A Walk Through Renewable Energy Sources: A Step Toward Net-Zero Emissions

In the call for a sustainable and eco-friendly energy future, renewable sources have emerged as a beacon of hope. Sunlight, wind, water, and other natural elements present a clean and limitless reservoir of power. In this article, we delve into the potential and various types of renewable energy sources that hold the promise of shaping a greener tomorrow.

Solar Energy

Sunlight reaches the surface of the earth with a peak intensity of around 1.0 kW/m², within a wavelength range of 0.3 to 2.5 micrometers (Oji et al. 2012). This is referred to as shortwave radiation, encompassing the visible light spectrum. In populated regions, this radiation varies between 3 to 30 mega-joules/m²/day, depending on location, time, and weather conditions (Sani et al. 2019). This represents a high-quality energy flow, originating from an easily accessible and heating source.

This renewable energy source involves two main technologies: 1) Photovoltaic (PV) cells and 2) Concentrated solar power systems (CSP).

a.       Solar PV systems

It directly transforms solar energy into electrical power using PV cells (mainly silicone), semiconductor devices that convert sunlight into direct-current electricity. These cells are connected to create PV modules, ranging from 50 to 200 W, usually integrated with various application-specific components like inverters, batteries, and other electrical devices to form a PV system. These modular systems allow the linking of modules to generate power ranging from small wattages to tens of megawatts (Ellabban et al. 2014).

PV systems are generally divided into two main categories: off-grid and grid-connected applications. Off-grid PV systems offer economic viability in areas without electricity in developing nations. Recently, centralized off-grid PV mini-grid systems have proven to be a dependable choice for electrifying villages (Branker et al. 2011).

In contrast, grid-tied PV systems utilize an inverter to transform direct current into alternating current, then feed the produced electricity into the electric grid. These installations are more cost-effective than off-grid systems because they do not require energy storage where the grid acts as a buffer.

Grid-connected PV systems are categorized into two application types: distributed and centralized. Distributed PV systems are set up to supply power either directly to a grid-connected customer or to the electric network. Centralized PV systems function as large power stations, providing bulk power to the grid without being tied to a specific customer or network function.

b.       Concentrated solar power systems (CSP)

CSP technology employs mirrors to gather sunlight and heating as either a fluid or solid material. The resulting steam drives a turbine, generating electricity. Large CSP facilities mainly use reflected sunlight, focusing either in a line (like Fresnel systems) or at a point (such as dish systems). CSP applications vary widely, from small distributed systems producing tens of kilowatts to large, centralized power stations generating hundreds of megawatts. The initial commercial CSP plants, such as the 354-MW solar electric generating stations in California, remain operational. Learning from these early experiences, the industry often utilizes trough systems as CSP industry expands.

In general, there are several types of solar panels including monocrystalline, polycrystalline, and thin film. Monocrystalline panels are known for their high efficiency and sleek appearance. Polycrystalline panels offer a more cost-effective option. Thin film panels are lightweight and flexible, making them suitable for specific utilizations.

Solar energy offers numerous benefits. It is an infinite energy supply, cost-free with no greenhouse gases, leading to zero air or water pollution. In addition, it promotes energy efficiency in buildings by utilizing natural heating and cooling systems and addresses the energy needs in areas lacking an electric network.

However, it comes with various disadvantages. Building solar power stations can be costly, making it potentially less economically viable. PV cells produce low output, and planar collector systems can pose health risks due to high temperatures and toxic heat transfer fluids. During the production of solar cells, workers may be exposed to hazardous substances. Moreover, electricity generation depends on adequate sunlight, impacting reliability and affecting storage capabilities.

Wind Energy

Wind energy is an ancient technology that is several millennia old. Wind turbines were initially deployed on a large scale in the US during the 1980s. However, their global production significantly increased during the mid-1990s (Moriarty & Honnery 2019). The Earth's rotation with uneven heating, and higher temperatures at the equator than at the poles, creates air movements known as wind. Producing electricity from wind involves converting the kinetic energy of moving air into mechanical and electrical energy that presents a challenge for industry to create efficient and affordable wind turbines and power plants.

The potential energy extracted from wind increases with the cube of the wind speed. Unfortunately, a turbine captures only a fraction of this energy (typically 40 to 50%). Therefore, wind turbine design aims to maximize energy capture across various wind speeds while minimizing the overall cost of wind energy, considering all relevant factors.

Wind turbines operate effectively during cloudy, windy, and rainy conditions, and their performance is significantly influenced by their placement. Typically positioned atop towers reaching approximately 30 m in height, wind turbines are spaced apart at distances from 5 to 15 times their blade diameter to prevent turbulence flow regimes affecting each other’s and can function in both horizontal and vertical axis configurations (Alrikabi 2014).

In a typical wind turbine, pitch bearings connect the rotor hub and blades, enabling adjustments to maximize energy extraction from the wind (Stammler et al. 2018). A yaw bearing aligns the turbine rotor with the wind direction. The method of alignment depends on the turbine's size. Larger turbines often use an active system involving a motor to turn the nacelle. In contrast, smaller turbines might employ a passive system, utilizing a tail fin to allow free movement of the nacelle based on wind direction. Passive systems are mainly used in smaller turbines, while micro wind turbines are suitable for taller buildings (Palraj & Rajamanickam 2020).

The power produced by a wind turbine, denoted as P and measured in watts, is determined by the following formula:

 

Where C_p is the coefficient of performance, p is the density of air (kg/m³), A is the swept area of the turbine blades (m³), and u is the wind velocity (m/s). The Betz limit, set at 59.3%, represents the theoretical maximum energy that turbines can extract from the wind (Ahmed et al. 2022).

It's important to mention that wind turbines require wind speeds of at least 13 km/h to function. They perform optimally when the wind speed averages around 22 km/h. Most wind turbines are horizontal axis models with three blades, ranging from 15 to 30 m in diameter, generating electricity between 50 to 350 kW (Alrikabi 2014). Furthermore, the continual expansion of wind energy has been facilitated by the introduction of new materials that allow for significantly larger blade diameters.

Wind energy is a universally accessible, cost-free, continuous, and widely available energy source. Like solar power, it is environmentally friendly, giving it one of the smallest carbon footprints among energy sources (Halkos & Gkampoura 2020). However, wind energy installations can cause noise pollution, disrupt telecommunication signals, detract from the visual appeal of landscapes, and sometimes cause harm to, or kill, migratory birds as they collide with turbines (Mohtasham 2015).

Wind turbines are effective only in areas with consistent strong winds, which limits their geographical applicability. To generate substantial electricity, a significant number of turbines are necessary, leading to notable visual changes in the environment. Storage during peak production times can be costly, and wind plants require extensive open spaces for installation. Lastly, the maintenance of wind turbines is expensive due to the wear and tear experienced by their mechanical components over time.

Geothermal Energy

Geothermal energy taps into the Earth's interior warmth through natural processes. It utilizes hot water, steam, and gases generated from the heat stored at varying accessible depths within the Earth's crust, where temperatures exceed atmospheric levels. In other words, geothermal energy is both clean and sustainable, and its sources range from shallow ground heat to hot water and rocks located several miles beneath the Earth's surface. It extends even deeper into the intensely high temperatures of molten rock, known as magma, with a geothermal gradient averaging about 30°C/km (Owusu & Sarkodie 2016). It can also be obtained through the radioactive decay of different isotopes within the Earth's interior and crust (Moriarty & Honnery 2011).

Various technologies including geothermal heat pumps, district heating, and hydrothermal reservoirs can be employed. For instance, geothermal heat pumps take advantage of the Earth's shallow ground, where temperature ranges between 10° and 16°C, to heat and cool buildings. These systems include a heat pump, ductwork, and heat exchangers composing a network of pipes buried in the ground near the structure. The heat pump extracts warmth from the heat exchanger during winter and circulates it indoors. Conversely, the heat pump transfers heat from indoor air to cool the building in summer and is used for free hot water as well (Alrikabi 2014). Nowadays, enhanced geothermal systems, and reservoirs adequately hot and enhanced through hydraulic stimulation are technologies under development (Halkos & Gkampoura 2020).

Geothermal energy was initially used on a small scale, however today it has started to occupy a much greater scale of application. The sources of this type of energy fall into three categories: 1) hydrothermal systems (comprising liquid and vapor-dominated types), 2) conductive systems (including hot rock and magma at varying temperatures), and 3) deep aquifers (involving fluids circulating in porous media or fractures beyond 3 km depth, lacking a localized magmatic heat source) (Ellabban et al. 2014).

At a depth of about 4 km beneath the Earth's surface, the temperature can reach around 90°C due to rock heat conductivity. Regions with geysers, hot springs, hot rocks, or volcanoes possess higher local potential for geothermal heat that may last for more than 1,000 years, as predicted by Sani et al. (2019).

A set of key formation properties play a vital role in determining the viability of a geothermal prospect.

a) Formation temperature: Higher formation temperatures typically lead to increased energy production value. However, excessively high temperatures, such as those related to magma chambers, can create adverse conditions and higher risks and costs, negating the benefits of greater energy potential.

b) Permeability: Higher permeability is advantageous as it allows for greater flow of injected and produced hot water. In contrast, excessively high permeability can cause an overly rapid water flow, resulting in insufficient energy transfer between the formation and circulating water, leading to decreased energy production.

c) Effective porosity: Greater effective porosity is preferred as it provides a larger surface area for transferring heat from the formation to the injected water.

d) Interconnected fractures: having better interconnected fractures opens onto good permeability, porosity, and surface area that facilitate efficient energy transfer.

Other features are also among the properties which affect the prospect viability, such as geomechanical properties, reservoir structure, lithology and geochemistry, and the formation's water chemistry (Bradley et al. 2019).

Geothermal energy offers numerous advantages, including its sustainability, relative cleanliness, affordability, and reliability. It is a readily available source of energy that can take millions of years to deplete. It has a minimal environmental impact, boasts high capacity and efficiency, and contributes to energy security and enhances living standards.

However, geothermal energy also comes with notable drawbacks: 1) While it releases fewer greenhouse gases than fossil fuels, certain harmful gases and minerals can still emerge from the ground, posing challenges in control, 2) Re-injection is necessary to manage emissions like hydrogen sulfide and carbon dioxide, 3) Despite being renewable, geothermal energy can face depletion and overexploitation concerns, and 4) Initial development expenses and ongoing maintenance costs, often due to corrosion, can present significant challenges.

4) Biomass

Biomass is a cornerstone in sustainable energy that transforms the potential of organic materials. It is considered one of the earliest fuels utilized by humans and remained an important global energy source until the mid-18th century. Recently, it is viewed as a carbon-neutral energy source, unlike fossil fuels that emit substantial carbon and contribute to global warming and ocean acidification (Sani et al. 2019).

Biomass refers to organic material sourced from trees, food crops, agricultural residues, municipal or industrial waste, grasses, wood plants, and even methane emissions from landfills (Mohtasham 2015). These materials generally store energy from sunlight in their chemical bonds. When these bonds are broken through combustion, gasification, anaerobic digestion, or decomposition, biomass energy is produced.

Alrikabi (2014) considers biomass as an indirect product of solar energy that comprises combustible organic compounds created through photosynthesis. Therefore, it is a proper conversion (mainly by combustion) of biomass sources into useful energy for transport, heating, electricity generation, and cooking (Owusu & Sarkodie 2016). Nowadays, bioenergy employs liquid biofuels, biogas, and various other technologies such as bio-refineries (Moon et al. 2022).

Biodiesel, ethanol, methane, butanol, vegetable oil, and others are typical examples of biofuels that are easily transported and stored. They enable on-demand heat and power generation, which is crucial in energy systems relying heavily on intermittent sources like wind power. Bioenergy is a main renewable energy source in the form of burned fuelwood, crop residues, and animal dung in developing nations, and contributes to about 4.3 exajoule (EJ) of the estimated global consumption of 59 EJ (Moriarty & Honnery 2019).

Anaerobic digestion is also a powerful process for waste transformation, including products like municipal solid and market wastes, using facultative bacteria that break down organic materials in the absence of oxygen, producing methane and carbon dioxide. The resulting effluent and digester residues contain valuable nutrients like nitrogen and phosphorus, which can be recycled as fertilizer. This method allows for the extraction of 70% of the energy from the biomass.

Biomass shows several advantages that contribute to its growing importance in the quest for sustainable energy solutions. The bioconversion process is renewable, environmentally friendly, and cost-effective. It is widely available and constitutes approximately 75% of the world's renewable energy sources (IRENA 2019). It is relatively low in pollution, and absorbs carbon dioxide and releases oxygen during growth, making it carbon neutral. Biomass energy is affordable and has significant potential for generating biogas. It contains less ash, sulfur, and trace elements and is even cheaper than fossil fuels (Ahmed et al. 2022). It also provides organic fertilizer, supports waste disposal, and contributes to rural employment and income, potentially alleviating poverty in developing nations.

Despite its merits, biomass energy also comes with many disadvantages. One major challenge is the need for a continuous supply of biomass to generate energy (Sani et al. 2019). Regardless of being carbon neutral, the environmental impact is significant when easily grown grains like corn and wheat are diverted to make ethanol, contributing to issues like vegetation degradation and deforestation (Halkos & Gkampoura 2020), and the consideration that burning biomass leads to air pollution.

In addition, cost-effectiveness is a concern, especially the high construction costs of biogas plants and their transportation pipelines over long distances that pose logistical difficulties. Furthermore, crops used for biogas production are seasonal and not available year-round and require blending with minor quantities of fossil fuels to enhance their effectiveness. Other technical challenges arise during thermochemical processing, such as slagging and corrosion (Ahmed et al. 2022).

5) Hydropower

It is produced from the movement of water and transforming the energy generated into electricity. Different water bodies, such as rain and rivers, flow naturally from higher to lower elevations, and can be properly regulated using dams and reservoirs to spin turbines and generate power. Dams provide significant storage capacity, allowing for large-scale energy production. In contrast, dam-less hydropower is suitable for smaller energy needs and is regarded as a more environmentally friendly choice.

This energy source has been utilized for many centuries, especially in the early days of electricity, and was generated from hydroelectric power plants. In general, hydropower turbines are employed to transform the pressure from water into mechanical energy that drives generators or other devices. The amount of power produced depends on the pressure head and the volume of flow.

Contemporary hydropower turbines can convert as much as 90% of the energy into electricity, though this efficiency diminishes with larger turbine sizes, and ranges between 60% and 80% in micro-hydro systems (Ahmed et al. 2022).

Note that some plants do not only generate energy, like pumped storage hydropower plants, but also serve as storage devices. In this setup, water is pumped from lower reservoirs to upper ones in low-demand periods, and during peak hours, the flow is reversed to generate power.

Hydropower plants come in various sizes that cover energy up to several gigawatts, depending most times on specific characteristics of the river system they are located in. Some of the largest projects include Itaipu in Brazil, generating 14,000 MW, and Three Gorges in China, producing 22,400 MW, and their output typically ranges from 80 to 100 TWh per year (Ellabban et al. 2014).

Hydropower is abundant, environmentally safe, producing no waste or greenhouse gas emissions. It allows adjustments in water flow based on power demand, can be upgraded quickly with a large future potential, and it can store energy for extended periods (Hamann 2015). Additionally, it contributes significantly to renewable energy, accounting for about 97% of electricity generated by renewables (Bagher et al. 2015).

On the flip side, hydropower comes with several drawbacks. Constructing dams is prohibitively expensive and often leads to water access issues. The process of building dams involves flooding nearby areas, impacting local wildlife and plant life. Furthermore, hydroelectric structures disrupt the natural ecology of rivers by altering their hydrological patterns and hindering the natural flow of sediment and fish migration due to the installation of dams, dikes, and weirs (Owusu & Sarkodie 2016). And finally, it relies on precipitation, and the availability of water cannot be always controlled (Halkos & Gkampoura 2020).

6) Ocean (Marine) Energy

Ocean energy refers to the several forms of renewable energy derived from the ocean's natural processes. These energy sources capture the power of ocean tides, waves, currents, temperature gradients, and salinity differences to generate electricity. Each category originates from distinct sources and demands unique technologies for conversion. In addition, it is influenced by the speed and duration of the wind and the distance it travels, and the resulting wave height as well. Some aspects of ocean energy potential follow.

Wave energy

It is the primary ocean energy source, generated when wind energy transfers to surface waters through shear forces and is captured by converters. Wave energy has a high power density of 2 to 3 kW/m², making it significantly more potent than solar or wind energy. It is available approximately 90% of the time and is well-matched to demand, with around 44% of the global population living within 150 km from a coastline (López et al. 2013).

Tidal energy

Different techniques are used to capture this power: 1) tidal stream (horizontal axis) systems using underwater turbines to extract energy from moving water, and 2) tidal range (vertical axis) systems that leverage the water-level disparity between high and low tides to produce electricity. Along coastlines, this energy density can reach 65 MW/mile, making tide-generated electricity economically viable. However, its worldwide potential is limited due to a small total resource base of 75 EJ, with approximately 73 EJ of it dissipated at coastlines where the tidal range is insufficient in many areas for practical utilization.

• Salinity gradient energy

It employs osmotic pressure differences in seawater of varying salinity levels. Researchers have modeled a power generation system, identifying key factors affecting its performance such as hydraulic pressure, draw solution flow rate, and feed flow rate. Optimizing the osmosis process under specific pressure conditions led to a significant increase in energy output, up to 54% higher than conventional methods (Altaee & Cipolina 2019).

• Ocean thermal energy

It is derived from the contrast in temperature between the warm surface waters and the chilly depths of the ocean. This variance in temperature can be exploited to produce electricity using technologies like ocean thermal energy conversion plants. In these systems, a fluid with a low boiling point, such as ammonia, is employed to vaporize and power a turbine, generating electricity as it expands (Liu et al. 2020).

Ocean energy provides numerous benefits. It has minimal environmental repercussions as it doesn't emit pollutants or generate waste. Additionally, it is consistently available, predictable, and abundant (Melikoglu 2018). Marine energy is particularly suitable for island nations, capturing energy that would otherwise go untapped, especially wave energy that holds promise as a renewable source for most countries with coastlines, as it can be utilized in open seas or near shores.

However, the construction of ocean energy facilities comes with high costs and the technology used is still in the developmental stages and requires further research, especially in terms of its socioeconomic and environmental impacts (Wright et al. 2016). Some environmental groups oppose it due to its potential negative effects on wildlife as it occupies significant space, posing challenges for maritime navigation. Moreover, wave energy faces challenges due to the variable nature of wave height, frequency, and direction changes, complicating the design of conversion devices. Wave power plants are vulnerable to cyclones and severe storms, causing interruptions in their operation during such periods (Alrikabi et al. 2014).

In summary, the investigation into renewable energy sources signifies a crucial stride toward a future that prioritizes sustainability and environmental awareness. Exploiting the potential of sunlight, wind, and water not only provides a clean substitute for conventional energy but also unlocks opportunities for innovative technologies and methodologies.

It becomes clear that the route to a more environmentally friendly future necessitates a shared commitment to adopting and progressing renewable energy solutions. Through ongoing research, advancements in technology, and widespread implementation, it is time to mark a transformative period for a cleaner and more environmentally conscious world.

For Further Reading

Ahmed, A., Ge, T., Peng, J., Yan, W. C., Tee, B. T., & You, S. (2022). Assessment of the Renewable Energy Generation Towards Net-Zero Energy Buildings: A Review. Energy and Buildings. https://doi.org/10.1016/j.enbuild.2021.111755

Alrikabi, N.K.M.A. (2014). Renewable Energy Types. Journal of Clean Energy Technologies. ‏ https://doi.org/10.7763/JOCET.2014.V2.92

Altaee, A., & Cipolina, A. (2019). Modeling and Optimization of Modular System for Power Generation from a Salinity Gradient. Renewable Energy.‏ https://doi.org/10.1016/j.renene.2019.03.138

Bagher, A.M., Vahid, M., Mohsen, M., & Parvin, D. (2015). Hydroelectric Energy Advantages and Disadvantages. American Journal of Energy Science.‏ https://www.researchgate.net/publication/275094706_Hydroelectric_Energy_Advantages_and_Disadvantages

Bioenergy and Biofuels. (2019). https://www.irena.org/Energy-Transition/Technology/Bioenergy-and-biofuels

Bradley, T., Aarnes, I., Outrequin, D., Monneyron, N., & Reseaux, E. (2019). Application of Oil and Gas Methodology to Geothermal Formation Evaluation: The Value of Data. GRC Trans.‏ https://publications.mygeoenergynow.org/grc/1034186.pdf

Branker, K., Pathak, M.J.M., & Pearce, J.M. (2011). A Review of Solar Photovoltaic Levelized Cost of Electricity. Renewable and Sustainable Energy Reviews. https://doi.org/10.1016/j.rser.2011.07.104

Ellabban, O., Abu-Rub, H., & Blaabjerg, F. (2014). Renewable Energy Resources: Current Status, Future Prospects and their Enabling Technology. Renewable and Sustainable Energy Reviews.‏ https://doi.org/10.1016/j.rser.2014.07.113

Halkos, G.E., & Gkampoura, E.C. (2020). Reviewing Usage, Potentials, and Limitations of Renewable Energy Sources. Energies.‏ https://doi.org/10.3390/en13112906

Hamann, A. (2015). Coordinated Predictive Control of a Hydropower Cascade. Carnegie Mellon University.‏ https://hydrofoundation.org/wp-content/uploads/2022/05/hamann_final_research_findings.pdf

Liu, W., Xu, X., Chen, F., Liu, Y., Li, S., Liu, L., & Chen, Y. (2020). A Review of Research on the Closed Thermodynamic Cycles of Ocean Thermal Energy Conversion. Renewable and Sustainable Energy Reviews.‏ https://doi.org/10.1016/j.rser.2019.109581

López, I., Andreu, J., Ceballos, S., De Alegría, I.M., & Kortabarria, I. (2013). Review of Wave Energy Technologies and the Necessary Power Equipment. Renewable and Sustainable Energy Reviews.‏ https://doi.org/10.1016/j.rser.2013.07.009

Melikoglu, M. (2018). Current Status and Future of Ocean Energy Sources: A Global Review. Ocean Engineering. https://doi.org/10.1016/j.oceaneng.2017.11.045

Mohtasham, J. (2015). Renewable Energies. Energy Procedia.‏ https://doi.org/10.1016/j.egypro.2015.07.774

Moon, M., Park, W.K., Lee, S.Y., Hwang, K.R., Lee, S., Kim, M.S., et al. (2022). Utilization of Whole Microalgal Biomass for Advanced Biofuel and Biorefinery Applications. Renewable and Sustainable Energy Reviews.‏ https://doi.org/10.1016/j.rser.2022.112269

Moriarty, P., & Honnery, D. (2011). Rise and Fall of the Carbon Civilization: Resolving Global Environmental and Resource Problems. London: Springer.‏ https://doi.org/10.1007/978-1-84996-483-8

Moriarty, P., & Honnery, D. (2019). Global Renewable Energy Resources and Use in 2050. In Managing Global Warming . Academic Press.‏ https://doi.org/10.1016/B978-0-12-814104-5.00006-5

Oji, J.O., Idusuyi, N., Aliu, T.O., Petinrin, M.O., Odejobi, O.A., & Adetunji, A.R. (2012). Utilization of Solar Energy for Power Generation in Nigeria. International Journal of Energy Engineering.‏ http://dx.doi.org/10.5923/j.ijee.20120202.07

Owusu, P.A., & Asumadu-Sarkodie, S. (2016). A Review of Renewable Energy Sources, Sustainability Issues, and Climate Change Mitigation. Cogent Engineering.‏ https://doi.org/10.1080/23311916.2016.1167990

Palraj, M., & Rajamanickam, P. (2020). Motion Control of a Barge for Offshore Wind Turbine (OWT) Using Gyrostabilizer. Ocean Engineering. https://doi.org/10.1016/j.oceaneng.2020.107500

Sani, G. D., Ibrahim, A., Sahabi, S., & Lailaba, B.B. (2019). Renewable Energy: Environmental Impacts and Economic Benefits for Sustainable Development. International Journal of Engineering Research.‏ Renewable Energy: Environmental Impacts and Economic Benefits for Sustainable Development (researchgate.net)

Stammler, M., Schwack, F., Bader, N., Reuter, A., & Poll, G. (2018). Friction Torque of Wind-Turbine Pitch Bearings–Comparison of Experimental Results With Available Models. Wind Energy Science.‏ https://doi.org/10.5194/wes-3-97-2018

Wright, G., O’Hagan, A.M., de Groot, J., Leroy, Y., Soininen, N., Salcido, R., et al. (2016). Establishing a Legal Research Agenda for Ocean Energy. Marine Policy.‏ https://doi.org/10.1016/j.marpol.2015.09.030