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Chapter 8
8. Conclusions and suggestion for further works
8.1. Summary and conclusions
In the present work we have studied the problem of evaluating the environmental impact of energy conversion systems, with special attention on the effects arising from the CO2
emissions of coal combustion processes. The aim of this thesis has been the attempt to consider into a common basis the different dimensions affecting the energy conversion systems, analyzing especially the possible connection with each other.
The internalization of the environmental impacts produced by the energy conversion systems into the classic Thermo-economic analyses has been already discussed in literature without any notable conclusive solution. Indeed, in the existing literature this problem has been repeatedly confronted unilaterally, that is, focusing the analysis on a single specific problem affecting the energy production. Neither the eco-efficiency paths nor the market based approach by themselves, have been enough to furnish a comprehensive view of the multidimensional energy problem.
In the opinion of the author, the different approaches carried out in order to broach the environmental problems related to the energy production systems, should identify a
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common ground. The apparent dilemma among the guidelines between the main issues of the different disciplines, -economic continuous growth or environmental protection- should be appropriately addressed not as an either/or dichotomy but as a either/and issue. An important initial part has been dedicated to the analysis of the different pollutant emissions control technologies. For the CO2 capture has been analyzed the energy
requirements of diverse chemical absorption processes. These processes, being the most commercialize technologies, are very energy intensive archiving consumption energy values that goes from 2 to 6 MJ/kgCO2, as evaluated in chapter 2.. This means a great
“waste” of energy inside the thermodynamic performance of the plant. Indeed, within the framework of coal, (1 kg coal combusted produces about 3.6 kg CO2), the use of these
technologies would cause a reduction of the coal calorific value, as an input, from 6 to 18 MJ/kg, which even if considering a high quality coal, (about 30 MJ/kg) it would be a reduction between the 20% to the 60% of its calorific value.
In a second part has been examined the state of the art of different attempts found in literature for integrating the three main aspects confronting the energy problems; the technological, the economical and the environmental, from reductionistic to general approaches and from technical to holistic ones.
In this context, we have proposed a classic multi-objective optimization methodology, for integrating the environmental dimension into the energy analysis, where the consumption of the resources, the energy degradation and the chemical impact are considered in order to define an environmental framework into the energy analysis. In spite of this objective function penalizes especially the energy degradation relegating the emissions in a secondary role, however, the use of the multi-objective utility function Ψp has offered some important considerations.
Certainly, when analyzing the case of NOx and SOx emissions either regulatory or
economic approaches have been very successful. These emissions are usually low, about 1g/kWh, requiring less energy from the system to be mitigated. In fact, both approaches agree with confining these emissions till their technological limits.
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On the other hand, for CO2 emissions control the result are much more controversial.
Under a pure economic point of view (carbon tax), it is generally concluded that because of the economic advantage of their sunken investment that most existing PC plants would probably just pay the tax. This raises the broader question of just, what incentives can current PC plant owners be offered to reduce CO2 emissions?
A high increase in the taxes would lead to solution for abating at any cost, which reduces significantly the efficiency of the plant and therefore increases the fuel consumption. Indeed, the economic approach is very sensitive to the tax value.
Furthermore, from an energetic point of view, the CO2 capture is non-sustainable. Either
if analyzed with a conventional objective function (thermal efficiency) or with a more elaborated one (Ψp) the results are always the same, that is, to not abate. This is due to
the fact that the capture of CO2 reduces the efficiency of the plant without any “energy
benefit”. For abating the CO2, more resource must be used producing more emissions, so
it could end up without any emissions reduction.
Therefore, the CO2 control emission problem must be confronted multi-dimensionally.
Actually, an energy analysis base exclusive on energy basis, can never give positive results to “end-of-the-pipe” CO2 capture technologies, since these technologies are very
energy intensive to assume their unconditional use. Further criteria as economic taxes or political constrains should be in some way applied in order to make the CO2 capture a
practicable solution.
However, even if with the function Ψp some interesting conclusions have been reached,
as revealed in chapter 7. It can be concluded that this approach is still far from any conclusive solution. Particularly, there is not a direct correlation among the term βp, the
pollutant emission factor, and the energy requested for the capture. In spite of the favourable effect on the Ψp function, caused by means of an emissions reduction in term
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of a reduction produced in βp, appears that this reduction is not compensated with its
spending energy. The most likely explanation is that the reference environmental state, atmospheric air, used for calculating the chemical exergy for the pollutant emission factor is very simple. Certainly, in the atmospheric composition the chemical exergy of the CO2 is definitively very low, as regards the reduction of the CO2 emissions is not
significantly rewarded. Indeed, a more refined reference state, for example including the fossil fuels, could accredit to the chemical exergy of the capture compound a bigger value.
8.2 Future perspectives
The future for the energy generation industry deals with the major challenges to stabilizing the atmospheric CO2 concentration. So far there are still enough coal reserves
to last at least 160 years at current consumption levels. The question here is if we can bear the risk to burn all this coal throwing all their related CO2 produced emissions to the
atmosphere?
The energy problem stems from the fact that no combination of carbon-free energies is currently capable of displacing fossil fuels as the main sources of the world’s base load energy requirements. The challenges turn out to be heavily technological in character. Indeed it is claiming for an optimal use of the world’s plentiful fossil and no-fossil fuel resources. With this intention, in order to launch the indispensable contribution of renewable energies into the world’s energy mix to stabilize climate; political will, research and development efforts, and international cooperation are required.
The approach presented in this thesis, based upon multi-objective analysis, leads to a better understanding of the thermodynamic, economic and environmental performance of the energy conversion process. However, it has been only applied to different fossil fuel conversion processes, remaining open the problem of energy systems base on renewable resources. This issue, certainly not unproblematic, merits further investigation being especially of relevance in order to define energy policy strategies for middle to long term.
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of irreversibility I. Indeed, using the irreversibility a very more “fair” comparison among fossil and renewable technologies can be achieved. In any case the results will be better well-founded that with a solely economic approach or the one based on the life cycle of the plant. This will confirm the fact that even if the renewable resources are for “free”, they should be used in the most possible correct way, that is, with the minimum generation of degraded thermal energy, i.e. avoiding the misuse of the resources.
Nevertheless, there are limits to the expansion or scale-up of these energy technologies. Expansion for hydroelectric power is limited by potential sites that also limit the geothermal energy. Furthermore, specific factors constrain solar and wind energy, namely, their intermittent generation and their storages difficulty. Furthermore, it is usually overlooked that the renewable energies, solar, wind and biomass for instance, are not only highly land using, but they are likely to draw heavily on available water supplies as well. Indeed, what is seen is that different indicators are relevant in order to describe renewable energies. Thus, present understandings of renewable energies require special considerations. Some specific descriptions for different indicators related to renewable technologies, specially the one concerning the first and second laws of thermodynamics, are summarized in appendix A.
