No 63 Em Português
Direct impact of Nuclear Generation
on Emissions of Ghe Gases in Brazil
Direct Impact of Nuclear Generation
on Emissions of ghe Gases in Brazil
Carlos Feu Alvim firstname.lastname@example.org
Frida Eidelman email@example.com
Olga Mafra firstname.lastname@example.org
Omar Campos Ferreira
The evaluation of the impact regarding the introduction of nuclear energy on emissions of GHE gases should be made comparing it with the case it would not be used. That is, this evaluation, as any substitution evaluation, is somewhat subjective: what scenario would we have should the implemented alternative not exist.
In the present study simple assumptions were made and whenever possible, avoiding arbitrary choices.
The basic criteria adopted were:
1. Considering that the thermal complementation is a demand of the Brazilian electric system and that the energy used (and to be used in the future) instead of nuclear energy would be generated using fossil fuels;
2. The fuel shares in the thermal generation would be the same as the one in each evaluated year if there was no nuclear energy;
3. A comparison was made for public service utilities (private and government-owned);
4. Only direct impacts of fuel used were calculated but the indirect costs for obtaining the fuels (nuclear and conventional) or in the construction and maintenance of plants were not.
This study evaluates the contribution of nuclear energy to the reduction of greenhouse effect using data from the Carbon Balance - 1970 to 2005. The electric generation of the public power plants was analyzed since they are responsible for 90% of the electricity generation in 2005 while the autonomous plants supply the remaining 10%. Actually nuclear energy will be used in public service power plants.
Figure 2.1 shows the marked predominance of hydraulic energy in electricity generation. In Figure 2.2 it can be noticed that the share of thermal plants has been decreasing since the petroleum prices shock of 1973 (with some recovery around 1986 due the Cruzado Plan and the cold shock of the petroleum prices) and it only increased again in absolute (Figure 2.1) and relative (Figure 2.2) terms from the mid of the nineties on.
It was in this context that the growth of thermal energy share in electricity generation had a more important role in this generation. This fact justifies the use of the existing thermal plants in each year in order to estimate the impact of carbon emission reductions since the probable alternative would be the increase of thermal plants to supply the necessary electric energy.
In order to evaluate the averted emissions in the past it is also necessary to evaluate the share of the different energy sources in electricity generation. As it is known, the carbon quantity emitted by fuels depends on their nature, in particular on the carbon/hydrogen ratio. The burning of natural gas, for example, has a lower emission than that of mineral coal, since all energy of the latter comes from the oxidizing of carbon that generates CO2, whilst in the former there is the contribution of hydrogen that generates H2O. Therefore the emissions averted through the use of nuclear energy depend on the composition of the fuels that would be used in the generation. For the purpose of the present evaluation, the same composition of the existing thermal generation would be considered for the year under evaluation.
Figure 2.1: Electricity generation by source showing that thermal generation increased from 1994 on. Hydraulic energy represents in the whole period more than 99.96% of the so called renewable energy
Share of the different electricity thermal
Hydraulic and nuclear energy do not directly participate in the formation of GHE gases; methane production by dams, due to the biomass decomposition with insufficient presence of oxygen, is not yet conclusively evaluated. The biomass fuels generation is also not taken into account in the emissions inventory since the emitted carbon is previously absorbed from the atmosphere by the plants.
The evaluation of GHE gases carried out using the Carbon Balance, developed by Economy and Energy - e&e OSCIP for the Ministry of Science and Technology, has shown that the gases that contribute most to the GHE are carbon dioxide (CO2) and methane (CH4). Therefore, carbon emission is an important parameter for evaluating the contribution of the different energy sources to increase the greenhouse effect.
In Table 3.1 it is analyzed the contribution in the year 2005 of the different energy sources to electricity generation and carbon emissions.
Table 3.1: Contribution of Energy Sources to Electricity Generation and Carbon Emissions
(*) 99,97% hydraulic energy
Table 3.1 was used to illustrate the process used in the evaluation of emissions averted by the use of nuclear energy. In order to evaluate these emissions for the whole available period (from1970 to 2005), it is necessary to retrieve the data shown in Table 3.1 for all the years. This was carried out in Annex 1.
Besides the impact of nuclear energy on emissions reduction, one can try to evaluate the effect corresponding to the use of hydraulic energy as it is also shown in Table 3.1. Even though this has been done as an exercise, the assumption that the thermal generation profile would be the same as the one valid for each year, it is more questionable in this case than that of nuclear energy since in Brazil hydraulic energy corresponds to the largest share of generated energy and the scenario of its substitution would be much more complex. For example, due to the non availability of large volumes of natural gas, it would be rather improbable that it would participate so intensively in the global generation as it participates in the subset of fossil energy. This would increase the impact to be assigned to hydraulic energy on emissions reduction.
The tables of Annex 1 were organized in the same way as the rows of Table 3.1. So Table A1.1 corresponds to row 1 of Table 3.1 and so forth until row 6 that corresponds to Table A1.6.
Row 1 of Table 3.1 (and Table A1.1) shows the energy values corresponding to the transformation of the energy contained in the different energy sources (or groups of energy sources) into electricity. Following the convention adopted in BEN, the values of “consumed” energy are represented as negative values while those of “produced” energy, namely electricity, are positive.
In row 2 of Table 3.1 and Table A1.2 are shown data relative to electric energy generation per energy source. The values of Table A1.2 were the base for constructing Figures 3.1 and 3.2. In row 3 and Table A1.3 are shown the values of the generated electric energy / consumed energy ratio that represent the apparent generation efficiency.
Efficiency is an important factor for determining past and future emissions. The evolution of the apparent efficiency of electric generation is shown in Figure 3.1 for natural gas, petroleum and natural gas products, mineral coal and nuclear energy in apparent values since they are based on recorded fuel consumption and electric generation. Besides the natural statistical uncertainties, in the nuclear case there is a natural difference between the record of fuel consumption (assumed to be accounted for when it is fed into the reactor) and its effective use since the uranium can remain in the reactor core for years. Some thermal plants are maintained in operation condition even when they are not generating electric energy which means loss of fuel and efficiency decrease. Therefore, it was expected the increase of efficiency with higher use of thermal plants in the last years.
Figure 3.1: Evolution of apparent efficiency that, as expected, has increased in the last years due to higher use of thermal energy in generation
It should be noted that in row 3 of Table 3.1 (and in Table A1.3) the marked efficiency of hydraulic energy is 1 (100%) which is thermodynamically not viable but results from the form the hydraulic energy is accounted for (by the value of the generated electric energy without taking into account the mechanical losses).
In order to obtain emissions data it was used the bal_eec software owned by ECEN Consultoria and developed by OSCIP Economy and Energy - e&e, described in Annex 2. This program permits also to calculate the CO2, CO, CH4, NMVOCs, N2O and NOx emissions.
Row 4 (and Table A1.4) presents data of row 1 (electric energy generation in toe) converted to GWh.
The evolution of carbon emissions, with considerable increase from the nineties on, is shown in Table A1.4 and in Figure 3.2. In the figure it is shown the share of emissions by type of fuel. In 2005 the contributions to carbon emissions were almost equally distributed regarding electric energy from natural gas, petroleum (and NG) products and mineral coal. As an illustration it is also indicated, as usual, the correspondent CO2 mass (carbon mass X 44/12). The unit used, teragram (1Tg = 1012 g), corresponds to one million tons.
Figure 3.2: Carbon emissions from electricity generation and the corresponding CO2 emissions (carbonic gas that would be generated from the carbon mass)
In Figures 3.3 (data from Table A1.3) and 3.4 (data from A1.2) are shown the shares in energy generation that are quite different from those of carbon emissions. Natural gas, responsible for half of the electric energy generation from fossil fuels in 2005, had a share of one third of emissions in this year. This is due to its higher efficiency and its lower carbon content per contained energy when compared with coal and with petroleum products.
3.3: Participation of sources in electricity
3.4: Participation of sources in carbon emissions in
As a result of fossil fuels composition variation and its efficiency regarding electricity generation, emission coefficients per unit of generated energy have varied along time as can be seen in Figure 3.4 (data from Table A1.6).
In order to evaluate the averted emissions, the value used was that corresponding to the average value of fossil fuels (see row 6 of Table 3.1 and column fossil in Table 1.6). The averted emissions are obtained multiplying the emission coefficient for fossil fuels (0.20 tC/MWh in 2005) by the electricity generated using nuclear energy.
The use of this coefficient is a consequence of the adopted assumptions (in case of no nuclear generation the shares of thermal energy in the total generation and the structure of fossil generation would be the same). The averted emissions by MWh of generated nuclear energy have decreased along the period following the curve of fossil fuels shown in Figure 3.4.
3.5: Carbon emissions by unit of electric energy
The averted emissions by MWh of generated nuclear energy are shown in Figure 3.6 in two scales (tC/tep and tC per MWh).
The averted emissions by MWh of generated nuclear energy are shown in Table 3.2 and compared with emissions averted by hydroelectric plants and carburant alcohol. In Annex 3 the process adopted to estimate the emissions averted by the use of carburant alcohol is described.
Figure 3.6: Carbon emission coefficient per electric energy generated by fossil fuels that was used to evaluate emissions averted by nuclear energy
Figure 3.7: Emissions averted by the use of nuclear energy compared with averted emissions assigned to hydraulic energy and use of carburant alcohol
Table 3.2 – Averted Carbon Emissions
The evaluation made shows that nuclear energy has averted the emission of 85 million tons of CO2 between 1984 (year when Angra I started to generate electricity) and 2005 of which 47 million in the 2000/2005 period. Comparatively the emissions averted by the use of nuclear energy between 2000 and 2005 would be about 40% of that corresponding to the use of carburant alcohol and 4% relative to hydraulic energy.
In the present study it was assumed for both the nuclear and hydraulic energies the same share of fuels presently used. The emissions averted by alcohol were calculated in terms of equivalent energy that takes into account the higher efficiency of alcohol relative to that of gasoline as well as its lower energy content. This equivalence varies year by year because the shares of anhydrous and hydrated alcohol are different.
The estimates are 2.04 tones of CO2 by m3 of alcohol used, as compared to 2.44 tCO2/m3 from the evaluation made for the First Brazilian Declaration to the United Nations Framework Convention on Climate Change. The differences in equivalences and emission rates used explain the discrepancy.
Concerning nuclear energy, the emission average value was 0.29 tC/MWh,, corresponding to 0.98 tCO2/MWh. In the mentioned Declaration two scenarios were defined for estimating the values of emissions averted by hydraulic energy. In the publication it is written that the same scenarios were adopted for nuclear energy but it does not explain the mix of energy sources that would substitute the nuclear energy. In scenario I the averted emission is 0.29 tCO2/MWh and in scenario II, 0.73 t CO2 /MWh. Since both scenarios include a share of hydraulic energy, the lower values in the Declaration could be assigned to this hypothesis. It should be remembered that the Declaration does not consider the thermal plants option that was intensified after the year 2000 and corroborated the hypothesis that the probable substitute of nuclear energy would be fossil fuel.
A impact evaluation could be carried out in three levels: 1) the direct impacts resulting from the comparison of using the fuel at issue and the alternative ones; 2) the impacts comprising the production, storage, transport and waste disposal steps (also compared with the alternatives) and 3) the impacts involving all indirect costs of supplies that are part of the productive chain.
The evaluation carried out here refers only to the direct effects of substituting the conventional thermal generation by the nuclear one until 2005 (level 1). Therefore it is not considered the energy used for extracting, processing and disposing wastes regarding the nuclear cycle relative to for example that involved in the corresponding steps of the coal cycle (level 2).
In a more complex evaluation (level 3) one could analyze the indirect emissions such as those from the fabrication of machines and equipment used in the different steps for obtaining the fuel, as well as the investments on infrastructure connected with the activity; this could be carried out using the input X product of IBGE and using carbon emission coefficients of each activity. Concerning the energy part, the Carbon Balance made by the Economy and Energy OSCIP – e&e would supply the necessary coefficients.
In the nineties some doubts were cast about the indirect energy costs in electricity production (doubts similar to the present ones regarding alcohol). Studies have shown that the energy balance was indeed positive when the costs concerning waste storage and the use of inefficient enrichment process such as the jet nozzle (demonstration yield). As the emissions resulting from industrial processes are directly linked to energy, the same would be true concerning indirect emissions.
The averted emissions considered here do not take into account the better use of hydraulic energy made possible by thermal complementation. This evaluation can be made using the methodology developed by the project of ECEN Consultoria for Eletronuclear.
Nuclear energy was considered as an option that is inserted in the thermal complementation to the Brazilian electrical system. It was adopted the hypothesis that nuclear energy would be substituted by a mix of other fuels used in thermal generation in public service plants in each year. As a result it was concluded that in Brazil nuclear energy has averted the emission of 85 million tons of CO2.
The impact evaluation of hydraulic plants is just indicative but a comparison assuming the same fuel profile used for the nuclear energy indicates that the emissions averted by nuclear energy with only two plants (47 million tons between 2000 and 2005) is equivalent to 4% of that averted by hydraulic energy and 40% of that averted by the use of carburant alcohol in the same period.