Implementation of Innovative Measurement Techniques for Precise Experimental Characterization of Frost Growth in a Heat Pump Evaporator

Refrigeration systems such as heat pumps may be subject to frost formation due to ambient air humidity. This phenomenon plays a major role in the HP overall performance drop due to the presence of an additional thermal resistance and an increase of the air-side pressure drop. The aim of this paper is to present a new innovative experimental technique to measure the amount of frost (or condensates) within the evaporator and therefore the air latent load. As the refrigerant distribution may vary with the frost apparition at the evaporator level, the exchanger cannot be weighted on its own to measure the frost mass over time. One of the main interesting features of the test rig consists in monitoring the weight of the heat pump as a whole. As the relative mass of the frost compared to the test rig is rather negligible, a counterweight has been designed to decrease the total load of the force sensor and therefore to increase the measurement accuracy dedicated to the frost mass measurement. The system is more elaborated and complex to set-up, but it brings much more confidant measurements on the mass, compared to a enthalpy balance based on humidity sensors. Mainly because it relies on only one sensor and there is no error propagation throughout the test. The first phase of the experimental campaign specifically focuses on characterizing the evaporator behavior (without coating) under frosting conditions. As superhydrophobic coatings might represent an efficient solution to delay the frost formation, a second testing phase (not presented in this paper) will take place in order to characterize such coated evaporators and compare them to the bare ones. The testing of different evaporators (coated and uncoated) will mark a fundamental contribution to the progress of understanding coated heat exchangers’ behavior under frosting conditions. Lastly, the creation of the experimental database will feed the calibration of numerical models

Critical Impact of Fin Thermal Conductivity in the Modeling of Evaporator Under Frost Conditions

peer reviewedRefrigeration systems such as heat pumps may be subject to frost formation. This phenomenon plays a major role in the heat pump overall performance drop due to the presence of an additional thermal resistance and an increase of the airside pressure drop. Deep experimental investigations show a typical pattern in frost distribution through the heat exchanger, in various operating conditions. A tube-by-tube simulation model is proposed. The dynamic model accounts for two-phase and overheated zones on the refrigerant side. On the air side, the frost may evolve independently on each tube and their corresponding fins, giving a non-uniform frost layer throughout the device. However, in a first time, important discrepancies are noticed between the distribution of frost predicted by the model and the obtained experimental data. The aforementioned model is then improved by taking the fin thermal conductivity into account. Unlike the first version of the model, the results from the new one match more closely with the experimental observations. This model is successfully validated by means of the empirical data. Such model can pave the way toward future designs of evaporator that requires accurate frost accretion prediction

Experimental On-site Performance and Numerical Analysis of a Mini Exhaust Air Heat Pump Integrated into a Low Energy Detached House

Nowadays, the building sector accounts for 40 % of the total EU energy consumption. In that context, the construction sector is today on a critical path to help decarbonize the European economy by 2050. For this purpose, sustainable cooling and heating technologies may be developed and on-site measurements may confirm the performance measured under laboratory conditions. This paper presents the on-site performance of a mini exhaust air heat pump integrated into a low energy detached house situated in Belgium. The system consists of five components: a simple exhaust ventilation system, an exhaust air heat pump, a backup electrical resistance for space heating only, a domestic hot water storage tank and fan-coil units to heat the building. In that system, the heat source of the heat pump is the air from the ventilation system and the heat pump heating capacity is limited to 1400 Watts. During the night, the exhaust air heat pump produces the sanitary hot water which is stored in a water tank. Consequently, the totality of the domestic hot water is produced by the heat pump. During the day, the heat pump can also be used to heat the building. Nevertheless, only a part the energy requirements related to heating are covered by the machine, due to the limited heating capacity. The remaining heating requirements are covered by the backup electrical resistance. For this reason, this machine is particularly suitable for apartment buildings characterized by a low heating demand and a significant energy demand related to domestic hot water production. In the first part of the paper, the characteristics of the building case study and the different components of the system are presented. The second part of the paper describes the sensors placed in the building used to measure the on-site performance of the machine. In the third part of the paper, the on-site performance of the machine is presented. The influence of the main variables (exhaust water temperature, supply air temperature, outside temperature) on the performance is also discussed. In the last part of the paper, the performance of the whole system is estimated for a typical meteorological year. The estimation is based on the Energetic Performance of Building certificate of the building, and on empirical relationships established with the on-site performance of the machine

Experimental Study and Modelling of a 2-Stage Compression R744 refrigeration System with Vapor Injection and Inter-Cooling

Current trends in CO2 supermarket refrigeration have leaned towards systems including parallel compression and ejectors. However, the possibilities that 2-stage compression system architectures can offer remain out of the main spotlight, thus, system optimization and comparison with the other alternatives are less studied. In this work, a Condensing Unit for CO2 supermarket refrigeration is tested in a climatic chamber. The system includes a 2-stage rolling piston compressor with vapor injection and inter-cooling. The condensing unit is tested in several operating conditions to characterize the system COP, with a special focus on also assessing the compressor performance. On one side, a system-cycle model is applied and calibrated to theoretically optimize the system performance and to evaluate the impact of the intercooler. On the other side, the experimental results allow for calibrating a 2-stage rolling piston compressor semi-empirical model. This model is for the first time validated not only for main mass flow rate and power consumption prediction but also for discharge temperature and vapor injection mass flow rate calculation. Results show that the semi-empirical model can predict these four key compressor variables satisfactorily. The usefulness of the model is thus highlighted. On the other side, the cycle analysis shows that the intercooler has the double positive impact of enhancing compressor performance and increasing the injection mass flow by rising the density of the fluid before the 2nd stage of compression.9. Industry, innovation and infrastructur

Designing And Testing An Air-PCM Heat Exchanger For Building Ventilation Application Coupled To Energy Storage

Due to the increase of energy costs, buildings energy consumption has tended to decrease in the past decades. This gives an opportunity for developing innovative renewable technologies that are more adapted to recent buildings with low energy demand. In this context, one main challenge is to manage non-simultaneous availability of heat source or sink and the energy demand of buildings. Hence, different technologies dedicated to energy storage have been developed recently; one of them is the use of Phase Change Materials (PCM). These materials are considered because they exhibit a high latent energy and a tunable phase change temperature according to their composition. This paper studies a PCM heat exchanger coupled to a building ventilation system. This PCM module can either store heat during the day (e.g. by cooling solar PV panels) and restore it to the building during the night for space heating purposes or store coolness during the night and give it back during the day and thus act as a free cooling system. This project aims to develop a performing air-PCM heat exchanger providing latent energy storage of 0.5 kWh, this energy is delivered between 15 and 30°C. This heat exchanger is based on corrugated cells that can be easily filled, lined up and then locked in a box, letting the air pass between the cells. The PCM used for the prototype is mainly composed of paraffin. In order to develop an efficient PCM ventilation module, two different ways of investigation were followed and used in parallel. The first one used CFD simulations and the second one, a semi-empirical model based on correlations. The CFD simulations were able to predict the convection coefficient on the air side and also show the flow repartition between the different channels whereas the semi-empirical model allowed a parametrical study in order to identify the best possible geometry. Once the heat exchanger geometry was optimized, a test bench was built and a prototype of air-PCM heat exchanger was manufactured in order to measure its thermal and hydraulic performances. The tests consist in either a complete solidification or liquefaction of the PCM starting from respectively liquid PCM at 30°C or solid PCM at 15°C. The airflow rate was set to 45m³/h, which corresponds barely to the ventilation rate of a traditional room in a residential building. A comparison between the models and the measurements was carried out in order to calibrate the semi-empirical model. Finally, the investigation of a free-cooling application in buildings was performed to calculate the annual performance of the system in different types of climates

Description d'un modèle thermo-aéraulique de bâtiment dans Modélica

peer reviewedNowadays, in newly built housings, energy losses due to the ventilation can represent up to 50 % of the total building energy consumption. As a result, heat recovery ventilation units are widely used in order to save primary energy and different control strategies for ventilation systems are investigated. For instance, demand control ventilation sounds like a promising solution to decrease the energy impact of the ventilation system in the residential sector. An accurate building model integrating the influence of ventilation (so called thermo-aeraulic building model) is necessary in order to investigate the control and the impact of the ventilation system on a yearly basis. The aim of the present paper consists in a description of a combined multi-zone airflow network model and thermal building model implemented in the Modelica language. The thermal model is a simplified dynamic model using equivalent thermal resistance and capacity. The airflow network is based on the traditional electrical circuit analogy. The model can be used for ventilation systems design, infiltration rate calculation, inside air quality calculation, energy consumption calculation, etc. The first part of the paper details the multi-zone thermal building model. The results obtained from the model are compared to experimental in situ results collected in the typical single family house test facilities. Those experimental results have been obtained in the frame of the IEA-EBC Annex 58. The second part of the paper introduces the multi-zone airflow network building model. Obtained model results are compared with the results provided by a typical multizone airflow analysis software, for a simple three zones test case. The third part of the paper describes the coupling between both thermal and airflow models. The different numerical problems encountered are described and solutions are discussed

Performance evaluation of a desiccant cooling system coupled with indirect evaporative cooling technologies under various climates

peer reviewedThis work contributes to assess the impact of coupling two indirect evaporative cooling technologies with a standard desiccant evaporative cooling system. In the first modified version, an indirect evaporative cooler (IEC) is added to the system. The secondary air of the IEC is the air extracted from the building. In the second investigated system, the process air is sensibly cooled in a dew-point indirect evaporative cooler (D-IEC). Part of the cooled process air is used as secondary air in the D-IEC. The performance of the three systems in terms of regenerative energy, electricity and water consumptions are compared over the cooling periods of seven climatic zones. It is shown that adding an IEC to the system reduces the overall consumption of the system, especially for warm, moderate and/or dry climate zones. In very hot and humid climate zones, the ambient specific humidity is too high and the system should be enhanced to provide more suitable supply conditions

Critical Impact of Fin Thermal Conductivity in the Modeling of Evaporator Under Frost Conditions

peer reviewedRefrigeration systems such as heat pumps may be subject to frost formation. This phenomenon plays a major role in the heat pump overall performance drop due to the presence of an additional thermal resistance and an increase of the airside pressure drop. Deep experimental investigations show a typical pattern in frost distribution through the heat exchanger, in various operating conditions. A tube-by-tube simulation model is proposed. The dynamic model accounts for two-phase and overheated zones on the refrigerant side. On the air side, the frost may evolve independently on each tube and their corresponding fins, giving a non-uniform frost layer throughout the device. However, in a first time, important discrepancies are noticed between the distribution of frost predicted by the model and the obtained experimental data. The aforementioned model is then improved by taking the fin thermal conductivity into account. Unlike the first version of the model, the results from the new one match more closely with the experimental observations. This model is successfully validated by means of the empirical data. Such model can pave the way toward future designs of evaporator that requires accurate frost accretion prediction

Experimental Study and Modelling of a Two-Stage Compression R744 Refrigeration System with Vapour Injection and Inter-Cooling

Current trends in CO2 supermarket refrigeration have leaned towards systems including parallel compression and ejectors. However, the possibilities that 2-stage compression system architectures can offer remain out of the main spotlight, thus, system optimization and comparison with the other alternatives are less studied. In this work, a Condensing Unit for CO2 supermarket refrigeration is tested in a climatic chamber. The system includes a 2-stage rolling piston compressor with vapor injection and inter-cooling. The condensing unit is tested in several operating conditions to characterize the system COP, with a special focus on also assessing the compressor performance. On one side, a system-cycle model is applied and calibrated to theoretically optimize the system performance and to evaluate the impact of the intercooler. On the other side, the experimental results allow for calibrating a 2-stage rolling piston compressor semi-empirical model. This model is for the first time validated not only for main mass flow rate and power consumption prediction but also for discharge temperature and vapor injection mass flow rate calculation. Results show that the semi-empirical model can predict these four key compressor variables satisfactorily. The usefulness of the model is thus highlighted. On the other side, the cycle analysis shows that the intercooler has the double positive impact of enhancing compressor performance and increasing the injection mass flow by rising the density of the fluid before the 2nd stage of compression.9. Industry, innovation and infrastructur