University of Twente Student Theses


Thermodynamic Optimization of Organic Rankine Cycles for Co-Generation from Low Enthalpy Ultra-Deep Geothermal Reservoirs

Wijk, R.F.O van (2019) Thermodynamic Optimization of Organic Rankine Cycles for Co-Generation from Low Enthalpy Ultra-Deep Geothermal Reservoirs.

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Abstract:In this report an investigation was carried out into the applicability of an Organic Rankine Cycle electricity plant in combination with ultra deep geothermal energy. The first step was to determine the possible lay-out of the plants. Four different ORC systems were analyzed: A subcritical ORC, a subcritical regenerative ORC, a supercritical ORC and a regenerative supercritical ORC. From these four systems it was found that the regenerative supercritical system would lead to the largest electricity production. To analyze these systems, a detailed model of the ORC system was built in MatLab. This model optimized three thermodynamic parameters of the ORC system: The condensation temperature, evaporation pressure and the maximum temperature in the cycle. Furthermore, the model maximizes the working fluid mass flow, which is restricted by the pinch-point temperature difference in the heat exchangers and by the minimum outgoing temperature of the brine. A large selection of potential working fluids was analyzed. It was found that the properties governing the the maximum electricity output, were the working fluid critical temperature and the molar weight of the fluid. For the supercritical systems the molar weight appears to be the governing property determining the suitability of a working fluid. For a selection of suitable working fluids, the sensitivity of various parameters was mapped. n-Butane was selected as a working fluid and multiple concepts of co-generation plants were investigated. These concepts were all based on the subcritical non-regenerative design of the ORC. The combined electricity and heat production was analyzed for two different situations with different ingoing geothermal brine properties and different heat demands. A system consisting of two separate ORC cycles condensing at different temperatures was found to be the most effective in all situations. Furthermore, it was found that using a co-generation plant, delivering heat at high temperature approximately halves the produced electricity compared to the system with no heat production.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Organic Rankine Cycle (ORC), Co-generation, Ultra deep geothermal energy, Thermodynamic optimization, Sustainable energy production
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