## 5 Energy Efficiency and Sustainability
### Sustainable Practices in Deploying RIS Technology  
Sustainable practices are crucial in deploying RIS technology in HAPs and are paramount to ensure long-term environmental benefits. These practices include energy-efficient hardware design, optimized power management algorithms, integration of energy-harvesting technologies [67], and the use of renewable energy sources for HAPs, such as solar or wind energy [127]. The deployment of RIS technology also involves considering the environmental impact and resource management [128]. This includes minimizing electromagnetic pollution and reducing the exposure of the end user to electromagnetic fields [129]. Moreover, energy-efficient algorithms and optimization techniques can be applied to maximize the energy harvested by RIS-enhanced HAPs, leading to a more sustainable and eco-friendly operation [130].  
Various approaches have been recently explored to ensure the long-term sustainability of RIS-enhanced HAPs. Factors such as energy consumption and resource allocation optimization have been considered [124]. These practices include optimizing power allocation, beamforming strategies, and integrating renewable energy sources into the HAP'sinfrastructure. For example, a novel mechanism for a new element on-off to improve the scalability and energy efficiency of RIS-enhanced HAPs is presented in [122]. This mechanism allows RIS elements to selectively activate or deactivate based on channel conditions and user requirements, reducing unnecessary energy consumption and improving overall system sustainability. Furthermore, implementing energy-saving mechanisms, such as sleep mode and dynamic power control, can further enhance energy efficiency in RIS-enhanced HAPs.  
Developing mitigation strategies to tackle the environmental impact based on assessment must be performed to identify potential risks. These strategies include considering the materials used in manufacturing RIS elements, ensuring proper disposal and recycling processes, and minimizing the overall carbon footprint of RIS-enhanced HAPs [128].  
Deploying RIS technology in HAPs will foster a more eco-friendly and energy-efficient wireless communication system. This can be achieved through several approaches, such as follows:  
* **Community pricing:** Rather than individual householders bearing the cost of RIS deployment, community developers can price the deployment of RISs [131]. This approach reduces complexity and promotes collective responsibility in implementing RIS technology.
* **Function pricing:** Customizing the deployment and size of RISs based on different communication needs can optimize resource allocation and promote efficient usage of RIS technology.
* **Renewable energy integration:** Integrating renewable energy sources, such as solar or wind power, into RIS deployment can reduce reliance on traditional energy sources and promote sustainability by reducing greenhouse gas emissions.
* **Lifecycle assessment:** Conducting a thorough lifecycle assessment of RIS technology, including its manufacturing, deployment, maintenance, and disposal, helps to identify areas where environmental impact can be minimized and sustainability practices can be improved.
* **Standardization:** Developing industry-wide standards for RIS technology ensures interoperability and compatibility, reducing unnecessary duplication of resources and promoting efficient deployment practices.
* **Recycling and reuse:** Implementing processes for recycling and reusing materials used in RIS technology, such as electronic components, can reduce waste and promote sustainability.
* **Energy-efficient infrastructure:** Designing RIS systems with energy-efficient components and technologies, such as low-power-consumption RIS types, can reduce the RIS technology's overall energy consumption and environmental impact.