Bahsoon, RamiHendley, RobertAlofi, Akram Mohammed Ali2023-08-242023-08-242023https://hdl.handle.net/20.500.14154/68973Blockchain technology is an emerging technology revolutionising information technology and represents a change in how information is shared. It has captured the interest of several disciplines because it promises to provide security, anonymity and data integrity without any third-party control. Although blockchain technology has great potential for the construction of the future of the digital world, it is facing a number of technical challenges. A most critical concern is related to its environmental sustainability. It has been acknowledged that blockchain-based systems' energy consumption and carbon emissions are massive and can affect their sustainability. Therefore, optimising the environmental sustainability of these systems is necessary. Several studies have been proposed to mitigate this issue. However, the literature needs to include models for optimising the environmental sustainability of blockchain-based systems without compromising the fundamental properties inherent in blockchain technology. In this context, this thesis aims to optimise the environmental sustainability of blockchain-based systems by balancing different conflicting objectives without compromising the decentralisation and trustworthiness of the systems. First of all, we reformulate the problem of the environmental sustainability of the systems as a search-based software engineering problem. We represent the problem as a subset selection problem that selects an optimal set of miners for mining blocks in terms of four conflicting objectives: energy consumption, carbon emissions, decentralisation and trustworthiness. Secondly, we propose a reputation model to determine reputable miners based on their behaviour in a blockchain-based system. The reputation model can support the enhancement of the environmental sustainability of the system. Moreover, it can improve the system's trustworthiness when the number of miners is reduced to minimise energy consumption and carbon emissions. Thirdly, we propose a self-adaptive model that optimises the environmental sustainability of blockchain-based systems taking into account environmental changes and decision-makers' requirements. We have conducted a series of experiments to evaluate the applicability and effectiveness of the proposed models. Finally, the results demonstrate that our models can enhance the environmental sustainability of blockchain-based systems without compromising the core properties of blockchain technology.234enBlockchainOptimisationSustainabilityDecentralisationTrustworthinessThesis