Wearable Thermal Management System
During my senior year, I worked on an independent research project to develop a wearable thermal management system. It was inspired by a movie called Mines, starring Armie Hammer. In this movie, he accidentally steps on a mine, and is stuck in the same position for multiple days while he waits for rescue. He was in a desert environment, so it got very hot during the day, and very cold during the night. After watching that, I started coming up with ideas for thermal management systems, and wanted to explore one that would cool you down during the day, but would store the heat it removed from you, to release it at night. Below is my final report, which explains the project in further detail. I have omitted some information, as it is unpatented intellectual property, I wish to keep private.
Executive Summary
This report outlines the progress made during the Fall 2020 semester to develop a wearable thermal management system. An initial concept was developed utilizing a Phase Change Material to store waste heat to be recycled from Peltier modules as they cool the body. The majority of testing was centered around quantifying the characteristics of the Phase Change Material (Beeswax). It was found that the phase change material was able to absorb about 1037.9 kJ/kg or 268 kCal/kg of heat, which equates to about two hours worth of exercise. After further research into the end user, the project was pivoted and slightly modified in function into a thermal contrast therapy device. A basic outline of the plan for next semester was created, to ensure that this product is developed as thoroughly as possible.
Background
The purpose of this project was to develop a wearable thermal management system geared towards space exploration and military use. The original idea spawned from a fictional story about a marine who stepped on a landmine in the middle of an African Desert. He was stuck there for multiple days, and the temperatures fluctuated significantly from hot daytime temperatures to cold nighttime temperatures. He had access to a solar panel, as do many people on longer missions both on land and in space. The original idea was to have a vest a soldier or an astronaut could wear that would actively pump heat from the body into a phase change material, which could be stored in a phase change material and recycled when the nighttime temperatures drop or temperatures fluctuate in space. The advantage of using this in a military application would be for increased comfort and better performance of soldiers in extreme environments. In particular, snipers and scouts aren’t allowed to move as much as other soldiers, so this would help them better regulate their temperatures. There is also an advantage to this system for space exploration in that it is a closed loop system, as opposed to the current system that rejects water into the surroundings [1]. Given that water is a precious and finite resource in space, this would be a more viable option to maintain functionality while conserving resources.
Technical Work
The initial goal set at the beginning of the semester was to develop a wearable thermal management system. That encompassed a set of sub-tasks, including developing the heat recycling system, the heat rejection system, and the fluid circulation system. These were based on the initial concept that Peltier modules would drive a fluid circulation system to remove and deliver heat to the body, while heat is rejected into a phase change material to be stored. This path was developed based on some initial analysis of available materials and systems. In particular, as can be seen in figure 1 and figure 2, the current commercially available personal heating and cooling systems.
Figure 1: Wearable resistive heating vest.
Figure 2: Ice water circulation cooling vest.
Figure 1 is an electrically powered resistive heating system, which is commonly used by winter athletes, military operations, and a variety of other groups that must function in extreme temperatures. This solution works, but is not very energy efficient, given that it uses resistive heating. A small battery pack will only power this for roughly 2 - 3 hours, which isn’t feasible for this application. Figure 2, is a water circulation driven cooling system, often used in stationary applications. This wouldn’t work in a dynamic military scenario, or space scenario, as it requires an input of ice. The criteria developed based on these findings state that the system must be portable, energy efficient, and must serve the function of removing body heat with the ability to store and recycle that heat.
Based on the design criteria, it was determined that the Peltier module would be the ideal candidate for this application as it is portable, compact, and can be energy efficient if used properly. As can be seen in Figure 3, the Peltier is a semiconductor that actively creates a temperature differential when supplied with current [2].
Figure 3: Peltier Module Diagram
The major drawback of this is the fact that a temperature gradient must be maintained by rejecting heat from the hot side. If heat isn’t adequately rejected, the cold side will not be able to cool properly. So that pointed the immediate focus of the project into heat rejection/storage. A thermal battery had to be created. The ideal concept was to use a phase change material (PCM) to store thermal energy, particularly because the phase change region can be tuned to the needs of the application. To give some background, refer to the flat phase change region in Figure 4. That is the region in which the material changes phase from solid to liquid, which takes energy. If that region is maximized, thermal energy can be stored in the material without raising the temperature. This is advantageous in that it significantly increases the energy density of the thermal battery and allows for adaptation/customization of the system.
Figure 4: Phase change graph
With that information, the next step was to develop a method for determining the phase change region of a material. Figure 5 displays the test setup, in which there was a “boat” of aluminum foil, on top of the hot side of a Peltier module. The cold side of the peltier module was attached to a heatsink, submerged in hot water (to keep the thermal gradient).
Figure 5: Initial test setup to determine the phase change region of beeswax
It was found that the beeswax had a very small phase change region, as can be seen in Figure 6. The peltier module was run at a constant 12.2 volts (V) and 2.5 amps (A). Equation 1 shows the relationship between voltage, current and wattage,
where W is wattage, V is voltage, A is amperage. Figure 6 depicts the graph of temperature with respect to time of the beeswax.
Figure 6: Temperature vs. Time of heating beeswax. Phase change region marked with red.
In this case, we can also use the wattage to find the amount of energy in Joules inputted to the wax. The phase change region was about 340 seconds, thus the amount of energy inputted to the wax was,
W =(Joules)/(second)
Joules = Seconds* (W)= 340s(30.5W) = 10379 J = 10.379 kJ
The phase change region is able to absorb roughly 10.379 kJ of energy with no significant raise in temperature. To give context, that is equivalent to about 2.68 kCal, and an average person running burns about 100 kCal per hour. Now this was with about 10 g of beeswax. So, if this yields roughly 2.68 kCal/10g, it can be estimated that this beeswax can absorb about 268 kCal/kg. Given that, it can be roughly established that 1 kg of beeswax as a PCM can absorb heat for about 2 hours. Now, upon holistically analyzing the necessary components, the system would weigh well over 5 lbs. There would be 2.2 lbs of PCM, about 3 lbs of batteries, and about 2 lbs more of electronics and packaging. Upon doing further research into the application of this system, it seems to be more advantageous and reliable to use a PCM lined vest to aid in heat absorption rather than have an active heat pump. The reasoning behind this lies in the fact that military operators already have a significant amount of gear and electronics to carry on their missions. Adding an extra few pounds, as well as an additional electronic interface wouldn’t quite be the best option. Based on where the design was headed, a pivot was made into a different end user with a similar technology.
Future Work
Given that the original idea of a wearable thermal management system doesn’t currently seem to hold the right amount of potential, the project needed to be pivoted to something else. The idea currently being pursued is to use a very similar technology for a thermal contrast therapy device. This idea was originally developed based on the data to comprise Figure 7, skin temperature and water temperature with respect to time for an Ice Bath.
Figure 7: Body surface temperature and water temperature with respect to time of Ice bath
Ice baths are incredibly useful for increasing circulation, decreasing inflammation, and promoting healing. They’re even more beneficial when paired with a hot bath or a sauna experience, because of the vasodilation and vasoconstriction. Essentially, cold causes vasodilation, where blood vessels contract to conserve heat, and the heat causes the vessels to dilate and reject heat. Rapidly switching from hot and cold environments causes a rapid constriction and dilation of the blood vessels, which then in turn increases circulation and removal of cellular waste. An NIH conducted study concluded that within their test group, contrast water therapy decreased muscle soreness, and prevented muscle strength loss compared to passive recovery. The idea would be to use the previously developed setup to create a Peltier module driven thermal contrast therapy device.
Although this idea is still in the infant stages, there is potential for it to be used as a physical therapy device and/or a preventative medicine. To provide context on the predicted function of the device, if a patient is having muscular pain or soreness, they would be able to attach this device to a specially designed sleeve which goes over the afflicted area. The device would then cycle hot and cold water through the sleeve at set or dynamic intervals based on the injury and the patient profile. This allows for a passive, low footprint, localized method to utilize the benefits of contrast water therapy. The end users of this would likely be physical therapists, and athletes, but there is also potential for regular home use.
Depicted in Figure 7, the initial concept for the system involves two peltier modules paired with a PCM thermal inertia unit, and water lines to the body sleeve. While the cold water is running, the Peltier module will reject heat into the PCM, and when the hot water is running, the opposing Peltier module will essentially pump the heat from the PCM to the water to be distributed into the sleeve. This design is made such that the device will be energy efficient, be “solid state” and silent excluding the water pumps. Please note that this is a preliminary concept, and will likely change throughout the life of this project. The goal is to make this device streamlined, user friendly, simple, and relatively inexpensive to produce. That will require some industrial design elements and market research to be applied.
Figure 8: Initial concept drawing of thermal contrast therapy device
Project Impact
This project as a whole has been very dynamic in scope throughout this semester, but appears to be converging on a single purpose going forward. The final product, a thermal contrast therapy device has potential to revolutionize the sports medicine and physical therapy industries. It pairs an age old technique with modern technology into a user friendly, simple, streamlined, and relatively inexpensive product. Furthermore, there is potential for new discoveries in thermal management and coolant system design, due to the abstract nature of this system.