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10:30   Session 6B: Heat Exchangers Chair: Florian Heberle 10:30 20 mins Simultaneous experimental investigation of nucleate boiling heat transfer and power output in ORC using R245FA and R1233ZD(E) Matthias Welzl, Florian Heberle, Dieter Brüggemann Abstract: In binary geothermal power plants based on the Organic Rankine Cycle (ORC) typically shell-and-tube heat exchangers are used as evaporators. In the shell-side, nucleate boiling of the working fluid takes place on the outer surface of the tubes. For the replacement of fluids with high global warming potential (GWP) or selection of efficient working fluids, a comprehensive evaluation has to be performed. Therefore, the knowledge about the nucleate boiling heat transfer coefficient (HTC) in combination with the electrical power output is necessary. In this study, the focus is led on the investigation of the replacement of R245fa by the low GWP fluid R1233zd(E) in geothermal applications. The nucleate boiling HTC on a horizontal tube and the electrical power of a 1 kW scroll expander are simultaneously measured with an ORC test rig for both fluids. The thermal input is provided by an electrically heated preheater and evaporator. Nucleate boiling takes place on a plain copper tube with an outer diameter of 32 mm and a heated length of 822 mm. The surface temperature of the copper tube is determined by thermocouples within the tube in consideration of thermal conduction. The obtained results, regarding power output as well as heat transfer characteristics, show that the working fluid R245fa performs better at equal saturation temperatures due to the higher density and pressure, and the lower viscosity. The HTC of R245fa is exemplarily up to 43.2 % increased in comparison to R1233zd(E). 10:50 20 mins System cost and efficiency optimization by heat exchanger performance simulations Andreas Möller, Vijaya Sekhar Gullapalli Abstract: Heat exchangers of any types are fully necessary for sourcing heat energy to, as well as disposing the low temperature waste energy from, the ORC system. Depending on the temperatures and the number of heat sources; the working fluids, choice of thermodynamic cycle and internal cycle operating conditions can be varied to determine the optimum fit of the system design for the available heat source and sink. Brazed Plate Heat Exchangers (BPHEs) are used for a wide variety of applications and are very suitable for ORC applications due to their compactness and high efficiency. In this article, we will present a tool which is useful for early phase decision making, such as system optimization based on brainstorming of the concept and choice of thermodynamic cycle, during the design phase involving system design and heat exchanger selections, and for the late post-launch phase where an existing system design might be adopted for use with different heat sources. The principle strength of the developed freeware tool is that it combines system efficiency calculations for various thermodynamic cycles and comes with a large fluid database as well as a heat exchanger price indicator. The tool communicates with a commercial heat exchanger selection software (SWEP SSPG7), in which detailed calculations at component level can be performed followed by possibility of configuration of heat exchangers, check stock availability and generate 3D drawings. Cases studies within low and medium temperature applications, based on different installations of the CraftEngine1, are presented in a separate performance verification section. Several variations of the same basic concepts are obtained by varying temperatures, internal recuperation, working fluid and secondary fluids, which all in all shows some of the possibilities which can be explored virtually with good reliability. 11:10 20 mins Application of the novel "emeritus" air cooled condenser in geothermal ORC Marco Astolfi, Lorenzo Noto La Diega, Matteo Carmelo Romano, Umberto Merlo, Stefano Filippini, Ennio Macchi Abstract: The present work aims to investigate the potential advantages in using a novel wet and dry configuration for heat rejection units in ORC power plants. The reference case is a geothermal power plant that exploits a medium temperature brine and uses a closed loop of cooling water to release the condensation heat. In the calculations, the off-design operation of the whole plant is optimized by a techno-economic point of view with a realistic part-load behaviour for the ORC and the use of experimentally validated correlations for the heat rejection section. The performance attainable with the novel LU-VE Emeritus® unit equipped with a water spray system and an adiabatic panels is compared with those achievable with the same unit in dry operation. Final results show a marked increase of revenues with Emeritus® units with respect to a dry unit. 11:30 20 mins Development of a direct concept helical-coil evaporator for an orc based micro-CHP system João S. Pereira, José Baranda Ribeiro, Ricardo A. Mendes, Gilberto C. Vaz, Jorge C. André Abstract: The combined production of heat and power (CHP) has been considered the major alternative to traditional systems in terms of energy savings and environmentalconservation. Micro-CHP systems are those suitable for a scale that ranges from the thermal demands of public/commercial buildings down to the needs of individual household. The residential market represents the most promising sector for the micro-CHP systems which has the potential to meet a number of energy, social and policy goals. Considering the available technologies, ORC based CHP solutions are recognized as one of the simplest and less likely to raise difficulties to retrofit the current residential heating systems. ORC based CHP systems points to a residential implementation almost with market available components. In fact, two of the most important components, the pump and the condenser, are considered off-the-shelf products while the expander has been adapted from a scroll compressor with success. On the other hand, for the ORC-evaporator there isn’t a ready-to-use component within this power range and the overwhelming majority of the ORC based systems developed by universities or research centers use an indirect way to vaporize the organic fluid which involves the implementation of an intermediate circuit, usually with water or oil, between the heat source and the ORC-evaporator. This intermediate circuit appears to be far from the ideal solution because it increases the thermal inertia, the system complexity and costs. This paper presents the conceptual design and the preliminary characterization results of a special developed evaporator, capable to perform the direct vaporization of an organic fluid, incorporated in an ORC based micro-CHP system. This research is sponsored by FEDER Funds through the Programa Operacional Factores de Competitividade under the contract Centro-01-0247-FEDER-003351 and by the Energy for Sustainability Initiative of the University of Coimbra. 11:50 20 mins Response time characterization of Organic Rankine Cycle evaporators for dynamic regime analysis with fluctuating load Manuel Jimenez-Arreola, Christoph Wieland, Alessandro Romagnoli Abstract: There is a strong urgency in developing technologies ensuring emissions reduction in line with global carbon mitigation policies. A valuable alternative approach to improving overall energy efficiency is to capture and reuse the waste heat that is intrinsic to all industrial manufacturing, transport sector, etc. Regarding industrial processes waste heat accounts for as much as 20% to 50% of the energy consumed. Organic Rankine Cycle (ORC) represents a well-established solution to recover waste heat. However, there is a fundamental issue that needs further investigation: the mismatch between the fluctuating nature of the waste heat available and the ORC system. Current design procedures for ORC systems’ components do not account for this fluctuating nature and this leads to sub-optimal component selection and poor cycle performance at off-design conditions. Since the dynamics of the ORC are dominated by the heat exchanger transients, this study aims to characterize the response times of the ORC evaporator and compare them to the frequency of waste heat fluctuation in order to identify different dynamic regimes of response: 1) behavior closer to a quasi-steady state, 2) response transient dominated and 3) fluctuations effectively filtered out. By using a finite-volume Modelica-based dynamic model of the heat exchanger a general characterization of the response times of ORC evaporators is performed and response time charts are built to highlight the inertia dependence on different factors such as geometry and wall material of the heat exchanger, as well as working fluid type and thermal conditions. Using this characterization, a methodology focused on the ratio of the characteristic response times to the representative periods of waste heat fluctuation has been developed and demonstrated with an exemplary industrial waste heat profile in order to compare how different evaporator designs and fluid selection affect the dynamics and operability of the ORC. The result of the study show that for a certain design parameters selection, a selected evaporator can effectively reduce some of the fluctuation components of the load, leading to a system less prone to wear and with smaller deviations from the design point. 12:10 20 mins A moving boundary modeling approach for heat exchangers with binary mixtures Donghun Kim, Davide Ziviani, James Braun, Eckhard Groll Abstract: In this paper, a modeling approach for a heat exchanger that uses a binary mixture is presented. A moving boundary method assuming a linear spatial distribution of enthalpy could result in a considerable steady state prediction error, due to the temperature glide of a binary mixture. Motivated from this, a more accurate moving boundary method that incorporates analytic enthalpy distributions is presented. The enthalpy profiles are derived by defining a specific heat capacity at each thermodynamic phase of a binary mixture and by solving crossflow heat transfer equations. The proposed approach results in accurate predictions compared with those of a detailed model using the Finite Volume Method (FVM). The corresponding computation time is two to three times slower than that of a conventional moving boundary approach, but the approach is still computationally advantageous compared with the FVM model.