**2. Renewable energy resources**

The term energy can be generally defined as the amount of force or power when applied can move one object from one position to another, or it defines the capacity of a system to do the work. The most important characteristic of energy is the possibility to convert one of its forms to another. Generally, the energy technologies are the man-made devices, equipment, and systems used to capture, convert, store, and transport energy from the energy resources [38]. Energy is an important demand in our daily life as a way of enhancing human development leading to productivity and economic growth. Energy is a key driver for agriculture, industries, and service sectors that influence economic development [39]. The term energy sources refer

to the output forms of energy from the man-made energy technologies, while the energy resources refer to the naturally available forms of energy [38]. The resources of energy are divided into three groups: nuclear resources, renewable resources, and fossil fuels [40]. Renewable energy is called "renewable" because the sources harnessed to create the energy renew and replenish themselves constantly and within a reasonably short period of time (i.e., months or years, not centuries) [41]. Thus, renewable energy sources renew themselves naturally without being drained in the earth [39, 41]. Renewable energy techniques give a great opportunity for reduction of greenhouse gas emission and decreasing global warming via replacing traditional energy sources [6]. Various types of renewable resources such as hydrothermal, geothermal, solar, wind, ocean (tide and wave), heat from the Earth's interior, and biogenic (biomass) energies are available and they give the possibility to produce consistent power [38]. These renewable energy sources are also often called alternative sources of energy [42]. Alternative or renewable energy sources include traditional renewable energy technologies (such as wind turbines) as well as innovative new technologies, such as hydrogen internal combustion engines, and hydrogen-based fuel cells [38].

There is a direct relationship between renewable energy and sustainable development via its effect on human development and sustainable economic growth [43]. Renewable energy sources supply a lot of opportunities in reduction of environmental and health impacts, energy access, energy security, climate change mitigation, social, and economic development [6, 38, 44–46]. Renewable energy resources will play an important role in the world's future [6, 40]. The technologies developed to exploit renewable energies are known as renewable energy technologies (RET) or clean technologies or green energy. Sustainable economic and industrial growth also requires safe and sustainable energy resources [47–49]. Maximizing resource (material and energy) recovery and minimizing environmental impacts such as contribution to the global warming are important objectives in the solid waste management (SWM) sector, which is considerably developed over the past century [50, 51].
