Bulgarian Chemical Communications, Volume 48, Special Issue E (pp. 64 - 70) 2016 Energy performance of a dual air and ground source heat pump coupled with a Flat-Panel ground heat exchanger Michele Bottarelli 1, * , Li Zhang 2 , Marco Bortoloni 1 , Yuehong Su 2 1 Department of Architecture, University of Ferrara, Via Quartieri 8, Ferrara 44121, Italy 2 Department of Architecture and Built Environment, University of Nottingham, University Park, NG7 2RD, UK Integrating air and ground source can be an effective solution to improve the performance of a heat pump by reducing the drawbacks of each individual technology. In fact, a dual source system not only greatly reduce the size of the ground heat exchanger, but also can achieve a higher efficiency by selecting the more thermally favourable source. Hence, the frosting/defrosting process, which regularly occurs in a common air source heat pump (ASHP) could be avoided. In the present contribution, the performance of a dual source heat pump (DSHP) has been numerically analysed. The energy demand for both heating and cooling of building has been estimated by means of the software EnergyPlus. Then, the resulting time series values are used as the boundary heat fluxes to model a ground heat exchanger. The commercial Finite Element Method (FEM) simulation package COMSOL Multiphysics is implemented to simulate the heat transfer in the ground which is produced by a horizontal Flat-Panel ground heat exchanger. A function has been properly implemented in COMSOL to control the switching between air and ground sources, according to their temperatures. Compared with an ASHP, the DSHP shows much higher efficiency because of the more favourable working conditions and the protection against frosting. Consequently, a DSHP should be a viable solution to combine the respective advantages of air source and ground source heat pumps. Keywords: dual-source heat pumps, horizontal ground heat exchanger, finite element model (FEM) INTRODUCTION Nowadays, the reduction of greenhouse gas emissions and the rational use of energy have become a major issue. In view of this, the recent environmental policies have been promoting the renewable technologies expanded worldwide. Among them, air-source heat pumps (ASHPs) and ground-coupled heat pumps (GCHPs) are regarded as viable and efficient technologies for applications of heating and cooling in residential and commercial buildings [1]. These systems are gradually being applied with significant savings in terms of primary energy, due to their universal applicability and versatility. Due to their universal applicability and versatility, these systems have been gradually applied with significant saving of primary energy use in the recent years. The performance of a heat pump is significantly affected by the operating conditions, which depend on the heating/cooling demand and the heat source feature. In order to achieve higher efficiency than the widespread ASHPs, the GCHP systems use the ground as a heat source/sink, which often provides more favourable and stable temperature than outdoor air temperature. As the depth increases, the ground temperature fluctuations are reduced. The annual average temperature of the shallow ground * To whom all correspondence should be sent: michele.bottarelli@unife.it depends on the location and it is approximately equal to the annual average air temperature [2]. In addition, the ground temperature can be significantly different between rural and urban areas, where the soil is usually warmer due to the urban heat island effect [3]. Besides the high efficiency, GCHPs have also higher purchase and installation cost than air-source systems due to the initial cost of the ground heat exchanger [4], which is recognized as the least efficient component of these systems. In addition, the performance of a GCHP is strongly affected by the ground heat exchanger, which can be installed in vertical boreholes or in shallow horizontal trenches (also referred as VGHE and HGHE, respectively). The HGHEs hold some advantages in terms of costs and installation but as well have drawbacks in terms of land area requirements and efficiency of soil heat transfer. In order to overcome the drawbacks of current available technology, recent studies have attempted to develop more efficient arrangements for the widespread HGHE configurations [5] or novel shapes such as the Flat-Panel, which has been developed at the University of Ferrara in 2012 [6]. In contrast, ASHPs have a low initial installation cost and are almost easily applied. However, during winter operations as well as under cold and humid weather condition, these systems are subjected to frosting on the evaporator. This phenomenon © 2016 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria 64