Modeling and Experimental Study of a Heat Pump Water Heater Cycle

Heat Pump Water Heaters are becoming more and more interesting technologies for efficient sanitary hot water production. The specificities of hot water production compared to the traditional use of heat pumps for space heating are the relatively constant energy needs for different outdoor temperatures and the more rapid dynamics associated with water temperature elevation. This study focuses on the modeling and performance evaluation of an R134a air to water heat pump water heater with an external mantle heat exchanger. By nature being a thermo-hydraulic kind of system, a heat pump water heater requires both the aspects of fluid mechanics and heat transfer to be covered when modeling the global system composed of the heat pump and the thermal storage tank. Hence, a detailed thermodynamic model of the heat pump cycle is developed using Modelica covering a description of all the components of the thermodynamic cycle from compressor to evaporator and all the possible operating conditions such as heating and defrosting. This model is associated with a zonal model accounting for the convective behaviour patterns of the water observed in the storage tank at different operating conditions and boundary conditions imposed by the heat pump cycle. This dynamic model is compared against experimental data acquired from an instrumented system tested in laboratory conditions for different phases such as draw-off, standby and heating. Good precision ( \u3c 5-10%) is attained for the heat flow rates, temperatures along the thermodynamic cycle and temperature profiles in the water tank for the different phases tested. It is shown that the water tank plays an important role in the performances of the system that is very sensitive to the operating conditions such as draw-off flow rate, heat pump operating capacity or thermal losses, that cause mixing and destruction of the thermal stratification and a reduction in the available energy for the end user

Modelling Of An Automotive Multi-Evaporator Air-Conditioning System

With the arrival of plug-in hybrid electric vehicles (PHEV) and battery electric vehicles (BEV) with significant autonomy, battery cooling becomes a necessity in driving mode to ensure their durability and ability to charge rapidly.  For these vehicles, the refrigerating system may be composed of two evaporators (for front and rear passengers) in order to afford cooled air in the cabin and a chiller or a built-in battery evaporator to cool down the traction battery. This kind of multi-evaporator air-conditioning system has a number of technological barriers that must be undone. They are related to the components sizing in a context of cost reduction and control of such complex systems. The study therefore focuses on understanding the dynamic coupling of the several loop components such as the three evaporators having different cooling capacities. Understanding the behaviour of their respective expansion devices and the choice of these latter is also essential to control properly the transient phase and ensure an optimal operation of the air-conditioning system. In the literature, the effect of battery cooling by means of a chiller on the automotive air-conditioning loop has been already proved by simulation in the Dymola® [1] environment. The simulation results for several driving cycles, refrigerants and ambient conditions emphasize the thermal discomfort caused by the use of the chiller loop. However, no global control strategy has been established. More recently, a first study of an air conditioning system model with three evaporators was carried out [2]. After the validation of their component models, a cool down test was performed to test the performance of their air conditioning system. From a control point of view, a simple PI control on the temperature of air blown at the front evaporator was used to regulate the speed of the compressor. In the building sector [3], the benefits of a supervisory controller to regulate the multi-evaporator air conditioning system was developed. Although this type of decentralized model seems to be robust and applicable to the car, it requires the use of sensors and components currently too costly and subject to a less restrictive environment than in automotive. The challenge of such a cooling loop lies in the dynamic coupling of components as well as their design. The model of a multi-evaporator automotive air conditioning system (two evaporators and a chiller) is thus produced using the 0D AMESim® software. In order to obtain more representative results in the transient state, the majority of components, including the chiller and regulators, are physical models giving a good representation of their internal geometries. These models were validated using experimental test maps. The first results highlight the importance of the regulators choice on the loop stability. A comparison of several types of expansion valves (orifice, thermostatic and electronic) will be conducted in order to select the most suitable to meet the price-performance compromise. Finally, control strategies are studied in transient state to further improve the stability and speed of convergence to the target instructions

Some important specificities of the boiling behavior of water used as (the most natural) refrigerant. (Invited keynote lecture)

International audienc

Compact evaporators for sorption chillers: from the fundamental phenomena to improved design opportunities: Invited keynote lecture

Invited keynote lectureInternational audienc

Some characteristics of bubble dynamics during pressure-induced subcooled boiling

International audienc

Thermodynamic investigation of quasi-isothermal air compression/expansion for energy storage

International audienc

Experimental study of RIA related fuel-coolant interaction

International audienc

Experimental study of RIA related fuel-coolant interaction

International audienc

Experimental validation of an analytical model for predicting the thermal and hydrodynamic capabilities of flat micro heat pipes

International audienceAn analytical model by Lefèvre and Lallemand [F. Lefèvre, M. Lallemand, Coupled thermal and hydrodynamic models of flat micro heat pipes for the cooling of multiple electronic components, Int. J. Heat Mass Transfer 49 (2006) 1375-1383] that couples a 2D hydrodynamic model for both the liquid and the vapor phases inside a flat micro heat pipe (FMHP) and a 3D thermal model of heat conduction inside the FMHP wall has been modified. It consists of superposing two independent solutions in order to take into account the impact of evaporation or condensation on the equivalent thermal conductivity of the capillary structure. The temperature, pressure and velocity fields can be determined using Fourier solutions. The model has been experimentally validated based on literature data from a grooved FMHP. Two new correlations for the equivalent thermal conductivities during evaporation and condensation inside rectangular micro-grooves have been proposed based on a numerical database. The influence of the saturation temperature and geometry on the maximum heat flux transferred by the system is presente

Pressure transient and vaporization process following the rapid heating of a liquid — Experiments and modelling

International audienceThe aim of this paper is to study the influence of the thermodynamic conditions on the consequences of the rapid vaporization of CO2. This study is motivated by the lack of experiments characterizing those transient phenomenaover a wide range of thermodynamic conditions. For that purpose, a complete test section was designed based on the Joule effect to deliver the energy discharge in pressurized CO2.The transient deposit of power in the liquid have two consequences: the generation of a pressure wave (of several bars) due to the transient creation of vapour, and a slow compression in the test section due to the creation and expansion of vapour (few cm3). This experiment had been performed 200 times over a wide range of thermodynamics conditions, (Pi, Ti, E)∈[2.55MPa, 6MPa]×[-12 ◦C, 22 ◦C]×[70 J, 310 J]. From this databank, significant tendencies are extracted from the maximum of overpressure generated and the mass of vapour created function of the test conditions. Lastly, a simple model permits to predict the first pressure peak as a function of the test conditions.This work, motivated by the so-called Fuel Coolant Interaction (FCI) nuclear safety related problematic, brings consistent data allowing to better characterize the small scale processes for such transient vaporization phenomena