Energy Harvesting across Temporal Temperature Gradients using Vaporization

Abstract

Energy harvesting is an attractive alternative to carrying onboard power for mobile robots, especially for long duration missions. While solar is a powerful option, alternatives are needed for situations where direct sunlight is unavailable. One intriguing concept was proposed in the 17th century to power clocks: energy harvesting based on temporal, rather than spatial, temperature gradients, using a low boiling point fluid that vaporizes at ambient temperatures. This concept has many strengths: it offers all-in-one energy harvesting and storage; direct high-force and large displacement mechanical output, eliminating the need for a motor; and temporal gradients are ubiquitous, due to diurnal thermal fluctuations. The challenge for robotic applications, however, is to create large enough amounts of work in a small enough package to power a mobile device while using a non-toxic and readily available working fluid. Here we present a simple, low-cost energy harvesting actuator, powered by the vaporization of butane and isobutane, with an isobaric energy density of up to 38000 J/m3 (i.e. energy extracted per total volume expansion) each time the temperature fluctuates 13.1°C, enough to power a small car to drive 10m. Two principles enable this: i) precompression of the working fluid, allowing us to tune the boiling point and choose among many non-toxic fluids that do more work than non-compressed fluids; and ii) a constant force profile of the return springs, allowing more work than a linear spring. We present a simple model of the actuator and experimental results characterizing its behavior. Our work lays the foundation for energy harvesting across temporal temperature gradients using vaporization as a viable option for powering mobile robots.