Simulation and analysis of a solar-assisted trigeneration system

UNIVERSITY OF TRIESTE _________________________________________________________________________ Simone De Luca – Master thesis - 19 - Abstract This work aims to assess energetic and economic performance of a solar-assisted trigeneration system that provides heating, cooling and electricity to a public hospital building. This analysis is based on a transient numerical model. The core objective is the development of a cost-effective control strategy for the solar field, which collects the thermal energy supplied by concentrating collectors and delivers this amount of heat to a cogeneration unit, represented by an Organic Rankine Cycle machine. Heating and cooling production are based on wasted thermal energy from the power unit, with the latter provided by an adsorption chiller. Such set of technologies represents an innovative solution for building size energy systems with few installed plants worldwide. Main critical issues are the variability of the solar energy source, the limited power cycle efficiency at low evaporation temperatures and the technical and control requirements for coupling low-grade rejected heat with an efficient cooling cycle. Energetic advantages of combined cooling, heat and power are expected to be remarkable even in small-scale applications, but the economic feasibility is still questionable. Therefore, a performance and economic analysis of the system is carried out using numerical simulation results. The final goal of this analysis is the definition of an optimal design for further development of the studied system. In order to achieve the proposed objectives, some tasks have been completed. Firstly, technical features of solar trigeneration systems are examined in order to define the state of the art of such systems. That investigation aims to rate the actual performance of such systems and understand which kind of improvements can be applied through an optimization process. After the theoretical study of each main component, a transient numerical model of the whole system is implemented using the TRNSYS simulation software. The thermal oil loop is completely modelled within this work, while heating and cooling circuits are simulated using an existing numerical model adapted to the studied system specifications. An overall control strategy is also elaborated, with particular attention to the minimization of the backup heater fuel consumption and the maximization of average solar output throughout the year. The control logic is implemented in the same transient model. Simulation results are used to evaluate performance of the obtained system. Electrical and thermal outputs are evaluated in terms of annual and seasonal amounts of energy, while consumption is considered through Final Energy and Primary Energy indicators. A comparison with two reference systems, for heating and cooling production, respectively, is the base for the evaluation of energy and economic savings related to the studied system. Last analysis aims to assess the economic feasibility of the project, by means of calculating an approximate return of investment, using actual costs of the components. Since the involved technology is costly, a further optimization of the system is considered to achieve higher economic performance by changing some model parameters. Chance of investigating this innovative trigeneration system is given by the European research project BRICKER.