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Session: Poster Session and Sponsors Exhibition
Session starts: Thursday 22 September, 14:00

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Christopher Steins ()
Martin Habermehl ()
Reinhold Kneer ()

Abstract:
The use of geothermal energy for power generation at brine temperatures of less than 200°C is usually achieved by the Organic-Rankine-Cycle (ORC). Thereby, the choice of a suitable fluid results in another level of complexity in the design of such a process. The study analyses criteria for the evaluation of an ORC related to a given brine flow. Due to the given temperature and the given heat capacity rate, the aim for ORC design is to maximize the power output. Since the heat flow and the heat capacity rate from the geothermal source are limited, a simple efficiency consideration for the ORC based only on the Carnot efficiency will not lead to the maximum power output. The heat transfer between the brine and the working fluid must also be included into efficiency considerations. Using the concept of power maximization [1] to characterise the process temperatures, the choice of the fluid is used as a design parameter to optimise the heat transfer into the ORC. Based on a preliminary developed theoretical concept, the study analyses eight different working fluids, which have been chosen by their thermodynamic properties in relation to the maximum temperature of the brine. Within under-critical conditions the simple power cycle is then calculated over an interval of process power. As a reference, the conditions for a prospected geothermal power station in the German Upper Rhine Valley are taken. It is shown how the choice of the fluid influences the power output and the ability of the ORC to transfer the brine’s heat into work. Furthermore, considerations for a consistent efficiency definition are presented.