###### Abstract  
This study presents the implementation of a new Type for TRNSYS based on the integral exponential function, which is fundamental in subsurface heat transfer modeling. This function can be expressed as a series expansion, and the developed Type enables the representation of multiple terms in this expansion. To validate its accuracy, the model's results are compared with experimental data from two geothermal boreholes located in Asturias, Spain: Q-Thermie-Uniovi 1 and Q-Thermie-Uniovi 2. These boreholes, spaced 7.3 m apart and each 48 m deep, were used in a thermal response test, where Q-Thermie-Uniovi 1 underwent thermal injection while temperature measurements were recorded in Q-Thermie-Uniovi 2 over a period of three months. This study evaluates how the number of terms in the series expansion influences the model's accuracy and demonstrates that the newly developed Type effectively replicates experimental data. This work provides a robust tool for optimizing the design of geothermal systems and enhancing the understanding of subsurface heat transfer dynamics. 25 March 2025222 April  
of the European Union [4], heating represents approximately 47% of household energy consumption, while DHW accounts for around 19%. These percentages vary depending on housing type and climatic region. For instance, in single-family homes located in the Mediterranean region, heating can account for up to 63.9% of energy consumption, whereas in apartments in the Atlantic region, this percentage decreases to 32.2%. Shallow geothermal energy has a range of practical applications, particularly in regions where extremely high temperatures are not required. Some of the most common applications of low-temperature geothermal energy include building heating and cooling and domestic hot water production, among others. Low-enthalpy geothermal energy represents a renewable source that plays an increasing role in the field of energy sustainability. Shallow geothermal systems, which use geothermal boreholes to capture or dissipate thermal energy in the subsurface, are becoming more common in residential and industrial applications [5]. However, the performance of these systems largely depends on the local ground conditions [6], as well as the thermal interactions between the boreholes, particularly when designing geothermal fields composed of multiple boreholes [7; 8; 9]. Recent studies have addressed the thermal interference between boreholes using numerical simulations and empirical models [10; 11; 12]. While these contributions have improved the understanding of borehole interaction dynamics, most rely on numerical solvers or simplified G-function approaches and are not directly implemented in widely used simulation environments such as TRNSYS. Moreover, few models incorporate the exponential integral function in a customizable format suitable for long-term simulations. This gap motivates the development of our module, which brings an analytical basis into TRNSYS and provides an accessible, validated tool for geothermal system design. In this context, the use of advanced simulation tools such as TRNSYS (Transient System Simulation Tool) has proven to be essential for studying and predicting the long-term thermal behavior of the subsurface, allowing for the optimization of geothermal exchange systems' design and ensuring their effectiveness and sustainability [13]. This program also helps to evaluate how boreholes interact with each other and can assist in the design and optimization of borehole location and quantity [14; 15; 16; 17]. In TRNSYS, there are several types/modules available for modeling boreholes as components. These modules aid in simulating the behavior of Ground Source Heat Pump (GSHP) systems and shallow geothermal energy systems. While TRNSYS offers several predefined types for modeling borehole heat exchangers, most rely on empirical G-functions or simplified analytical solutions that do not include the exponential integral function explicitly.