Energy, exergy and exergo-economic analysis of an ORC biomass cogeneration system integrated with an hybrid solar thermal solution

Concerns related to climate change have encouraged the search for renewable and efficient technologies to be installed in plants with high energy consumption, since there would be resulting with a greater possibility of primary energy saving and emission reduction. Several studies have shown that indoor swimming pools require, in the services sector, the highest energy consumption to ensure comfort and compensate for the losses. Nevertheless nowadays there are only few totally renewable cogeneration prototypes, which operate in the low power and temperature ranges required by the pools, due to the low efficiencies achieved so far. The purpose of this thesis is to expand the information available in the scientific community through the analysis of a low-temperature and biomass-powered cogeneration prototype, innovative in the Portuguese market, proposed by the SCIVEN company, spin-off of the University of Coimbra. Is presented the case study of a swimming pool, whose water requires a thermal load between 80 and 150 kWth, with an inlet temperature between 25 and 27 °C and an outlet temperature between 30 and 32 °C. The peculiarity of the plant is that the water heated in the combustion chamber reaches temperatures below the boiling point, ie. water at 95 °C, to have for fewer legislative constraints during construction and maintenance. This thermal source is so used to evaporate and overheat the R245fa organic fluid up to 90 °C. Moreover, particular attention was dedicated to the heat exchangers, to which was applied the moving boundary method. Through the MATLAB software, the energetic, exergetic and exergoeconomic conditions of the combustion chamber and the cogeneration cycle were analysed in design and off design conditions. Thermodynamic analysis showed a maximum electrical efficiency of 6.62% for a condenser capacity of 140 kWth. For different condenser thermal powers, there was also discovered a pool temperature control strategy, able to increase the electrical efficiency of 0.45% and the net electric power produced of 16%. Finally, for each benchmark, the ranges of the thermodynamic conditions have been reported to carry out future thermo-mechanical analyses. The exergetic and exergoeconomic analysis were conducted on the full range of operating conditions on the condenser to minimize the cost of exergy. It emerged that the component with the greatest irreversibility is the combustion chamber, between 89.6 and 93%, followed by the evaporator, between 5.9 and 3.3%. Finally, since the combustion chamber has the highest sum of the cost rate and the exergy destruction, it was decided to reduce its contribution to the plant by preheating the organic fluid with an hybrid solar thermal system. The solar system has been sized, thought Polysun software, both to preheat the organic fluid and to generate hot water for the showers of a system of 250 daily users. The plant was analysed with two different technologies, the vacuum tube and the flat plate. The optimization on panel surface allows a pellet savings of 21.9 [ton/year] and 19.4 [ton/year] respectively. Finally, was evaluated the NPV of the solutions proposed without incentives and considering those of the Italian and Portuguese states.