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Abstract The use of night cooling ventilation in addition of phase change materials (PCMs) is a very powerful strategy for reducing the cooling demand of buildings. Nevertheless, there are inherent drawbacks in the way things have been doing so far: (a) The limited area of contact between PCM and the air; (b) the very low convective heat transfer coefficients which prevents the use of significant amounts of PCM and (c) the very low utilization factor of the cool stored due to the large phase shift between the time when cool is stored and time when it is required by the building. In this paper, we present innovative solutions using PCM to overcome the above situation. Compared with existing solutions, innovative solutions proposed, increase the contact area between PCM and air by a factor of approximately 3.6, increase the convective heat transfer coefficient significantly, and improve the utilization factor due to the inclusion of active control systems which allow the cold stored be actually used when required.

The purpose of this study was to relate regional variation in litter mass-loss rates (first year) in pine forests to climate across a large, continental-scale area. The variation in mass-loss rate was analyzed using 39 experimental sites spanning climatic regions from the subarctic to subtropical and Mediterranean: the latitudinal gradient ranged from 31 °N to 70 °N and may represent the the largest geographical area that has ever been sampled and observed for the purpose of studying biogeochemical processes. Because of unified site design and uniform laboratory procedures, data from all sites were directly comparable and permitted a determination of the relative influence of climate versus substrate quality viewed from the perspective of broad regional scales. Simple correlation applied to the entire data set indicated that annual actual evapotranspiration (AET) should be the leading climatic constraint on mass-loss rates (Radj2 = 0.496). The combination of AET, average July temp. and average annual temp. could explain about 70% of the sites' variability on litter mass-loss. In an analysis of 23 Scots pine sites north of the Alps and Carpatians AET alone could account for about 65% of the variation and the addition of a substrate-quality variable was sufficiently significant to be used in a model. The influence of litter quality was introduced into a model, using data from 11 sites at which litter of different quality had been incubated. These sites are found in Germany, the Netherlands, Sweden and Finland. At any one site most ( ≫ 90%) of the variation in mass-loss rates could be explained by one of the litter-quality variables giving concentration of nitrogen, phosphorus or water solubles. However, even when these models included nitrogen or phosphorus even small changes in potential evapotranspiration resulted in large changes in early-phase decay rates. Further regional subdivision of the data set, resulted in a range of strength in the relationship between loss rate and climatic variables, from very weak in Central Europe to strong for the Scandinavian and Atlantic coast sites (Radj2 = 0.912; AET versus litter mass loss). Much of the variation in observed loss rates could be related to continental versus marine/Atlantic influences. Inland locations had mass-loss rates lower than should be expected on the basis of for example AET alone. Attempts to include seasonality variables were not successful. It is clear that either unknown errors and biases, or, unknown variables are causing these regional differences in response to climatic variables. Nevertheless these results show the powerful influence of climate as a control of the broad-scale geography of mass-loss rates and substrate quality at the stand level. Some of these relationships between mass-loss rate and climatic variables are among the highest ever reported, probably because of the care taken to select uniform sites and experimental methods. This suggest that superior, base line maps of predicted mass-loss rates could be produced using climatic data. These models should be useful to predict the changing equilibrium litter dynamics resulting from climatic change.

Abstract This present paper is focused on the study of high temperature thermal energy storage (TES) using phase change materials (PCM) to be applied on cooling and refrigeration systems by solar cooling. Thus, a pilot plant with a working temperature range between 150 and 200 °C was designed and built at the University of Lleida (Spain). Hydroquinone was selected for the specific application from different PCM candidates as the most suitable material after a literature review and a differential scanning calorimetry (DSC) analysis. This PCM has a phase change temperature range between 166 °C and 173 °C and a melting enthalpy of 225 kJ kg −1 . Two storage configurations were evaluated using the same PCM to have preliminary results before the final storage tank design. From the pilot plant results and experience, a 5 Tn PCM storage tank was designed and built to work in a real solar cooling installation in Seville (Spain).

Abstract The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.

Solar concentration technology is fully dispatchable due to the use of thermal energy storage systems, in particular, commercial parabolic trough power plants use indirect storage systems based on molten salts. This system includes a heat exchanger to transfer energy between the thermal oil from the solar field and the salts used in storage. In addition, some content of water is considered by cross-contamination in the steam generator of the power block. This study analyses the compatibility between both fluids in the event of cross-contamination in the equipment. The degree of compatibility between a triphasic system made up of a mixture of sodium nitrate and potassium nitrate salts (60:40), heat transfer fluid (DP:DPO), and water, when they were subjected to 390 ºC for a period of 312 h (two weeks). The study shows how thermal oil undergoes complete thermal-oxidative degradation without high impact on the composition of the salts. This work was partially funded by the Ministerio de Ciencia e Innovación de España (ITC-20111061-FEDER- Innterconecta- Composol) (PID2021-123511OB-C31 - MCIU/AEI/FEDER, UE), by the Ministerio de Ciencia e Innovación (TED2021-132216A-100), and by the Ministerio de Ciencia, Innovación y Universidades de España - Agencia Estatal de Investigación (AEI) (PID2021-123511OB-C31 - MCIN/AEI/10.13039/501100011033/FEDER, UE). The authors at University of Lleida would like to thank the Catalan Government for the quality accreditation given to her research group (2021 SGR 01615). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia

Abstract A new type of ventilated facade (VF) with macro-encapsulated phase change material (PCM) in its air cavity is presented in this paper. The thermal performance of this special building envelope is experimentally tested to analyze its potential in reducing the cooling demand during the summer season in the Continental Mediterranean climate. Two identical house-like cubicles located in Puigverd de Lleida (Spain) were monitored during summer 2012, and in one of them, a ventilated facade with PCM was located in the south wall. Six automatized gates were installed at the different openings of the channel in order to control the operational mode of the facade. This versatility allows the system to be used as a cold storage unit, as an overheating protection system or as a night free cooling application. The experimental results point out the night free cooling effect as the most promising operational sequence to reduce the cooling load of the cubicle. On the other hand, the thermal resistance of the outer skin of the facade must be increased; otherwise the cold storage system cannot be used efficiently.

Abstract Refrigeration industry is adopting a proactive strategy to phase out fluorinated greenhouse gases by more sustainable working fluids. R744 is a natural refrigerant widely proposed for commercial refrigeration. Its use in cascade and booster cycles allows a combined cooling and freezing production. However, when single-stage evaporation at low temperature is required, the adoption of R744 in transcritical cycles is scarce. The main reasons are due to the low Coefficient of Performance (COP) achieved, as well as the technical limitations to reach extreme pressure ratios using commercial compressors. In light of this, this paper proposes to use the CO2 Ejector-Expansion Refrigeration Cycle (EERC) to overcome these drawbacks. To assess the feasibility of the proposal, a thermoeconomic optimization is conducted for low-temperature refrigeration in warm climates. The analysis has been conducted considering a two-phase flow ejector, a commercial double-stage compressor, and evaporating conditions ranging from −10 °C to −38.11 °C, which was revealed the minimum temperature to avoid the triple point inside the ejector. The results showed that the EERC allows using smaller commercial compressors within a broader operating envelope, improving the annual average COP about 5.5% compared to the reference cycle, besides reducing investment and yearly energy consumption costs.

Concentrating Solar Power (CSP) is a proven and mature technology for energy supply. In recent years, electricity generation based on this technology has increased worldwide, to a large degree due to its high dispatchability when coupled with an energy storage system. Commercially, most plants store the energy in molten salts in a two-tank configuration. This study focuses on this storage configuration and proposes a mathematical model for the thermal losses in these tanks, both at nominal conditions and during transients. With this model at its core, a computational tool for thermal performance analysis in OpenModelica is developed. This dynamic thermal model includes the estimation of local heat loss due to assembly defects, which are heat flows that cannot be determined by theoretical modelling. The development of a semi-empirical correlation for estimating local heat loss is also presented. Simulation results showed that this local heat loss may represent a share about 40% of the total heat loss in a small-scale tank. A comparison of the characteristics of the model proposed in a previous work is also presented to establish the innovation of the model. Two thermal storage systems with different tank designs and sizes were simulated to compare the results using the present model with data available in the literature. Results show good agreement in transient thermal behaviour of heat flows, temperatures, and cooling rates when compared with data from other authors for the same tank. This project was funded by the European Union's Horizon 2020 Research and Innovation Programme under No. 838311 (CSP plus). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREiA (2021 SGR 01615). GREiA is a certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work was partially funded by the Ministerio de Ciencia e Innovación - Agencia Estatal de Investigación (PID2021-123511OB-C31 - MCIN/AEI/10.13039/501100011033/FEDER, EU, TED2021-132216A-I00 and RED2022-134219-T). This work is partially supported by ICREA under the ICREA Academia programme.