Abstract
Aquifer thermal energy storage systems in the sediments of the Upper Jurassic in the north-eastern part of the Bavarian Molasse Basin seem to be feasible in terms of the hydrogeological and hydrochemical setting. This study presents unique results from the first large-scale high-temperature heat storage test in these sediments and a hydrogeochemical model based and validated with the field data. The test was run in a single well setting with five injection and production cycles and temperatures from 65 to 110 °C. The flow rates were 15 L/s. Due to the very high transmissivity, mixing and density–driven flow have been observed and confirmed by hydrochemical analyses. Mixing was quantified using the natural contrast of the sodium ion concentrations as a natural tracer. Using the mixing ratios, a deduction of the effects of mixing on the temperature of the produced water was possible and a correction was applied to the recovered energy. The temperatures of the produced water show that 48% of the injected energy was recovered during the field test and the remaining energy is “charging” the aquifer. A kinetic hydrogeochemical model including 1D-transport was developed with PhreeqC to quantify the reactions in the reservoir and calibrated with the hydrochemical data of the first and second phase of the field test. The other three phases of the field test were used for validation. Model and measurement data were in excellent agreement and show significant dissolution of carbonates which can be attributed to an undersaturation of the water as it equilibrates with the matrix at lower temperatures. Based on field data from the single well test and the calibrated model, the operation of an ATES system was designed and simulated. Model results indicate that a doublet setting for ATES cannot be operated for more than a few cycles, regardless of the conditioning methods. In a triplet system, however, the time frame for successful operation can be extended to decades.
Highlights
Combined heat and power plants (CHP) are efficient and environmentally friendly because excess heat produced during power generation is used for heating purposes
A kinetic hydrogeochemical model including 1D-transport was developed with PhreeqC to quantify the reactions in the reservoir and calibrated with the hydrochemical data of the first and second phase of the field test
The carbonates of the Upper Jurassic start with 20 m (264 m true vertical depth (TVD)) limestones followed by dolomites and dolomitic breccia
Summary
Combined heat and power plants (CHP) are efficient and environmentally friendly because excess heat produced during power generation is used for heating purposes. While the power demand remains rather constant throughout the year, the heat demand shows seasonal variations. In a worst-case scenario, the heat production in winter is not sufficient, and the power production in summer has to be ramped down because the excess heat cannot be released to the environment. Storage of excess heat from a CHP is highly beneficial from an economic and an ecological point of view. Water from an aquifer is produced, heated up by excess heat from the CHP and injected through a second borehole back into the aquifer (Fig. 1)
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