| dc.description.abstract | Many of the synthetic refrigerants used in heat pump and air conditioning
systems are potent greenhouse gases. In light of the increasing concern regarding
climate change, there has been an increased interest in natural refrigerants such as
carbon dioxide which have a comparatively negligible impact on climate change.
Direct expansion geothermal heat pumps require a very large volume of refrigerant,
making the use of a natural refrigerant particularly beneficial from an environmental
perspective.
In this study a numerical model has been developed to analyze the steady state
performance of a direct expansion geothermal heat pump water heater using carbon
dioxide in a transcritical cycle. The system incorporates a compressor, a counter-flow
gas cooler, an expansion device and a ground heat exchanger which is the system
evaporator. The model was developed by means of thermodynamic, heat transfer and
fluid flow analysis of each component of the system. A comparison between predicted
component performance and experimental results available in the literature indicated
that the simulation can provide a reasonably accurate representation of an actual
system. Given this verification, the simulation was used to gain an understanding of
the direct expansion CO2 geothermal heat pump's performance under varying design and operating parameters. The salient parameters which were studied include:
compressor speed, ground coil length, number of ground circuits and gas cooler
length. The effect of monthly soil temperature variation was also investigated.
The parametric study revealed several factors which are important for system
optimization. First, at any given soil temperature an optimum mean evaporation
temperature exists; even a small deviation from this optimum can have a significant
impact on coefficient of performance. Another important factor is the number of
evaporator circuits. Finally, the gas cooler and evaporator capacities were shown to
have a large impact on performance; heat exchanger capacities should be matched for
optimum performance. Utilizing the findings of the study, an optimized system was
simulated and compared to the baseline. The optimized system achieved a coefficient
of performance of 2.58, representing an 18% improvement over the baseline system.
Heating capacity increased 17% to 12.3 kW. The study suggests that with further
research and optimization, carbon dioxide can perform well in a direct expansion
geothermal heat pump and is a suitable replacement for more environmentally
degrading refrigerants. | en_US |
