Elsevier

Renewable Energy

Volume 120, May 2018, Pages 478-487
Renewable Energy

Conversion of atmospheric variations into electric power – Design and analysis of an electric power generator system

https://doi.org/10.1016/j.renene.2017.12.080Get rights and content

Highlights

  • An electro-mechanical power generator is proposed for a phase change vapor based atmospheric energy harvester.

  • Mathematical models for the system are identified and analyzed through dynamic simulations.

  • Observed power generating capabilities are indicative of energy harvesting and use in operating microelectronic circuits.

Abstract

Given its abundant availability, ambient thermal energy harvesting has the potential to power standalone microelectronic systems. The challenge in efficiently harvesting temperature and pressure variations is the low thermal to electric conversion ability of current harvesters. Most thermal harvesters require high temperature gradients. This paper presents the design, analysis, and implementation of an energy harvesting system that effectively harnesses naturally occurring temperature variations using ethyl chloride filled mechanical bellows. A mechanical drivetrain scales the bellows displacement and a coil spring stores the potential energy. This energy is periodically released and converted into useable electric power by a DC generator. A series of mathematical models are developed and accompanying numerical analyses completed on the harvester system. For a low frequency sinusoidal temperature cycle of ±1 °C about 22 °C, 9.6 mW of electrical power was produced using a 1.5 V micro DC generator for a 24 h harvesting period. The power generation capacity of the proposed harvester is sufficient to indefinitely operate low power sensors and microelectronics in environments with small temperature gradients.

Introduction

The process of harnessing or “capturing” energy from ambient sources that are natural or artificial, and converting it to useable electric power is referred to as energy harvesting. Strict environmental regulations and rising interest in power capability of electronic devices, wireless sensor networks and autonomous devices have created a rising market for energy harvesting technologies. Currently, mass manufactured energy harvesting devices are targeted to run a range of low-power and mid-power electronic equipment. Energy harvesting allows implementation of self-sustaining, portable smart devices that have increased life and minimal maintenance requirements [1]. These energy harvesters can facilitate the use of smart computers, low power sensors [2] and LED lighting systems in remote regions throughout the world with minimal battery storage requirements. Ideally energy harvesters are compact, miniature systems that can be integrated into devices without significant structure or space requirements. Advanced material technologies, micro-manufacturing and three dimensional (3D) printing have enabled the development of micro components that satisfy functional requirements, at lower costs. Energy storage techniques have also received considerable attention in offering dense power capacities for uninterrupted power when coupled with energy harvesting systems.

The power requirements for wireless sensors can often be satisfied with energy scavenging strategies including photovoltaic cells, mechanical vibrations, and atmospheric variations [3]. Commonly used piezoelectric, electrostatic and electromagnetic harvesting devices have found applications in low-power personal devices. Ambient temperature variations can be harvested for electric power generation but at a lower frequency consistent with natural cyclical behaviors. Thermal energy harvesters have the advantage of power scalability but the disadvantage of space requirements. Typical thermoelectric harvesters operate on the Seebeck effect and produce very small voltages [4]. Some thermoelectric harvesters studied for low-power autonomous electronics were observed to be inefficient in converting large temperature gradients into useable electric power [5]. One approach to increase the power output is to use vapor pressure changes to harvest atmospheric energy. The proposed design incorporates an intermediate mechanical drivetrain linked to a coiled spring storage system between a thermal driven bellows and an electric generator. J. L. Reutter patented a mechanism for converting temperature and pressure variations of a vapor into mechanical motion in 1928, which was implemented in the Atmos clock [6]. The clock operation depends on atmospheric temperature and/or pressure variations for timekeeping; originally designed to operate with mercury, it now uses ethyl chloride within the motor [7]. Rawlings [8] explored the dynamic forces associated with time keeping gear trains and discussed the effect of energy losses, including friction, on system performance. The auto-winding mechanisms, gear arrangements, and mechanical motor systems involved in mechanical clock movements are now being widely implemented in energy harvesting systems for human motion [[9], [10], [11], [12], [13]]. Various methods have been proposed to overcome the hurdles of variable motion requirements and size constraints [14] in applying conventional piston-crank mechanisms as motion rectifiers in energy harvesters. For example, rectifier beams for small amplitude-high frequency motion [15], rack and pinion based systems for varying amplitude-low frequency motion [16,17], and other motion rectification systems have been integrated into vehicle shock absorbers, building dampers, and ocean buoys to generate energy [18,19]. Ali et al. [20] used an ethyl-chloride filled mechanical bellows with a return spring to harness atmospheric temperature variations. The resulting mechanical motion of the bellows, due to the thermodynamic behavior of the vapor was slow and irregular since it followed the thermal variations. It was unable to effectively transfer energy directly into a micro generator [21]. Therefore, the proposed system was connected to an energy storage device and then interfaced to the micro generator for electric power production. The proposed energy harvester offers versatility in that the electric generator has been decoupled from the energy harvester, to allow accumulation of mechanical energy from multiple atmosphere cycles and then released for a controlled burst of electrical power production. The research problem was to identify whether small temperature and pressure gradients may be harvested and efficiently used to power microelectronic systems.

The renewable energy device consists of an ethyl chloride filled mechanical bellows which is linked through a mechanical drivetrain to store energy from low frequency temperature variations in a coil spring over a defined time period. The mechanical drivetrain serves two purposes. First, the translational motion of the bellow's end plate is converted into rotational motion that is accommodated by the storage element (e.g., coil spring) [22]. Second, the gear ratios scale this displacement to optimize the potential energy stored in the coiled spring for a given charging cycle profile. The release of the stored potential energy in the spring winding is controlled and transferred to a DC generator to provide sufficient voltage and current for battery charging. A ‘hold and release’ ratchet mechanism periodically engages and disengages to isolate and engage the DC generator circuit and the storage component. The design of the drivetrain, spring and generator (refer to Fig. 1), may be optimized into a compact and portable unit with size scalability to accommodate bellows of varying sizes.

The feasibility of attaching an electric generator with an accompanying mechanical driveline, to the atmospheric sensitive bellows will be evaluated through studies based on dynamical models in Matlab/Simulink. The remainder of the paper is organized as follows. Section 2 describes the energy harvester system in terms of the mechanical bellows, drivetrain and coil spring assembly, and electrical generator. A comprehensive lumped parameter system mathematical model is presented in Section 3. Representative numerical results are offered in Section 4 followed by the conclusion in Section 5. Simulation graphs are presented in the Appendix.

Section snippets

Design of energy harvester

The concept of using an ethyl chloride filled mechanical bellows to harvest atmospheric temperature and pressure variations with conversion to useable mechanical energy was first implemented in Atmos Clocks. Ethyl chloride is a stable gas (CAS 75-00-3) that has a boiling point of 12.3 °C at standard atmospheric pressure [[23], [24], [25]]. At room temperatures, the vapor pressure (refer to Fig. 2) is sufficiently low to be safely used in small mechanical systems with thin metal bellows, similar

Mathematical model

A lumped parameter mathematical model can be derived to describe the dynamics of the energy harvester system. The process may be divided into three sections – thermodynamics, mechanical dynamics and electrical dynamics.

Numerical results

The energy harvester mathematical model was implemented in Matlab/Simulink to investigate the system dynamic behavior. To drive the system, a sinusoidal temperature profile was applied to the bellows' ethyl chloride vapor. The resulting pressure changes were converted into a force that actuates the bellows expansion and contraction. The bellows end plate provides a bidirectional displacement to the rack. This displacement is converted into unidirectional rotational motion and supplied to the

Conclusion

The proposed energy harvester is a self-contained device that operates on atmospheric temperature and pressure variations to produce “clean” electric power. Although the amount of power is relatively small, it is sufficient to run miniature electronic circuits. The integration of the mechanical drivetrain, storage spring, and DC generator with the thermodynamic driven bellows has been mathematically modeled and numerically simulated to explore the functionality of the system. Such a green

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