Hydrogeological and environmental impact assessment of a geothermal closed loop ground source energy installation


Courtesy of ESI Ltd - Environment Specialists

ESI in Milan was approached to give support for designing one of the largest closed loop systems for heating and cooling in Europe.  The system was installed at the IKEA store in Corsico, Milan, with the need to provide some 1.4 MWth and covering thermal energy demand in the range from 40 to 100%.  The closed loop system is expected to save the Client over 500 t/y in CO2 emissions.  Pay back time was calculated in 7 to 10 years depending on climate conditions. After this period, capital costs will be recovered and only running costs (maintenance and management) will be required.

The regulatory authority of the Milan Province requested a detailed hydrogeological and environmental impact assessment in support of the licence application.  This was required due to the presence of a public water supply borehole 300 m downgradient from the proposed ground source energy scheme, constituted by more than 300 boreholes at 90 and 125 m deep.

The assessments were undertaken by developing groundwater and heat transport models and through desk studies.  The scope of work was to:

Define the influence of the heat exchange system on the local groundwater flow regime

Quantify the induced temperature changes within the aquifer

Predict changes in the physical-chemical and biological properties of the groundwater used for the public water supply system

Both the main shallow aquifer and the deeper aquifer were analysed within the model.  The 3D numerical model simulated the operation of the ground source energy system for a period of 5 years. Results from the model predicted that:

Abstraction from the public water supply will not significantly affect the flow lines in the area of the ground source energy exchanger

The installation of the heat exchange system will not induce any local changes to the water table or alter the abstraction rate at the public water supply borehole

Changes of seasonal temperatures at the abstraction well were not expected to exceed 0.03 ۫C

Bacterial proliferation due to heating of the aquifer is considered a potential threat only in the occurrence of nutrients within the groundwater. However, in this situation it is not considered an issue due to their likely absence and, in the worst case, due to soil cooling during winter.

A monitoring programme was established to assess the possible changes in groundwater chemistry occurring from induced changes of the groundwater temperature.  The monitoring programme included the definition of the baseline conditions prior to the installation and operation of the ground source energy system.

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