A contemporary outlook on wireless sensor networks for solar power harvesting

1. Introduction
A recent emerging technology,wireless sensor network (WSN) is the backbone of many advances in the Internet of Things (IoT).The applications of these networks have ranged from disaster management,ecological studies,medical monitoring systems,and air quality metrics [2]. One of the major functions of WSNs currently is environmental monitoring which is characterized by its function to transmit data over a large area wirelessly to the gateway node.Amongst the notable standards for wireless sensor networks commonly researchers cite ZigBee,Z-wave,WiFi, and Bluetooth [2, 3].The novel and inviting features of WSNs are their reliability which gives the system a large operational lifetime,especially when flexible architectures such as wireless mesh are utilized.At the heart of the performance of any WSN are the wireless sensor nodes and their specifications.

At their core, ever wireless sensor node is characterized by its size, stand-alone power supply, its transmission latency and its sensory accuracy. Over the recent years,sensors have become increasingly accurate at miniscule sizes,therefore it can be deduced that the primary bottleneck in the size of a wireless sensor node is its powering mechanism or the size of the battery.It is observed that a significant correlation exists between the useful life of the individual nodes and their duty cycle as well as the volume of data flow within the system.While there are IoT objects that use high power per unit time, these tend to be less sustainable,therefore it is a standard practice to maintain a design constraint of being powered by a readily available source such as 1.5V IEC 60086 size R6 batteries [2].Though WSNs are mainly low power systems that do not need large batteries,Inherently the use of batteries applies a limit on the operational time of the network after which the system will have to be taken out of service to recharge or replace the batteries.This process interrupts the operation where the wireless sensor is being used and in cases where the network is spread over several kilometers the replacement can be time intensive and impractical.Therefore, a source of energy is needed that provides energy at the place of operation itself without the need for interrupting service,human intervention, or connection to grid. Some such viable technologies are stated in the Table 1.

Tabla 1. Energy Resources viable for use in Wireless Sensor Networks.

From Table 1 it can be noted that solar power demonstrates the optimal trade-off between efficiency and power density amongst other comparable sources of energy.Furthermore,it should be noted that the size of harvesters of solar energy are becoming smaller by the day,recent cells using nanotechnology have been manufactured in the order of nanometers.The growing market for solar panel manufacturers also provides ample investment and resources for its integration with WSNs.Maurya et al.have stated that near about 15 mW/cm3 of photovoltaic power can be harvested from solar energy for wireless sensors[2].Converters are used to transform this energy to the DC that drives the internal circuitry of the WSNs. Additionally,advancements in storage technology allows the system to function in conditions that are not favorable for harvesting solar energy.The advantages of solar energy that make them optimal for powering WSN nodes, which are given as follows [1]:

• From a theoretical perspective the output from harvesting solar energy is inexhaustible.
• Solar power tends to show a superior power density than other quotidian renewable energy resources.
• Apart from the production of the solar cells,the pollution and carbon footprint of solar energy is null.
• Solar energy shows compatibility with a most devices having diverse ranges rated power characteristics.
• Photovoltaic harvesters are silent,more durable,and less location restricted than other harvesters.

The major objective of this critical review is to extensively encompass the features of a Solar Energy Harvesting based Wireless Sensor Node,to illustrate the known and potential shortcomings faced by such systems,and review in detail the solutions proposed in various recent studies to improve this technology or extend its application.The remaining portion of this paper is organized as follows:Section II aids in a more comprehensive overview of Solar Energy Harvesting Wireless Sensor Networks and provides an outline of the components involved in such a system.Section III provides the problem description or major considerations in designing an optimal system.Section IV reviews literature which proposes new technology to optimize the designs with respect to the considerations in the previous section.Section V provides some simulation platforms developed to aid in analyzing performance of the Photovoltaic Harvester.Section VI gives the overall summary of the review along with the author’s insight into the most efficient technologies in every domain.

2. Major components of solar energy harvesting wireless sensor network

1. Photovoltaic Harvester Subsystem
   • - Photovoltaic panel / Solar cell
   • - DC Boost Converter
   • - Mechanism for Maximum Power Point Tracking (MPPT)
   • - Addition Intelligent Mechanism for Energy Forecasting
   • - Storage Media
2. Sensory Array
   • - Sensor Development / Interface Board (SENSOR-PUCK, FLORA, Thunderboard Sense)
   • - Analog to Digital Converter
3. Computing and Processing Unit
   • - Central Microcontroller Device (TI MSP430F1232, STM32F103, Atmel ATmega328)
   • - Analog to Digital Converter
4. Communication Unit
   • - Transceiver Module (Zigbee, LoRa, etc)

The sensor unit performs as the central instrumentation platform which detects physical quantities like temperature,light, humidity,or pressure, see Figure 1.The computational unit then processes this data to useful markers that would provide the system with decision making feedback.As per the mesh architecture these data packets of useful information are relayed to nearby nodes until it reaches the central database or gateway node.The end user can then access this information or it can be used to autonomously drive a control system.The energy harvesting unit, which is a unique feature of this build of WSNs,supply stable power at the operating voltage (3V) to the WSN node to allow continuous supply of energy and uninterrupted service.

3. Modern Wireless sensor mechanisms for solar power harvesting
The following section covers the literature survey related to the key aspects of Solar Energy Harvesting Wireless Sensor Networks where the key obstacles lie and can lead to gross optimization improvements in existing systems.

1. Storage Technologies for Wireless Sensors

   Solar energy can be continuously collected and utilized directly by the WSN node,which is known as battery-less operation.This reduces the cost of the solar energy harvesting WSN as well as battery related maintenance.However,it creates contingency issues as power through the sun may become erratic or insufficient in certain environmental conditions.Recent advancements in high efficiency storage devices have promoted supercapacitors to the primary media for SEH storage, whereas traditional batteries are now often only integrated in these systems as a contingency [1].various media for energy storage in solar energy harvesting wireless sensor node have been summarized from various studies in Table 2.

   Tabla 2. Storage Technologies Used in Solar Energy Harvesting Wireless Sensor Nodes

   

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b. Maximum Power Point Tracking Maximum power point tracking (MPPT) has often been availed in solar energy harvesting in order to ensure the maximum amount of power that can be harnessed through the solar panels may be collected.The maximum power point refers to the to the constant voltage level that provides peak power levels being produced in the photovoltaic cell.In most new designs of solar power systems and charge controllers,some version of the MPPT technique is employed.At its core, MPPT technique aims to make the solar module’s resistance equal to that of the load of the circuit or the battery. Generally, MPPT can boost energy generation by 20%–30% [9, 13].All MPPT algorithms fall into the following wide classifications:

- Perturbation and observation (P&O): This method alters the operational parameters directly to try to achieve the MPP.This algorithm can be very complex and has been modelled to have optimal performance in various studies but essentially it boils down to the sequence shown in Figure 2a.

- Incremental conductance:This algorithm has been visualized in Figure 2b. Essentially it operates by comparing the incremental conductance of solar energy harvester to the instantaneous conductance.Depending on the feedback, which is either a positive or a negative value,it varies the voltage until a stopping condition (MPP) is reached. Unlike the first method once the stopping condition is met there are no further alterations in the voltage.

c. Solar Tracking for Wireless Sensors As mentioned in the earlier sections of this paper,the main constraint with respect to WSN nodes is their size.Therefore,until recent times tracking,which is common in many commercial PV systems,was not used as the energy used by the tracker would be more than the energy harvested by the system overall.This would put the design into an energy debt and therefore was mostly deemed infeasible.However,many studies in recent literature have proposed trackers that ensures minimal energy is lost with low power consumption which are ideal for WSN nodes.Some of these studies are highlighted in Table 4.

Tabla 3. Novel Solar Trackers in recent literature

Source:[12-14]

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4. Overall summary with highlights of best noted technologies in every domain
Finally,the Table 7 summarizes the domains of literature studied as part of this survey on solar energy harvesting for wireless sensor nodes.Overall,the main features that have led to development in more efficient,long-lasting and compact solar energy harvesting nodes have been storage technologies,maximum power point tracking, solar tracking,intelligent algorithms and developed simulation platforms.The table summarizing this information also provides a critical evaluation of the most optimal and novel technology that has been studied in literature (as per analysis by the author).The reasoning as to why this particular technology has been chosen is also provided.

Tabla 4. Overall Summary of Review with Critical Analysis

5. Conclusiones
Solar Energy harvesting continues to make its way into the market as the energy resource of the new age. The use of solar power for niche applications such as for powering WSN nodes, proves its versatility as a power source even for smaller machines. This survey provides the following contributions.The fundamental concepts and components of a Solar Energy Harvesting based WSN system have been detailed.This is followed by a review of the challenges and design considerations that are currently experienced in building such systems.Finally, efforts at tackling these challenges made in recent notable literature have been reviewed comprehensively.The future scope of Solar Energy Harvesting based WSN systems is very promising in the field of smart homes,smart cities, ecological monitoring, process control and automation in industries,and IoT applications.This survey is aimed at serving as a reference for future engineers,researchers,and scientists while designing and manufacturing improved Solar Energy Harvesting based WSNs.