## 1 Introduction  
Currently, the main environmental challenges worldwide are to increase the share of renewable energy sources in final energy use and achieve an increase of energy efficiency in order to contribute to reducing greenhouse gas emissions. One of the most promising measures is to increase the utilization of solar energy. However, due to its intermittent nature, there is a need for thermal energy storage technologies in most of areas where solar energy is utilized [1; 2]. This is especially relevant to domestic hot water (DHW) systems. Water heating is still a large share of the total energy consumption of households worldwide [3]. For example, it represents 19% of the total energy consumption of households in the United States [4] and 14.8% of the total energy consumption of households in the European Union [5]. Therefore, if this energy demand is fulfilled with the help of solar water heater systems, it will help to increase energy efficiency, decrease fossil fuels dependency and reduce greenhouse gas emissions. Actually, the energy efficiency in the water heating sector has increased by 25% worldwide since 2000, and it is estimated to increase by 43% until 2040 [6], for which developingmore efficient solar thermal energy storage solutions that also increase the share of renewable heating systems is a necessity. In general, thermal energy storage can be accomplished through sensible, latent, or thermochemical (e.g., sorption) processes. Compared with conventional sensible and latent thermal energy storage processes, adsorption can deal with the temporary thermal energy storage in an easier, more compact and efficient way, even for long periods, with high energy densities and low or even null heat losses. In addition, adsorption thermal energy storage systems are able to use a wide range of heat sources, e.g., solar thermal, biomass, geothermal, thermal surplus, or available waste heat. It is thus a promising alternative to conventional heat storage systems and also to integrating or partially or fully replacing heating from fossil fuels or electric systems [2; 7; 8; 9; 10]. This technology is still under development, since different issues need to be solved at both material and system levels, to become a feasible technology for commercial applications. However, adsorption heat storage is gaining more and more attention in the scientific community as an emerging technology; its viability has already been demonstrated, and different adsorption cycles have already been applied for thermal energy storage in several research works [1; 8; 11; 12; 13; 14; 15]. In this context, a new adsorption thermal energy storage system was presented and optimized in previous works [16; 17; 18; 19], trying to reduce some limitations of conventional sensible DHW heat storage systems. It is especially appropriate for solar energy systems, where energy supply does not typically coincide with hot water demands and is not possible to control. The system combines the main features of the adsorption thermal energy storage with a conventional hot water storage tank. It is composed of a hot water tank, an adsorber (filled with silica gel) located inside the hot water tank, a condenser, and an evaporator. Water is the working fluid in the adsorption system. The adsorber stores the thermal energy received from the hot water in the storage tank (desorption heat) and releases it later to heat up the water in the tank (adsorption heat). The condensation of the operating fluid in the condenser releases heat into the storage tank, and the evaporation of the operating fluid in the evaporator extracts heat from the ambient. This work aimed to evaluate the performance of the DHW storage system with an adsorption module, previously optimized for the Portuguese climate conditions, when operating in some representative locations around the world. It was thus an assessment on how a given product behaves in distinct world locations without further modifications of the system itself.