System design of a geothermal sourced CO2 network in a residential district

Many countries and cities in the world have pledged to drastically reduce their CO2 emissions during the next decades. Given the high degree of urbanization in occidental society, district energy systems present a high potential to increase the efficiency of heating and cooling systems. The present work focuses on a specific type of district network, based on CO2 as a working fluid, which allows to exchange heat through condensation and evaporation. The low distribution temperature increases the potential of heat recovery, and thus the energy and exergy efficiency of the global system. Previous studies have shown that very high efficiencies can be reach with the use of heat pumps to supply heat to the buildings, and harvesting heat from lake water. The aim of this work is to study the performance of such a system, based on a case study, the Eglantine district located near Lausanne. The focus is set at evaluating the integration of a geothermal field, as a heat source. The main research questions that will be tried to answer are: "How does the CO2 district energy network perform - energetically as well as financially - in the Eglantine district, and under which conditions does it perform better than concurrent solutions?" "What are the characteristics of a typical district that favor the choice of the CO2 district energy network technology?" The results show a very high energy and exergy performance (COPglobal of 6.1)for the CO2 network that is combined with a geothermal field, which is considerably higher than for the conventional GS-HP system (COPglobal of 4.6). The difference is principally explained by the use of a direct expansion system of CO2 into the ground. Moreover, it is shown that for different uses of the buildings there is an optimal ground temperature, or in other words an optimal borehole depth, and the other way around. This work identiffies the key parameters that the CO2 network technology can leverage to compete against modern, well performing, heat pump systems.