Impact of management strategy on green methane production from wind energy

Mitigating the effects of global warming by reducing greenhouse gas emissions requires the adoption of sustainable practices and the promotion of renewable energies. However, in an energy scenario strongly dominated by intermittent energy sources, storage systems are becoming increasingly important. In this context, the conversion of renewable energy peaks into green hydrogen can be considered an interesting possibility. Furthermore, the use of power-to-gas systems solves, at least in a transition phase, the problems associated with the lack of infrastructure dedicated to hydrogen. In this study, a power-to-gas system producing synthetic methane from wind energy was modelled. Three management strategies were implemented and compared to assess the flexibility and versatility of the system. Results showed the importance of using an intermediate hydrogen storage tank to reduce the amount of surplus hydrogen. However, the choice of a management strategy depends on the purpose for which the power-to-methane system is designed

Multi-Effect Distillation (MED) plants for seawater desalination: thermodynamic and economic improvement

The growing global demand of fresh water coupled with the increasing interest in renewable energy and in waste heat recovery has resulted in a growing attention to the Multi-Effect Distillation (MED) desalination process because it requires relatively low temperature sources and maximum Top Brine Temperature (TBT) of 70-90°C. The study of MED configurations that could increase the efficiency and reduce desalted water cost of production is an actual field of research. Differently from previous studies present in literature, a Parallel/Cross flow (P/C) MED was studied with variable feed mass flows in each effect to obtain the maximum allowable Recovery Ratio (RR). The maximum allowable RR is related to the need of avoiding scaling problems due to gypsum precipitation (function of the effect temperature and of the feed water salinity). Starting from a base P/C MED, several configurations have been simulated to find the best ones from a thermodynamic and economic point of view. The simulations have been conducted through an Aspen Plus model by varying the Bottom Brine Temperature (BBT) and the TBT. Configurations with a preheater for each effect showed the highest increase of Performance Ratio (PR) relative to the base configuration, in particular for the highest TBT and the lowest BBT (+ 10 % with TBT=75°C and BBT=35°C for a 4 effects-MED) due to the better exploitation of the energy content of desalted water streams

Critical aspects of green hydrogen production from renewables

Renewable hydrogen production can have an important role in the context of global decarbonization and to increase the penetration of renewable energies (REs) in the electric system. Renewable hydrogen can contribute to balance REs production fluctuations, reduce curtailment, and ensure grid stability by acting as a long-term storage of large amounts of energy. Green hydrogen can be converted into other fuels or used as feedstock in industry where its production is now mainly based on fossil fuels. Research interest in electrolysis has risen in the last decades since it is the main process to produce hydrogen by water splitting from renewable electricity. The research focus aims to increase conversion efficiency and reduce costs. When coupled with intermittent and variable sources, electrolyzers are subjected to part-load and dynamic operation. This can accelerate the electrolyzer stack degradation and worsen its performance. In the first part of this thesis, an alkaline electrolyzer is modelled in Matlab by considering electrochemical and thermal aspects. In addition, a degradation model is proposed to include the stack degradation, and the performance reduction due to electrolyzer variable operation and number of on/offs. Currently, in the literature, very few electrolyzer models including stack degradation are deployed in simulations of electrolysis systems. The electrolyzer model is then applied to a case study in which an electrolysis system is coupled to a wind farm for green hydrogen production. Because of RE intermittency, the sizing of the electrolysis plant is a trade-off between the maximization of RE utilization, high electrolyzer utilization factors and cost minimization. In the analyzed case study, the sizing of the electrolysis system and the size and number of separated electrolysis groups into which it is divided has been investigated by performing a techno-economic analysis of several configurations. On one hand, the system is more flexible by having a greater number of separated groups. On the other hand, each electrolysis group must have its own balance of plant, which results in higher capital costs. At the same overall electrolysis capacity, the presence of two separated groups resulted to be enough to significantly increase hydrogen production compared to the configurations with only one larger group and minimized the specific cost of hydrogen production in the case of an overall electrolysis nominal power greater than about two-thirds of the nominal wind power. In the second part of the thesis, the potential, sustainability, and feasibility of hydrogen production through electrolysis are investigated in future scenarios of the Italian electric grid with increasing PV and wind capacities. Both electrolytic hydrogen production with solely RE curtailment (green hydrogen) and electrolytic hydrogen production also with additional electricity from natural gas-based plants are analyzed and compared from techno-economic and CO2-emission points of view. Very low utilization factors of the electrolyzers were obtained (up to a maximum of 22.35% in the highest RE penetration scenario), and consequent high specific hydrogen production costs, by assuming to produce only green hydrogen. By considering additional capacities of electric storage (up to 200 GWh) to cover the electric demand, the RE curtailment available for electrolysis is further reduced and the maximum green hydrogen production potential results far lower than the current national consumption. By allowing the use of additional non-RE electricity for electrolysis, the specific cost of hydrogen can be reduced but CO2 emissions increase. Under the theoretical hypothesis of producing all the current national hydrogen demand through electrolysis, specific CO2 emissions resulted higher than SMR ones, with the sole exception of the highest RE penetration scenarios and at the highest installed electrolysis capacity considered and, consequently, at the highest costs. Furthermore, electrolysis from renewables potentially competes with the electric grid decarbonization since the electricity used could be alternatively stored and provided back to the grid when electricity demand exceeds RE production

Multi-effect distillation plants for small-scale seawater desalination: thermodynamic and economic improvement

The growing global demand for fresh water coupled to the increasing interest in renewable energies and waste heat recovery has resulted in flourishing attention to the multi-effect distillation process for seawater desalination. The low operating temperature makes this technology attractive in the case of low temperature heat sources such as geothermal, solar or waste heat recovery. The low energy density of these heat sources requires small-scale desalination systems whose layout and operation may differ from large-scale plant. In this work, new plant configurations for a small-scale multi-effect distillation system are proposed and analyzed from a thermodynamic and economic point of view. Each configuration tends to better exploit the energy content of the various streams by improving heat recovery, according to an increasing layout complexity. These configurations were studied in two layouts, differing in the way seawater and brine fed the various effect. The feed mass flow in each effect was varied to maximize the recovery ratio by imposing the maximum salt concentration in the brine related to calcium sulphate precipitation. Numerical simulations were conducted in Aspen Plus environment by varying the top brine temperature with a fixed bottom brine temperature of 40 °C. The electrolyte non-random two liquid equation of state was adopted to evaluate saltwater properties and an inter-model comparison with a validated algebraic model was carried out. The configurations implementing seawater preheating increased the performance ratio up to 10% due to the better exploitation of the energy content of distillate streams. The proposed solutions with the maximization of the recovery ratio demonstrated to be cost-effective with respect to the base multi-effect distillation configuration when thermal energy cost became relevant. In the case of negligible thermal energy cost (waste heat recovery) the base configuration was the preferable solution in terms of water cost, despite the lower performance ratio

Techno-economic analysis of hydrogen production from PV plants

Hydrogen production through electrolysis from renewable sources is expected to play an important role to achieve the reduction targets of carbon dioxide emissions set for the next decades. Electrolysers can use the renewable energy surplus to produce green hydrogen and contribute to making the electrical grid more stable. Hydrogen can be used as medium-long term energy storage, converted into other fuels, or used as feedstock in industry thus contributing to decarbonise hard-to-abate-sectors. However, due to the intermittent and variable nature of solar and wind power, the direct coupling of electrolysers with renewables may lead to high production fluctuations and frequent shutdowns. As a consequence, accelerated electrolyser degradation and safety issues related to low load operation may arise. In this study, simulations of hydrogen production with an electrolyser fed by a PV system are performed in Matlab for a reference year. The effect of PV power fluctuations on the electrolyser operation and production is investigated. The impact of the electrolyser size for a fixed nominal power of the PV plant is also analysed from both energetic and economic points of view

Impact of management strategy on green methane production from wind energy

Mitigating the effects of global warming by reducing greenhouse gas emissions requires the adoption of sustainable practices and the promotion of renewable energies. However, in an energy scenario strongly dominated by intermittent energy sources, storage systems are becoming increasingly important. In this context, the conversion of renewable energy peaks into green hydrogen can be considered an interesting possibility. Furthermore, the use of power-to-gas systems solves, at least in a transition phase, the problems associated with the lack of infrastructure dedicated to hydrogen. In this study, a power-to-gas system producing synthetic methane from wind energy was modelled. Three management strategies were implemented and compared to assess the flexibility and versatility of the system. Results showed the importance of using an intermediate hydrogen storage tank to reduce the amount of surplus hydrogen. However, the choice of a management strategy depends on the purpose for which the power-to-methane system is designed