## 4 Energy Harvesting  
An important consideration in the design of FSSs is what happens to the energy contained in the blocked frequency bands. In typical FSS designs, energy from blocked bands can either be reflected back towards the source or absorbed by the surface, where it may be dissipated as heat. However, there is a growing interest in utilizing this absorbed energy through RF energy harvesting, which converts electromagnetic waves into usable electrical energy.  
By integrating electronic circuitry into an FSS design, the absorbed RF energy can be converted into DC electrical energy, a process that opens up new possibilities for sustainable power solutions. This process generally involves the following steps:  
1. Absorption of RF Energy: The FSS unit cells are designed not only to filter or block certain frequencies but also to absorb a portion of the electromagnetic energy within those frequencies.
2. Conversion to DC Power: The absorbed RF energy can be converted into electrical energy using rectifying circuits, such as rectennas (rectifying antennas). These circuits convert the high-frequency AC energy from RF waves into DC energy.
3. Energy Storage: The harvested energy can be stored in capacitors, super capacitors, or small batteries to provide a stable power source.  
4 Regulation: Regulation circuits ensure that the power output is stable and sufficient to power low-consumption devices.  
This energy can be used to power Internet of Things (IoT) devices within smart buildings, contributing to the larger vision of energy-efficient and autonomous building infrastructures. Some examples of RF energy harvesting systems based on FSS using the unit cell designs presented in the previous sections are summarized in [83].  
The use of Internet of Things (IoT) technology in construction is growing [84], with smart buildings using connected services for lighting, heating, security, and other functions. This proliferation of IoT devices, however, increases the demand for energy, posing a challenge in terms of sustainable power sources. RF energy harvesting from FSSs could provide a wireless, environmentally friendly solution to supply power to these devices, especially for low-power sensors or communication nodes in common building areas.  
While RF energy harvesting is not a complete solution for providing all the energy needs of a building, it can lead to significant cost savings in specific scenarios. For example, large-scale FSS installations on building exteriors, covering surfaces like windows or walls, could harvest enough energy to power low-consumption devices, such as environmental sensors, security systems, or small IoT appliances.  
Authors have reported various conversion efficiencies and voltage outputs from RF energy harvesting systems [10; 11; 12; 85], depending on the design of the FSS unit cells and the specific operating conditions; even partial energy harvesting from blocked frequencies can help reduce the energy footprint of modern buildings, particularly in common maintenance areas.  
Overall, integrating RF energy harvesting into FSS technology provides an exciting path toward creating self-sustaining smart buildings, capable of powering low-energy devices using the very electromagnetic signals they are designed to filter. This dual functionality could become an essential feature in the development of green, energy-efficient architecture.