What This Book Is About
With the recent advances in wireless communication technologies, sensors and actuators and highly integrated microelectronics technologies, wireless sensor networks (WSNs) have gained worldwide attention by facilitating the monitoring and control of physical environments from remote locations, which can be difficult or dangerous to reach. WSNs represent a significant improvement over wired sensor networks with the elimination of the hardwired communication cables and associated installation and maintenance costs. The possible uses of WSNs for real-time information in all aspects of engineering systems are virtually endless, from intelligent building control to health care systems, environmental control systems, and more. As electronic hardware circuitries become cheaper and smaller, more and more of these WSN applications are likely to emerge, particularly as these miniaturized wireless sensor nodes offer the opportunity for electronic systems to be embedded unobtrusively into everyday objects to attain a “deploy-and-forget” scenario.
In the great majority of autonomous sensor nodes in WSNs, electrical energy that is necessary for their operation is provided primarily by batteries. Batteries take up a significant fraction of the total size and weight of the overall system. Moreover, they are also the weakest link but yet the most expensive part of the system. Another important factor to be considered is the requirement for the proper maintenance of batteries, with the need either to replace or recharge them. This is a serious limitation of WSNs, in which there are dozens or hundreds of sensor nodes with batteries to maintain. Generally, the highest reported energy for present-day battery technologies ranges around 3.78 kJ/cm3 [1], which implies that an ultralow-power miniaturized wireless sensor node with a volumetric size of 1 cm3 operating at an average power consumption of 100 μW to have a 10-year life span needs a battery as large as 10 cm3. Thus, energy supply is one of the major bottlenecks for the lifetime of the sensor node and is constrained by the size of the battery.
The major hindrances of the “deploy-and-forget” nature of WSNs are the their limited energy capacity and the unpredictable the lifetime performance of the battery. To overcome these problems, energy harvesting (EH)/scavenging, which harvests/scavenges energy from a variety of ambient energy sources and converts it into electrical energy to recharge the batteries, has emerged as a promising technology. With the significant advancement in microelectronics, the energy and therefore the power requirement for sensor nodes continues to decrease from a few milliwatts to a few tens of microwatts. This paves the way for a paradigm shift from the battery-operated conventional WSN, which solely relies on batteries, towards a truly self-autonomous and sustainable energy harvesting wireless sensor network (EH-WSN). Various types of EH systems and their respective main components (i.e., energy harvester [source], power management circuit, energy storage device, and wireless sensor node [load]) are investigated and analyzed in this work. EH systems, based on wind energy harvesting (WEH), thermal energy harvesting (TEH), vibration energy harvesting (VEH), solar energy harvesting (SEH), hybrid energy harvesting (HEH), and magnetic EH, are designed to suit the target applications regarding ambient conditions and event/task requirements and then implement into hardware prototypes for proof of concept. To optimize these EH systems, several different types of power-electronic–based management circuits, such as an active alternating current-direct current (AC-DC) converter, DC-DC converter with maximum power point tracking (MPPT), energy storage and latching circuit, and more have been introduced.
Like any of the commonly available renewable energy sources, WEH has been widely researched for high-power (greater than a few megawatts level) applications. However, few research works on the small-scale WEH, which are used to power small autonomous sensors, can be found in the literature. A small-scale WEH system has the problems of low-magnitude generated output voltage and low harvested electrical power; as such, they pose severe constraints on the circuit design of the power management unit of the WEH wireless sensor node. To overcome the problems mentioned, an optimized WEH system that uses an ultralow-power management circuit with two distinct features is proposed: (1) an active rectifier using a metal-oxide semiconductor field-effect transistor (MOSFET) for rectifying the low-amplitude AC voltage generated by the wind turbine generator (WTG) under a low wind speed condition efficiently; and (2) a DC-DC boost converter with a resistor emulation algorithm to perform MPPT under varying wind speed conditions. As compared to the conventional diode-bridge rectifier, it is shown that the efficiency of the active rectifier has been increased from 40% to 70% due to the significant reduction in the on-state voltage drop (from 0.6 V to 0.15 V) across each pair of MOSFETs used. The proposed robust low-power microcontroller-based resistance emulator is implemented with a closed-loop resistance feedback control to ensure close impedance matching between the source and the load, resulting in efficient power conversion. From the experimental test results obtained, an average electrical power of 7.86 mW is harvested by the optimized WEH system at an average wind speed of 3.62 m/s, which is almost four times higher than the conventional EH method without using MPPT.
For space constraint applications where a small-scale WEH system needs to be as small as possible and highly portable, this type of conventional, large, and bulky WTG is not that suitable. As such, a novel method to harvest wind energy using piezoelectric material lead zirconate titanate (PZT) has been presented. The overall size of the proposed PZT structure is much smaller compared to the WTG. Energy harvested from the piezoelectric-based wind energy harvester is first accumulated and stored in a capacitor until there is sufficient stored energy to power the sensor node; a trigger signal is then initiated to release the stored energy in the capacitor to the wind speed sensor node. Experimental results show that the harvested stored energy of 917 μJ is used to detect wind speed beyond a certain threshold level of 6.7 m/s for an early warning storm detection system.
In some places where a wind energy source is not available, TEH from ambient heat sources with low temperature differences have recently received great attention but has been impeded by the challenges of low energy conversion efficiency, inconsistency, low output power due to temperature fluctuation, and higher cost. To supplement the TEH scheme, an efficient power management circuit that could maximize power transfer from the thermal energy source to its connected electronic load is desirable over a wide range of operating conditions. In this work, a DC-DC buck converter with a resistor emulation-based maximum power point (MPP) tracker is presented for an optimal TEH scheme in sustaining the operation of wireless sensor nodes. From the experimental test results, an average electrical power of 629 μW is harvested by the optimized TEH system at an average temperature difference of 20 K, which is almost two times higher than the conventional EH method without using an MPPT scheme.
Electrical cables that are used in residential and industrial buildings to connect an appliance to a control switch on the wall have been a cause of nuisance as well as led to higher installation costs. Undesirable recabling implications may also arise should the cable become faulty over time. To overcome the problem, a batteryless and wireless remote controller is proposed to switch electrical appliances such as lights and fans on/off in a wireless manner. In this work, two types of piezoelectric-based VEH systems are presented to harvest impact or impulse forces from a human pressing a button or using a switch. By depressing (1) the piezoelectric push-button igniter or (2) the pre-stressed piezoelectric diaphragm material, electrical energy is generated and stored in the capacitor. Once sufficient energy is harvested, the batteryless and wireless remote controller is powered up for operation.
An EH system itself has an inherent problem: the intermittent nature of the ambient energy source. The operational reliability of the wireless sensor node may be compromised due to unavailability of the ambient energy source for a prolonged period of time. To augment the reliability of the wireless sensor node operation, two types of HEH approaches are investigated. A hybrid WEH and SEH scheme is proposed to harvest simultaneously from both energy sources to extend the lifetime of the wireless sensor node. When the two energy sources with different characteristics are combined, there is bound to be impedance mismatch between the two different sources and the load. Hence, each energy source has its own power management unit to maintain at its respective MPP. The WEH subsystem uses the resistor emulation technique, while the SEH subsystem uses the constant voltage technique for MPP operation. Experimental results show that an average electrical power of 22.5 mW is harvested by the optimized HEH system at an average wind speed of 4 m/s and an average light irradiance of 80 W/m2, which is almost three times higher than the single wind-based energy source.
In another HEH research work, a hybrid of indoor ambient light and a TEH scheme that uses only one power management circuit to condition the combined output power harvested from both energy sources is proposed to extend the lifetime of the wireless sensor node. By avoiding the use of individual power management circuits for multiple energy sources, the number of components used in the HEH system are reduced, and the system form factor, cost, and power losses are thus reduced. An efficient microcontroller-based ultralow-power management circuit with fixed voltage reference-based MPPT is implemented with a closed-loop voltage feedback control to ensure near maximum power transfer from the two energy sources to its connected electronic load over a wide range of operating conditions. From the experimental test results, an average electrical power of 621 μW is harvested by the optimized HEH system at an average indoor solar irradiance of 1010 lux and a thermal gradient of 10 K, which is almost triple that obtained with a conventional single thermal-based energy source.
Other than EH, this work also demonstrates an alternative means to remotely power low-power electronic devices through a wireless power transfer (WPT) mechanism. The WPT mechanism uses the concept of inductive coupling (i.e., harvesting the stray magnetic energy in power lines to transfer electrical power without any physical connection). The AC voltage and current in the power lines are 230 V and 1 to 4 A, respectively. Experimental results show that the implemented magnetic energy harvester is able to harvest 685 μJ of electrical energy from the power lines to energize the radio-frequency (RF) transmitter to transmit 10 packets of 12-bit encoded digital data to the remote base station in a wireless manner. To extend the WPT distance, self-resonating coils, operating in a strongly coupled mode, are demonstrated. Experimental results show that the WPT system is capable of delivering wireless output power up to 1 W at an efficiency of 51% over a separation distance of 20 cm to power a small lightbulb.
Until this stage, the proof of concepts for the developed EH prototypes have been demonstrated. The performance of the EH systems in powering wireless sensor nodes are investigated and tested under various operating conditions simulated in the laboratory. In addition, the EH prototypes are optimized according to their designed applications. However, in reality, the environmental conditions of the deployment area are not as ideal as in the laboratory environment. Therefore, the next stage of this EH research, which is considered future work, is to carry out a series of application-specific field trials to evaluate the performance of EH systems under real-life deployment conditions for a prolonged period of time. For the EH mechanism to be successful, an overall system optimization with respect to energy consumption, taking into account the duty-cycling operation of the WSNs for the entire chain (i.e., from sensing the environmental parameter to transmitting and delivering the sensed parameter reliably), is to be investigated. This part of the work is beyond the scope of this book and therefore is proposed as future research work.