Topical Article: Analysis of the use of ground source heat pumps for Brazilian climate and soil conditions

The use of Ground Source Heat Pumps (GHSP) is a technological alternative to produce higher efficiency for air conditioning for cooling/heating buildings coupled with the reduction of the carbon emissions from building operations. In Brazil, this sort of technology started to drive attention and become an alternative for reducing carbon emissions and was included as one of the strategies in the Brazilian Energy Plan for 2050.

The ground temperature along the depth can be divided into two regions, a zone in the shallow ground, where the ground temperature varies with the seasonal change and a zone in the deeper ground (a depth of more than ~10 m below the surface) where the ground temperature remains relatively constant. Considering that the Brazilian climate is diverse, a ground temperature variation from south to north is expected. For this reason, the feasibility of GHSP may also be variable in this country. In the tropics, the constant ground temperature in the deeper ground zone varies between 20^oC and 25^oC, which still permits the cooling of buildings.

Another important local soil factor which influences the performance of GHSP system is that unsaturated soils cover a large part of the Brazilian territory, and in these areas, due to the alternation of dry and rainy periods, the water content of the soils vary seasonally. Consequently, the soil thermal conductivity, the most important thermal soil parameter, also varies seasonally.

The applications of GHSP in Brazil are limited to, so far, two projects in a residence, one in a research centre building (Centre of Innovation in Sustainable Construction) and one in a soft drink plant, and there are two other projects that will be built in the next two years for two hotels in the southeast region of Brazil. Regarding research and technical studies, there have been over ten research projects mainly focused on technical feasibility studies of the implementation of GHSP in different regions and building typologies.

This study shows that the energy consumption has a significant reduction, where the typologies that provide the highest savings are hotels (43%) and shopping malls (54%). For the typologies using water-chilled systems, there is also a considerable reduction in water consumption (from 40 to 60%) due to the elimination of the cooling towers from the system.

Despite the significant amount of energy and water savings, the cost to implement the GHSP is relatively high, especially for the vertical loops, due to the cost of borehole drilling. Therefore, the payback time for the analysed typologies ranges from 10 to 15 years. Also, due to the Brazilian climate conditions, the need for cooling the building is much higher than heating. This imposes an unbalanced load on the soil, resulting in an increase in soil temperature close to the bore loops, reducing the efficiency of the GHSP system as well as its lifespan.

Alberto Hernandez Neto; Cristina de Hollanda Cavalcanti Tsuha, Brazil

This text has been shortened by the HPC team

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