Numerical methods for a sustainable future: indoor environmental quality and innovative energy systems analysis

In the face of the profound challenges posed by the 21st century, sustainable development stands as an imperative, reflecting our shared responsibility to address environmental, social, and economic concerns, ensuring a better quality of life for present and future generations. This doctoral dissertation offers a pathway to a safer and more sustainable future, seeking novel solutions that address both the immediate need for healthier indoor spaces and the long-term goal of achieving a carbon-neutral landscape. It is divided into three main parts, each consisting of self-contained chapters. Part I and II use experimental and Computational Fluid Dynamics (CFD) approaches to study indoor air quality and thermal comfort (the first two pillars of indoor environmental quality) in both large and micro indoor environments. Specifically, in Part I, the focus is initially on a car cabin micro-environment: the risk of infection from SARS-CoV-2 Delta variant of passengers sharing a car cabin with an infected subject for a 30-min journey is estimated through an integrated approach combining a predictive emission-to-risk approach and a validated CFD numerical model. The findings demonstrate that CFD approaches are essential for properly assessing individual risk in such confined spaces, and that simple zero-dimensional models produce misleading results. One limitation of this study is the simplified geometry adopted for the numerical analyses, which is addressed in Part II of this thesis. Later, the attention shifts to a large indoor environment: a university lecture room. A CFD tool is developed and validated to study the dynamics of droplets emitted by an infected subject speaking in front of the class for 2 h, replicating a 2-h lesson. The influence of three different air supply rates (3.75 ACH, 7.5 ACH and 15 ACH) on particles age, distribution throughout the room and concentration within selected control volumes surrounding the rows of benches is analysed. The main finding is that better control of local airflow patterns is needed in indoor environments, as increasing the airflow rate provided by Heating, Ventilation and Air Conditioning (HVAC) systems might have adverse local effects. Consequently, the last chapter of this section introduces the design of a portable personal air cleaner intended to control local airflow patterns in large indoor spaces and reduce the airborne transmission of respiratory pathogens. CFD analyses revealed that the effectiveness of the device is 96% in close proximity scenarios and 99.5% in large indoor environments. Part II, relying on the results presented in the preceding section, presents a comprehensive benchmark with Particle Image Velocimetry measurements and CFD simulations of a real car cabin. A comparison is made between unsteady Reynolds-averaged Navier-Stokes (URANS) and large eddy simulation approaches, evidencing that the URANS produces satisfactory results and is suitable for studying the thermal-fluid dynamics fields inside real car cabins. The URANS model is then applied to characterize the mean thermal-fluid dynamics fields in winter and summer conditions, and to perform a thermal comfort analysis. This analysis leads to the conclusion that passengers sharing the same car may have contrasting comfort perceptions. This point must be considered in the design and operation of the HVAC systems, since the environment must meet the minimum comfort requirements for all the occupants. In this regard, validated CFD tools become essential as they allow to faithfully predict the airflow patterns in this and other indoor environments. Part III differs from the first two sections and addresses sustainability from an environmental and energy perspective, investigating solutions to promote the diffusion of renewable energy sources and the replacement of fossil fuels. This is achieved by evaluating the feasibility of small-scale Power-to-Hydrogen (PtH2) systems distributed across the national natural gas transmission network (NGTN), in correspondence of the several regulating and measuring stations present there. The idea is to use the surplus electricity produced by photovoltaic power systems to produce green hydrogen, which is then injected into the natural gas pipelines. This solution, known as line-pack storage, could provide the storage capacity needed to manage the future large energy production from renewables. A mathematical model has been developed to study the hourly operation of the proposed PtH2 system and to determine the optimal sizing for each subsystem comprising the plant. Findings reveal the need for a hydrogen buffer to balance its production (dependent on the intermittent electricity production from renewables) with its injection in the NGTN (dependent on the natural gas consumption by end-users). The results also show that in the best-case scenario, the cost of hydrogen production is 5.10 EUR/kg and the PtH2 plant allows to save about 32 tonnes of CO2 emissions per year.