Thermal characterisation of compact heat exchangers for air heating and cooling in electric vehicles

[EN] The use of air conditioning in all-electric cars reduces their driving range by 33% in average. With the purpose of reducing the energy consumption of the vehicle and optimising the performance of the batteries, the mobile air-conditioning can be integrated with the temperature control system of the powertrain by means of a coolant loop. In such layouts, the air-to-coolant heat exchangers must operate efficiently in both air heating and cooling modes. Dynamic simulation tools comprising the entire thermal system are essential to assess its performance. In this context, fast but accurate models of the system components are required. This paper presents the thermal characterisation of a commercial compact louvered-fin flat-tube heat exchanger (heater core) for this novel application, based on an experimental campaign comprising 279 working points that reflect real air-conditioning (heating and cooling) working conditions. A general methodology to fit a single correlation of the global heat transfer coefficient for both dry and wet working conditions is explained. The semiempirical correlation developed is employed in a single-node model of the heat exchanger that requires minimal computation time. The present model predicts the heat transfer rate with an average deviation of 3.5% in the cases with dehumidification and 1.9% in the cases when the heat exchanger remains dry.This work has been supported by the European Commission under the 7th European Community framework program as part of the ICE project ‘‘MagnetoCaloric Refrigeration for Efficient Electric Air-Conditioning”, Grant Agreement no. 265434. B. Torregrosa-Jaime acknowledges the Spanish Education, Culture and Sport Ministry (Ministerio de Educación, Cultura y Deporte) for receiving the Research Fellowship FPU ref. AP2010-2160.Torregrosa-Jaime, B.; Corberán, JM.; Payá-Herrero, J.; Delamarche, JL. (2017). Thermal characterisation of compact heat exchangers for air heating and cooling in electric vehicles. Applied Thermal Engineering. 115:774-781. https://doi.org/10.1016/j.applthermaleng.2017.01.017S77478111

A Discussion about the Methodology to Validate the Correlations of Heat Transfer Coefficients and Pressure Drop during the Condensation in a Finned-Tube Heat Exchanger

As already demonstrated by others authors, when the performance of a heat exchanger is analyzed, a semi-empirical model allows getting good prediction of the experimental results provided that it is accompanied by the application of the suitable correlations for calculating heat transfer coefficients (HTC) and pressure drop (PD) in both refrigerant and air side. Many correlations for calculating these coefficients are available in literature, therefore choose the more suitable of them turns out to be not an easy task. This paper presents a discussion about the methodology to perform a comparison aimed to define the criterion for choosing the most suitable correlation to be used in a simulation model for prediction of heat exchangers performance in refrigeration systems. Differently from others works, where the results of each correlation are compared with those obtained by means specific experiments focused on the local phenomenon visualization, in this work the correlations are compared against the measurements of a complete finned-tube condenser integrated in a traditional air-to-water heat pump. The comparison has been carried out using a model able to discretize the heat exchanger through a finite volume method (FVM). The discussion is supported with a case study which was experimentally tested and numerically modeled. The condenser is a finned tube coil that characterized by two asymmetric circuits. The tube diameter is 7 mm and the fin pitch is 2.6 mm. The experimental test bench includes a single-speed 34.3 cm3 reciprocating-hermetic compressor, a plate evaporator and an electronic valve as expansion device. An accumulator allow adjusting the condenser sub-cooling. A specific test campaign has been designed in order to cover a wide range operating points of the condenser: different air velocities (from 1.5 m/s to 4 m/s), inlet temperatures (20-46°C) and sub-cooling (0-5-10°C) have been taken into account. All the correlations considered were programmed and simulated with the software IMST-ART. In order to avoid the overlapping of the effects of the different correlations, during the analysis of the two-phase flow, the heat transferred in the one-phase section of the exchanger and the air-side heat transfer coefficient were maintained constant. The analysis of air-side heat transfer coefficient and pressure drop was carried out in the same way but keeping constant the coefficients of the refrigerant side. Different accuracy parameters were defined in order to assist the final decision about which correlation is able to provide the best agreement with experimental data. The paper aims to present a discussion about the validation methodology, the analysis of the correlations impact on the model results rather than define the parameters to choose the best correlation

Exploring Graphs with Time Constraints by Unreliable Collections of Mobile Robots

A graph environment must be explored by a collection of mobile robots. Some of the robots, a priori unknown, may turn out to be unreliable. The graph is weighted and each node is assigned a deadline. The exploration is successful if each node of the graph is visited before its deadline by a reliable robot. The edge weight corresponds to the time needed by a robot to traverse the edge. Given the number of robots which may crash, is it possible to design an algorithm, which will always guarantee the exploration, independently of the choice of the subset of unreliable robots by the adversary? We find the optimal time, during which the graph may be explored. Our approach permits to find the maximal number of robots, which may turn out to be unreliable, and the graph is still guaranteed to be explored. We concentrate on line graphs and rings, for which we give positive results. We start with the case of the collections involving only reliable robots. We give algorithms finding optimal times needed for exploration when the robots are assigned to fixed initial positions as well as when such starting positions may be determined by the algorithm. We extend our consideration to the case when some number of robots may be unreliable. Our most surprising result is that solving the line exploration problem with robots at given positions, which may involve crash-faulty ones, is NP-hard. The same problem has polynomial solutions for a ring and for the case when the initial robots' positions on the line are arbitrary. The exploration problem is shown to be NP-hard for star graphs, even when the team consists of only two reliable robots

Experimental investigation of the temperatures and performance of a commercial ice-storage tank

[EN] This paper presents the results of an experimental installation with an internal melt-ice-on-coil tank which has a total capacity of 172 kWh. The aim of this work is to analyse the freezing process in a tank with counter-current spiral-shaped coils immersed in around 1855l water. An experimental campaign has been performed with different inlet temperatures and mass flow rates of the heat transfer fluid. This study analyses (i) the chiller performance, (ii) the ice-formation process and (iii) the energy consumption of the installation. Supply temperatures between 2.5 C and 5.2 C have been sufficient to charge the tank without using any nucleating agents. The lowest energy consumption has been achieved for the fastest charging tests.The authors gratefully acknowledge ACCIONA Infraestructuras for the financing support and collaboration. Parts of the experimental results used in this work were collected at Federico II University during the project "Layout d'impianto e metodi di controllo innovativi per la refrigerazione domestica, commerciale e professionale". A.W. Mauro acknowledges the financial support provided by Regione Campania.López-Navarro, A.; Biosca Taronger, J.; Torregrosa-Jaime, B.; Martínez Galván, IO.; Corberán, JM.; Esteban-Matías, JC.; Payá-Herrero, J. (2012). Experimental investigation of the temperatures and performance of a commercial ice-storage tank. International Journal of Refrigeration. 36(4):1310-1318. doi:10.1016/j.ijrefrig.2012.09.008S1310131836

Sizing of a reversible magnetic heat pump for the automotive industry

[EN] This paper focuses on the design of an innovative air-conditioning system, namely a magnetocaloric air-conditioner for an electric minibus. An integrated design of the complete system is necessary, as the hot and cold side of the regenerator will work under dynamic conditions which depend on the instantaneous thermal load in the cabin. In order to assist the design of the system, a dynamic model has been developed for the cabin, the hydraulic loops and heat exchangers, and the magnetocaloric unit. This paper presents (i) a description of the dynamic models, (ii) an analysis of the operating conditions of the magnetocaloric unit and (iii) a discussion on the design of the magnetocaloric air-conditioner. The results show that the electric minibus requests 1.60 kW of cooling power over a span of 37 K in cooling mode, and 3.39 kW of heating power over a span of 40 K.This work has been supported by the European Commission under the 7th European Community framework program as part of the ICE project "MagnetoCaloric Refrigeration for Efficient Electric Air-Conditioning", Grant Agreement no. 265434. B. Torregrosa-Jaime acknowledges the Spanish Science and Innovation Ministry (Ministerio de Ciencia e Innovacion) for receiving the Research Fellowship FPU ref. AP2010-2160.Torregrosa Jaime, B.; Corberán Salvador, JM.; Vasile, C.; Muller, C.; Risser, M.; Payá Herrero, J. (2014). Sizing of a reversible magnetic heat pump for the automotive industry. International Journal of Refrigeration. 37:156-164. https://doi.org/10.1016/j.ijrefrig.2013.06.018S1561643

Experimental validation of a short-term Borehole-to-Ground (B2G) dynamic model

[EN] The design and optimization of ground source heat pump systems require the ability to accurately reproduce the dynamic thermal behavior of the system on a short-term basis, specially in a system control perspective. In this context, modeling borehole heat exchangers (BHEs) is one of the most relevant and difficult tasks. Developing a model that is able to accurately reproduce the instantaneous response of a BHE while keeping a good agreement on a long-term basis is not straightforward. Thus, decoupling the short-term and long-term behavior will ease the design of a fast short-term focused model. This work presents a short-term BHE dynamic model, called Borehole-to-Ground (B2G), which is based on the thermal network approach, combined with a vertical discretization of the borehole. The proposed model has been validated against experimental data from a real borehole located in Stockholm, Sweden. Validation results prove the ability of the model to reproduce the short-term behavior of the borehole with an accurate prediction of the outlet fluid temperature, as well as the internal temperature profile along the U-tube.The present work has been supported by the FP7 European project "Advanced ground source heat pump systems for heating and cooling in Mediterranean climate" (GROUND-MED), and by the "Resource-Efficient Refrigeration And Heat Pump Systems" (EFF-SYS+) program.Ruiz Calvo, F.; Rosa, MD.; Acuña, J.; Corberán Salvador, JM.; Montagud Montalvá, CI. (2015). Experimental validation of a short-term Borehole-to-Ground (B2G) dynamic model. Applied Energy. 140:210-223. https://doi.org/10.1016/j.apenergy.2014.12.002S21022314

Ice formation modelling around the coils of an ice storage tank

This paper aims to develop a dynamic model of the charging process of a commercial ice-storage tank. Firstly, three different 1st order and 2nd order numerical schemes have been compared to solve the transport equation of the heat transfer fluid. Euler s method has finally been chosen as the mass flow rate can vary throughout the charging and it avoids the oscillations which are introduced by Lax-Wendroff s and MacCormack s method. Secondly, the heat transfer outside the coils is analyzed. The numerical complications involved in the creation of the first ice layer around the tubes are discussed and an electrical resistance model is introduced to avoid this problem. The model results have provided a very good agreement with experimental measurements of charging tests which have been performed on a CALMAC ICEBANK tank with a capacity of 172 kWh. The model helps to predict the final part of the latent heat transfer process, where the thermal power is decreased due to the contact between the ice layers around adjacent tubes of the tank.Biosca Taronger, J.; Payá Herrero, J.; López Navarro, A.; Corberán Salvador, JM. (2012). Ice formation modelling around the coils of an ice storage tank. Journal of Physics: Conference Series. 395:12133-12142. doi:10.1088/1742-6596/395/1/012133S1213312142395Zalba, B., Marı́n, J. M., Cabeza, L. F., & Mehling, H. (2003). Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering, 23(3), 251-283. doi:10.1016/s1359-4311(02)00192-8Gebhart, B., & Mollendorf, J. C. (1978). Buoyancy-induced flows in water under conditions in which density extrema may arise. Journal of Fluid Mechanics, 89(4), 673-707. doi:10.1017/s0022112078002803Chen, S.-L., & Lee, T.-S. (1998). A study of supercooling phenomenon and freezing probability of water inside horizontal cylinders. International Journal of Heat and Mass Transfer, 41(4-5), 769-783. doi:10.1016/s0017-9310(97)00134-

Borehole modelling: a comparison between a steady-state model and a novel dynamic model in a real ON/OFF GSHP operation

The correct design and optimization of complex energy systems requires the ability to reproduce the dynamic thermal behavior of each system component. In ground source heat pump (GSHP) systems, modelling the borehole heat exchangers (BHE) dynamic response is especially relevant in the development of control strategies for energy optimization purposes. Over the last years, several models have been developed but most of them are based on steady state approaches, which makes them unsuitable for short-term simulation purposes. In fact, in order to accurately predict the evolution of the fluid temperatures due to the ON/OFF cycles of the heat pump, it is essential to correctly characterize the dynamic response of BHE for very short time periods. The aim of the present paper is to compare the performance of an analytical steady-state model, available in TRNSYS environment (Type 557), with a novel short-term dynamic model. The new dynamic model is based on the thermal-network approach coupled with a vertical discretization of the borehole which takes into account both the advection due to the fluid circulating along the U-tube, and the heat transfer in the borehole and in the ground. These two approaches were compared against experimental data collected from a real GSHP system installed at the Universitat Polit ècnica de Val ència. The analysis was performed comparing the outlet temperature profiles predicted by both models during daily standard ON/OFF operating conditions, both in heating and cooling mode, and the between both approaches were highlighted. Finally, the obtained results have been discussed focusing on the potential impact that the differences found in the prediction of the temperature evolution could have in design and optimization of GSHP systems.De Rosa, M.; Ruiz Calvo, F.; Corberán Salvador, JM.; Montagud Montalvá, CI.; Tagliafico, L. (2014). Borehole modelling: a comparison between a steady-state model and a novel dynamic model in a real ON/OFF GSHP operation. Journal of Physics: Conference Series. 547:1-10. doi:10.1088/1742-6596/547/1/012008S11054

Copper-catalyzed diastereo- and enantioselective desymmetrization of cyclopropenes: Synthesis of cyclopropylboronates

This document is the accepted manuscript version of a Published Work that appeared in final form in Journal of American Chemical Society 136.45, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see DOI: 10.1021/ja510419zA novel Cu-catalyzed diastereo- and enantioselective desymmetrization of cyclopropenes to afford nonracemic cyclopropylboronates is described. Trapping the cyclopropylcopper intermediate with electrophilic amines allows for the synthesis of cyclopropylaminoboronic esters and demonstrates the potential of the approach for the synthesis of functionalized cyclopropanesWe thank the European Research Council (ERC-337776) and MINECO (CTQ2012-35957) for financial support. M. T. and A. P. thank MICINN for RyC and JdC contract