Economy & Energy
Year IX -No 63:
August - September 2007 
ISSN 1518-2932

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Direct Impact of Nuclear Generation on GHE Gases Emissions in Brazil

The bal_eec Program – User’s Manual Contained Carbon, Equivalent and Final Energy

Auctions of New Energy: Vectors of Offer and Demand Crisis or Adjustment 

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 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 feu@ecen.com

Frida Eidelman frida@ecen.com

Olga Mafra olga@ecen.com

Omar Campos Ferreira

Rafael Macêdo

1- Introduction

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[1] 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.

2- Participation of Thermal Plants in Electricity Generation

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[2]. 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

Figure 2.2: Share of the different electricity thermal
generation with an amplified scale

3- Emission of GHE Gases in Electricity Generation

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.[3]

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[4].

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 NO_x 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.

Figure 3.3: Participation of sources in electricity
generation from public service plants

Figure 3.4: Participation of sources in carbon emissions in
electricity generation from public service plants

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.

Figure 3.5: Carbon emissions by unit of electric energy
generated and average value of fossil 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