## 1 Introduction
However, there is still a lack in literature of research that integrate PCM with metal foams in order to include hybrid water-PCM TES in dynamic thermo-energetic building simulations. Recently, the authors simulated the use of Al foams to improve the heat transfer capabilities of paraffin waxes in hybrid water TESs based on Trnsys type 860 [20; 21]. They demonstrated the suitability of these enhanced surfaces for speeding up both the loading and unloading processes. The results were based on simulations with experimental validation. Nevertheless, no study concerning energy and economic evaluation of a complex HVAC plant (i.e., using solar heating/cooling and a multi-source heat pump) comparing PCM vs. enhanced PCM vs. sensible heat storage is available, to the best of the authors' knowledge. [PERSON] et al. [22] conducted a similar study, but it is limited to the comparison between sensible vs. PCM TES. This paper reports a study of a dual source (solar thermal and ground) absorption heat pump system. The solar thermal energy is used directly for domestic hot water (DHW) and for heating, and it is also used as heat source for the heat pump, or to regenerate the ground when the system operates for summer air conditioning. In this case, condensation heat from the absorption chiller can be usefully directed to the post-heating coils of the air handling units, or to regenerate the ground as well. Annual simulations by the dynamic simulation software Trnsys are carried out based on an existing building that will be retrofitted during 2019 to become a nearly zero energy building (nZEB). The building is an old school sited in Feltre, northern Italy. The dual source and solar cooling/heating plant presents three thermal storage tanks: the first producing hot water for heating, the second for DHW, and the third producing cold water for cooling. An optimization of the solar and ground field designs from both energy and economic point of view is performed by dynamic simulations. Subsequently, the energy performance of the plant is evaluated by considering different cases: all the three tanks modeled as sensible (water) storage, or one of the tanks modeled as PCM storage, or as enhanced PCM with metal foam. In the first part of the paper, the Trnsys type 860 is validated against some data collected during experimental tests. Then, two issues are examined with the aim of minimizing the primary energy consumption and maximizing the economic advantage with respect to a traditional heating/cooling plant: the evaluation of the best size of the solar field/ground probes/thermal storage tanks, and the best choice for designing one of the three storages as a \"PCM storage\" or \"enhanced PCM storage\". It is suitable to introduce the analysis by a brief description of PCM-water hybrid thermal storage from the modeling point of view, and its validation with experimental data (Section 2.1). Subsequently, the retrofitting intervention on the building is described with the aim to calculate the heating, cooling, and DHW thermal loads (Section 2.2). Then, the HVAC plant and its control logic modeled in Trnsys are described (Sections 2.3 and 2.4). Section 2.5 reports on the correct setting of the parameters of type 860 to simulate PCM and enhanced PCM with Al foam. In Section 3, the main energy and economic results of the optimization analysis are reported and discussed.