09:00
Session 6D: Supercritical-Transcritical Cycles
Chair: George Tsatsaronis
09:00
20 mins
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Performance Comparison of Transcritical CO2 Power cycle, Organic Rankine Cycle and Kalina Cycle for Low Temperature Geothermal Source
Fanxiao Meng, Enhua Wang
Abstract: The wide utilization of renewable energy can relieve the energy crisis effectively. As a type of the renewable energy, geothermal energy is sustainable, abundant, environmentally-friendly, independent of various weather conditions and easily coupled with the conventional system. Among the low-temperature recovery technologies, transcritical CO2 Rankine cycle (TCRC), and organic Rankine cycle (ORC) and Kalina cycle have their own characteristics. This paper explored the thermodynamic performance of TCRC with the various turbine inlet pressure and temperature, and determines the optimal design operating condition for the 120℃geothermal heat source at first. Then, a comparative analysis of the thermodynamic performance of TCRC, ORC and Kalina cycle was presented under the optimal design operating condition. Results show that the net power output of TCRC is the largest while the ORC has advantages in term of thermal efficiency, whose value is 12.28%. However, the net power of ORC is the lowest, whose value is only 1556.34kW. The thermal efficiency and net power of Kalina cycle are 9.77% and 2704.68kW, respectively.
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09:20
20 mins
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Combined Heat and Power Generation by Enhanced Geothermal Systems: Comparison of Direct and Indirect Concepts for Water and Supercritical CO2 as Heat Carriers
Christopher Schifflechner, Fabian Dawo, Sebastian Eyerer, Christoph Wieland, Hartmut Spliethoff
Abstract: While the utilization of hydrothermal resources is limited to certain geological regions, Enhanced Geothermal Systems (EGS) have a tremendous technical potential without major regional restrictions [1]. In the last years, an increasing focus is laid on the utilization of EGS with supercritical CO2 (sCO2) instead of water due to its beneficial fluid characteristics [2].
First studies discuss several different concepts for power generation from sCO2 EGS. Next to the direct expansion of the sCO2 in a turbine [2], also the application of the sCO2 solely as heat carrier for an ORC system as well as the combination of a direct expansion and an ORC system is investigated [3,4]. Concerning the utilization of EGS by water, several potential plant layouts are discussed [5]. Direct expansion by single or double flash plants as well as their combination with an additional ORC as binary cycle are possible. Depending on the achievable temperature level of the brine and as well avoiding potential corrosion issues within the components in case of direct utilization, also the sole application of an ORC as a binary power plant may be advantageous [6].
However, the current development of the geothermal sector in Central Europe increasingly emphasizes the combined heat and power generation (CHP). Consequently, the annual available heat flow for power generation might differ significantly due to the varying heat demand. Thus, an assessment of the potential different power plant types for EGS should consider the part load characteristic of the plant layout, instead of assuming constant full load operation.
Based on the heat demand characteristic of an actual district heating network, this study investigates the performance of the above-ground plant, comparing sCO2 and water as heat carrier for the utilization of the EGS. The performance of both options is evaluated with respect to different temperatures of the geothermal reservoir. The results present a recommendation regarding the most suitable plant layout and heat carrier medium for EGS projects in case of CHP operation. The work provides a valuable contribution to the debate about suitable plant concepts for EGS projects against the background of combined heat and power generation.
References:
[1] Chamorro et al. Enhanced geothermal systems in Europe: An estimation and comparison of the technical and sustainable potentials. Energy (2014)
[2] Garapati et al. Brine displacement by CO2, energy extraction rates, and lifespan of aCO2-limited CO2-Plume Geothermal (CPG) system with a horizontal lproduction well. Geothermics (2015)
[3] Wang et al. Working fluid selection for organic Rankine cycle power generation using hot produced supercritical CO2 from geothermal reservoirs. Applied Thermal Engineering 149 (2019)
[4] Ruiz-Casanove et al. Use of supercritical CO2 heated with geothermal energy for power production through direct expansion and heat supply to an ORC cycle. Proceedings of the ASME 2017 International Mechanical Engineering Congress and Exposition (2017)
[5] Hu et al. A Selection Method for Power Generation Plants Used for Enhanced Geothermal Systems (EGS). Energies (2016)
[6] Lu, S.-M., A global review of enhanced geothermal system (EGS). Renewable and Sustainable Energy Reviews (2017)
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09:40
20 mins
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The Dynamic Performance and Control Strategy of a CO2 Transcritical Cycle for Waste Heat Recovery of Internal Combustion Engine
Rui Wang, Gequn Shu, Hua Tian, Xuan Wang, Jinwen Cai, Zhijie Li, Xianghua Kong
Abstract: Waste heat recovery technologies are regarded as one of the most promising method to improve efficiency of internal combustion engines of trucks. Therein, CO2 transcritical cycle can make full use of waste heat of exhaust and jacket water at the same time under the design working condition. However, since the working conditions of trucks are complex, the waste heat recovery system also needs to work under different conditions. When the working condition of CO2 transcritical cycle changes, the properties of CO2 vary obviously and the cycle may not be able to use up the waste heat of exhaust and jacket water anymore. Therefore, a dynamic model of CO2 transcritical cycle which recovers both of exhaust and jacket water waste heat is established in this work by Simulink. After being verified by experiment data, the model is used to research the dynamic and off-design performance of the CO2 transcritical cycle. Based on these, a control strategy is proposed to help the cycle be able to recover the waste heat of exhaust and jacket water at the same time as much as possible.
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10:00
20 mins
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The Role of Operational Variability on the Non-ideal Flow in Supersonic Turbines for Supercritical Organic Rankine Cycles
Alessandro Romei, Davide Vimercati, Alberto Guardone, Giacomo Persico
Abstract: The potential efficiency gain in supercritical Organic Rankine Cycles (ORC) must face an increase of complexity in the fluid-dynamic design of the first nozzle cascade, as severe non-ideal gas effects may possibly occur. Examples of these non-ideal effects are a non-monotonic variation of the Mach number along isentropic expansions, oblique shocks featuring an increase of the Mach number and a significant dependence of the flow field on the thermodynamic conditions at the turbine inlet.
In this work, two uncertainty-propagation scenarios, targeting the field operational variabilities, are analysed for representative first-stage nozzle cascades, whose expansion processes occur in the so-called non-ideal gasdynamic regime (0 < Γ < 1, where Γ is the fundamental derivative of gasdynamics). Realistic variabilities, derived from field measurements in running ORC power plants, are propagated through a turbulent compressible flow solver featuring generalised thermodynamic treatment via non-intrusive Polynomial Chaos representations to compare the cascade performance when Γ is either ⪅ 1 or ≪ 1.
The analysis of cascade-loss distributions indicates that the considerable dependence of the flow field on the upstream total conditions induces different cascade operations from a stochastic perspective. Given uncertainties of ≈ 1% in cycle design conditions, the turbine cascade operating with Γ ≪ 1 exhibits cascade-loss variations as high as ±0.75%pts with respect to the mean value, compared to approximately ±0.15%pts when Γ ⪅ 1. Finally, the decomposition of variance contributions reveals that the most influencing parameter on the turbine performance migrates from the expansion ratio to the upstream total temperature when approaching supercritical conditions characterised by Γ ≪ 1. This finding suggests that, when devising supercritical ORCs, a realistic estimate of the heat-load variability during plant operations should be taken into account in the early stage of the turbomachinery design.
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