The open-source Python package Geoloop introduces a novel, semi-analytical model for predicting the performance of deep (>500 m depth) vertical borehole heat exchangers (BHEs), with a focus on capturing depth-dependent variations in subsurface thermal properties, i.e., geothermal gradient and thermal conductivity. Conventional computationally efficient semi-analytical models based on load-aggregation of g-functions often assume uniform subsurface thermal properties. Geoloop addresses this gap by implementing a vertically stacked approach, allowing for realistic simulation of depth-variability in both the subsurface and borehole material properties. The model is benchmarked in the shallow domain against standard depth-uniform g-function implementations (up to 100 m depth) and for deeper conditions with a numerical finite volume model, demonstrating strong agreement and validating its accuracy and efficiency. Simulations for typical Dutch conditions show that deeper BHEs (up to 2000 m) can achieve significantly higher thermal power supply than shallower systems, and results in terms of resulting inlet/outlet temperatures for given heat extraction rates can strongly deviate (>4 °C) from results obtained by depth-uniform assumptions in thermal properties. Application of the model to the Dutch context reveals a non-linear increase in heat extraction potential with depth, surpassing values assumed in common practice by Dutch industry. The results highlight the importance of considering local geological heterogeneity and depth-dependent properties for accurate deep borehole heat exchanger (BHE) performance assessment and system optimization. Geoloop thus offers a robust, versatile platform for advancing the design and analysis of deep vertical BHE systems. Korevaar, Z.; Brett, H.; Lourens, A.; van Wees, J.-D. Geoloop (v1.0)—An Efficient Semi-Analytical Deep Borehole Heat Exchanger Model. 2026, 19, 2697. https://doi.org/10.3390/en19112697 Korevaar Z, Brett H, Lourens A, van Wees J-D. Geoloop (v1.0)—An Efficient Semi-Analytical Deep Borehole Heat Exchanger Model. . 2026; 19(11):2697. https://doi.org/10.3390/en19112697 Korevaar, Zanne, Hen Brett, Aris Lourens, and Jan-Diederik van Wees. 2026. "Geoloop (v1.0)—An Efficient Semi-Analytical Deep Borehole Heat Exchanger Model" 19, no. 11: 2697. https://doi.org/10.3390/en19112697 Korevaar, Z., Brett, H., Lourens, A., & van Wees, J.-D. (2026). Geoloop (v1.0)—An Efficient Semi-Analytical Deep Borehole Heat Exchanger Model. , 19(11), 2697. https://doi.org/10.3390/en19112697
