Computational performance assessment of very shallow energy geo-structures for multi-storey buildings in moderate climates

Energy geo-structures have been studied mainly for shallow foundation elements and predominantly in heating-oriented applications, whereas the performance of very shallow foundation-integrated heat exchangers under cooling-dominated building operation remains insufficiently quantified. This study presents a coupled thermo-hydraulic numerical assessment of two very shallow energy geo-structure configurations, namely a foundation slab and a retaining wall, integrated into a single-building closed-loop ground source heat pump (GSHP) system for a multi-storey nearly Zero Energy Building (nZEB) residential case in Cyprus. Hourly heating and cooling loads derived from a building energy model were coupled to COMSOL simulations and used together with preliminary ground-loop sizing to evaluate both short-term and yearly thermal behaviour. The investigated dwelling was strongly cooling-dominated, (August cooling demand was 4.4 times higher than February heating demand). For the foundation-slab configuration, the average outlet fluid temperature was 29.5 °C in August and 16.6 °C in February, with average HP coefficient of performance, COP HP , 5.4 and 6.5 respectively. Compared with the Air Source Heat Pump (ASHP) benchmark, the GSHP configurations showed higher COP HP (4.6 vs 6.5 in February, and 4.3 vs 5.6 in August), corresponding to an improvement of approximately 41%. Preliminary sizing also showed that full-load coverage would require approximately ∼1800 m of horizontal ground-loop pipe, whereas only ∼300 m pipe could be integrated into the foundation elements investigated. Both very shallow foundation-based GSHP configurations outperformed ASHP, but their thermal response remains strongly influenced by ambient conditions due to their very shallow depth. The retaining-wall configuration exhibits more stable thermal behaviour than the foundation slab, while the overall analysis shows that such systems are better interpreted as hybrid or partial-load geothermal solutions rather than stand-alone full-load replacements for the entire building demand. The study therefore clarifies both the performance potential and the operational limitations of very shallow foundation-integrated geothermal systems for cooling-dominated residential applications.