Étude et modélisation d'une pompe à chaleur géothermique à expansion directe au CO2

Abstract

Geothermal heat pump technology is currently one of the most interesting technologies used to heat buildings. There are two designs used in the industry: geothermal heat pumps using a secondary ground loop and Direct Expansion (DX) ground source heat pump. The latter is less used, with one of the possible reasons being that less research has carried out into the design of this sort of heat pump. In this study, a DX ground source heat pump CO2 was analyzed to fill this gap. After a review of the state of the art of the aspect, three articles are going to respond to the objective of this study: - Development of a transient ground evaporator model with R22 and CO2; - Development of a DX geothermal heat pump model with R22; - Development of a DX geothermal heat pump model with CO2; - Analysis of a transient DX geothermal heat pump with CO2 in heating mode. In the first phase, a model of a transient ground exchanger in the evaporator mode with R22 was developed. This model was validated using a comparison with an experiment realized in the CTT of ÉTS Montréal. The difference between the output of the model (heat extraction and pressure drop) and the experiment was less than 10%. After that, a parametric study shows that to optimize the heat extraction; the length of the borehole needs to be optimal (complete evaporation with a small superheat), the angle between the borehole and the horizontal and the mass flow rate need to be small. This model was used in the second part of this thesis to create a complete transient model of a DX heat pump in heating mode using R22. This model contained the four elements of the heat pump: exchanger R22-water, the geothermal exchanger, the compressor and the expansion valve. The complexity of this study is the transient coupling between this model and the control of the superheat at the entry of the compressor during time. Again here, this model was validated using a comparison with an experiment; the difference was low (less than 15%) for a 24-hour test. Using this model, a parametric study of the geometry of the ground exchanger was developed. In conclusion, to have the best heat extraction of the ground, the pipes shank spacing need to be maximized without increasing the borehole diameter. The last part of this thesis is a study of a DX geothermal heat pump using CO2 using the model previous developed. The thermodynamic cycle is changed in this model; a transcritical cycle is used with an intermediate heat exchanger between the high and low pressure. This model was validated using tests realized in CANMET Énergie at Varennes. A study on the use of the intermediate exchanger and a new control law of the pressure in the gas cooler were developed. This study concludes that to have the best performance, the intermediate heat exchanger needs to be used at 100% (all the flow going to the exchanger) and the new law created needs to be implemented in the valve control system. To conclude, the DX geothermal heat pump was been studied with different configurations: Carnot cycle and transcritical cycle with CO2. Two transient models were developed and validated using experimental results. Using these models a lot of recommendations for the design of this technology was developed. A more complex design method can be developed using these models in a future study.

Date	6 Sept 2017
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Louis Lamarche (Supervisor) & Mohamed Ouzzane (Co-supervisor)