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Session: New applications: Process Integration
Session starts: Tuesday 08 October, 10:00
Presentation starts: 10:20
Room: Willem Burger Zaal

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Stefan Schimpf (Ruhr-Universität Bochum)
Roland Span (Ruhr-Universität Bochum)

Abstract:
Stefan Schimpf*, Roland Span *Ruhr-Universität Bochum, Faculty of Mechanical Engineering Chair of Thermodynamics Universitätsstraße 150, 44801 Bochum, Germany e-mail: s.schimpf@thermo.rub.de, http://www.thermo.rub.de/ ABSTRACT The coupling of a ground source heat pump and solar thermal collectors in a domestic solar combisystem providing both space heating and domestic hot water is an established technology. As there is no demand for space heating during summer the area of the collector array is overdimensioned and the collectors come to a standstill whenever the maximum temperature of the storage is reached. This stagnation can be circumvented by the application of an ORC harnessing the excess heat. In the ORC the working fluid of the heat pump is evaporated by solar heat and is expanded through a scroll expander before it is condensed in the ground heat exchanger recharging the ground. The additional investments of the combined system only comprise a scroll expander, a pump and advanced controls and could therefore be greatly reduced if the already existing scroll compressor of the heat pump could also be used as expander. Simulation runs are performed to discuss the energetic benefit of the ORC. The feasibility of two analytical solutions for the ground heat exchanger is evaluated. The first one is Eskilson’s g-function approach [1], which can be regarded as state-of-the-art for the calculation of the long term response of a ground heat exchanger. This approach is compared to a short-term solution developed by Javed et al. [2]. Aspects of the regeneration of the soil are discussed. The simulation results for the location Denver show that the reduction of the total electricity demand of the combined system resulting from the addition of an ORC is 51 kWh using the Javed-model, respectively 66 kWh using the Eskilson-model. Thus the overall savings are rather low, which indicates that the addition of the ORC can only become competitive in the future assuming rising energy costs. The results however indicate that for the simulation of systems in which the condensation of an ORC is carried out in a ground heat exchanger the application of long term response models is not sufficient. More advanced short-term solutions are required for this purpose. REFERENCES [1] P. Eskilson, “Thermal analysis of heat extraction boreholes”, Dissertation, 1987. University of Lund, Department of Mathematical Physics, Lund, Sweden. [2] S. Javed and J. Claesson, “New analytical and numerical solutions for the short-term analysis of vertical ground heat excahngers”, ASHRAE Transactions, Vol. 117, pp. 3-12, (2011).