A Wireless Sensor System Supplied by a Solar Energy Harvester toward IoT Environmental Monitoring

Harmful environments can cause seriously health problems to humans. Thus, it is should be develop a generation of wireless sensor systems that are energy self-powered to monitor physical parameters of an ambient environment in real-time and sending these parameters remotely to an IoT cloud service. In this paper, a wireless sensor system is proposed for an environmental monitoring. This system is based on two sensors and a NodeMCU board that includes a microcontroller with a Wi-Fi chip. This system is built to measure the ambient temperature, relative humidity, atmospheric pressure, and ultraviolet (UV) index. The power supply of the sensor system is a solar energy harvester, which consists of a solar cell, a DC-DC converter, and a rechargeable battery. This harvester is practically tested outdoors under direct sunlight. The wireless sensor system experimentally consumes an average power of 40 mW over one hour and the life-time of this system is 123 hours in active-sleep mode. The results demonstrate that the wireless sensor system has long-term and sustainable operation of the monitoring of an environmental data .


INTRODUCTION
In the last decade, wireless sensor systems had a robust effect in environmental monitoring applications [1,2]. These systems are extensively utilized to measure an environmental parameters in real-time [3][4][5][6]. Batteries are in often utilized to power the sensor systems, but they have a bounded life-time [7,8]. Energy harvesting (EH) methods are utilized to extend the life-time of the batteries [9][10][11][12]. These methods utilize various energy sources for instance solar [13], wind [14], thermal [15], mechanical [16], and radio frequency [17]. Solar energy is characterized by the high power density compared to other energy sources [18]. However, the illumination rates of solar energy are in instantaneous change. So, energy-storage elements and DC-DC converters are needed to store and regulate the power extracted via harvesting methods.
Two storage-elements "a super-capacitor as well as rechargeable battery", which are utilized as energy storages. Super-capacitors have lower energy densities than rechargeable batteries [19,20]. So, in this paper, a battery is utilized. Moreover, a harvester depends on the solar was implemented to overcome the generic battery problem of the systems [21][22][23][24][25]. This proposed harvester grants the sensor system a more long-term life-time.
In the previous researches, different sensor systems were presented for environmental monitoring applications [26][27][28]. S. Senivasan et al. [26] implemented a harvester based on solar energy to power a wireless sensor network mote, which includes two NiMH batteries, a voltage regulator, and a temperature sensor, the power consumption was 94.29 mW; moreover, its lifespan of 38 hours, and the solar cell was large in the area. J. C. Lim et al. [27] developed a sensor system, has a rechargeable battery with a capacitance of 1500 mAh. This system had a ZigBee module and a temperature sensor, consumed a high power of 91.41 mW, as well as worked for forty hours. L. Joris et al. [28] introduced a sensor system has a sigfox module and a SHT21 sensor, the supplying was a 90 mAh battery, the system consumed power of 107.7 mW and the life-time was 20 hours. This paper proposes a self-sustainable wireless sensor system for the perpetual monitoring of environmental parameters. The primarily goal of the proposed system is to prolong the life-time of wireless sensor systems. A low-power software algorithm is developed to enhance the system life-time to be 123 hours. Thereby, the proposed system could be work without changing the battery. Additionally, the system is powered depending on a perpetual solar energy harvester to reach a higher life-time for the wireless sensor system. A rechargeable battery has high energy density-based the harvester. The system is implemented with the testing outdoors and consumes 40 mW.
The sensor system measures the four physical parameters of ambient temperature, relative humidity, atmospheric pressure, and ultraviolet (UV) index. The data of the system are gathered by two sensors, monitored on a serial monitor application and sent to a cloud service via Wi-Fi. The primarily contributions of the paper are in two points:  Development of a self-powered wireless sensor system for continuous monitoring of ambient temperature, atmospheric pressure, relative humidity, and ultraviolet index;  Design a solar energy harvester to power the system depend on a solar cell and a rechargeable battery with saving the power consumption of the system using sleep commands and low-power hardware components; The paper is organized as follows: Section 2 introduces the proposed architecture of the solar energy harvester and sensor system. The hardware implementation of the wireless sensor system with solar energy harvester is described in Section 3. The results are demonstrated in section 4. Section 5 discusses the proposed work. The conclusion of the paper is introduced in section 6. Fig. 1 demonstrates the proposed structure of the solar energy harvester as well as the wireless sensor system. This structure includes a solar cell to detect the sunlight irradiance. It also has a DC-DC converter to step-up the solar cell output and a rechargeable battery is used as energy storage and a NodeMCU board for processing the physical signals of two sensors: One to measure the humidity, temperature, and pressure, the other sensor to sense ultraviolet index. Further, a Wi-Fi to transmit an environmental data to a cloud service. Eventually, the environmental data of sensors are monitored on a serial monitor application. Fig. 2 illustrates the schematic diagram of the harvester with the sensor system. In this Figure, the utilized solar sell is a MPT 3.6-75, which is connected to the input of a LTC3105 DC-DC converter to charge a

Simulation Model of the Solar Energy Harvester
The designed solar cell circuit and the DC-DC converter of the solar energy harvester is demonstrated in Fig. 3. The circuit is simulated by the LT-Spice software. Additionally, C3 and C2 are linked to the LTC3105 utilized to smooth the voltages of output and input, L1 is a coil to charge the input current and the Vout of the converter is fed to a suitable rechargeable battery.
(2) Fig. 3. The designed solar cell circuit connected to the DC-DC boost converter.

Calculations of Energy and Lifetime
The life-time and energy consumption of the sensor system are theoretically calculated in case the no connection of energy harvester. Suppose the proposed system works in the active-sleep mode, and the process time cycle (T) of the system is chosen to be 60 minutes (3600 seconds), is comprised of 600 seconds active time (tactive) and 3000 seconds sleep time (tsleep) and the operating voltage (V) of 3.3 V. Fig. 4 demonstrates a public power profile for the wireless system. The Active period tactive is equal to (tactive -0), the Sleep period tsleep is equal to (T-tactive), where T = tactive + tsleep.
The overall capacitance of the rechargeable battery is supposed to be 3800 mAh, From the solar cell, the average energy EPV is obtained via Eq. (6). W is the sunlight irradiance, ƞ is the solar cell efficiency, A is the area of the solar cell, it is supposed 0.00432 m 2 , and tPV is the time that the solar cell is giving energy. Let, the cell efficiency is 7 %, the solar cell operates for six hours/day, and the irradiance of 1000 W/m 2 .
Thereby, the EPV of the solar cell is 6531.84 Joules. Additionally, the charging time (TCharge_PV) from the solar cell to battery is 0.022 days (0.52 hours), as obtained via Eq.
(7). So, the discharging time of the system will be higher than the charging time of the harvester. Thereby, the solar cell of harvester is perpetual for the sensor system.

Hardware Implementation
The setup of wireless sensor system with energy harvester is demonstrated in Fig.   5. It is implemented using the following hardware components "solar cell, DC-DC converter, rechargeable battery, BME280 sensor, GY1145 sensor, and NodeMCU board". This board has total area of 3.0 cm × 7.0 cm.

Fig. 5.
The setup of the wireless sensor system with energy harvester.

Software Implementation
The flowchart of the wireless sensor system is described in Fig. 6. It comprised of sequential steps. The BME280 and GY1145 sensors are configured, then I 2 C communication protocols are initialized for the mentioned sensors. Then, an initialization is carried out to the Wi-Fi, then, the NodeMCU is identified on the Ubidots platform. The NodeMCU of the system working in two modes: active and sleep. So, the sensor system is worked in the active mode. Then, the sensor system verify a connection of the network of Wi-Fi, if its status is equal to one, the NodeMCU continuously reads the environmental data of the utilized sensors. After sending these data to the Ubidots platform for 10 seconds through the Wi-Fi and then, the sensors are turned off, the Wi-Fi goes into sleep, then, the NodeMCU board is transferred into the sleep mode for a duration of 50 seconds. A serial monitor application is used for monitoring the data, which was received from the system. In active mode, the BME280 and GY1145 sensors are configured and I 2 C protocols are initialized to these sensors.
The NodeMCU board also is configured. Finally, the cloud service receives the data and also the serial monitor application is used to display the ambient temperature, relative humidity, atmospheric pressure, and ultraviolet (UV) index. Fig. 6. The flowchart of the wireless sensor system.

Wireless Communication
The Wi-Fi chip type "ESP8266MOD" is utilized. The performance of the Wi-Fi chip depends on many factors for instance, data rate, energy consumption, and security.
The Wi-Fi has power consumption of 70 mA at active mode, as well as its data rate is 72.2 Mbps. The Wi-Fi is secure in transmission for certain data from a device to others.
It provides multi security modes in authentication and data encryption and authentication. The operating frequency of the Wi-Fi is 2.4 GHz. It is a wireless protocol convenient for wireless systems [35].
In the proposed system, the utilized Wi-Fi is version 5.8, and it can save energy; the Wi-Fi is experimentally tested and the measured current consumption was 0.2 mA at sleep mode, thereby, the Wi-Fi protocol is efficient. Moreover, the utilized Wi-Fi is so suitable for energy harvesting (EH) systems and convenient with wireless systems including sensors. Wi-Fi chip compatible with ASCII commands, which is utilized in configuring communications. The utilized protocol in the Wi-Fi was depend on the IEEE 802.11 b/g/n. additionally, the data of utilized sensors connect to the NodeMCU board of the wireless sensor system through I 2 C protocols.

RESULTS
From the simulation of the solar energy harvester, Fig. 7          From this perspective, one can notice the environmental information by a direct method. In the First, the ultraviolet index reading of an environment is shown to be 0.00. Also, the relative humidity reading is 31.79% and the ambient temperature reading of an environment is monitored at 36.56 °C. In addition, the atmospheric pressure is 1008.28 hPa. Then, the ultraviolet index, and relative humidity, ambient temperature, and atmospheric pressure are 0.00, 31.71%, 36.60 °C, and 1008.26 hPa, respectively. So, these data would be changed continuously. Thereby, this app provides humans information foe an environmental status of an ambient environment over the time. Fig. 11. A screenshot of the monitored physical environmental data in the serial monitor application. Table 1 illustrates comparisons between recent wireless sensor systems. In the proposed work, the developed system has 2 sensors (BME280 and GY1145), but the systems in [26][27][28] have only one sensor. Numbers of physical parameters of the proposed work is 4, while the systems parameters in [26][27][28] were less. The utilized Wi-Fi technology has a higher range than the technologies in [26,27]. Additionally, the life-time of the system is the highest compered to researches [26][27][28], as well as the system power consumption is fewer than works in the literature [26][27][28]. Various tradeoffs in these systems for example, energy storages types "super-capacitors and batteries" which have difference in energy density, size, and charging cycles. Also, a challenge among amount of environmental data sent and power consumption from the system and a trade-off among various wireless technologies to send environmental data whether in long or short range as well as the power consumption of these technologies.

CONCLUSION
This paper introduces a perpetual solar energy harvester for supplying a wireless sensor system towards environmental monitoring. The solar energy harvester is designed to stretch the life-time of the wireless sensor system from 2 to 123 hours. The power consumption of the system is 40 mW. The sensors' data of the system are instantaneously monitored on the serial monitor application. The system provides a strolling solution for sustainable monitoring of an ambient temperature, relative humidity, atmospheric pressure, and ultraviolet index. In the future, we will consider a battery has a higher capacity than 3800 mAh. Further, testing the power performance of the solar cell at different bending angles. Additionally, a product of the wireless sensor system could be manufactured using a printed circuit board.

Conflict of interest
There is no conflict of interest regarding the publication of this paper.
Funding No funding was received for this manuscript.

Data availability
The paper has no associated data.