Simulation of a solar assisted combined heat pump- Organic Rankine Cycle-system Stefan Schimpf 1,* , Karsten Uitz 2 , Roland Span 1 1 Ruhr-Universität Bochum, Thermodynamics, Bochum, Germany 2 SIMAKA Energie- und Umwelttechnik GmbH, Argenbühl, Germany * Corresponding author. Tel: +49 2343226390, Fax: +49 2343214163, E-mail: s.schimpf@thermo.rub.de Abstract: In conventional collector systems for the supply of domestic hot water and space heating the collectors come to a standstill during summer whenever the maximum temperature in the storage tank is reached. The resulting excess heat can be harnessed by a combined heat pump-Organic Rankine Cycle-system. The aim of this work is to simulate such a system in order to determine the optimum operating conditions and impacts on power requirement and cost. For this purpose models for collector, storage tank, heat pump and geothermal heat exchanger are implemented. First results indicate that the isentropic efficiency of the scroll expander has the largest influence on the ORC-revenue. For a system consisting of 12 m² flat-plate collector area with an expansion efficiency of ,exp 0.7 s η = the power requirement for space heating and domestic hot water is reduced by 3.6%, whereas the costs decrease by 42 € or 12.3% respectively compared to a conventional system. The results suggest that an installation is more reasonable in larger dwelling units like hotels, senior citizens’ homes and multiple family dwellings. Keywords: Solar Heating, Organic Rankine Cycle, Heat pump. Nomenclature A c collector aperture area................... m² c coefficients of the characteristic line of the collector c p,CF isobaric heat capacity of collector fluid ......................................... kJ/kgK COP coefficient of performance of the heat pump C tot total electricity costs......................... € E HP,DHW electricity consumed by the heat pump for domestic hot water ....... kWh E HP,SH electricity consumed by the heat pump for space heating ............... kWh E ORC electricity generated in the ORC . kWh E tot total consumed electricity............ kWh F’ collector efficiency factor G b beam radiation ........................... W/m² G d diffuse radiation ......................... W/m² h enthalpy ...................................... kJ/kg K θ incidence angle modifier .................... ex m  external mass flow......................... kg/s ex m  mass flow between nodes .............. kg/s n ser number of collectors in series q  specific heat flow......................... W/m² q cond specific heat of condensation ..... kJ/kg q in supplied specific heat ................. kJ/kg DHW Q  heat transferred to the generation of domestic hot water ........................... W Loss Q  heat loss of a node ........................... W Q λ  conductive heat flow across nodes... W t ORC operating time in ORC mode ............h T a ambient temperature ...................... °C T ground temperature of the ground.............. °C T DHW temperature of domestic hot water. °C T SH space heating flow temperature ..... °C T in collector inlet temperature............. °C T m mean collector temperature ........... °C T ORC scroll expander inlet temperature .. °C T out collector outlet temperature........... °C w t,comp specific work for compression.... kJ/kg w t,exp specific work of expansion ......... kJ/kg V  volume flow ..................................... l/h str V  volume flow per collector string ..... l/h β collector slope η s,c isentropic compression efficiency η s,p isentropic pump efficiency η s,exp isentropic expansion efficiency ρ CF density of the collector fluid .......kg/m³ (τα) n normal transmittance absorptance product 1. Introduction The application of ground coupled heat pumps and solar combisystems providing both space heating and domestic hot water is a proven technology. In these conventional systems the collectors come to a standstill whenever the maximum temperature in the storage tank is 3937