Conceptual Design of a Stirling Engine-Powered Phone Charger
with Nathan Dekray, Logan Esser, Nicola Imponenti, Taryn Jones, Christopher Nevins, Charles Gardner, and Brad Wallace. Completed for UF’s Senior Design course in Spring 2016.
Overview
For our final project in UF's Senior Design class, as a team of 8 our goal was to thoroughly design and analyze a phone charger powered by a Stirling engine. This included the mechanical design, material selection, and manufacturing analysis of the system. We completed kinematic, dynamic, thermodynamic, and heat transfer analyses to investigate the potential power output capability of the proposed device, which ideally produces enough power to charge a cell phone from a flame causing natural expansion/contraction of air in a piston chamber. We also completed assembly, cost, and safety analyses. I lead the mechanical design, kinematic, and dynamic analysis of the device.
What is a Stirling engine?
A heat engine is a system that converts the thermal energy of some heat input into usable mechanical energy. A Stirling engine is a heat engine that operates on the cyclic expansion/compression of a working fluid, such as air. The cycle is driven by the temperature difference between a “hot” and “cold” side. The expansion of the fluid drives a power piston, which in turn drives the rotation of a flywheel. The momentum of the flywheel rotation completes the cycle, aiding in the compression/expansion of the fluid. During the heating (and cooling) portions of the ideal cycle, heat is extracted from (and stored in) a regenerator, which is contained within the closed system. The regenerator is a heat exchanger that captures a portion of the thermal energy that would otherwise be output to the environment in order to increase the overall cycle efficiency. This feature differentiates the Stirling engine from other hot air engines.
Design
Our design was based on a gamma-type Stirling engine configuration, modified with a Ross Yoke mechanism to reduce lateral loading on the pistons and thus ensure greater conservation of linear momentum. The heat source used was a standard candle/alcohol burner, which applied heat to a hot cylinder made of brass, which housed the displacer. The displacer in this design doubled as a regenerator, made of a tightly wound copper mesh. The piston housing was made of cast aluminum, and was separated from the hot cylinder by a thermal insulator in an effort to minimize heat transfer between the hot and cold sides. Once the engine was heated and started, expansion of the air inside drove the power piston, which turned the flywheel. The flywheel was coupled to a hub on the generator shaft via belt, with an effective gear ratio of 0.66 (speed increased at the generator side). The generator was wired to buck converter-USB interface, which regulated the voltage and current output to 5W. Clear plastic safety covers were used to keep the user safe from the rapidly moving parts, and a thermal cover was placed around the heat source to protect the user from the open flame.
Kinematics & Dynamics
Beyond the mechanical design, I was responsible for a full analysis of the kinematics and dynamics of the device. These steps are critical as they bridge the gap between the thermodynamic input stage (i.e., heat causes air in the chamber to expand, causing increased pressure on the power piston), and the mechanical power output stage (i.e., flywheel rotation) that drives the generator and charges the device. I generated kinematic and dynamic models for all components in our design, and developed a simulation in MATLAB to analyze the mechanical power output to different heat input conditions.
