Abstract
The aim of this paper is to present the viability of an energy-harvesting system prototype, based on thermoelectric generators (TEGs), to be embedded in a Long-Range Wide Area Network (LoRaWAN)-based wireless sensor node, allowing continuous quasi-real-time data transmission even for low temperature gradients and for frequent transmissions. To this end, an RFM95x LoRa module is used in the system. The energy management of the entire node is achieved by exploiting a nano power boost charger buck converter integrated circuit, which allows power extraction from the energy-harvesting source and, at the same time, regulates the charging/discharging process of a Li-Po battery that supplies the wireless node. The first phase of the project was dedicated to understanding the electrical characteristics of the TEG. A series of tests were performed to study the open circuit voltage, the current and the power generated by the TEG at different temperature gradients. Following this first phase, tests were then set up to study the charging/discharging process of the battery by changing two crucial parameters: the temperature between the faces of the TEG and the frequency of the transmissions performed by the transceiver. Experimental results show a positive balance for the battery charging at different conditions, which suggests two important conclusions: first of all, with high temperature gradients, it is possible to set relatively high transmission frequencies for the LoRaWAN module without discharging the battery. The second important consideration concerns the operation of the system at extremely low temperature gradients, with a minimum of 5 °C reached during one of the measurements. This suggests the usability of thermoelectric energy-harvesting systems in a wide range of possible applications even in conditions of low temperature gradients.
Highlights
The widespread diffusion of Internet of Things (IoT) networks has always gone hand in hand with the study of new techniques for energy provisioning and with the development of devices and programming techniques based on a low power perspective.A crucial aspect of every IoT node, is its energy autonomy, which represents a major challenge for those applications deployed in remote and hostile areas, without access to the energy grid and where the human intervention must be limited
A significant aspect that emerged during the tests is the autonomy of the system at relatively low temperature gradients: the values used during the last tests are available in a plethora of scenarios, from rural areas to industrial facilities
The adoption of Long-Range Wide Area Network (LoRaWAN) technology offers different advantages in terms of power consumption and employment: thanks to the LoRaWAN performances, relatively high transmission rates are achievable for larger temperature gradients, close to the limits posed by the law regulations related to the system duty cycle which has to be lower than
Summary
The widespread diffusion of Internet of Things (IoT) networks has always gone hand in hand with the study of new techniques for energy provisioning and with the development of devices and programming techniques based on a low power perspective.A crucial aspect of every IoT node, is its energy autonomy, which represents a major challenge for those applications deployed in remote and hostile areas, without access to the energy grid and where the human intervention must be limited. The low cost and the reliability over long periods of time are among the most important requirements for the IoT nodes; energy-harvesting systems can be crucial to extend the average life of the battery and, of the entire node. A preliminary study of the application scenario needs to be carried out with the objective to find the most suitable harvesting source. The fact that TEGs only need a temperature gradient to operate makes them suitable solutions especially for indoor applications, for which other powergenerating systems cannot be conveniently used because of poor illumination, absence of winds, surfaces subjected to vibrations, etc. The design simplicity and the high scalability of TEGs makes them applicable to heat sources of considerably different sizes and temperature gradients. Other strengths are the long life span and the fact of being environment friendly
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