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Available online Keywords: Borehole heat exchanger Model validation Model optimization TRNSYS Transient behavior a b s t r a c tThe short-term behavior of borehole heat exchangers (BHE) is investigated using three 2 h test sequences each with constant inlet temperature and constant mass flow rate. Three models of the transient systems simulation program (TRNSYS) have been compared to these measurements. In order to assess the deviations a 3D-FEM model, built in COMSOL Multiphysics ® , is compared to the measurement as well.The "Erdw€ armesonden" model (EWS) shows a very good accuracy with a deviation less than 10%. The superposition borehole model (SBM) and duct ground heat storage model (DST) neglect the internal borehole heat capacity of fluid and grout. In order to improve their short-term behavior, the models are modified with an adiabatic pipe model with wall capacity in front of the BHE model. Different parameter sets for the pre-pipe are investigated. The optimum parameter set reduces the error between simulation and measurement of the injected heat from 50% to about 5%. With this model modification, all investigated models SBM, DST, and EWS perform as accurate as the COMSOL model.

There are two major approaches to mitigate energy-related emissions of dwellings: Insulating the building's envelope on the one hand and modernizing its heat supply system on the other hand. Often the insulation of dwellings leads to lower energy savings than expected (Greller et al., 2010), which is assumed to be related to the heat losses of the heat distribution system itself. So attention needs to be drawn to the modernization of the heat supply system, which can lead to a significant reduction of the building's final energy demand (Jahnke et al., 2015). This paper is following this approach and focuses on the heat supply and distribution system of multi-family houses (MFH), representing a significant share of Germany's residential market. The present work compares different systems for multi-family houses by means of its primary energy demand, energy efficiency and economic aspects. Heat distribution losses are analyzed and their relevance for efficient systems is discussed. Based on this analysis, the work highlights effective integration routes of solar thermal supply systems which may lead to increased efficiency of the overall heat supply system.

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