**4. Clean energy: introduction and overview**

Energy is one of the main sectors, which contributes a major share in the environmental degradation, particularly in air phase of the environment. Conventionally, fossil fuels are used for meeting the environment requirements of industrial, commercial, transportation, domestic as well as agricultural operations and activities. Fossil fuels are excavated from the earth and after going through a series of refining processes are consumed in the energy generation systems. Burning of such fuels in the firing system acts as a baseline in the formation of primary air pollutants. Hence, technical aspects of the fossil fuel burning in the firing system are extremely important to minimize the environmental impacts. Such measures include use of refined fuels, design of combustion systems, firing methods, air to fuel ratios, filtration and scrubbing systems for smoke in the stacks as well as quality of fuel. Despite several measures, there is still likely chance of release of harmful gases from the burning of fossil fuels, which may seriously affect the air environment. Moreover, rapid depletion of fossil fuels results in high energy prices around the world. In this case, this causes both high economic costs and environmental costs. It is dire need of the time to introduce alternative energy generation ways, which can replace the need of fossil fuels (**Figure 3**) [43].

The economic, social as well as environmental constraints caused by the use of fossil fuels in energy generations system have pushed the community to use the renewable energy resources for meeting the energy requirements. Having no emissions and effluents, renewable energy resources are considered clean energy resources. The renewable energy resources, that is, clean energy resources are categorized in different groups such as Solar Energy (SE), Wind Energy (WE), Geothermal Energy (GTE), Bioenergy (BE), and Hydro Energy (HE) as shown in **Figure 4**.

Having a natural cycle in the earth's atmosphere, all kinds of clean energy resources (**Figure 4**) have zero or minimal impacts on the environment and hence

**Figure 4.** *Types of clean energy.*

contribute to mitigate the air pollution as well as greenhouse gases. This creates more balance in the earth's environment contrary to the fossil fuels that disturb the natural balance and cause natural hazards. Further, there are several benefits to invest in such alternative energy generation techniques including lowering the energy costs, fulfillment of energy gaps due to rapid population growth, minimal environmental impacts, improved public health sustainable, creation of employment and business opportunities, and hence, development of societies and communities [44]. The modern technological advancements and innovations have made such resources more efficient and cost-effective and easier to use in place of conventional fuels and primary energy systems [45].

Different incentives and subsidies are being provided around the world for the promotion and adoption of clean energy resources in domestic, commercial as well as industrial applications around the world. However, there is a need of more friendly policies for the encouragement of such technologies so that social implications can be avoided for the successful adoption of clean energy [46].

It is important to study and understand the basis and technical aspects of clean energy resources for successful adoption. The major clean energy resources and their systematic flow of energy generation in line with its connection to mitigate air pollution are discussed as below.

#### **4.1 Solar energy as clean energy resource**

Sun is the most prominent and basic source of energy for the Earth. This is also considered to be the first-stage energy source for all kinds of clean energy resources as the energy flows in the form of solar radiation from the sun's surface toward earth's environment and is absorbed or captured by various ways. This is considered as renewable resource as a continuous flux of solar energy (solar rays) rays is received by the earth. A portion of these rays is absorbed by the clouds, dust particles, or moisture/gas molecules, another portion is reflected back to the environment, and a significant part is absorbed by the earth surface [47, 48]. If a device is placed in the path of solar rays, the energy possessed in the solar radiation can be captured to process further to the desired form of energy (**Figure 5**).

The energy flux coming from the solar source can be captured for two purposes, that is, 1—Solar Photovoltaic (PV) generation and 2—Solar Thermal Energy (**Figure 5**). The former is based on a simple P-N junction diode cell to capture photons present in the sunlight to produce a flow of electrons, which is used as an electrical energy input for various applications. Another application of solar energy is to capture and concentrate the solar radiation to produce a high-density flux of radiation resulting

*Air Pollution and Clean Energy: Latest Trends and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.112226*

#### **Figure 5.**

*Outputs of solar energy [49].*

in a significant temperature output, which can be used in various thermal energy applications such as water heating, drying of agricultural products, cooking, and processing of chemicals and solutions. The device used for such purpose is termed as Solar Collector and Contractors. In solar collectors, the solar radiation is captured and absorbed by the black body present in the solar collector, while in case of solar concentrators the solar radiation is contracted with the help of mirrors and focused lenses to produce a high temperature. By using both techniques, the energy requirement in both forms, that is, electricity and thermal energy can be fulfilled, which is conventionally provided by the burning of fossil fuels and other primary energy resources. A multiscale installation of such system can provide sufficient energy for town. There are certain technical challenges such as solar fluctuations, day of the time, and climate and weather conditions, which hinder the proper availability of solar rays. However, these aspects can be managed by using storage systems as well as installation of auxiliary energy system, which can be used during the time of no or less solar energy. The use of solar energy is helpful to reduce dependence on fossil fuels to meet both electricity and thermal energy for industrial and domestic applications [49].

Adoption of solar energy also provides a promising solution for the transport sector, which is one of the significant contributors for air pollution and environmental degradation. Electric vehicles (EVs) powered with solar energy are one of the alternative solutions to the conventional transportation vehicles being run by firing the fossil fuels like gasoline, diesel. The use of EVs does not release any harmful pollutants into the air. This is particularly important for the big and populated cities where transportation is major cause of respiratory and heart diseases. Availability of solar PV powered recharging stations can further help to further disseminate this environmentally friendly technology [50].

Solar energy could be combined with other clean energy sources, like wind, bioenergy, and hydroelectric power, to make a more reliable and efficient energy system. There are still certain challenges and barriers that make it hard for solar energy to be used at a large scale. However, this also highlights a lot of opportunities and future directions for introduction of modern tools and systems, which can eliminate the technical problems and promote further sustainability. In this regard, life cycle assessment, energy balances as well as energy routing of solar PV cells and collectors are also needed for the technological advancement of this environmental technology.

#### **4.2 Bioenergy as clean energy resource**

Bioenergy is another type of clean energy resources, which is also considered to be an alternative to many fossil fuels and conventional energy resources. Bioenergy is based on the energy extraction from biomass materials, i.e., organic matter generated from living bodies such as plants and animals. Use of biomass materials for the production of energy is considered as net zero emission technique on the basis of natural cycle of plants and organic matter. Under this concept, the same amount of carbon dioxide as well as temperature is returned to the natural environment, which was taken during the growth of biomass matter such as plants (**Figure 6**). This makes it clean source of energy as compared to the fossil fuels that are taken up from the earth or mines and after burning an additional volume of greenhouses gases is added in the atmosphere [51–53].

Biomass materials are generated as by-products during the decay of forest trees, crop production as well as raring of livestock animals. Such materials consist of significant energy contents in the form of calorific value or heating value (MJ/k), which can be recovered to meet the energy requirements. This recovery can be executed in various ways (**Table 2**).

In simple and basic energy recovery techniques, the raw biomass material is cut, chopped, and physically processed to produce a more refined form of organic matter, which can be used in other processing techniques with higher efficiency. For example, biomass materials are crushed and ground into fine particles, which are then densified with high pressure and are allowed to pass through the dyes of known size resulting in small fuel structures in the form of pellets or briquettes termed as Refused Derived Fuel. In case of oily biomass and seeds, the oil extraction technique is used. Similarly, in Thermo-Chemical Processing, biomass materials are treated and destructed under controlled temperature conditions to produce high temperature, pyrolysis oil, or more refined gaseous fuels, which can be used in place of conventional fuels. In Biological Processing, the breakdown of biomass materials occurs biologically, that is, by the microorganisms to produce secondary fuels like biogas, biodiesel (**Table 2**). Being the part of natural cycle, biomass can be used to replace

*Air Pollution and Clean Energy: Latest Trends and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.112226*


**Table 2.**

*Energy recovery of biomass [51–53].*

the fossil fuel to minimize environmental impacts and air pollution, in particular. However, attention must be given to use the suitable energy conversion technique for better outcomes [52, 53].

#### **4.3 Wind energy as clean energy resource**

A very high energy potential is possessed by winds blowing at high velocities. This is because of the natural convective loops caused by the temperature difference in different regions. Due to this factor, this is considered as a by-product of solar energy. The high speed and impact of wind is converted into mechanical energy and then to electricity by using wind turbines. Various designs of wind turbines and mills are used for this purpose, depending on the available potential of winds in a particular region. Primarily, wind turbines are classified into 1—Horizontal Axis Wind Turbines and 2—Vertical Axis Wind Turbines. In the first category, the wind turbine rotates in the horizontal plane and it is governed by lift force of the winds. On the other hand, Vertical Axis Wind Turbines rotate in the vertical plane by drag force of winds. There are certain drawbacks and benefits for both the categories. However, it is important to check the feasibility of both designs for the desired location. The speed of the wind can change a lot over time, which can make it hard to rely on wind energy alone to meet energy needs. Therefore, a stable and reliable energy supply requires energy storage systems or backup power sources [54, 55]. The feasibility of optimum design of wind turbine can be achieved by using different simulation tools such as CFD, which can provide a true insight of thrust and impact of winds on solid body of the turbines. Based on such simulation results, the turbine height can also be adjusted for maximum output. Many countries have set high goals for generating renewable energy. Wind energy is expected to continue to play a big role in reducing air pollution and slowing climate change in the future. Modern energy generation systems particularly at seashore are equipped with such technologies for harvesting of maximum energy from winds. As per an estimate, this technology is expected to meet around 18% of the world's electricity demand by 2025 [56, 57].

#### **4.4 Geothermal energy as clean energy resource**

Geothermal energy refers to the energy extracted from earth core. The major benefit of this technique is the continuous and persistent potential of energy as compared to the other renewable energy resources. The high temperature in the earth core is extracted with the help of drilled pipelines filled with thermal fluid, the most commonly water. Sometimes, earth is taken as heat sink to increase or decrease ambient temperature. This is based on the concept that earth's temperature beneath the earth's surface remains constant throughout the year. Depending upon the local climatic condition, if a fluid is allowed to be injected in the earth surface with the help of heat conductor material (i.e., copper pipe), this will help to maintain the temperature as per earth's temperature. However, optimum depth, length, and orientation of pipe and the ambient weather conditions are the important factors influencing this technique. Buildings equipped with such technique will reduce the energy consumption for both cold and hot climatic conditions [58–61].

#### **4.5 Hydro energy as clean energy resource**

Hydro energy also sometimes termed as hydropower refers to the energy extraction from the kinetic energy possessed in the water. This is most suitable for hilly areas or natural water retaining locations where water has sufficient potential energy. In other words, a large volume of water is captured at a high altitude from where it is allowed to pass through a narrow path resulting in a very high speed of water stream. The hydro turbine is placed in this path, which converts the kinetic energy of water to electrical energy [62]. It is a source of energy that has been used for centuries, but in recent decades, it has been increasingly popular to reduce the emissions of greenhouse gases and ultimately mitigate air pollution. According to the International Hydropower Association, adoption of this technology, that is, hydropower contributes to prevent around 4 billion tons of CO2 emissions every year across the globe [63, 64]. This is correlated with removing more than 1 billion cars from the road. There is a wide range of sizes and configurations available for hydropower plants, and they can be constructed on rivers, streams, or other bodies of water. Small-scale hydropower plants are only capable of producing a few kilowatts of electricity, whereas large-scale hydropower plants can produce thousands of megawatts [65].
