## 4 RF Energy-Harvesting Techniques
### Hybrid Energy Harvesting  
Hybrid energy-harvesting systems are designed to capture and convert energy from multiple ambient sources, such as solar radiation, mechanical vibrations, and thermal gradients, into electrical power. By integrating different energy conversion mechanisms, these systems enhance overall efficiency and reliability, particularly in environments where a single energy source may be inconsistent or insufficient. The multi-modal design of hybrid harvesters allows them to operate across a wider range of environmental conditions, increasing their duty cycle and enabling continuous power delivery, even when one energy source becomes unavailable. This robustness is critical in off-grid or intermittently powered applications. The integration of solar and vibration-based energy harvesting has been explored as a means of capturing energy from both light and mechanical motion. A notable example is the development of hybrid harvesters that combine photovoltaic (PV) cells with piezoelectric, electromagnetic, or electrostatic mechanisms. This approach allows for simultaneous energy conversion from sunlight and mechanical oscillations, resulting in improved performance over standalone energy-harvesting systems. For instance, PV cells generate electricity from photons, while piezoelectric components exploit strain-induced charge separation to harvest energy from vibration or motion. Electromagnetic harvesters use relative motion between magnets and coils to induce current, and electrostatic systems rely on variable capacitance. Their joint operation can offer complementary power outputs--steady outputs from solar energy, and pulsed outputs from vibrations. Recent studies have demonstrated that such hybrid systems can significantly enhance energy output, making them suitable for self-powered devices in remote locations [67; 68; 69]. Hybrid systems that merge solar and thermal energy harvesting leverage the synergy between photovoltaic cells and thermoelectric generators (TEGs). Photovoltaic thermal hybrid solar collectors (PVT collectors) exemplify this approach by integrating solar cells with thermal collectors to convert solar radiation into both electrical and thermal energy. This combination increases overall efficiency by utilizing waste heat from photovoltaic cells to generate additional power. Traditional PV cells suffer from thermal losses, which reduce electrical conversion efficiency. By harvesting this waste heat using TEGs, PVT systems increase total energy yield. The TEGs exploit the Seebeck effect, where voltage is generated across two different conductors in the presence of a temperature gradient, to recover otherwise lost thermal energy. Research on PVT collectors has shown that they provide a higher energy yield compared to standalone photovoltaic or thermal systems, making them an attractive option for renewable energy applications. The integration of vibration and thermal energy harvesting has also been investigated to take advantage of both piezoelectric and thermoelectric effects [70; 71]. Piezoelectric materials convert mechanical vibrations into electricity, while thermoelectric materials generate power based on temperature differences. Researchers have developed hybrid systems that combine these two mechanisms, resulting in increased power generation compared to single-source energy harvesters. These hybrid configurations are particularly suitable for environments like industrial plants or vehicles, where both mechanical vibrations and thermal gradients naturally coexist. Efficient coupling between the two subsystems is critical to avoid energy loss and to allow simultaneous operation without signal interference or thermal degradation. Such systems have demonstrated improved energy capture efficiency and have potential applications in wireless sensor networks and wearable electronics.