Since batteries they relied on can only provide a finite period of time, wearable electronic devices are currently seeking power sources which are smaller and lighter than conventional batteries that could enable the desired portability and energy autonomy. Likewise, wireless sensor nodes are usually deployed in the places which cannot be easily reached by the electricity grid while the backup battery is a finite resource and also difficult to be replaced. Considerable research effort has been therefore devoted to exploiting energy sources in order to raise the possibility of inï¬nite lifetime for low-power applications. The fact has been revealed that the most intriguing way to reduce the dependence of batteries is to harvest energy from environment. This paper reviews the background of power harvesting as well as the latest development in the field of vibration, motion, thermal, electromagnetic radiation (RF) and solar energy harvesting.
2 Literature Review
2.1 Background and intentions
Nowadays, power harvesting, the term used to describe the process of converting the ambient energy into electricity, is becoming a more and more attractive technique in electronic industry not only because of the increasing number of research reports for the last decade but also the market need for self-powered micro-system. Since many decades ago the evolutionary trends in technology have allowed the decrease in both size and power consumption of electronic systems. However, due to the technical and technological limitations, manufacturers cannot make batteries in smaller size that is capable of storing more energy.
Fig. 1. Electronic evolution since 1990 [1].
As shown in Fig.1, battery technology has slowest evolution in the context of portable devices. Since almost all portable electronic devices currently are utilizing batteries as the source of energy, batteries have already become a major bottle neck for systems since it adds unwanted weight and volume and also limits the operational time and performance of portable devices as it need to be either replaced or recharged periodically [1]. The reality of changing batteries for electronic devices is quite costly while less power needed devices being produced due to the advanced technology has inspired researchers the idea of powering an electronic device by capturing the energy from ambient sources. The environmental energy harvesting has therefore emerged as a viable technique to supplement battery supplies. Furthermore, such technique of growing interest may also lead a way to solve the issue of how to sustain the present day civilization with the high energy prices [2, 3]. Several types of energy harvester are already on the market for automotive, home, industrial, medical, military and aerospace energy harvesting applications and report also shows the number of units will increase 12 times in 5 years [4]. Therefore, the huge development potential in this area is the main aim of this project. In order to evaluate the performance of solar power system, a prototype will be designed and constructed to power the low-power applications. By the end of this project a system with three main parts including solar cell, energy store device and solar tracker should be obtained.
2.2 Revision of previous works
The primary target of wireless remote sensors is to run unattended for a long time therefore the energy source could be problematic for such systems. Batteries would limit the lifetime of sensors, as a result, it is not a recommended power source for wireless sensors and several approaches have been made in order to harvest energy from the environment to power wireless sensor networks. Various kinds of sources including kinetic (wind, waves, gravitational, vibration), electromagnetic (photovoltaic, antenna/rectanna), thermal (solar-thermal, geothermal, temperature gradients, combustion), atomic (nuclear, radioactive decay) or biological (biofuels, biomass) could be the energy sources for energy harvesting systems. Such broad and diverse technology therefore can be a viable alternative to batteries [4]. Previous researches related to this area have already attempted to examine various energy sources stated above for power harvesting systems.
2.2.1 Vibration, motion and mechanical energy harvesting [5, 6]
Electrostatic, piezoelectric or electromagnetic are established transduction mechanisms for converting motion or vibration. In piezoelectric transducers, a voltage can be generated subjected to movements or vibrations which cause the deformation of a piezoelectric capacitor. In electromagnetic transducers, an AC voltage across the coil is generated as the relative motion of a magnetic mass with respect to a coil causes a change in the magnetic flux.
Fig. 2. Schematic overview of a vibration harvester.
Numbers of papers have been focusing on the resonant vibration harvesters which could be treated as a velocity damped mass spring system using the mechanical vibration. According to (1), if 0 is at the maximum possible displacement, the amount of power generated is therefore proportional to the cube of the vibration frequency .
(1)
One research has successfully examined that energy harvesting devices can be based on human kinematical energy which is a kind of readily available energy source. The sport sneaker is chosen for the experiment from other ordinary shoes because of its energy dissipating sole.
Fig. 3. Force/displacement curve for a sneaker sole
According to Fig. 3, when the sole springs back after the step it does not exert as much force as before, the returning energy is less than was put into it and that is the energy aimed to be obtained. Three different devices which adopt the principles stated above are examined. Two of them are embedded into shoe by using piezoelectric material for generating electricity when subjected to motion, vibration or mechanical energy. The third one is a rotary magnetic generator mounted on the shoe.
Harnessing the bending of the sole is the first attempt the researchers tried to capture the energy, which is achieved by moulding a hexagonal laminate of piezoelectric foil directly into a shoe sole. When the stave is bent, the voltage is produced as the polyvinylidene fluoride (PVDF) sheets on the outside and inside surfaces have differing radii of curvature. The high pressure exerted in a heel strike also attracts researchers` attention. A unimorph strip of spring steel bonded to a patch of piezoceramic material modified to be flexible is selected to tap power from pressure. The Lead Zirconate Titanate (PZT) shows a high efficiency of mechanical to electrical energy conversion. Therefore, two Lead Zirconate Titanate (PZT) strips with different sizes and capacitances are boned to two different strips of spring steel forming the devices. However, they cannot be reverse bend, otherwise the piezoceramic material will crack. Another technique is examined for extracting power from foot pressure is to adapt a standard electromagnetic generator, which is a well-proven technology, capable of very high conversion efficiency. However, a conventional rotary generator is a comparatively large, which is always considered to its major difficulty.
Fig. 4. Rotary magnetic generator Fig. 5. Exploded view of integration of piezos
The magnetic generator was affixed as shown in Fig. 4 and the PVDF and PZT elements were mounted between the removable insole and rubber sole as indicated in Fig. 5. As fully stated in the performance results, the output of Thunder Unimorph could reach to peaks approaching 150 Volts while the PVDF stave produces peaks of roughly ±60 Volts. According to Fig. 6, the average net energy transfer of the PZT is 2 mJ/step which is the twice of PVDF. In contrast, shown in Fig. 7, the shoe-mounted AC magnetic generator could provide peak powers of roughly a Watt and a quarter-Watt on average.
Fig. 6. Power output from piezo generators Fig. 7. Performance of magnetic generator
Facts has been revealed that the PZT Unimorph device can convert more energy which is enough to drive lots of low-power applications with improved efficiency by adding a simple sole retrofit to strong the piezoelectric coupling while the conversion efficiency of PVDF system is 1% lower than the assumed efficiency of stretching the foil. Moreover, the design of mounting rotary generator on the shoe can unobtrusively scavenge enough energy from ambient environment and its capability of generating electricity allows it to power a wide variety of low-power applications, such as transistor radio with loudspeaker.
A mechanical device which can generate electricity by using manual power is also mentioned in [7], namely electrostatic generator. Such device is harvesting electrostatic (capacitive) energy based on the changing capacitance of vibration-dependent varactors. It has a significant advantage that this type of energy harvesting system can be integrated with microelectronics and do not need any smart material. Therefore, such type of harvesting is of great interest to engineers. Since its basic operating principle is that the mechanical energy is converted into electrical energy as vibrations separate the plates of an initially charged varactor (variable capacitor), an additional voltage source is therefore needed to initially charge the capacitor, which is one of the disadvantages of using electrostatic converters.
Roundy et al. have classified the electrostatic generators into three types, namely in-plane overlap, in-plane gap closing, and out-of-plane gap closing converters. A comparison of the energy harvesting capabilities of these three different types is provided in [8]. Several types of electrostatic MEMS based on three different converters stated above have presented by other researchers as well. For instance, an AC output power of 1.2μW with a load of 5MΩ at 1.87 kHz is obtained from an electrostatic MEMS energy harvester of in-plane gap closing mechanism with a 1cm2 chip area according to the research taken by Chiu et al. Furthermore, A simple analytical method to optimize the efficiency of two types of electrostatic Vibration Energy Harvesters (VEH): the out-of-plane (OPGC) and in-plane (IPGC) gap-closing converters has been presented in the [9].
2.2.2 Thermal energy harvesting
Significant progress also can be observed in thermal energy which is another form of energy readily present in the environment. Humans and animals, machines are common sources of thermal energy and according to [10], assumptions are made in order to calculate the maximum power which can be recovered from the human body and transformed into electricity. It is of about 3.7-6.4 W, indicating a huge potential. The thermoelectric generator is another kind of thermal energy harvester converting heat from fluids or flames to electricity energy by means of semiconductor materials. Such devices are based on the Seebeck effect that an electrical current will be produced when two junctions, made of two dissimilar conductors, are kept at a different temperature. The Fig. 8 shows the schematics of a thermocouple which is the simplest voltage generator based on the Seebeck effect [5]. The thermopile is the core element of a thermal energy harvester, formed by a large number of thermocouples placed between a hot and a cold plate and connected thermally in parallel and electrically in series. As shown in Fig. 9, if the temperature of the source is the core temperature of the body, the thermal resistance of the source would be the same as the body and the thermal resistance of the sink would therefore become the factor limiting the heat exchange between the thermopile and the air. Hence, proper design of thermopiles and other parts constitute the system is necessary in order to optimize the thermoelectric generator. Bi2Te3 is the most widely used material for the fabrication of thermoelectric generators operating at room temperature while Poly-SiGe has also been used especially for micromachined thermopiles. Researchers are putting more attention on nano-structured materials and super-lattices in order to optimize thermoelectric properties, seeking new materials that might replace Bi2Te3 in the future.
Fig. 8. Thermocouple (left) and thermopile (right) Fig. 9. Thermal circuit of the source
Fig. 10. Left: The principle of the thermoelectric micro-system proposed by [11]. When the heat flows across the PN junction, an electrical power current is generated by the Seebeck effect. Right: Schematic of the step-up circuit.
As technology advanced, various thin film techniques have emerged and been utilized to produce small thermoelectric devices, enables the possibility of harvesting very small amounts of heat for low-power applications such as wireless sensor networks, mobile devices, and even medical applications. Report also shows a thermoelectric micro-converter for energy harvesting systems is fabricated using thin films of bismuth and antimony tellurides (Fig. 10 left). By using the Seebeck effect, it can supply individual electroencephalogram (EEG) modules and a 1 cm2 thermoelectric micro-converter can provide a power of about 18 mW with the aid of a simple step-up circuit (Fig. 10 right) [2]. Thermoelectric generators are all solid-state devices with some important characteristics. They contain no moving parts and are therefore completely silent. They are also considered to have a long lifetime, low maintenance and high reliability. Therefore, they are used in a wide variety of applications, such as the deep space probe and waste heat recovery in automobiles [12]. A three-dimensional numerical model of thermoelectric generators in fluid power systems has also been developed and simulated in [13] in order to maximize the system performance by simplifying the co-design and co-optimization of the fluid or combustion system and the thermoelectric device.
2.2.3 RF energy harvesting
RF energy harvesting is another promising method to generate energy by capturing ambient RF-electromagnetic wave which is available through public telecommunication services, such as broadcast, radio, television, mobile telephony and wireless networks. An output power of 15 mW received at 30 cm distance by a commercial system in ideal conditions has encouraged researchers to put more attention on RF energy for low-power devices [14]. Research in this area is ongoing worldwide and researchers from Nokia currently are working towards a prototype that could harvest up to 50 mW of power which is enough to slowly recharge a phone that is switched off from many different frequencies [15]. A novel method to capture the RF energy is also stated in [16]. The system which consists of four parts including rectenna, DC-DC converter, switch control circuit and final output controller as well as low dropout voltage regulator is designed for wireless sensor nodes and it is later fabricated by a 0.6 μm-CMOS process in order to form the RF induced one-chip power supply system which can be widely applicable to the power supplying system for integrated micro sensor nodes.
Fig. 11. Conceptual diagram of the wireless power supply system on a silicon chip (Left).
Fig. 12. Output waveform of the system when the low dropout voltage regulator outputs the voltage to the load (Right).
Such system can provide a stable DC voltage to feed the sensors circuits from received RF magnetic wave energy (cell phones) by boosting and regulating the received microwave power. By using the technique of deep reactive ion etching process, four surface mount devices are mounted on deep holes of the CMOS chip to realize a very small one-chip power supply system for ubiquitous micro sensor nodes. With RF-power source of a mobile phone, the final experimental results show this system can supply regulated 4.0 V/1mA to the load for 10 ms every 1.8 s which can be seen as a great success. As stated in [17], an experiment has been taken to compare the performances of a modified form of existing CMOS based voltage doubler circuit and a traditional CMOS based voltage multiplier circuit.
Fig. 13. Traditional Villard voltage multiplier circuit using the NMOS transistors (Left).
Fig. 14. Villard voltage multiplier circuit with one self-biasing circuit (Right).
Compared to traditional circuit as shown in Fig. 13, the modified circuit has a self-biasing circuit reducing the load seen by the output voltage and a NMOS transistor replaced with PMOS. As the simulation results shown in table 1, 160% increase in output power is finally achieved over traditional circuits at 0dBm input power.
Table. 1. Output voltage comparison of traditional and modified circuits.
2.2.4 Solar power harvesting
Solar energy, radiant light and heat from the sun, represents the earth`s largest energy resource, which is renewable and environmentally friendly. The average solar power reached on the surface of earth is about 215 W/m2 if polar regions are ignored. In areas with high insolation level, such as the Sahara Desert or New Mexico, the solar power at ground level can be measured up to 260 W/m2 [18]. Due to its high power density, solar cell is therefore considered to a convenient solution for areas where are rich of sunlight while usage of batteries and other means of power supply are not feasible or expensive. An overview of state-of-the-art cell development of photovoltaic solar cells has been discussed in [19], involved in materials and methods used for fabricating photovoltaic solar cell devices.
Fig. 16. Global PV energy capacity [21].
As one of the most intriguing alternative sources of energy, solar power is usually considered to be a mature technology for electricity generation. As shown in Fig. 16, a fast increase of PV capacity can be observed that the solar PV system has been widely used. For instance, as one of common energy harvesting devices, residential solar panel can help people reduce their electricity bills. Furthermore, some electronic devices, like digital calculator, have solar panels embedded in their systems acting as their backup power supply. However, several factors in the solar photovoltaic (PV) system design could have impact on its efficiency of energy harvesting. In [22], authors have described various considerations and tradeoffs that are involved in the design of a solar power harvesting module with comparisons to the conventional battery based systems. Since the power management strategy is especially important in a distributed harvesting system, such as a sensor network, a prototype system is also developed to try to perform such function for sensor network.
Fig. 17. Schematic design of the MPPT system. Fig. 18. Solar powered wireless sensor node.
Another problem has also arisen that existing solar power systems are with an on/off-thresholded charge mechanism simply relying on a diode connecting the cell with the rechargeable battery. As the solar power is intermittent due to weather conditions, this mechanism appears to have some disadvantages, such as low efficiency of energy transfer in changing environment and no power generated during not optimal solar conditions. In order to resolve these issues and improve the efficiency of energy conversion, an adaptive system for optimal solar energy harvesting has been proposed in [23]. Such system features a maximum power point tracker (MPPT) circuit which is a power transferring circuit for optimally conveying solar energy into rechargeable batteries even in not optimal weather conditions. Another effective method to greatly improve the amount of energy generated by PV array is by using solar tracker. A solar tracker system in PV with digital control has been developed in [24], achieving outcomes of increasing solar energy utilization ratio and reducing the overall cost of PV power generation.
Apart from photovoltaics, solar powered electrical generation is also based on heat engines to drive the steam turbine to generate electricity, namely solar thermal conversion. As reported in [25], existing solar thermal power system (STPS) still suffers from some drawbacks. A hybrid system with characteristics of STPS and conventional thermal power plant that could be a promising mode to utilize the solar power in the future has been analyzed.
2.3 Discussion
Table. 2. Summary of some energy harvesters [5].
Table 2 presents a clear overview of some energy harvesters that have already been analyzed by other researchers. Apart from the solar power systems, the systems harvesting from ambient vibrations can typically produce power from few μW up to tens of microwatt. Vibrations may not generate as much power as solar energy system, but they can consistently and reliably produce energy, becoming competitive to solar power. The capability of collecting energy thus makes them more suitable to power the microelectromechanical system (MEMS) devices. A wide variety of vibration harvesters which can deliver tens of mW's of power at around 50-120 Hz have already been emerged on the market. Currently, vibration harvesters still used for niche applications as they are expensive systems. However, as the technology advanced, the available of micro-machined devices will allow the application to be applied in mass-market systems. As stated above, harvesting energy from shoes is a promising method based on kinematic energy. The outputs of three designed systems are quite promising. Although the magnetic rotary generator can supply much more power than other two piezoelectric systems, the large size makes it still not practical to integrate such device smoothly into the design of conventional footwear. Another reason that electromagnetic harvesters can generate more power is they have been fabricated using a combination of micromachining and mechanical tooling techniques due to the technological problem on the creation of coils with sufficient windings. In contrast, from a process perspective, the piezoelectric harvesters are easy to fabricate and devices with lateral sizes between 1 and 10 mm have already been developed in literature [5]. The modifications on piezoelectric systems like terminating these piezoelectric generators into inductive loads to produce an LC resonance in order to be tuned to the frequencies arising from the walking excitation, could improve the efficiency of power extraction and energy storage. One problem has left behind in the paper that how to move power off the shoe to power other wearable systems. Another problem is the lifetime of piezoelectric generator, which could be affected by various factors including the long-term wear, dynamic forces, and potential moisture and abrasion [6].
As stated in [10], if energy harvesting is based on the temperature difference of human body, the maximum power available from human body is up to 2.8-4.8 W. Since it is calculated with some assumptions, the power is overestimated. Furthermore, problems like location of the device to capture the heat of the human body still exist and further efforts are needed to more accurately evaluate the feasibility of thermal energy harvesting from human activity. Likewise, current RF energy harvesters suffer from the low efficiency. As the the output power of RF devices is restricted by regulations due to safety and health concerns offered by EM radiations [17], the RF energy harvested from one frequency is only few μW therefore it is necessary to harvest energy from multiple frequencies. However, the energy collected may be enough to power the wireless sensor nodes as most commonly used wireless sensor nodes consume dozens μW in sleep mode and hundreds μW in active mode but it is still far smaller than the portable devices require.
Table. 3. Characteristics of various energy sources [5].
Table 3 shows the power generation capability of solar cells compared to some other common energy harvesting sources. Solar power system can normally generate a few mW/cm2 [20] and up to 100 mW/cm2 during sunny daytime [26], which makes it competitive to other energy harvesting devices. However, there are a number of disadvantages of solar power limiting its practical widespread use. First of all, large collection areas are required to collect the solar energy widely spread on the surface of earth. Secondly, the solar power is not reliable and consistent since it is depended on the factors including weather conditions, location, time of day, and time of year. The last one is the initial cost of installing a solar power system. Although the solar energy is free, the equipment required to capture the energy is quite costly, which limits the competitiveness of current solar systems [18].
3 Conclusions
This paper presents the overview and progress of energy harvesting from the environment to power the low-power applications, particularly wireless sensors and portable devices. Except the solar power which is usually considered to be a mature technology for electricity generation in large scale, other alternatives still provide a low level of energy that could not meet the power needs of today's electronic devices. Currently, the combination of conventional rechargeable battery and energy harvester could form an almost infinite energy source. However, it could be replaced by combining several types of energy harvesters together to increase the capabilities of energy harvester to obtain energy under different situations. As the technology advanced, small-size energy harvesters could be fabricated with new materials to generate more energy and power consumption of microelectronic systems could be decreased. Thus, it is expected that energy harvesting will play an important role in future microelectronic devices.
