Low-Carbon Scenario Comparison
by Giuseppe Zollino
(from http://astrolabio.amicidellaterra.it/node/3122#)
The author, a professor of Energy Technology and Economics at the University of Padua, has been refining his analysis of the effectiveness and efficiency of technologies for the energy transition for several years. In this case, he has deepened the evaluation of land use for us. In the comparison between different scenarios, once again, nuclear energy proves to be essential. The novelty is that, now, the professor is responsible for energy at Azione, and intends to commit himself to anchoring climate and energy policy choices to the accurate measurement of costs and benefits. We hope he succeeds.
1. Introduction
Reducing global emissions of carbon dioxide and other greenhouse gases to zero in the long term is an extraordinary challenge. Predicting exactly when the goal will be reached is a challenge within a challenge. The only certainty is that the transition to a prosperous economy for both advanced and developing countries, respectful of ecosystems and biodiversity, and free of anthropogenic emissions of climate-altering gases (as the European Green Deal envisages) corresponds to a long march in a territory almost unexplored, with routes all to be built and to be traveled keeping economic and social sustainability always in sight (and therefore aware that there will be setbacks, side steps and even backsliding) and to be completed as soon as possible. And the optimal route for each country is not necessarily the same as the others, because the boundary conditions and potentials are different, although they all have many common elements and requirements that create constraints that must necessarily be taken into account in the search for the optimum.
With reference to energy demand, the subject of this contribution, the transition from fossil fuels to those with zero greenhouse gas emissions (renewables and nuclear) and the consequent increase in the electrification of final uses are common elements to all zero-emission scenarios. Scenarios that are indispensable tools for identifying the optimal technological mix for each country, that is, the one with the least impact on the territory, the least use of materials, the lowest cost of the entire electricity system, that is, the cost of all generation and storage facilities necessary to meet, hour by hour, the expected electricity demand in that country in a long-term zero-emission scenario, taking into account the generation profiles of variable renewable sources (solar and wind) and the load profile, as well as the unit costs of each technology.
We will examine here some Italian scenarios with only low-carbon technologies from the point of view of efficiency and sustainability. For the long-term Italian electricity demand, reference is made to the "Italian Long-Term Strategy on the Reduction of Greenhouse Gas Emissions" [1], which hypothesizes an electricity demand of at least 650 TWh, due to the increased uses in all sectors: for the production of green hydrogen for those so-called hard to abate, for the electrification of public and private transport, for the electrification of domestic and tertiary uses (heating, domestic hot water, cooking) and for the greater electrification of industrial uses. Naturally, all the electricity demand must be met, hour by hour, by the power generated from sources and technologies that do not emit greenhouse gases: renewable sources (hydroelectric, geothermal, solar, wind, biomass) and nuclear, as provided for in the European Green Taxonomy.
The results presented here are reworkings of optimized scenarios obtained with the COMESE code, developed at the RFX Consortium in Padua, already published [2][3][4]. In place of future fusion power plants, present in the publications cited, here we consider fission power plants of the type included in the European Green Taxonomy, that is, of evolved third generation.
A methodological note is necessary at this point. Although each of the so-called low-carbon (low-carbon) technologies, if analyzed over the life cycle, does not have zero emissions (see Figure 1), in the search for optimal scenarios we did not use the "life cycle emissions" parameter in any way, neither as a design variable, nor as a constraint. As if they were all effectively zero emissions. However, it is possible to evaluate ex post the emissions associated with each of the 650 TWh of demand, by distributing them over them the total life cycle emissions associated with all the kWh generated in each scenario.
In paragraph 2 we will examine the effectiveness (in reducing emissions) and efficiency (in land use and material use) of the different low-carbon technologies.
In paragraph 3 we will compare different low-carbon electricity scenarios with each other and with the Italian electricity system of 2023, from the point of view of installed power, electricity generated, overall system cost and land area actually occupied or only scattered with plants, while still being able to use the land or the underlying water body for other uses (this is the case of large wind farms).
Fig.1 Life cycle greenhouse gas emissions, in grams of CO2 per kWh generated (Source: United Nations Economic Commission for Europe: Life Cycle Assessment of Electricity Generation Options - last update 2022)
2. Efficiency and effectiveness of low-carbon technologies
Figure 1 shows the greenhouse gas emissions, expressed in grams of CO2 equivalent per kWh generated during the useful life, for four so-called low-carbon electric technologies: hydroelectric, nuclear fission, photovoltaic, and wind. These emissions are due to various phases of the life cycle, from the procurement of materials and construction of the various plant components, to the extraction and preparation of the fuel (in the case of nuclear), maintenance, and decommissioning. The data are taken from the literature [5]. With reference to hydroelectricity, the two very different values refer to plants with a reservoir (those with the highest emissions) and flowing water.
As you can see, the specific emissions (per unit of electricity generated) of nuclear energy are by far the lowest, on average 9 times lower than those of silicon photovoltaic plants, 6 times lower than those of offshore wind and more than 2 times lower than those of onshore wind. From the point of view of the effectiveness in reducing greenhouse gases, nuclear energy is certainly in first place.
Fig.2 Main materials used in the life cycle, in grams per MWh generated during the plant's operating life (Source: United Nations Economic Commission for Europe: Life Cycle Assessment of Electricity Generation Options - last update 2022)Figure 2 shows the quantities of materials used in the life cycle, expressed in grams per MWh generated during the plant's operating life, for a selected set of materials. In this case too, the data are taken from the literature [5]. As you can see, for the same amount of electricity generated, nuclear energy requires, on average, 7 times less materials than photovoltaics and about 3.3 times less than wind. With regard to offshore wind, it should be noted that only the case of fixed foundations (on the seabed) is shown in the graph. In the case of floating wind, the amount of materials (in particular steel) increases: the literature reports values up to 50% higher than in the case of fixed foundations. Therefore, even from the point of view of the need for materials (which will concern the whole world), nuclear energy is certainly the most efficient among low-carbon technologies.
A further particularly significant aspect is the land area occupied by the plants. In the case of wind farms, this is actually a surface "disseminated" by wind turbines, which create various disturbance effects (noise, shadow-flickering effects, visual impact, etc.) but allow the use of the soil for agricultural activities, grazing, etc.
In this regard, it should be noted that wind turbines suitable for predominantly weak winds such as those in Italy, at the same nominal power, have larger blades (even 50% longer) than wind turbines suitable for sustained winds. At the same time, the height of the support towers is increased, in order to intercept winds of higher speed. In this way, at the same nominal power of the wind turbine, increasing the length of the blades can reduce (even to 2.5-3 m/s, compared to 4 m/s of a rotor suitable for sustained winds) the so-called attack speed of the wind, that is the speed at which the machine begins to generate electricity, even if at a power much lower than the nominal one, and also the nominal speed, that is the wind speed from which the machine generates the nominal power. With the result that, at the same nominal power, it is possible to increase the load factor, that is in the end the electrical energy generated, in a way at most proportional to the increase in the rotor diameter (to understand with simple numbers, if, at the same nominal power, the rotor diameter doubled, twice the electrical energy would be produced in a year).
However, wind turbines must be spaced apart to prevent perturbations (vorticity), induced on the "lines" of the wind as it passes through a rotor, from excessively altering, even to the point of compromising it, the operation of the next rotor. It is good practice that in an offshore wind farm the towers are spaced apart by about 6 times the diameter in the direction of the prevailing wind, and at least 4 times the diameter in the direction perpendicular to the prevailing wind. And then, if the energy generated increases in proportion to the diameter and the surface to be "reserved" for a wind turbine increases with the square of the diameter, it can be concluded that "gigantism" does indeed help the "bankability" of an offshore wind farm, but it "involves" ever larger areas. Furthermore, at the same overall area involved by the park, it reduces the electrical energy generated, but it generates it at lower costs. Therefore, we are faced with a classic problem of finding the optimum point, considering all the "interests" involved.
Explanation of some key terms and phrases:
- Aerogeneratori: Wind turbines
- Venti deboli: Weak winds
- Venti sostenuti: Sustained winds
- Velocità del vento di attacco: Attack speed of the wind
- Velocità nominale: Nominal speed
- Fattore di carico: Load factor
- Vorticosità: Vorticity
- "Bancabilità": Bankability
Summary of the main points:
- Wind farms occupy a significant amount of land, even though the turbines are spaced apart to avoid interference.
- Wind turbines suitable for weak winds have larger blades and taller towers than those suitable for sustained winds. This reduces the attack speed and nominal speed of the wind turbine, allowing it to generate more energy from weaker winds.
- However, the larger blades and taller towers also increase the area required for each turbine.
- As a result, there is a trade-off between the amount of energy generated and the area required. The optimal point will depend on a variety of factors, including the cost of land, the availability of wind, and the environmental impact of the turbines.
To provide empirical evidence of the difference in surface area occupied by electrical generation plants using different technologies, figure 3 shows the layouts of the Barakah nuclear power plant in the United Arab Emirates and a recently proposed wind farm in the territory of the municipality of Manciano, in the province of Grosseto. The Barakah plant consists of four 1,360 MW reactors for a total of 5,450 MW. It was built in 11 years, 8 years per reactor, one year apart; it generates 44 TWh/a, continuously and will do so for at least 60 years. As shown in the aerial photo, it occupies a total surface area of about 200 hectares.
The Manciano wind farm consists of eight 6 MW wind turbines with a diameter of 170 m and a hub height of 115 m. It is expected to generate an average of ~125 GWh of electricity per year during its 25-year lifespan. Due to the necessary spacing of the turbines, the plant occupies a surface area of about 550 hectares, for an average of about 12 hectares/MW.
In theory, if it were possible to extend the plant ad libitum (assuming that there is a wind-blown area free of any type of constraints, as large as you like), to generate 44 TWh of electricity, variable hour by hour, almost simultaneously (when there is sufficient wind, all the wind turbines generate the same power that is added together; when there is none, all generate zero!), it would take 350 plants like the one in Manciano, for a total of 16.8 GW, on a surface of about 200,000 hectares. To give you an idea, 200,000 hectares are just under half the size of the Molise region.
Similar calculations can be made to find that, if you want to generate 44 TWh of electricity with single-axis tracking photovoltaic plants, considering the average Italian load factor, you would need 27 GW, which would cover about 40,000 hectares.
The numbers we have seen regarding the "sizes" of the variable renewable replacement plants (16.8 GW wind or 27 GW photovoltaic), to which the evaluations on the necessary storage plants and grid infrastructure should be added, significantly reduce the easy objection that the 11 years necessary for the construction of a plant like Barakah is too long, especially considering that, during the 60-year lifetime of the plant, the replacement plants would have to be rebuilt one or two times (depending on the type).
Explanation of some key terms and phrases:
- Impianti eolici: Wind farms
- Fattore di carico: Load factor
- Inseguimento mono-assiale: Single-axis tracking
Summary of the main points:
- The Barakah nuclear power plant is much more compact than a wind farm of equivalent capacity.
- To generate the same amount of electricity as the Barakah plant, a wind farm would need to occupy an area 100 times larger.
- A photovoltaic plant would need to occupy an area 20 times larger.
- The construction time of the Barakah plant is comparable to the lifetime of a wind or photovoltaic plant.
Conclusion:
The Barakah nuclear power plant is a more compact and efficient way to generate electricity than wind or photovoltaic plants. The construction time of the Barakah plant is also comparable to the lifetime of a wind or photovoltaic plant, so the objection that nuclear power plants are too slow to build is not valid.
Fig.3 Layout of the Barakah nuclear power plant, in the United Arab Emirates
Fig.3 bis Layout of a wind farm proposed in the municipality of Manciano, province of Grosseto.
3. Long-term Italian electricity scenarios with zero emissions
Here we present four long-term (around 2050) electricity scenarios that can fully meet, hour by hour, the Italian electricity demand, which is assumed to be 650 TWh, with a peak power of 114 GW. The solution is obtained with the COMESE code, as briefly described in paragraph 1 and in more detail (for those who want to go deeper) in the articles mentioned in the same paragraph.
Figure 4 shows the installed capacities for each technology included in the mix with reference to the current electricity system and four possible zero-emission alternatives, two with only renewables and two with nuclear and renewables. The two 100% renewable scenarios (100%RES1 and 100%RES2) differ in the type of photovoltaic systems used (in the first 70 GW on both civil and industrial roofs and the rest on the ground, all with fixed tilt; in the second still 70 GW on roofs and the rest on the ground, all with single-axis tracking) for the limit on the installed capacity of floating offshore wind power placed on the optimizer (15 GW in the first case, and the optimizer "uses" them all; 35 GW in the second case and the optimizer "uses" 31 GW), and for the hourly daily profile of winter thermal demand. The two scenarios with nuclear power differ in the limit on nuclear power: 42 GW in the first and the optimizer "uses" them all; 80 GW in the second and the optimizer is given the constraint of maintaining the same installed renewable capacity in 2023 and of the 80 GW "uses" 72.
As can be seen, today a total capacity of 125 GW is installed, equal to about 2.3 times the peak demand; if the RES100%1 scenario were chosen, it would be necessary to install just under 900 GW, more than 7 times the current value and 8 times the expected peak demand in 2050; with the RES100%2 scenario, about 670 GW would be needed, five times the current capacity and six times the peak demand. If nuclear power plants with a total of 42 GW are also used, then it is necessary to install about 360 GW, 2.8 times the current capacity and about 3 times the peak in 2050; finally, if 72 GW of nuclear power are installed, the total capacity returns to be slightly higher (1.2 times) than the current one.
Explanation of some key terms and phrases:
- Scenario 100%RES1: A 100% renewable scenario with 70 GW of photovoltaics on roofs and the rest on the ground, all with fixed tilt, and 15 GW of floating offshore wind power.
- Scenario 100%RES2: A 100% renewable scenario with 70 GW of photovoltaics on roofs and the rest on the ground, all with single-axis tracking, and 35 GW of floating offshore wind power.
- Scenario 42GWn: A scenario with 42 GW of nuclear power and 360 GW of renewable energy, including 70 GW of photovoltaics on roofs and the rest on the ground, all with fixed tilt, and 31 GW of floating offshore wind power.
- Scenario 80GWn: A scenario with 80 GW of nuclear power and 72 GW of renewable energy, including 70 GW of photovoltaics on roofs and the rest on the ground, all with single-axis tracking, and 31 GW of floating offshore wind power.
Summary of the main points:
- The four scenarios presented all meet the Italian electricity demand of 650 TWh with zero emissions.
- The 100% renewable scenarios require the installation of much more capacity than the scenarios with nuclear power.
- The 42GWn scenario is the most cost-effective option, with a total installed capacity of 360 GW, compared to 900 GW for the RES100%1 scenario and 670 GW for the RES100%2 scenario.
- The 80GWn scenario is the most ambitious option, with a total installed capacity of just over 100 GW.
Fig.4 Power installed in Italy in 2023 and in 4 electricity scenarios without CO2 emissions in 2050, with annual demand of 650 TWh and a peak of 114 GW
Figure 5 shows the electricity generated by all installed plants. It can be seen that the higher the installed capacity of non-controllable sources such as photovoltaics and wind power, the greater the surplus electricity generated and that dissipated in the charging and discharging processes. Nevertheless, the mixes presented are those with the minimum overall cost, under the given boundary conditions; that is, with the total costs (of all generation and storage plants) as low as possible. The LCOTE (Levelised Cost of Timely Electricity) parameter shown in figure 5 is the resulting cost for each of the 650 TWh of demand, when all the costs (investment, maintenance, fuel, for all the plants present), discounted, are distributed among those 650 TWh.
Explanation of some key terms and phrases:
- LCOTE: Levelised Cost of Timely Electricity. This is a measure of the cost of electricity, taking into account all the costs associated with generation, including investment, maintenance, and fuel.
- Surplus electricity: Electricity that is generated but not consumed.
- Dissipated energy: Energy that is lost in the charging and discharging processes of storage devices.
Summary of the main points:
- The higher the installed capacity of non-controllable sources, the greater the amount of surplus electricity generated and dissipated.
- The mixes presented are those with the minimum overall cost, even though they generate more surplus electricity.
- The LCOTE is the cost of electricity for each of the 650 TWh of demand.
Fig.5 Electricity generated in 2023 and in the 4 electricity scenarios to 2050 and cost of energy made available at the time of demand, LCOTE.
For simplicity, the indicated LCOTE values do not include grid costs, which are certainly more relevant for the two 100% renewable scenarios. The costs of each technology used the values indicated in the Net Zero scenario of the International Energy Agency. For an analytical definition of the LCOTE parameter, refer to the publications cited in paragraph 1. As can be seen in figure 5, the LCOTE decreases as the installed nuclear power increases.
Finally, figure 6 shows the total area occupied or affected (in the sense explained in paragraph 2 for wind farms) by all generation and storage plants, excluding network infrastructure. In the two 100% renewable scenarios, the areas increase considerably compared to the current situation (3.5 and 16 times, respectively). In the scenario with 42 GW of nuclear power, it remains much larger than the current one, but it is less than half of that required for the 100% renewable scenarios. In the scenario with 72 GW of nuclear power, the required area is less than the current one.
Still in figure 6, the greenhouse gas emissions due to the life cycle of all installed generation and storage plants are also indicated, expressed in grams of CO2 equivalent per each kWh of demand. From this point of view, the second of the two 100% renewable scenarios is preferable; moreover, the two scenarios with a nuclear share are by far those with the lowest emissions in the life cycle; the second about 3 times less impactful.
Therefore, considering the installed capacities (and, therefore, the materials to be used), the areas to be occupied, the total costs of the system and the emissions in the life cycle, the scenario with 72 GW of nuclear power would be the most sustainable, followed by the scenario with 42 GW of nuclear power. For example, in the first case, it would be necessary to install 13 power plants of the type of the Barakah plant, seen in paragraph 2, in the second 8. Naturally, many other intermediate scenarios are possible, but from what has been seen, the positive impact of the presence of a nuclear share is clear.
Explanation of some key terms and phrases:
- LCOTE: Levelised Cost of Timely Electricity. This is a measure of the cost of electricity, taking into account all the costs associated with generation, including investment, maintenance, and fuel.
- Grid costs: The costs of transporting electricity from the point of generation to the point of consumption.
- Surface occupied or affected: The area of land required to install a power plant or storage facility, including the area required for access roads, parking, etc.
- Greenhouse gas emissions: The emissions of gases such as carbon dioxide, methane, and nitrous oxide that contribute to climate change.
Summary of the main points:
- The scenario with 72 GW of nuclear power is the most sustainable, followed by the scenario with 42 GW of nuclear power.
- The scenario with 72 GW of nuclear power has the lowest LCOTE, the smallest surface area required, and the lowest greenhouse gas emissions.
- The scenario with 42 GW of nuclear power is also a good option, with a lower LCOTE than the 100% renewable scenarios.
- The 100% renewable scenarios require much more land than the scenarios with nuclear power.
Fig.6 Surfaces occupied or affected by generation and storage plants, excluding all distribution and transmission network infrastructures, and specific emissions relating to the electricity actually used.
4. Conclusions
Decarbonizing the Italian economy will be a cyclopean undertaking, comparable to crossing a terrifying ravine on a tightrope. Claiming to do it by picking and choosing the technologies you like best among those included in the European taxonomy is like walking on a tightrope with one eye blindfolded and your hands tied.
Instead, all the technologies are needed: gas with carbon capture and possible reuse or storage, renewables, and nuclear. How much of each? The only tool to decide this is scenario simulations that evaluate the balance between generation and demand hour by hour and keep an eye on essential parameters such as the area occupied, the materials used, the emissions in the life cycle, and the total costs of the system. All costs. Because, as Totò said, “the sum makes the total!”
Acknowledgements. In addition to all the colleagues co-authors of the articles mentioned in paragraph 1, the author wishes to thank Dr. Nicola Menna, a recent graduate, for his contribution during his final internship.
Explanation of some key terms and phrases:
- Decarbonization: The process of reducing greenhouse gas emissions to zero.
- Tassonomia europea: A classification system developed by the European Union that defines which economic activities are considered sustainable.
- Gas con cattura e possibile riuso o stoccaggio della CO2: A technology that captures carbon dioxide from the flue gases of fossil fuel power plants and stores it underground.
- Rinnovabili: Energy sources that do not produce greenhouse gas emissions, such as solar, wind, and hydroelectric power.
- Nucleare: A low-carbon energy source that generates electricity by splitting atoms.
- Simulazioni di scenario: Computer models that are used to predict the future behavior of a system.
- Equilibrio tra generazione e domanda: The balance between the amount of electricity generated and the amount of electricity consumed.
- Area occupata: The amount of land required to install a power plant or other infrastructure.
- Materiali impegnati: The materials that are used to build a power plant or other infrastructure.
- Emissioni nel ciclo di vita: The total amount of greenhouse gases emitted over the lifetime of a product or process.
- Costi totali del sistema: The total cost of building, operating, and maintaining a power system.
Summary of the main points:
- Decarbonizing the Italian economy will be a challenging task that requires a combination of technologies.
- The European taxonomy is a useful tool for identifying sustainable technologies, but it is not the only factor to consider.
- Scenario simulations are essential for making informed decisions about the mix of technologies that will be needed to decarbonize the economy.
- All costs, including the environmental costs of emissions, must be considered when making these decisions.
NOTE
[2] C. Bustreo, U. Giuliani, D. Maggio, G. Zollino: “How fusion power can contribute to a fully decarbonized European power mix after 2050”; Fusion Engineering and Design 146, Part B (2019) 2189-2193
[3] U. Giuliani, S. Grazian, P. Alotto, M. Agostini, C. Bustreo, G. Zollino: “Nuclear Fusion impact on the requirements of power infrastructure assets in a decarbonized electricity system”. In: Fusion Engineering and Design 192 (2023), 113554
[4] U. Giuliani, M. Agostini, C. Bustreo and G. Zollino, "The Fusion to Hydrogen Option in a Carbon Free Energy System" in IEEE Access, vol. 11, pp. 131178-131190, 2023, doi: 10.1109/ACCESS.2023.3332917
The CAPABLE project described by the Polish specialized press and exhibited at the national conference on the training of technicians in the presence of the ministers of energy and public works. 
