Impact of Suction and Injection Gas Superheat Degrees on The Performance of a Residential Heat Pump With Vapor Injection and Variable Speed Scroll Compressor

The market share of heat pump systems has grown significantly in Europe in the past decades and, in residential applications, air-source heat pumps (ASHP) are usually considered due to their relatively low cost. In the literature, it has been widely demonstrated that injection cycle can improve the system performance and operating range. This paper presents experimental results of an air-to-water residential heat pump using a variable speed scroll compressor with vapor injection. The first part of the paper focuses on the experimental results collected from a vapor injection and variable speed scroll compressor air-to-water residential heat pump. The unit is a 10 kW residential system working with R410a as working fluid and capable of providing floor heating and domestic hot water. It was tested in a controlled environment in order to achieve a wide range of outdoor and indoor conditions. The impact on the system performance of the vapor superheat degrees at both injection and suction ports is discussed. It is shown that a better control of these variables could improve the system COP and heating capacity by respectively 10 and 15%. It is also shown that the control of the superheat degrees is a coupled problem and the use of standard gain-scheduled SISO PIDs is not optimal. The second part of this paper presents a model of the system. Finite-volume models are used for the heat exchangers and the split lines. A thermodynamic model of the vapor injection scroll compressor is developed using empirical correlations for the volumetric efficiency, isentropic efficiency and the ratio between the injection and suction mass flow rates. A simple model is proposed for the four-way valve. Finally, a static decoupler-based controller is presented in order to take into account the coupling between both superheat degrees and shows increased performances compared to the SISO PID controller

Comparison of moving boundary and finite-volume heat exchangers models in the Modelica language

When modelling low capacity energy systems such as a small (5–150 kWel) organic Rankine cycle unit, the governing dynamics are mainly concentrated in the heat exchangers. As a consequence, accuracy and simulation speed of the higher level system model mainly depend on the heat exchanger model formulation. In particular, the modelling of thermodynamic systems characterized by evaporation or condensation, requires heat exchanger models capable of handling phase transitions. To this aim, the finite volume (FV) and the moving boundary (MB) approaches are the most widely used. The two models are developed and included in the open-source ThermoCycle Modelica library. In this contribution a comparison between the two approaches is performed. Their performance is tested in terms of model integrity and accuracy during transient conditions. Furthermore the models are used to simulate the evaporator of an ORC system and their responses are validated against experimental data collected on an 11 kWel ORC power unit

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

Geometric Design of Scroll Expanders Optimized for Small Organic Rankine Cycles

The application of organic Rankine cycles (ORCs) for small scale power generation is inhibited by a lack of suitable expansion devices. Thermodynamic and mechanistic considerations suggest that scroll machines are advantageous in kilowatt-scale ORC equipment, however, a method of independently selecting a geometric design optimized for high-volume-ratio ORC scroll expanders is needed. The generalized 8-dimensional planar curve framework (Gravesen and Henriksen, 2001, “The Geometry of the Scroll Compressor,” Soc. Ind. Appl. Math., 43, pp. 113–126), previously developed for scroll compressors, is applied to the expansion scroll and its useful domain limits are defined. The set of workable scroll geometries is: (1) established using a generate-and-test algorithm with inclusion based on theoretical viability and engineering criteria, and (2) the corresponding parameter space is related to thermodynamically relevant metrics through an analytic ranking quantity fc (“compactness factor”) equal to the volume ratio divided by the normalized scroll diameter. This method for selecting optimal scroll geometry is described and demonstrated using a 3 kWe ORC specification as an example. Workable scroll geometry identification is achieved at a rate greater than 3 s⁻¹ with standard desktop computing, whereas the originally undefined 8-D parameter space yields an arbitrarily low success rate for determining valid scroll mating pairs. For the test case, a maximum isentropic expansion efficiency of 85% is found by examining a subset of candidates selected the for compactness factor (volume expansion ratio per diameter), which is shown to correlate with the modeled isentropic efficiency (R² = 0.88). The rapid computationally efficient generation and selection of complex validated scroll geometries ranked by physically meaningful properties is demonstrated. This procedure represents an essential preliminary qualification for intensive modeling and prototyping efforts necessary to generate new high performance scroll expander designs for kilowatt scale ORC systems.United States. Environmental Protection Agency (SU 83436701

Experimental results of a variable speed scroll compressor with refrigerant injection

Experimental data of a variable speed scroll compressor with refrigerant injection for residential applicationsThis CSV file provide the data of an experimental campaign on a variable speed scroll compressor with refrigerant injectio

Experimental investigation and dynamic modeling of an air-to-water residential heat pump with vapor injection and variable speed scroll compressor

peer reviewedThis paper presents experimental results of a residential air source heat pump using a scroll compressor with variable speed and vapor injection. The system is modeled using the Modelica language and a comparison with the experimental results is performed. It was shown that there is still room for improvement on the control of the system. In fact, the tested gain scheduled PID controller showed limitation to maintain a low and steady superheat on both the suction and injection lines due to dynamics associated to the vapor split line or heat exchangers. The presented model of the system could be used in order to develop model based control and increase the system performances

Comparison of a dynamic model and experimental results of a residential heat pump with vapor injection and variable speed scroll compressor

peer reviewedThe first part of this paper presents a Modelica-based dynamic model of the system. Finite-volume models are used for the heat exchangers and the evaporator model takes into account frost formation. A thermodynamic model of the vapor injection scroll compressor is developed using empirical correlations for the volumetric efficiency, isentropic efficiency and the ratio between the injection and suction mass flow rate. The second part presents experimental results of a vapor injection and variable speed scroll compressor air to water residential heat pump. The unit is a 10kW residential system working with R410a as working fluid and capable of providing floor heating and domestic hot water. It was tested in a controlled environment in order to achieve a wide range of outdoor, including frosting conditions, and indoor conditions. The model predictions and experimental results are compared in order to validate the component models in steady state. The experimental investigations showed that there is room for improvement on the control side of the system and the developed model can be used in order to develop model based control of the injection and outdoor electronic expansion valves

RÉSULTATS EXPERIMENTAUX ET MODÉLISATION DYNAMIQUE D’UNE POMPE À CHALEUR AIR-EAU UTILISANT UN COMPRESSEUR SPIRO-ORBITAL À VITESSE VARIABLE ET INJECTION DE VAPEUR

peer reviewedCet article présente les résultats expérimentaux d’une campagne de tests et le modèle de simulation dynamique d’une pompe à chaleur résidentielle air-eau fonctionnant avec un compresseur spiro-orbital (« scroll ») à vitesse variable et injection de vapeur. Il s’agit d’un système résidentiel de 10kW fonctionnant avec du R410a comme fluide de travail, et capable d’alimenter un système de chauffage et de produire de l'eau chaude sanitaire. Les performances du système à charge partielle, le contrôle de la surchauffe ainsi que la validation en régime étable du modèle de simulation sont présentés

Empirical model of a variable speed vapor injection compressor for air source heat pump dynamic modeling

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