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: A survey of multi-source energy harvesting systems 
Chapter Title:  III. CLASSIFICATION OF EXISTING ARCHITECTURES 
1. Abstract of Cited Paper (BIB001):  This paper presents the design, implementation and characterization of an energy--efficient smart power unit for a wireless sensor network with a versatile nano-Watt wake up radio receiver. A novel Smart Power Unit has been developed featuring multi-source energy harvesting, multi-storage adaptive recharging, electrochemical fuel cell integration, radio wake-up capability and embedded intelligence. An ultra low power on board microcontroller performs maximum power point tracking (MPPT) and optimized charging of supercapacitor or Li-Ion battery at the maximum efficiency. The power unit can communicate with the supplied node via serial interface (I2C or SPI) to provide status of resources or dynamically adapt its operational parameters. The architecture is very flexible: it can host different types of harvesters (solar, wind, vibration, etc.). Also, it can be configured and controlled by using the wake-up radio to enable the design of very efficient power management techniques on the power unit or on the supplied node. Experimental results on the developed prototype demonstrate ultra-low power consumption of the power unit using the wake-up radio. In addition, the power transfer efficiency of the multi-harvester and fuel cell matches the state-of-the-art for Wireless Sensor Networks. 
2. Abstract of Cited Paper (BIB002):  The increasing attention on energy autonomous sensing and computing systems which can operate unattended tens of years, have made energy harvesting and power conversion techniques key technologies for the future. The goal is to power systems nearly perpetually if the scavenger is exposed to reasonable environmental energy conditions. However, the system is still threatened to run out of energy, if a prolonged lack of energy intake happens. The last frontiers of perpetual operating systems is combining cutting edge technologies for energy generation from the environment and long-term and green energy supply using small factor fuel cells with few cm3. In this paper we introduce an hybrid power architecture which improves embedded systems power availability. We present a Smart Power Unit (SPU) that is a power supply architecture which manages both energy harvesting and novel fuel cells technologies. SPU provide an efficient air-flow and solar energy harvesting stage and a hydrogen micro fuel cell interface. Each harvester stores energy in a local supercapacitor and, when full, a lithium-ion battery is charged. Micro fuel cell acts as reservoir source for recharging battery in low environmental power condition. The core of the SPU is the microcontroller based power manager that exploits MPPT, energy prevision, battery monitoring and communications with user node.

Literature Review:
Seven prominent systems that have been reported in the literature or released as commercial demonstrators are detailed in Table I . This table lists the relevant features of each device including the types of energy resource it supports and its energy monitoring capabilities. Please note that, where the storage or EH hardware are listed as 'swappable', this means that the devices are not soldered onto the board and may be connected to the terminals. It is important to ensure that the alternative EH device has similar characteristics to the original, and that it does not violate the requirements of the input power conditioning circuitry. System B is the only one that is not sensitive to this, because each energy harvester/storage device has an interface circuit that brings its characteristics into line with those required by the power unit. The most commonlyused harvester types are photovoltaic (PV) cells, wind turbines, and vibration energy harvesters BIB002 , BIB001 , BIB003 . This section reviews the capabilities of the existing technologies.