You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review 
Chapter Title:  Pyroelectric EH/(nano)generators 
1. Abstract of Cited Paper (BIB001):  This paper presents a brief history of energy harvesting for low-power systems followed by a review of the state-of-the-art of energy harvesting techniques, power conversion, power management, and battery charging. The advances in energy harvesting from vibration, thermal, and RF sources are reviewed as well as power management techniques. Examples of discrete form implementation and integrated form implementation using microelectromechanical systems (MEMS) and CMOS microelectronic processes are also given. The comparison between the reviewed works concludes this paper. 
2. Abstract of Cited Paper (BIB002):  In this paper, a hybrid of indoor ambient light and thermal energy harvesting 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 hybrid energy harvesting (HEH) system is reduced and the system form factor, cost and power losses are thus reduced. An efficient microcontroller-based ultra low power management circuit with fixed voltage reference based maximum power point tracking is implemented with 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 obtained, 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 of that can be obtained with conventional single-source thermal energy harvesting method. 
3. Abstract of Cited Paper (BIB003):  Nanotechnology is an enabling technology that provides an extensive range of resources to resolve the energy-related problems, as the developing components and appliances are smaller than 100 nm they provide the new ways to catch, store and exchange energy. Every day, the sun shines a huge amount of energy which is generated through a process of nuclear fusion. Even the sun radiates more energy in one second than people have practiced since the beginning of time. It has been noted that the technical potential of solar energy all over the world is many times larger than the current total primary energy demanded. In this study, the solar harvesting technology with the help of nano-materials has been thoroughly studied. The different types of modern solar collecting technologies that use the nano- materials effectively and successfully have been discussed. Among a number of devices, the solar collector, the fuel cell, photocatalysis and solar photovoltaic have used the nanomaterials to increase the efficiency. It is found that by using nanomaterials the incident radiation can be increased by 9 times while the efficiency of the solar collector is 10% higher compared to that of a conventional flat plate solar collector. The generation ways solar cell technologies have been also discussed here. At the end of this article, few challenges in using nanotechnology are also addressed in detail. 
4. Abstract of Cited Paper (BIB004):  Randomly oriented poly(vinylidene fluoride) (PVDF) nanofibre webs prepared by a needleless electrospinning technique were used as an active layer for making mechanical-to-electrical energy harvest devices. With increasing the applied voltage in the electrospinning process, a higher β crystal phase was formed in the resulting PVDF nanofibres, leading to enhanced mechanical-to-electrical energy conversion of the devices. The power generated by the nanofibre devices was able to drive a miniature Peltier cooler, which may be useful for the development of mechanically driven cooling textile. In addition, the needleless electrospinning also showed great potential in the production of nanofibres on a large scale. 
5. Abstract of Cited Paper (BIB005):  This paper presents a fully autonomous power converter IC for energy harvesting from multiple and multitype sources, such as piezoelectric, photovoltaic, thermoelectric, and RF transducers. The converter performs an independent self-adapting input power tracking process for each source. The peak power conversion efficiency measured during single-source operation is 89.6%. With all sources enabled, the intrinsic current consumption is as low as 47.9 nA/source. A self-starting battery-less architecture has been implemented in a 0.32-μm STMicroelectronics BCD technology with a 2142 μm × 2142 μm die area. The IC only requires a single-shared inductor and an external storage capacitor for the basic working configuration. With respect to other multisource energy harvesters, this design specifically introduces a series of nanopower design techniques for extreme minimization of the intrinsic consumption during operation. The small chip size combined with the limited number of required external component, the high conversion efficiency, and the state-of-the-art intrinsic nanocurrent consumption make the IC suitable for many critical applications with very limited available power, such as wearable devices or unobtrusive wireless sensor networks. 
6. Abstract of Cited Paper (BIB006):  This paper presents a power management unit (PMU) architecture for multi-source energy harvesting system in wearable devices. The energy harvesting system utilizes a thermal and vibration harvesters as energy sources. In addition, the PMU is designed to control the harvested energy as well as to manage the power delivered to an ECG processor. Further, the PMU generates different signals to enable and disable the processor depending on the available energy. The ECG processor analyzes ECG signals and predicts ventricular arrhythmia (VA). The ECG processor is a multi-voltage design that uses a 0.6V for the processing core and a 1V for the SRAM. Simulation results of the PMU shows the operation of the ECG processor at different energy states in different modes of operation. The proposed architecture could help wearable electronics to run at their optimum energy point to increase their life time. 
7. Abstract of Cited Paper (BIB007):  In this paper, we present a novel hybrid scheme of magnetic, thermoelectric, and vibration energy harvesting (EH) system, which is directed towards the low-power sensing applications within power grids. The topology aims to address the energization of wireless sensor networks that are deployed for the condition-based monitoring purposes of electric facilities. To this end, we have investigated the energy conversion properties of the three EH modalities by means of simulation studies and in-lab experiments. Thereafter, a hybrid energy management system has been established using an ultra-low power consumption circuitry to regulate and integrate individual outputs from the front-end harvesters. During a proof-of-concept, we have observed from a developed demonstrator that a DC output voltage held steady and the output was fed into a ZigBee sensor to keep it operational meanwhile without downtime, which indicates that the power consumption of a ZigBee node can be fully covered by the harnessed energy in the context of this article. Hence, the ambient energy scavenging methodologies discussed herein are well-suited to empower energy-autonomous sensors. The work is characterized by the first demonstration of thorough assessment of the three stray energy forms in the grid and the joint utilization thereof to enhance the system loading capacity. Rather than exploiting a single ambient energy source of some kind, this hybrid and versatile option is proven to accomadate more stable power throutput, and the system signifies feasibilities and potentials to improve the problematic power supplies of the sensors that are deployed within the grid. 
8. Abstract of Cited Paper (BIB008):  Abstract With the rapid development of wearable electronics like artificial e-skins and smart patch, harvesting biomechanical energy and realizing self-powered sensing are of essential importance for achieving sustainable and efficient function of the system. Here we report a flexible hybrid device that can be conformally attached on soft surface like human skin to harvest diversity touch energies based on electrospun nanofiber mat. Facilitated by the working mechanisms of triboeletric and piezoelectric, the device can generate maximum peak power up to 84 μW/cm 2 and 0.11 μW/cm 2 for the TENG and PENG part when stimulated by a compressive stress, which can enhance the energy harvesting efficiency and expand its application areas. By virtue of the high sensitivity of the piezoelectric nanomaterial, the device can also be attached on different parts of body for real-time monitoring the human physiological signals such as respiratory information and radial artery pulse, which shows potential value in self-powered e-skins and healthcare monitoring systems. 
9. Abstract of Cited Paper (BIB009):  Abstract Energy harvesting is a key technology for the self-powered mode of wireless sensor nods and mobile terminals. A large number of devices have been developed to convert mechanical energy into electrical energy. Whereas great efforts have been made to improve the output performance, problems like energy dissipation, device life and response range still need to be addressed. Herein, we report a hybridized triboelectric-piezoelectric-electromagnetic nanogenerator efficiently harvesting vibration energy. Three harvest modes are integrated into a single device, whose core component is a magnetic levitation structure. On the one hand, it presents higher sensitivity than conventional spring or cantilever designs due to low energy loss, which favors the tiny energy harvesting like the slapping desk vibration and the running car vibration. On the other hand, the mechanical fatigue or damage can be avoided by the special structure design. Under 20 Hz, triboelectric nanogenerator (TENG) can deliver a peak output power of 78.4 μW, while the top (EMG2) and the bottom (EMG1) electromagnetic generator can provide a peak output power of 36 mW and 38.4 mW, respectively. Piezoelectric generator located at top (PEG2) and bottom (PEG1) can contribute a peak output power of 122 mW and 105 mW, respectively. The capacitor charge measurement reveals that unit combination performance is remarkably stronger than individual performance, and the combination of TENG+EMG1+EMG2+PEG1+PEG2 has the highest energy harvesting capacity. Finally, this device has been integrated into a wireless sensor system. Results show that the wireless sensor system can be activated and transmit temperature and vibration signal to control computer. This work has a vital significance to the development and application of the internet of things. 
10. Abstract of Cited Paper (BIB010):  Abstract Many types of nanogenerators (NGs) have been developed in a rapid pace, the technological focus and trends of this fantastic emerging technology deserve in-depth exploration. This paper presents the outcomes of a technology roadmapping study based on the research on existing position of NGs. An integrating approach that combines bibliometrics, patent analysis and technology roadmapping method (TRM) workshops was applied. The bibliometrics and patent analysis provide a means of visualizing detailed information about the past trajectory and technological focus of the NGs technology. Its results show that China has been with the international forefront regard to research into NGs since 2013, especially in the field of triboelectric nanogenerators (TENG). Based on these quantitive results, key external factors which may shape the growth of NGs-based industry have been identified according to the experts’ opinions. Finally, the future development roadmap of NGs-based industry in China was proposed from technology, industry dynamics, production and market& applications perspectives. This work provides a comprehensive means for visualizing more information about the present focus and future development trends of NGs, based on a quantitive and qualitative coupling method.

Literature Review:
With relative higher conversion efficiency; Can be fabricated as micro/nanoscale. Require high-frequency temperature fluctuations and high efficiency of energy extraction cycles. The source characteristic, conversion efficiency and harvested power are referenced from the introduction to energy harvesting concluded by an applications engineer with the single cell battery charge management group at Texas Instruments in 2015  . In the literature, there are numerous examples of the integration of more than two EH technologies. For instance, the magnetic, thermal and mechanical energy were scavenged by electromagnetic, thermoelectric, and piezoelectric techniques implemented in a hybrid scheme to supply enough power for power grid condition monitoring using ZigBee sensor nodes BIB008 . Triboelectrification, piezoelectricity and electromagnetics were incorporated to achieve a hybrid nanogenerator with high sensitivity, efficiency and stable power consumption for transmission of temperature and vibration signals BIB010 . Hybrid energy harvesters provide reliable and stable power backup support for the self-sustainable WSNs. But the implementation of a hybrid energy harvester, like other individual energy harvesters, requires a power management circuit and should have a multiple-input power converter BIB002 to obtain a regulated desired voltage level to power WSNs and store excess electricity. Because different sources with EH technologies produce voltages and currents with different characteristics, such as a thermoelectric generator (which produces very low output voltages BIB006 ) or a piezoelectric generator (which provides sinusoidal sources and needs to be rectified BIB001 ). Some researchers have explored various effective integrated circuits of AC/DC or DC/DC converters. For example, a converter is developed for working on the condition and combination of energy generated with photovoltaic and thermoelectric harvesters BIB003 . Another converter is designed to regulate and combine the energy harvested with piezoelectric EH and thermoelectric EH technologies to successfully support the operation of ECG processor in wearable electronics in BIB007 . Dini et al. BIB006 has investigated an autonomous power converter which can be used for single or multiple sources and achieved the conversion efficiency up to 89.6%. In general, most converters implement single-source voltage conversion and energy storage. Converters used for multi-source energy harvesting should be designed according to the requirements because the conversion efficiency of autonomous converters is not high. As a result, the improvement of power management converters is beneficial for the application of EH for self-sustainable WSNs in machine condition monitoring. As mentioned previously, with the drive of the hybrid EH technique integration and the limitation of the device volume, nanogenerators are a promising industry developing at a rapid pace. They are expected to provide effective and efficient solutions for self-powered WSNs due to the characteristics of flexibility, lightweight and integrability BIB004 BIB005 . The statistics made by Wang et al. BIB011 reports that nanogenerator technology is rapidly developing as an emerging technology in China. Chen et al. BIB009 has devised and made a triboelectric-piezoelectric nanogenerator based on flexible fibres to attach to a soft surface to harvest kinetic energy. However, some critical challenges have a significant influence on large-scale industrial production and applications, such as the acquisition and lifespan of nanomaterials, development of nanotechnology, optimization of the nanostructure and conversion ratio of nanogenerators. There are lots of difficulties and barriers waiting for solutions during the development process of nanogenerators.