Green Energy: Sources and Types

By Ajnish Ghimire and Ram Chandra Poudel 10th August 2020

A global trend of long-term sustainability on global energy has become a growing concern. To mitigate the several issues and threats of non-renewable resources, the potential pathways for renewable energy are getting attention. There has been more research and development on the domain of energy, and various technologies are already on use to reduce the dependence on non-renewable fossil fuels like coal, oil, and natural gas. The types of green energy sources determine the variety of green energy.


The natural sources, such as sunlight, wind, biomass, water, wave, tide, and geothermal heat, are the green energy sources. Unlike fossil fuels, these energy sources are renewable, infinite, and naturally replenished. The negative consequences of the green energy are very less, and their direct contribution to greenhouse gas emissions is almost negligible. Therefore, it is essential to develop renewable energy as a critical player in the global energy sector to promote an environmentally friendly and sustainable energy future. To meet the global clean and affordable energy demand which is one of  the United Nations Sustainable Development Goals (UN SDGs), an accelerated rate of investment on renewable capacity and technological advancement in favour of sustainable green energy is essential.


In the future, the integration of the technological development made in the on-going fourth industrial revolution may establish the sustainable green energy industry as a triumphant global industry. However, it is crucial to take the different factors, such as environmental, social, economic, technical, plan and policy, subsidy, geographical, and market availability into consideration for the sustainable development of the global green energy sector. The common types of renewable energy are described below.


Solar Energy: The sunlight, being the source of solar energy is available all the time on the planet and is renewable. Almost 174-petawatt (PW) worth of energy falls on the earth's atmosphere in the form of sunlight and is equivalent to 10,000 times the energy consumed by humans. Even by harnessing a fraction of this energy, the global energy insecurity may reverse (Rhodes, 2010). 

The photovoltaic cells harness the sunlight and then transform it into electricity. The solar radiation that reaches on the surface of the earth gets distributed over various wavelengths; and the semiconductor absorbs the light with high energy with shorter wavelength. Then, the semiconductor get in contact with electrodes and then exposed to solar light (Inganas and Sundstorm, 2016). It has the most significant potential among all the sources of renewable energy.


Wind Energy: The air that flows on the surface of the earth is the source of wind energy. Wind power demands stable and unidirectional wind speed which significantly affects the potential of wind power. However, the wind speed differs in both time and space and altered by geographical and weather conditions (Tong, 2010). For instance, the higher altitude and near to offshore areas provide the best options for harnessing the wind energy (Caduff et al., 2012).


The wind turbine generates the energy and consist of the tower blades, the nacelle, and the tower. The turbines are made of glass-fibre coated with polyester and are coloured light grey to reduce the reflection of light during lightning conditions. The turbine blades rotate when faced into the wind, and each blade is angled like an aeroplane's wing to generate the lift that makes the blades spin. In the nacelle, a sensor detects the wind direction and speed. The shaft connected to a generator uses an electromagnetic field to convert the mechanical energy into electrical energy (Caduff et al., 2012).


Geothermal energy: The below of the earth’s crust possesses a massive amount of thermal energy generated by the decay of radioisotope of elements, such as uranium and thorium. This renewable energy source is an independent source of energy of the sun and gravitational attraction (Younger, 2015).


The temperature difference in the earth's core and the surface of the earth causes a continuous generation of thermal energy from the centre to the exterior. The production of geothermal carries out by drilling into underground reservoirs and the turbines generate electricity. The natural steam and hot water, a form of geothermal energy are utilized for decades to generate electricity (Barbier, 1997). In some countries, this energy is used for cooking and heating purposes. It is one of the cheaper energy generation methods as it is continuously available.Similarly,  the emissions such as carbon dioxide from geothermal power plants are very low.


Hydropower: Hydropower, also known as hydroelectric power is the generation of energy in the form of electricity with natural earth’s cycle of evaporation, rainfall, and the force of water created in a dam. In principle, the increased water level allows water to fall forcefully. The turbine gets rotated with the force of falling water that causes the blades of the turbine to spin. The generator connected to the turbine converts mechanical energy into electric energy. Thus, the generated electricity connects to transmission lines for its distribution (Kaunda et al., 2012).


Hydropower is one of the largest global renewable energy sources and is the primary renewable energy source in the G7 countries. It has higher flexibility and storage capacity and has technical, economic, and environmental benefits. Therefore, hydroelectric power will play a significant role in the future global energy mix, especially in developing countries, like Nepal, Brasil, Ethiopia and Malaysia (Chala et al., 2019). However, flooding riverside lands, alteration of land and aquatic ecosystem, hindrance of the natural flow of sediments are the few negative consequences of hydropower.


Bioenergy and Biofuels: Biomass, such as agricultural residue, forest residue, animal excreta, microbial biomass, weed, and other organic waste is the source for bioenergy and biofuel. Bioenergy and biofuel have been used for heating, cooking, and lightning purposes since the beginning of human civilization. As biomass contains the stored energy derived from the sun, it is one such energy that produces far fewer greenhouse gases than other petroleum-based fuels.


Bioenergy and biofuel exist in solid forms, such as pellets, charcoals, and chips of wood and firewoods, liquid forms, such as bioethanol and biodiesel, and gaseous forms, such as biogas and syngas (Guo et al., 2015). The energy generated from biomass plays a significant role in the global energy system; and based on the current use of it, it is likely to increase its utilization for the substitution of fossil fuels (Reid et al., 2020). Guo et al.  (2015) estimated that 30% of the world’s sustainable green energy demand will come from bioenergy and biofuel by 2050. As biomass energy is a strong contributor to the world energy supply, the technological development and the production of first, second, third and fourth-generation biofuels from different carbon sources are increasing greatly.


Wave Energy: The kinetic power of the water generates wave power. When the wind passes across the surface of the seawater, the waves are generated. The waves are converted into wave energy by the  converters, such as oscillating water columns, overtipping devices, and oscillating body systems (Aderinto and Li, 2019). The features of the wave conversion equipment, the economic part, and the wave conditions determine the quantity of the harvested wave energy at the desired location (Mattiazzo, 2019).


The wave power can be integrated with solar and wind power to connect with the electric grid. As compared to solar and wind energy, wave energy has the highest energy density and delivers constant and predictable power (Aderinto and Li, 2019; Mattiazzo, 2019). However, solar and wind energy are dominating the commercial market due to their cost-effective technologies compared to wave energy.


Tidal Energy: The gravitational pull of the sun and moon, and the centrifugal force developed due to the rotation of the earth-moon system generates tidal energy (Rourke et al., 2010). The tidal barrage approach harnesses the potential energy formed due to the rise and fall of the tides as tidal energy. Similarly, the tidal current turbines approach converts the kinetic energy generated by the flood and ebb current into tidal energy (Rourke et al., 2010; IRENA, 2014).


The tidal current turbine technology is expensive and therefore, demands further research to make it cost-effective at the commercial level (Rourke et al., 2010). Like wave energy, tidal energy is also very predictable energy as the energy source is almost not influenced by the weather conditions.


To sum up, renewable energy is a global phenomenon and is the energy demand of the future; therefore, the growth of global green energy is set to accelerate. The priorities and collaborative decisions made by global political leaders, policymakers, investors, and the scientific community will determine the future deployment of green renewable energy. Similarly, the local government plan and policy regarding the upliftment of the renewable energy sector is equally important.


With the growing concern in green energy, there is an increasing demand for a better-integrated system and technology for its sustainable applications. It is also equally important to understand and evaluate the financial feasibility and other optimization parameters, such as safety and durability to make clean, safe, reliable, and affordable green energy. The integration of different green renewable energy will be the sustainable energy system of the future in the world.


For citation: Ghimire, A. and Poudel, R.C. 2020. Green energy: sources and types. Sustainable green energy,


Green Energy Renewable Energy Sustainable Green Energy SDGs Solar energy Wind energy Bioenergy Biofuels Biomass Electricity grid Hyropower Wave energy Tidal energy Geothermal energy

Memory Quiz


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