## 1 Introduction  
One of the main aspects of solar systems is storage. Storage density, in terms of the amount of energy per unit of volume or mass, is an important issue for applications in order to optimize the solar ratio, that is, how much of the solar radiation is useful for the heating/cooling purposes. Efficiency of appliances such as solar thermal collectors and absorption heat pumps/chillers and room consumption are important issues as well. For these reasons, it is important to investigate the possibility of using phase change materials (PCMs) in solar system applications. PCMs can potentially increase the energy density of small sized hybrid water thermal energy storage (TES) tanks, thus reducing solar storage volume for a given solar fraction, or increasing the solar fraction for a given available volume [1]. For example, for a 10 \({}^{\circ}\)C temperature drop that is common in many applications such as ambient heating or single stage absorption cooling, the specific volume required can drastically decrease from values in the order of 0.1 m\({}^{3}\) kWh\({}^{-1}\) (36 MJ m\({}^{-3}\)) to values as low as 0.02 m\({}^{3}\) kWh\({}^{-1}\) (180 MJ m\({}^{-3}\)). As a further advantage, heat storage and delivery normally occur over a fairly narrow temperature range (the transition zone) [2]. [PERSON] et al. [3] report a review of several studies developed during recent years concerning PCM heat storage. They are all related to the performance of the storage itself rather than of the system. [PERSON] [4] reports a study on a solar cooling and refrigeration plant in which different PCM have been tested under different conditions. Recently, many studies have been devoted to identifying possible solutions to increase the thermal conductivity of PCMs as this is usually very low (in the order of 0.2 W m\({}^{-1}\) K\({}^{-1}\)), causing slow melting and solidifying processes [5; 6]. In this regard, open-cell metal foams seem to be a promising way to enhance the PCM heat transfer performance. They consist of a stochastic distribution of interconnected pores almost homogenous in size and shape, allowing a high heat transfer area per unit of volume and high thermal conductivity [7]. [PERSON] et al. [8] carried out some experiments measuring the improvement of the heat transfer by a metal foam during the solid-liquid phase change process of different paraffin waxes. More recently, [PERSON] et al. have conducted an extensive campaign measuring the temperature distribution and loading and unloading times of hybrid water PCM TES with and without aluminum (Al) foams [9; 10; 11]. They demonstrated significantly higher conduction heat transfer rates with the use of metal foams, as a consequence of their higher thermal conductivities. Some numerical models for integrating PCM systems into commercial programs have been proposed in literature [12]. Among the different available simulation tools, Trnsys [13] is a widespread transient simulation tool for dynamic simulation of heating, ventilation, and air conditioning (HVAC) plants using renewables and heat pumps (HP). For this reason, it has been selected and implemented in the present work. During the last decades some attempts to incorporate PCM systems in Trnsys have been made, above all into building components [14; 15]. Active systems with PCMs as part of the thermal storage system have been investigated as well [16; 17]. [PERSON] and [PERSON] [18] developed a Trnsys type called \"type 860\" to simulate water storage tanks including PCM modules (vertical cylinders, plates, or spherical beds). The model takes conduction and convection into PCM into the account as well as at the interface between PCM and water, and it is based on an enthalpy approach. Hysteresis and subcooling of PCM are also considered. To evaluate the accuracy of the model, numerical results were validated against some experimental data, showing a good agreement [19].