Small-scale trigeneration systems based on biomass combustion: organic rankine cycle optimization
- Autores: Daniel Maraver de Lemus
- Directores de la Tesis: Francisco Javier Royo Herrer (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2014
- Idioma: español
- Tribunal Calificador de la Tesis: Luis Miguel Romeo Gimenez (presid.), Eduardo Ferrer Benedí (secret.), Wojciech Kostowski (voc.)
- Materias:
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- Resumen
The main objective of this thesis was to contribute to the development of small-scale trigeneration systems based on direct biomass combustion, by means of evaluating their performance from an energy, economic and environmental perspectives, analyzing the thermodynamic integration of the possible subsystems involved and systematically optimizing organic Rankine cycles. The work developed under the framework of this thesis to accomplish such a goal can be found in different published articles and it can be summarized into four different blocks or sections, described within the manuscript.
Firstly, as an introduction to the main work performed in this thesis, it was necessary to study how to integrate different subsystems in order to generate cooling, heating and power in a combined way, and also how to quantify their possible benefits in comparison to the equivalent separated generation scenario. The introduction to the study of combined generation systems based on biomass combustion was carried out by means of two examples in which two of the most interesting technologies present in systems at a larger scale were coupled in a trigeneration plant layout: organic Rankine cycles and absorption chillers. The preliminary conclusions of this work stage confirm the interest of the proposed systems from different perspectives and motivate the in-depth analysis carried out in the rest of the thesis, both from the points of view of the overall benefits of small-scale trigeneration systems based on biomass combustion and the possible further prospects for improvement of the subsystems involved to optimally fit the requirements of such plants.
Secondly, an extensive review of the literature was undertaken, with respect to the current state of development of bio-fuelled trigeneration plants and the subsystems with high potential as prime movers and thermally-driven cooling technologies: Stirling engines, organic Rankine cycles, absorption and adsorption chillers. Such review work was the key to understanding the main current barriers to the development of CCHP plants based on biomass combustion, particularly at small- and micro-scale. These barriers are, in general, related to the technical operating limitations of the subsystems and, more specifically, to the lack of prime mover ad hoc units.
When considering the integration of the state-of-the-art subsystems in a third stage of the work carried out within the framework of this thesis, it was however concluded that these technologies can, indeed, be part of a trigeneration plant based on biomass combustion that complies with a series of benefits, with respect to separated conventional generation. Such analysis, along with the previously developed state-of-the-art review revealed both the interest of ORCs as prime mover units in small-scale bio-fuelled trigeneration systems and its possible improvement margin, existing with respect to the current state of development. In other words, it was necessary to reduce the technology gap regarding small-scale ORCs in order to be optimally integrated in such systems.
Therefore, the fourth section of this document is focused on the development of optimal ORCs. To that end, a methodology based on a thermodynamic approach which was subjected to a series of characteristic parameters of the heat exchangers, expander and feed pump, was generated in order to obtain practical design guidelines. Hence, this methodology considers both thermodynamic and practical engineering constraints with the objective of their being applied to an ORC, integrated within a small-scale trigeneration plant. As a result, practical design guidelines were obtained, including the working fluid selection, the expander inlet pressure and temperature, the superheating degree, the operation mode (sub- or transcritical) and the presence of a recuperator, considering the constraints of the main components of the ORC. In addition, to fulfill the work carried out in this thesis, it was considered necessary to include a final analysis of the benefits achieved, by the use of the optimal CCHP system, in contrast to conventional generation. Such analysis showed an important improvement step with respect to the results obtained, considering the integration of the current small-scale ORC units present in the market. However, it is noteworthy to remark that trigeneration plants integrated by ORCs in which a great amount of cooling is generated in contrast to the heating are not feasible from energy saving and environmental perspectives. Such a barrier could be, partially, overstepped through the development and use of direct fired units.
