Multidisciplinary Senior Design Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York 14623 Project Number: P15441 MINI-AIR: PERSONAL COOLING DEVICE Alexander Cornell Mechanical Engineering Joshua Erbland Electrical Engineering Usama Haq Mechanical Engineering Daniel Probst Electrical Engineering Nathan Serfass Mechanical Engineering Abstract When performing moderate activities outside, many people, especially elderly and the obese, wish for a method of personal cooling. Also, the United States Department of Energy has stated that money can be saved by heating and cooling the individual instead of the whole area [2]. The objective of this project is to create a personal air cooler that will hang from an individual's neck and will blow cool air on the user. Based on the customer requirements, five parameters were set focusing on power consumption, cooling effect and comfort. The final prototype for the Mini-Air device weighs seven ounces and was tested to meet project-defined comfort requirements. It produces a temperature differential of ten degrees Fahrenheit and the exit velocity of the fan blows at one and a half meters per second. This feat is achieved by using a Peltier module in combination with a small DC fan. The device keeps the user cool for 30 minutes using a rechargeable lithium battery. The next step is to use the prototype to create a product that can be manufactured cost effectively. Introduction With the advent of new technologies, this idea of personal cooling has become a reality. However, few devices currently exist on the market to fill this niche for portable air conditioners. The most common devices use a sponge that is saturated with water and a fan that draws the hot air across a sponge, which acts as a thermal storage, and then the device blows the air onto the user [4]. These devices are limited in areas with high humidity. Other devices are also currently being developed that use contact thermoelectric modules (TEMs) on the body at both the wrists and neck [1, 3]. The aim of this project is to analyze these existing devices and to create a small, easy to use, energy efficient, and cost effective personal air conditioning product that will keep the user at a comfortable temperature. The final result is a prototype that can be manufactured, is consistent with the customer s provisional patent, and meets necessary federal safety requirements in order to have a successful transition into the marketplace. Copyright 2015 Rochester Institute of Technology
Page 2 Process Customer and Engineering Requirements The design of the Mini-Air was driven by the requirements given to the team by the customer. Through the use of both the project readiness package and conversations with the customer, the needs and constraints of the project were determined and specifically, which of them would heavily affect and influence the design process. The main needs and constraints included: use scenarios, the device hanging from the neck, using a TEM and fan for cooling, low manufacturing cost, and small size. Based on these needs and constraints, the engineering requirement metrics were defined. Throughout the design process and with further conversations with the customer, the final engineering requirements were determined as seen in Fig 1. These requirements were also ranked from one to three, with one being a must and three being a preference only. This process allowed all stakeholders to feel confident going into the design process that the final design would be a useful product. Figure 1: Engineering Requirements Design Process To first understand the interaction of the components with one another, hand sketches were created in order to determine all the possible ways they could be positioned and possible overall shapes that could be used. Then, a mock-up design was made using a small fan, a thermoelectric module and foam. The mock-up helped us to understand what the overall size of our device was going to be, and if we were on the right track with airflow and temperature. This mock-up led to computer-aided design (CAD) models which were used to print out numerous 3D prototypes. The 3D prototypes helped determine what aspects were overlooked as well as what required the most focus. These challenges included the fan underperforming due to a pressure drop in the device and ensuring that the TEM was given enough power to generate the temperature differential we required. The largest pressure drop occurred as the air traveled between the heatsink fins. The air enters through an area the size of the fan but then has to be compressed into the tiny space between the fins. By calculating the pressure drop across the fins [5] and generating a curve based on the fin parameters, the pressure drop curve can be placed on the performance graph of the fan. The point where the fan performance curve and pressure drop curve intersect is the true air flow generated by the fan. Figure 2 below shows the curve for the selected fan and fins. Other losses occur from our design of the flow path and material choice, but these are negligible compared to the fins pressure drop. Another challenge after determining the true air flow from the fan was being able to have the cold air exit fast enough at the user s neck. The air needed to reach the width of the user s neck while still travelling fast enough to feel the cold air. By reducing the size of the cold side exit to a rectangle the width of the TEM and only a few millimeters tall, a blade of air is created that travels to the user s neck. The hot side exit maintains a large area for the air to remain slow and dissipate without noticeably being felt. Project P15441
Page 3 Using the CAD model, a negative solid of the space or air in the device was created and used to run an ANSYS simulation for the heat transfer to both the user and back to ambient air for the cold and hot air respectively. The results helped determine the exit velocity and temperature of the air to meet the requirements. Using the mock-up testing, similar inputs were used to compare with the simulation results. Once it was certain the model was accurate, components were chosen to meet the parameters and the subsequent subsystems were created. Figure 2: Performance vs Pressure Drop curves showing the expected air flow Subsystems The final iteration of the Mini-Air design consists of three major subsystems: thermal, airflow and electrical. The thermal subsystem involves a TEM and heatsinks, the airflow subsystem contains the housing, fan and nozzle, and the electrical subsystem includes the batteries, charging accessories and the printed circuit board (PCB). Thermal Subsystem One of the constraints on the project was to use a thermoelectric module (TEM) to provide the cooling for the device. A TEM uses dissimilar junctions between ceramic plates to generate voltage from a temperature differential or vice versa. The challenge of a TEM is transferring the temperature from both sides of the thermoelectric to the air. The chosen method involves applying heatsinks to increase the surface area of both sides of the TEM. As the air rushes over the heatsinks on their respective sides of the device, the heat rate across the TEM cools the air going to the user and applies it to the air going to the hot side away from the user. During testing, a K-type thermocouple was used to measure and monitor the temperatures. Measurements of the temperature were taken at both the base of the heatsink and at the exit of the nozzle. The probe made it possible to reach into the device while it functioned and with proper placement, measure the temperatures. It was found during testing that the TEM has a range in its quality, leading to some of the received TEMs to operate at a much lower power. The lower the TEM operates, the less heat it will transfer. Using a TEM we found to be within the specification, our temperature readings came out as expected. Copyright 2015 Rochester Institute of Technology
Page 4 Airflow Subsystem The air flow subsystem while drawing the least amount of power, provides an important role. It pushes the air through the device and onto the user. The only thing necessary for determining how much airflow is needed was determining the pressure drop that occurs as the air travels through the device. A schematic of the device and the components layout can be found in Appendix B. To measure the airflow and performance of the fan, an anemometer was used. The air velocity was measured at a distance of four inches from the nozzle exit. This distance was chosen for how far the device would be from the user s neck. Electrical Subsystem The main functions of the electrical subsystem are to interact with the user and provide power management for the system. The final revision schematic for the PCB can be found in Appendix A. All parts in the final design were chosen carefully to lower manufacturing costs. The user interface is composed of a toggle switch, for turning the device on and off/charging, and two LED lights. When the Micro USB charger is connected and the switch is in the charging position, the LED lights will denote battery status. If blinking orange, there is a fault in the battery connection to the circuit. If steady orange, the batteries are charging. If steady blue, the batteries are fully charged to 4.2V. This is accomplished using the open drain status pins of the AAT3620 charging chip. During charging, the chip will control the current to the batteries to maintain a safe charging environment. During constant current mode, charging will occur at 5V, 500mA. When the switch is in the on position and the battery voltage is above 3.4V, power will be supplied to the TEM and fan motor boost converter enable pins allowing power to flow to the loads. When voltage is below 3.4V, the TL431 shunt regulator will disable the two boost converters in order to shut the system down when the batteries have a low charge in order to preserve battery charging capabilities. The TEM boost converter increases the voltage supplied from the batteries to the TEM to 6V in order to supply a current of approximately 1.7A to achieve the necessary temperature differential. The fan motor boost converter increases the voltage supplied from the batteries to the fan motor to 12V in order to supply a current of approximately 160mA. The only major difference between the two converters is that the TEM boost converter and its constituent parts are rated for higher ampacities. In order to meet the engineering requirement for battery life, two 1400mAh lithium ion battery cells were connected in parallel. Two battery cells were used in order to attempt to meet the engineering requirement for size as well. A figure with the results of the battery discharge test can also be found below in Figure 3. The results show that the battery will last at least 33 minutes before turning itself off which is three minutes greater than the marginal requirement. Figure 3: Battery discharge test Project P15441
Page 5 Results and Discussion The final product involved all the components housed in a 3D printed case. Using the testing procedures developed and instruments specified, the results to the tests are listed below in Figure 4. Many of the values fall close to the marginal. The design, shown in Figure 5, minimized all the requirements as much as possible while still ensuring the device could meet what was important. Figure 4: Test Results for components and sub-system Figure 5: Final, assembled design Conclusions and Recommendations In summary, the Mini-Air is a personal device that will keep individuals cool. The device contains a DC fan, a Peltier module, two heatsinks, and a lithium battery within an enveloped housing which provides sufficient airflow to the user. A customized DC fan could improve air efficiency and potentially reduce the size, weight, and power consumption. However, to create a custom fan requires a scope and knowledge of how fan blades operate and need to be made. Battery life in the device was restricted by its size and current, commercially available power density materials. As battery technology advances, a more efficient battery or higher density material could increase battery life and potentially reduce the size. The Peltier module was restricted by size requirements and cost. A higher priced Peltier module would increase efficiency by a small amount but this would not be a cost effective decision. Peltier module technology needs to improve and smaller Peltier modules need to have a higher efficiency. The Peltier module drives many other requirements in the device. A Peltier module that draws less power would significantly increase battery life. Before continuing with the project, the team recommends developing a chart detailing the relationship between ambient air and the power consumption of the TEM. This chart could show the correlation between cooling at various ambient temperatures and give future teams an idea of the battery power and life that would be needed. Based on the battery technology, they could then determine the size of the battery and therefore the size of the device. Copyright 2015 Rochester Institute of Technology
Page 6 Acknowledgements Our team would like to thank Gary Werth for his contributions in guiding our team to success and our sponsors Rick Rubin and Richard Kahn. References [1] "CLIMAWARETM CryothermicTM Scarf." Dhama Innovations. Web. <http://dhamainnovations.com/products/medical-healthcare/climaware/>. [2] "Energy Saver." Energy Saver. Office of Energy Efficiency & Renewable Energy. Web. <http://www.energy.gov/energysaver/>. [3] "Wristify by Embr Labs." EMBR Labs. Web. <http://www.embrlabs.com/>. [4] "Small Fan & Mini Air Conditioner: Stay Cool Anywhere with Handy Cooler." Genexus LLC. Web. <http://www.myhandycooler.com/>. [5] Simmons, Robert. "Estimating Parallel Plate-fin Heat Sink Pressure Drop." Electronics Cooling Magazine Focused on Thermal Management TIMs Fans Heat Sinks CFD Software LEDsLighting. Electronics Cooling. Web. <http://www.electronicscooling.com/2003/05/estimating-parallel-plate-fin-heat-sink-pressure-drop/>. Project P15441
Page 7 Appendix Appendix A - Electrical Schematics Figure A1: Charging block Figure A2: Fan motor boost converter Copyright 2015 Rochester Institute of Technology
Page 8 Figure A3: TEM boost converter Appendix B Mechanical Schematics Figure A4: Full Assembly Drawing, Sheet 1 Project P15441