A Comprehensive Review of Green Energy Technologies: Towards Sustainable Clean Energy Transition and Global Net-Zero Carbon Emissions

Abstract

The present paper seeks to showcase the significant potential of alternative energy technologies in driving clean energy transition. Renewable energy sources, including hydro, geothermal, biomass, solar, and wind energy, are developed and marketed as low- or non-carbon alternatives to conventional energy sources. However, the high upfront costs of these energy resources, coupled with their intermittency, are demerits that must be dealt with. Since certain nuclear technologies generate significantly less waste than coal and oil, nuclear energy is occasionally regarded as a green energy source, though the primary source of nuclear energy, namely uranium, is a finite resource. The main goal of developing green energy technologies is to provide energy in a sustainable manner while cutting down on waste and greenhouse gas emissions, thus reducing the overall carbon footprint of energy production.

1. Introduction

By 2050, the world’s population is expected to grow from 7.8 billion in 2020 to 9.9 billion, requiring 80% more energy [1,2]. Unfortunately, over 759 million people do not have access to consistent electrical energy [3,4], and they are unable to engage in the contemporary economy, which is a significant economic barrier. Lack of energy access has a detrimental influence on women’s chances, education, public and household health, and the already precarious vulnerabilities that exist in impoverished communities.

The absence of contemporary energy infrastructure, such as power plants, transmission lines, and subterranean pipelines for the delivery of energy supplies like natural gas, is one factor contributing to energy poverty. Even if energy is readily available, people with lower incomes may not be able to afford it; therefore, they avoid utilizing it. Energy poverty is more and more at the center of the global debate on sustainable development, especially in developing countries. The top 20 least electrified nations worldwide as of 2019 were all found in Africa, accounting for roughly eight out of every ten people without electricity in 2021 [5,6].

The most recent analysis predicts that under the current policy scenario, 660 million people—the majority of who live in Sub-Saharan Africa—will still not have access to energy in 2030 [4,5]. Significant disparities in access between urban and rural areas can also be seen in the region. Considering the foregoing, energy, especially clean energy and its preservation, effective administration, and a variety of eco-friendly energy technologies are significant subjects that require specific attention from the global scientific community [7]. There is a need to speed up the development of alternative energy and energy efficiency measures [8].

Numerous renewable energy sources, including hydro, geothermal, solar, wind, and bioenergy, are never-ending sources of energy that replenish themselves continuously. Most of the renewable energy that comes from the sun, whether directly or indirectly, can be utilized to heat water, cool buildings, power various commercial and industrial devices, and generate electricity.

To produce chemicals, polymers, materials, feed, and bioenergy, among other bio-based goods, it is extremely desirable to utilize biomass resources. While significant progress has been made, research is still needed to overcome constraints that are both technological and non-technological.

Although many authors have dwelt on the importance of alternative and green energy resources towards the achievement of a sustainable energy transition and a secure energy future for mankind, the present paper is intended to (i) discuss various sources of renewable energy, (ii) highlight a few of the recent case studies of small-, medium-, and large-sized plants based on green energy technologies, notably solar thermal and biomass technology, and (iii) underscore some emerging green energy technologies that have the potential to accelerate global energy transition and achievement of net-zero emissions.

2. Green Energy

Energy is important for life and operations throughout the universe. Energy resources are divided into finite and renewable energy. Energy sources can be generally divided into two groups: renewable or green energy (infinite) sources, which can be used repeatedly, and non-renewable energy (finite) sources, which cannot be reused [9].

Green energy is created with the least amount of adverse environmental effects possible. The usage of fossil fuels, in addition to other major energy sources, emits greenhouse gases, which are thought to be the main contributor to the phenomenon known as global warming or climate change. Renewable energy sources, including hydro, geothermal, solar, and wind energy, are developed and marketed as substitutes that contribute little or nothing to the global warming process.

Owing to the fact that some nuclear technologies produce far less waste than coal and oil, even nuclear energy is sometimes regarded as a green energy source [10]. Nuclear and renewable energy are two viable alternatives to fossil fuel-based energy generation [10].

2.1. Up-to-Date Information/Status on Nuclear Energy

Nuclear energy is generated by heavy atom splitting or the fusion of light atoms; more specifically, there are two processes: fission and nuclear fusion [11]. The fission of uranium, for instance, ref. [10] is a mature process and the principle on which all reactors built up to it are based. Its main criticality is the production of radioactive waste [12].

Arising from the global energy crisis of the 1970s and the consequent sharp rise in oil prices, an increasing number of nations made the decision to start nuclear energy programs [13]. In fact, most of the world’s reactors were built between 1970 and 1985.

Currently, there are an estimated 440 nuclear reactors in operation across 32 nations, including Taiwan, with a total capacity of about 400 GWe (Figure 1). In 2023, these countries supplied 2602 TWh, around 9% of the global electricity. Thirteen nations generated at least 25% of their electricity from nuclear power in 2020, with the United States, Russia, China, and France holding a significant market share [10,14]; in fact, as of 2021, nuclear reactors produced about 68% of electricity in France [15].

Overall, in the last 40 years, the number of plants has been fairly stable based on the quantity of installed power. Many countries are planning or are building new power reactors. As of 1 July 2024, 59 reactors (58.6 GW) were under construction, and four out of five reactors are under construction in Asia or Eastern Europe. Nuclear power facilities are being built in sixteen nations, which include Egypt and Brazil. Just four nations—China, India, Russia, and South Korea—have construction underway in several locations (Figure 1).

In 2022, construction of ten reactors began around the world, including five in China and the other five being deployed by Russia to Egypt (2), Turkey (1), and domestically (2). Construction of three more reactors began in the first half of 2023, with two of them in China and one from Russia to Egypt.

On the technology side, as shown in Figure 2, as of mid-2023, Russia has 24 units under construction, dominating the global market. South Koreans and French are constructing overseas in the United Arab Emirates and the United Kingdom.

In recent years, nuclear power has been gaining traction as part of the backbone of a zero-emission energy mix, thanks in part to the ability to replace end-of-life nuclear power plants with advanced new nuclear power. The number of fusion devices around the world is given in Figure 3.

Advanced nuclear technologies can be classified into two groups:

Small Modular Reactors (SMRs), which exploit current technology on a small scale, in a circuit-type configuration (loop) or in an integral configuration (primary circuit components all installed in the reactor vessel);

Advanced Modular Reactor (AMR), derived from fourth-generation technologies, uses new cooling systems (e.g., liquid lead) or innovative fuels to offer better performance, new functionalities (cogeneration, hydrogen production, solutions for closing the fuel cycle, and, therefore, nuclear waste management), and a change in pace for higher economic competitiveness, sustainability, passive safety, and reliability, as well as proliferation resistance and physical protection.

There are more than 80 projects and concepts on these issues around the world. While some are said to be short-term deployable, most are still in different phases of development. Several established and emerging nuclear energy nations are engaged in SMR research and development, and four SMRs are presently in advanced stages of construction in Argentina, China, and Russia.

However, it should be noted that there are still uncertainties on the part of investors about business models and whether, linked to the high costs of technologies, advanced reactors guarantee better investment returns than other business models.

As a research activity on this specific topic, the International Thermonuclear Experimental Reactor (ITER) project is cited, whose goal is to replicate the fusion processes that take place on the Sun [18].

A prototype fusion reactor to produce electricity is the goal of the continuing project, which is being carried out in collaboration between Europe, the United States, China, India, Japan, Russia, and South Korea [18]. With an estimated cost of around EUR 20 billion (USD 25 billion), the majority of construction costs (45.6%) are borne by Europe, with China, India, Japan, Korea, Russia, and the United States sharing the remaining 9.1%. In total, 35 countries share the costs of the project.

ITER is intended to generate 500 MW of fusion energy from 50 MW of input heating power, or ten times more energy return (Q = 10). By 2025, the project will have more than half of its superheated plasma tested, and by 2035, it will have achieved its first full-power fusion.

2.2. Merits and Demerits of Nuclear Energy

Divergent views on nuclear energy have existed since the 1950s, when the first nuclear reactor came online. It is true that nuclear energy is a safer substitute for fossil fuels and that nuclear power generation is less polluting than electricity production from coal and oil [19]; in fact, coal and oil are “invisible killers”. It is estimated that, in 2018 alone, fossil fuels caused the death of 8.7 million people worldwide [20].

However, the fact remains that nuclear energy poses many concerns, just to mention the risk associated with the management of radioactive waste and poisonous by-products of nuclear reactors [21]. Thus, a global stockpile of 250,000 tons of highly radioactive nuclear waste has been accumulated and dispersed since the 1950s; 90,000 tons of this material are stored in storage in the United States alone [22,23]. Nuclear energy, therefore, has the potential for catastrophic failures [24]. A recent example is the Fukushima accident in 2011. Although it did not directly kill anyone, it caused nearly 150,000 displaced people, thousands of deaths from evacuation-related causes, and billions of dollars in clean-up costs.

While there have been only three major nuclear accidents in the nearly 70 years since the invention of nuclear energy—Fukushima in 2011, the Chernobyl Accident in 1986, and the 1979 Three Mile Island Accident—of which the accident at the Ukrainian Chernobyl reactor was the most serious and resulted in numerous fatalities [25], it has caused widespread concern that even leads to popular belief about the pollution of their cooling towers. Contrary to popular belief, the cooling towers of nuclear plants only release water vapor into the atmosphere; no pollutants or radioactive materials are released [26].

On the other hand, there is no doubt that there are some advantages that nuclear energy offers over the most widely used green energy sources. Nuclear power has, by far, the highest capacity factor, is widely available, is affordable, and has almost unlimited fuel. According to the U.S. Office of Nuclear Energy, these plants are three times more reliable than wind and solar plants because they require less maintenance, can run for up to two years before they need to be refueled, and can produce power at full output for more than 93% of the time of year [27].

The European Commission has clarified its position by listing environmentally friendly economic activities and classifying nuclear energy as a green energy source [28]. This is mainly due to the fact that up to 90% of the radioactive waste produced during the creation of nuclear energy is recyclable [29]. In fact, because fission extracts very little energy from the fuel, the fuel used in reactors, usually uranium, can be treated and used in other reactors [14].

The use of this type of energy is opposed by the anti-nuclear movement for several reasons. The first and most discussed drawback of nuclear energy now is the spread of nuclear weapons. This discussion was triggered by the devastating atomic bombings of Hiroshima and Nagasaki in Japan during World War II and has recently been reignited due to growing fears of a nuclear escalation in the conflict between Russia and Ukraine [30].

In 1970, the Treaty on the Non-Proliferation of Nuclear Weapons came into effect. Its objectives were to encourage the peaceful use of nuclear energy and stop the spread of these weapons in order to eventually achieve nuclear disarmament.

Critics of nuclear energy still see this source of energy as closely linked to nuclear weapons technologies and think that as nuclear technologies become more widely available, there is a greater chance that they will end up in the wrong hands, particularly in nations with high levels of instability and corruption [31]. The second reason is that nuclear energy is considered to be one of the most expensive and time-consuming sources compared to other energy sources. Building a nuclear power plant takes years, often even more than ten years, and costs billions of dollars. It is also the most expensive type of green energy infrastructure.

Nevertheless, nuclear power plants are more competitive because, despite their high construction costs, they are generally inexpensive to operate [14,30] and can play an important role in global decarbonization. It has been estimated that by 2050, a total of 300 GW of new nuclear power will be needed in the United States to make an impact on reducing carbon emissions.

Many proponents of nuclear energy also argue that the fastest decarbonization effort in history was triggered by nuclear energy, citing the construction of nuclear reactors in major nuclear-producing nations such as South Korea, France, Saudi Arabia, and Canada as examples of these nations’ clean energy transitions and the most rapid decline in carbon intensity [26].

One concept on which everyone agrees, however, is the diversification of sources and that instead of relying heavily on a single source in the future, it will be necessary to define the optimal energy mix that will include various energy-generating techniques.

3. Renewable Energy Sources

It is widely accepted that increasing the use of renewable energies is a viable option and one of the key pillars to decarbonize the energy system and help lessen the effects of climate change, even though fossil fuels still account for around 80% of global energy consumption. Two-thirds of the world’s energy supply is predicted to come from renewable sources by 2050. Of these, solar energy appears to be a promising technological option to carry out the shift to a distributed and decarbonized energy supply because it can be installed anywhere in the world.

Among renewables, solar energy has been used directly or indirectly for many years for both commercial and industrial applications, including domestic and industrial hot water heating, solar cooling, and electricity generation. Solar energy, both in direct application and as photovoltaics (PV), can be crucial to cutting-edge energy-saving initiatives, particularly in new construction. According to Bernreuter Research, solar energy with a worldwide operational capacity of 7.4 TW [32] by 2028 will certainly be a promising technological alternative to achieve the transition to a decarbonized energy supply.

Wind energy, often known as wind power, is a renewable energy source that uses the force of the wind to create electricity. The main character of Miguel de Cervantes’ masterpiece, Don Quixote, is arguably one of the most iconic representations of wind power. Wind energy now ranks second among renewable energy sources due to ongoing technological advancement and innovation; in 2023, the total installed wind power capacity worldwide was estimated to be 1021 gigawatts. Eight to ten percent of the world’s electricity generation during the past ten years has come from wind power, according to statistics from a 2019 report prepared by the International Renewable Energy Agency (IRENA). By meeting the growing need for energy worldwide and reducing environmental issues, wind energy has the ability to support long-term economic prosperity [33,34,35].

With an offshore wind capacity of 72,663 megawatts, the total installed wind power capacity globally in 2023 was almost 1021 gigawatts (Figure 4). At over 946 gigawatts that year, onshore wind power made up the majority of the total wind power capacity. The nations with the most offshore wind farms are China, Germany, the United Kingdom, and Vietnam. An increased supply of electricity from offshore wind energy is made possible by higher wind speeds that occur offshore as opposed to on land.

The global wind power market is expected to reach USD 278.43 billion by 2030, up from USD 112.23 billion in 2022, according to a new Kings Research analysis.

Geothermal energy is a significant renewable energy source since it uses heat that is stored in the earth’s crust. As of 2020, its installed capacity was estimated to be 15 GW worldwide. Leading nations include the United States, Indonesia, the Philippines, Italy, Turkey, Mexico, Kenya, and New Zealand. For instance, GSI predicted that 10 GW might theoretically be collected from geothermal energy in India. To verify the economic and technical feasibility of generating electricity from improved geothermal systems by means of innovative exploration research, resource management, and assessment plans, more sophisticated and reasonably priced drilling and stimulation technologies and more efficient power cycles are required [37,38].

Hydropower, also known as hydraulic energy, is a sustainable primary energy source that converts the gravitational potential energy of a mass of water at a specific altitude. One of the most popular renewable energy sources is hydropower. The world’s biggest contributor to hydroelectric capacity is China.

Depending on their size, hydropower projects are divided into two categories: large and small. The size requirements used to categorize small hydropower plants vary by nation. In 1878, the Cragside country house in Northumberland, England, employed the first hydroelectric plant in history to light a single bulb. Within ten years, hundreds of hydropower plants were operating in the United States, with the first plant serving a system of private and commercial clients opening its doors in Wisconsin four years later. As the new technology evolved, hundreds of tiny hydropower plants were operating globally by 1900. A 500 kW installed hydroelectric station was constructed in China in 1905 on Xinjian Creek close to Taipei. In 2023, the total hydropower capacity worldwide was roughly 1416 gigawatts. After the United States, Canada, and Brazil, China has 29% of the world’s installed hydroelectric capacity. Numerous steps were taken in India to promote small hydropower, with a 100 MW aim.

In 2023, hydropower generated 4210 TWh, ranking it as the third biggest source of electricity, behind coal and gas. According to a recent report by the International Renewable Energy Agency, hydroelectricity is still the most affordable electricity source globally, with a cost of USD 0.05/kWh. According to the Net Zero Emissions by 2050 Scenario, hydropower is expected to generate over 5500 TWh of electricity year between 2023 and 2030.

The most effective method of producing electricity is hydropower. Up to 90% of the available energy can be converted into electrical power by modern hydro turbines. With generation costs ranging from 2 to 4 cents per kilowatt-hour, hydropower is thought to be reasonably cheap to run. The levelized cost of electricity (LCOE) range for small hydropower projects in developing countries, as evaluated by IRENA, remained relatively stable between USD 0.02 and USD 0.10/kWh throughout the past decade.

That year, the average cost of hydropower was about 6.1 cents per kilowatt-hour. Hydropower prices in India are relatively low, ranging from INR 2.50 to INR 4.00 per kWh. This is due to the fact that hydropower plants have a long lifespan and low operating costs. The average cost of producing hydropower in the United States is 0.85 cents per kilowatt-hour (kWh). According to recent data, hydropower in Wisconsin is generated for less than one penny per kilowatt-hour (Figure 5). This is around one-third the price of fossil fuels and half the price of nuclear.

New Ocean concepts to increase the availability and predictability of deliverable energy, along with better installation and production procedures and harmonized testing methods to promote the development of cheaper and safer onshore and offshore systems [40]. This ocean energy is derived from the kinetic energy of ocean waves. Ocean energy systems use variations in water temperature and salinity, along with the force of tides and waves, to generate electricity. In 2023, the global capacity of marine energy reached 527 megawatts (South Korea and France, Russia, and the Netherlands, generating 256, 212, and 5 megawatts, respectively).

The hybrid photovoltaic–thermal (PVT) system is a potential innovation that can generate both electricity and heat energy at the same time, making it ideal for use in both residential and commercial structures. Building thermal energy demands may be satisfied using solar-powered heat pump PVT systems. There have also been reports of PVT systems’ potential advantages in terms of longevity, dependability, and energy efficiency [41].

There have been reports of numerous ongoing scientific investigations to improve the technology’s dependability and use in decarbonizing buildings [42]. Nevertheless, the reality is that there is currently a dearth of information about the environmental implications of PVT systems’ industrial applications. In summary, hybrid PVT systems undoubtedly provide a wide range of applications, but significant cost and upgrade improvements are necessary for their market penetration. For PVT technology to reach its full potential and become commercially viable, ongoing research and development is necessary.

According to the STEPS, renewables will provide 80% of new electricity capacity by 2030, with solar photovoltaics alone making up more than half of this total. It is important to remember, nevertheless, that the growth of renewable energy not only impacts the environment but also generates employment. According to a survey by IRENA, renewables in 2023 will employ over 16 million people, with an increasing trend over the years and, above all, with strong growth in 2023 in the photovoltaic sector, certainly linked to the strong reduction in technology costs that have occurred in recent years.

As regards photovoltaics specifically, in 2023, there were approximately 7.1 million people employed in this particular sector, of which approximately 4.6 million, equal to 65% in China, followed by India with approximately 310,000 new employees (see Figure 6).

Over 38% of the electricity produced in the European Union came from renewable sources, including solar and wind power, which combined to generate 624 TWh. Similar to this, a sizable portion of the power generated in the Asia Pacific region, particularly in nations like China, Japan, and India, comes from renewable sources like solar, wind, and hydro. For example, China’s solar PV generated 423 TWh of electricity in 2022, while India’s combined wind and solar power generation was 3.7 times greater than that of nuclear reactors. As regards the production of electricity from renewables, in 2023 a power of approximately 4000 GW has been installed in the world. Among the various sources, wind and photovoltaic energy continue with strong growth, while the other sources remain stationary (see Figure 7).

It is important to note that in 2024, global energy investment is expected to surpass USD 3 trillion for the first time, with USD 2 trillion allocated to clean energy infrastructure and technology [32].

3.1. Solar Thermal Energy and Its Evolution with Time

This is a method of harvesting thermal energy (heat) from the sun for practical purposes. Currently, dependable and well-developed solar thermal technologies available on the market are contributing significantly to meeting the low-temperature (<100 °C) global heating demands. Moreover, advanced energy-saving initiatives, particularly those involving new buildings and structures, can greatly benefit from the application of these technologies.

2020 saw the addition of an expected 25.2 GWth of new solar thermal capacity, bringing the total to approximately 501 GWth worldwide, while global CSP capacity increased to 6.2 GW [33]. The year 2023 witnessed a newly installed capacity of 30 million square meters of collector area or a capacity of 21 GWth. The global solar thermal market size was valued at USD 21.5 billion in 2021 and is projected to reach 984.39 GW by 2032. In Europe, only a handful of countries recorded market growth in 2023. By the end of 2023, 336 solar district heating systems with an installed capacity of 1.9 GWth (2.73 million square meters) were in operation.

India, a significant solar thermal market, saw a 27% increase in solar thermal energy. Both Latin America and southern Africa are seeing the emergence of expanding marketplaces. South Africa’s market grew by 12%, while Mozambique’s grew by an astounding 40%. Additionally, 5% and 3% growth rates were reported by Mexico and Brazil, respectively. At least 116 new industrial process heat systems with 94 MWth of output were put into place globally in 2023. In addition to the two PV district heating systems constructed in 2023, there are an increasing number of solar combination systems for space and hot water heating in residential buildings. According to the REN21 Renewables 2022 Global Status Report, India ranks fourth in the world for installed renewable energy capacity with 167.75 GW as of 31 December 2022 and fourth for both wind and solar power capacity. The overall solar heat capacity by the end of 2023 was 560 GWth, which is equal to 800 million m² of collector area and a net reduction of 158.4 million tons of CO₂ and 49.1 million tons of oil. Furthermore, according to the International Energy Agency’s World Energy Outlook 2023 study, the highest emissions reductions to 2030 in the NZE Scenario will be achieved by tripling the installed capacity of renewable energy sources worldwide to 11,000 GW by 2030. International organizations and governments around the world have set a number of goals to be accomplished by the end of 2020, 2030, 2040, and 2050. Increasing the proportion of renewable energy sources in energy generation is the primary focus of these aims.

3.2. Solar Thermal Technologies and Their Prospects in the Future

Undoubtedly, because of availability, accessibility, capacity, and efficiency, solar thermal technologies can be used for both low-to-medium temperature applications such as drying, heating, cooling, and cooking and for a variety of industrial applications for sustainable energy systems in industries.

However, on an industrial scale, concentrated solar power (CSP) technologies are utilized to generate electricity, while concentrated solar thermal (CST) technologies are used to meet such heating requirements. There are currently four different kinds of CSP technologies in the market [44]: (i) solar dish collectors (which concentrate power by focusing ST energy onto a single point above a reflector dish); (ii) Fresnel mirrors; (iii) power towers (which are arrays of thousands of sun-tracking reflecting mirrors placed in a field to concentrate solar radiation to a single point); and (iv) parabolic troughs.

It has been observed that CSP works well in areas with little to no cloud cover or haze (Figure 8).

Furthermore, it is important to note that both large commercial systems and household systems are undergoing a change. A total of 598 large-scale solar heat plants with a combined installed capacity of 2.3 GWth—or 3.3 million m² of collectors—were operational by the end of 2023. A total of 560 GWth, or 800 million m² of collector area, was the entire solar heat capacity by the end of 2023. This translates to a net reduction of 158.4 million tons of CO₂ and 49.1 million tons of oil. Additionally, solar-powered air conditioning and cooling systems, including solar thermal and PV-driven systems, have a lot of potential. Systems powered by solar thermal energy employ natural refrigerants like water and ammonia and use less conventional energy. With more than 2000 systems installed worldwide as of 2022, solar thermal cooling is still a specialized market. Europe is home to 70% of the world’s small and medium-capacity (less than 350 kW) solar cooling systems. Nonetheless, there are a number of worldwide projects and growing awareness of small- to medium-sized solar thermal-driven systems. Commercial solar thermal cooling, for instance, is the subject of ongoing research initiatives in China, the United States, Mexico, Mali, Uganda, Nigeria, Morocco, Egypt, Jordan, Dubai, Greece, Spain, Austria, the Netherlands, Ukraine, India, and Thailand. International organizations and governments around the world have set a number of goals to be accomplished by the end of 2020, 2030, 2040, and 2050. Increasing the proportion of renewable energy sources in energy generation is the primary focus of these aims.

3.2.1. Solar Heat for Industry

Solar Heat for Industrial Processes, showing significant growth in recent years, represents an incredibly promising market segment. Industrial procedures, such as washing or dyeing fabrics, can employ low and medium temperatures. Heat is used in the dairy industry for pasteurization and washing. It can be reached in other sectors, such as mining. As a result, there are many different applications for low- and medium-temperature heat in industrial operations.

The market niche for Solar Heat for Industrial Processes is very promising and has shown rapid expansion in the last several years. This expansion is seen in both the quantity and the scope of the applications, which are becoming more and more prevalent throughout Europe and beyond in many industrial sectors [34].

The required temperature range dictates which solar thermal collectors to utilize. Evacuated tube collectors, flat plate collectors, and air collectors are used for temperatures up to 150 °C. Concentrating solar thermal collectors, such as concentrating dishes, Fresnel collectors, and parabolic troughs, can give larger temperatures—up to 400 °C or more when needed [35,44,46].

At least 116 new SHIP plants with a 94 MWth capacity were put into service globally in 2023. Given its enormous potential and the total number of SHIP plants (about 1200 systems, with a collector area of 1.4 million square meters and a capacity of 951 MWth), this technology can be crucial to industrial decarbonization initiatives and the achievement of net-zero emissions [47]. In view of the European pulp and paper industry’s set target of attaining carbon neutrality by 2050 [48], Solar Heat for Industrial Processes, when applied in both the pulp and paper sector, has significant potential to accelerate the decarbonization of industrial processes. For such applications, non-concentrating solar collectors, such as evacuated tube collectors and conventional or evacuated flat plate collectors, can generate heat up to 150 °C by both direct and indirect radiation [38,49].

Concentrating solar collectors, on the other hand, by using Fresnel mirrors or lenses, small or large parabolic reflectors or troughs, or other techniques, can reach temperatures of more than 400 °C. They are especially effective in sunny regions of Europe since they utilize direct sunlight and function best on cloudless days [35].

Three primary factors that significantly affect the cost of the energy produced by a solar thermal system are the system’s initial cost, lifespan, and performance. Under ideal circumstances, the cost of solar heat can range from 20 to 50 EUR/MWh. The 2012 IEA report states that the investment costs for large systems in Europe might range from 315 to 936 EUR/kWth (335 to 1040 USD/kWth). Energy expenses (Figure 9) might vary from 40 to 150 USD/MWhth (36 to 135 EUR/MWhth) in Europe and from 20 to 70 USD/MWhth (18 to 63 EUR/MWhth) in the Southern United States.

3.2.2. Solar Energy for District Heating

The number of megawatt-scale solar district heating systems for industrial and district heating applications is steadily increasing. Solar district heating is the largest sub-sector of large-scale solar thermal heating systems. With an installed capacity of 1795 MWth (2.56 million m²), 325 large-scale solar district heating systems (>350 kWth, 500 m²) were operational by the end of 2022. Apart from one concentrating system in Mexico and four concentrating systems in China, which would have increased the above total by 5300 m², 218 MWth (311,700 m²) were installed outside of Europe. In 2019, 47 big building and district heating systems (307,000 m²) were installed in China. In 2019, Zhongba installed the biggest district heating system in China, with a 35,000 m² collector area. With a collector area of 21,700 m², Latvia had the largest system deployed in Europe in 2019, aside from the systems in Denmark. The largest solar district heating system has a collector area of 18.300 m² and a thermal capacity of about 10 MW [39].

The lack of inexpensive seasonal heat storage, the low cost of other fuels, and the thermal utilities’ skepticism about solar heating are the main barriers to expansion. By the end of 2023, there were 598 systems operating in residential buildings and connected to district heating networks, totaling 2285 MWth of capacity, or 3.3 million square meters of collector area (Figure 10).

Apart from solar district heating, the second area of interest in the large-scale industry is solar applications for public, commercial, and residential buildings. Globally, 246 large-scale solar thermal systems (>350 kW at 500 m²) were providing heat to public, commercial, and residential structures at the end of 2022. These systems have a combined installed capacity of 353 MWth (504,422 m²).

3.2.3. Combined DHW and Space Heating (Combi-Systems)

Over the last ten years, a growing number of solar-powered space and hot water heating systems—often called “combi systems”—have started to gain traction. Installing combi-systems—which can heat both residential hot water and space—has become standard practice in Central and Northern Europe. These combi-systems usually have a collector size of 7–20 m² and a tank(s) capacity of 300–2000 L. Combination systems are often more advanced than solar systems that only provide DHW. At the end of 2021, 1428, 151 combi-systems with a combined collector area of 16,659,891 m² were in use for space heating and household hot water supply [51].

Even while combi-systems have a lot of potential to provide residential hot water all year round, air conditioning in the summer, and space heating in the winter, they are still not very common in Southern Europe.

3.3. Solar Thermal for Electricity Production

Based on how they focus the sun’s rays and the technology they use to capture the energy, the four main tracking solar concentrating technology families are Parabolic Trough Collector, Parabolic Dish, Linear Fresnel Reflectors, and Solar Tower.

The fact that CSP technologies are integrated into traditional thermal facilities is an inherent advantage. Moreover, solar thermal plants with thermal storage can offer stable capacity without the requirement for independent backup power plants or random grid disruptions.

The main goal of using the concentrated solar thermal system for the production of electricity is to reduce the cost of electricity produced to 0.05 EUR/kWh. Through the adoption of novel ideas for inexpensive, dependable, and efficient systems and parts for non-electrical processes, high-temperature chemical solar reactors produce high-value materials such as hydrogen [52].

An innovative design (Figure 11) of a new type of concentrator based on thinner mirrors with an innovative design for a receiving tube, large-scale heat storage enabling the plant to provide heat to the steam generator at a constant rate 24 h a day, and the use of an alternative heat transfer fluid (molten salt eutectic mixture, 60% NaNO₃-40% KNO₃), was proposed, designed, and investigated experimentally by ENEA.

4. Case Studies

Most operational CSP stations are located in Spain, Italy, and the United States. Other countries, like Finland, Denmark, Israel, Ukraine, and Algeria, can also produce any portion of their electricity consumption. Spain and the United States lead the ranking with the highest installed and operating power using this technology, with 2.3 GW and 1.8 GW, respectively, followed by China, Morocco, and South Africa.

More than 350 MW of CSP systems were installed in California in the 1980s. More recently, CSP has experienced a rebirth. In the U.S., for example, utilities have announced plans for at least eight new projects totaling more than 2000 MW. Numerous integrated CSP/combined-cycle gas turbine power plants are under development in North Africa and California.

Noor Complex, planned to produce up to 582 MW, is the world’s largest concentrated solar power plant with a production capacity of 510 MW, located in the Sahara Desert.

4.1. Solar Thermodynamic Power Plant Installed in Italy

Solar thermodynamic energy has a long history, likely discovered in 212 B.C. in Sicily. The first CSP plant with an installed capacity of 4.26 MWe was built in Trapani. So far, Italy is concerned; over 730,000 solar power plants with a total capacity of 19.7 GW were built by 2017. The capacity surpassed 20 GW in 2018, with a set target to achieve 50 GW by 2030 in the framework of the “National Energy Strategy”, or SEN, announced in 2017.

In Italy, Since the early 2000s, ENEA has focused its research activities on the main components of concentrating solar plants, the study of new heat transfer fluids, the development of innovative surface coating materials for receiver tubes, and the construction of advanced thermal storage systems for the development of an innovative and still unique technology worldwide. A novel fluid heat carrier (molten salt eutectic combination, 60% NaNO₃-40% KNO₃) was employed to boost operating temperature and heat storage capacity [53,54].

Another ENEA invention was the design of a new type of concentrator based on thinner mirrors, which saves money on construction and installation. Another significant innovation in the Archimedes project was the use of large-scale heat storage, which allows the plant to send heat to the steam generator at a consistent rate 24 h a day. The steam generator is made up of ‘tube and shell’ heat exchangers that transfer heat to water to produce superheated steam, which is then used in a traditional thermoelectric plant.

The innovative technology was used to build a five-megawatt capacity solar facility with an unusual ability to gather and preserve thermal energy from the sun for several hours, allowing the plant to generate electricity both during off-sun hours and when the sky is overcast. The construction of the plant cost roughly 60 million euros.

It is true that the cost per kWh produced is roughly five or six times more than the cost of energy created using conventional fuels, but with 2100 tons of oil saved and 3250 tons of CO₂ emissions cut in a year, the current achievement is unquestionably significant.

4.2. Thermodynamic Solar Power Plant in Ottana, Sardinia

A 10,000 m² thermodynamic solar-powered plant with a 1 MW total electrical output capacity has been built in the industrial zone close to Ottana on the Italian island of Sardinia, thanks to EU financing. The plant’s connection to the national electrical grid makes it one of the first of its sort in the nation. In order to heat water, the plant uses unique curved photovoltaic lenses that focus sunlight. A generator that generates energy is powered by the 600 kW steam turbine at the facility, which is driven by the generated steam. Thermal and electrochemical storage systems, which enable energy to be supplied for up to four hours in the absence of sunlight, are used to manage power distribution. The plant’s utilization of contemporary technology reduces the amount of land required for energy generation, despite its industrial scale. Among the primary purposes of the installation are testing and research. On the one hand, it provides for the evaluation of the performance of energy storage systems and small-scale thermodynamic solar technology; on the other hand, it permits the testing of different supply management models. Even in areas of the grid that are not directly connected to the plant, such models may save energy prices.

4.3. Solar Thermodynamic Power Plant Installed in North Africa

Another solar thermodynamic power station based on ENEA technology, which uses molten salts at a maximum temperature of 550 °C as a working fluid and a thermal storage system to distribute energy even in the absence of solar radiation, has been erected in North Africa. The power plant, when combined with other ‘traditional’ fuels, can reliably supply high-temperature heat up to 5 MW, electricity up to 1 MW, and approximately 250 m³ of desalinated water per day. The plant has been integrated into the local electricity, gas, and water distribution networks, but it can also operate off-grid [55].

Project MATS (Multipurpose Applications by Thermodynamic Solar) was a consortium that enjoyed international funding of 22 million euros, of which 12.5 was from the European Union, as well as ENEA. It includes other research institutes (French CEA, the German Fraunhofer, and the Egyptians ASRT and NREA), universities (British University of Cranfield), and industrial partners (including the Italians KT-Kinetics Technology of the Maire Tecnimont Group and Archimede Solar Energy for the construction and supply of the most innovative components and the Egyptians Orascom Construction Industries and Delft Environment). A 100 MW parabolic trough project is being constructed in China, bringing the global installed capacity of CSP to 6.2 GW [43].

4.4. Solar Disk Powered by Air Microturbine

ENEA introduced the world’s first solar disk, which, thanks to its integration with an innovative microturbine, can convert approximately 70 kW of captured radiant power to electric power of up to 15 kW (enough to power five flats).

In contrast to Sterling, which is frequently used in this kind of application, this system is novel because it used an air microturbine engine for the first time on a solar disk (Dish Power System). Its modularity and ease of operational management also make it suitable for small shopping malls, businesses, supermarkets, and schools that are both connected to and disconnected from the power grid.

The unique feature of this system, which consists of a solar disk (Dish Power System), is the use of an air microturbine engine in comparison to Sterling, which is commonly used in this type of application, as well as the ease of operational management and modularity, which allows it to be used for small shopping malls and businesses, supermarkets, and schools, both connected and disconnected from the power grid. Sun mirrors may focus up to 2000 times the sun’s radiation onto a small focal area.

5. Solar Photovoltaics

Early in 1839, the development of solar cells began with the observation of the photoelectric effect on specific materials. The cells advanced to the point of crystallization decades later. Two types of silicone technology began to dominate the market: “Poly” by directed solidification and “Mono” through the Czochralski technique. Over time, other silicon-based technologies have also emerged, including perovskites, copper indium gallium diselenide (CIGS), and cadmium telluride (CdTe). The global market is dominated by three types of PV cell technologies: thin film, polycrystalline silicon, and monocrystalline silicon [42]. Monocrystalline solar panels were the most efficient kind of solar panels a few years ago.

Since the first photovoltaic panels were created for the space industry rather than for terrestrial use, the P-type (positively charged silicon) market has become dominant despite the fact that the first commercial silicon solar cell was based on the N-type (USA Bell Labs 1954). This is because the P-type technology has been shown to be more resistant to degradation from cosmic rays. Commercial panels achieved 14–15% efficiency by the 1990s, increasing the viability of solar energy for broad use. The efficiency of solar panels has increased exponentially in the twenty-first century. The efficiency of solar panels on the market nowadays usually falls between 15% and 22%.

Nevertheless, current research and development efforts have produced a novel tandem solar cell that combines silicon and perovskite materials. Because of its enhanced ability to harvest sunlight, the new perovskite-silicon tandem has attained a world record efficiency of 33.89 percent. The early 1970s saw the beginning of a large-scale terrestrial photovoltaic deployment. Since then, the market for c-Si photovoltaics has dominated, holding a current 90% dominance, primarily due to the device’s affordability and robustness. We can only hope that it will continue to do so in the foreseeable future. Perovskite solar cell is an emerging technology for manufacturing solar cells based on an unusually three-dimensional material called perovskite. Photovoltaic proves to be of strategic importance to achieve a sustainable, energetic supply. Perovskite silicon tandem solar cells perhaps have a very high potential to reach a low levelized cost of electricity.

By 2024, it is anticipated that the power sector will have invested more than USD 500 billion in solar photovoltaic (PV) technology. New legislative initiatives, well-run public tenders, and upgraded grid infrastructure are all responsible for the progress made in Brazil, India, and portions of Southeast Asia and Africa. Africa will invest just over USD 40 billion in clean energy in 2024. A substantial influx of fresh investment is also anticipated in the United States, China, Europe, Asia-Pacific, and sections of Latin America, particularly in Colombia, Chile, and Brazil. The amount of solar PV installed worldwide is expected to increase from roughly 220 GW in 2022 to 500 GW in 2030. In view of the above and, moreover, continuously falling PV module prices (with current costs per kilowatt-hour typically around 6–8 cents), solar photovoltaics remain central to the power sector’s transformation. With a global PV cumulative capacity of 1.6 TW in 2023, needless to say, photovoltaics should be looked at as the most important instrument for socioeconomic development, the eradication of poverty and unemployment, and an important instrument of rural development.

Moreover, the widespread diffusion of photovoltaics in countries such as India, China, Saudi Arabia, Japan, etc., and the North American and European regions, with an estimated solar power capacity of nearly 3800 GW in 2030, photovoltaics appears to be the only option that offers the theoretical potential to serve long-term power requirements worldwide.

As is obvious from Figure 12, the solar capacity of China, Europe, and the US will reach 1000, 600, and 400 GW, respectively, by 2030.

6. Energy from Biomass

Biomass is part of the solution to conserving and enhancing natural wealth, as well as developing a more sustainable economy and society and a low-carbon future. Biomass today provides numerous answers to the energy transition, including a variety of energy production technologies, sustainable fuels, bio-based products, and chemicals. In addition to the industrial uses of bioenergy and sustainable biofuels, bio-products help build resource-efficient and low-carbon economies.

Several biomass technologies are now mature enough to play a significant role in economic decarbonization as part of a circular economy and sustainable development. Consequently, the exchange of knowledge and ideas on the latest scientific advances and applications of industrial biomass in industrial sectors is crucial.

Studies aimed at the generation of biofuels from second-generation biomass are focused on both thermochemical processes and those of biochemical conversion of lignocellulosic products. The gasification of biomass into carbon monoxide, hydrogen, and methane reflects a versatile system for the production of energy and liquid biofuels, for example, through the Fischer–Tropsch process [57].

Another very current topic is the production of biogas from biomass and waste. Global research activities are focused on the production and upgrading of biogas for electricity generation or grid applications. Innovative technologies for the valorization of digestate energy.

Numerous research and development initiatives have focused on the production of second-generation bioethanol for use in the transportation industry using a variety of feedstocks, including agroforestry biomass and perennial and herbaceous biomass crops. The process steps are biomass pretreatment, fermentation, enzymatic hydrolysis, and alcohol separation. Saturated steam is used to pre-treat biomass at a moderate temperature (approximately 200 °C). This allows for the profound degradation of biomass, which facilitates the separation of its constituent parts (cellulose, hemicellulose, and lignin) [58].

Biorefinery techniques can be used to convert sugars from lignocellulosic biomass into a variety of advanced chemicals and biofuels. This strategy involves maximizing biomass energy and increasing conversion efficiency. These initiatives consist of the fermentation of carbohydrates into bioethanol and the production of biohydrogen through the fermentation of wet biomass and biofuels from microalgae cultures.

6.1. Areas of Intervention and Innovative Aspects

The two transformation processes, thermochemical and biotechnological, are at the center of biomass-related research activity. Since gasification and pyrolysis are the basis of thermochemical conversion processes, research and development (R&D) efforts in this area help industrial systems to identify applications of these new cutting-edge technologies to convert biomass into thermal energy, electricity, and advanced biofuels.

Biotechnological processes applied to lignocellulosic biomasses and the numerous biorefinery schemes, through which raw materials can be broken down into sugars through hydrolysis and fermentation processes, allow the production of a wide variety of compounds [59].

Research and development (R&D) is conducted on three main steps of biomass conversion. These steps are: (i) biomass pretreatment/fractionation to promote enzymatic digestion; (ii) high solids hydrolysis, also known as high gravity hydrolysis; and (iii) sugar upgrading and conversion into various biobased products, mainly through fermentation (e.g., bioethanol, microbial lipids, and lactic acid) [60,61].

Amongst various pretreatment processes, the use of steam explosion using medium-pressure saturated steam can be considered a more effective and flexible process scheme. In mild conditions, the process could be catalyzed by the addition of modest amounts of chemicals. A separate fractionation technique could be determined by the type of additive.

Future technological issues are mainly related to the use of lignin, which is now considered a by-product and is used to generate energy and heat to make biorefineries self-sufficient. The question is: If lignin is partially used to create bio-aromatics or biochemicals, how does the energy balance of the biorefinery look like?

6.2. Thermo-Chemical Processes for the Exploitation of Biomass, Residues and Wastes

Thermochemical conversion refers to the disintegration of biomass at a temperature range of 523 to 1673 °C [2,62,63]. It consists of several types of technology, as shown in Figure 13.

The bioenergy industry expanded at an annual growth rate of 2% between 2000 and 2018 [65]. In 2019, the world production of wood pellets (38.9 million tonnes) and wood fuel (1.9 billion m³) was recorded [66]. On the other hand, there was a lack of information regarding turn-down rates, pollution, cost factors, efficiency, and real-world operating hours. The generally poor performance of different prototypes led to limited actual working experience and low confidence in the technology.

Fixed bed gasifier facilities with power capacities of 25–30 and 80 kWe best suited for their use and widespread dissemination, particularly in developing nations, were designed, fabricated, and thoroughly investigated both theoretically and experimentally [51].

Also, a 150 kWth reactor-based updraft fixed bed gasification plant using high biogenic fraction feedstock and steam/air gasifying agents and generating LHV 5–6 MJ/Nm³ dry product gas has been investigated. A biodiesel-powered wet filtration system is installed in the factory.

The generated gas was thought to be capable of producing hydrogen suitable for fuel cells or a high H₂/CO ratio (>2), which might be employed in the production of biofuels like methanol. In order to achieve this, the gas stream is directed to a portion for gas upgrading and CO₂ removal following the biodiesel scrubbing [52].

To examine the most promising sector of use (production of fuel gas for power generation), the plants have been experimentally investigated via a variety of technological pathways, including internal combustion engines, gas turbines, and high-temperature fuel cells (MCFC).

It has also been thought of using biomass gasification to produce syngas and convert it into biofuels. The development of gasifiers specifically designed to produce hydrogen, sun-diesel, and methanol—as well as syngas—for internal combustion engines is the focus of more recent endeavors.

The bioenergy industry has expanded at a 2% yearly growth rate between 2000 and 2018 [53]. Global manufacture of wood pellets (38.9 million tons) and wood fuel (1.9 billion m³) was recorded in 2019 [67]. On the other hand, there was a dearth of information regarding turn-down ratios, pollution, cost factors, efficiency, and real running hours. The generally poor performance of the different prototypes has resulted in limited actual working experience and low faith in the technology.

With a range of feedstocks, Atmospheric Circulating Fluidized Bed Gasifiers (ACFBGs) have shown to be incredibly dependable. They are also reasonably simple to scale up from a few MWth to 100 MWth. There is assurance that the industry will be able to supply dependable running gasifiers, even with capacities greater than 100 MWth. For large-scale applications, it seems to be the preferable method, and most industrial enterprises utilize it. As a result, ACFBGs are technically sound and have a strong commercial appeal.

6.3. Status of Operational Gasification Plants

The gasification process, through the production of syngas (a mixture of H₂, CO, and CH₄), is considered one of the best routes for energy recovery from biomass. The wide availability of biomass and its use as a feedstock is a potential remedy for the sustainable production of gaseous and liquid fuels.

Biomass gasification is a thermochemical conversion process of biomass that allows the production of biochar and syngas. Biochar is mainly a product obtained by pyrolysis during the gasification process. In addition to the CO₂ sequestration capacity, it contains physical–chemical properties similar to fertilizers.

The quality of gasification products depends on several factors, including gasifying agent, type of gasifier, design of gasifier, type of raw material, feeding rate, temperature, and pressure. Table 1 below shows a comparative analysis.

According to the IEA Bioenergy report published in 2020, there are 686 gasifiers operating in 272 large-capacity facilities with a syngas production capacity close to 200 GWth, which equates to approximately 200 MWth per facility [62]. The use of gasification in more than 27 industrialized countries, as well as the diversity of its products, illustrates the enormous potential for the continued growth of the gasification industry.

The database released by the U.S. Department of Energy (DOE) shows that the worldwide gasification capacity has continued to grow for the past several decades and is now at 70,817 megawatts thermal (MWth) of syngas output at 144 operating plants with a total of 412 gasifiers [62].

The new database is available in Microsoft Excel format, making it easily accessible to parties who are interested in understanding the world’s use of gasification for energy and chemical production [62]. Due to the high concentration of tar in the fuel gas and the resulting problems with gas cleaning, updraft fixed bed gasifiers are essentially unattractive for the energy application market.

For small-scale applications (less than 1.5 MW), downdraft fluidized bed gasifiers are attractive in both developed and developing countries.

With a range of feedstocks, atmospheric fluidized bed gasifiers (ACFBGs) have proven to be incredibly reliable. They are also reasonably easy to scale from a few MWth to 100 MWth. There is a guarantee that the industry will be able to provide reliable operating gasifiers, even with capacities exceeding 100 MWth.

For large-scale applications, it appears to be the preferred method, and most industrial companies are using it. As a result, ACFBGs are technically sound and have strong commercial appeal. Six plants in Europe have commercialized the FICFB gasifier, either in the planning, development, or operation phase, following the successful operation of the Güssing plant.

In both small- to medium-sized commercial applications and pilot-scale testing, bubbling fluidized bed gasifiers (BFBGs) have demonstrated reliability with a feedstock range of up to about 25 MWth. BFBGs are most cost-effective for volumes in the small to moderate range [69].

In summary, gasification is a key technology for biomass valorization. However, there is still much work to be conducted to solve the problem of efficient tar removal, and more automated operations are needed, especially for small-scale industrial applications. It should be said that current advances in catalytic tar conversion offer reliable alternatives, and consequently, there are excellent prospects for advanced applications.

6.4. Economic Aspects

The feasibility and prospects of biomass gasification have been the subject of numerous economic studies. Electricity investments above EUR 5000/kW are not uncommon, making economic viability difficult. To reduce Capex investments, value engineering efforts and operational experience are needed. The cost of running the business, especially the raw material price and availability, is another crucial factor. Raw material input costs can be negative (cheap), for example, for municipal waste, or positive (expensive), for dedicated short-rotation crops. Raw material costs increase with energy density, handling, processing, and transportation.

In addition, automation and process management are needed to reduce labour labor expenses. Practical experience is needed to estimate maintenance costs. The cost of heat and electricity can also have a significant impact on the economy.

However, due to the learning curve, it is expected that investment costs can be reduced to around EUR 2000/kW electricity in the next ten years. In the immediate and distant future, energy from biomass gasification will not be able to compete with fossil fuels. Research indicates that when favorable conditions and lower capital costs are achieved, biomass gasification can be as cost-competitive as other renewable energy sources.

6.5. Ethanol Production from Lignocellulosic Biomass

The interest in this type of application is primarily linked to the possibility of influencing the production of sustainable biofuels using waste materials. In actuality, vegetable oils (20% rapeseed oil, 25% soybean oil, and 30% palm oil) or used cooking oils (20%) still make up around 75% of biodiesel produced worldwide [70].

With 38% of all feedstock used in 2020, rapeseed oil is the most common biodiesel feedstock in the EU. Rapeseed oil’s domestic availability and the resulting rapeseed methyl ester’s (RME) superior winter stability over alternative feedstocks are the main factors contributing to its popularity. Palm oil, which is mostly used in Spain, Italy, France, and the Netherlands and to a considerably smaller amount in Belgium, Finland, Germany, and Portugal, was the third-largest source of raw materials in 2020 (18%). As biofuels from high-risk ILUC crops are phased out, palm oil use will be impacted.

Used cooking oil, commonly defined as oils and fats that have been used for cooking or frying in the food industry, restaurants, and households, was the second most important raw material in 2020, accounting for 23% of the total raw material.

The technology for producing bioethanol from starch and sugar crops (sugar cane, sugar beet, corn, etc.) is sufficiently advanced to enable a liquid fuel to compete with petrol and diesel in terms of price and performance [71]. It will be essential to discover substitute feedstocks to avoid penalizing the food market as they become more widespread [72].

Within this context, research efforts have focused on the use of lignocellulosic biomass, which can be easily recovered from agricultural, forestry, or agro-industrial wastes or specifically cultivated [33]. Unlike sugar syrups made from cane or beet, these substances require the addition of acids or enzymes to produce simple sugar, which initiates fermentation. This explains why, compared to the production of sugar and starch, the process of producing ethanol from lignocellulosic biomass is now somewhat more expensive.

In addition to the valorization of current lignin and hemicellulose processes, it is important to develop technologies for pretreatment, enzymatic hydrolysis, and isolation of alcohol from broth, regardless of the quality and availability of the raw material.

Green chemistry and biorefineries are the two main areas of interest for current research efforts. The main operations in this field include developing novel methods for the multipurpose production of biofuels, biochemicals, and bioenergy from lignocellulosic feedstocks, oil crops, and residual oils; co-products of the manufacture of bioethanol and biodiesel; and food industry byproducts.

In order to increase conversion efficiency, the biorefinery strategy maximizes the use of the biomass barrel and offers a tangible means to promote sustainable development. The objective of the laboratory is to promote the development of new holistic biorefinery models through the implementation of processes and technologies for the production and conversion of sugars, lignin, and oils. However, there are still many technological and non-technological barriers that need to be overcome.

There are three main methods for converting lignocellulosic feedstocks: acid hydrolysis, enzymatic hydrolysis after effective pretreatment, and liquefaction in ionic liquids. The process based on hydrolytic enzymes has reached industrial scales, also thanks to the marked improvement in the performance of commercial blends in the last decade.

The three main steps of biomass conversion—biomass pretreatment to promote enzymatic digestion, high-solids hydrolysis or “high-gravity hydrolysis”, and fermentation-based sugar conversion—are the subject of ongoing studies in collaboration with industry and research organizations.

Since steam explosion pretreatment and fractionation produce high degrees of biomass destructuration and facilitate further fractionation of its macro components, they can be considered adaptable process schemes. Under mild conditions, the process could be catalyzed by the addition of modest amounts of chemicals. A separate fractionation technique could be determined by the type of additive.

The severity of the process is a combination of treatment time and temperature. High severity reduces the degree of polymerization of cellulose and increases its enzymatic hydrolyzability but, at the same time, increases the acidity level of the product. Therefore, compromise conditions are necessary.

An important issue is the reduction in microbial inhibitors. In a high solids loading process, i.e., 20–30% solids concentration, the acid level could be in the range of 4–10 g/L. The effect of these degradation products on the fermentability of the product will depend on the microorganism and the process setup (e.g., inoculum amount, pH, and process strategy).

One of the main problems in the enzymatic hydrolysis of biomass is the treatment of concentrated slurries containing at least 20% [34].

High-gravity hydrolysis is the term for this procedure. Its advantage is that it produces a higher concentration of the final product, which reduces the amount of waste fluids produced, the capacity of the bioreactor, and the cost of distillation.

The difficulty in high-gravity hydrolysis comes from high viscosity, which limits mass transfer and leads to poor mixing. It also causes inhibition of by-products. In fermentation, this often indicates a high level of microbial inhibitors, osmotic stress caused by high solute concentration, and the harmful effects of ethanol (synergistic inhibition). The result is a lower process yield.

Therefore, the research and development effort is focused on the creation of appropriate bioreactor geometries for concentrated sludge processing, on the optimization of fed and batch bioreactor feeding strategies, on the identification of the best process strategy (hybrid, SSF, or SHF), on the reduction in toxicity of hydrolysates for fermentation microorganisms, and on the cultivation of resistant microorganisms.

The traditional conversion strategy includes separate hydrolysis and fermentation (SHF) and simultaneous conversion (SSF). Both strategies have advantages and disadvantages. A hybrid process has proven to be more effective with the concentrated suspension. It consists of a preliminary liquefaction of the biomass followed by a second step in which the hydrolysis is completed and the sugars are simultaneously fermented.

These processes must be optimized according to the subsequent fermentation strategy (e.g., fermentation or co-fermentation, type of microorganism). In this regard, the liquefaction time is an important variable [65].

As for the production of bioethanol, compared to the traditional process, hybrid conversion produces an increase in ethanol concentration, which also depends on the inoculation method of the microorganism and yeast and, above all, on the enzyme dosage. Based on the experience gained on bioethanol, it is known that the process and the productivity of biomass degradation products depend on the concentration of the pair inhibitor and on the selected strain.

The conversion of cellulosic carbohydrates derived from lignin into biological products requires that microorganisms are resistant to the by-products of biomass degradation. In some cases, detoxification of hydrolysates by chemical-physical methods is necessary to increase the efficiency of microbial conversion.

Alternatively, the robustness of the microorganism can be increased by microbial adaptation to toxic products or by fermentation at high cell concentrations, as in the case of immobilized cell fermentation.

In collaboration with researchers from the University of Salerno, ENEA has recently started the analysis of various flow sheets to produce a biobased product in a bioethanol-producing biorefinery [73]. Part of the data for the simulation came from the research activities of ENEA Trisaia.

The starting point is given by the general flow sheet to produce second-generation bioethanol. The bioconversion unit was imagined as a separate process to obtain an easier comparison of the different options for the conversion of the hemicellulose stream. This is a draft balance sheet of the process that indicates the possibility of co-producing a significant amount of green electricity from lignin. Then, we simulate the in situ production of enzymes with the aim of obtaining a limited price of commercial mixtures, making the in situ production cost-effective.

Preliminary analysis indicates that for an enzyme cost above EUR 1.5/kg, in situ production could be economically advantageous.

Another option is the conversion of hemicellulose into polyols, such as xylitol. The process simulated here is a fermentation process, while the actual process is a chemically catalyzed process. The equilibrium is as in the previous case, and more energy is needed for the separation of xylitol through crystallization. Finally, the hemicellulose can be dehydrated into furfural, which can be an intermediate to produce biofuels or negative-value bioproducts.

However, in this case, the process balance indicates that this process consumes significantly more thermal energy than the previous one for the separation of furfural through distillation. Preliminary assessment data indicate that, considering the expected high market price for succinic acid, this biorefinery scheme could make the co-production of xylitol or furfural more cost-effective; however, the process is no longer self-sufficient for energy needs, and this implies that it needs to be assessed for its full impact.

Steam explosion (Figure 14) was chosen as a suitable technology for biomass pretreatment as it produces a highly biodegradable substrate with minimal environmental impact. The method is based on the intrinsic ability of water vapor to permeate plant materials at high temperatures (200–250 °C) and pressures (20–25 bar), dissolving the chemical bonds that hold together cellulose, hemicellulose, and lignin. The final result is the release of sugars from the raw material, which would otherwise be difficult to metabolize for the microorganisms used in the various bioconversion phases.

7. Merits and Demerits of Some Green Energy Sources

Even though solar energy technology has advanced over time, installation costs can still be high, and the system still needs a large piece of land. Concerns about wind turbines harming the surrounding bat and bird populations are also present. Regarding hydro energy, the ecology may be harmed by damming a river and building an artificial lake. Relocating is required to build the reservoir.

The cost of the technology needed to use geothermal energy limits its ability to feed a sizable section of the world.

Table 2 shows the different energy sources and some of their attributes.

Table 2

Comparisons of renewable energy resources.

Energy Type	Energy Source	Merits	Source	Demerits	Source
Non-Renewable	Fossil Fuels	Established technology	[65,66,74]	CO2 and greenhouse gas emission	[68,69,74]
		High employment		Limited quantity
		Largest energy industry		Rising costs of fossil fuels
	Nuclear	Low carbon emission	[31,75,76]	Emission of nuclear waste	[17,24,72]
				Nuclear accidents
				Land degradation
Renewable	Solar	No carbon emissions	[77,78,79,80]	Weather dependent	[81,82]
		Cheapest and most abundant energy source		Large expanse of land required
				High costs of installation
	Bioenergy	Removes wastes from the environment	[71,83,84,85,86]	Lack of public acceptance	[69,86,87]
		Carbon emissions are minimized		Competition with food production
		Provide diverse forms of energy to satisfy different needs		Harvesting, collection, and transportation is still a challenge
	Geothermal	No carbon emission	[34,88]	High infrastructure costs	[89,90]
		Abundant resources		Resistance by Indigenous citizens
		Environmentally friendly		Seismicity hazard
	Wind	Minimal emissions	[91,92,93]	Large expanse of land required	[87,91,94]
		optimum use of the land		Low-frequency collection
		Low operational costs.		Non-uniform wind flows

The deployment of some of the most important clean technologies may be delayed or hampered by certain hazards. According to the IEA’s recently released report, “World Energy Outlook 2023”, each technology has a distinct risk profile that could jeopardize its function in clean electrification (Figure 15).

Figure 15

Primary risks associated with key clean electrification technologies [95].

[Figure omitted. See PDF]

8. Hydrogen as an Energy Carrier

Hydrogen offers a way out of carbon-heavy industries, but its contribution to net-zero emissions will require major investment and coordination issues to be addressed. Undoubtedly, hydrogen is a crucial component in promoting the energy transition. From basic research on materials to applied research and demonstration in real-world environments, ENEA’s Department of Energy Technologies and Renewable Energy Sources conducts research and development activities spanning the entire hydrogen value chain. These activities deal with the development of processes, components, and systems in the areas of hydrogen production, storage, and end use.

Prototype design and fabrication, simulation, technical and economic assessment and evaluation, and bench and field testing under specific operating circumstances are all components of innovative applications. In 1989, research on hydrogen technologies got underway, with a focus on polymer electrolyte fuel cells. In the years that followed, the focus progressively shifted to the creation of additional technologies related to the hydrogen supply chain, ranging from production systems to various fuel cell types, including solid oxide and molten carbonate fuel cells. Numerous thermochemical and electrochemical processes using carbonaceous materials and/or water as feedstock have been investigated throughout the years to produce hydrogen. The conversion and use of hydrogen have also been studied.

Research at ENEA is currently concentrated on solar-assisted thermo-catalytic conversion of biogas, high-temperature electrolysis, enhanced alkaline electrolysis, and thermochemical water splitting for hydrogen production. Processes such as “Power to Methane”, which produces synthetic methane through catalytic and biological methanation (carbon dioxide and carbon monoxide hydrogenation), and the production of liquid fuels (Fischer–Tropsch synthesis products, methanol and DME) have all been researched and tested in relation to hydrogen conversion. The entire hydrogen technology value chain, from production to end uses, is covered under the research program. Nowadays, hydrogen is produced commercially and utilized as a feedstock in refineries and the chemical industry, as part of a mixture of gases in the steel industry, and to generate heat and electricity. Around 75 MtH2/yr of pure hydrogen and 45 MtH2/yr of mixed gases are produced worldwide each year. Currently, grey hydrogen (produced from natural gas) is the leading source of hydrogen used as an industrial intermediate in many parts of the world [5].

Clean hydrogen can be used both as an alternative fuel for light-duty vehicles and also as an industrial raw material. In addition to water, many organic molecules also include hydrogen. It is the element that is most prevalent on Earth. However, it is not a gas that occurs naturally. It is always mixed with other elements, such as oxygen, to create water.

However, concerted efforts are now geared to produce green hydrogen via electrolysis of water using renewable power [96]. Hydrogen fuel cells are a method of producing clean energy that can be useful in cars and in a variety of other applications (Figure 16). In reference to a recent report published by researchers from the University of Colorado, Boulder, and the University of California, Los Angeles, as an important research breakthrough, the use of tiny platinum alloy catalysts could be crucial for making hydrogen fuel cells both more viable and efficient at converting water into energy [source: Cu Boulder report 2024].

It is worth noting that fuel cells, by creating energy out of typically only hydrogen and oxygen and without producing planet-overheating air pollution while in use, make them a very attractive renewable energy source.

Hydrogen can be used as fuel or transformed into energy once it is isolated from another element. Global hydrogen use reached 95 Mt in 2022, a nearly 3% increase year-on-year, with strong growth in all major consuming regions [97,98].

An increasing number of people are considering switching to automobiles that run on hydrogen due to the growing popularity of hydrogen fuel as a sustainable energy alternative. However, where and how to fill up their tanks is one of the biggest worries for prospective owners of hydrogen fuel vehicles. Hydrogen is expensive to produce and difficult to handle. Neither of these things is going to change with advancing technology, as they are linked to basic physics, which cannot change.

Hydrogen can be transported in different states, such as liquid hydrogen (LH₂), liquid methane (green hydrogen), ammonia (NH₃), liquid organic hydrogen carrier (LOHC), and methanol (MeOH) [99]. Green hydrogen is produced when its source of energy is renewable energy, while grey hydrogen is produced when the energy source is fossil fuels (Figure 16).

9. Clean Energy Matters

It is generally accepted that the use of clean energy is a better option, not only to meet the ever-increasing global energy demand but also to enhance economic growth, energy independence, health, and well-being of people compared to conventional combustible fuels, such as coal, natural gas, and petroleum oil.

Power generated using renewable sources, i.e., wind, solar, bioenergy, geothermal, and hydro, does not produce harmful carbon dioxide emissions that lead to climate change, which causes drought, wildfires, flooding, poverty, health risks, species loss, and more. In 2023, the global solar thermal energy yield from all installed systems corresponds to savings of 49.1 million tons of oil and 158.4 million tons of CO₂. This underscores the substantial contribution of this technology toward mitigating global greenhouse gas emissions.

Also, when considering the requirement of land for infrastructure, renewable energy systems often have a smaller footprint compared to conventional energy sources. Clean energy technologies can play a very important role in advanced energy-saving projects, especially in the transport sector and new buildings and structures. It has been estimated that with 70% of all power generation by 2030, renewable electricity could allow a net 55% reduction in greenhouse gas emissions.

Decarbonization, no doubt, is complex yet critical and still needs global tech solutions to cut emissions strategically. Reducing emissions, particularly from hard-to-abate industries, is critical to achieving net zero targets. Needless to say, a calculated combination of factors that adequately support the innovation, engineering, and execution necessary to enable low-carbon hydrogen to reach its full potential can surely help to make a leap toward global decarbonization goals.

Worldwide, solar resources are abundant and cannot be monopolized by one country. Amongst various renewable energy sources, solar energy appears to be a key technology option for implementing the shift to a decarbonized energy supply and can be deployed almost everywhere on this planet. There are two main technologies involved in the exploitation of solar energy, which differ in the way that solar radiation is harvested and converted to electricity. These are the solar photovoltaic (PV) and concentrated solar power (CSP) technologies.

Biomass offers several solutions in this energy transition through a range of technologies to produce energy, sustainable fuels, and biobased materials and chemicals.

10. Conclusions

Nuclear energy compares favorably with other renewable energy sources. Renewable resources prove to be of strategic importance to achieve a sustainable energy supply. Today, low-temperature (<100 °C) thermal solar technologies are reliable and mature for the market.

For a generation of electric power using solar energy, a novel concept developed and experimented with by ENEA, consisting of a solar field, a storage system, a steam generator, a solar disk powered by an air microturbine, etc., needs to be implemented on an industrial scale. Due attention needs to be focused on the installation of fixed bed gasifier plants of power capacity 25–30 and 80 kWe, most appropriate for their diffusion in the agricultural sector, especially in developing countries. Several biomass technologies can play an important role in the decarbonization of the economy as part of a circular economy in the context of sustainable development.

Advanced biofuels will play an important role in ensuring the security of the energy supply as well as the decarbonization of critical sectors that are difficult to electrify, mostly aviation, maritime, and heavy road transport.

Author Contributions

Conceptualization, V.K.S. and G.B.; methodology, V.K.S. and G.B.; validation, All; formal analysis, G.M.; investigation, G.M.; resources, N.N.A.; data curation, C.N.A.; writing—original draft preparation, V.K.S.; writing—review and editing, C.N.A. and N.N.A.; visualization, G.M.; supervision, G.B.; project administration, G.B. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data sharing is not applicable. No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Nuclear Plants Operational in the World [16].

Figure 2. Nuclear power reactors under construction in the world [16].

Figure 3. Number of fusion devices under study in the world [17].

Figure 4. Cumulative installed wind power capacity (GW) worldwide from 2001 to 2023 [36].

Figure 5. Levelized cost of generating electricity (LCOE) by technology [39].

Figure 6. Evolution of Global RE Employment by Technology. Source: [43].

Figure 7. Energy production from renewables in 2023 [43].

Figure 8. Global installed CSP capacity (MW) distribution (This information was obtained from the National Renewable Energy Laboratory “NREL” official website) [45].

Figure 9. Cost of solar heating and cooling (USD/MWhth) [50].

Figure 10. Global Status of Solar District Heating in 2023. Source [51].

Figure 11. Novel Solar Thermal Electricity Generation Plant by ENEA.

Figure 12. Deployment of Solar PV by Continent [56].

Figure 13. Biomass Processing Technologies. Source [64].

Figure 14. Continuous steam explosion pilot plant designed by Stake Technology Ltd., Ontario, Canada, installed at the ENEA Trisaia Centre in Southern Italy.

Figure 16. Type of hydrogen based on the energy that produced it.

Table 1

Comparison of principal gasification technologies for different energy applications [68].

Gasification Technologies	Size	Common Applications	Advantages	Disadvantages
Updraft Fixed bed	few 10 kWth-MWth	Heat/CHP applications	High efficiency	Higher tar production and more effluent treatment are needed.
Downdraft fixed bed	up to a few MWth	Heat/CHP applications	Syngas production of superior quality (lower tar content)	Regulation of the humidity and input granulometry
Single fluidized bed (bubbling or circulation)	500 kWth-50 MWth	Heat/CHP applications, ongoing developments for SNG, H2 production	Managing a range of inputs, this technology is a strong contender for SNG if “cheap” oxygen is available.	Air blowing was shown; however, low LHV syngas and oxygen blowing are more costly, and steam blowing is not conducted on a commercial scale.
Dual fluidized bed	500 kWth-100 MWth	Heat, CHP, SNG (ongoing development for H2, chemicals, liquid fuels)	High LHV syngas production and substantial methane output.	Cooler systems are extremely vulnerable to contaminants, and in high-velocity regions, erosion-related restrictions on the refractory
Direct plasma gasifier	>50 MWth	CHP applications	Syngas production of superior quality	excessive power usage
Entrained flow bed	large scale > 100 MWth	Liquid FT-fuels, chemicals	Syngas production of superior quality with a high H2 concentration	The requirement for feedstocks that have been finely crushed and torrefied
Two-stage processes (pyrolysis + gasification)	Depends on the gasification technology downstream of pyrolysis	H2, liquid FT-fuels, and chemicals (depends on the gasification)	Low levels of tar and hydrocarbons, a high H2/CO ratio, and the potential to produce biochar	Reduced conversion efficiency and moderate feedstock flexibility

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