## 4 Conclusions and Future Research Directions  
The present research adds knowledge to the existing gap regarding the energy utilization of SuDS. Therefore, a novel hybrid experimental-numerical methodology has been proposed to determine the thermal properties of entire cross-sections of SuDS, using a dry swale as the first SuDS technique to validate the methodology. This will enable a holistic design of swales in combination with shallow geothermal systems, incorporating the energy viewpoint and paving the way in the field for further experimentation in other SuDS.  
The study also evaluated the inclusion of new materials in the cross-section of a green dry swale (expanded clay and CDW). It was observed that the implementation of these materials improved the swale's thermal properties. The equivalent thermal conductivity of the most favorable swale cross-section (model type 3) for implementing an SGS was found to be 0.152 W/mK under dry conditions and 0.271 W/mK under wet conditions. These values represent an 87% improvement under dry conditions and a 51% improvement under wet conditions compared to the equivalent thermal conductivity of the conventional green swale cross-section. In addition, the type 2 and 3 sections, formed by expanded clay, had a greater hydraulic storage capacity than the conventional section. This again highlights the importance of the energy-water nexus.  
It has been observed that the thermal properties of the thermally designed SuDS has the potential to improve the performance of the GSHP compared to a conventional SGS installation in conventional soils.  
Complementary tests were carried out to thermally characterize the materials used in the green swale; these were based on a non-destructive test using the MTPS method. The results obtained confirm that the actual thermal conductivity values of the materials, measured with traditional techniques, do not reproduce the real thermal behavior of swales. This is due to the fact that the materials and their arrangement are heterogeneous and other heat transfer phenomena occur between the green swale layers and their environment.  
Transient numerical models have been shown to be a useful tool to simulate the thermal insulation capacity of SuDS under real climatic conditions to improve green swale designs. This will allow us to determine their impact on the performance of geothermal heat pumps when combined with SGS, since one of the fundamental design parameters is the temperature at which the GHE is located in the ground [34]. This will help to avoid design errors identified in field tests carried out by other authors, thus improving the performance of GSHP [18; 24; 39]. Future developments of simplified tools will allow engineers and specialists to determine the best possible design for these systems for the geothermal utilization of SuDS infrastructures.  
\begin{table}
\begin{tabular}{c c c} \hline
**Green Swale** & **Dry Conditions** & **Wet Conditions** \\ \hline Type 1 & 2.270 & 2.776 \\ \hline Type 2 & 1.772 & 2.522 \\ \hline Type 3 & 1.765 & 2.544 \\ \hline \end{tabular}
\end{table}
Table 10: Temperature increases (\({}^{\circ}\)C) registered at the bottom of green swale models.  
As future research steps, field tests should be conducted to evaluate the thermal performance of the combined system of a green s