Pfalzwerke (Germany)

Extreme environments on Earth host a large diversity of microbial life. Bacteria, archaea, and fungi are able to survive under one or several extreme conditions including extreme ranges of temperature, pressure, pH or salinity. Despite extensive research on extremophilic microorganisms, a relatively unexplored frontier within the study of the deep biosphere is the survey of the diversity of microorganisms inhabiting deep geothermal reservoirs used for energy production. These sites offer unique access to investigate life in the deep biosphere. The conditions in these reservoirs are often within the range of the known limits of life, which makes them a suitable habitat for various extremophilic microorganisms. Moreover, microbial-driven processes such as microbially induced scaling or corrosion can decrease the efficacy of geothermal power plant systems. The present review summarizes the current knowledge and uncertainties surrounding microbial life in deep geothermal reservoirs. As the knowledge in deep geothermal fluids is still scarce, the microbial diversity in analogous environments, such as surface geothermal springs, deep-sea hydrothermal vents or deep subsurface environments, is also summarized here. The high diversity of microorganisms inhabiting these analogous environments suggests that deep geothermal fluids may host an unsuspected microbial diversity. Moreover, the challenges associated to the study of microorganisms in geothermal fluids are reviewed. These include notably challenges linked to sampling, DNA extraction from low biomass samples, DNA amplification and sequencing of unknown communities, and biases induced by comparison of the sequences obtained to reference databases. Such biases are even stronger concerning fungi and archaea, as specific databases are less extensive than those for bacteria. A broader knowledge on microorganisms in deep geothermal fluids may not only allow to reduce the negative impact of microbial activity in geothermal power plants, but could also provide new insights into the evolution of microorganisms and their survival in extreme environments.

The European EGS project of Soultz-sous-Forets (France) which was first investigated in 1987 is aimed at producing power by the extraction of the heat stored in deep, fractured crystalline rocks. Extensive research and development contributed to get a better understanding of the geothermal reservoir: large-scale hydrothermal circulation occurs within a network of large permeable faults developed within a Tertiary graben. 3 deep wells have been drilled down to 5 km depth, where a rock temperature of 200degC was reached. At the bottom depth, the horizontal distance between the producing wells is over 1.3 km length, knowing that the injection well is located in the middle. Several hydraulic and chemical stimulations were performed and combined in order to reactivate the system of fractures, which are often sealed by natural hydrothermal deposits. This led to improvement of injectivity and productivity of the wells. After a successful 5-months circulation test done in 2005, we are now building a pilot power plant. Two types of production pumps will be tested for this purpose and a first ORC conversion module of 1.5 MWe will be installed and tested in the beginning of 2008. If the results are promising, a second module should be later installed.

Programs for optimizing energy procurement are incorporated in the control center of the Pflazwerke AG. A main element in the optimizing strategy is load management. The authors deal with the procedures for energy import optimization based on the results of a load forecast program. Special emphasis is put on load management and the investigations for modeling the load behavior depending on season, kind of day, and temperature. In addition, practical aspects in exceptional situations such as equipment failures are handled. Load management is achieved by ripple control of storage heaters with an installed capacity of approximately 65% of the peak load.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>