Sizing of the buffer tank in chilled water distribution air-conditioning systems

[♪EN] This paper presents a sizing study of the buffer tank in chilled water A/C systems. In order to find out the adequate sizing criteria for buffer tanks in these kind of installations, a review of different manufacturers' guidelines was carried out and it was concluded that there are three main operation parameters affected by the volume of the tank: the ON cycle time, the OFF cycle time, and the number of starts per hour of the chiller compressor. In order to better understand the influence of each parameter, a theoretical analysis was carried out where the impact of the building thermal load in different design criteria was also studied. After a thorough analysis of all the studied criteria, it was concluded that the minimum ON cycle time which is necessary to ensure that the oil returns to the compressor is the most critical criterion. Finally, a design guideline is proposed by the authors in order to determine the minimum volume of the buffer tank, mainly depending on the capacity of the chiller and the temperature deadband of the thermostat control.This work was supported by the "Programa de Ayudas de Investigacion y Desarrollo (PAID)" of the Universitat Politecnica de Valencia. This work was also supported under the FP7 programme "Advanced ground source heat pump systems for heating and cooling in Mediterranean climate" (GROUND-MED project).Cervera Vázquez, J.; Montagud Montalvá, CI.; Corberán Salvador, JM. (2016). Sizing of the buffer tank in chilled water distribution air-conditioning systems. Science and Technology for the Built Environment. 22(3):290-298. https://doi.org/10.1080/23744731.2016.1131569S29029822

Performance of a scroll compressor with vapor-injection and two-stage reciprocating compressor operating under extreme conditions

The current paper presents a comparative study between a scroll compressor with vapor-injection (SCVI) and a two-stage reciprocating compressor (TSRC) operating under extreme conditions. The present work is divided into two parts: in the first part, both compressors are compared in terms of compressor efficiency, volumetric efficiency, coefficient of performance (COP), and cooling capacity with R407C refrigerant; in the second part, the seasonal performances of both compressors working in cooling and heating modes are estimated and analyzed. Results show that the SCVI presents better efficiency and COP than the TSRC for pressure ratios below 7.5. This compressor can be used in air conditioning systems and heat pumps which work under moderate temperature conditions. For higher pressure ratios, the TSRC has better efficiency which subsequently gives higher COP. This type of compressor is more suited to be used in sanitary hot water systems operating in harsh climates and in low-temperature freezing systems (under −20°C).Fernando M. Tello Oquendo acknowledges financial support provided by the "CONVOCATORIA ABIERTA 2013-SEGUNDA FASE" program, which was funded by the SENESCYT ("Secretaria Nacional de Educacion Superior, Ciencia, Tecnologia e Innovacion") (Grant No 104) of Ecuador.Tello Oquendo, FM.; Navarro-Peris, E.; Gonzálvez Maciá, J.; Corberán Salvador, JM. (2016). Performance of a scroll compressor with vapor-injection and two-stage reciprocating compressor operating under extreme conditions. International Journal of Refrigeration. 63:144-156. https://doi.org/10.1016/j.ijrefrig.2015.10.035S1441566

Analysis and optimization of different two-stage transcritical carbon dioxide cycles for heating applications

[EN] Increased interest in the environmental impact of refrigeration technology is leading toward design solutions aimed at improving the energy efficiency and use of eco-friendly refrigerants with low GWP. The aim of this paper is to theoretically analyse R744 air to water heat pump cycles for heating applications up to 80 °C. This work studies the following cycle configurations: two-stage with injection (with and without intermediated cooling between compressors) and a single-stage circuit coupled with an auxiliary circuit. Internal heat transfer among the different streams of refrigerant is included, and the cycles have been optimized with regards to COP in terms of the intermediate conditions and gas cooler pressure. Finally, these cycles have been compared and analysed among each other and with a subcritical injection cycle working with R134a and a single-stage R744 cycle. The improved cycle with R744 can represent a global improvement of 15% in terms of COP.This work has been developed in the 7 framework program of the European Union by the project Next Generation of Heat Pump Technologies (NEXTGHP) grant agreement 307169. The authors give thanks for the given support. Part of the work presented was carried by Miquel Pitarch-Mocholi with the financial support of the Phd scholarship from the Universitat Politecnica de Valencia.Pitarch Mocholí, M.; Navarro-Peris, E.; Gonzálvez Maciá, J.; Corberán Salvador, JM. (2016). Analysis and optimization of different two-stage transcritical carbon dioxide cycles for heating applications. International Journal of Refrigeration. 70:235-242. https://doi.org/10.1016/j.ijrefrig.2015.08.013S2352427

Analysis and optimization of subcritical two-stage vapor injection heat pump systems

Two of the major problems of heat pump systems working in extreme conditions are the loss of efficiency of the system and the high compressor discharge temperatures. One possibility in order to overcome these issues is to perform the compression in two stages. In this frame the use of vapor injection two stage cycles represent an economic and effective solution. This study analyzes the influence of design parameters and injection conditions for two different configurations of two stage cycles, for four refrigerants (R407C, R290, R22 and R32). Design parameters, such as the displacement ratio, are optimized in terms of COP in ideal conditions for both injection systems. A deeper analysis taking into account the efficiencies of the compressor is done finding that two stage systems could reach improvements of 30% in terms of COP compared with one stage systems and that a bad design of this type of systems could represent a loss of improvement between 6% and 10%. Finally a method to control the system at any operating point in order to make it works in its optimum is done. From all the analysis, guidelines for the optimum design and control of such systems are obtained in terms of capacity, Coefficient of Performance (COP), seasonal performance factor and discharge temperature criteria.This work has been partially supported by the Spanish Ministerio de Economia y Competitividad through the project rsf. DPI 2011-26771-C02-01. The authors are grateful for the given support.Redon Climent, A.; Navarro Peris, E.; Pitarch, M.; Gonzálvez Maciá, J.; Corberán Salvador, JM. (2014). Analysis and optimization of subcritical two-stage vapor injection heat pump systems. Applied Energy. 124:231-240. doi:10.1016/j.apenergy.2014.02.066S23124012

Reproducibility of solidification and melting processes in a latent heat thermal storage tank

This study analyzes the reproducibility of solidification and melting tests in a tank containing 181 kg of paraffin for cold storage at around 8 °C. Firstly, an experimental campaign of 10 identical tests was carried out. The performance is practically the same in terms of PCM temperatures and thermal power, with a maximum deviation of 2% in the capacity of all tests. In a second campaign, the impact of the initial conditions was studied. The results indicate that fixing a same mean PCM temperature at the beginning of the tests is insufficient to ensure an accurate reproducibility. Depending on the heat transfer rate during the preparation tests, the capacity differed in up to 33%. In tanks with such quantities of PCM, fixing a uniform initial PCM temperature is hardly possible, thus it is important to prepare the tank with same operation conditions.Bosholm, F.; López Navarro, A.; Gamarra, M.; Corberán Salvador, JM.; Payá Herrero, J. (2016). Reproducibility of solidification and melting processes in a latent heat thermal storage tank. International Journal of Refrigeration. 62:85-96. doi:10.1016/j.ijrefrig.2015.10.016S85966

Transient thermal model of a vehicle's cabin validated under variable ambient conditions

[EN] This paper presents a lumped-parameter thermal model of the passengers' compartment of a vehicle. The model is completely dynamic and has been thoroughly validated under variable ambient conditions including solar radiation. The proposed model reproduces accurately the warm-up and cool-down of the cabin and can help analyse capacity reduction actions for air-conditioning systems. In this study, the thermal loads have been calculated by means of the model. For the tested minibus in real outdoor conditions, air renewal represents from 7% up to 53% of the thermal load, while the solar radiation accounts for 18% 31%. According to the results, a decrease of 0.2 in the glazing transmissivity can lead to a 3.3% reduction of the installed cooling capacity.This work has been supported by the European Commission under the 7th European Community framework program as part of the ICE project "MagnetoCaloric Refrigeration for Efficient Electric Air-Conditioning", Grant Agreement no. 265434. B. Torregrosa-Jaime acknowledges the Spanish Education, Culture and Sport Ministry (Ministerio de Educacion, Cultura y Deporte) for receiving the Research Fellowship FPU ref. AP2010-2160.Torregrosa Jaime, B.; Bjurling, F.; Corberán Salvador, JM.; Di Sciullo, F.; Payá Herrero, J. (2015). Transient thermal model of a vehicle's cabin validated under variable ambient conditions. Applied Thermal Engineering. 75:45-53. https://doi.org/10.1016/j.applthermaleng.2014.05.074S45537

Improvement of transversal professional skills through cooperative work and group dynamics in the UPV Master's Degree in Energy Technologies for Sustainable Development (MUTEDS)

[EN] This article presents an updating of the structure, methodology and evaluation results after 4 years of teaching (courses 2015-2016, 2016-2017, 2017-18 and 2018-19) of a subject entitled Applied energy technology - Project course belonging to MUTEDS (Master's Degree in Energy Technology for Sustainable Development) at the Universitat Politècnica de València). The presented subject is mainly focused on improvement of professional skills as multidisciplinary teamwork and leadership, lifelong learning, competitiveness, planning and managing of time and effective communication in English. To work these competences students have to do, in 8 sessions, the exercise of creating a company (including name and logo) where there are 4 -6 students randomly selected and with different roles, this company evaluates technical-economical-environmental feasibility of an energy project (related with energy efficiency and renewable sources in and specific location) during 8 technical session (in which roles change in each session), and present the project proposal in a public competitive event where the students vote the best work according to their point of view. In parallel, competences and academic quality of the reports are evaluated by the professors.Alfonso Solar, D.; Vargas Salgado, CA.; Montagud Montalvá, CI.; Corberán Salvador, JM. (2020). Improvement of transversal professional skills through cooperative work and group dynamics in the UPV Master's Degree in Energy Technologies for Sustainable Development (MUTEDS). Editorial Universitat Politècnica de València. 239-248. https://doi.org/10.4995/INN2019.2019.10134OCS23924

Hot sanitary water production with CO2 heat pumps: Effect of control strategy on system performance and stratification inside the storage tank

[EN] In this work three different control strategies for the production of sanitary hot water by means of an electric heat pump working with CO2 are investigated. The heat pump is a prototype, here modelled in the vapour-compression software package IMST-ART. By simulating this model, the performance of the heat pump is correlated to the boundary conditions and is scaled to different sizes, namely 1, 1.5, and 2 times larger than the reference system. After having chosen an application for which the load profile of sanitary hot water during the year is known, these heat pumps are simulated in a TRNSYS16 model where the production of sanitary hot water and the consumption are buffered by the presence of a tank. Key parameter in guaranteeing comfort and good performance of the system is the stratification inside the storage tank. The size of the tank necessary to keep a certain level of comfort at the user is then determined through a parametric analysis for each size of the heat pump. The energetic performance is also evaluated for each system in terms of seasonal performance factor. Then, the results obtained are compared with a different system where the heat pump is equipped with an inverter and the circulation pump follows a different control logic. The size of the tank and the seasonal performance factor are therefore determined in this case too. Moreover, a night&day control logic is compared to these first two options to have a baseline of comparison in terms of volume of storage needed to guarantee a same level of comfort and performance. To provide information also on the running costs, a parametric analysis was run varying the type of control, the heat pump and the tank sizes for different load profiles. The results show that the size of the heat pump has a significant effect on the comfort of the user, which usually leads to oversizing of the storage tank when the load profile is unknown. With regard to this, the results obtained for the alternative control system show a 20% reduction of the volume of the tank, given a certain level of comfort, and is therefore useful to reduce the size of the storage tank.The study related to thiswork has been partially supported by the FP7 European project ‘Next Generation of Heat Pumpsworkingwith Natural fluids’ (NxtHPG) Grant agreement no: 307169. The work of M. Tammaro on electric heat pumps is financially supported by Universita degli Studi di Napoli Federico II throughthe FP7 European project 'Next Generation of Heat Pumps working with Natural fluids' (NxtHPG).Tammaro, M.; Mauro, AW.; Montagud Montalvá, CI.; Corberán Salvador, JM.; Mastrullo, R. (2016). Hot sanitary water production with CO2 heat pumps: Effect of control strategy on system performance and stratification inside the storage tank. Applied Thermal Engineering. 101:730-740. https://doi.org/10.1016/j.applthermaleng.2016.01.094S73074010

A novel TRNSYS type for short-term borehole heat exchanger simulation: B2G model

[EN] Models of ground source heat pump (GSHP) systems are used as an aid for the correct design and optimization of the system. For this purpose, it is necessary to develop models which correctly reproduce the dynamic thermal behavior of each component in a short-term basis. Since the borehole heat exchanger (BHE) is one of the main components, special attention should be paid to ensuring a good accuracy on the prediction of the short-term response of the boreholes. The BHE models found in literature which are suitable for short-term simulations usually present high computational costs. In this work, a novel TRNSYS type implementing a borehole-to-ground (B2G) model, developed for modeling the short-term dynamic performance of a BHE with low computational cost, is presented. The model has been validated against experimental data from a GSHP system located at Universitat Politecnica de Valencia, Spain. Validation results show the ability of the model to reproduce the short-term behavior of the borehole, both for a step-test and under normal operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.The present work has been supported by the FP7 European project Advanced ground source heat pump systems for heating and cooling in Mediterranean climate (GROUND-MED).De Rosa, M.; Ruiz Calvo, F.; Corberán Salvador, JM.; Montagud Montalvá, CI.; Tagliafico, L. (2015). A novel TRNSYS type for short-term borehole heat exchanger simulation: B2G model. Energy Conversion and Management. 100:347-357. https://doi.org/10.1016/j.enconman.2015.05.021S34735710

Control del subenfriamiento: una manera de mejorar el rendimiento de condensadores para la producción de agua caliente con un elevado salto de temperatura del agua

El uso de ciclos subcríticos en bombas de calor para la producción de agua caliente se limita a temperaturas relativamente bajas y a saltos de temperatura del agua a calentar también pequeños. Es una buena solución, por ejemplo, para la producción de agua caliente para aplicaciones de calefacción, porque la temperatura del agua es admisible: 35 a 50°C y el salto de temperatura del agua es bajo, por ejemplo, alrededor de 5K. Para la producción de agua caliente sanitaria sin embargo, el salto de temperatura del agua es mucho mayor, por ejemplo, 50K cuando el agua se calienta de 10 a 60ºC. La temperatura del refrigerante en el proceso de condensación es bastante constante, lo que hace que la eficacia del condensador disminuya a elevados saltos de temperatura del agua. Recientemente se han desarrollado calentadores de agua que utilizan CO2 en ciclo transcrítico, ya que han demostrado un buen funcionamiento a elevados saltos de temperatura del agua, ja que se ve beneficiado por un elevado salto de temperaturas en el lado del refrigerante. Por lo tanto, una manera de mejorar la eficacia del condensador en un ciclo subcrítico será produciendo una gran cantidad de sub-enfriamiento, con el consecuente aumento de capacidad y el COP del ciclo. El siguiente artículo presenta dos ciclos diferentes con el fin de obtener el subenfriamiento deseado y la instalación experimental para la comba de calor agua/agua trabajando con propano. Finalmente, algunos resultados preliminares del COP se dan para diferentes condiciones de funcionamiento, tales como temperatura de la fuente de la bomba de calor en el lado caliente y el grado de subenfriamiento. Además, los resultados se comparan con un punto de referencia con cero subenfriamiento y una bomba de calor comercial.Los resultados de este estudio se han desarrollado marco del proyecto europeo FP7 European Project, Next Generation of Heat Pumps working with Natural fluids, NxtHPG. Parte de los trabajos presentados se realizaron por Miquel Pitarch Mocholí con el apoyo financiero de la beca de doctorado de la Universitat Politècnica de València