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Evaluation of ultrasonic degasification as a tool to mitigate total dissolved gas supersaturation downstream hydropower plants
Abstract
This Doctoral Thesis evaluates ultrasonic degasification as a potential tool for mitigating total dissolved gas (TDG) supersaturation in the river downstream a hydropower plant. The volumetric liquid-gas mass transfer coefficient, kLa, is identified as a benchmark for comparing the efficiency of different parameters influencing liquid-gas mass transfer. Natural degasification in case of TDG supersaturation in a river downstream of a Norwegian hydropower plant is evaluated based on TDG saturation measurements, enabling the calculation of kLa through certain assumptions and simplifications. Laboratory experiments are conducted to explore the degasification of TDG supersaturated water in a controlled environment. A method is developed to increase TDG saturation levels in water for both small- and medium-scale experiments. This water is utilized to test the influence of selected parameters on liquid-gas mass transfer during degasification by technical methods, with a focus on ultrasonic degasification, in both a small-scale batch reactor and a medium-sized degasification test rig. The resulting kLa values are in agreement with literature, and ultrasonic degasification is identified as a highly effective method for increasing liquid-gas mass transfer, both in static and kinematic conditions, hence reducing TDG saturation levels. An empirical model, derived from batch reactor data and adjusted using test rig results, offers possibilities for estimations regarding application in real-world scenarios.
The study contributes to the understanding of important parameters driving liquid-gas mass transfer during degasification of TDG supersaturated, flowing water. In addition, it advances the understanding of the complexities of degasification mechanisms within formerly unknown fields of application. While offering significant insights, remaining questions, and emerging challenges highlight the necessity of further research. Collaborative efforts among academia, industry, and authorities are vital to expand knowledge, implementation, and regulatory frameworks, in order to enhance aquatic environmental protection and advance hydropower as an environmentally friendly technology. Continuing the pursuit of knowledge is encouraged to build upon the foundation this Thesis establishes.
... Its application to mitigate TDG supersaturation has been mentioned early, e.g. by [45], but only recent studies showed its actual capability of effectively reducing the TDG saturation level of flowing water, e.g. [81]. Yet, field applications or economic assessments of ultrasonic degassing are to-date missing. ...
Downstream hydropower plants, a change in water chemistry can lead to the occurrence of a widely unknown problem: total dissolved gas (TDG) supersaturation. It takes place when air is entrained in a water body and exposed to high pressures, which leads to gas dissolution in the water. Re-exposure to atmospheric pressure downstream the power plant results in TDG supersaturation. This is a potential danger for the aquatic environment living in these waters, as the increased saturation poses the risk of experiencing gas bubble disease (GBD). Studies about TDG supersaturation are found in North America (USA and Canada), China, Brazil, and Norway (minor studies include Austria, Germany, and Sweden). Yet, knowledge about the risk of the problem is not widespread, which leads to the repetition of mistakes. Moreover, shifting precipitation patterns induced by climate change are expected to lead to an increase in TDG supersaturation occurrences, as those are associated with flooding. An overview of methods to either prevent or mitigate the problem of TDG supersaturation downstream hydropower plants is presented and recent study results are disseminated. These include civil engineering, operational, and technical methods. Where hydropower plants are in a planning phase, this can contribute to preventing the occurrence of TDG supersaturation in the first place, while existing hydropower plants can implement different measures to reduce the risk of producing TDG supersaturation in the downstream waterways. This helps maintain the aquatic environment as well as local habitat for fish and invertebrates, and therefore counts towards hydropower taxonomy and increases social acceptance of hydropower.
... Therefore, adaptive management based on systematic risk assessments and screening of hydropower plants for TDGS and its biological effects are recommended, followed by monitoring and the implementation of mitigation measures where needed. Mitigation measures, including enhanced vacuum intakes, secondary intake adjustments, improved aeration of supersaturated water, dam deflectors, ultrasonic degassing, operational adjustments and alert systems, are partly available (Pulg et al., 2018;Nielsen and Szabo-Meszaros, 2022;Kuhn, 2023;Zhang et al., 2023). This knowledge holds significance for global river management and both existing and new hydropower installations. ...
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