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Design and development of a wireless networking platform of environmental sensors with Energy Harvesting supported power supply

Gkionis Nikolaos

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Summary

The aim of the present thesis is to design and develop two original environmental sensors systems, with energy harvesting supported power supply. The first system is autonomous and is used in marine environments, and the second is an autonomous, highly expandable and versatile terrestrial system with telemetry capabilities, which can function both as a sensors interface platform and as a node in a sensors network. Both systems can improve their autonomy by incorporating energy harvesting devices. Several different energy harvesting techniques where studied, as well as their impact on the terrestrial system's autonomy and a special software was developed in order to support the retrieval, processing and storage of the data collected by the proposed sensors systems.The first part of the thesis discusses the theoretical background behind the technologies and the basic components used in sensors systems and networks. Furthermore, various energy harvesting techniques are presented, as well as the required equipment needed in order to efficiently harvest energy from renewable energy sources in low power applications.The second part presents the design of the two prototypes: The first system is used for underwater measurements of the water column. It utilizes sensors for conductivity, water temperature, pH, hydrostatic pressure, and turbidity, and its data are stored locally in a flash memory. The second system is a sensors' interface platform, which provides great versatility in the number and type of sensors to be connected. In this thesis, a temperature, barometric pressure and relative humidity sensor, an anemometer, a UV and solar radiation sensor, a dust sensor and a rain sensor are used. The collected measurement data can either be stored locally, or transmitted to the base station via LoRa. A very important aspect of both systems designs was the minimization of their power consumption, by utilizing highly effective power supplies as well as energy saving techniques. The second system also utilizes energy harvesting to further improve its autonomy. Both systems were constructed by using prototype PCB printing equipment.Emphasis was given in the development of prototype energy harvesters as well as their energy management circuits. A photovoltaic, a thermoelectric, an electromagnetic and a piezoelectric "flag type" harvester were constructed. The third part presents the design of the experimental setups used to study and verify the functionality of both sensors systems and the energy harvesting systems. The marine sensors system was placed in a suitable waterproof housing, with the sensors protruding from it, and was attached to a diving cage. The terrestrial system was also placed in a waterproof housing, to protect its electronic circuits, and was attached to a mast together with the peripheral sensors. Furthermore, experimental setups were designed to study the performance of the energy harvester systems, by simultaneously monitoring and recording the harvesters' output and the environmental conditions during the experiment, and also by simulating realistic environmental conditions, where needed.The fourth part of the thesis presents and discusses the results of the conducted experiments. The energy harvesters' analysis showed that the photovoltaic harvester achieved a maximum efficiency of 9.41\%, and its power was sufficient to provide total energy autonomy to the terrestrial system, for sampling periods greater than 10 minutes. The thermoelectric harvester provides less power and it requires significantly larger space in order to be installed, and subsequently is overall inferior to the photovoltaic harvester, especially considering that in the proposed application, both harvesters utilize the same energy source (solar radiation). The piezoelectric harvester proved to be fully functional, however, more piezoelectric crystals have to be used in order for the harvester to have a noteworthy contribution to the autonomy of the sensors system. Lastly, the electromagnetic harvester had a very low output voltage, and therefore, it was not used.Moreover, different power consumption and autonomy scenarios and strategies are discussed for the second system, investigating the effect of the sampling period and the photovoltaic energy harvesting. Furthermore, suitable software was developed for the base station to retrieve and preprocess the transmitted data from the terrestrial system, and to preprocess the data of the marine system. The user can then access the data using the graphical interface of the prototype SensorDataPlotter program, select the time period and measurements of interest, and either plot them or store them in a dedicated file.The fith part presents in detail the conclusions of the thesis and several improvements and extensions to the proposed systems are suggested. Finally, several possible ways are discussed for an improved utilization of energy harvesters, depending on the application's specifications.

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