MUSE pilot area activities – RESULTS – #3 Zaragoza

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Defining the exploitation patterns of groundwater heat exchanger systems in Zaragoza city pilot area (Spain)

Alejandro García Gil (IGME-CSIC), 31 May 2021

Within the MUSE project activities in the pilot area of Zaragoza city, we have collected information from 27 groundwater heat exchanger systems operating in the alluvial aquifer of the city of Zaragoza (Figure 1). Firstly, the obtained datasets allowed us to examine the exploitation regimes using descriptive statistics and identifying linear relationships between captation and injection temperatures. Data obtained provided us with the first evidences of thermal interferences between geothermal wells of the systems.

Figure 1: Field works conducted in the MUSE pilot area of Zaragoza city.

Afterwards, time series of flow rate, operation temperature and energy transfer were modelled by means of spectral analysis and sinusoidal regression methods, followed by the definition of the relative exploitation patterns. The exploitation regimes examined presented a clear cooling bias and a similar cyclicality. The amplitudes correlated to the different end-user’s activities when high frequency cycles were observed, while climatization strategies did so when low frequency cycles were detected. The time series models allowed defining the relative operational pattern of a system and the groups of systems following such patterns. The biases in exploitation regimes of groundwater heat pump systems existing in Mediterranean areas require correction measures to ensure a more balanced exploitation of the shallow geothermal energy resources. The definition of the characteristic exploitation pattern (Figure 2) proposed could be applied to guide resource managers by identifying unbalanced systems, understanding existent exploitation strategies and proposing corrective alternative plans.

Figure 2: Relative exploitation patterns identified for the investigated groundwater heat exchanger systems in pilot area of Zaragoza. A-C plots represent the normalized flow rates of the 27 systems, reported as minimum (MIN), quantile 25% (Q1), quantile 50% (Q2), quantile 75% (Q3) and maximum (MAX) values. Same nomenclature is shown for normalized injection temperatures in D-F plots. G-I plots present median values (Q2) of the 27 systems for pattern identification. Time 0 represents 1st of April as the starting point of the cooling season.

Shallow geothermal energy systems are the most efficient and clean technology for the air-conditioning of buildings and constitutes an emergent renewable energy resource in the worldwide market. Undisturbed systems are capable of efficiently exchanging heat with the subsurface and transferring it to human infrastructures, providing the basis for the successful decarbonisation of heating and cooling demands of cities. Unmanaged intensive use of shallow geothermal energy resources in urban environments with a high concentration of systems threatens the resources´ renewability and the systems´ performance due to the thermal interferences created by a biased energy demand throughout the year. Therefore, results obtained from MUSE project activities have provided some clues about how to overcome thermal interferences.

Further information in: García-Gil, A. et al., 2020. Defining the exploitation patterns of groundwater heat pump systems. Science of the Total Environment, 710: 136425. DOI: https://doi.org/10.1016/j.scitotenv.2019.136425

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