5th International Seminar on
ORC Power Systems
Athens Greece

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16:00   Session 5D: Heat Exchangers (1)
Chair: Marco Astolfi
20 mins
Evaluation of Heat Transfer Correlations for Flow Condensation in Plate Heat Exchangers and their Impact on the Design of Organic Rankine Cycle Systems
Ji Zhang, Enrico Baldasso, Roberta Mancini, Brian Elmegaard, Fredrik Haglind
Abstract: A well-verified heat transfer correlation to predict the thermal performance of flow condensation is essential for optimal condenser design in organic Rankine cycle systems. This paper aims at evaluating the prediction accuracy of existing correlations for the flow condensation in plate heat exchangers, and studying the impact of using different correlations for the design and performance estimation of organic Rankine cycle units. In order to achieve these goals, an experimental test campaign was conducted with the working conditions that commonly prevail in condensers of organic Rankine cycle units, and the test data were subsequently utilized for comparison with the predicted values calculated using existing correlations. Moreover, a simulation framework was applied for a case study of waste heat recovery. The results indicate that the correlations from Yan et al. (1999) show best predictive performance to the test data, resulting in a mean absolute percentage deviations below 20 %. With respect to the case study, the required heat transfer area for the plate condenser ranged from 75.8 m2 to 132 m2, and the estimated power output of the unit, for a given condenser design, varied in the range -7.2 % to +8 % compared to the target value. The results hence indicate the need for more accurate prediction methods and generally applicable heat transfer correlations.
20 mins
Sizing Criteria and Performance Evaluation of Direct Air Cooled Heat Rejection Units for Supercritical CO2 Power Plants
Dario Alfani, Marco Astolfi, Marco Binotti, Matteo C Romano, Ennio Macchi, Stefano Filippini, Umberto Merlo
Abstract: Supercritical CO2 power plants are recognized as a promising solution for the exploitation of different energy sources: from fossil fuel combustion and nuclear energy to solar energy and waste heat recovery. The advantage of sCO2 systems with respect to ORC for small power outputs is represented by the possibility to reach higher temperatures thanks to the high thermal stability of CO2. On the contrary, for large scale power plants sCO2 technology can compete against steam cycles being likely able of fast transients thanks to the compact dimension of turbomachinery and to show outstanding part load performance. However, the reliability of sCO2 power plants is not proven yet on large scale systems and it will strongly rely on the availability of specific components. The design of the compressor is critical because of the small dimension, the high rotational speed and the need to handle a fluid close to the critical point. The recuperators design is also challenging because of the large pressure differences between cold and hot side and the need to enhance the heat transfer coefficients. The last main component, with the exception of the primary heat exchanger which depends on the specific application, is the heat rejection unit (HRU). The large range of possible applications and possible scarcity of water resources for sCO2 systems makes the use of direct air cooled HRU of great interest. This paper deals with the numerical modelization supported by experimental data of direct air cooled HRU for supercritical CO2 with pressures ranging from 70 to 100 bar and maximum temperatures between 70-150°C. Main design criteria are derived from the design of HRU of CO2 refrigeration systems that likely share many features with next generation power plants components. The effect of different heat exchangers arrangements on the overall heat transfer area, fan consumption, internal volume and footprint are discussed providing useful insights on the design and the performance of supercritical CO2 heat rejection units.
20 mins
Optimization of an Exhaust Heat Exchanger Using Metal Foam Baffles for ORC Waste Heat Recovery System
Tianyu Chen, Gequn Shu, Hua Tian, Xiaonan Ma, Tingting Zhao, Hongfei Zhang, Zhao Zhang
Abstract: Heat exchanger is an important component in ORC waste heat recovery systems (WHRs), and shell and tube heat exchanger is one of the most common heat exchangers. Compared with traditional metal baffle heat exchanger, the heat transfer area of metal-foam baffle heat exchanger (MF-STHE) is enhanced due to the geometry of metal-foam baffles, which results in a significantly increase in heat transfer performance. Properly designed segmental metal foam baffles can reduce pressure loss and enhance heat transfer in MF-STHE. In present investigations, a 3D numerical model of an exhaust heat exchanger using metal-foam segmental baffles is established to optimize its performance. Firstly, the vector distribution is analyzed. Then, some MF-STHE models with various porosity and pore density are simulated under same mass flow rate (1 kg/s) for parametric study. The area goodness factor j/f1/2 is considered as one evaluation factor to reveal the comprehensive performance of different MF-STHEs. The results show that the comprehensive performance of the MF-STHE with ε = 0.71 and PPI = 20 is the optimum one in all MF-STHEs considered in this paper.