Greenhouse gas | EPFL Graph Search

Greenhouse gases are those gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by water vapor (), carbon dioxide (), methane (), nitrous oxide (), and ozone (). Without greenhouse gases, the average temperature of Earth's surface would be about , rather than the present average of . Human activities since the beginning of the Industrial Revolution (around 1750) have increased atmospheric methane concentrations by over 150% and carbon dioxide by over 50%, up to a level not seen in over 3 million years. Carbon dioxide is the cause for about three quarters of global warming and can take thousands of years to be fully absorbed by the carbon cycle. Methane causes most of the remaining warming and lasts in th

La négociation des risques technologiques

Marc Audétat

This thesis considers technological risks and their constitution as public problems. It supports that the main difference among risks is not to see between proved risks and uncertain risks, on the basis of the objective knowledge, but rather separates risks which have already reached compromises at both levels of evaluation and responsibility and those which are still controversial. The starting observation is that social acceptability of risk is out of the scope of the objective approach. Actors use categories of "objective risk" and "perception of risk" within struggles to define what is at stake. It shows how risks, as soon as their evaluation starts, are negotiated within controversies, mobilizing experts, stakeholders, and actors of society. With many environmental risk, and with the globalization of hazards such as climate change, the preventive paradigm is overflowed; and with the emergence of new uncertainties, such as the introduction of genetically modified organisms in agriculture and food, expertise as the only method to deal with risk is contested in society. The precautionary approach, which asks to act against uncertainty before the completion of scientific evidence of risk is declining in the United-States while its integration is increasing in public policy in Europe. Have we entered the risk society? Or, is the society of the developed countries safer than it has ever been? Are risk increasing or decreasing? These apparent paradoxes shows there is different social theories of risk with different possible balance between objective and social factors to explain them. The role of technology is ambivalent, generating new risk and in the same time privileged as a solution to uncertainty. The thesis puts forward "science, technology and society" approach in risk studies, and it aims at a sociology where experts are not considered neutral actors. In this perspective, robustness of expertise does not rely directly on objective knowledge, but on the relation of this knowledge with its context of application. After studying the role of risk in modernization and rationalization processes, the thesis focuses the mobilization of actors in risk controversies and the role of some new institutions in the public sphere such as the ethical committees and the technology assessment agencies. Two case studies are investigated which take place in Switzerland end of the 1990s. The first relates to the introduction of genetically modified organisms in agriculture and environment. A controversy occurred about field test, i.e. scientific experiments refused by the authorities in 1999 and 2001. The role of research in biotechnology is considered as well as the different role of expertise in the public sphere. Expertise in biosafety and social acceptability of biotechnology are negotiated simultaneously. The second case study is the climate change issue and the negotiation of measures to control greenhouse gas. The role of research policy in the governance of climate risk is studied. Both case studies are interested with the role of experts in the public sphere, with the coordination of expertise and decision making, and with the degree of democracy achieved in sociotechnical controversies. The main result of this study is that a linear model of risk dominates approaches and constitutes an obstacle to the democratization of technological change. Instead, the thesis proposes a model where rationality is not defined beforehand, but appears as the proper result of controversies. In this view, risks are socially negotiated, already during the production of expertise, and all along the decision making and the implementation processes.

EPFL2004

Characterization of bacterial communities in Lake Geneva by comparing genetic and physiological methods

Coralie Chappelier

Over the last century, the level of atmospheric CO2 has increased to the highest concentrations on Earth within the past 800'000 years. Current predictions anticipate that the effects of greenhouse gases will lead to a rise in air temperature ranging between 1.4 and 5.8°C. Oceans and freshwater ecosystems absorb large quantities of CO2, but are sensitive to global climate change that directly influences water temperature and pH. Specifically, phytoplankton accounts for approximately 50% of the total photosynthesis on Earth, and contributes to the mobilization of CO2 through sinking or transferring fixed carbon to deeper water layers and sediments. Bacteria are among the most abundant entities in plankton and play a major role on the biological pump for the cycling of carbon and other elements. In order to understand and anticipate the response of aquatic ecosystems to climate change, we need to characterize how planktonic organisms respond and adapt to environmental change. In this project, we will merge microbial ecology and genomics with the ultimate goals of (i) characterizing current ecological niches of the operational taxonomic units highlighted in bacteria populations of Lake Geneva, (ii) identifying candidate adaptive genes responsible for adaptation of freshwater bacterial communities to contrasting freshwater habitats, (iii) projecting current bacteria distributions as a function of the expected change and (iv) identifying possible alterations in ecosystem functioning and services as a function of climate change. By doing this, we expect to contribute expanding our ability to characterize ecological niches using genetic data and sensitizing society about the importance of protecting freshwater ecosystems.

2020

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008