Variants Comparison: Probing for Performance
Summary: In the planning stages of projects, GreenCity enables an early and transparent comparison of different system variants. In this example, a small residential neighborhood is evaluated. The simulation shows how probe field size affects backup heat demand, heat pump operation, cold network performance and electricity use.
In this first example, a small neighborhood with several residential buildings is evaluated for the implementation of a cold district heating network. A key planning question is the dimensioning of the geothermal probe field: should the system be designed with 40, 55 or 70 double-U probes? The answer depends not only on the installed capacity, but also on how the geothermal source, PVT collectors, heat pumps, network effects, backup boiler and building heat demand interact over several years of operation.
The simulation model makes it possible to compare these variants under realistic operating conditions. It answers questions such as: How strongly does the number of geothermal probes influence the long-term source temperature? How much backup heat is required in each case? What contribution can the PVT collectors and network gains provide? And how robust are the variants if heat demand increases or source availability is reduced?
The model is based on the GreenCity simulation library and includes building load profiles, borehole probe FMUs, network loss and network gain modelling, PVT integration, brine/water and air/water heat pumps, a peak-load or backup gas boiler and the corresponding system control strategy. The investigated variants compare probe field sizes of 40, 55 and 70 double-U probes, each in combination with approximately 300 PVT modules, a 300-kW brine/water heat pump, a 100-kW air/water heat pump and limited backup boiler operation.
The simulation results show that all three probe-field variants 40, 55 and 70 double-U probes are technically feasible. The 40-probe variant requires slightly more backup heat and relies more strongly on the air/water heat pump, while the 55-probe variant provides a balanced solution with very low boiler use after several years of operation. The 70-probe variant offers the highest robustness, with almost no boiler heat and the lowest electricity demand, although the reduction in electricity use is only around 2%.
The cold network balance confirms that geothermal heat extraction is comparable across the variants, while peak capacity increases with the number of probes. Since the PVT area remains unchanged, both heat and electricity contributions from the PVT system are nearly identical in all cases. Overall, the comparison highlights the trade-off between a compact, cost-efficient probe field and a more robust but larger geothermal design.
The simulation therefore provides more than a simple feasibility check. It supports concrete decisions on the appropriate size of the geothermal probe field, the role of the PVT area, the heat pump capacity and the required level of backup heat. At the same time, it helps identify overdimensioning, unfavorable operating states and low self-consumption shares at an early stage. Planners, operators and investors receive a transparent basis for comparing the 40, 55, and 70 probe variants and selecting a robust system configuration.
In conclusion, the 55 double-U probe variant was selected as the recommended design because it offers the best balance between technical performance, operational reliability and efficient dimensioning. Compared with the 40-probe variant, it reduces backup boiler use and dependence on the air/water heat pump, resulting in more stable long-term operation. Although the 70-probe variant provides slightly higher robustness, its additional benefit is limited, with only a small reduction in electricity demand, while requiring a larger probe field.