5th International Seminar on
ORC Power Systems
Athens Greece

 
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11:10   Session 7C: Heat Exchangers (2)
Chair: Antonios Charalampidis
11:10
20 mins
Economic Optimization of Heat Exchanger Design for Geothermal Applications
Willem Faes, Steven Lecompte, Johan Van Bael, Robbe Salenbien, Kim Verbeken, Michel De Paepe
Abstract: Geothermal power plants are a promising alternative to polluting fossil fuels. In Belgium, geothermal brines can have production temperatures up to 130°C. This makes them interesting to be used with an Organic Rankine Cycle. The brines have however a high salinity and contain a high amount of carbon dioxide. Most common metallic construction materials will suffer corrosion when in contact with the brine, requiring a high material thickness. This is problematic for heat exchangers, since they are typically designed with an as small as possible wall thickness. Therefore, a heat exchanger model is combined with a corrosion model and an optimal solution is determined with a genetic optimization algorithm. This solution will have the lowest possible total cost of ownership, taking into account not only the investment cost, but also the operational costs.
11:30
20 mins
Droplet Detachment during Condensation Process on a Downward-facing Grooved Surface
He Wang, Zilong Deng, Chengbing Zhang
Abstract: A hybrid thermal lattice Boltzmann method is employed to numerically simulate the condensation process on a two-dimensional downward-facing grooved surface. Nucleation, growth, coalescence and detachment process on surfaces with different wettability are reproduced with a focus on the droplet dynamic behavior during the detachment. The droplet detachment during condensation in pendent state is investigated in a wide range of equilibrium contact angle. The numerical simulation identified a single-droplet mode and a coalescence-induced mode for droplet detachment during condensation process on the downward-facing grooved surface. In the single-droplet detachment mode, the critical departure radius is negatively correlated with the equilibrium contact angle with two inflection points. The first inflection point is related to the transition of droplet wetting morphology from Wenzel to Cassie mode, and the second is corresponding to the transition of departure morphology from breakup to non-breakup mode.
11:50
20 mins
An Innovative Method to Enable Stable, High Efficient, Evaporation in Thin Volume ORC Evaporator Modules
Cor Rops, Patrique Boerboom
Abstract: Over the last decade flow boiling in thin spaces has gained attention due to the developments on Waste Heat Recovery (WHR) systems in the industry. For example, WHR systems for heavy duty vehicles are explored and WHR opportunities for hybrid and fuel cell powered vehicles are identified as well. The elevated fluid temperatures, commonly dumped to the environment, are now used in an ORC to create a pressurised vapour which drives the expander. This expander provides mechanical or electrical energy. The more heat extracted, the more vapour massflow created, the more energy the expander can realise [1]. A counterflow heat exchanger is known to be the most efficient heat exchanger type [2]. However, due to boiling explosions occurring in small diameter/ thin volume channels, most evaporators employ of some kind of crossflow heat exchanger layout. Our investigations [3] on boiling explosions in small diameter/ thin volume channels has given us understanding on the origin of the explosive vapour bubble growth, see Figure 1. This knowledge has led to the insights to control them properly and even use the phenomenon to our advantage. The control over the boiling explosions has led to the elimination of the large pressure fluctuations causing the unwanted fluid backflow. Additionally the fast liquid propulsion through the evaporator is annihilated, providing enough time to fully evaporate. Currently, the above mentioned knowledge is used to realise numerical models describing the flow boiling heat transfer in our innovative structure. Furthermore, heat transfer models on the hot fluid side (eg. Flue gasses) incorporating fin structures are developed. Combining these models, amongst others, allows us to propose evaporators based on a counterflow heat exchanger layout, see Figure 2. Further optimisation studies show a potential vapour massflow gain of about 25% compared to currently conventional crossflow evaporators. Additionally, the counterflow principle allows more compactness and/or significant pressure drop reductions.
12:10
20 mins
Experimental Study on the Influence of Heat Exchanger on the Organic Rankine Cycle Performance
Yingzong Liang, Xianglong Luo, Xiaosheng Zheng, Jianyong Chen, Zhi Yang, Ying Chen
Abstract: Organic Rankine cycle (ORC) is a common accepted low-grade heat-to-power technology. Although the ORC has been extensively investigated in the past decades, experimentally validated works occupy only a small fraction of published literature. The experimental test rig in most of the previous studies are mostly focus on the expander, cycle configuration, working fluid, and system performance. The experimental investigation on the heat exchanger is quite limited though the heat exchanger occupies important role in investments cost and irreversible loss. In the present study, a novel ORC experimental test rig with switchable heat exchangers and scroll expander is introduced. The steady test is first conducted. Then, the influence of working fluid flowrate from 0.1kg/s to 0.16kg/s on the cycle performance is conducted for ORC1 with 6.56m2 evaporator and 13.59m2 under design conditions. Next, the component and cycle performances are investigated under different heat source temperature range from 120oC to 140oC and heat sink temperature range from 15 oC to 25 oC. Finally, the system performance test and comparison are conducted for six ORCs with different heat exchanger area, namely ORC1, ORC2 with 6.56m2 evaporator and 10 m2, ORC3 with 6.56m2 evaporator and 5.42m2, ORC4 with 3.71m2 evaporator and 13.59m2, ORC5 with 3.71m2 evaporator and 10 m2, ORC6 with 5.42m2 evaporator and 13.59 m2. Results show that the thermal efficiency of ORC first increases then decreases with working fluid mass flowrate for ORC1 under design heat source/sink conditions. The thermal efficiency of ORC increases with the inlet temperature difference of heat source and heat sink. ORC2 features the maximum thermal efficiency at working fluid flow rate 0.12kg/s, heat source temperature 150oC, and heat sink temperature 15oC. The comparison between different ORCs show that the condenser area is large than assumed and the pinch point temperature difference is very small in most of the studied operating conditions. The results show that the ORC performance is remarkably affected by the heat exchanger size and the present experimental study provide a valuable guidance for the reasonable heat exchanger design and to improve the ORC performance.