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SOLAR SCENERY

J. Sol. Energy Eng. 2002;124(3):205. doi:10.1115/1.1504094.
Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS

J. Sol. Energy Eng. 2002;124(3):206-214. doi:10.1115/1.1487882.

The working fluid in solar receivers, utilized for effecting chemical reactions, is usually flown through a sealed enclosure provided with a quartz window. When one of the reactants or products of reaction is a powder, care must be taken to prevent contact of the incandescent powder particles with the window, in order to obviate its destruction by overheating. Attempts made in the past to screen the window against particle deposition by a “curtain” of an auxiliary gas stream showed that very substantial flow rates of auxiliary gas (30—80% of the main stream flow rate) were necessary for perfect window screening. The heat absorbed by the auxiliary gas stream represented a major loss of energy. In an effort to reduce the auxiliary stream flow rate to a minimum, a certain flow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory. It enabled effective reactor window screening by an auxiliary gas flow rate less than 5% of the main gas flow rate. The tornado effect is discussed and demonstrated by a smoke flow visualization technique.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):215-222. doi:10.1115/1.1488164.

This paper deals with the material testing under extreme conditions, mainly around space programs, using the French CNRS (Centre National de la Recherche Scientifique) solar facilities for planetary entry (Earth, Mars), solar corona in situ exploration, and cryogenic propulsion. For these purposes, different facilities were developed around the various solar concentrators: MESOX (Moyen d’Essai Solaire d’OXydation), for the study of the atomic oxygen recombination at the surface of heated materials and the oxidation kinetics of ceramics under plasma atmospheres up to 2300 K. (Some results are given for several materials.); MEDIASE (Moyen d’Essai et de DIagnostic en Ambiance Spatiale Extre⁁me), for the thermophysical characterization of thermal shield materials up to 2400°K under high vacuum: mass-loss kinetics, mass spectrometry (neutral and ionic species) and thermo-radiative properties (total or spectral directional emissivity). (Results are presented for various carbon/carbon composite materials.); and DISCO (DISpositif de Caractérisation Optique), for the measurement of changes in the reflectivity of materials at temperatures up to 2500°K and its correlation with the surface behavior (aging, ablation and oxidation). Results concerning a copper alloy used for the combustion chamber of cryogenic motors are given.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):223-229. doi:10.1115/1.1488668.

A method for remote optical measurement of the geometry of nonimaging concentrators is presented. A concentrator installed in a solar tower was measured by observation of transmission patterns from the heliostat field, and comparison of the measured patterns to a ray tracing simulation. The actual geometry of the concentrator was derived from optimization of the match between real and simulated patterns. The measurement was sensitive and accurate enough to detect small errors in the concentrator geometry, such as 1 millimeter in linear dimension and 0.1° in concentrator tilt angle. The measurement procedure is simple and can be easily adapted to a wide range of nonimaging optical systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):230-236. doi:10.1115/1.1488666.

The development and performance of a full-scale carbon particle cloud generator together with the evaluation of nine commercial carbon blacks is reported. Large variations were found in the dispersability and settling properties of the investigated powders. Scanning electron microscope analysis of cloud samples from different powders showed unequal state of agglomeration and particle size. The particle population distribution of the most suitable powder was determined, showing that the particle cloud consisted of 99.8% spheroid primary particles (25–570 nm dia) and 0.2% large irregularly shaped agglomerates. Although the numerical fraction of the agglomerates was only 0.2%, they contributed 40% to the cloud’s geometrical cross section. Significant variations in the population distribution were found from different batches of the same particle powder. The developed full-scale particle generator was capable of sustained operation, creating a particle cloud with an extinction coefficient exceeding 40 m−1 at a nominal flow rate of 25 SLPM. The dispersal efficiency of the system with the optimal ejection nozzle was 25%, compared to less than 1% for free ejection. The particle dispersal rate was 30 g/hr at 25 SLPM at an evacuation efficiency of 87%. Specific extinction cross-sections of 5.8 m2/g were measured for particle mass loading higher than 2 g/m3.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):237-242. doi:10.1115/1.1488665.

This paper deals with a method of performing a parametric sensitivity analysis. Such a study is very important for modellers as it can provide useful information. Indeed, it can point out a model’s weaknesses and allow us to identify the most important parameters in the model, which the modeller must know accurately to provide reliable results. After describing the approach, an application of the method in building thermal simulation is discussed. The study concerns a real test cell and gives coherent results as some of the most influential factors can be physically interpreted.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):243-249. doi:10.1115/1.1488669.

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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):250-255. doi:10.1115/1.1498847.

In a PV cooling duct, heat transfer from the heated side to the cooling air flow takes place partly by convection at the walls and partly by radiation exchange between them. A method is developed for representing these effects in combination, avoiding the uncertainties and iterations involved in treating the two mechanisms as independent and parallel. Though the radiative element introduces two further parameters, the procedure has a straightforward closed form, convenient for routine engineering calculations. An approximation, that treats the radiation exchange as determined by the local wall temperatures, is validated by comparison with published results in which the diffusion due to the axial temperature distribution is fully represented. The method is applicable to both laminar and turbulent flows, employing coefficients already available in the literature. The incorporation of duct heat transfer within thermal models of the PV installation is discussed briefly, highlighting further areas which are being refined by on-going work.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):256-261. doi:10.1115/1.1487886.

Heat transfer from a perforated, sinusoidal plate with suction to air flowing over the plate, perpendicular to the corrugations, has been studied numerically and experimentally. This study used a numerical model, validated by wind tunnel tests and hot wire anemometer/resistance thermometer measurements, to determine the heat loss to the air stream over the plate as a function of wind speed, suction velocity, and plate geometry. Both attached and separated flow regimes were observed, and the criterion for flow attachment was determined to be ReV0,P≥6.93 ReU∞,A0.5. Correlations were developed for heat transfer to the air stream for each flow regime. For attached flow, the heat transfer can be represented as Nuatt=Nuflat{1+0.81(A/P)0.5}. For separated flow, the following correlation applies: Nusep=2.05(A/P)1.40 Re1.63.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):262-267. doi:10.1115/1.1488165.

Standard analyses of solar collector thermal performance are based upon an energy balance in which the environments adjoining the front and rear of the collector are assumed to be at the same temperature. This assumption is inappropriate for some collector designs, particularly building-integrated collectors. An approach for analyzing such situations is presented based upon a new conceptual temperature termed the “equivalent ambient temperature.” The concept is explained, the new temperature is defined, and the approach is applied to a typical collector geometry. The approach retains the convenience of the standard analysis while accounting for the unequal front/rear ambient temperatures and permits collector characterization in terms of the conventional parameters: plate efficiency factor, F; heat removal factor, FR; overall heat loss coefficient, UL, and effective transmittance–absorptance product, (τα)e.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):268-275. doi:10.1115/1.1488166.

An approach based on a new conceptual temperature termed the “equivalent ambient temperature” has been introduced for analyzing solar collector thermal performances when the environments to the front and rear of a collector are at different temperatures. Using a specially-designed solar simulator, experimental work is presented which validates the new approach as applied to a wall-integrated covered profiled metal solar air collector. Using both the new and traditional approaches, collector thermal performances are predicted to reveal the practical conditions for which use of the new approach is warranted. The latter findings will be of importance to designers. Performance characteristics for this collector geometry are also presented for use by designers.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):276-282. doi:10.1115/1.1487885.

This paper investigates the technical feasibility of using a compact, air-cooled, solar-assisted, absorption air conditioning system in Puerto Rico and similar regions. Computer simulations were conducted to evaluate the system’s performance when subjected to dynamic cooling loads. Within the computer model, heat and mass balances are conducted on each component of the system, including the solar collectors, thermal storage tank, the air-cooled condenser, and the air-cooled absorber. Guidance on component design and insight into the effects of such operating factors as ambient air temperature were gained from exercizing the simulation model. Comparisons are made with an absorption air conditioning system that uses a cooling tower instead of air-cooled components. The particular absorption system of study is one that uses lithium bromide and water as the absorbent and refrigerant, respectively. The heat input to the absorption system generator is provided by an array of flat plate collectors that are coupled to a thermal storage tank. Systems having nominal cooling capacities of 10.5, 14, and 17.5 kW were considered. Useful information about the number of collectors needed, storage tank volume, and efficiency of the overall system is presented.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):283-290. doi:10.1115/1.1487883.

Mass recovery can play an important role to better the performance of adsorption refrigeration cycles. Cooling capacity can be significantly increased with mass recovery process. The coefficient of performance (COP) of the activated carbon/ammonia adsorption refrigeration cycle might be increased or decreased with mass recovery process due to different working conditions. The advantage is that its COP is not sensitive to the variation of heat capacity of adsorber metal and condensing and evaporating temperature. The cycle with mass and heat recovery has a relatively high COP.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):291-299. doi:10.1115/1.1498849.

A new, bi-directional thermodiode designed for energy-efficient buildings was constructed and tested. Experimental results are presented and discussed for solar-heating applications. The thermodiode system consisted of a number of rectangular loops filled with water. The tilting angle of the loops can be altered to reverse the direction of natural convection within the loops for bi-directional operations. The horizontal segments of the loops were attached to metallic panels facing indoors or outdoors. The amount of thermal radiation incident on the outdoor-facing surfaces can be adjusted by rotating the panels or by installing a removable shading device in front of the surfaces. Results of the indoor tests for winter use of the diode showed an onset time between 7 to 20 min for natural convection to be induced throughout the loops in the thermodiode. Before the throughflow started, the fluid in the heated copper tubes reached its maximum temperature. A sudden drop and rebound in this temperature was observed immediately after the onset of throughflow. After that, temperatures at different locations on the thermodiode rose at approximately the same rate until a steady state was reached. During the cool-down phase, the temperatures decreased at the same rate without humps, indicating only conduction took place in the rectangular loops when the thermodiode was reverse-biased. A simple analytical model was developed to estimate the temperature variations and heat transfer rates in the diode system. The diode under forward-biased condition increases the heat transfer rate by nearly 100 times for an incident radiation of 600 W/m2 .

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):300-304. doi:10.1115/1.1488667.

A new variable water (VW) flow system for chilled water system application is presented. Design, operational, and control issues are discussed. Analytical models of pump power and evaporative temperature are developed. The VW systems consume considerably less pump and compressor energy than the primary secondary (PS) systems under partial load conditions for most existing chillers. The VW system consumes less compressor energy since it results in higher refrigerant temperature in the evaporator. The VW systems also have the decoupling capability of the primary and secondary pumping systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):305-310. doi:10.1115/1.1498850.

The experimental work described in this paper evaluated the efficacy of dyes as sensitizing agents for solar photochemical detoxification and disinfection of water. Methylene blue and rose bengal were evaluated as photosensitizing agents. While neither dye was effective for detoxification, methylene blue showed some efficacy for photodisinfection over natural sunlight. In all sunlight experiments, methylene blue effected a coliform reduction ranging from 96% at pH 7 and lower methylene blue concentrations, to 99.5% for all concentrations at pH 10 and the 10 mg/L concentration for pH 7. The 99.5% reduction at pH 7 is comparable to 74% reduction at the same pH using sunlight alone. Significant coliform reduction was also observed in dark experiments with 10 mg/L of methylene blue.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):311-314. doi:10.1115/1.1498848.

Thermal and productivity measurements and flow visualization experiences were performed on a real scale module of a basin type solar still, whose geometry and thermal conditions could be changed in a controlled way. The convective stage was studied with the aim of acquiring information about the nature of the medium inside it and the influence of different parameters over the productivity. Literature shows a great number of experimental and numerical works dealing with different aspects of the performance of solar stills: thermal losses, vapor losses, salt deposit on the tray, geometry, thermal inertia, etc. Few works are reported that take into account convective phenomena and the fluiddynamic characteristics of the medium inside the still. Most of these works are based on Dunkle’s and Copper’s models of the still that does not take into account the characteristics of the environment. A new physical model based on these experiments is presented.

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Sol. Energy Eng. 2002;124(3):315-317. doi:10.1115/1.1488163.

The glazing of thermal barrier coatings using concentrated solar energy has already been demonstrated, but crack initiation appeared as a major drawback of this processing. In this paper, the thermo-mechanical effects resulting from the fast heating at high temperature were modeled. Crack initiation was found to result from high temperature gradients in the ceramic coating. Experiments performed on plasma-sprayed ZrO2-Y2O3 layer showed that a significant reduction of crack initiation was achieved when the specimen was preheated at temperatures in the range 500°C–800°C. The potential of the solar processing was improved and this technique becomes competitive with conventional laser processes.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2002;124(3):317-319. doi:10.1115/1.1487884.

Experiments in photocatalysis often involve the use of blacklight UV lamps to simulate the solar UV spectrum under indoor laboratory conditions. solar UV radiometers (such as the Eppley TUVR) which are calibrated for solar UV spectrum require correction when used to measure the energy from a lamp array. This paper describes a transfer calibration procedure that was used to determine instrument response to the blacklight lamp energy spectrum. It also shows theoretically and experimentally that a correction factor of about 1.15 is needed to use a solar UV radiometer for measurements of outputs from the lamps.

J. Sol. Energy Eng. 2002;124(3):319-321. doi:10.1115/1.1488162.

This paper discusses the technical, financial, and economic principles underlying the levelized cost method of computing the cost of solar electricity. Topics include the levelized cost method, solar radiation, solar panel efficiency, depreciation, cost of capital, fixed and variable operating and maintenance costs, and taxes. The paper includes the worksheet, “Levelized Cost Worksheet for a 1 kW Solar Electric Generating Plant.” Its benchmark values are for a model solar plant located in Chicago, IL. The paper discusses these benchmark values as it analyzes the worksheet’s constants (capacity-1 kW, 8,760 hr/yr), independent variables (capital cost-$/kW, cost of capital-%, physical life-yr, standard sun hours, fixed and variable O & M expense), and dependent variables (capital amortization expense, capacity factor, cost of solar electricity).

DISCUSSION

J. Sol. Energy Eng. 2002;124(3):322. doi:10.1115/1.1487928.
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Commentary by Dr. Valentin Fuster

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