Unfortunately this was ruled out due to how difficult it would be to make 7 of these without any leaking, and how the vibration of a forklift may end up buckling the thin metal sheet over time.
 	After our client mentioned the idea of having our interfacing system be modular, where each battery could have its own cooling system independent of the others, we came up with this design. This is also when we decided to use thermal epoxy to connect the batteries to each copper tube. Some advantages include the ability for the batteries to be placed vertically within the assembly, rather than needing to slide or be placed at an angle, and the need for only one pipe configuration. The disadvantages for this, such as the lack of cooling in certain areas on each side of the battery and the number of bends necessary for each part, disqualified this design.
 	This third option has less of a heat gradient than the previous one, while still maintaining the modularity. Unfortunately, our group was unable to find a manufacturer who would be willing to anneal the tight radii necessary for this design, for such a small order quantity. We recommend our sponsor pursues this design for its final build, due to how unlikely it is to fail compared to our prototype's build.
 	A product of necessity, this removal interface requires brazing every bent joint of the previous concept. This is a significant amount of labor, with high potential for leaking and failure in an applied scenario, so although it may work for our prototype, it is not recommended for the final model.
=Concept Design=
The most recent concept as of midterms for the Spring 2020 semester, most of the refrigeration-cycle components are salvaged from last year's project. Some key features include the nested tube design and custom inlet and outlet manifolds, both of which are detailed in the Spring Engineering Release Presentation. Although our system will be controlled manually, the final, temperature-dependent control systems will be handled by our sponsor after project hand-off.
=Prototype Manufacturing Plan=
The 1/2" copper tubes used to interface between the batteries need to be pressed to fit within each 10mm gap. This tube pressing jig will let the team flatten each tube evenly prior to cutting, and finally brazing the joints together. Due to the complicated nature of this process, it is not recommended for final product manufacturing.
The three manifolds (one inlet, two outlets) will be manufactured using 0.060in aluminum sheet metal. This will be plasma cut, then bent to shape, and finally welded together as shown. The various pipes and fittings used to connect the manifold to the system will be welded on over each of the cut holes.
=Design Validation Plan (Pre-COVID-19)=
Because the system consists of these three major components, we hope to run the following tests once their necessary parts are finished:
==Pump and Manifold==
By hooking the pump up to a water reservoir and measuring how fast it drains, we can get the total volume flow rate our pump can output. The outlet of the pump will run into our inlet manifold, and each outlet of the manifold will run to its own separate container. This will validate that the manifold splits the flow rate evenly between our interfacing tubes.
==Refrigeration System==
After all of the parts of the refrigeration system are assembled, we will pressurize the system to ensure there are no leaks or failures. When it is filled with refrigerant, we will run the refrigeration loop and measure the temperature of the fluid entering the heat exchanger, to ensure our refrigeration system cools well below ambient temperature.
==Heat Removal Interface==
Our prototype will feature one separate heat removal "mockup" in which the interfacing tubes are thermally epoxied to sheet metal and pressed against a 180W heat source. While the heat source is running, we can measure the average temperature of the sheet metal plates for an approximation of how well the interfacing tubes remove heat from the batteries
==Proof of Concept Prototype==
After testing these components separately, we will assemble everything into a proof of concept prototype to make sure everything fits within our spatial restrictions, and our various pipes and connections are watertight. This prototype should be easy for our sponsor to "drop in" to a functioning battery pack.
=Design Validation (Post-COVID-19)=
The main goal of our design validation plan was to validate the heat removal of the batteries and determine how much of a temperature gradient forms during the battery operation. Although the image shows a heat gradient forming of approximately 20°C, this could be minimized by increasing the flow speed, and the hottest temperatures are still below our 45°C goal
=Final Deliverables=
Please click here to view the Final Presentation, and see the attached final report.
=Team Members=
	Jack Gonzalez 
Major: Mechanical Engineering
Hometown: Boise, ID
Responsibility: Communication Head, CAD Modeling 
Email: gonz4673@vandals.uidaho.edu
I will be graduating this May with a B.S. in mechanical engineering and will hopefully be proceeding to graduate school. Eventually, I hope to find myself designing rocket engines and bring humans to other planets. Some personal passions of mine include hosting murder mysteries and conquering tabletop dungeons.
	Jesus Barrera 
Major: Mechanical Engineering
Hometown:American Falls, ID
Responsibility:Budget, Thermal Interfacing 
Email: barr6006@vandals.uidaho.edu
I'm from American Falls, Idaho where agriculture is very prevalent. I grew up on a farm where we grew potatoes and wheat among other crops. I started working on the farm at a young age and was I always curious about how the tools and machines we used functioned. That curiosity is what inspired me years later to pursue a degree in mechanical engineering.
	Donald Funk 
Major: Mechanical Engineering
Hometown:Coeur d' Alene, ID
Responsibility: Materials Selection, Co-System Design 
Email: funk5542@vandals.uidaho.edu
From a young age two of my biggest interests have been art and design, and tinkering with cars and various machines. Through mechanical engineering I have found a way to mesh the two while working towards solving real world problems.
	Tarrin Funderburg 
Major: Mechanical Engineering 
Hometown: Pocatello, ID
Responsibility: Documentation, Wikipage 
Email: fund6621@vandals.uidaho.edu
For as long as I can remember, I've known that technology and STEM were my passion. Thanks to my aptitude for math, everyone recommended that I pursue engineering and now that I'm in my last year of Mechanical Engineering I hope to make an impact in people's lives with all of my work.
=Additional Documentation=
Project Schedule
	The final, executed schedule for this project is attached on the left
Meeting Minutes
Meeting Minutes can be found in the following Google Drive folder 
Presentations
Meeting Presentations can be found in the following Google Drive folder