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.
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,
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
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).
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).
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).
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.
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.
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).
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
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
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