A direct expansion geothermal heat pump (DX GHP) operates on a thermodynamic vapor compression cycle as the secondary loop geothermal heat pump (SL GHP). However, it has a feature that distinguishes it from its counterpart in secondary loop. The ground heat exchanger is an integral component of the heat pump, which makes this complex system because the ground is the seat of two phase flow phenomenon.
Despite its benefits compared to SL GHP and according to the statistics in the number of geothermal heat pumps installed worldwide in general, the conclusion is that direct expansion geothermal heat pumps are still not often used because of lack of research and technical information on its performance.
This thesis, far from filling all information gaps with regard to the technology, aims to analyze and propose a simulation model of the geothermal heat pump direct expansion. To achieve these objectives, this paper is structured in three parts that are the main focus of our research.
In a first phase, a performance analysis highlighting the potential for a R22 DX heat pump and the influence of some parameters impacting its performance have been made. The geothermal heat exchanger consists of three geothermal loops 30 m deep, installed in parallel. Successive tests between 2010 and 2014 have shown that the coefficient of performance of the heat pump varies between 2.70 and 3.44, with a daily average of 2.87. Heating capacity reached a daily average of 8.04 kW for a cooling water flow rate of 0.38 L.s-1. The heat extraction rate from ground reached an average of 58.2 W.m-1. The impact of factors such as the cooling water temperature and flow rate of the condenser, the condensation temperature, the pressure drop in the evaporator, the thermal properties of soil on the performance of DX GHP are also presented. Finally, a comparative study between the use of electricity and DX GHP as a source of domestic heating shows that DX GHP realizes savings of approximately 70 % compared to electricity.
In the second part, artificial neural networks were selected to model the DX GHP in heating mode. The goal is to build a model that simulate and develop control strategies for a more efficient use of the heat pump. The methodology of data collection based on the experimental design and Taguchi algorithms has been presented. Among the four algorithms tested in this study with a variable number of neurons in the hidden layer, the Levenberg-Marquardt (LM) with 28 neurons in the hidden layer seems to be the best with an average coefficient of multiple determinations (R2) of approximately 0.9991, an average root mean square (RMS) of 0.16330 and an average coefficient of variance (COV) of 2.9319.
Finally, a comparative analysis of the performance of direct expansion geothermal evaporator using R410A, R407C and R22 as refrigerants has been presented in the third part. The main goal is to choose the best refrigerant that can replace R22. The later will be soon to be eliminated under the entry into force of the Protocol of Montreal signed in 1989. A geothermal evaporator model developed and validated by our research team has been updated to take into account of new media alternatives to R22. The simulation results show that for a low refrigerant flow rate, a R410A DX evaporator shows a better performance than R22. From the pressure drop and the superheating recorded, it is concluded that R407C is the best fluid to replace R22 in the field of DX GHP. However, R410A would be a better choice for new DX systems designed to minimize pressure drop especially for high refrigerant flow rates and to solve the high pressure problems by reducing the compressor size.
The results of this thesis are presented in the form of three articles. The first two articles are published in the journal Energy and Buildings and the third was submitted to Applied Thermal Engineering journal. Two other articles have been published in reports of refereed conferences. The first one describes the modeling and validation of the water-refrigerant heat exchanger in the Comsol software environment and the second one outlines how the methodology of optimal Taguchi plans has been exploited to determine the optimum operating conditions of the DX GHP. Both papers are available in the Appendices. Finally, this work is structured principally into four separate chapters that will be detailed in different parts of the thesis.
| Date | 11 Mar 2015 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Louis Lamarche (Supervisor) & Stanislaw Kajl (Co-supervisor) |
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Fannou, J.-L. C. (Author),
Lamarche (Supervisor) &
Kajl (Co-supervisor),
11 Mar 2015Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering