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  • 1
    Language: English
    In: Energy Conversion and Management, 01 November 2016, Vol.127, pp.80-89
    Description: In previous studies, the amount of exploitable shallow geothermal energy was estimated by assuming a uniform temperature drop of 2–6 °C in the aquifer. In this work, a more comprehensive numerical model has been employed to evaluate the available amount of shallow geothermal energy by using Borehole Heat Exchanger coupled Ground Source Heat Pump systems. Numerical experiments have been performed by simulating the long-term evolution of the subsurface temperature field, which is subject to the operation of borehole heat exchangers and varying parameters like subsurface thermal conductivity and groundwater flow velocity. The concept of equivalent temperature drop is proposed as an auxiliary quantity for the subsurface. With the help of this parameter, a procedure has been established to quantify the amount of shallow geothermal potential. Following this approach, a realistic equivalent temperature reduction is found to be from −1.8 to −4.4 °C in the subsurface over a period of 30 years. This can be translated to an annual extractable geothermal energy value in a unit surface area, and it ranges from 3.5 to 8.6 kW h m a . The exact value is site specific and heavily depends on the soil thermal conductivity, groundwater velocity, and borehole arrangement.
    Keywords: Shallow Geothermal Potential ; Borehole Heat Exchanger (Bhe) ; Ground Source Heat Pump (Gshp) ; Opengeosys (Ogs) ; Engineering
    ISSN: 0196-8904
    E-ISSN: 1879-2227
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  • 2
    Language: English
    In: Energy Conversion and Management, 15 October 2017, Vol.150, pp.392-402
    Description: This study analyzes the possibility of determining the parameters of an adsorption equilibrium model based on a reduced number of isotherms for the working pair water/zeolite 13X. The employed models rely on the Dubinin-Polanyi theory of micropore adsorption. The reliability of the adsorption equilibrium model based on sparse data is evaluated in terms of the error in the adsorption equilibrium and in terms of the error in loading lift and heat storage density for an adsorption cycle typical for heat storage applications. It is found that as little as three measured adsorption isotherms are sufficient to yield a description of the adsorption equilibrium of zeolite 13X in a wide pressure and temperature range, if the following criteria are obeyed: (i) the measured isotherms should cover the entire range of the characteristic curve and (ii) it is recommended to include isotherms at temperatures close to the working cycle limits. Based on these considerations, temperature ranges for the experimental determination of a reduced set of adsorption isotherms are recommended that yield a reliable description of the adsorption equilibrium in a wide pressure and temperature range. Thereby it is demonstrated that the experimental effort can be reduced significantly while maintaining the predictive capability of the theoretical model.
    Keywords: Water/Zeolite Adsorption ; Equilibrium Characterization ; Thermal Energy Storage ; Sparse Experimental Data ; Engineering
    ISSN: 0196-8904
    E-ISSN: 1879-2227
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  • 3
    Language: English
    In: Energy Conversion and Management, 15 January 2019, Vol.180, pp.977-989
    Description: Solid heat storage is an attractive solution for a wider utilisation of solar power for domestic and commercial applications alike. Thermal efficiency and long-term stability are of major concern both scientifically and industrially. There are many aspects that influence the thermal performance of a specific solid thermal energy storage (TES), such as the energy storage capacity of its storage material, the characteristics of its heat transfer fluid, and the mechanical integrity of the solid storage medium. In the present study, we develop a thermo-mechanical phase-field approach to fracture in order to examine the thermal performance degradation of the solid sensible heat storage (SHS) caused by potential thermally induced cracking of the heat storage medium around the embedded heat exchangers. Two representative solid SHS structures are examined: one applied in a low-temperature setting, the other used at high temperature levels. In both cases, fracturing caused by the mismatch of thermal expansion coefficients of the storage medium and the heat exchanger occurs under certain conditions. The open fractures form a space that will be filled by a fluid whose nature is determined by the system concept and which may well have a low thermal conductivity. Hence, heat transfer can be disturbed in the damaged regions, causing high temperature anomalies, which further may lead to significant fluctuations/loss of heating power during the heating phase. In the two specific scenarios investigated here, the highest loss of heat flow was estimated to be 7.7% in the water-saturated low-temperature SHS and 20.5% in the high-temperature SHS.
    Keywords: Phase-Field ; Brittle Fracture ; Thermal Cracking ; Thermal Energy Storage ; Opengeosys ; Engineering
    ISSN: 0196-8904
    E-ISSN: 1879-2227
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  • 4
    Language: English
    In: Energy Conversion and Management
    Description: This paper presents a case study using calibrated numerical models to evaluate the thermal impacts and long-term sustainability of intensive geothermal use on a neighborhood scale. The subsurface heat transport model is configured with site-specific parameters and validated against monitoring data from a typical urban living quarter in Germany. Based on the simulated ground temperature profile, the heat pump performance is approximated. In addition, the effects of groundwater flow on the thermal interaction and economical operation of the shallow geothermal systems are examined. The results indicate limited thermal impacts as the groundwater temperature will maintain above 3.2 °C and that the area undergone severe temperature drop is less than 1% size of the neighborhood. Since the estimated seasonal coefficients of performance (SCOPs) are at least 3.8, the sustainability of the shallow geothermal applications is confirmed economically. Nevertheless, financial disadvantages up to 92 € are anticipated due to gradual efficiency losses of the heat pump, which are meant for the owners of downstream installations. In addition, uncertainties in groundwater flow rate are also analyzed. For the negligible advection case, simulation results suggest that some systems can only operate sustainably for eight years. Conclusions are drawn regarding the general feasibility of neighborhood-scale shallow geothermal utilization and the importance of hydrogeological site investigations during the planning phase of such projects.
    Keywords: Ground Source Heat Pump (Gshp) ; Shallow Geothermal Energy ; Thermal Impact ; Long-Term Sustainability ; Opengeosys (Ogs) ; Engineering
    ISSN: 0196-8904
    E-ISSN: 1879-2227
    Source: ScienceDirect Journals (Elsevier)
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