# The Latest Advances in Wireless Communication in Aviation, Wind Turbines and Bridges
## 4 WSN Systems in Different Applications: Aerospace, Bridge Structures, and Wind Turbines
### WSN in Aerospace
#### 4.1.5 Further limitations for Wireless SMH within Aerospace  
In addition to the signal propagation problems experienced when operating within an aerospace environment, the implementation of SHM has further hurdles that must be surpassed in order to be efficiently implemented. The power of the restrictions mentioned above of wireless sensors is of particular concern for aerospace. The use of solar panels, commonly found in civil infrastructure monitoring devices [52], is not feasible. Furthermore, the use of batteries is often limited as, in addition to safety concerns, temperature fluctuation in flight is large, which significantly affects their performance. Degradation of batteries over time with continual charging also requires regular replacement to be carried out. A proposed alternative to batteries' use is instead using capacitors, which, although having a lower energy density, do not degrade over time or suffer from temperature fluctuations [53]. Super and ultra-capacitors with higher performance through narrower dielectrics further increase their feasibility.  
Whether batteries or capacitors are utilized, for a wireless node to achieve long term use without manual charging, some form of energy harvesting is essential. Vibration harvesting is an area of great interest due to the abundance of energy available in flight. This is harvested in three ways, though either piezoelectric, electromagnetic, and electrostatic methods [54]. Piezoelectric devices are often favorable due to their simplicity and ability to be applied as a Micro-Electro-Mechanical System (MEMS) device [55]. Although recent advances have significantly increased vibration harvesters' energy output, their output is realistically limited to under 1 mW. The actual outcome depends on operating conditions and size. Additionally, although researchers have attempted to broaden their resonance [56; 57], which harvesters have at some meaning, making appropriate selection of a device for a given application is essential.  
Although it has been less researched, the generation of electrical energy from thermal gradients seems to be another promising energy harvesting approach; on an aircraft's outside skin, a large temperature change (upwards of 50 \({}^{\circ}\)C [58]) takes place during take-off and landing. Through the phenomenon known as the Seeback effect, Thermoelectric Generators (TEG) convert this change into electricity. Although less efficient than heat pumps, TEG is maintenance-free, contains no chemical reactions or moving parts, and has a long life span [59]. Researchers have successfully applied TEG in a lab setting, generating 126 J over a simulated 80-min flight cycle [60]. A different, smaller device was also substantially flight tested, where an average of around 20 J was generated per flight cycle from the fuselage skin [60].  
Although developments are being continually made in energy harvesting, it is clear that there is a limit to the available power. This can make operations that would be simple in some situations very complex onboard an aircraft. Data communication is of primary concern. It is typically the highest energy drain for a wireless node [61]. Minimizing the transfer of recorded data is one aspect of reducing this expense. However, other essential operations must often be considered. Time synchronization of nodes, for instance, is vital for specific SHM applications, such as vibration or Acoustic Emission (AE) monitoring. Required accuracy is heavily dependent on the technology in use. Vibration-based monitoring must be within 50 \(\upmu\)s [62], whereas, for AE, microsecond accuracy is required for some location approaches to be accurate. Although time synchronization of this accuracy is possible, the power necessary to do so may make it unviable for self-powered applications, making alternative approaches where lower accuracy is required advantageous.