• heat pumps
• integration
• distributed energy systems
• combined cooling heating and power
• cogeneration
• optimization

Distributed Energy Systems (DESs) based on polygeneration plants can play an important role in the development of a more sustainable energy paradigm. The high energy efficiency resulting from the optimal integration of different energy technologies and vectors, and the high penetration of renewable energy sources allow for significant primary energy savings and reduced greenhouse gas emissions, while guaranteeing the economic sustainability.
Nevertheless, the inherent complexity of DESs makes the economic, environmental and energy performance highly dependent on equipment capacity and operational strategy, thus requiring the development and adoption of advanced design methodologies and optimization tools. Moreover, the availability of many different energy technologies entails the issue of the selection and layout of the components to be installed in the system. The synthesis and evaluation of novel configurations of DESs is a rich and complex topic, which is worthy to be investigated. In particular, the integration of heat pumps, which represent a link between different energy vectors, can be especially favorable in DESs, by providing operational flexibility and increasing the overall energy efficiency.
In this framework, this thesis aims to contribute to the development of the scientific knowledge in the field of DESs, by pursuing three main objectives: (i) a systematic overview of simulation-based approaches for the design of DESs, (ii) the development of methodologies for the optimal synthesis, sizing, and operation of DESs, and (iii) the proposal and evaluation of novel configurations of DESs.
An overview of cogeneration-based DESs and related energy technologies is given, and benefits and drawbacks compared to traditional energy systems are discussed. The models and simulation of DESs are presented, as well as the objectives and methodologies of this approach. The issue of uncertainty in DESs is dealt with, identifying uncertain parameters in DESs and presenting probabilistic models and simulation techniques, such as the Monte Carlo method.
Moreover, the topic of the design of DESs is thoroughly analyzed. The synthesis-sizing-operation paradigm is defined, and both deterministic and stochastic indicators for evaluating DES performance are presented. Then, a comprehensive review of traditional and innovative methodologies for the definition of operational strategies and design techniques is given. In this regard, a wide-ranging selection of literature works is presented and discussed.
Closely related to the topic of simulation-based design of DESs, a complete overview of optimization techniques adopted in this field is given. Single-objective optimization methods are presented, with a focus on genetic algorithm and mixed integer linear programming. Then, the multi-objective optimization problem and scalarizing functions are introduced, with a highlight on the achievement function approach. Furthermore, optimization-under-uncertainty approaches and dimensionality reduction methods (namely data clustering and rolling horizon) are discussed.
In the second part of the thesis, innovative methodologies for the optimal design of cogeneration-based DESs are developed and tested in case studies. A comprehensive methodology for the integrated optimal sizing and operation of cogeneration systems with thermal energy storage is defined. A probabilistic approach to consider long-term uncertainty in energy demands in the optimal design of cogeneration system is proposed. An operational optimization method for trigeneration system based on real-time measurements of energy demands and ambient conditions is developed.
Furthermore, innovative system configurations are proposed and evaluated, with a view to increase the overall energy efficiency and penetration of renewable energy sources, mainly focusing on the integration of heat pump technologies in polygeneration systems. To this end, exergy and levelized cost of energy analysis are performed, and optimization tools are adopted to evaluate the optimal design of the proposed configurations in case studies. A novel trigeneration system including a high-temperature vapor-compression heat pump is investigated. The high-efficiency integration of reversible absorption heat pumps and internal combustion engine is examined. Vapor-compression heat pumps are integrated within a hybrid renewable trigeneration system.