Hydrogen based polygeneration for energy communities.

Renewable energy application in the residential sector is one of the key solutions considered by the European Community to lead through the energy transition and the greenhouse gasses emissions reduction. In particular, the photovoltaic panels are considered as the most suitable technology in this contest. However, the energy storage system is the key issue linked to renewable energy, due to the weather-dependence and the mismatch between the production and consumption by the users. The battery is the most common storage device, characterized by a high round-trip electrical efficiency. In last few years also hydrogen storage systems have been evaluated as a competitive storage mean. These are composed by the electrolyzer, to produce hydrogen through the exceeding amount of electrical energy during the day, a tank, to store and release the hydrogen, and a fuel cell, to produce electrical energy through the hydrogen. These storage systems are characterized by a lower electrical efficiency than the battery. However, these show some advantages as the possibility of seasonal storage and the polygeneration. Indeed, the electrolyzer generates heat during the hydrogen production, while the fuel cell during the hydrogen consumption. Therefore, the thermal energy could be recovered to obtain an increasing of the overall efficiency of the hydrogen storage system. The aim of the thesis is to make a comparison of the two proposed storage technologies. Moreover, due to the ongoing evolution of hydrogen fueled internal combustion engine electro-generators, the possibility to substitute fuel cells with this machine is considered. For this purpose, an energy analysis of the photovoltaic plant with the different storage and generation systems has been carried out. These plants are further matched with the solar water heating system, to consider the possibility to recover the thermal energy. These are considered applied in a residential building, lived by ten families. The energy balance analyses are firstly carried out on annual base by considering the average annual energy production and the consumption. Further, a sensitivity analysis is carried out by increasing the PV power from 20 kWp to 40 kWp, and, consequently, the energy production. Thus, for each case, the size of the storage systems is designed with the aim to store all the average exceeding amount of electrical energy during the day. This design method is applied to maximize the energy supplied to the users, in order to make the plant as much as possible independent by the grid. Then, the functioning performances of the systems are obtained, to make a comparison of the different storage technologies. These are obtained by considering both the only electrical balance and the overall energy balance. Further, to understand the functioning performance of the system as a function of the different weather conditions, the analysis is applied considering three different Italian cities: Milan, Rome and Syracuse. On average the plants show better performances respectively in Syracuse, Rome and Milan. Then, once the hydrogen storage systems look like be competitive with battery, a more detailed analysis is carried out on monthly base. In this way, it is possible also to study as the seasonal variation of the produced and consumed energy influence the performance of the systems, As a conclusion, results shows that if full energy balance is considered the hydrogen based systems present a better functioning in comparison to the battery based.