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 theoretical analysis is presented for the performance study of a Latent Heat Thermal Storage (LHTS) system that contains a phase change material (PCM) dispersed with high conductivity particles. The effect of fraction of dispersed particles in the PCM on energy storage time and heat flux is presented for laminar and turbulent flows, and also analytical expressions are presented for various quantities of interest to study the energy storage capabilities. The combined effect of thermal and flow properties of both the heat transfer fluid (HTF) and the PCM-mixture is also included in the study. It is observed that there exists an optimum fraction of particles to be dispersed in the PCM for maximum energy storage/extraction.

Description: Two sets of experimental data for cylindrical canisters with thermal energy storage applications were examined in this paper: 1) Ground Experiments and 2) Space Experiments. A 2-D computational model was developed for unsteady heat transfer (conduction and radiation) with phase-change. The radiation heat transfer employed a finite volume method. The following was found in this study: 1) Ground Experiments, the convection heat transfer is equally important to that of the radiation heat transfer; Radiation heat transfer in the liquid is found to be more significant than that in the void; Including the radiation heat transfer in the liquid resulted in lower temperatures (about 15 K) and increased the melting time (about 10 min.); Generally, most of the heat flow takes place in the radial direction. 2) Space Experiments, Radiation heat transfer in the void is found to be more significant than that in the liquid (exactly the opposite to the Ground Experiments); Accordingly, the location and size of the void affects the performance considerably; Including the radiation heat transfer in the void resulted in lower temperatures (about 40 K). [S0199-6231(00)00304-X]

Description: The set of optical models that is implemented in ray-tracing software determines the accuracy of its output. A sensitivity analysis was carried out using a powerful in-house program, which provides a large number of surface reflectance and scattering models and in addition, can also run spectral simulations. A linear Fresnel collector was selected as a test case together with the most accurate data that can be found in the literature for the optical properties of its components. The test results indicate that simulations based on constant values, such as mostly provided by the manufacturer, are generally inaccurate and a spectral simulation is not essential for thermal applications.

Description: Research work on inter-zone convective heat transfer in buildings is reviewed and the parameters that govern this process are discussed. A limited comparison between the heat transfer rates calculated by existing correlations and those obtained in a full-scale test facility is presented. A number of research areas where more work is needed are identified.

Description: The flux distribution delivered by a Fresnel lens when concentrating solar energy is characterized. The flux is measured with a Si photo-detector equipped with an integrating sphere. Flux mapping is performed by scanning lines at discrete positions on one plane. The peak concentration is determined as well as the distribution of the flux density in 3-D inside the focal area. Future utilization of this Fresnel lens for solar processing and surface modifications of materials is discussed. The analysis is made on the basis of the optical characteristics of the device and of the results of previous works in the same field. The size of the focus and the peak flux density are key parameters for examining the candidate processing and for discussing the dimensions of the treated components.