ALBERT

Description: Temperature fields and their transient behaviors are essential subjects to be considered for modeling and design of absorber tubes in concentrated solar power plants. Both subjects have been addressed by various authors. However, the first subject has been primarily solved in the steady state. While the second has been solved by considering transient variations in the environmental or operating conditions, but with a heat conduction model in steady state. To the best of our knowledge, there are no analytical transient two-dimensional (2D) (r, φ) solutions involving nonuniform heat flux distribution (NUHFD) on the absorber tube of a parabolic trough solar collector (PTC). This study aims to obtain an analytical solution for the transient heat conduction in 2D of the absorber tube. The analytical solution was obtained using the method of separation of variables and the superposition principle. Two NUHFD functions were analyzed: a step function and a local concentration ratio (LCR) function. To the first function, the effect of the inlet fluid temperature and efficiency were also studied. The results agree with experimental and numerical results from the literature. The maximum average root-mean-square was near 6.4% for the step function, while the maximum average error was 1% for LCR function. The theoretical energy balances corroborate the validity of the analytical solution. The analytical solution could be useful to compare other theoretical studies (e.g., to prove new numerical schemes), to simulate other parameters of design, and to calibrate experimental tests. Even this work could be extended for nonlinear boundary conditions.

Description: An experimental investigation was conducted to demonstrate the effects of materials on the heat transfer characteristics of R410A during evaporation and condensation inside two horizontal plain tubes with the same inner diameter of 6 mm, but with two different materials of aluminum and stainless steel. The variation of vapor quality for the test section was kept in the range of 0.2–0.9, while mass velocities were allowed to vary from 100 to 400 kg/m2/s1. First, a series of single-phase and repetitive experiments were conducted to verify the accuracy and reliability of the test rig. Results of the evaporation experiments show that the plain aluminum tube performs best for all tested mass velocities. Several different correlations were employed to predict the present data, and their predictive ability was compared and discussed. Results indicate that the Liu and Winterton correlation could accurately predict the present results except for low mass velocities. Roughness effects were accounted for employing a correction factor. The larger roughness of the stainless steel tube was supposed to make the stainless steel tube perform better if roughness effects were accounted for, so the better performance of the aluminum tube was mainly attributed to the material effects. The pool boiling heat transfer as predicted by the VDI model was compared with the experimental results, and more obvious material effects have been found for pool boiling conditions. The minor differences between the two tubes in this case may be explained by the nucleate boiling suppression and incomplete wetting. For the condensation experiments, little difference was found between the two tested tubes, which means that the material and roughness effects may have had little influence on the thermal performance during condensation.

Description: The abundant spatial and temporal availability of solar energy has been fueling many researches and have been the reason for the proliferation of solar energy applications in the past decades. Many of these applications involve heavy investments and thus require highly accurate and reliable long-term average solar data for efficient deployment of solar energy technologies. Since ground stations are costly, site-specific, scarce and cannot provide long-term solar data, satellite-derived data is the next best alternative. However, satellite models are often unable to capture the complex local climatological variations of a given site. As such, short-term high precision solar ground measurements are used to train the satellite model so as to improve the accuracy of long-term solar estimates. There exist several site adaptation techniques to perform this task. However, to the knowledge of the researchers, no comparative study has been conducted to establish which site adaptation technique is the most effective. In this study, a robust methodology has been proposed to compare the effectiveness of four site adaptation techniques for monthly and yearly data sets using novel key performance indicators. Ground measurements from 12 stations in the tropical islands of Mauritius, Rodrigues, and Agalega were used to adapt satellite data obtained from HelioClim-3 database using different techniques. Three new nonlinear site adaptation techniques have been proposed: adjustment technique (Technique 2), compensation technique (Technique 3), and relationship technique (Technique 4). The first part of the study showed that 67–100% of the data sets were best approximated with sixth-order polynomials for the three nonlinear techniques. The second part revealed that Technique 1 (linear method) and Technique 2 were most appropriate for maximum and average data sets, respectively. The results were such that Technique 2 and Technique 1 provided best approximations for77.9–83.3% and 40.7–58.3% of average and maximum data sets, respectively. In the third part of the study, only Technique 2 provided remarkable improvements for all statistical metrics with respect to the original monthly data sets (113–118 data sets). The analysis reported 57.6–89.9%, 49.8–68.0%, 67.4–87.3%, 53.8–63.1%, 45.0–64.0%, 7.7–9.6% and 2.7–4.7% mean improvements for mean bias error (MBE), mean absolute bias error (MABE), mean percentage error (MPE), mean absolute percentage error (MAPE), root-mean-square error (RMSE), Nash–Sutcliffe (NSE), and coefficient of determination (COD), respectively, for Technique 2. Similar results were observed for yearly average data sets while the appreciation was shared among all four techniques for yearly maximum data sets, with Technique 1 having a slight advantage.

Description: Satellite-based solar power data is becoming more and more important because of its continuous temporal and spatial availability. However, its reliability can be enhanced through quality control and calibration against ground-based measurement data. Here, a holistic methodology is employed for the adaptation of satellite-based data for estimating solar energy. For the purpose, high-quality ground-based measurement data and satellite-based datasets are assessed across 12 sites in three small islands located in the Indian Ocean. Initially, both datasets go through a rigorous quality control process. A quantitative analysis of irradiance and insolation data is then conducted. Eventually, site adaptation of satellite-based data is performed using bias removal technique and statistical analysis of datasets. A set of seven statistical performance indicators is used to support the assessment. Analysis of datasets shows that adaptation of peak values should be performed separately. Results showed that despite the small surface areas of the islands studied, a spatial variation of insolation can be depicted. A temporal variation of insolation is also noted with a peak in the summer and low insolation levels in winter. Peak irradiance values tend to exceed solar constant for all sites. Variations of peak irradiance can only be noticed in ground-based measurement data. While insolation levels are comparable in the summer season for all the sites, insolation levels in the winter season are higher in the sites with lower latitudes. Calibration factors for peak irradiance, monthly and annual average irradiance as well as yearly insolation are presented.

Description: A performance assessment was conducted for a solar–biogas hybrid micro gas turbine integrated with a solar power tower technology. The considered system is a solar central receiver integrated with a micro gas turbine hybrid with biogas fuel as a backup. The Brayton cycle is designed to receive a dual integrated heat source input that works alternatively to keep the heat input to the system continuous. The study considered several key performance parameters including meteorological condition effects, recuperator existence and effectiveness, solar share, and gas turbine components performance. This study shows a significant reduction in CO2 emissions due to the utilization and hybridization of the renewable energies, solar, and biogas. The study reveals that the solar–biogas hybrid micro gas turbine for 100-kW power production has a CO2 emission less than a conventional fossil fuel gas turbine. Finally, the study shows that the method of power generation hybridization for solar and biogas gas turbines is a promising technique that leads to fuel-savings and lower CO2 emissions.

Description: A novel multi-generation system (MGS) that comprises two absorption cycles, two Rankine cycles (RCs), and a hot water (HW) production chamber is studied in this research. It is designed to utilize the waste heat from the first Rankine cycle as a thermal energy input for the second Rankine cycle and a double-effect absorption cycle (DEAC). The waste heat from the second Rankine cycle serves as heat input to a single-effect Rankine cycle. Regeneration and reheat principles are also applied to the Rankine cycles. The objective of the study is to develop an MGS without a gas cycle that can achieve higher energy and exergy efficiencies. Two concentrated solar technologies, namely, parabolic trough collectors (PTCs) and heliostats are used to power the designed system. The environmental benefit of the system is also analyzed. The energy and exergy efficiencies of the novel MGS presented in this study are 73.11% and 50.72%, respectively. The application of solar thermal technologies to power the system reduces the overall energy and exergy efficiencies, respectively, to 56.12% and 38.39% for the solar PTC and 41.89% and 29.06% for heliostats. The energy and exergy coefficient of performances (COPs) are 0.754 and 0.349 for the single-effect absorption cycle (SEAC), respectively. As much as 752.7 kg/h of CO2, 2.13 kg/h of NOx, and 4.21 kg/h of SOx will be saved from being emitted to the atmosphere.

Description: The structural and architectural elements of building-integrated photovoltaic-thermal (BIPVT) systems are made up of photovoltaic (PV) modules and these are required to be fixed at an optimum inclination angle for generating maximum exergy. This work presents an attempt to determine the amount of exergy generated by an optimally inclined double-storied BIPV thermal system by considering the actual cyclic nature of insolation, surrounding air temperature, PV cell temperature, intermediate slab temperature, and the chamber temperature. The insolation value, which is computed by an anisotropic sky model along with these cyclic variables, is used for solving the set of governing differential equations for evaluating the exergy of the system. Other influencing parameters of the BIPV thermal systems such as air changes in both chambers, packing factor of PV module, the orientation of PV module, and thickness of the intermediate slab are considered for finding its effect on the total exergy of the system. Numerical results show that for packing factor more than 0.6, there is no significant change in total heat exergy with respect to the inclination angle. For packing factor more than 0.3, the generation of electrical exergy exceeds the heat exergy, and the overall exergy of BIPVT system decreases with rise in packing factor (βm) up to 0.3 and then rises nonlinearly.

Description: The interest of the direct normal irradiation (DNI) estimation is important in the evaluation of the solar potential and, consequently, for data correction and expansion of the historical series. In this study, a review of the performance of 16 models of radiative transfer was performed. These models are used to estimate DNI on a clear day in Botucatu/SP region located in Brazil. The revised models are categorized into two classes: simple models (11 models: ASH, MAJ, ALLEN, GH, P1, HLJ, FU, KU, H1, IP, and INC) and complex models (five models: BIRD, IQ, MRM5, P2, and YANG). The evaluation methodology used here is consistent with the literature. The input parameters were estimated and a statistical analysis using relative-mean-bias-error (rMBE), root-mean-square-error (rRMSE), and mean absolute percentage error (MAPE) indicators were performed to validate those models. The results indicate that the complex models (that require more atmospheric inputs) generally performed better than simpler models. Despite the consistent limitations in the use of estimated parameters, the performance of the models can be considered satisfactory. The best performances are highlighted for models MRM5 and YANG. Simple models ASH and IP performed similar to complex models. These results were confirmed using frequency distribution and the cumulative frequency analysis. These results are important for engineers of solar systems to use the best model and select the most suitable locations for installing a small or large solar plant.

Description: A method for optimizing the geometrical layout for a façade-mounted solar photovoltaic array is presented. Unlike conventional studies, this work takes into account the finite height of the façade, which is more realistic. The proposed analytical relationships and optimization routine evaluate the best tilt angle and the number of panels such that the whole layout receives the maximum solar radiation, year-round. This is achieved while ensuring that the panels are at a safe minimum distance to avoid mutual shading issues. Validation was performed by simulating the scenarios and comparing the results with manual measurements taken in a three-dimensional drafting program. The method was then used to evaluate designs for facades with a variety of orientations, hypothetically located in Auckland, New Zealand. For this case study, the per-panel and total year-round energy accumulation associated with the number of panels were determined. The results showed that more panels can be integrated into constrained fields by sacrificing the year-round best value of the tilt angle. Therefore, increasing the number of panels may decrease the energy accumulation performance.

Description: This paper analyzes the direct solar vapor generation of acetone by solar radiation falling on the heat pipes of an evacuated tube collector (ETC) that can activate a domestic scale organic Rankine cycle (ORC). The irradiance from the sun determines the mass flow of acetone along the horizontal manifold of the ETC to produce vapor at the collector outlet. A scilab code is developed to simulate the flow of acetone inside the manifold where subcooled acetone undergoes heating and evaporation process. Simulation is run from 60 °C to a saturation temperature of 120 °C at a pressure of 604 kPa, vapor qualities from 1% to 100%, and solar radiation from 300 to 1100 W/m2. The Kattan–Thome–Favrat flow boiling model is used to obtain the two-phase local heat transfer coefficients along the horizontal manifold, and it is validated with the numerical and experimental values of ammonia. The ORC system can generate 218 kWh/year of electrical energy, a thermal power capacity of 1616 kWh/year and achieve an ORC efficiency of 84.4%. The solar-ORC has a thermal efficiency of 3.25% and an exergy efficiency of 21.3% with a solar collector of 2.84 m2.