Globally, the energy sector is responsible for around three-quarters of current greenhouse gas (GHG) emissions and is the key to averting the worst effects of climate change [1,2]^[1][2]. The transformation of the energy sector is an essential aspect of achieving climate neutrality, mainly based on the deployment of low carbon technologies. This ongoing transformation towards renewables is not just a shift from one set of fuels to another but involves a much deeper transformation of the world’s energy systems that will have major social, economic, and political consequences that go far beyond the energy sector.

On 4 November 2016, the Paris Agreement entered into force after being adopted by 196 Parties at the United Nations Framework Convention on Climate Change (UNFCCC) Conference (COP21) on 12 December 2015. The Paris Agreement’s main objective is to hold “the increase in the global average temperature to well below 2 °C above pre-industrial levels”; the goal has been updated to 1.5 degrees Celsius (°C) recently ^[3]. This agreement requires all countries to submit Nationally Determined Contributions (NDCs) that set out their climate targets. As of September 2023, 94 countries and the European Union (EU) have legally committed to a net zero emissions target, and some countries have announced long-term strategies, although non-legally binding, to contribute to global climate neutrality ^[4]. This is clearly huge progress, although most of these commitments have not yet been followed by the adoption of short-term policies and measures ^[1]. Amongst advanced economies, the EU ^[5], the United States (USA) ^[6] and Japan ^[7] have declared a commitment to become carbon neutral by 2050, while, within emerging markets, the People’s Republic of China (hereinafter China) will do so by 2060 ^[8].

The electricity sector is expected to provide the main contribution to limiting global warming to no more than 1.5 °C ^[9]. In May 2021, the International Energy Agency (IEA) published its landmark report, “Net Zero by 2050: A Roadmap for the Global Energy Sector”, highlighting that the electricity sector is projected to transition from being the highest-emitting sector in 2020 to becoming the first sector to achieve zero emissions by 2040 ^[1]. In September 2023, an update to the net zero roadmap was published by IEA ^[4]; although the pathway remains open it has been narrowed because global energy-related carbon dioxide emissions rose in 2022, reaching a new record of 37 gigatonnes CO_2eq, mainly due to the global energy crisis triggered by Russia’s invasion of Ukraine. While advanced economies see an emission reduction of 4% compared to the 2019 value, for emerging markets and developing economies a 4.5% increase, with respect to the 2019 level, has been observed.

In 2022, fossil fuels still generated 60.94% of global electricity, resulting primarily from coal (35.72%) and gas (22.12%), while low carbon sources accounted for the resulting 39.06% (nuclear 9.15% and renewables 29.91%) ^[10]. Although demand for conventional fuels did not fall compared to 2019 levels, the last two years have also seen remarkable progress in developing and deploying some key clean energy technologies (solar photovoltaics, wind, heat pumps, and batteries). Nevertheless, much more remains to be done to align with the IEA net zero roadmap, which requires each country to strengthen its ambition and set policy packages for an effective deployment of clean energy technologies.

The Life Cycle Assessment (LCA), with its holistic approach to assess the potential impact throughout the life cycle of a system or product, is an effective methodology for assessing the environmental impact of energy technologies. The Life Cycle Assessment (LCA) is widely acknowledged as the most advanced methodology for acquiring verified and comparable information regarding the environmental performance of products, technologies, and services. This encompasses both qualitative and quantitative aspects across their entire life cycle, including raw material extraction, design and formulation, processing, manufacturing, packaging, distribution, utilization, re-use, recycling, and waste disposal.

LCA is a method for quantifying the environmental impacts of products, technologies, and services through their whole life cycle; in other words, “from cradle to grave”. Enhancing comprehension of the environmental impacts during upstream and downstream phases is crucial for preventing the transfer of environmental burdens from one life cycle stage to another. It also serves to mitigate the likelihood of burden shifting from one country to another ^[11].

Lately, the significance of the Life Cycle Assessment (LCA) concept has grown within environmental policy, playing a pivotal role in fostering and facilitating the transition to a green economy. Outcomes derived from LCA can help decision-makers, enabling them to evaluate the multitude of environmental impacts associated with diverse energy options. This involves identifying both the advantages and disadvantages inherent in selecting from various alternatives. LCA includes four phases: definition of goal and scope, inventory analysis, impact assessment, and interpretation of results [12,13]^[12][13].

In the transition phase towards climate neutrality, the global energy mix must necessarily combine variable renewable energy technologies with other environmentally friendly technologies that can guarantee continuous energy production regardless of weather and climate conditions.

As of December 2022, electricity generated by coal-fired power plants worldwide was 10,191 TWh (35.72% of the total) ^[10] and contributed about 20% of total greenhouse gas emissions, while gas power plants generated 6309 TWh (22.12% of the total) ^[10] and contributed about 7% of total greenhouse gas emissions [18,19]^[14][15].

The United Nations Economic Commission for Europe (UNECE) compares, through LCA methodology, the potential environmental impacts of the different electricity generation technologies, coal and natural gas, with and without carbon capture and storage (CCS); wind power, onshore and offshore; polycrystalline and thin-film photovoltaics; concentrated solar power; hydropower; and nuclear power [15]^[16].

Regarding coal-fired power plants, with or without CCS, the study considers:

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subcritical “pulverized coal” (PC) thermal power plants, which use finely ground coal for combustion;

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supercritical power plants, based on PC technologies, but at much higher pressures and temperatures;

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the integrated gasification combined cycle (IGCC), which relies on converting coal into a synthetic gas before combustion.

Coal-fired plants highlight the highest scores, with a minimum of 751 gCO_2eq/kWh (IGCC, USA) and a maximum of 1095 gCO_2eq/kWh (pulverized coal, China). Equipped with a carbon dioxide capture facility, and accounting for the CO₂ storage, these values decrease to 147–469 gCO_2eq/kWh, respectively [15]^[16].

As for natural gas-fired power plants, the main technology used today is the natural gas combined cycle (NGCC), which uses the coupling of a gas turbine and a steam turbine to maximize the overall efficiency of the system. A natural gas combined cycle plant can emit 403–513 gCO_2eq/kWh from a life cycle perspective, and anywhere between 49 and 220 gCO_2eq/kWh with CCS. In 2020, CO₂ eq emissions related to 1 kWh of electricity generation from natural gas, in Europe, were 434 and 128 gCO_2eq/kWh, without and with CCS, respectively. The operational phase is the one that contributes most significantly to the CO₂ emissions (more than 80%). Both coal and natural gas models include methane leakage at the extraction and transportation (for gas) phases; nonetheless, direct combustion dominates the life cycle GHG emissions. The National Renewable Energy Laboratory (NREL) published original results from the Life Cycle Assessment Harmonization Project and updated estimates of electricity generation GHG emissions factors [16]^[17]. NREL reviewed hundreds of LCA studies, concluding that renewable technologies emit less GHG than fossil fuel-based technologies. For coal-fired plants, the median value was 1001 gCO_2eq/kWh, with estimates between 675 to 1689 gCO_2eq/kWh, while for electricity generated from gas-fired combustion turbine systems the median was 486 gCO_2eq/kWh, with a minimum at 307 and a maximum at 988 gCO_2eq/kWh [16]^[17].

2.2. LCA of Renewable Energy Systems

As of 31 December 2022, according to the International Renewable Energy Agency (IRENA), worldwide renewable generation capacity was 3372 GW, accounting for 40% of global installed power capacity [28]^[18]. Hydropower still accounts for the largest renewable power source in terms of installed capacity, followed by solar and wind. In 2022, the global installed capacity of hydropower (excluding pumped hydro) achieved a milestone of 1256 GW, constituting 37% of the total capacity derived from renewable sources. Solar reached 1053 GW (almost entirely PV for 1047 GW, with a very small part being CSP—6.5 GW), representing 31% of total renewable capacity. Wind accounted for 27% of total renewable capacity, with a total installed capacity of 899 GW. The rest of the renewable capacity includes 149 GW of bioenergy, 15 GW of geothermal, and 524 MW of marine energy. UNECE is the most comprehensive and up-to-date source of data, since it covers almost all energy technologies and dates back to 2022. In 2022, the organization published a report [29]^[19] aimed to evaluate the life cycle environmental impacts of energy technologies for power generation. The report highlights that renewable technologies exhibit substantially lower emission values by about an order of magnitude than fossil technologies, to be attributed mainly to infrastructure, capacity factor, and system lifetime. This leads to considerable variations in life cycle impacts due to raw material origin, energy mix used for production, the mode of their transport during various phases of manufacturing and installation, etc. The National Renewable Energy Laboratory (NREL) assessed and harmonized hundreds of LCAs of electricity generation technologies. GHG emissions from renewable energy systems are usually lower than those from fossil fuel-based technologies [16]^[17].

2.2.1. Hydropower

Globally, hydropower is the largest renewable technology in terms of installed capacity, with 1256 GW (37%) [28]^[18]. Run-of-river hydropower plants are characterized by smaller size and capacity, while a hydropower plant with a reservoir provides more energy and can potentially store energy by pumping water from a lower reservoir to a higher one. UNECE [29]^[19] reports that hydropower shows significant variability, as emissions exhibit significant site-specific characteristics, ranging from 6 to 147 gCO_2eq/kWh. The report also performed a LCA study for a 360 MW reservoir plant located in Europe with an associated GHG emission mean value of 10.7 gCO_2eq/kWh. The main GHG emissions are from transportation during construction (more than 80%), followed by materials of the dams and turbines. However, it should be noted that biogenic emissions from sediment accumulating in reservoirs can be very relevant in tropical areas. Motuziene et al. [30]^[20] found high values in hydropower systems with high dams and large reservoirs (152–237 gCO_2eq/kWh) because the dams were built using huge amounts of concrete and steel and thus emissions. Run-of--river plants show very low values of GHG emissions (2.06–13 gCO_2eq/kWh) because of their much smaller dam heights, reservoir sizes, and flooded land areas. The paper also highlighted intermediate values in a couple of cases (both run-of-river). The first one was a 3-kW hydroelectric plant in Thailand (52.7 gCO_2eq/kWh) and the second one refers to five large-scale plants (120–790 MW) in Myanmar showing GHG values in the range 31.17–39.23 gCO_2eq/kWh. 

2.2.2. Wind

At the end of 2022, onshore wind was the third largest source of renewable electricity after hydropower and solar photovoltaics, with 835.6 GW of installed capacity. Onshore wind power holds a dominant position in the wind market (93%), while offshore wind power accounted for a global capacity of only 63.2 GW (7%) [28]^[18]. According to UNECE, in Europe in 2020 mean emission values were 12.4 and 14.2 gCO_2eq/kWh for onshore and offshore plants, respectively. These values were obtained by performing a LCA on updated life cycle inventories of wind technology [29]^[19]. Wang et al. [36]^[21] evaluated and compared the environmental impacts of three technologies (hydro, nuclear, and wind) in China using LCA. As far as concerns wind power, the study selected a wind power project sited in Inner Mongolia (an onshore plant) as the reference case study. The plant has a capacity of 49.5 MW. This plant produces 28.6 ± 3.2 gCO_2eq/kWh for GWP throughout its life cycle, approximately 65% arising from the manufacturing stage. Alsaleh and Sattler [39]^[22] carried out a complete LCA for large onshore wind power plants in the USA. They found a mean GWP of 18 gCO_2eq/kWh, more than 80% attributable to raw material acquisition and manufacturing phases. The analysis of the aforementioned papers reveals that larger wind farms, equipped with bigger turbines, and particularly those with higher capacity factor values, yield lower CO₂ emissions. The primary portion of emissions is generated during the pre-operational phases. Two primary factors contribute to the diminished environmental impact per kWh generated by wind systems: scaling capacity and increase in size of the wind turbine. At the device scale, wind turbines have become increasingly efficient due to their larger size, particularly their height and diameter. Height is crucial as it allows for the capture of more wind energy at higher wind shear factors and hub heights. Diameter is associated with the area swept by the blades and the kinetic energy harnessed by the turbine. The technological learning encompasses the experience accumulated over time (proportional to cumulative installed capacity), leading to enhanced design and manufacturing efficiency, as well as technological advancements such as the utilization of less and more efficient materials for the blades. Collectively, these two factors have been estimated to reduce the life cycle environmental impacts of wind power by 14% for every doubling in capacity.

2.2.3. Photovoltaics

At the end of 2022, solar PV was the second largest source of renewable electricity after hydropower, with 1046.6 GW of installed capacity (31%) [28]^[18]. Over the past decade, photovoltaic (PV) technology has matured and emerged as the fastest-growing source of electricity production from renewable energies. PV involves the conversion of light into electricity through semiconductors, which exploit the photoelectric effect. The primary types of PV cell and module technologies include crystalline silicon (mono and multi), thin-film (Copper Indium Gallium Selenide (CIGS), Cadmium Telluride (CdTe), amorphous silicon (a-Si), perovskite), and multi-junction (utilizing multiple p-n junctions of different semiconductor materials to absorb various wavelengths of light) modules. The PV systems can be ground-mounted or roof-mounted (building-mounted or building-integrated). Based on how the generated electricity is managed, PV systems can be categorized as grid-connected or stand-alone. These systems come in various main types: residential, commercial, or utility-scale. The fundamental components of a PV system include photovoltaic modules, the tracking system, the balance of the system, and the inverter. The PV sector achieved its initial terawatt milestone in the spring of 2022, with a cumulative installed PV capacity of 1 terawatt peak and an annual production of solar cells and modules ranging between 200 to 230 gigawatts peak (GWp). The forthcoming terawatt milestone, reaching 1 terawatt peak of annual production, is anticipated within the next 5 to 7 years, and it is projected to escalate to a 2 terawatt peak by the beginning of the next decade. This advancement aims to curtail worldwide greenhouse gas (GHG) emissions, aligning with the objective of limiting the global temperature increase to 1.5 °C by the mid-century, as outlined in the 2016 Paris Agreement. Asdrubali et al. [20]^[23] examined 33 case studies regarding PV applications. GWP values were found between 9.4 and 167 gCO_2eq/kWh. After the harmonization procedure, the GWP median value was 29.2 gCO_2eq/kWh. Li et al. [44]^[24] performed a LCA on an onshore 40 MW wind plant and compared its environmental performance with other renewable and fossil technologies. They considered reference GWP values for photovoltaics in the range 16–40 gCO_2eq/kWh. Paulillo et al. [38]^[25] studied the environmental impact of geothermal energy with LCA, but also compared it with the environmental impact of other renewable energy sources. They found a median GWP value for photovoltaics equal to 48 gCO_2eq/kWh.

2.2.4. Concentrated Solar Power

At the end of 2022, global concentrated solar power accounted for 6.5 GW, less than 0.2% of the global renewable generation capacity [28]^[18]. Up to now, concentrated solar thermal technologies have developed to a commercial scale but have played only a small role in decarbonising the energy system. Global CSP market growth remains modest and on current trends may not reach levels foreseen by the IEA roadmaps. Nonetheless, considerable potential exists, although the development of the market relies on the design of effective auctions which can potentially reward the flexibility that the technology provides. As a technology, over the last 10 years CSP has made big steps forward in terms of cost reduction and in establishing a track record as a reliable option (benefiting from the good performance of the Spanish fleet and that of some recent international projects). However, to become more competitive, further standardisation in design and manufacturing can be key to attracting the levels of investment needed to bring deployment rates back on track. R&D has a major role to play in this; as shown by the PV sector, mass-production processes can accommodate major innovations and cost cutting. Digitisation in all phases needs also be fully embraced. Concentrated Solar Power (CSP) plants generate electricity in a manner analogous to conventional power stations, employing high-temperature steam or gas to propel a turbine. However, in CSP plants, the hot fluid is generated through the concentration of solar radiation rather than the combustion of fossil fuels. A comprehensive review of the scientific literature about environmental performances of CSP plants published in recent years was carried out by Guillen-Lambea and Carvalho [65]^[26]. Only seven studies out of 96 passed the screening criteria adopted by the authors in order to obtain high quality studies. GWP values of the seven studies, most of which referred to CSP parabolic trough with molten salt-based thermal energy storage (TES), were in the range 26–60 gCO_2eq/kWh. The contribution of pre-operational phases to global emissions ranges from 52% to 80%. Differences are due to the different energy mix (plants are located in Spain, USA, United Arab Emirates, and South Africa), sizes of the plants, DNI values, and the lifetime of the CSP plants. Higher values were found for synthetic molten salt compared with mined salt. 

2.2.5. Geothermal

As of 31 December 2022, global geothermal capacity accounted for almost 15 GW, about 0.44% of the global renewable generation capacity [28]^[18]. A natural hydrothermal geothermal reservoir is characterized by porous and permeable rocks saturated with hot water or steam, possessing both an ample heat supply and a reliable recharge mechanism. Geothermal energy can be found at various depths and temperatures. Three primary power plant technologies are used to convert the energy in geothermal resources to electricity: dry steam, flash/double steam, and the binary cycle [71]^[27]. In the first, steam is piped directly from underground wells to the power plant, where it is fed into a turbine/generator. This typology is not considered in the review because it is less common. Dominant water tanks are used to power systems in single or double flash. In the binary cycle, geothermal fluid is used to vaporize, through a heat exchanger, a second liquid, with a lower boiling point than water. The Organic Rankine Cycle (ORC) is a technology that converts low-temperature heat sources into mechanical energy and can be employed to generate electrical energy within a closed-loop system. Enhanced geothermal systems (EGS) improve the permeability of geothermal systems through hydraulic, chemical, and thermal stimulation through pumping of the water into the rock fractures, thereby creating an artificial reservoir. Lacirignola and Blanc [75]^[28] examined the environmental impact of enhanced geothermal systems using LCA. EGS systems, despite the great amount of energy and materials they require in the construction phase, show environmental performances like other renewables. They analysed ten case studies of EGS located in Central Europe. GHG emissions were in the range 16.9–49.8 gCO_2eq/kWh. Menberg et al. [76]^[29] carried out a LCA to evaluate the environmental performance of a binary geothermal plant located in Germany. The GHG value found for this plant was 38.2 gCO_2eq/kWh. The main contribution to this value comes from the refrigerant used as a working fluid (64%).

2.3. LCA of Nuclear Systems

According to the International Atomic Energy Agency (IAEA) [78]^[30] (pp. 9–10), as of 31 December 2022 there were 411 nuclear power plants in operation all over the world with a total electricity generating capacity of 371 GW. Light water reactors (LWRs) are by far the most prevalent with 333.2 GW installed capacity (90% of total installed capacity: 78% pressurized water reactors (PWR) and 12% boiling water reactors (BWR)). As reported by IAEA, the nuclear power fleet generated about 2486.8 TWh of low-emission electricity during 2022 (excluding Ukrainian reactors), accounting for about 10% of electricity generation globally. Essentially, two fuel cycle types are used nowadays, the “open” and the “partially closed”, also called “once through cycle” and “twice through cycle” (TTC), each in order. A distinction should be made between the “partially” closed fuel cycle as it applies to thermal reactors, which is limited to twice-through, and the “fully” closed cycle, which applies to fast reactors. In this review the term “closed” fuel cycle refers to the “partially” closed cycle (TTC). In 2021, Joint Research Centre (JRC), the European Commission’s science and knowledge service, carried out a review on the ‘do no significant harm’ (DNSH) aspects of nuclear energy, by examining the entire life cycle of nuclear energy also in terms of environmental impact, also focusing on the management of high-level radioactive waste and spent nuclear fuel [79]^[31]. Most of the LCAs consulted are complete, as they include all the phases including the disposal phase. The JRC study analysed many LCA studies from the last two decades. Three references were given more weight for their comprehensiveness. All the three studies are very recent. These studies resulted in the following values for GHG emissions: 5.3 gCO_2eq/kWh for PWR reactors (mixed cycle), 5 gCO_2eq/kWh for III generation EPR (European Pressurized water Reactor) reactors (open cycle), and 4.6 gCO_2eq/kWh for EPR (closed cycle). Kadiyala et al. [81]^[32] examined published LCAs identifying a quite significant range of variations. They reviewed 26 nuclear power generation LCA studies (49 case representations). Most of these studies defined the system boundary conditions including all the life cycle phases of nuclear power plants. The paper highlights that the centrifuge enrichment method produces lower GHG emissions than the gaseous diffusion enrichment method. This study does not include small modular reactors (SMRs). They found the following mean life cycle GHG emissions for several reactors: 6.26 (fast breeder reactor—FBR), 28.2 (high water reactor—HWR), 11.87 (PWR), 14.52 (BWR) gCO_2eq/kWh.  Portugal Pereira et al. [86]^[33] evaluated the effects of four alternative electricity generation scenarios in Japan after Fukushima. The use of LCA methodology allows to assess scenarios in terms of non-renewable energy (NRE) consumption, global warming potential (GWP), terrestrial acidification potential (TAP), and particulate matter formation (PMF). Emissions from energy technologies were calculated using the GEMIS (Global Emission Model for Integrated Systems) software. The functional unit was 1 kWh of produced electricity. The global warming potential of nuclear power was 16.95 gCO_2eq/kWh for PWR reactors, of which 13.80 gCO_2eq/kWh were from upstream processes. 

3. Conclusions

Both renewable energy technologies and nuclear power can contribute to the EU’s goal of zero emissions by 2050 by choosing the most appropriate energy mix, depending on the country’s geographical and geological characteristics. A summary of the gCO_2eq/kWh associated with each technology considered is shown in Table 16.