Thermal performance of energy retaining walls in hot-dominant climates: experimental evaluation in unsaturated Brazilian soil

Ground source heat pump (GSHP) systems are widely adopted across many countries as efficient solutions for space heating and cooling, harnessing shallow geothermal energy—a renewable and low-carbon resource. In Brazil, where a substantial share of electricity consumption is linked to buildings, particularly due to air conditioning in the predominantly warm tropical climate, the integration of earth-retaining structures as ground heat exchangers—known as energy walls—presents a promising and cost-effective alternative to improve GSHP performance. A significant portion of Brazil’s landmass consists of unsaturated soils, where seasonal transitions between dry and rainy periods result in fluctuating groundwater levels and degrees of saturation. These changes affect essential ground thermal properties, such as thermal conductivity, and must be considered in the design of geothermal systems. This study presents preliminary findings from the first field investigation on energy retaining walls in Brazil, conducted at a representative tropical unsaturated soil site at the University of São Paulo in São Carlos. Two thermal performance tests (TPTs) were conducted during the rainy summer season—a period of increased cooling demand —on concrete wall panels embedded in unsaturated lateritic clayey sand. The objective of the tests was to assess the thermal performance of the lower section of an energy wall in contact with unsaturated soil on both sides, and to examine how different operating conditions (intermittent versus continuous operation) influence its performance in building cooling applications. Temperature sensors installed within the wall and surrounding soil recorded thermal variations throughout the heating and recovery phases. In addition, both numerical and analytical models were validated against the experimental results, exhibiting good agreement and demonstrating their suitability for long-term predictions. The presented results offer valuable insights for the future design and implementation of energy retaining walls in tropical unsaturated soil environments.