## 2 Experimental Setup  
The experimental setup was developed in parts through teamwork [31; 32]. The choice of the geographic location of the experimental setup was influenced by several factors, such as soil properties (a soil sample from the EAHE site contains 20% gravel, 59% sand, and 21% silt/clay, with a predominance of silt composition in the site) [15], ease of excavation, proximity to a power point, and distance from heat sources that could interfere with heat exchange. The experience was made in February 2020 near the Electromechanics Building of Beira Interior University. The design of the EAHE system was chosen as a spiral, vertical loop shape. This design requires less space than the horizontal serpentine design [33], due to lack of space, especially in urban areas, or due to geological aspects, such as rocky soils. The layout of the EAHE system is shown in Figures 2 and 3. From Figure 1, it can be seen that there are two fans. In the closed loop, Fan 1 is responsible for blowing the air that comes from the GH to EAHE to GH again (Figure 2a), and Fan 2 is responsible for the air change of both GHs with the ambient air. While in  
Figure 1: Layouts of GH + EAHE with the reference GH. (**a**) Closed-loop schematics. (**b**) Open-loop schematics. Figure 2: Path of the air exchange in the two models. (**a**) Closed-loop schematics. (**b**) Open-loop schematics. the open loop, Fan 1 is responsible for blowing the air from the ambient to EAHE to GH (Figure 2b), and Fan 2 is responsible for the air change of the second greenhouse. The airflow was measured with the Testo 416 anemometer positioned inside the PVC pipe with the telescopic anemometer probe inserted in the geometric center of the flow cross-section, at a distance of about 30 cm from the fan outlet. A previous calibration process allows us to use this velocity to evaluate the mean integrated velocity in that section. Several measurements were performed. The average value was used to calculate the number of air changes in the inner volume of the greenhouse (see Table 1). \begin{table}
\begin{tabular}{c c} \hline \hline
**Element** & **Technical Specification** \\ \hline \hline  & Configuration: spiral, vertical shape (open-loop \(\mid\) closed-loop) \\  & Pipe of PVC (Polyvinyl chloride): density 1380 (kg.m\({}^{-3}\)). \\  & Specific heat Capacity 900 (J.kg\({}^{-1}\).K\({}^{-1}\)). \\  & Thermal conductivity 0.16 (W.m\({}^{-1}\).K\({}^{-1}\)) \\ EAHE—earth air heat & Pipe length: 29 (m) \\ exchanger & External diameter: 0.075 (m) \\  & Pipe thickness: 0.0015 (m) \\  & Pipping depth: 3 (m) \\  & Distance between pipes: 0.15 (m) \\  & Inlet/outlet air temperature sensors position are 3.20 (m), 24.77 (m) \\ \hline \hline  & Materials: fir-wooden bars with a square profile of 30 \(\times\) 30 mm section, assembled with steel joinery, screws, and anchored into the ground up to about 20 cm with six steel tubes with a 35 \(\times\) 35 mm\({}^{2}\) profile. \\ GH- greenhouse & Greenhouse volume is 2 \(\times\) 1.4 \(\times\) 1.4 m\({}^{3}\) with extra triangle volume (0.5 \(\times\) 1.4) x 0.7 \(\times\) 2 (m\({}^{3}\)) = 5 m\({}^{3}\) \\  & Distance between greenhouses is 3 m \\  & 0.1 mm-thick Polyethylene film was cut, stretched, and fixed to the structure using VD tubes (PVC  rigid tubes applied in electrical installations), cut in segments of 7 cm length, and screws. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics and technical specifications of the equipment used in the experimental setup. Figure 3: Details of the two scaled-down models. (**a**) Open-loop photographs. (**b**) Closed-loop photographs. The materials used are fir wood piles with a square profile of 30 \(\times\) 30 mm cross-section, connected with steel joints and screws, and anchored in the ground 20 cm deep with six steel pipes with a square profile of 35 \(\times\) 35 mm. The structure was covered with a 0.1 mm thick polyethylene film.