Dynamic simulation and exergetic optimization of a Concentrating Photovoltaic/ Thermal (CPVT) system

The development of a dynamic, theoretical model suitable for the prediction of the long-term performance of a parabolic-trough Concentrating Photovoltaic/Thermal CPVT system is discussed in the present study. The formulation of the mathematical model and the considered geometrical and operational parameters of the system, such as the characteristics of the employed PV modules and active cooling system are described in detail. The effect of heat capacity is taken into consideration in the thermal balances and thus the model is able to capture the transient behavior of the system. Besides, the model is validated using available experimental data of a manufactured prototype CPVT system. The daily performance of system is predicted for different values of the cooling fluid flow rate and temperature under various environmental conditions. At a second stage, an exergy analysis is conducted in order to point out the effect of the characteristics of the main system sub-components on the exergetic efficiency and exergy output of the CPVT system. It was established that the system exergetic performance is primarily influenced by the optical quality of the parabolic trough and the electrical efficiency of the PV module. Increasing these two factors to achievable values, e.g. ηopt = 0.75 and ηel = 0.25, can yield an increase of the system exergetic efficiency from 12% to 24%

Design and optimization of a micro heat sink for concentrating photovoltaic/thermal (CPVT) systems

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.An optimization methodology for a microchannel heat sink suitable for the cooling of a parabolic trough CPVT system is presented in this study. Two different microchannel configurations are considered, Fixed (FWμ) and stepwise Variable-Width (VWμ) microchannels respectively. The performance evaluation criteria comprise the thermal resistance of the heat sink and the cooling medium pressure drop through the heat sink. Initially, the effect of the geometric parameters on the heat sink thermal and hydrodynamic performance is investigated using a thermal resistance model in order to save computational time. The results of the 1-D model enable the construction of surrogate functions for the thermal resistance and the pressure drop of the heat sink, which are considered as the objective functions for the multiobjective optimization process that leads to the optimal geometric parameters. In a second step, a 3-D numerical model of fluid flow and conjugate heat transfer in the optimized FWμ heat sink is developed in order to investigate in detail the flow and thermal phenomena. The overall analysis demonstrates that microchannel heat sinks achieve very low values of thermal resistance and that the use of variable-width channels can significantly reduce the pressure drop of the cooling fluid. Furthermore, it is proven that the 1-D model is capable of providing a good estimate of the behavior of the heat sink

Multi-objective design optimization of a micro heat sink for Concentrating Photovoltaic/Thermal (CPVT) systems using a genetic algorithm

An optimization methodology for a microchannel, plate-fin heat sink suitable for the cooling of a linear parabolic trough Concentrating Photovoltaic/Thermal (CPVT) system is applied in this study. Two different microchannel configurations are considered, Fixed (FWμ) and stepwise Variable-Width (VWμ) microchannels respectively. The performance evaluation criteria comprise the thermal resistance of the heat sink and the cooling medium pressure drop through the heat sink. Initially, the effect of the geometric parameters on the heat sink thermal and hydrodynamic performance is investigated using a thermal resistance model and analytical correlations, in order to save computational time. The results of the 1-D model enable the construction of surrogate functions for the thermal resistance and the pressure drop of the heat sink, which are considered as the objective functions for the multi-objective optimization through a genetic algorithm that leads to the optimal geometric parameters. In a second step, a 3-D numerical model of fluid flow and conjugate heat transfer for the optimized FWμ heat sink is developed in order to investigate in detail the flow and thermal processes. The overall analysis demonstrates that microchannel heat sinks achieve very low values of thermal resistance and that the use of variable-width channels can significantly reduce the pressure drop of the cooling fluid. Furthermore, it is proven that the 1-D model is capable of providing a good estimate of the behavior of the heat sink

Effect of secondary flows due to buoyancy and contraction on heat transfer in a two-section plate-fin heat sink

The effect of buoyancy forces on laminar heat transfer inside a variable width plate-fin heat sink is numerically analyzed: the configuration under investigation comprises an array of rectangular fins, the number of which is doubled at the streamwise middle length of the plate, leading to a stepwise reduction in the respective channel width and hydraulic diameter. The mixed convection problem is thoroughly examined for Archimedes numbers in the range Ar = 1.32-5.82 and Reynolds numbers, based on the channel hydraulic diameter before the stepwise reduction, in the range Re = 559-667, under the thermal boundary condition of axially constant heat flux. It is illustrated that the secondary flow pattern emanating from the flow contraction and manifested through the presence of a pair of counter-rotating horseshoe vortices and a pair of counter-rotating (fin) sidewall vortices interacts with longitudinal rolls created by buoyancy forces. In fact, the lower horseshoe vortices that are co-rotating with the buoyancy-induced rolls are significantly enhanced in magnitude and cause intense fluid mixing in the vicinity of the channel bottom wall, with a substantial distortion of the temperature field. The numerical results indicate that the joint action of the buoyancy-induced rolls and the combined secondary flow pattern has a beneficial impact on the heat sink thermal performance, a fact quantified through the circumferentially-averaged local Nusselt number distributions. The effect of the top lid thermal conductivity on the heat transfer inside the heat sink is also discussed. Finally, a comparative investigation is conducted between the present variable-channel-width configuration and two configurations of fixed-width heat sink designs. The comparative results reveal that the introduction of stepwise channels leads to superior heat transfer performance, i.e. lower values of the total thermal resistance with mitigated pressure drop penalty and increased temperature uniformity on the cooled surface

Design and experimental evaluation of a parabolic-trough concentrating photovoltaic/thermal (CPVT) system with high-efficiency cooling

The design and performance evaluation of a novel parabolic-trough concentrating photovoltaic/thermal (CPVT) system are discussed in the present study. Initially, the system design and manufacturing procedures as well as the characteristics of the system sub-components are thoroughly illustrated. At a second stage, the findings in regard to the optical quality of the parabolic trough are presented, as obtained through an experimental procedure that utilizes a custom-made measuring device. The device bears a grid of sensors (photodiodes), so that the irradiation distribution on the receiver surface and the achieved concentration ratio can be determined. Besides, the main factors that have a significant effect on the trough optical quality were identified through ray-tracing simulations. The system electrical and thermal performance was subsequently evaluated in a test rig specially developed for that reason. Three variations of the system receiver incorporating different PV-module and heat-sink designs were evaluated and the prototype CPVT system was found to achieve an overall efficiency approximately equal to 50% (44% thermal and 6% electrical efficiencies, respectively) mainly limited by the trough optical quality, however with a very weak dependency on the operating temperature

Flow and Temperature Fields in Cooling Devices with Embedded Serpentine Tubes

The turbulent flow (Re = 5124) and conjugate heat transfer in heat-sink designs of the tube-on-plate type are numerically investigated. The cooling configurations employ a serpentine tube partially (or fully) embedded inside the plate. Two-and four-pass configurations are investigated. A constant heat flux is applied at the bottom surface of the heat-sink plate. The SST k-ω model is used for the prediction of the turbulent flow and heat transfer. Two pairs of longitudinal vortices, as well as secondary flow separation, have been found to set in at the tube curved section. The combined secondary flow pattern enhances heat transfer at the tube sections over a considerable distance downstream of the 180° bends. In the last part of the analysis, the overall performance of the two configurations is compared using a number of evaluation criteria suitable for heat exchanging devices. The four-pass configuration with fully embedded tubing exhibits the best thermal (energetic) and exergetic performance

Three-dimensional flow effects on forced convection heat transfer in a channel with stepwise-varying width

A numerical investigation based on the finite volume methodology of the three-dimensional laminar flow and conjugate heat transfer inside a plate-fin heat sink with stepwise-varying channel width is presented. Results are obtained for the three-dimensional flow developed in the vicinity of the flow contraction as manifested by the existence of two pairs of contra-rotating longitudinal vortices, one at the corners of the fin leading edge and the other at the flow separated regions. The topology of the flow is thoroughly examined and the contributions of the endwalls and the front surface of the flow-contracting fin to the mechanisms that cause the emergence of the three-dimensionality are determined. It is established that the corner longitudinal vortices are in fact horseshoe vortices, which emerge due to the deceleration effect of the fin front surface on the oncoming fluid. Furthermore, the interaction between the corner longitudinal vortices and the downstream recirculation bubble formed over the fin tip is also elucidated. The analysis covers a wide range of flow conditions within the laminar region in order to monitor the effect of the Reynolds number on the flow topology and to verify that the flow remains symmetrical in this region. From the numerical results, it is deduced that the recirculation downstream of the flow contraction has a beneficial impact on the heat sink thermal performance, a fact quantified through the local Nusselt number distributions

Outcomes of the SARS-CoV-2 omicron (B.1.1.529) variant outbreak among vaccinated and unvaccinated patients with cancer in Europe: results from the retrospective, multicentre, OnCovid registry study

BACKGROUND: The omicron (B.1.1.529) variant of SARS-CoV-2 is highly transmissible and escapes vaccine-induced immunity. We aimed to describe outcomes due to COVID-19 during the omicron outbreak compared with the prevaccination period and alpha (B.1.1.7) and delta (B.1.617.2) waves in patients with cancer in Europe. METHODS: In this retrospective analysis of the multicentre OnCovid Registry study, we recruited patients aged 18 years or older with laboratory-confirmed diagnosis of SARS-CoV-2, who had a history of solid or haematological malignancy that was either active or in remission. Patient were recruited from 37 oncology centres from UK, Italy, Spain, France, Belgium, and Germany. Participants were followed up from COVID-19 diagnosis until death or loss to follow-up, while being treated as per standard of care. For this analysis, we excluded data from centres that did not actively enter new data after March 1, 2021 (in France, Germany, and Belgium). We compared measures of COVID-19 morbidity, which were complications from COVID-19, hospitalisation due to COVID-19, and requirement of supplemental oxygen and COVID-19-specific therapies, and COVID-19 mortality across three time periods designated as the prevaccination (Feb 27 to Nov 30, 2020), alpha-delta (Dec 1, 2020, to Dec 14, 2021), and omicron (Dec 15, 2021, to Jan 31, 2022) phases. We assessed all-cause case-fatality rates at 14 days and 28 days after diagnosis of COVID-19 overall and in unvaccinated and fully vaccinated patients and in those who received a booster dose, after adjusting for country of origin, sex, age, comorbidities, tumour type, stage, and status, and receipt of systemic anti-cancer therapy. This study is registered with ClinicalTrials.gov, NCT04393974, and is ongoing. FINDINGS: As of Feb 4, 2022 (database lock), the registry included 3820 patients who had been diagnosed with COVID-19 between Feb 27, 2020, and Jan 31, 2022. 3473 patients were eligible for inclusion (1640 [47·4%] were women and 1822 [52·6%] were men, with a median age of 68 years [IQR 57–77]). 2033 (58·5%) of 3473 were diagnosed during the prevaccination phase, 1075 (31·0%) during the alpha-delta phase, and 365 (10·5%) during the omicron phase. Among patients diagnosed during the omicron phase, 113 (33·3%) of 339 were fully vaccinated and 165 (48·7%) were boosted, whereas among those diagnosed during the alpha-delta phase, 152 (16·6%) of 915 were fully vaccinated and 21 (2·3%) were boosted. Compared with patients diagnosed during the prevaccination period, those who were diagnosed during the omicron phase had lower case-fatality rates at 14 days (adjusted odds ratio [OR] 0·32 [95% CI 0·19–0·61) and 28 days (0·34 [0·16–0·79]), complications due to COVID-19 (0·26 [0·17–0·46]), and hospitalisation due to COVID-19 (0·17 [0·09–0·32]), and had less requirements for COVID-19-specific therapy (0·22 [0·15–0·34]) and oxygen therapy (0·24 [0·14–0·43]) than did those diagnosed during the alpha-delta phase. Unvaccinated patients diagnosed during the omicron phase had similar crude case-fatality rates at 14 days (ten [25%] of 40 patients vs 114 [17%] of 656) and at 28 days (11 [27%] of 40 vs 184 [28%] of 656) and similar rates of hospitalisation due to COVID-19 (18 [43%] of 42 vs 266 [41%] of 652) and complications from COVID-19 (13 [31%] of 42 vs 237 [36%] of 659) as those diagnosed during the alpha-delta phase. INTERPRETATION: Despite time-dependent improvements in outcomes reported in the omicron phase compared with the earlier phases of the pandemic, patients with cancer remain highly susceptible to SARS-CoV-2 if they are not vaccinated against SARS-CoV-2. Our findings support universal vaccination of patients with cancer as a protective measure against morbidity and mortality from COVID-19. FUNDING: National Institute for Health and Care Research Imperial Biomedical Research Centre and the Cancer Treatment and Research Trust

Characteristics of Different Systems for the Solar Drying of Crops

Solar dryers are used to enable the preservation of agricultural crops, food processing industries for dehydration of fruits and vegetables, fish and meat drying, dairy industries for production of milk powder, seasoning of wood and timber, textile industries for drying of textile materials. The fundamental concepts and contexts of their use to dry crops is discussed in the chapter. It is shown that solar drying is the outcome of complex interactions particular between the intensity and duration of solar energy, the prevailing ambient relative humidity and temperature, the characteristics of the particular crop and its pre-preparation and the design and operation of the solar dryer