Renewable and Green Energy: Major Sources and Challenges

Renewable energy benefits both the environment and humans, but what is it?

Some natural resources provide energy that can be replenished in less time and without diminishing the planet’s resources. These resources are accessible in one way or another almost everywhere, including sunshine, rain, wind, biomass, waves or tides, and thermal energy stored in the earth’s crust, called renewable energy resources. They are nearly limitless. More importantly, they do not significantly harm the ecosystem or climate.

On the other hand, fossil fuels like natural gas, crude oil, etc., are only found in finite volumes. They eventually end up if we continue to extract them. Even if natural processes produce them, their replenishment does not occur as rapidly as our consumption.

Fossil fuels are being used extensively today. In the meantime, the pollution they contribute has risen to record levels, ranging from harmful particles to greenhouse gases that harm the environment. Additionally, the repercussions are severe when anything goes wrong, as in 2010 when the Deepwater Horizon oil facility collapsed.

The growth of renewable energy has outpaced that of all other energy sources since 2011. The installed power capacity of renewable energy increased by more than 314 gigawatts (GW) in 2021, which is a record-breaking growth. Nearly 25-30% of our power is generated using renewable energy, and this percentage is rising.

1 Key Benefits of Renewable Energy

All energy sources influence our environment, just like any human activity. Every energy source has trade-offs, and renewable energy is no different. However, no one denies the benefits of renewable energy over the destructive effects of fossil fuels, including reduced water and land consumption, less air and water pollution, minimal loss of species and habitat, and no or lower emissions of greenhouse gases.

Additionally, their local and decentralized nature as well as the advancement of technology, provide significant positive effects on the inhabitants and economy.

1.1 Renewable Energy Emits minimum or no Greenhouse Gases

Fossil fuels are used to produce energy, which releases a considerable volume of greenhouse gas that results in global warming. The bulk of renewable energy sources emits extremely little to no emissions, even when the technology’s whole life cycle is considered.

1.2 Renewable Energy Cause low or no Air Pollution

Raising air pollution is due to the increasing use of fossil fuels for power production, industrial processes, transportation, and the open burning of waste in many areas. Poor indoor air quality is a problem in many developing nations where heating and cooking are based on charcoal and firewood. Cities are suffocated by particles and other air pollution from fossil fuels. The World Health Organization (WHO) has shown that their presence over metropolitan skies causes millions of preventable deaths and costs billions.

1.3 Renewable Energy can be Produced at Low Costs

Energy price fluctuation and restricted resource access are frequently associated with geopolitical instability. Local production means renewable energy is less impacted by supply chain breakdowns, price increases, or geopolitical crises.

2 The difference between Renewable, Green, and Clean Energy

Although terms like renewable, green, and clean energy are frequently used interchangeably, some significant characteristics truly distinguish them. Here, we will examine many forms of energy sources, how they differ from one another, and how they assist us in transitioning from dependence on fossil fuels.

2.1 What is Green Energy?

The energy source with the least negative effects on the environment is green energy or green power. You may effectively lower your carbon footprint by using energy sources that do not emit harmful carbon dioxide. Solar, wind, geothermal, and other low-environmental impact sources are types of green energy.

2.2 Green vs. Renewable Energy

The renewable energy sources with the most significant positive effects on the environment are categorized as “green energy,” a subset of renewable energy. Therefore, all green energy sources are renewable, but not all renewable sources are green. For example, burning wood is renewable (since more trees may be grown) but not green; yet, wind power is green and renewable.

2.3 What is Clean Energy?

Clean energy is energy that is produced without the release of any damaging greenhouse gas or pollutants. By definition, all renewable and green energy sources are clean. However, a source of energy does not need to be renewable to be perceived as clean. Nuclear power is an example of a clean energy source, even though it does not naturally regenerate over time.

2.4 What is Sustainable Energy?

Sustainable energy derives from non-depletable sources, negating the need for routine replenishment. Solar and wind energy are two examples of sustainable energy sources. We can compare these sources to biomass resources like wood, crops, and other materials that can be exhausted and need time and potential human involvement to replenish.

3 Solar Energy

Sunlight is one of our planet’s most readily available and plentiful energy source. The amount of solar energy reaching the earth’s surface in a single hour outweighs our yearly energy requirements. Although solar energy appears to be the perfect renewable energy source, the amount we can use varies depending on the time of day, the season of the year, and geography.

Solar energy has recently experienced massive growth as a result of both technological advances that have reduced costs and government policies that support the development and use of renewable energy sources. This is because there is a growing demand for clean, environmentally friendly, and sustainable energy sources.

We have always used solar energy, but recently we have developed cutting-edge technology to convert it into useful heat and power. It is quickly establishing itself as a useful and affordable renewable energy source, which implies it has a significant role to play in our urgent effort to stop our present reliance on energy derived from climate-destroying fossil fuels.

3.1 Different Methods of Generating Solar Energy

Three main techniques are used to harvest solar energy and transform it into a useable form:

• Using solar photovoltaic technology, solar panels
• Concentrated solar power (CSP), a solar thermal application
• Solar water heating, another type of solar thermal energy

3.1.1 Solar Panels (Photovoltaics)

The most common method of using solar energy for electricity generation is through solar panels, sometimes called photovoltaic (PV) panels.

The photovoltaic effect is used by a network of unique semiconducting solar cells in solar panels to convert sunlight’s photons into direct current (DC) electricity. The DC energy is often routed into a solar converter, transforming it into the AC energy utilized in our homes and the electric grid.

You can set up as many solar panels as you require to fulfill your power needs.

3.1.2 Concentrated Solar Power (Solar Thermal)

Concentrated solar power (CSP) is a technology that uses special reflectors to produce energy on a big scale.

Helioscopes, a specific type of mirror used in CSP facilities, are used in hundreds to reflect sunlight onto a central collector. The heat energy from the sun is either captured in a heat storage medium, such as molten salt or utilized to power a steam turbine.

CSP power stations absorb heat from the sun, unlike PV plants that produce energy from light. So CSP plants are solar thermal facilities.

The application of CSP technology at the home or commercial level is not practicable due to its high cost. However, because it may be utilized as a kind of energy storage, it is viewed as a potential technology at the utility level.

3.1.3 Solar Water Heating

Another method of using solar thermal energy is through solar water heaters. Here, water stored in an insulated cylinder is heated using rooftop solar thermal panels—not to be mistaken with PV solar panels.

Solar hot water systems could fulfill most of a home’s hot water requirements even when it is freezing outside.

3.2 Common Types of Solar Projects

Solar energy is mainly divided into three common types: residential, commercial, and utility-scale.

• Residential solar:This term describes electric energy produced in residences, typically using rooftop solar panels. Residential solar panel systems typically range from 1 to 10 kilowatts (kW).
• Commercial solar:This term, often known as commercial and industrial solar, refers to the use of solar energy by for-profit organizations, enterprises, and government institutions. The most prevalent sizes of commercial solar systems are between 10 kW and 100 kW.
• Utility-scale solar:This includes substantial solar power facilities that provide enormous amounts of electricity for the grid. A power plant is deemed utility-scale by the US Energy Information Administration (EIA) if its overall generation capacity is 1 megawatt (MW) or more.

Solar energy is highly scalable, making it a feasible and cost-effective energy alternative for any size project.

3.3 Solar Batteries and their Use

Solar batteries are specialized devices developed for the efficient storage of solar energy. When solar energy is produced, they effectively store it and swiftly releases it when needed.

Any power produced by solar panels that are not used immediately by the load is delivered to the battery to be stored. The battery may discharge its stored energy and power household appliances later when the panels are not generating enough electricity. During darkness, a solar battery may also be utilized as backup power.

3.4 Challenges and Limitations in Getting Solar Energy

Solar technology harnesses radiations for various purposes, such as power generation, commercial and industrial usage, interior lighting, and water heating. But before solar truly replace fossil fuels as an energy source, it still has to overcome several challenges. Entrepreneurs are discovering unforeseen difficulties with solar electricity when conducting business in this emerging age.

3.4.1 High Capital Cost

The initial expenses of installing solar panels are high, although they would have significant long-term advantages.

Many countries have offered tax cuts and subsidies to promote the installation of solar panels, but the cost may be prohibitively expensive if you do not have funds for this purpose.

3.4.2 Problem of Efficiency

There is a lot of info available regarding how ineffective solar energy is. The majority of people are unaware of what efficiency is. When people see 20% efficiency, they believe it to be inadequate. They believe that 100% efficiency would be perfect. However, even in the near future, there is no possibility of using solar energy with 100% efficiency in the real world. The area needed to produce energy is the key to efficiency.

Due to space constraints, higher efficiency is essential for compact power devices. However, a roof can generate enough electricity for any family’s needs. It is crucial to keep in mind that more effective panels cost more money.

3.4.3 Installation Area

Solar farms need substantial area since electricity production directly correlates with the surface area covered. As a result, deserts and vast open expanses are where the world’s largest solar farms are located. However, this is impractical for smaller countries with constrained landmasses or even for bigger countries with higher agricultural land dependencies.

3.4.4 Reliability

One significant issue with solar electricity is reliability. A solar panel can only generate power for a maximum of 12 hours daily and only reaches its maximum output around midday. During the peak generating time, solar panels with a sensor can swiftly follow the sun’s path. Even yet, it still means that panels operating at full capacity spend relatively little time of the day. Solar panels can charge storage batteries during peak production periods, which helps to provide electricity at night, but it increases the capital and operational cost of the solar system. The repeated charging and discharging cycles of batteries result in quick wear out.

4 Hydro Energy

Solar energy is also collected indirectly by using hydropower. It is recognized as the most developed and environmentally friendly form of renewable energy. Water falls from a high potential to a low potential, producing the renewable energy known as hydropower. Hydro energy is generated by utilizing the potential energy of the falling water to power a hydro turbine. This turbine is linked to the electrical generator’s rotor.

The sun’s energy is necessary for the water cycle. In oceans, rivers, and lakes, the upper surface molecules warm up as sunlight hits the water’s surface. When they have enough energy, they evaporate off the surface and enter the atmosphere. When these molecules ascend into the atmosphere, they get colder. These molecules condense into water droplets when they pass through the cold regions where the air is at low temperature. Since cold air has less ability to hold droplets than warm air, these water droplets produce rain and keep water moving throughout the sky. When it rains in mountain ranges, the precipitation freezes to form glaciers. The cycle restarts if it rains in the fields because it somehow makes its way to the rivers, lakes, and seas.

Hydropower has lower energy costs, operating expenses, and maintenance requirements than other renewable sources. In addition, hydropower is more efficient than both traditional and other energy sources. It is an environmentally friendly energy source since it does not release any pollutants that pose a health risk, such as NOx and SOx. Since it does not produce carbon dioxide, it does not contribute to global warming.

4.1 Basic Components of the Hydropower Plant

4.1.1 Dam

A dam is a construction that separates two rivers so that water may be stored in huge quantities. This reservoir provides controlled and large discharge water for hydropower generation and agriculture purposes.

4.1.2 Penstock

Penstocks are pipes used to transport water from dams to turbines. Valves or gates are utilized to manage water flow to the turbine on the dam side. The penstock might be a steel or polyvinyl chloride (PVC) pipe that has been cemented, and its length, diameter, and thickness have all been precisely determined for the head and the available flow rate.

4.1.3 Turbines

In order to turn water’s potential energy into the mechanical rotational energy needed to turn the rotor of the electrical generator, a turbine is utilized in any hydropower system. The flow rate and capacity of the power plant determine the kind of turbine that would be appropriate for the hydropower project. For big heads, only Pelton turbines are appropriate. From around 500 meters, Francis turbines can be employed in the middle area. The Francis turbine is preferred because of its higher efficiency and better operating flexibility with varying heads and smaller dimensions in the region where the head overlaps with the Pelton turbine by roughly 100-500 m. The Pelton turbine shows more resistance to wear in this region. Kaplan turbines power smaller heads. Figure 2 illustrates the main turbine types utilized in hydroelectric plants.

4.2 Challenges and Limitations in Getting Hydro Energy

The presence of water resources at a high elevation typically makes hydro potential available in isolated mountainous places.

Large dams are thought to be highly concentrated ground loads. Due to the lack of building materials in the region, the construction of the storage dam in distant places may cause a delay in commercial operations. Deforestation may also be necessary, which might be bad for the ecosystem.

In order to build a dam and an electricity facility, the land must be acquired and restored.

There is the possibility of a flood in the communities downstream, which has to face a heavy rainy season, so the safety of the dams is vital. It also involves the destruction of ecosystems for wildlife and fish.

5 Wind Energy

A plentiful source of renewable energy is the wind. It is produced for three primary reasons: the rotation of the earth, the uneven surface characteristics, and the inconsistent warmth of the atmosphere by the sun. Environmental circumstances, the planet’s topography, and buildings influence wind flow patterns. Power can be produced if this wind energy is harvested using wind turbines. Although there are many uses for windmills, those that produce energy are known as wind turbines. Wind kinetic energy is transformed through these turbines into a source of usable energy. The wind’s kinetic energy drives the rotation of the turbine blades. These blades are linked to a shaft that runs the electrical generator. After that, the generator turns the mechanical energy into electrical energy.

5.1 Advantages of Wind Energy

Here are a few of the main advantages of using wind energy:

• It is cost-effective: One of the least expensive types of renewable energy, wind power, is very cost-effective. Since there is now little to no market for wind energy, it might be seen as a free power source. The operating expenses of wind farms are also very inexpensive compared to other electricity sources because they are considered free energy sources.
• A domestic power source:It is necessary for the local area to have a plentiful wind source for wind power conversion installations. Since domestic wind power plants mostly serve local consumers, they are advantageous.
• Renewable and eco-friendly: Most places appropriate for wind farms have year-round constant wind production. Even though there are daily variations, wind energy is generally considered a reliable renewable source. Additionally, wind energy is eco-friendly and minimal in its carbon impact.
• Provides power to remote locations:Wind power is practical in areas where we have constant strong winds most of the year. These areas are typically unpopulated and geographically distant, including hillsides and beaches. In many circumstances, wind power systems can power domestic populations that live in such places.

5.2 Challenges and Limitations in Producing Wind Energy

Even though wind energy is becoming increasingly popular, the industry still faces several challenges. Here are a few of the primary problems with wind energy:

5.2.1 Unpredictable

The inability to generate energy from wind constantly is its most considerable drawback. The wind must be blowing to generate power.

The wind speed affects how much electricity is generated by turbines. As a result, wind power is unsuitable as a base load energy source.

However, it could be conceivable to rely more on wind power if energy storage technology becomes more affordable. Currently, to satisfy our electricity needs, wind turbines need to be utilized in conjunction with other energy sources due to their unpredictable nature.

5.2.2 Establishing Wind Farms

Most wind farms are situated in isolated areas, distant from major cities and infrastructure. As a result, building a wind farm demands substantial time, money, and resource commitments.

5.2.3 Compromising on Profitable Land

Although wind energy is environmentally good, windmills are an inefficient method of land use due to their size and scope. Such wind turbine construction sites might be better utilized to generate an immediate financial gain.

5.2.4 Location Specific Nature

Wind turbines are installed in an area where they can generate enough power to be economically feasible. The tops of hills, open plains, and other places with strong, consistent winds are good places for wind farms. Most of these appropriate locations are often found in rural or offshore areas, distant from cities and towns. Due to this separation, new infrastructure, including power lines, needs to be constructed to link a wind farm to the electrical grid. This can be expensive and even harmful to the environment (i.e., cutting down trees to make room for power lines).

6 Tidal Energy

The influx of ocean waters during tide changes generates tidal energy, which is a renewable energy source.

Engineers discovered techniques to harness the area between high and low tides to produce power in places with many tidal ranges throughout the 20th century. Each process turns tidal energy into electricity using specialized turbines.

Generating power from tides is still considered in its infancy. The quantity of energy generated so far has been minimal. There are hardly many commercial-scale tidal power facilities in operation worldwide. The first commercial-scale tidal power was at France’s La Rance. There are very few places in the United States where tidal energy could be generated at a competitive price, but there are no tidal plants. Canada, China, England, France, and other countries have considerable potential for using this kind of energy.

6.1 Tidal Energy Generators

There are currently three main ways to get tidal energy: tidal streams, barrages, and tidal lagoons.

6.1.1 Tidal Streams

The same astronomical factors that eventually produce the tide also generate periodic horizontal water flow known as a tidal stream (tidal is vertical fluid movement). Turbines are installed in tidal streams for the majority of tidal energy generation. Since water is far denser than air, tidal energy can produce more force than wind energy. In contrast to wind, tides are consistent and predictable.

Turbine placement is challenging since the many machines disturb the tide they seek to harness. The environmental effect might be severe depending upon the size of the turbine and the location of the tidal stream. The best conditions for turbines are in shallow water where electricity is generated, and ships can travel around the turbines. Tidal generator turbine blades rotate slowly to prevent marine living creatures from becoming entangled.

6.1.2 Barrage

A barrage is a large dam that is used by another type of tidal energy producer. Water can flow through the turbines in the dam or over the top with a barrage.

Similar to how a river dam uses river power, turbines within the barrage use the power of tides. As the flood rises, the barrage gates are opened. The barrage gates close during high tide, forming a pool or tidal lagoon. Engineers can manage the pace at which the water is discharged through the barrage’s turbines, producing energy.

6.1.3 Tidal Lagoon

The building of tidal lagoons is another form of tidal energy producer. A body of ocean water partially surrounded by an artificial or natural barrier is called a tidal lagoon.

Similar to a barrage, a tidal energy producer can be built utilizing tidal lagoons. Tidal lagoons may be built along the existing shoreline, unlike barrages. In addition, a tidal lagoon power plant has the potential to provide continuous electricity. The turbines work when the lagoon fills and drains.

6.2 Challenges and Limitations in Getting Tidal Energy

One of our most significant engineering problems is converting the seas’ enormous power into reliable energy to light our grid and make our houses habitable. A dazzling array of tidal turbines, wave-energy technologies, and coastal-barrage concepts are being developed to address this challenge. However, it seems unlikely that they will ever be able to compete with solar and wind power due to complications in developing, constructing, and utilizing these devices.

The following types of devices are included in tidal and wave equipment:

• Equipment used in power-conversion systems that employ turbines, blades, and other parts to transfer the kinetic energy of moving rivers and ocean currents into a stream of useable electricity.
• Cables that keep these devices in place and deliver electricity from the devices to the mainland are occasionally needed.
• Cable accessories that effectively extend the capabilities of cable arrays and devices by connecting them and transmitting the produced power.
• Moorings use chains, cables, or ropes to moor objects to the ocean floor.

When one piece of this equipment malfunctions, the entire system might be brought to a halt. The following are some equipment issues that tidal and investors and tidal engineers may have to deal with.

6.2.1 The Expense of Early-stage Development

Since there are no cost savings from manufacturing ocean energy devices in bulk, their prices remain high. Each piece of gear requires much conceptualization, testing, and production in laboratories before it can be developed into a prototype and tested in open ocean conditions.

The lengthy process of trial-and-error requires funding to survive. Governments are currently providing major funding since private firms do not see many prospects to achieve huge investment returns.

6.2.2 Unproven Prototypes

There are a dozen or even more favorable devices for capturing waves’ movement and tides’ rise and fall. Devices using wave energy can move up, down, and side to side to generate motion that can be turned into electricity.

However, wave action is very different and challenging to record consistently since it depends on surface winds, weather, and local terrain. Furthermore, it is challenging to develop a standard design since an ocean-energy device that performs well in one location may be disastrous in another.

6.2.3 Environmental Disruption

Any significant mechanical device introduced into a dynamic ocean ecosystem can cause issues. Spinning blades can be harmful to aquatic species.

Additionally, mechanical equipment may leak lubricants or make irritating sounds to aquatic animals and fish. It is challenging to resolve these issues on paper or analyze them in a software simulation environment before they become significantly expensive. Engineers can modify their designs to reduce environmental harm, but they can’t get complete manipulation of how their prototypes will behave in the waves.

6.2.4 Unpredictable Weather

Waves continue to hit on the coast and other hurdles indefinitely; their amplitude and frequency are constantly changing because of wind impact on the surface water. Harsh circumstances frequently benefit these devices since they produce more significant motion and energy. However, those who use electricity need a consistent, steady energy supply—not one that fluctuates each time. For this reason, battery storage is occasionally taken into consideration.

The potential of equipment being destroyed instantaneously by hurricanes, typhoons, tsunamis, and other catastrophes is substantial; that’s why regular maintenance is needed.

6.2.5 Corrosion and Bio-fouling

The strength of metal alloys is often required for devices powerful enough to transform waves into energy. The problem is that sturdy, affordable metals like steel corrode in seawater. Special attention must be taken to prevent corrosion during the design, building, and installation phases. Although several metallic alloys have remarkable corrosion resistance, it will be expensive to construct whole energy arrays out of these metals.

Then there are the aquatic species cling to everything we introduce to the water. The moving elements of underwater equipment can become attached by small creatures and plants, increasing the risk of expensive failures and maintenance.

7 Geothermal Energy

Renewable energy derived from the earth’s core is known as geothermal energy. It originates from heat produced during the planet’s initial formation and the radioactive decay of elements. Rocks and fluids contain this energy in the earth’s core.

Geothermal energy is more eco-friendly than conventional fossil sources. A geothermal power plant also has a negligible carbon footprint. Although there is some pollution from geothermal energy, it is significantly less than that from fossil fuels.

Geothermal energy is a renewable resource that has been available for a long time. It is sustainable and renewable since the earth’s heat reserves are constantly being replenished by nature.

Compared to other renewable energy sources like wind and solar power, geothermal power offers a dependable energy supply; unlike wind or solar energy, the resource is constantly accessible.

The quantity of energy generated by this resource is easy to estimate since it does not fluctuate as significantly as energy from solar and wind sources. This means we can accurately anticipate the electricity production from a geothermal plant.

There is no requirement for fuel because geothermal energy is a naturally occurring resource; in contrast, fossil fuels are finite resources.

Extensive study is being conducted on geothermal energy, leading to the development of innovative tools to improve the extraction and utilization of this resource. Numerous initiatives are being undertaken to advance and expand this sector of the economy. This rapid improvement will mitigate many of the problems now associated with geothermal energy.

7.1 Challenges and Limitations in Getting Geothermal Energy

7.1.1 Location Restricted

The primary disadvantage of geothermal energy is its dependence on geographic location. Geothermal plants can only be installed in areas with access to sufficient geothermal energy.

7.1.2 Environmental Side Effects

Greenhouse gases are not produced during the normal usage of geothermal energy, but many gases are stored under the surface and are released when excavations are performed. These gases are released into the atmosphere naturally, although the rate is higher around geothermal plants. However, these greenhouse gases are still significantly lower than those brought on by fossil fuels.

7.1.3 Earthquakes

There is a possibility that geothermal energy may trigger earthquakes. This issue is common with large geothermal power plants, which inject water into the earth’s crust to generate cracks and gain more energy. However, the effects of these earthquakes are generally limited because most geothermal units are located far away from populated areas.

8 Biomass Energy

We get renewable energy in the form of bioenergy when we burn biomass. Biomass fuels are derived from several sources of organic matter, including crop residues, animal residue, etc.

Biomass generates power at a significantly reduced environmental and financial cost by turning home, industrial, and agricultural waste into gas, fuel, etc.

Biomass technology is significantly less expensive than fossil fuel extraction, requiring substantial capital investments in oil drilling, pipelines, and fuel collection

8.1 Challenges and Limitations in Getting Geothermal Energy

Energy from biomass gives many benefits, but it also has significant drawbacks and limitations, such as:

8.1.1 Effectiveness

Some alternative fuels, including ethanol, are less efficient than traditional gasoline. Fossil fuel support is required in order to be as successful as possible.

8.1.2 Cleanliness

Animal waste raises methane emissions, which harms the environment. Additionally, pollution from the combustion of wood, plants, and other natural resources may be compared to that generated by coal combustion and other energy sources.

8.1.3 Deforestation

Wood is considered among the most popular biomass source of energy. Substantial wood and other waste materials are required to burn to provide the necessary energy. This increases the possibility of future deforestation.

Although biomass energy has significant drawbacks, more research and innovation are still being put into the sector to make it a more accessible, less expensive alternative, and valuable replacement for conventional electricity and other energy sources.

9 References

• Chowdhury, M. S., Rahman, K. S., Selvanathan, V., Nuthammachot, N., Suklueng, M., Mostafaeipour, A., & Techato, K. (2021). Current trends and prospects of tidal energy technology. Environment, development and sustainability, 23(6), 8179-8194.
• Gielen, D., Boshell, F., Saygin, D., Bazilian, M. D., Wagner, N., & Gorini, R. (2019). The role of renewable energy in the global energy transformation. Energy Strategy Reviews, 24, 38-50.
• Güney, T., & Kantar, K. (2020). Biomass energy consumption and sustainable development. International Journal of Sustainable Development & World Ecology, 27(8), 762-767.
• Hansen, K., Breyer, C., & Lund, H. (2019). Status and perspectives on 100% renewable energy systems. Energy, 175, 471-480.
• Kumar, Y., Ringenberg, J., Depuru, S. S., Devabhaktuni, V. K., Lee, J. W., Nikolaidis, E., … & Afjeh, A. (2016). Wind energy: Trends and enabling technologies. Renewable and Sustainable Energy Reviews, 53, 209-224.
• Lund, J. W., & Toth, A. N. (2021). Direct utilization of geothermal energy 2020 worldwide review. Geothermics, 90, 101915.
• Moriarty, P., & Honnery, D. (2016). Can renewable energy power the future?. Energy policy, 93, 3-7.
• Sen, S., & Ganguly, S. (2017). Opportunities, barriers and issues with renewable energy development–A discussion. Renewable and Sustainable Energy Reviews, 69, 1170-1181.
• Twidell, J. (2021). Renewable energy resources. Routledge.