Rafael Almanza

Direct steam generation in parabolic trough concentrators with bimetallic receivers

This document shows the results obtained in direct steam generation for low powers in parabolic trough concentrators with bimetallic Cu–Fe wall receivers. The objective is to study its thermal behavior under transient conditions and stratified two-phase flow. Experimental results in transient state appear considering the variants of mass flows between 60 and 150 kg/h, the direction of the solar beam irradiance on the receiver is from below and on one side of the receiver, this last being the most critical, in particular in steel receivers because of low thermal conductivity. Some conclusions in this work are that the most significant deformation is in a transient state and it happens only in one part of the receiver, when it changes from the liquid phase to the steam phase during the boiling process. The bimetallic receiver reduced the transient deformation to the degree that it does not seem a problem for some critical parts of the absorber system during the direct steam generation (DSG). Whereas in steel receivers the deformation rises to 7 cm, in the Cu–Fe receivers the most drastic deformation was only of 18 mm upwards with a mass flow of 150 kg/h and wall temperature of 200 oC. So, the use of the bimetallic receiver is recommended for the DSG for low power applications.

DSG Under Two-Phase and Stratified Flow in a Steel Receiver of a Parabolic Trough Collector

The bending of a receiver tube in two-phase flow under stratified conditions when water is first introduced to the hot steel receiver of a 14.5-m long parabolic trough concentrator is presented in this paper Thermal gradients were observed on the absorber wall at the inlet of the receiver tube during the boiling of water, at low mass flow of 1.6 ×10 -5 m 3 /sec (I liter/min), and low pressure (4 ×10 2 kPa). It should be noted that the solar concentrator was focused on the receiver tube, which contained static air before the water was introduced. The introduction of the water produced a change in the temperature difference between the upper and lower sides of the receiver from 40-60 K to much lower temperatures, in about 45 seconds. The bending of the steel receiver tube occurred when the two-phase flow began. Maximum deflection was observed when the thermal gradient reached a minimum value. We conclude that, when the flow of steam, water, and air exist in a stratified pattern, the combination of these three elements produces the bending phenomenon. The theoretical model, developed to evaluate the experimental data, confirms that the change in temperature gradient produces the bending of the steel receiver tube during this transient stage.

Further option for solar concentrators: Aluminum first surface mirrors

Abstract The research and development of aluminum first surface solar mirrors is presented. Two protection films for the aluminum layer are discussed: Si 2 O 3 and SiO 2 . Two electron guns (e-gun) are used to manufacture aluminum first surface solar mirrors. One, for aluminum evaporation, eliminates or minimizes pinholes observed when aluminum is evaporated with tungsten filaments. The other e-gun allows the evaporation of SiO and SiO 2 without the mirror contamination previously seen due to the air when the chamber was opened. Better adherence between the aluminum film and the Si 2 O 3 or SiO 2 is obtained due the use of two electron guns that does not permit the chamber to open. Si 2 O 3 is a material obtained by oxidation of SiO by admitting some oxygen into the evaporation chamber (10 −4 Torr). The optimum thickness of the aluminum layer was 1000 A or higher, about 2500 A for the Si 2 O 3 , and 3200 A for the SiO 2 . The specular reflectance of these mirrors is about 0.89. These mirrors were tested in the environmental chamber for accelerated weathering without any important degradation, making them another option for solar concentrators in solar energy applications.

Energy requirements for a swimming pool through a water-atmosphere energy balance

The methodology displayed here is to calculate the energy requirements for heating a swimming pool to a desired temperature. This methodology consists of an energy balance between water-atmosphere as is used in the temperature evaluation of cooling ponds in power plants. Different mathematical expressions are given to calculate such a balance. It is necessary to know the month of the year, the ambient temperature, relative humidity, wind velocity, and solar radiation. With these parameters it is possible to know the natural temperature of the water, natural evaporation, energy needed to reach a determined swimming pool temperature and the evaporation of the heated pool.

Total solar radiation in Mexico using sunshine hours and meteorological data

An empirical method is used to obtain global radiation in Mexico, with deviations of less than 10%, using meteorological data that has been registered in Mexico for several years. In addition to insolation data, the method requires parameters such as latitude, mean length of day per month, number of rainy days and mean humidity. A cylindrical projection is used to present contour maps of mean total radiation. (SPH)

A survey of solar pond developments

We discuss experimental and theoretical advances on solar ponds. Using a theoretical model that involves internal and external heat- and mass-transfer phenomena, the temperature behavior of the solar pond is shown as a function of time. The temperature is strongly dependent on the thicknesses of the three solar-pond layers, especially the non-convective layer. Clay liners are presented as an alternative for use in solar ponds. For these, the thermal conductivity and vertical permeability are similar to or better than membrane liners. Some applications are discussed such as water desalination, generation of electric power and salt refining and mining.

Development of aluminum first surface mirrors for solar energy applications

Abstract The progress in obtaining first surface mirrors by using Si2O3 and SiO films as the front surface over aluminum films thermally evaporated over soda lime glass substrates is given in this study. The size of the mirrors is 30 × 30 cm with a 3-mm substrate thickness. A glow discharge produced during 20 min (3 kV and 250 mA) was sufficient to clean the substrate. An aluminum film was deposited after this cleaning. Two types of first surface mirrors were formed. One with SiO, and the other one with Si2O3 as the front surface, each one with a thickness of about 3000 A. The reflectances were 0.82 and 0.86, respectively. After one year of exposure to the environment, no degradation was detected, but after two years, a slight amount of corrosion appeared. Some alternatives are being studied in order to minimize or eliminate this corrosion.

Study of a kaolinite clay as a liner for solar ponds

Abstract A technical option of liners for solar ponds is the use of compacted clays. This aspect is under study using a small laboratory experiment that simulates the influence of the lower convective zone over the clay liner. In this experiment it is possible to measure thermal and mechanical properties of the clay as well as the ion exchange characteristics. Properties for CH-type clay (inorganic clay of high plasticity) that exhibited a kaolinite structure are reported here. When fresh hot water was used, a thermal conductivity of the order of 0.88 W/m°C for a temperature range of 50–60°C was measured in the clay; the vertical permeability was on the average 2.2 × 10 −7 cm/s. When hot saturated NaCl brine was used (50°C), the thermal conductivity of the clay became 0.77 W/m°C when the effluent coming from the model, after filtering through the clay, had a salinity value between 17 and 31% that of saturation, after about one month under the hot brine. After six months under the hot brine at the same conditions, the salinity in the effluent became 60% that of saturation and the thermal conductivity was 0.57 W/m°C. The vertical permeability at the end of the experiment was reduced to 1.4 × 10 −7 cm/s. The cations and anions identified in the effluents were Na + , Cl − , Ca ++ , Mg ++ and SO 4 − . As a conclusion it is possible to affirm that the thermal conductivity and the permeability of this type of clay are reduced by the influence of a hot brine during long periods of exposure, and it can be used as liner.

Water remediation by UV–vis/H2O2 process, photo-Fenton-like oxidation, and zeolite ZSM5

AbstractThe present work shows the application of the UV–vis/H2O2 process, Fenton-and photo Fenton-like reactions in presence of zeolite material ZSM5 in the degradation of a pharmaceutical pollutant recently found in environmental aquatic systems, the anti-inflammatory drug naproxen (NPX). Three commercial iron oxide powders (hydrated hematite, hematite, and magnetite) for Fenton-like reaction were tested against 10 mg L−1 of NPX. An UV–vis simulator source was employed in order to contrast the performance of the photo Fenton-like reaction during 120 min of irradiated conditions in a no-buffered almost neutral pH 6.5 ± 0.5. ZSM5 zeolite was tested as adsorbed material in mechanical mixtures with or without iron oxide powders in order to improve the physical removal along with the oxidative stress. Effects of non-photocatalytic control test (such as UV–vis photolysis, H2O2 oxidation, and oxide iron complex formation) were previously evaluated. Negligible effect was evidenced for Fenton-like reaction and a...

Behavior of Bimetallic Absorber in Parabolic Collectors During DSG to Low Powers

Thermal and mechanical behaviors of absorbers of different material for its application in parabolic collectors during the direct steam generation, with low mass flows (90 kg/h to 120 kg/h) and low pressures (9 X 10 2 kPa to 16X 10 2 kPa) were studied. The distribution of temperatures in the absorber periphery of copper, steel, and bimetallic copper-steel were analyzed using a theoretical model with the influence of a stratified two-phase flow in the internal wall of the absorber, as well as heat flux in a section of the external wall; the previous situation led to the construction of the copper-steel absorber and experimentally to a comparison with a steel absorber. In the experimental part, the thermal and mechanical analysis of the three absorbers is included, emphasizing the thickness of the wall and the thermal conductivity of each material, under the same thermohydraulic conditions. The theoretical and experimental work is analyzed for solar beam irradiance arriving from below the absorber for an east-west orientation.