Open-loop system evaluation

In order to study the hydrodynamic properties of the shallow aquifer, the experimental data resulting from the pumping test were interpreted with the software Aqtesolv. The Theis solution was implemented via a type curve matching method, as shown in Fig. 3.1, which represents a log-log graph of time versus drawdown. As represented in the graph, the curve did not reach a well-defined stabilization. However, this interpretation method provided the required values for the transmissivity and the storage coefficient, or specific yield, equal to 0.017 m²/s and 0.156, respectively. Once the transmissivity is known, it was possible to calculate the aquifer hydraulic conductivity, expressed as follows:

𝐾 =𝑇

𝑏=0.017𝑚² 𝑠

3.5 𝑚 = 4.91 ∙ 10⁻³𝑚

𝑠 (3.5)

Using the results of the single-well pumping test, it was also possible to evaluate the specific capacity of the well and thus its productivity, which is quite similar to the transmissivity value:

𝑞_𝑠𝑝 =𝑄

𝑠 =4.8 ∙ 10⁻³𝑚³ 𝑠

0.22 𝑚 = 0.022𝑚²

𝑠 (3.6)

The aquifer transmissivity value is very high, thus, it could be confirmed that the well productivity and the aquifer performance are excellent; also considering that the water table is quite shallow and the unconfined aquifer has a reduced saturated thickness. In this case, most probably, the anthropic activity has significantly contributed to feed the groundwater flow: in fact, the irrigation network supplies large water discharges through its several channels across the neighboring fields.

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Fig. 3.1 Aquifer diagnostic curve interpretated with Aqtesolv.

As it is shown in the Fig. 2.5, the site is located between the equipotential line corresponding to hydraulic heads of 145 m and the line which connects points with hydraulic heads equal to 140 m. The distance between these two imaginary lines is approximately equal to 1800 meters.

Therefore, it was possible to determine the hydraulic gradient which is about 3‰. The properties of the shallow aquifer are summarized in the Tab. 8 below.

In accordance with the results of the single-well pumping test, the open-loop configuration was designed with a pumped rate equal to 5 l/s. The temperature difference between the extraction and the injection wells can be expressed as follows:

∆𝑇 = 𝑃𝑡ℎ

𝑄 ∙ 𝜌_𝑤𝑐_𝑤= 40 𝑘𝑊 5 ∙ 3600

1000 𝑚³

ℎ ∙ 1.167 𝑘𝑊ℎ

°𝐶 𝑚³

= 2 °𝐶 (3.7)

This thermal alteration, induced in the aquifer, complies with technical regulations, which require a maximum variation for the water temperature of ±7 °𝐶.

Moreover, in order to avoid the recycling of the cold plume, the minimum distance between the abstraction well and the reinjection one must be equal to the following length:

𝐿 = 2𝑄

𝜋𝑇𝑖= 2 ∙ 5 ∙ 10⁻³𝑚³ 𝑠 𝜋 ∙ 0.017𝑚²

𝑠 ∙ 2.8 ∙ 10⁻³

≅ 67 𝑚 (3.8)

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However, the supply well is already present near the pressure reduction station, and its position may not be changed. Concerning the recharge well, it will be drilled in the property of the Alba4 plant, thus, the designed distance between the two wells is fixed at approximately 90 meters. This length will ensure that a null share of the reinjected flow rate will be recycled.

Tab. 8 Hydrodynamic properties of the aquifer.

PROPERTIES VALUE U.M.

Transmissivity 1.72∙10^-2 m²/s

Storage capacity 1.56∙10^-1 -

Hydraulic conductivity 4.91∙10^-3 m/s Specific capacity 2.18∙10^-2 m²/s Hydraulic gradient 2.78∙10^-3 -

All the parameters presented above were then implemented in the FEFLOW model described in Chapter 2.3.2, in order to verify the correct operation of the designed open-loop system. The simulation time was set to 20 years and the thermal plant was assumed to work with a constant power of 40 kW and pumping flow rates equal to 5 l/s and -5 l/s for the extraction and the reinjection wells, respectively. The numerical simulation of groundwater flow and heat transport provided the temperatures over time at the two wells points, which are represented in the graph in Fig. 3.2, and the distribution of the thermal plume after 20 years of operation, as shown in the Fig. 3.3. The aquifer temperature was of 14 °C, whereas the reinjected water temperature was initially at 12 °C and, subsequently, gets colder and colder. In fact, during the system operation, the temperature of the abstracted flow rate decreases due to the cold plume propagation. In particular, the temperature of the supply water decreases of about 1 °C in the first three years of operation and, then, remains quite constant in the range of 13.5-12.5

°C in next years. Obviously the recharge water temperature undergoes to the same trend, but it is 2 °C lower, because of the constant temperature change imposed by the groundwater heat pump. The temperature variations, caused by the reinjected water flow rates, are not so high and the resulting cold plume extension is limited and towards the groundwater flow direction, thus, the extraction well is not affected by hydraulic and thermal recycling phenomena, as can be seen in the Fig. 3.4 below.

This shallow open-loop configuration seems to be a good and sustainable solution in order to replace the conventional preheating system of the Alba4 plant. However, the hydrological framework should be analysed more in detail: in particular, the piezometric levels should be monitored over time and also other wells near the site should be investigated, in order to properly evaluate the aquifer hydraulic heads and the groundwater flow direction.

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Fig. 3.2 Temporal distributions of the wells temperatures during 20 years of operation.

Fig. 3.3 The resulting extension of the thermal plume at the end of the simulation.

9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5

0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300

Temperature (°C)

Simulation time (days) Extraction well Injection well

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Fig. 3.4 Zoom on the temperature distribution at the two wells points after 20 years.