BUSCADOR RECOLECTA

The use of carbon dioxide in transcritical state has become one of the most used solutions to comply with the F-Gas directive and reduce greenhouse gases emissions from refrigeration systems at high ambient temperatures. For low-medium power units, the commonly used solutions to improve the efficiency such as the ejector, multiple compressor arrangements, mechanical subcooler, etc., add complexity and increase the cost of the refrigeration facility, which is not ideal for small units. In this low-medium power range, two technologies stand out to increase the performance of a carbon dioxide transcritical cycle: the internal heat exchanger and the thermoelectric subcooler. This study brings a complete research in which both solutions have been tested in the same experimental transcritical carbon dioxide refrigeration facility under the same working conditions. It focuses on the real performance of both systems and discusses the strengths and weaknesses of using an internal heat exchanger or a thermoelectric subcooler. The results show that the thermoelectric subcooler outperforms the internal heat exchanger in both the coefficient of performance and the cooling capacity while also being a more controllable and flexible solution., The authors would like to acknowledge the support of the Spanish
Ministry of Science, Innovation and Universities, and European Regional
Development Fund, for the funding under the RTI2018-093501-B-C21
and RTI2018-093501-B-C22 research projects. We would also like to
acknowledge the support from the Education Department of the Government of Navarra with the Predoctoral Grants for Phd programms of
Interest to Navarra and the Official School of Industrial Engineers of
Navarre with the scholarship Fuentes Dutor. Open access funding provided by Universidad Pública de Navarra.

To improve the performance of vapour compression refrigeration cycles, the inclusion of a thermoelectric subcooler for low-medium power units has been the focus of recent studies due to its robustness, compactness and simplicity of operation. In thermoelectric systems, it has been demonstrated that the heat exchangers used in the hot and cold side of the thermoelectric modules have a critical impact in the performance of the system. This influence has not yet been studied for thermoelectric subcooling systems in vapour compression cycles. This work, for the first time, evaluates the impact that the heat exchangers of a thermoelectric subcooler, included in a transcritical carbon dioxide refrigeration cycle, have, in the performance of the refrigeration cycle. The influence is quantified in terms of: optimum working conditions, coefficient of performance and cooling capacity. The results show that, through an optimization of the heat exchangers of the thermoelectric subcooler, the performance improvements on the coefficient of performance using this technology are boosted from 11.96 to 14.75 % and the upgrade in the cooling capacity of the system rises from 21.4 to 26.3 %. Moreover, the optimum gas-cooler working pressure of the system is reduced and the optimum voltage supplied to the thermoelectric modules increases., The authors would like to acknowledge the support of the Spanish Ministry of Science, Innovation and Universities, and European Regional Development Fund, for the funding under the RTI2018-093501-B-C21 and RTI2018-093501-B-C22 research projects. We would also like to acknowledge the support from the Education Department of the
Government of Navarra with the Predoctoral Grants for Phd programmes of Interest to Navarra and the Official School of Industrial Engineers of Navarre with the scholarship Fuentes Dutor. Open access funding provided by Universidad Pública de Navarra.

The nowadays European educational framework boosts applying the learned theoretical concepts to real situations. Hence, practice sessions are key resources to present students direct applications of the theoretical concepts shown in class. Thus, developing new educational equipment and practice sessions oriented to bringing theoretical knowledge closer to practice should be one of the objectives of teachers. The present work describes a solution proposed by lectures of two Spanish universities looking to increase the knowledge of their engineering students. Along the years, these docents have noticed the lack of connection between the theoretical and practical knowledge among their students, drastically harming their learning procedure. Thus, in order to deepen into practical learning, a teaching methodology involving low-cost prototypes of vapor compression systems and a gamification method to help the students understand the concepts is proposed. The proposed methodology is expected to make a big positive impact on the results obtained by the students, taking into account the preliminary results reached., The authors are indebted to the Spanish Ministry of Science, Innovation and Universities, and European Regional Development Fund for the economic support to this work, included in the RTI2018-093501-B-C21 and RTI2018-093501-B-C22 research projects and to the Public University of Navarre for the economic support to this work included in the PJUPNA2003 research project.

One of the measures to fight against the current energy situation and reduce the energy consumption at an industrial process is to recover waste heat and transform it into electric power. Thermoelectric generators can be used for that purpose but there is a lack of experimental studies that can bring this technology closer to reality. This work presents the design, optimizations and development of two devices that are experimented and compared under the same working conditions. The hot side heat exchanger of both generators has been designed using a computational fluid dynamics software and for the cold side of the generators two technologies have been analysed: a finned dissipater that uses a fan and free convection biphasic thermosyphon. The results obtained show a maximum net generation of 6.9 W in the thermoelectric generator with the finned dissipater; and 10.6 W of power output in the generator with the biphasic thermosyphon. These results remark the importance of a proper design of the heat exchangers, trying to get low thermal resistances at both sides of the thermoelectric modules, as well as, the necessity of considering the auxiliary consumption of the equipment employed., This research was funded by the Spanish Ministry of Science, Innovation and Universities, and European Regional Development Fund research project number RTI2018-093501-B-C22; and the Education Department of the Government of Navarra with the Predoctoral Grants for Phd programms of Interest to Navarra.

Plants communicate with microorganisms by exchanging chemical signals throughout the phytosphere. Such interactions are important not only for plant productivity and fitness, but also for terrestrial ecosystem functioning. It is known that beneficial microorganisms emit diffusible substances including volatile organic compounds (VOCs) that promote growth. Consistently, soil application of cell-free culture filtrates (CF) of beneficial soil and plant-associated microorganisms enhances plant growth and yield. However, how this treatment acts in plants and whether it alters the resident soil microbiota, are largely unknown. In this work we characterized the responses of pepper (Capsicum annuum L.) plants cultured under both greenhouse and open field conditions and of soil microbiota to soil application of CFs of beneficial and phytopathogenic fungi. To evaluate the contribution of VOCs occurring in the CFs to these responses, we characterized the responses of plants and of soil microbiota to application of distillates (DE) of the fungal CFs. CFs and their respective DEs contained the same potentially biogenic VOCs, and application of these extracts enhanced root growth and fruit yield, and altered the nutritional characteristics of fruits. High-throughput amplicon sequencing of bacterial 16S and fungal ITS rRNA genes of the soil microbiota revealed that the CF and DE treatments altered the microbial community compositions, and led to strong enrichment of the populations of the same beneficial bacterial and fungal taxa. Our findings show that CFs of both beneficial and phytopathogenic fungi can be used as biostimulants, and provide evidence that VOCs occurring in the fungal CFs act as mediators of the plants’ responses to soil application of fungal CFs through stimulation of the beneficial soil microbiota., This work was supported by the Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (Spain) (grants BIO2013-49125-C2-1-P, BIO2016-78747-P, and PID2019-104685GB-100), the Government of Navarra (refs. P1044 AGROESTI, P1004 PROMEBIO, and P1046 MICROBIOME), and the project “Plants as a tool for sustainable global development” (registration number: CZ.02.1.01/0.0/0.0/16_019/0000827) within the program Research, Development and Education (OP RDE).

Thermoelectricity, in the form of thermoelectric generators, holds a great potential in waste heat recovery, this potential has been studied and proved in several laboratory and theoretical works. By the means of a thermoelectric generator, part of the energy that normally is wasted in a manufacturing process, can be transformed into electricity, however, implementing this technology in real industries still remains a challenge and on-site tests need to be performed in order to prove the real capabilities of this technology. In this work, a computational model to simulate the behaviour of a thermoelectric generator that harvest waste heat from hot fumes is developed. Using the computational model an optimal configuration for a thermoelectric generator is obtained, also an experimental study of the performance of different heat pipes working as cold side heat exchangers is carried out in order to optimize the performance of the whole thermoelectric generator, thermal resistances of under 0,25 K/W are obtained. The optimized configuration of the thermoelectric generator has been built, installed and tested under real conditions at a rockwool manufacturing plant and experimental data has been obtained during the 30 days field test period. Results show that 4.6 W of average electrical power are produced during the testing period with an efficiency of 2.38%. Moreover, the computational model is validated using this experimental data. Furthermore, the full harvesting potential of an optimized designed that takes advantage of the whole pipe is calculated using the validated computational model, resulting in 30.8 MWh of energy harvested during a sample year which could meet the demand of 8.34 Spanish average households., The authors are indebted to the Navarra Government for economic support of this work, included in the 0011-1365-2018-000101 Research Project, also to the State Research Agency of Spain for economic support, included in the RTI2018-093501-B-C22 Project from the Research Challenges Program. We would also like to acknowledge the support from the FPU Program of the Spanish Ministry of Science, Innovation and Universities (FPU16/05203).

The inclusion of a thermoelectric subcooler as an alternative to increment the performance of a vapour compression cycle has been proved promising when properly designed and operated for low-medium power units. In this work, a computational model that simulates the behaviour of a carbon dioxide transcritical vapour compression cycle in conjunction with a thermoelectric subcooler system is presented. The computational tool is coded in Matlab and uses Refprop V9.1 to calculate the properties of the refrigerant at each point of the refrigeration cycle. Working conditions, effect of the heat exchangers of the subcooling system, temperature dependent thermoelectric properties, thermal contact resistances and the four thermoelectric effects are taken into account to increment its accuracy. The model has been validated using experimental data to prove the reliability and accuracy of the results obtained and shows deviations between the ±7% for the most relevant outputs. Using the validated computational tool a 13.6 % COP improvement is predicted when optimizing the total number of thermoelectric modules of the subcooling system. The computational experimentally validated tool is properly fit to aid in the design and operation of thermoelectric subcooling systems, being able to predict the optimal configuration and operation settings for the whole refrigeration plant., The authors would like to acknowledge the support of the Spanish Ministry of Science, Innovation and Universities , and European Regional Development Fund , for the funding under the RTI2018-093501-B-C21 and RTI2018-093501-B-C22 research projects. We would also like to acknowledge the support from the Education Department of the Government of Navarra, Spain with the Predoctoral Grants for Phd programs of Interest to Navarra and the Official School of Industrial Engineers of Navarre with the scholarship, Spain Fuentes Dutor.

La producción de frío de manera artificial es considerada una de las contribuciones más relevantes de la historia, jugando un papel clave en el desarrollo de la sociedad humana, gracias a su contribución en la conservación de productos perecederos y como herramienta para conseguir confort térmico tanto en edificios como vehículos. Debido a su importancia, el sector de la refrigeración cuenta con un gran volumen y se encuentra ampliamente extendido tanto en el ámbito industrial como doméstico. Como consecuencia de su gran volumen, el sector viene ligado de un considerable impacto sobre el medioambiente debido al consumo eléctrico de las instalaciones y a los refrigerantes utilizados. Además, la demanda de refrigeración se espera que siga creciendo
durante las próximas décadas y las estimaciones más recientes predicen que llegará a duplicarse para el año 2050. Estos hechos, junto con la actual problemática medioambiental, ponen de manifiesto la importancia de disminuir el impacto medioambiental del sector de la refrigeración.
El impacto medioambiental de los equipos frigoríficos se debe a la emisión de gases de efecto invernadero a la atmósfera los cuales contribuyen al calentamiento global del planeta. Estas emisiones se deben por un lado al consumo eléctrico del sistema de refrigeración y por otro lado a la fuga de refrigerantes con alto potencial de calentamiento atmosférico de las instalaciones.
Con el objetivo de reducir las emisiones de los equipos frigoríficos existen dos principales vías de acción: el uso de refrigerantes naturales con bajo poder de calentamiento atmosférico, como el dióxido de carbono, y el desarrollo de equipos más eficientes para disminuir el consumo eléctrico de los sistemas de refrigeración.
Esta tesis doctoral combina ambas líneas de acción centrándose en el desarrollo de un sistema de subenfriamiento termoeléctrico para la mejora de la eficiencia de un ciclo de refrigeración por compresión de vapor que utiliza dióxido de carbono en estado transcrítico como refrigerante. Esta combinación de tecnologías se presenta como una solución novedosa y prometedora para la reducción de las emisiones de los equipos frigoríficos y es posible gracias a la robustez, escalabilidad,controlabilidad y versatilidad que ofrecen los sistemas termoeléctricos.
Para ello, en primer lugar se ha desarrollado un modelo computacional capaz de simular el comportamiento de un ciclo de compresión de vapor de dióxido de carbono en estado transcrítico junto con un sistema de subenfriamiento termoeléctrico. El modelo computacional desarrollado ha sido validado de manera experimental y los resultados muestran que es capaz de predecir el comportamiento real del sistema con desviaciones dentro del +/-7% de error.
Una vez desarrollado y validado el modelo computacional, este ha sido utilizado para el estudio y diseño del sistema de subenfriamiento termoeléctrico con el objetivo de optimizar el funcionamiento del sistema global de refrigeración. En el estudio se ha realizado una caracterización térmica de los intercambiadores de calor utilizados en el subenfriador termoeléctrico y mediante la utilización del modelo computacional se ha cuantificado el impacto de los interacambiadores de calor en el sistema global de refrigeración. Los resultados obtenidos muestran que mediante la utilización de intercambiadores optimizados, el aumento en potencia frigorífica con el sistema de subenfriamiento termoeléctrico se incrementa desde un 21.4% a un 26.3%. Asimismo, la mejora obtenida en el coeciente de operación pasa de un 11.96% a un 14.75%. Los resultados obtenidos demuestran el gran impacto que los intercambiadores de calor del subenfriador termoeléctrico tienen en el funcionamiento global del sistema de refrigeración.
Gracias a la información obtenida mediante el modelo computacional, se ha sido diseñado, construido e incorporado un sistema de subenfriamiento termoeléctrico en una instalación experimental de compresión de vapor que utiliza dióxido de carbono en estado transcrítico como refrigerante. La planta experimental ha sido ensayada bajo diferentes condiciones climáticas y de operación para comprobar de manera experimental el efecto del subenfriador termoeléctrico en el funcionamiento global del sistema. Además, los resultados obtenidos mediante la incorporación del subenfriador termoeléctrico han sido comparados con la utilización de un intercambiador recalentador subenfriador, una tecnología comúnmente utilizada para mejorar la eficiencia de ciclos de refrigeración por compresión de vapor. Los resultados obtenidos muestran como las mejoras obtenidas mediante el subenfriador termoeléctrico superan las obtenidas mediante el intercambiador recalentador subenfriador tanto en coeficiente de operación como en potencia frigorífica.
La inclusión del sistema de subenfriamiento termoeléctrico resulta en un aumento de potencia frigorífica de hasta un 20.8% y una mejora del coeficiente de operación del sistema de hasta el 16.2%. Por último, debido a la versatilidad y controlabilidad del subenfriador termoeléctrico, esta tecnología se ha combinado junto con el intercambiador recalentador subenfriador, con el objetivo de comprobar el funcionamiento del ciclo de compresión de vapor trabajando con ambas tecnologías simultáneamente. La incorporación de un subenfriador termoeléctrico junto con el intercambiador recalentador subenfriador resulta en un aumento de la potencia frigorífica de un 22.5% y en un incremento del coeficiente de operación del 22.4%. Estos resultados muestran que
mediante la combinación de estas dos tecnologías se obtienen mejoras superiores a las obtenidas a través de cada una de ellas de manera independiente.
Los resultados de esta tesis demuestran que la utilización de un sistema de subenfriamiento termoeléctrico es una solución tecnológicamente viable para la mejora de la eficiencia de sistemas de refrigeración por compresión de vapor con dióxido de carbono en estado transcrítico, disminuyendo así, las emisión de gases de efecto invernadero de los sistemas de refrigeración y contribuyendo a la producción de frio de manera sostenible y respetuosa con el medio ambiente., The refrigeration sector plays a critical role in the development and well-being of society by aiding in the conservation of perishables and providing thermal comfort in vehicles and buildings. Due to its undeniable importance and necessity, the vast volume of this sector is linked to a considerable impact on the environment. The demand of refrigeration is expected to grow over the next decades and according to the most recent estimations, the volume of the sector could double by 2050. This fact, alongside the current environmental problematic, reveals the importance of reducing the environmental impact of refrigeration systems.
The environmental impact of a refrigeration system is produced by the emission of greenhouse effect gases onto the atmosphere which contributes to the global warming of the planet. The emissions are related to the electrical consumption of the refrigeration system and the leakage of refrigerants with high global warming potential. Therefore, the two main lines of action are the use of natural refrigerants with almost negligible global warming potential, such as carbon dioxide, and the development of more efficient systems in order to reduce the electrical consumption of refrigeration facilities.
The present Ph. D. dissertation focuses on the development of a thermoelectric subcooling system to improve the performance of a vapour compression refrigeration cycle that uses transcritical carbon dioxide as a refrigerant. This combination of technologies presents itself as a novel and promising solution, possible due to the robustness, scalability, controlability and versatility that thermoelectric systems offer.
Firstly, a computationl model has been developed. It simulates the behaviour of a transcritical carbon dioxide vapour compression cycle working with a thermoelectric subcooler. The model has been experimentally validated and the results show that it is capable of representing a real system with deviations for the main outputs between the +/-7% interval of error.
Then, the computational model is used to aid in the design and to study the working principles of thermoelectric systems in order to maximize the performance of the global refrigeration system. A thermal characterization of the heat exchangers used in the thermoelectric subcooler has been performed and with the aid of the computational model developed, the impact that those heat exchangers have in the global refrigeration system has been quantified. The results demonstrate that when optimized heat exchangers are included in the thermoelectric subcooler, the enhancement in the cooling capacity of the system rises from 21.4% to 26.3%. Likewise, the improvement obtained in the coeficient of performance of the cycle rises from 11.96% to 14.75%. These results display that the heat exchangers of the thermoelectric subcooling system have a critical impact on the performance of the global system.
Considering the information obtained with the computational model, a thermoelectric subcooling system is designed, built and included in an experimental vapour compression facility that uses carbon dioxide in transcritical state as a refrigerant. The experimental plant is tested for multiple climatic and operation conditions to prove through experimental results the effect of including a thermoelectric subcooling system in the global performance of the refrigeration facility. In addition, the results with the thermoelectric subcooler are compared with the inclusion of an internal heat exchanger, a commonly used technology to boost the performance of vapour compression refrigeration cycles. The results show that, in terms of cooling capacity and coeficient of performance, the improvements reported with the thermoelectric subcooler surpass the ones obtained through the inclusion of the internal heat exchanger. Using the thermoelectric subcooler the cooling capacity of the refrigeration system is enhanced by 20.8% and the
coeficient of performance is boosted by 16.2%.
Moreover, considering the versatility and controllability of the thermoelectric subcooler, this system is used in combination with the internal heat exchanger to test the performance of the cycle working with both technologies at the same time. The inclusion of the thermoelectric subcooler in combination with the internal heat exchanger reported a maximum improvement in the cooling capacity of 22.5% and and increase in the coeficient of performance of 22.4%. These results showcase that improvements obtained with both technologies working together surpass the ones obtained with each of the technologies on their own.
The results of the present Ph. D. dissertation demonstrate that the inclusion of a thermoelectric subcooling system is a viable solution to boost the performance of vapour compression refrigeration systems that use natural refrigerants, reducing the emission of greenhouse gases from refrigeration systems and contributing to the production of cold in a sustainable way., The research is embedded in the LOWTEWI project (RTI2018-093501-B-C22), entitled 'Greenhouse attenuation in stand-alone commercial refrigeration systems'., Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011), Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011)

Regulations in the refrigeration sector are forcing the transition to low global warming potential fluids such as carbon dioxide in order to decrease direct greenhouse gases emissions. Several technologies have arisen over the past years to compensate the low performance of the transcritical carbon dioxide vapour compression cycle at high ambient temperatures. For low-medium power units, the inclusion of a thermoelectric subcooler or an internal heat exchanger have been proven as effective solutions for enhancing the coefficient of performance. However, the combination of a thermoelectric subcooler and an internal heat exchanger working simultaneously is yet to be explored theoretically or experimentally. This work presents, for the first time, an experimental transcritical carbon dioxide refrigeration facility that works simultaneously with a thermoelectric subcooler and with an internal heat exchanger in order to boost the cooling capacity and coefficient of performance of the refrigeration system. The experimental tests report improvements at optimum working conditions of 22.4 % in the coefficient of performance and an enhancement in the cooling capacity of 22.5 %. The 22.4 % increase in coefficient of performance would result in a decrease of energy consumption along a reduction of the greenhouse gases emissions. The proposed combination of a thermoelectric subcooler and an internal heat exchanger outperforms each of the technologies on their own and presents itself as a great controllable solution to boost the performance and reduce the greenhouse gasses emissions of transcritical carbon dioxide refrigeration cycles., The authors would like to acknowledge the support of the Spanish Ministry of Science, Innovation and Universities, and European Regional Development Fund, for the funding under the RTI2018-
093501-B-C21 and RTI2018-093501-B-C22 research projects. We
would also like to acknowledge the support from the Education Department
of the Government of Navarra with the Predoctoral Grants for Phd
programms of Interest to Navarra and the Official School of Industrial
Engineers of Navarre with the scholarship Fuentes Dutor. Publication
funding provided by Universidad Pública de Navarra.

CO2 is an excellent natural refrigerant that can be used in almost any commercial cooling application thanks to its useful range of evaporative temperatures and excellent environmental properties. However, due to its low critical temperature, CO2 has an important issue related to the low performance of the simplest transcritical refrigeration cycle. To overcome it, the subcooling technique is a well-known method to improve the energy performance of any refrigeration cycle especially the CO2 transcritical one. The IHX is a widely used example of this method that is implemented in almost all standalone systems that use CO2 as a refrigerant. As an alternative of this element, in this work, a thermoelectric subcooling system is presented and tested in a CO2 transcritical refrigerating plant. The experimental tests have been performed at two ambient temperatures: 25 and 30 degrees C, maintaining a constant evaporating level at-10 degrees C and varying the voltage supply to thermoelectric modules and the heat rejection pressure. The results from these experimental tests revealed that the COP and the cooling capacity of the refrigerating plant can be enhanced up to 9.9% and 16.0%, respectively, operating at the optimum operating conditions. Moreover, the experimental tests corroborate the existence of an optimum voltage which maximizes the COP, and the almost linear capacity regulation easily adjustable by varying the voltage supply., The authors would like to acknowledge the support of the Spanish Ministry of Science, Innovation and Universities, and European Regional Development Fund, for the funding under the RTI2018-093501-B-C21 and RTI2018-093501-B-C22 research projects.