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Content archived on 2024-06-18

Multi-criteria assessment and optimization of geothermal heat pump systems

Final Report Summary - ECO-GHP (Multi-criteria assessment and optimization of geothermal heat pump systems)

The use of shallow geothermal energy is continuously rising and, in particular, the application of vertical ground source heat pump (GHP) systems is expanding. Such applications are attractive for their potential to reduce greenhouse gas emissions while offering economic advantages in comparison to fossil-fuel based space-heating. Single GHP systems are well established for small residencies. Growing interest is directed towards bigger scale applications with galleries of multiple GHPs, for which much less experience exists and little research on appropriate planning and design is available. During this project an innovative computer-based assessment and optimization methodology is developed. Further, it contrasts a Life Cycle Assessment (LCA) based evaluation of GHP systems with respect to different impact categories with technico-economic criteria. The different criteria are quantified in a flexible manner for a broad range of multiple GHP configurations.
By implementation of technico-economic criteria into a mathematical optimization problem, optimal GHP gallery design and operation is computed. The methodology allows categorising sensitive status, design and control parameters with respect to various, potentially contradicting criteria. We employ mathematical optimization techniques to strategically operate and arrange BHEs in such fields. Linear programming and evolutionary algorithms are applied in combination with numerical and analytical equations to solve hypothetical and realistic problems. The studied scenarios represent variable environmental conditions, such as with and without groundwater flow, as well as homogeneous and heterogeneous hydrogeological conditions. We inspect GHP fields of different size, with seasonally variable heating and cooling energy demands, and for a range of total system lifetimes. It is demonstrated, by taking the maximum temperature decline in the ground as objective, that the GHP field performance can be improved both by case-specific ideal arrangement and time-dependently regulated individual GHP operation. It is found that instead of standard lattice arrangements, optimized geometries are favourable, with GHPs concentrated along the fringe of a field. Apparently, this enhances lateral conductive heat provision into the field. Groundwater flow means additional energy provision by advection from a given direction towards the field. Optimized operation under such conditions adopts a seasonally variable loading of individual BHEs that is oriented at the groundwater flow direction. The results of our analysis show that several degrees of freedom in the ideal geometric arrangement and long-term operation of multiple GHPs exist. Computer-based combined optimization-simulation reveals the improvement potentials in comparison to standard practice. The presented methodology is flexible, robust and it can be applied to various conditions. Uncertainties that exist in the description of hydrogeological and geothermal characteristics at the field site will be limitations for the approach in practice. However, if the case-specific characterization is reliable, improvement of GHP performance, more sustainable energy exchange with the ground, and economic advantages by decrease of required GHP total number can be achieved