Hybrid solar and piezoelectric power supply system for IoT environments
Author: Aljaž Mlinarič
Mentor: Matej Rojc
Degree: Level 2 study program Electrical Engineering, course Electronics
Date: October, 2021
DKUM: ALJAŽ MLINARIČ
Abstract
In this master’s thesis, we present methods for obtaining energy from the environment and procedures for transforming the energy into electric power. We detail the structure, properties, operation and use of piezoelectric sensors for use in the IoT systems. In the thesis we also developed a system for efficient energy harvesting from the environment using solar cells and piezoelectric sensors, which can produce energy during cycling, and can be converted into electric power. Together with solar cells, we have developed a hybrid power supply system for powering a mobile sensor system with a connectivity for smart cities that can be installed on the bicycle, and serves to measure environmental parameters such as temperature, humidity, carbon dioxide and organic compounds anywhere in the smart city.
Smart City
Technological advancement in the world today allows people to be connected with the environment and devices more than anytime before, and that information can be accessed even globally almost instantly. The amount of information that we are dealing is increasing, therefore, it is important to acquire statistically relevant information, patterns and forecasts, which can ensure more efficient management of the environment. It is also very important that we can access this information anytime, wherever we want. In addition to connecting with users, all Internet applications have a common concern about the power for running such systems. Depending on the purpose, developers still need to limit functionalities by considering the available power of the embedded systems in IoT.
In the master’s thesis, we wanted to present several methods for obtaining electricity from renewable energy sources in the IoT environment. In this context, we also proposed a hybrid system for generating electricity from the energy in the environment by using solar cells and a piezoelectric sensors. As part of the task, we also want to determine which piezoelectric sensor would be most suitable for use in such systems. We also want to see how the sensors can be installed, for example, on the wheel, in order to obtain the most electricity during cycling.
Using the proposed hybrid system, we want to store as much electricity into a suitable battery-powered system, so that it will be sufficient for mobile measuring system to monitor several environmental parameters in a smart city. The system has also LoRaWAN communication module, which can be used to report in real-time the status of a specific system and about captured sensory data even at larger distances in the smart city.
With a large number of embedded devices, we can create powerful concepts for a smart city. With the help of a large number of sensors and machine learning, we can analyze real-time data throughout the city, and according to the results we can determine patterns on the basis of which systems can automatically react, even before the event occurred. With the help of such systems, operators in places can predict traffic jams, electricity consumption, public security can increase etc.
Energy Harvesting from IoT environment
The acquisition and conversion of solar energy into electricity is now in place for one of the most effective methods for obtaining and converting energy from the environemnt. The basic energy generation module is a photovoltaic generator. The basic part of the photovoltaic generator is a solar cell or a photovoltaic cell, which consists of slices of a semiconductor or a mixture of semiconductors. Solar cells are most commonly made of silicon. Monocrystal solar cells have high efficiency, but they are expensive. The polycrystal solar cells, in comparison with the monocrystal ones, are less efficient and therefore cheaper. There are also amorphous solar cells, solar cells CdTe and Culnse2.
When the piezoelectric material is exposed to pressure, stress or vibration, due to the polarization of ions within the crystal, a charge that generates the electrical current is generated, and a voltage drop is also created. This phenomenon is called a piezoelectric effect and appears in solid substances, such as crystals, ceramics and biological substances (e.g. bones). Piezoelectric elements have the largest energy density on a given surface, so in the case of power supply for embedded systems, they can be helpful. The problem of such materials is still a high price.
Development of hybrid power supply system
The mobile weather station contains solar cells that are small enough to be installed on the bicycle, and together with the selected piezoelectric sensor form a hybrid power supply system. The selected piezoelectric sensor was attached to the bike so that we obtained as much electrical energy as possible. The hybrid system powers a battery with sufficient capacity for powering a microcontroller, sensors for measuring the environmental parameters, which we capture while cycling. The battery capacity is checked using the sensor that measures the battery’s voltage. We send all the measured data using the Lorawan module to the web server.
We decided for the FS-2513P piezoelectric sensor, from MIDAS manufacturer. The sensor is built of PVF2 material. The manufacturer specifies that a sensor has a resonance frequency of 80 Hz, where the sensor has high sensitivity and capacitance of 660 pF. The sensor measures 72 mm x 14 mm. This sensor is very flexible, resistant to moisture and is flexible. The FS-2513P sensor is also relatively cheap. When the piezoelectric sensor is exposed to mechanical load, the sensor produces one-way electrical voltage. If the load is removed, it creates the sensor electrical voltage in the opposite direction. If the sensor exposes recurrent mechanical load, an alternating voltage is generated, which has approximately the same frequency, such as the frequency of the recurring mechanical load. Therefore, the obtained alternating voltage must first be processed by using a diode bridge. The resulting voltage is then smoothened with a capacitor, and then converted it to the appropriate voltage using the DC/DC converter.
We decided for the FS-2513P piezoelectric sensor, from MIDAS manufacturer. The sensor is built of PVF2 material. The manufacturer specifies that a sensor has a resonance frequency of 80 Hz, where the sensor has high sensitivity and capacitance of 660 pF. The sensor measures 72 mm x 14 mm. This sensor is very flexible, resistant to moisture and is flexible. The FS-2513P sensor is also relatively cheap. When the piezoelectric sensor is exposed to mechanical load, the sensor produces one-way electrical voltage. If the load is removed, it creates the sensor electrical voltage in the opposite direction. If the sensor exposes recurrent mechanical load, an alternating voltage is generated, which has approximately the same frequency, such as the frequency of the recurring mechanical load. Therefore, the obtained alternating voltage must first be processed by using a diode bridge. The resulting voltage is then smoothened with a capacitor, and then converted it to the appropriate voltage using the DC/DC converter.
Mobile weather station for IoT environments
The hybrid system powers a mobile weather station. Weather stations are usually stationary and can send weather data only from one location. Our idea is that the user of the bicycle with an installed mobile weather station around the smart city can collect temperature data, humidity, air pressure, CO2 values from several locations. The weather station then sends captured information to the server by using a suitable connectivity. The mobile weather station is powered by a lithium-ion battery 18650 Samsung with a capacity C = 2500 mAh. In order to increase the life of the battery and maintain the original capacity as long as possible, the battery with a piezoelectric sensor must be charged with a constant current, and then at a constant voltage, or CC/CV. The sensors and the Lorawan module are powered by 3.3 V. Such a stabilized voltage is obtained from the DC/DC voltage converter TPS63001.
We have created a PCB with a LoRaWAN communication module and a sensors for measuring moisture, temperatures and air pressure BME280, sensor for measuring organic compounds and CO2 values MADO4410. There are also connectors for connecting solar cells and a piezoelectric sensor. On the underside there is a battery holder. With SSHI circuit, we increase the generation of electrical power by using a piezoelectric sensor. On the PCB, we also added circuit to prevent the reverse polarity of the power supply. We also created a module for tracking the point for maximum power from the solar panel. In order to protect the hybrid power supply system with the mobile weather station from weather conditions, we put it in a dedicated housing.
Discussion and results
