# Energy-Aware System Design for Autonomous Wireless Sensor Nodes: A Comprehensive Review
## 4 Energy Management
### Hybrid Energy Management for Multiple Harvesters  
Hybrid energy management techniques are developed depending on the properties of the hybrid sources and the application requirements [80]. In [81], an effective technique of harvesting the energy coming from multiple sources is realized through a diode based \"Power ORing\" architecture. This modular approach collects energy from an arbitrary number of connected harvesting subsystems in a concurrent and independent way. The presence of a diode at the output of each subsystem prevents the exchange of energy between these different harvesting sources, which makes each of them completely independent from the others. They are also able to independently perform MPPT of the transducers,  
Figure 7: General structure of energy management for (**a**) DC; and (**b**) AC sources. leading to an important increase in the tracking efficiency for each harvester source. A self-synchronized operation is also ensured by using the diodes in this technique, which reduces the complexity of the power management unit (PMU). However, an extra power loss is caused by the forward voltage drop of the diodes. In [82], a complementary use of energy sources approach is implemented. This technique mainly extracts energy from a thermal generator as a first source to power the active interface of the circuit and then uses a piezoelectric converter as a second source as an energy supply for the auxiliary circuits in the PMU. In the case of insufficient energy income, the cold start-up of the system is ensured by a passive circuit. One limitation mentioned in [82] is that this approach does not combine the energy coming from the hybrid sources to deliver it to the load because only the thermal generator in this case is used as the main power supply for the load circuit while the piezoelectric transducer is applied to charge a small capacitor to provide the supply voltage for some bias and reference circuits. Another technique is implemented in [83], named shared-inductor DC-DC converters, where buck-boost converters are used to realize impedance matching for MPPT purposes. The hybrid energy sources are assembled, by considering their types, and then combined using a buck-boost based energy combiner technique. Thereby, they all share the same inductor, which reduces the number of components required. The complex controller in this topology must give all the inputs access to the single inductor, while at the same time ensuring that a maximum power transfer for each one of them is reached. None of the above energy management techniques achieve a highly efficient combination of the hybrid energy sources extracted from each input because the energy coming from the different sources is not combined to be delivered to the load circuit. Therefore, the authors of [84] proposed an approach to overcome this limitation, named energy combining through linear regulators. All the energy extracted from the hybrid sources are thereby rectified separately one by one depending on the electric properties of the source, and then combined by connecting the output of individual linear regulators. An external capacitor is used as a single storage device to save the energy. In [84; 85], an effective energy management combining hybrid sources is proposed. Firstly, the energy sources are rectified one by one with the suitable rectifier fitting to the source properties, and then they are combined using operational amplifier circuits for adding or multiplying the voltages. The input sources are connected to a non-inverting op-amp. The purpose is to obtain a non-inverted wave on the output voltage across the resistor. Furthermore, the output of op-amp is directly connected to the designed boost converter, which raises the low voltage using a MOSFET. Hybrid energy harvesting is a key issue for improving energy availability and reliability to ensure a disturbance free operation of sensor nodes. Nevertheless, the efficiency of energy management needs to be considered.