Influence du stockage thermique dans la structure du bâtiment sur les systèmes de pompes à chaleur géothermiques

The length reduction potential of a vertical, closed loop geothermal heat exchanger linked to a thermally activated building system (TABS) that uses the thermal energy storage in the concrete floor is analysed. The ground floor of a typical commercial building in Montréal (Canada) is modeled in the TRNSYS simulation environment to study the coupling of geothermal heat pumps with a slab-on-grade TABS during the heating season. Different daily operation scenarios of the geothermal heat exchanger are studied. Results show that a 24 hour leveling of the ground thermal load during the heating season produces only a 4 % length reduction of the geothermal heat exchanger, which demonstrates the need to reduce the installed geothermal heat pump capacity to impact significantly the heat exchanger's length. Three design parameters of the TABS are analysed: the slab thickness and the depth and spacing of its embedded piping. It is shown that increasing the slab thickness from 100 to 200 mm (4 to 8 inches) compensates an 8.3 % reduction of the installed capacity of the heat pumps. An increase of the heat pump's energy consumption is observed with an increased depth of the slab's embedded piping, caused by the increased thermal resistance between the piping and the floor and the increased thermal losses beneath the slab-on-grade. Finally, a tightly spaced embedded piping results in a more efficient heat transfer between the fluid and the concrete, which increases the heat pump's coefficient of performance (COP). A comparison with an equivalent fan-coil heating system shows that the required length of the geothermal heat exchanger is 25 % greater for the TABS. This difference can be explained by the reduction of the fluid heating temperature for the TABS, which increases the heat pump's average COP. This is also accompanied by a 21 % reduction in the energy consumption. Finally, comparing the thermal energy storage in the concrete with water tanks shows that the former is beneficial for the TABS where the latter offers better results with the fan-coil heating system. Finally, it is shown that a reduction of the heat pumps installed capacity of 8.3 % leads to a maximum reduction of the heat exchanger's length of 10.1 % and 8.4 % for the TABS and fancoil heating systems, respectively.