ALBERT

Description: In the present work, a numerical method has been applied to model the water evaporation rate of a glazed collector/regenerator component of an open-cycle absorption refrigeration system. This two-dimensional model calculates local heat and mass-transfer coefficients as part of the solution. The air flow in the glazed channel is driven by the combined buoyancy of both heat and mass transfer (water evaporation). Since the heat and mass-transfer coefficients each depend on both of the driving potentials determined by local conditions in the falling film, a solution of the conjugate problem is required. The resulting nonuniform air-film interface conditions cause the local heat and mass transfer to differ significantly from the uniform boundary condition case. The glazed collector/regenerator is much less sensitive to the ambient temperature and humidity than the unglazed collector. The addition of a glazing over the collector/regenerator provides a significant performance improvement and enhances solution regeneration in a windy humid climate. The glazed collector/regenerator water evaporation rate is higher relative to the unglazed case because the reduction in convective and radiative heat losses increases the absorbent temperature and vapor pressure sufficiently to overcome the concomitant reduction in the mass-transfer coefficient.

Description: Background: Due to cost effectiveness and operational safety online monitoring of rotor blades is recommended, especially for offshore wind energy converters. Method of Approach: Statistic evaluation of wind speed and power output of a wind energy converter is used to monitor the overall rotor performance including increased blade surface roughness. Nacelle oscillation spectral analysis methods are applied to monitor the rotor faults mass imbalance and aerodynamic asymmetry. Results: Results of ISET’s research work related to online rotor condition monitoring are presented. A description of the fault effects on the rotor, the sensor and data acquisition equipment and a description of the developed signal processing and fault prediction algorithms are given. The paper also presents results from experiments and field tests. Conclusions: The developed algorithms have been verified due to their monitoring capabilities and suitability in commercial online condition monitoring systems.

Description: The thermodynamic dissipations in crystalline silicon solar cells are identified and evaluated. The ratio of the exergy of the output electrical power to the exergy of the input solar radiation is the effectiveness of the solar cell. The input exergy is converted to the output exergy (the electrical power delivered) with a series of dissipations. These dissipations are identified and evaluated for crystalline silicon cells in terms of the thickness and certain fundamental properties of the light absorbing silicon semiconductor (in this case a P-type material). It is assumed that the N-type material is very thin and absorbs no radiation. For representative values of these properties and a range of thicknesses, it is found that the dissipations due to transmission and thermalization and in the photogeneration process are dominant. The dissipations due to the dark current and recombination are small.

Description: This paper presents a numerical method for predicting the atmospheric boundary layer under stable, neutral, or unstable thermal stratifications. The flow field is described by the Reynolds’ averaged Navier-Stokes equations complemented by the k‐ϵ turbulence model. Density variations are introduced into the momentum equation using the Boussinesq approximation, and appropriate buoyancy terms are included in the k and ϵ equations. An original expression for the closure coefficient related to the buoyancy production term is proposed in order to improve the accuracy of the simulations. The resulting mathematical model has been implemented in FLUENT. The results presented in this paper include comparisons with respect to the Monin-Obukhov similarity theory, measurements, and earlier numerical solutions based on k‐ϵ turbulence models available in the literature. It is shown that the proposed version of the k‐ϵ model significantly improves the accuracy of the simulations for the stable atmospheric boundary layer. In neutral and unstable thermal stratifications, it is shown that the version of the k‐ϵ models available in the literature also produce accurate simulations.

Description: Background: The solar photo-Fenton process has enormous potential for becoming a viable alternative to conventional processes for the treatment of industrial wastewater. However, the costs associated with the use of artificial irradiation have hindered many times industrial application of these processes. Method of Approach: In this work, the photo-Fenton remediation of various industrial wastewaters (containing silicones, pesticides, phenol and hydrocarbons, model, and real) in aqueous systems has been studied using Fe(II), H2O2, and UV-visible sunlight. Experiments were carried out using a concentrating parabolic trough reactor (PTR) and a nonconcentrating falling-film reactor. Results: In general, at low contaminant concentration, more than 90% of the total organic carbon content could be converted to inorganic carbon within about 2-3h, using sunlight, in reactors of different geometry. Conclusions: Solar light can be used either as an effective complementary or alternative source of photons to the photo-Fenton degradation process of a diversity of chemical pollutants.

Description: This paper presents a mathematical framework to properly account for uncertainty in wind resource assessment and wind energy production estimation. A meteorological tower based wind measurement campaign is considered exclusively, in which measure-correlate-predict is used to estimate the long-term wind resource. The evaluation of a wind resource and the subsequent estimation of the annual energy production (AEP) is a highly uncertain process. Uncertainty arises at all points in the process, from measuring the wind speed to the uncertainty in a power curve. A proper assessment of uncertainty is critical for judging the feasibility and risk of a potential wind energy development. The approach in this paper provides a framework for an accurate and objective accounting of uncertainty and, therefore, better decision making when assessing a potential wind energy site. It does not investigate the values of individual uncertainty sources. Three major aspects of site assessment uncertainty are presented here. First, a method is presented for combining uncertainty that arises in assessing the wind resource. Second, methods for handling uncertainty sources in wind turbine power output and energy losses are presented. Third, a new method for estimating the overall AEP uncertainty when using a Weibull distribution is presented. While it is commonly assumed that the uncertainty in the wind resource should be scaled by a factor between 2 and 3 to yield the uncertainty in the AEP, this work demonstrates that this assumption is an oversimplification and also presents a closed form solution for the sensitivity factors of the Weibull parameters.

Description: This paper deals with multivariable pitch control design for wind turbines, including load reducing control objectives. Different design approaches, including collective and cyclic pitch, and robustness aspects are discussed. A control design with decoupled controllers for collective and cyclic pitch is worked out in detail, based on the H∞ norm minimization approach. The control design is verified by simulations with a full nonlinear model of the wind turbine, showing the potential of multivariable pitch control to actively increase damping of the first axial tower bending mode and to reduce 1p fluctuations in blade root bending moments. Multivariable control design provides a convenient way of including additional load reducing objectives into the pitch controller of wind turbines. Fatigue loading of certain components, as tower and blades, could be reduced significantly.

Description: A passive summer cooling technique that utilizes the underground soil temperature has application in climate control of residential as well as agricultural buildings. The soil temperature stays fairly constant at a depth of eight feet or more (references [1], [2]). Earlier studies [3, 4] have shown the usefulness of this technique for an open-loop system. However, the previous analyses in the literature did not evaluate the usefulness and limitations of this method for closed-loop air conditioning. In this study an analysis of the “Coefficient of Performance” (COP) of a closed-loop system, based on the above technique, in combination with a conventional air conditioner, has been done. In this system, the cooling needed to neutralize the heat gain of the conditioned space is provided by the air cooled in an underground air pipe in combination with an air conditioner. The underground air tunnel is used for hot parts of days and is off for cooler parts of days and nights. The analysis has been done by a computer model solution, using central finite difference method. When the system is on, the air temperature and the soil temperature are calculated. When the system is off, the heat is transferred within the soil and a new set of soil temperatures around the pipe are calculated for the next day. As the soil temperatures around the pipe increase, the COP of the system decreases. The COP is calculated for each hour until it decreases to the COP of an air conditioner. This shows us the length of time for which the underground cooling method will be useful. Since the knowledge of soil properties is very important, a computer model solution has been developed to predict the soil thermal properties by using an approximate analytic method based on simple temperature measurements.

Description: The direct steam generation (DSG) is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi megawatt range. According to Price, H., Lu¨pfert, E., Kearney, D., Zarza, E., Cohen, G., Gee, R. Mahoney, R., 2002, “Advances in Parabolic Trough Solar Power Technology,” J. Sol. Energy Eng., 124 and Zarza, E., 2002, DISS Phase II-Final Project Report, EU Project No. JOR3-CT 980277 a 10% reduction of the LEC is expected compared to conventional SEGS like parabolic trough power plants. The European DISS project has proven the feasibility of the DSG process under real solar conditions at pressures up to 100 bar and temperatures up to 400°C in more than 4000 operation hours (Eck, M., Zarza, E., Eickhoff, M., Rheinla¨nder, J., Valenzuela, L., 2003, “Applied Research Concerning the Direct Steam Generation in Parabolic Troughs,” Solar Energy 74, pp. 341–351). In a next step the detailed engineering for a precommercial DSG solar thermal power plant will be performed. This detailed engineering of the collector field requires the consideration of the occurring thermohydraulic phenomena and their influence on the stability of the absorber tubes.

Description: Based on the experimental results of a 300kW solar chemical pilot plant for the production of zinc by carbothermal reduction of ZnO, we performed a conceptual design of a 5MW demonstration plant and of a 30MW commercial plant. Zinc can be used as a fuel for zinc-air batteries and fuel cells, or it can be reacted with water to form high-purity hydrogen. In either case, the chemical product is ZnO, which in turn is solar recycled to zinc. The proposed thermochemical process provides an energy efficient route for the conversion, storage, and transportation of solar energy in the form of solar fuels.

Description: A proportional plus integral plus derivative (PID) controller is used to obtain usable energy from the sun in almost all the solar systems in Japan. However, it is difficult to collect the heat continuously close to a prescribed temperature using a PID controller because the solar radiation is often interrupted by passing clouds. We investigated, therefore, a Model Reference Adaptive Control (MRAC) system. In order to demonstrate its effectiveness, we constructed a MRAC system and introduced it into the collector loop of a solar system. This paper gives an outline of the MRAC algorithm and describes the experimental results for the outlet fluid temperature response of the loop by the MRAC and PID. From these results, it is shown that the MRAC algorithm is suitable for controlling a system affected by irregular disturbances in the insolation.

Description: This study concerns experimental evaluation of heat transfer during energy storage and release for the phase change of paraffin wax in spherical shells. Measurements are made using air as the heat transfer fluid (HTF), copper spheres with diameters of 2, 3, 4, and 6 cm. A detailed temperature field is obtained within the spheres using 10 thermocouple wires. Values of the air velocity and temperature used in the experiments are 4–10 m/s and 60–90°C, respectively. Measured times for melting and solidification varied over a range of 5–15 and 2–5 minutes, respectively. Calculations show that the Nusselt number in the phase change material (PCM) during melting is one order of magnitude higher than during solidification. Results indicate that the Nusselt number for melting has a strong dependence on the sphere diameter, lower dependence on the air temperature, and a negligible dependence on the air velocity. Variations in the Fourier number for melting and solidification show similar trends. An increase in the Nusselt number for a larger sphere diameter is attributed to increase in natural convection cells in the PCM inside the spheres. The larger volume allows for the free motion for the descent and rise of cooler and hotter molten wax. During the solidification process, the solid wax is evenly formed through the sphere, starting from the outer surface and moving inward. As the solidification proceeds, the melt volume decreases with a simultaneous decrease in the magnitude of natural convection within the melt. The higher values of Fourier number for melting indicate the consumption of a large part of the HTF energy in heating the molten wax rather than melting of the solid wax.

Description: Molten salt technology represents nowadays the most cost-effective technology for electricity generation for stand-alone solar power plants. Although this technology can be applied to both concentrating technologies, parabolic through and central receiver systems (CRSs), CRS technology can take advantages from its higher concentration, allowing to work at higher temperatures and therefore with a reduction in the size and cost of the storage system. The receiver system is the “door” for which the energy passes from the field collector to the thermal-electric cycle; it represents, therefore, the core of the CRS and its performance directly affects plant production. Starting from the published lessons from SOLAR TWO receiver technology, the validation of an improved receiver for molten salt technology was assumed as part of the SOLAR TRES solar thermal power commercial plant development. Main challenges for the new receiver were to increase its allowable peak flux up to 1MW∕m2 in order to maximize the thermal efficiency of the CRS solar power plant, and to improve its safe life without limiting the incident fluxes that the field of heliostats is able to deliver with an optimized pointing strategy. Several advanced features in geometric and thermodynamic aspects and in its material selection have been implemented on the receiver. With the results of a sensitivity analysis carried out with an own code developed by SENER (SENREC), a prototype receiver panel was designed, fabricated, and installed in a proper test bed at the PSA. Test validation on this panel was carried out in 2007. The initial test results show a very good behavior of the prototype receiver, which allows to anticipate that the objectives of its design can be fulfilled. SENER and CIEMAT have joined forces to face up the challenge of sizing and designing a new molten salt receiver of high thermal efficiency, able to operate at high fluxes without compromising its durability (at least 25years). Main challenges for the new receiver design were to optimize the receiver dimensions and receiver tube sizes and material selection to surpass the operating conditions in the new plants with respect to SOLAR TWO.

Description: It is generally felt that the application of line source theory for ground coil design usually resulted in excessive overdesign. It was anticipated that in order for the ground coil heat pump systems to be economically competitive with other residential heating and cooling systems, ground coil overdesign had to be kept to a minimum. A new ground coil model was derived, which based on energy balance rather than the traditional line source theory. It was aimed to more accurately predict the operation of ground coils. It is the intention of this study to compare this ground coil model with models based on line source theory, a simple line source model and a modified line source model, by using them to simulate the same field test data for both summer and winter ground coil operations. The results indicated that for winter coil operation, the new model predicted the coil liquid exit temperature less than 2°C maximum deviation from the measured values, with an average deviation less than 1°C. The modified line source model had an average deviation of more than 1.5°C. For summer operation, all models underpredicted the measured soil temperatures because the effect of thermal backfill material was not included in the models. The new model still predicted the test results better than the other two models. However, when the effect of sand thermal backfill was included in the new model, which was not easy for the other two models, the calculated soil temperatures were almost identical to the test results.

Description: A solar receiver-reactor has been designed to conduct solid-gas chemical reactions, using concentrated solar radiation as the energy source of high-temperature process heat. It consists of a conical cyclone gas-particle separator that has been modified to let concentrated solar energy enter the cavity through a windowless (atmospheric-open) aperture. It combines the advantages of cavity receivers and volumetric reactors, and permits continuous mode of operation. A small-scale prototype reactor to conduct the thermal decomposition of calcium carbonate at 1300 K was experimentally investigated in a solar furnace. Its thermal performance was evaluated. The mean energy absorption efficiency, based on the optically measured power incident on the receiver aperture, was 43 percent. Reaction products showed high degree of calcination.

Description: The objectives of the analysis reported in this paper are to evaluate the environmental impacts of the electricity produced in a 17MW solar thermal plant with central tower technology and a 50MW solar thermal plant with parabolic trough technology, to identify the opportunities to improve the systems in order to reduce their environmental impacts, and to evaluate the environmental impact resulting from compliance with the solar thermal power objectives in Spain. The methodology chosen is the life cycle assessment (LCA), described in the international standard series ISO 14040-43. The functional unit has been defined as the production of 1kWh of electricity. Energy use needed to construct, operate, and dismantle the power plants is estimated. These results are used to calculate the “energy payback time” of these technologies. Results were around 1yr for both power plants. Environmental impacts analyzed include the global warming impacts along the whole life cycle of the power plants, which were around 200g∕kWh generated. Finally, the environmental impacts associated with the compliance of the solar thermal power objectives in Spain were computed. Those figures were then used to estimate the avoided environmental impacts including the potential CO2 emission savings that could be accomplished by these promotion policies. These savings amounted for 634kt of CO2 equiv./yr.

Description: Postbuckling analysis of composite laminates representative of wind turbine blade substructures, utilizing the commercial finite element software ANSYS, is presented in this paper. The procedure was validated against an existing postbuckling analysis. Three shell element formulations, SHELL91, SHELL99, and SHELL181, were examined. It was found that the SHELL181 element with reduced integration should be used to avoid shear locking. The validated procedure was used to examine the variation of the buckling behavior, including postbuckling, with lamination schedule of a laminate representative of a wind turbine blade shear web. This analysis was correlated with data from a static test. A 100% postbuckling reserve in a composite structure representative of a shear web was quantified through test and analysis. The buckling behavior of the shear web was improved by modifying the lamination schedule to increase the web bending stiffness. Modifications that improved the buckling load of the structure did not always equate to improvements in the postbuckling reserve.

Description: The conventional Pennington Cycle desiccant cooling system offers a clear opportunity for heat-actuated air conditioning. However, efforts to translate this opportunity into commercially viable hardware have not been successful. The performance of the hardware has been inadequate, resulting in excessive solar collector requirements or, in the case of gas-fired equipment, uneconomical use of natural gas. Two methods for improving the coefficient of performance (COP) of these systems are: (1) the addition of inert heat capacity to the desiccant matrix, and (2) “staging” the regeneration air stream. An analysis is presented in this paper which explains the benefits and drawbacks of these methods based upon the wave nature of the heat and mass transfer processes occurring within the desiccant bed. The results indicate that the best overall system performance is obtained by staging the regeneration process while minimizing the amount of inert heat capacity.

Description: A solar updraft tower power plant—sometimes also called “solar chimney” or just “solar tower”—is a solar thermal power plant utilizing a combination of solar air collector and central updraft tube to generate a solar induced convective flow which drives pressure staged turbines to generate electricity. The paper presents theory, practical experience, and economy of solar updraft towers: First a simplified theory of the solar tower is described. Then results from designing, building and operating a small scale prototype in Spain are presented. Eventually technical issues and basic economic data for future commercial solar tower systems like the one being planned for Australia are discussed.

Description: Background. The development of alternative processes to eliminate pathogenic agents in water is a matter of growing interest. Current drinking water disinfection procedures, such as chlorination and ozonation, can generate disinfection by-products with carcinogenic and mutagenic potential and are not readily applicable in isolated rural communities of less-favored countries. Solar disinfection processes are of particular interest to water treatment in sunny regions of the Earth. Solar light may be used to activate a photocatalyst or photosensitizer that generates, in the presence of molecular oxygen dissolved in water, reactive oxygen species (ROS), such as the HO• radical, singlet oxygen (O21), or superoxide (O2•), which are toxic to waterborne microorganisms. Method of Approach. Wild and collection-type Escherichia coli have been selected as model bacteria. Inactivation of such bacteria by either TiO2 nanoparticles, water-soluble tris(2,2′-bipyridine)ruthenium(II) dichloride or Rose bengal (RB) subject to simulated sunlight have been compared. Although TiO2 is the prototypical material for heterogeneous photocatalysis, the other two dyes are known to generate significant amounts of O21 by photosensitization but have different chemical structures. The concentration of dye, illumination time, photostability, presence of scavengers, and post-treatment regrowth of bacteria have been investigated. Results. After 1hr of solar illumination the Ru(II) complex produced a strong loss of E. coli culturability monitored with solid selective agars. Both the collection- and wild-type bacteria are sensitive to the treatment with 2-10mgL−1 of dye. This photosensitizer showed a better inactivation effect than TiO2 and the anionic organic dye RB due to a combination of visible light absorption, photostability, and production of O21 and other ROS when bound to the bacterial membrane. A complete loss of culturability was observed when the initial concentration was 103CFUmL−1, with no bacteria regrowth detected after 24hr of the water treatment. At higher initial microorganism levels, culturability still remains and regrowth is observed. Scavengers show that the HO• radical is not involved in bacteria inactivation by photosensitization. Conclusions. A higher quantum yield of ROS generation by the sensitizing dyes compared to the semiconductor photocatalyst determines the faster sunlight-activated water disinfection of photodynamic processes. The homogeneous nature of the latter determines a more efficient interaction of the toxic intermediates with the target microorganisms. Solid supporting of the Ru(II) dye is expected to eliminate the potentials problems associated to the water-soluble dye.

Description: The Fixed Mirror, Distributed Focus (FMDF) hemispherical bowl is a solar thermal power technology which has been studied for several years as a potentially attractive means of producing electricity. This paper discusses projections of the collector (concentrator and receiver) performance and cost that could be achieved by five variations of the FMDF technology, based on the assumption of continued technology development and high volume production. The major loss mechanisms were determined for the concentrators and receivers and then design point losses and efficiencies were developed. The Pacific Northwest Laboratory (PNL) computer code SOLSTEP was then used to determine the annual performance. The cost analysis was based on a “commercial” solar thermal industry that could exist by the late 1990s. The industry is assumed to be installing several hundred megawatts per year of electric capacity, and is assumed to have improved the maturity of solar thermal components so that technical and economic risks are similar to other energy systems.

Description: This work is devoted to application of data mining methods for monitoring of state of a solar thermal plant. The methods discussed are illustrated by example of a study performed for the DISS direct steam generation facility at the Plataforma Solar de Almeria (Spain). In order to deal with the problems of large dimensionality and high correlation among the data the methods of latent variables, principal component analysis and partial least squares, were applied. Results showed that normal and abnormal states during plant operation could be identified.

Description: Desalination plants play a fundamental role in the fight against fresh water shortage in places with seawater availability. On the other hand, optimization of industrial processes is an important goal in order to increase operation efficiency and to minimize costs. This paper deals with an experimental hybrid solar-gas desalination system designed, built, and operated at the Plataforma Solar de Almería (Spain) under the framework of a European R&D project. This desalination system, based on a multieffect distillation process, is partially powered by a low temperature static solar collector field of a compound parabolic concentrator type. With the aim of performing an optimal operation of the plant working with the solar resource, process dynamics have to be studied and modeled. This paper shows control-oriented low complexity models for the solar field and thermal storage subsystems as well as a model of the distillate production rate depending on process temperature. These models are validated with real data with the objective of being useful in the evaluation of optimal operating points, the design of operational procedures, and the establishment of adequate control references to maximize the use of the solar resource and to improve process efficiency.

Description: Hexagonal and cubic mesostructured TiO2 anatase were synthesized by a templating sol-gel method using triblock copolymers as structuring agents, and used as photocatalysts for providing geometrical assistance to a photocatalytic reaction. The visible light and UV photocatalytic removal of gas-phase concentrated toluene (110ppm) within an annular flow-reactor was used as a tool to evidence the benefit to use mesostructured photocatalysts. The coupling of low amounts of WO3 with mesostructured anatase led to high efficiency using visible light and UV activation. The highly positive effect of WO3 on the toluene removal efficiency was attributed to the coupling between TiO2 and WO3 semiconductors, leading to an improved photogenerated charge separation and thus a weaker charge recombination. The hypothesis of a confinement effect of the reactants inside the mesostructured photocatalyst was put forward to explain the photocatalytic performances obtained under visible light and UV activation. This confinement effect would be based on the inner partial pressure concept. It consists of an increase in the partial pressure of the reactants next to the active sites inside the nanometric cavities of the ordered structure, while the apparent macroscopic partial pressures, outside the mesostructure, would remain unchanged. According to a traditional reaction rate law, such an increase results in the increase in the reaction rate. The photon-assisted reaction could be considered as “structure-assisted,” the geometrical assistance being provided by the surrounding ordered TiO2 walls of the mesostructure.

Description: In this work, the influence of the reactor configuration and the characteristics of the catalysts on the photodegradation of trichloroethylene (TCE) vapors are studied under sunlight illumination. The photocatalytic activity tests were carried out using two types of continuous flow reactors: (i) a compound parabolic collector (CPC) and (ii) a simple flat reactor. Three different photocatalysts based on TiO2 were utilized: (i) commercial powders calcined at 500°C (ii) a TiO2−xNx sample synthesized by treating the commercial sample at 500°C in an NH3 gas flow, and (iii) TiO2 thin film coatings on differently shaped borosilicate glass supports prepared by a sol-gel procedure. The obtained data reveal that the photonic efficiency for the removal of TCE is quite high but slightly decreases with increasing the light intensity. The commercial TiO2 sample presents the highest efficiency while nitrogen doping seems to be slightly detrimental for photoactivity, despite the fact that certain photoresponse in the visible can be envisaged. In contrast, transparent sol-gel TiO2 coatings present the highest TCE degradation rate per mass of catalyst. Regarding the type of reactor, it is found that the use of CPCs can be advantageous, especially when dealing with high volumes of effluent and elevated concentration of TCE, although flat reactor also shows a considerable efficiency.

Description: Gardon calorimetric transducers are widely used to measure concentrated solar radiation flux on solar thermal areas. These measurements need some correction to adapt their response from thermal to solar irradiance measurement. The authors propose the use of concentrating photovoltaic PV-cells to measure concentrated solar radiation flux. This paper shows the results obtained from a comparative test carried out in a solar furnace measuring concentrated solar irradiance with calorimetric and photovoltaic sensors, Gardon, and PV-Cells, respectively.

Description: Our main purpose was to study the disinfection of a secondary wastewater anaerobic effluent by a combined photolytic-photocatalytic continuous reactor using solar irradiation. This reactor had a rectangular section and included back-and-forth-flow internal channels. It comprised two compartments: the first one (6.4l) corresponded to photolytic disinfection with a 1h residence time and the second one (12.8l) corresponded to photocatalytic disinfection with a 2h residence time. The photocatalyst used was Degussa P-25 TiO2 fixed on an ordinary tile. By use of this reactor between 9:00h and 16:00h with a water flow rate of 3.4lh−1, the total coliform concentration diminished by four orders of magnitude, reaching values lower than 102MPN (most probable number)/100ml, which are below the regulation. Even if photolysis was important, this single process did not permit one to obtain an outlet coliform concentration below 103MPN∕100ml; however, it helped to improve the photocatalytic performance. A decrease in chemical oxygen demand (COD) was also observed in the photocatalytic compartment, illustrating an oxidation of organic pollutants. In parallel, batch experiments were carried out to get information about the residence times, the photocatalytic performance of TiO2 fixed on an ordinary tile, and the photolytic effect. The results suggest that this low cost and easy to operate treatment system could be a promising alternative to usual systems for the decrease in COD and the disinfection of domestic wastewaters.

Description: Transparent counter electrodes were prepared on transparent conductive glass (TCG) substrates from a hexachloroplatinic acid (H2PtCl6) solution applying the thermal decomposition method in combination with the spin-coating deposition technique. The effect of the precursor concentration and the number of deposited platinum layers on the surface characteristics of the Pt films was examined, and the relation between those surface characteristics and the electrochemical properties of the corresponding modified Pt/TCG electrodes was defined. Four types of counterelectrodes were prepared, differing in the concentration of the H2PtCl6 solution (0.03M and 0.15M) and in the number of Pt layers (one or two Pt layers); their performance as counterelectrodes was evaluated after their incorporation into dye-sensitized solar cells (DSSCs) employing a solid state redox electrolyte. The obtained results show that solar cells using counterelectrodes prepared from the 0.03MH2PtCl6 solution and consisting of two Pt layers (Pt032 electrode) exhibited the best performance characteristics (diffusion coefficient D*I3−=1.58×10−5cm2s−1, conversion efficiency η=2.16%, fill factor ff=62.14%, and short circuit photocurrent Isc=4.71mAcm−2). The electrochemical behavior of the modified counterelectrodes is consistent with the surface characteristics of the Pt film that formed on the conductive glass substrate, which seems to be significantly affected by the adopted method and the adjusted experimental parameters (Pt concentration and number of Pt layers). Specifically, this type of electrodes beside their low roughness (Rq=11.5nm), also presents a high complexity (Df=2.3). As a result, for this kind of solid state DSSCs, the less rough but the more complex the Pt/TCG electrode surface, the higher the efficiency of the corresponding solar cells.

Description: Metasystox, Ultracid, Sevnol, and Laition are commercial pesticides, whose active ingredients are, respectively, oxydemethon-methyl, methidathion, carbaryl, and dimethoate. Laboratory scale experiments were carried out to treat solutions of the pure active ingredient and the commercial formulation employing a solar simulator as irradiation source. Degradation of the active ingredient followed in all cases a pseudo-first-order kinetic and rate constants, k, indicated that reaction was faster when the thiophosphate moiety was present. Additives existing in the commercial formulations resulted in an important decrease in the reaction rate, as k values were at least 50% lower than those obtained with the corresponding pure active principle (0.013min−1 for Ultracid and 0.033min−1 for methidathion, 0.011min−1 for Laition and 0.026min−1 for dimethoate, and 0.007min−1 for Sevnol and 0.016min−1 for carbaryl). Important increase in the surface tension was measured in all cases, indicating that the photocatalytic treatment is able to remove the surface active species present in the commercial formulation. Finally, assays based on the inhibition of the respiration of activated sludge indicated a decrease in the toxicity of all four commercial mixtures from initial values in the range 40–80% to achieve complete detoxification when the elimination of the active ingredient is reached.

Description: The degradation processes of dibutyl phthalate (DBP), an important pollutant, are rather slow and do not lead to the complete decomposition. In the present work the photocatalytic degradation of dibutyl phthalate in the presence of TiO2 was studied. Experiments were carried out in suspensions and with immobilized layers, prepared from powder suspensions and by sol-gel technique. Two different polychromatic light sources (band maxima at 350nm and 365nm) were used for irradiation. The results confirmed that the photocatalytic degradation of DBP using TiO2 is an efficient degradation process and proceeds even at very low concentration of photocatalyst (0.001gdm−3). Reaction rates were significantly higher in the case of the lamp with the maximum at 365nm, which is due to the higher light intensity of this lamp. At comparable TiO2 amount P25 layers exhibit about 50% of photoactivity using TiO2 suspension.

Description: The photocatalytic degradation of an endocrine disruptor (resorcinol) and of two fungicides (pyrimethanil and triadimenol) has been studied and compared. The effect of pH, oxygen, and H2O2 on the photocatalytic degradation of these compounds has been established. The three organics were analyzed by means of high pressure liquid chromatography (HPLC) and their mineralization by total organic concentration (TOC) measurements. The evolution of the toxicity to Lemna minor of the aqueous solutions of the three organics during their photocatalytic treatment has also been studied. The obtained results have been interpreted according to Fourier transform infrared studies on the interaction of the molecules with the catalyst surface and their reaction mechanisms by gas chromatographpy-mass spectrometry (GC-MS) analyses. The toxicity studies have shown that some intermediates acted as nutrients or toxicity antagonists as negative growth rate inhibitions were obtained. After 30min of reaction, the resorcinol and pyrimethanil solutions were detoxified, although some amount of the organics still remained. In the case of triadimenol, a 92% detoxification was achieved after 60min of reaction. The solar photocatalytic degradations of the pollutants have resulted to be comparable with those obtained with UV lamp. The obtained results suggest that the type of interaction of pyrimethanil and triadimenol with the TiO2 surface decides their degradation mechanism by which the effect of pH, H2O2, and dissolved oxygen is determined. It has also been confirmed that the photocatalytic techniques are very efficient at the detoxification of wastewaters contaminated with these fungicides.

Description: Ni-ferrite (NiFe2O4) is a promising reactive ceramics of the ferrite for the solar hydrogen production by a two-step water splitting process using concentrated solar energy. However, it should be pretreated before H2-generation reaction by grinding the Ni-ferrite sintered after the O2-releasing reaction to make a fine powder. If the Ni-ferrite and yttria stabilized zirconia (YSZ) form a solid solution between these oxides (YSZ∕NiFe2O4 solid solution=YSZ(Ni,Fe)), it is expected that the two-step water splitting process with the Ni-ferrite system can proceed without treatment of the reduced product because of the high thermal stability of the YSZ∕NiFe2O4 solid solution. The YSZ∕NiFe2O4 solid solution was prepared by calcination of the mixture of the YSZ balls and NiFe2O4 powder at T=1823K for 1h, and its reactivity and thermal stability were examined for the two-step water splitting process. During the ten times repetition of the two-step water splitting reaction (T=1773K for O2-releasing, and 1473K for H2-generation) with the YSZ∕NiFe2O4 solid solution using infrared imaging furnace, the reactivity for O2-releasing and H2-generation was kept constant. The molar ratio of the released O2 gas volume (the average O2 gas, 1.9cm3∕g) and the generated H2 gas volume (the average H2 gas, 3.8cm3∕g) was nearly 1:2, indicating that the water decomposition process via two steps proceeds. The X-ray diffractometry (XRD) measurement showed that the YSZ(Ni,Fe) keeps the YSZ phase structure during the ten times repetition of the two-step water splitting process. The successive H2 gas production by the two-step water splitting process was performed (ten times repetition of the two-step water splitting process). From comparative study on the reactivity and the thermal stability for the two-step water splitting reaction among the YSZ∕NiFe2O4 solid solution, NiFe2O4 and ZnFe2O4, it is concluded that the YSZ∕NiFe2O4 solid solution is superior to the others.

Description: This study shows the results of pressure distribution measurements on a rotor blade of a horizontal axis wind turbine under various yawed operations. The experiments are carried out in a wind tunnel with a 2.4m diameter test rotor. In the measurements, the power curve and pressure distributions are measured for different azimuth angles. By increasing yaw angle, the maximum value of power coefficient of the rotor decreases. The sign of the yaw angle does not have any effect on power performance. The aerodynamic forces are discussed using the axial and rotational force coefficients for each azimuth angle. In the case of higher tip speed ratios, the blade section passing on the upstream side in yawed operations has a greater contribution to the rotor torque than that on the downstream side. In this tip speed range, the aerodynamic forces at the 70% radius section appear proportional to the angle of attack. In the case of the lower tip speed ratios, the blade on the downstream side does not contribute to rotor torque, which appears to result from separation.

Description: Hydrolysis of Zn is investigated as the second step in a ZnO∕Zn redox solar water splitting process. Zinc is evaporated and hydrolyzed with steam in a hot wall flow tubular reactor. The influence of the reactor temperature distribution and residence time on hydrogen conversion was measured for furnace set point temperatures of 1023K and 1073K. The yield of ZnO aerosol was measured in situ using a scanning differential mobility sizer. The composition and morphology of the solid product were characterized with X-ray diffraction and microscopy. Hydrogen conversions of 87–96% at temperatures above zinc saturation are attributed primarily to hydrolysis of zinc(g) at the wall of the reactor at temperatures from 800Kto1077K.

Description: The aerodynamic efficiency of a variable-speed wind turbine operating in Region 2, or below-rated wind speeds, is greatly affected by the identification of accurate parameters for the controller. In particular, the power coefficient (Cp) surface must be well known for optimal efficiency to be achieved with a constant-gain controller. However, adaptive control can overcome the inefficiencies caused by inaccurate knowledge of the Cp surface. Previous work focused on adaptive torque gain control to cause a variable-speed turbine to operate, on average, at the tip-speed ratio λ* for which the maximum Cp occurs. This paper considers the effects of adaptive blade pitch angle control on a turbine’s aerodynamic efficiency. Computer simulations and tests on a field turbine are used to verify the adaptive pitch control scheme. Simulation and field test results demonstrate that the adaptive pitch controller causes the pitch angle to approach its optimal value. Adaptive pitch control can be used to seek the optimal pitch angle for energy capture in Region 2 operation. Additional field operation is required before a statistically significant improvement in energy capture can be demonstrated.

Description: Extrapolation of extreme loads using turbulent wind samples of various mean speeds and random starting points is addressed using probability distribution functions that are suitably distorted to fit the peak extremes. The tail of the extreme value distribution of the simulated loads is required to fit accurately and this tail is extrapolated to a 50‐year exceedance probability to determine the characteristic load. The Gumbel distribution with a quadratic distortion is especially addressed due to its asymptotic theoretical validity for Gaussian loads. The blade root moments and the hub moments are studied here with respect to their behavior under extrapolation using a quadratic Gumbel distribution. Verification with a large number of random seeds at various mean wind speeds is done, so as to assess the accuracy of the extrapolation and the convergence of the extrapolated load. Methods of accounting for the variance in the extrapolated load with changes in the random wind seeds are proposed.

Description: Our objective here is to establish long-term loads for offshore wind turbines using a probabilistic approach. This can enable one to estimate design loads for a prescribed level of return period, generally on the order of 20–50years for offshore wind turbines. In a probabilistic approach, one first needs to establish “short-term” distributions of the load random variable(s) conditional on the environment; this is achieved either by using simulation or field measurements. In the present study, we use field data from the Blyth offshore wind farm in the United Kingdom, where a 2MW wind turbine was instrumented, and environment and load data were recorded. The characteristics of the environment and, hence, that of the turbine response at the site are strikingly different for wind regimes associated with different wind directions. Here, we study the influence of such contrasting environmental (wind) regimes and associated waves on long-term design loads. The field data, available as summary statistics, are limited in the sense that not all combinations of environmental conditions likely to be experienced by the turbine over its service life are represented in the measurements. Using the available data, we show how distributions for random variables describing the environment (i.e., wind and waves) and the turbine load of interest (i.e., the mudline bending moment) can be established. By integrating load distributions, conditional on the environment with the relative likelihood of different environmental conditions, long-term (extreme/ultimate) loads associated with specified return periods can be derived. This is demonstrated here by carefully separating out the data in different wind direction sectors that reflect contrasting wind (and accompanying wave) characteristics in the ocean environment. Since the field data are limited, the derived long-term design loads have inherent uncertainty associated with them; we investigate this uncertainty in such derived loads using bootstrap techniques.

Description: To reduce the level of uncertainty associated with current rotor aerodynamics codes, improved understanding of rotor aerodynamics is required. Wind tunnel measurements on model rotors contribute to advancing our knowledge on rotor aerodynamics. The combined recording of blade loads and rotor wake is desired, because of the coupled blade and wake aerodynamics. In general, however, the small size of model rotors prohibits detailed blade load measurements; only the rotor wake is recorded. To estimate the experimental blade flow conditions, a measurement analysis tool is developed: the inverse vortex wake model. The rotor wake is approximated by a lifting line model, using rotor wake measurements to reconstruct the vortex wake. Conservation of circulation, combined with the Biot–Savart law, allows the induced velocity to be expressed in terms of the bound circulation. The unknown bound circulation can be solved for, since the velocity is known from rotor wake measurements. The inverse vortex wake model is subsequently applied to measurements on the near wake of a model rotor subject to both axial and yawed flow conditions, performed at a TUDelft open jet wind tunnel. The inverse vortex wake model estimates the unsteady experimental blade flow conditions and loads that otherwise would have remained obscured.

Description: The investigation focuses on the analysis of the airfoil segment performances along rotor blades in the parked configuration. In this research, wind tunnel experiments on two twisted blade geometries with different airfoils played a dominant role. These measurements were carried out by the Swedish Aeronautical Research Institute, former FFA, and by the American National Renewable Energy Laboratories (NREL) during the Unsteady Aerodynamic Experiment. The spans of the blades were 2.375m and 5m, the STORK 5 WPX and the NREL Phase VI blade, respectively. Five span locations (inboard, midspan, outboard, and tip regions) were considered and compared with the 2D airfoil characteristics. Wing model experiments with similar blade aspect ratio were included in the research. Furthermore, the commercial computational fluid dynamics code FLUENT was used for the validation and analysis of the spanwise lift and drag coefficients at four different pitch settings, 20deg, 30deg, 45deg, and 60deg. The computed pressure distributions compared reasonably well, but the derived lift and drag showed quite some differences with the blade measurements. The lift coefficients for the sections beyond the leading-edge stall angle of the STORK blade were larger than for the NREL blade and were close to that of a wing model with similar airfoil and aspect ratio. Lift and drag coefficients for the sections of the two blades were always much smaller than the 2D results. The drag values for both blades showed quite some agreement, and airfoil and blade dependency seemed to be small.

Description: Seeking relevant criteria for testing the quality of turbulence models, the scale of turbulence and the gust factor have been estimated from data and compared with predictions from first-order models of these two quantities. It is found that the mean of the measured length scales is approximately 10% smaller than the IEC model for wind turbine hub height levels. The mean is only marginally dependent on trends in time series. It is also found that the coefficient of variation of the measured length scales is about 50%. 3s and 10s preaveraging of wind speed data are relevant for megawatt-size wind turbines when seeking wind characteristics that correspond to one blade and the entire rotor, respectively. For heights exceeding 50–60m, the gust factor increases with wind speed. For heights larger than 60–80m, present assumptions on the value of the gust factor are significantly conservative, both for 3s and 10s preaverages. The usually applied value of kp≈3 should probably be reduced.

Description: A new aerodynamic wake model has been developed for horizontal axis wind turbines. The aim is to develop an engineering tool for investigation and design of furling turbines. The prescribed vortex wake code HAWTDAWG, developed at the University of Glasgow, has been extended for dynamic flow conditions. This dynamic prescribed wake model is built into the aerodynamic code AERODYN and linked to the structural dynamics code FAST. The new model has been compared to unsteady aerodynamic experiment Phase VI wind tunnel data. Comparisons are also made to blade element momentum and generalized dynamic wake models built into AERODYN. Results are encouraging and justify further investigation.

Description: This paper discusses the results of an extensive investigation to assess the added value of various techniques of health monitoring to optimize the maintenance procedures of offshore wind farms. This investigation was done within the framework of the EU funded Condition Monitoring for Offshore Wind Farms (CONMOW) project, which was carried out from 2002 to 2007. A small wind farm of five turbines has been instrumented with several condition monitoring systems and also with the “traditional” measurement systems for measuring mechanical loads and power performance. Data from vibration and traditional measurements, together with data collected by the turbine’s system control and data acquisition (SCADA) systems, have been analyzed to assess (1) if failures can be determined from the different data sets; (2) if so, if they can be detected at an early stage and if their progress over time can be monitored; and (3) if criteria are available to assess the component’s health. Several data analysis methods and measurement configurations have been developed, applied, and tested. This paper first describes the use of condition monitoring if condition based maintenance is going to be applied instead of only scheduled and corrective maintenance. Second, the paper describes the CONMOW project and its major results, viz., the assessment of the usefulness and capabilities of condition monitoring systems, including algorithms for identifying early failures. Finally, the economic consequences of applying condition monitoring systems have been quantified and assessed.

Description: This work undertakes an aerodynamic analysis over the parked and the rotating NREL Phase VI wind turbine blade. The experimental sequences from NASA Ames wind tunnel selected for this study respond to the parked blade and the rotating configuration, both for the upwind, two-bladed wind turbine operating at nonyawed conditions. The objective is to bring some light into the nature of the flow field and especially the type of stall behavior observed when 2D aerofoil steady measurements are compared to the parked blade and the latter to the rotating one. From averaged pressure coefficients together with their standard deviation values, trailing and leading edge separated flow regions have been found, with the limitations of the repeatability of the flow encountered on the blade. Results for the parked blade show the progressive delay from tip to root of the trailing edge separation process, with respect to the 2D profile, and also reveal a local region of leading edge separated flow or bubble at the inner, 30% and 47% of the blade. For the rotating blade, results at inboard 30% and 47% stations show a dramatic suppression of the trailing edge separation, and the development of a leading edge separation structure connected with the extra lift.

Description: In this paper, we present a theoretical analysis and experimental results of a pyrometric temperature measurement method based on a two-color pyroreflectometer (working wavelengths of 1.3μm and 1.55μm) equipped with an optical fiber probe. By measuring simultaneously the radiance temperature and the normal reflectivity, the true temperature of the measurement target can be determined without the previous knowledge of the optical properties of the sample. The method is based on the introduction and the definition of the diffusivity factor ηd and the assumption that it is equal for the two wavelengths. Besides the experimental results, the sources of errors and their impact on the method accuracy and the performance of the setup are discussed. The quality of the measurement results demonstrates the utility of the pyroreflectometry in a domain where fundamental parameters, such as the temperature and radiative properties, present the major measurement difficulties. The diffusivity factor appears not only a step to determine the true temperature but also as a parameter very useful to evaluate the diffusing behavior of opaque materials.

Description: The work detailed in this paper is focused on updating and refining a coning rotor wind turbine concept. The coning rotor combines the load shedding properties of flapping hinges with gross change in rotor area, via large coning angles, to affect increased energy capture at nominally constant system cost. Previous studies have indicated that the large cost of energy reductions is possible, compared to the state-of-the-art machines then, particularly for abundant but presently uneconomic low-wind sites. Almost ten years later, the fundamentals of the design remain sound, but bear reevaluation relative to current machines, both exploiting modern power electronics and control technology. The coning rotor was never optimized in its own right, so an integrated design tool suitable for human and computer refinement of the design has been developed. Incorporated into the tool is a corrected blade element momentum method that more properly accounts for coned rotor aerodynamics. A discussion of the development of coning rotors is presented, along with a comparison to present operational strategies. Results obtained for nondimensionalized rotors and specific machine optimization studies are presented, followed by a discussion of further issues to be addressed.

Description: Previous investigators have shown that an internally reversible Carnot cycle, operating with heat transfer limitations between the heat source and heat sink at temperatures TH and TL, achieves maximum power at an efficiency equal to 1−TL/TH independent of the heat exchanger transfer coefficients. In this paper, optimization of the power output of an internally irreversible heat engine is considered for finite capacitance rates of the external fluid streams. The method of Lagrange multipliers is used to solve for working fluid temperatures which yield maximum power. Analytical expressions for the maximum power and the cycle efficiency at maximum power are obtained. The effects of irreversibility and economics on the performance of a heat engine are investigated. A relationship between the maximum power point and economically optimum design is identified. It is demonstrated that, with certain reasonable economic assumptions, the maximum power point of a heat engine corresponds to a point of minimum life-cycle costs.

Description: Previous results from a stability analysis of natural convection in a water-saturated porous medium with externally imposed downward flow of the fluid and distributed internal heat generation are used in this study for a preliminary design of a saltless solar pond. The pond is filled with a water-saturated porous medium to inhibit natural convection. Downward flow of water is steadily imposed to allow the absorbed solar radiation, dependent upon the extinction coefficient of the water-saturated porous medium, to be carried down to the bottom of the solar pond from whence warm water is conveyed to the point of use. The downward velocities used have small magnitudes to allow useful bottom temperatures to be obtained. A steady-state analytical study was performed to determine the maximum temperature that can be obtained at the bottom of the saltless solar pond which resulted in a relationship involving the internal and external Rayleigh numbers, extinction coefficient, and Peclet number of the imposed downward flow. Results from this analytical study are combined with results from the stability analysis in a design procedure. This procedure is used to design a saltless solar pond of the concept described above for several cases.

Description: The objective of the experiments was to quantify how attic ventilation would affect the performance of a radiant barrier. Ceiling heat flux and space cooling load were both measured. Results of side-by-side radiant barrier experiments using two identical 13.38 m2 (nominal) test houses are presented. The test houses responded similarly to weather variations. Indoor temperatures of the test houses were controlled to within 0.2°C. Ceiling heat fluxes and space cooling load were within a 2.5 percent difference between both test houses. The results showed that a critical attic ventilation flow rate of 1.3 (l/sec)/m2 of the attic floor existed after which the percentage reduction in ceiling heat fluxes produced by the radiant barriers did not change with increasing attic airflow rates. The ceiling heat flux reductions produced by the radiant barriers were between 25 and 35 percent, with 28 percent being the percent reduction observed most often in the presence of attic ventilation. The space-cooling load reductions observed were between two to four percent. All results compiled in this paper were for attics with unfaced fiberglass insulation with a resistance level of 3.35 m2 K/W (nominal) and for a perforated radiant barrier with low emissivities (less than 0.05) on both sides.

Description: A half-factorial, two-level experimental design is used to determine the effects of changes in collector area, storage tank volume, collector flow rate, recirculation flow rate, and storage tank design on thermal rating of a solar drain-back water heating system. Experimental ratings are determined in accordance with the Solar Rating and Certification Corporation guidelines. Storage tank design is varied by using a stratification manifold in place of the standard drop tube. Variations in other component sizes and operating factors are based on current industry standards. Statistical analyses indicate that a change in collector area accounts for nearly 90 percent of the variation in heat output. Doubling collector area from 2.78 m2 to 5.56 m2 increases delivered solar energy by 31 percent. Use of a stratification manifold increases the delivery of solar energy by six percent. Doubling collector flow rate from 0.057 to 0.114 1/s increases solar output by approximately three percent; however, the increase in pumping energy outweighs the benefits of increasing collector flow rate. The effects of recirculation flow rate and tank volume are obscured by experimental error.

Description: Entropy generated by operation of a two-phase self-pumping solar water heater under Solar Rating and Certification Corporation rating conditions is computed numerically in a methodology based on an exergy cascade. An order of magnitude analysis shows that entropy generation is dominated by heat transfer across temperature differences. Conversion of radiant solar energy incident on the collector to thermal energy within the collector accounts for 87.1 percent of total entropy generation. Thermal losses are responsible for 9.9 percent of total entropy generation, and heat transfer across the condenser accounts for 2.4 percent of the total entropy generation. Mixing in the tempering valve is responsible for 0.7 percent of the total entropy generation. Approximately one half of the entropy generated by thermal losses is attributable to the self-pumping process. The procedure to determine total entropy generation can be used in a parametric study to evaluate the performance of two-phase hot water heating systems relative to other solar water heating options.

Description: Many previously studied natural convection enclosure problems in the literature have the bounding walls of the enclosure responsible for driving the flow. A number of relevant applications contain sources within the enclosure which drive the fluid flow and heat transfer. The motivation for this work is found in solar thermal storage tanks with immersed coil heat exchangers. The heat exchangers provide a means to charge and discharge the thermal energy in the tank. The enclosure is cylindrical and well insulated. Initially the interior fluid is isothermal and quiescent. At time zero, a step change in the source temperature begins to influence the flow. The final condition is a quiescent isothermal fluid field at the source temperature. The governing time-dependent Navier-Stokes and energy equations for this configuration are solved by a finite element method. Solutions are obtained for 103≤RaD≤106. Scale analysis is used to obtain time duration estimates of three distinct heat transfer regimes. The transient heat transfer during these regimes are compared with limiting cases. Correlations are presented for the three regimes.

Description: The cost of disposal of liquid industrial waste is of intense concern to all industrial waste generators, but especially so to smaller generators which are penalized by the economy of scale. As a last resort to on-site treatment or recycling, the smaller generator is forced to depend upon commercial waste disposal at costs approaching $6.00 per gallon. The use of solar energy to evaporate water from aqueous wastes is a potentially viable alternative if the contaminants are salts having low volatility. This paper describes a field test conducted with a small solar evaporator installed in a machine shop. A naturally vented, basin-type evaporator was used to evaporate water from a spent plating solution used to anodize aluminum. Measured evaporation data validated performance predictions made earlier using TRNSYS and TMY weather data. The data also indicated that accepted correlation models for predicting performance of solar stills underpredict evaporator performance by about 20 percent.

Description: A model is developed to predict the total solar irradiance received by a tilted collector in the presence of a large shielding tilted surface. The model is suggested to replace the commonly accepted isotropic model which does not account for a shielding surface. The present model is found to depend mainly on the configuration factors between the collector and the sky and the ground in the presence of a shielding surface. Expressions for these configuration factors are derived for a general geometry of the collector and a large shielding surface. These expressions are found to depend on the tilt angles of the collector and the surface, the orientation angle and the location of the collector relative to the surface, and the height and width of the shielding surface. The expressions are verified numerically and also, by comparing them with the results obtained from the literature for special geometries. The expressions are simplified to facilitate their use for the case of a vertical collector and a vertical shielding surface. This special case is needed to accurately estimate the solar irradiance received by glass windows of buildings.

Description: Recent progress to better understand the environmental threat of micrometeoroid and space debris to the solar dynamic radiator for the Space Station Freedom power system is reported. The objective was to define a design which would perform to survivability requirements over the expected lifetime of the radiator. A previous paper described the approach developed to assess on-orbit survivability of the solar dynamic radiator due to micrometeoroid and space debris impacts. Preliminary analyses were presented to quantify the solar dynamic radiator survivability. These included the type of particle and particle population expected to defeat the radiator bumpering. Results of preliminary hypervelocity impact (HVI) testing performed on radiator panel samples were also presented. This paper presents results of a more extensive test program undertaken to further define the response of the solar dynamic radiator to HVI. Tests were conducted on representative radiator panels (under ambient, nonoperating conditions) over a range of particle size, particle density, impact angle, and impact velocity. Target parameters were also varied. Data indicate that analytical penetration predictions are conservative (i.e., pessimistic) for the specific configuration of the solar dynamic radiator. Test results are used to define more rigorously the solar dynamic radiator reliability with respect to HVI. Test data, analyses, and survivability results are presented.

Description: A porous, packed bed, volumetric solar collector consisting of two dissimilar layers of spherical beads is numerically modeled. The bed is irradiated on the top surface by concentrated solar flux isotropic within a known cone angle. A gas stream perfusing the bed is heated by convection with the solid particles. The equation of radiative transfer, which accounts for absorption, emission, and linearly anisotropic scattering in the bed, is simplified by employing the P1 differential approximation. The bed materials are spectrally selective in the solar and infrared wavelengths. Sensitivity studies are used to identify the critical input parameters of the system, and a baseline configuration, which incorporates the key requirements of an efficient solar collector, is adopted. Parametric studies are conducted on the mass flow rate, incident solar flux, top layer porosity, solar absorptivity, particle diameter, and degree of back scatter. Tailoring of the particle and fluid temperature profiles and enhancing the efficiency of the collector by an appropriate selection of these critical parameters is demonstrated. Various high-temperature ceramics with suitable radiative properties are identified and their relative performance in the collector is assessed.

Description: This paper presents the development of a three-dimensional numerical model to study the distributions of indoor air velocity, air temperature, contaminant concentration, and ventilation effectiveness in a two-zone enclosure. The numerical model is based on the k–ε two-equation model of turbulence and the SIMPLE algorithm. The false-time step and ADI iteration procedure are employed. The results of the computed velocity and temperature profiles and convective heat transfer by the model are in good agreement with the measurements as well as with the prediction of the PHOENICS code.

Description: This work addresses the entropy generation aspects of a latent heat storage in which the energy delivered by a hot gas flowing through a cylindrical tube induces melting of the material surrounding the tube. The heat transfer for conduction-dominated melting is analyzed, taking into account the two-dimensional effects. The storage process irreversibilities associated with both the gas flow and the heat transfer (including entropy generation in the melted layer) are considered. The number of entropy generation units, which is a measure of the thermodynamic imperfection of the energy storage process, is expressed as a function of the main design parameters of the system. Analytic bounds and simplified asymptotic expressions for this quantity are derived. The results are compared with earlier one-dimensional studies.

Description: A great deal of experimentation and analysis has been performed to quantify penetration thresholds of components which will experience orbital debris impacts. Penetration has been found to depend upon mission-specific parameters such as orbital altitude, inclination, and orientation of the component; and upon component specific parameters such as material, density, and the geometry particular to its shielding. Experimental results are highly dependent upon shield configuration and cannot be extrapolated with confidence to alternate shield configurations. Also, current experimental capabilities are limited to velocities which only approach the lower limit of predicted orbital debris velocities. Therefore, prediction of the penetrating particle size for a particular component having a complex geometry remains highly uncertain. This paper describes the approach developed to assess on-orbit survivability of the solar dynamic radiator due to micrometeroid and space debris impacts. Preliminary analyses are presented to quantify the solar dynamic radiator survivability, and include the type of particle and particle population expected to defeat the radiator bumpering (i.e., penetrate a fluid flow tube). Results of preliminary hypervelocity impact testing performed on radiator panel samples (in the 6 to 7 km/sec velocity range) are also presented. Plans for further analyses and testing are discussed. These efforts are expected to lead to a radiator design which will perform to Space Station Freedom requirements over the expected lifetime.

Description: This paper develops a four-parameter change-point model of energy consumption as a function of dry-bulb temperature, along with accompanying error diagnostics for the model’s parameters. The model is a generalization of the widely used three-parameter, or variable-base degree-day method. The model is applied to data from a case study grocery store, is compared to the three-parameter PRISM CO model of the store data, and is shown to provide a statistically better fit to consumption data below about 15°C. This model appears to be useful for diagnosing unexpected energy use in some buildings and should be useful for determining retrofit energy savings from monitored pre-retrofit and post-retrofit data for the class of buildings whose pre-retrofit consumption is fit by a four-parameter linear change-point model.

Description: Horizontal axis wind turbine (HAWT) performance is usually predicted by using wind tunnel airfoil performance data in a blade element momentum theory analysis. This analysis assumes that the rotating blade airfoils will perform as they do in the wind tunnel. However, when stall-regulated HAWT performance is measured in full-scale operation, it is common to find that peak power levels are significantly greater than those predicted. Pitch-controlled rotors experience predictable peak power levels because they do not rely on stall to regulate peak power. This has led to empirical corrections to the stall predictions. Viterna and Corrigan (1981) proposed the most popular version of this correction. But very little insight has been gained into the basic cause of this discrepancy. The National Renewable Energy Laboratory (NREL), funded by the DOE, has conducted the first phase of an experiment which is focused on understanding the basic fluid mechanics of HAWT aerodynamics. Results to date have shown that unsteady aerodynamics exist during all operating conditions and dynamic stall can exist for high yaw angle operation. Stall hysteresis occurs for even small yaw angles and delayed stall is a very persistent reality in all operating conditions. Delayed stall is indicated by a leading edge suction peak which remains attached through angles of attack (AOA) up to 30 degrees. Wind tunnel results show this peak separating from the leading edge at 18 deg AOA. The effect of this anomaly is to raise normal force coefficients and tangent force coefficients for high AOA. Increased tangent forces will directly affect HAWT performance in high wind speed operation. This report describes pressure distribution data resulting from both wind tunnel and HAWT tests. A method of bins is used to average the HAWT data which is compared to the wind tunnel data. The analysis technique and the test set-up for each test are described.

Description: A one-dimensional porous-medium model to determine the thermal characteristics of energy storage for the utilization of phase change material (PCM) in packed capsules has been developed. Comparisons of this theory with experimental data obtained in the previous work show good agreement. Predictions from the present model have also been compared with the lumped capacitance model which assumes temperature uniformity in the packed capsules and in the coolant flow. The results show that the one-dimensional model has the advantage of predicting the temperature distributions of PCM and coolant. For engineering application, a useful correlation of the overall heat-transfer coefficient for ice-water cool storage in packed capsules has also been presented.

Description: A detailed numerical model is presented for high-temperature, catalytically enhanced, solar absorption chemical reactors. In these reactors, concentrated solar energy is volumetrically absorbed throughout a porous absorber matrix impregnated with a catalyst. The catalyst promotes heterogeneous reactions with fluid-phase reactant species flowing through the absorber. This paper presents a description of a numerical model and the basic concepts of reactor operation. The numerical model of the absorber includes solar and infrared radiation, heterogeneous chemical reactions, conduction in the solid phase, and convection between the fluid and solid phases. The model is nonlinear primarily due to both the radiative transfer and the heterogeneous chemistry occurring in the absorber. The nonlinear two-point boundary value problem is solved using superposition with orthonormalization and an adaptive solution point scheme. This technique preserves accuracy throughout the domain. The model can be modified for other chemical reactions and can be simplified to model volumetric air-heating receivers.

Description: This paper presents the results of scoping deaeration experiments conducted with warm surface seawater under open-cycle ocean thermal energy conversion (OC-OTEC) conditions. Concentrations of dissolved oxygen in seawater at three locations (in the supply water, water leaving the predeaerator, and discharge water from an evaporator) were measured and used to estimate oxygen desorption levels. The results suggest that 7 percent to 60 percent of the dissolved oxygen in the supply water was desorbed from seawater in the predeaerator for pressures ranging from 35 to 9 kPa. Bubble injection in the upcomer increased the oxygen desorption rate by 20 percent to 60 percent. The data also indicated that at typical OC-OTEC evaporator pressures, when flash evaporation in the evaporator occurred, 75 percent to 95 percent of the dissolved oxygen was desorbed overall from the warm seawater. The results were used to find the impact of a single-stage predeaeration scheme on the power to remove noncondensable gases in an OC-OTEC plant.

Description: The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) is a 100 kWchemecal test reactor currently in operation. This type of high-temperature chemical reactor volumetrically absorbs concentrated solar energy throughout a catalytic porous absorber matrix volume, promoting heterogeneous reactions with fluid-phase reactant species flowing through the absorber. A numerical model of these reactors has been developed to provide guidance in the catalytic matrix design for CAESAR. In the CAESAR reactor, methane is reformed using carbon dioxide and a rhodium catalyst. In addition, the model is being used to evaluate both the reactor performance and test data. This paper presents the thermal and chemical characteristics of the reactor for varying incident solar flux, fluid mass flow, convective heat-transfer coefficient, solar and infrared extinction coefficients, and catalyst loading. Predicted CAESAR performance is based on a prototype absorber and anticipated operating conditions. Model results suggest the mass flux must be proportioned to the incident solar flux radial distribution to prevent unacceptably high local temperatures and to provide a reactor having more uniform exit conditions. Either the catalytic loading or geometric thickness of the absorber should be increased for conversion to approach equilibrium levels. Also, the optical density of the matrix (particularly at the sunlit side of the reactor) should be decreased to distribute solar energy more uniformly in depth and decrease matrix temperatures at the front of the absorber.

Description: Since 1986, the 3355 m2 salt gradient solar pond facility in El Paso, Texas, has operated with a temperature difference between the upper and lower zones of 55 to 75° C while delivering industrial process heat, grid-connected electrical power, and thermal energy for the experimental production of desalted water. Because the El Paso solar pond is an inland facility, it is necessary to recycle the salt in a sustainable salt management system. A method that uses the main pond surface for initial brine concentration and short-term storage was developed after it became evident that the original evaporation pond system was undersized. This paper examines the method for brine concentration and storage, the effects of a brine storage zone on pond operation, and the installation of an enhanced evaporation net system and an automatic scanning injection system. A short description of the performance history and current status of the project is also included.

Description: Direct measurements of fluid flow rates in thermosyphon solar water heaters in the field are difficult. This paper reports on an activity to develop, calibrate, and evaluate a simple device for such measurements. A suitable device should have the following general characteristics: low pressure drop, rugged, inexpensive, simple, nondisruptive, low power requirements, and reasonably accurate. Moderate success has been achieved in satisfying all of these requirements with a hydrogen bubble generator (utilizing the electrolysis of water) in a clear plexiglass tube mounted in a vertical section of the flow line in which the flow is upward. The rate at which the hydrogen bubbles are swept along with the water can be used (when calibrated) to indicate the flow rate.

Description: Design of a vapor transport solar water heater using R-123 is optimized to maximize thermal performance and minimize life cycle cost. Optimal systems for one and two-story homes are identified in a parametric study of collector area, condenser heat transfer area, water storage tank volume and circulating refrigerant volume for the Solar Rating and Certification Corporation rating procedure. Selection of collector and heat exchanger areas is critical to performance. With warm and cloudless ambient conditions, the most economic system is a large collector area, low efficiency system capable of meeting nearly 100 percent of the load. Under more realistic operating conditions, the best design will place more emphasis on efficiency. The condenser should have sufficient surface area to avoid significant increases in collector operating temperatures. Thermal performance is relatively insensitive to either refrigerant or water storage volume.

Description: Experimental and analytical analyses are presented for the evaluation of heat transfer through floating solar swimming pool covers. Two improved floating solar swimming pool cover designs are proposed and investigated. The results conclusively show that both new cover designs should have significantly better performance than conventional floating solar swimming pool covers.

Description: We analyze the advantages of solving building energy management problems with the techniques of optimal control. Our approach consists of describing the dynamic behavior of a heated building with a simple model and controlling the whole system by minimizing a criterion defined for a time horizon of a few days. The two control components are the heat delivered to the building, and the variable heat exchange through the building envelope. In Part I, input (control and meteorological data) and output (indoor temperature) are related through a simplified state-space representation of the building. Part II is devoted to the actual computation of the control input. Results are given for two categories of buildings: The first is characterized by important direct solar gains. The inside structure is of low thermal inertia and so is the heating system. The second type of building is well insulated, with less glazing and less solar gain. The heavy internal structure of the building and the distribution of heat give a large thermal inertia to the system.

Description: A technique for simultaneously measuring thermal conductivity and thermal diffusivity was evaluated for applications with insulation materials and liquids. The thermophysical properties are determined from a single, transient measurement. The measurement apparatus employs an encapsulated bead thermistor as the sensing probe. The results from using a glass-encapsulated and a teflon-encapsulated bead thermistor are described.

Description: Experiments were carried out to observe the solidification sequence and void distribution for two different experimental liquids (cyclohexane and butanediol) enclosed in Pyrex tubes. Both liquids exhibited about ten percent volumetric shrinkage during the phase transition from liquid to solid. To evaluate the possibility of regulating void formation by soluble gases, tests were conducted both in the presence and absence of dissolved air. A physical model has been developed which predicts the essential features of the solidification pattern under earth’s gravity field of 1-g for cylindrical geometries and allows extrapolation of the results to outer space conditions of 0-g. Finally, an attempt was made to determine analytically the final void shape which would result from each nucleated bubble in 0-g. Understanding these aspects of the solidification process is vital to development of better thermal energy storage systems for space power applications.

Description: A study of infiltration-type air flows through a room-size enclosure with an obstructing wall was conducted. A finite volume (cell finite difference) method was used, and a k-ε turbulence model was retained in the simulations. Detailed monitoring of how a simulated gaseous pollutant was diluted and dispersed over time was accomplished, and showed that nearly stagnant zones exist. The simple air change (ACH) method of rating the ventilation capabilities of an enclosure was judged to be marginally acceptable, with a local air age scheme a suggested alternative for multizone cases. Based on the simulations, it was argued that the tracer-gas empirical method of determining ventilation capability may be prone to large uncertainties if flow obstructions or multiple zones exist and only a few sampling locations are used.

Description: Steady natural convection heat transfer has been studied in porous wall, solar collector systems. The boundary conditions were: two isothermal walls at different temperatures, two horizontal bounding adiabatic walls, and either uniform or nonuniform heat generating porous layer without vents. The aspect ratio A was from 0.5 to 1.4. The Rayleigh number varied from 103 to 108. The Darcy number was from 10−8 to 10−2 but the detailed studies were carried out only for 10−4, an optimum value. The results are presented in terms of thermal parameters (θ, θmax, Nu) as a function of Ra and other nondimensional parameters (A=H/L, B=1/L, F=d/L, kr). The isotherms and streamlines within the system are also produced. The overall results indicate that geometrical parameters are the most important parameters affecting the system performance.

Description: Silica gel-water vapor is commonly suggested as a working media for solar-powered desiccant cooling systems since the system can be regenerated at relatively low temperatures. In the present study the sorption rates of water vapor by regular density silica gel particles were measured in the pressure range of 1–25 torr using a constant volume/variable pressure apparatus. The sorption rate was determined from recorded time variation of water vapor pressure in a test unit of known volume during the sorption process. The mass transfer film resistance was eliminated by evacuating the system and by introducing pure water vapor into the test unit. The apparent solid-side diffusivity was obtained by matching the analytical solution of the simultaneous heat and mass transfer governing equations to the experimental data. The uptake measurements had been performed for three particle sizes of silica gel (150 μm, 1 mm, and 3 mm). The tests were performed sequentially in small steps over a range of initial silica gel moisture content ranging from near zero up to 0.25 kg H2O per kg dry silica gel. The effect of moisture content and particle size on the sorption rate and apparent diffusivity were determined. The effect of charging on time variation of pressure was evaluated and used for correction on all sorption data.

Description: The effect of the circulation number (the number of times the entire storage mass is circulated through the system each day) on the stratification coefficient is investigated. An earlier model ignored the effect of the circulation number, assuming that a large value (a value 〉 2) did not have any effect. In this paper, an analytical model is developed to consider the effect of low circulation number. This model is combined with the previous model and a general expression for the stratification coefficient is developed that is valid for the entire range of circulation number. The extended analytical model is verified through numerical simulations. The results are valid for liquid-based as well as air heating systems. Clarifications are made regarding the definition of the stratification coefficient by introducing a concept of an instantaneous stratification coefficient.

Description: Global, spectral isolation data measured every minute over a four-year period at Dhahran (26° 32′ N, 50° 13′ E) Saudi Arabia, using five Schott filters with cutoff at “285, 500, 530, 630, and 695” nm, and ultraviolet radiation sensor, 295–385nm, are analyzed. Monthly averages of the diurnal variations of these bands are presented. The rainfall, dust/sand storm, cloud and air mass effects on the band radiation are also investigated. Comparisons of the yearly average of band radiation measured at Dhahran and those reported in Goldberg and Klein (1977) for Jerusalem (32° N) and Rockville (39° N) are presented. Monthly average band radiation are also presented. The data showed that the ratio of the monthly average of the diurnal band radiations to the total radiation for winter and summer are nearly the same. The data also showed that the rainfall increases the percentage of radiation in the bands 385–500nm, 500–530nm, and 630–690nm, and it decreases the percentage radiation of the band 690–2800nm and that the opposite is true for dust/sand storm effect. The change in the band radiation due to cloud cover is small. The data also showed that the monthly and yearly averages of the radiation in the bands “630–695, and 695–2800” nm are latitude independent. The monthly radiation values at each band over the year are almost constant.

Description: Much of the seasonal cooling operation of the heat pump occurs at part-load conditions when the unit cycles on and off to meet the cooling load. The seasonal efficiency under part-load conditions of the heat pump is typically estimated from a laboratory measurement of the degradation coefficient (CD). Manufacturers are only required to estimate CD at a single test condition where the indoor coil performs sensible cooling only. The effects of transient dehumidification losses are not accounted in estimating the seasonal efficiency. In hot and humid climates, dehumidification performance of a heat pump is as important as the sensible cooling performance. Therefore, a series of tests were designed to quantify the part-load dehumidification characteristics of a three ton residential air-to-air heat pump. The tests include: cycling rates from 0.8 to 10 cycles per hour (cph), percent on-times of 20, 50, and 80 percent, indoor dry-bulb temperature between 22.2°C and 26.7°C, and indoor relative humidity between 20 to 67 percent. The outdoor conditions and the indoor air flow rate were constant for all test runs. All experiments were performed in psychometric chambers under controlled conditions. The dehumidification process started between 60 to 150 seconds after start-up depending on the test conditions. During start-up, the losses in the latent capacity were greater than the losses in the sensible capacity. The dehumidification response increased with indoor dry-bulb temperature at constant relative humidity and decreased with indoor temperature at constant dew-point temperature.

Description: In this paper, alternative approaches for synthetically generating a wind speed time series are discussed. These approaches include: (1) the use of independent values from a specific probability distribution; (2) the use of an algorithm based on the statistical behavior of a one-step Markov chain; (3) the use of an algorithm based on the behavior of a transition probability matrix that describes the next wind speed value statistically as a function of the current wind speed value and the previous wind speed value; (4) the use of Box-Jenkins models; (5) the use of the Shinozuka algorithm; and (6) the use of an embedded Markov chain. The ability of each approach to capture the statistical properties of the desired wind speed time series is discussed. In this context the statistical properties of interest are the probability distribution of the wind speed values, the autocorrelation function of the wind speed values, and the spectral density of the wind speed values.

Description: Analyses are presented for the performance of direct contact jet and spray condensers in the presence of noncondensables. A solution procedure is developed which obtains analytically the liquid-side transient temperature profile, including the thermal entrance effect. The approach avoids iteration at the liquid-vapor interface, which usually is required to determine the interface temperature, by locally linearizing the energy and mass transfer relationship at the gas-liquid interface. Comparisons are carried out with some available data and correlations and show good agreement. The analyses are shown to provide a rational basis for correlating the experimental data. Unlike previous correlations, which are basically curve fits to the applicable data, the present analysis shows clearly the effects of noncondensables, flow, and geometric parameters on the condenser performance. Results are presented for conditions typical of open-cycle ocean thermal energy conversion (OC-OTEC).

Description: The steady-state laminar natural convection for air bounded by a hot plate and a cold cylindrical enclosure has been studied numerically for the case of cold isothermal cylinder and hot isothermal plate. A correlation is presented for the average Nusselt number over the range of Rayleigh number from 105 to 106 for different values of the width-aspect ratio Sw and thickness aspect-ratio St of the plate. It is found that the average Nusselt number increases with increasing Sw and Rayleigh number. A two-cell pattern is observed for Sw=1.5 and less. The effect of Sw on the average Nusselt number is found to be stronger than that of St.

Description: The state-of-the-art of parabolic dish solar concentrators is the faceted, glass-metal dish. The mass production costs of glass-metal dishes may be high because they do not incorporate the innovations of design and materials developed over the last eight years. Therefore, Sandia National Laboratories has undertaken to develop two stretched-membrane parabolic dish concentrators for the Department of Energy’s Solar Thermal Program. These solar concentrators are being designed for integration with an advanced solar receiver and a Stirling engine/generator in a 25-kWe power production unit. The first dish, which builds on the successful design of the stretched-membrane heliostats, is to be a low risk, near-term commercial solar concentrator. This solar concentrator comprises 12 large, 3.6-m diameter, stretched-membrane facets that are formed into parabolic shapes either with a large vacuum or by performing the thin membranes plastically. The focal length-to-diameter ratios (f/Ds) for the facets are about 3.0, relatively large for a dish but much lower than heliostats where they typically range from 50 to 100. Two contractors are currently fabricating facets for this dish, and a third contractor is designing the facet support structure and pedestal for the dish. The second stretched-membrane concentrator is a single-element monolithic dish with an f/D of 0.6. The dish is shaped into a parabola by plastically yielding the membrane using a combination of uniform and nonuniform loading. Initial measurements of the dish indicate that it has a slope error of 2.6 milliradians (one standard deviation) relative to a perfect parabola. In this paper, the designs of the two stretched-membrane dishes are analyzed using the computer code CIRCE to model the optical performance of the concentrators and a thermal model, which includes conduction, convection, and radiation heat transfer, to calculate the thermal losses from the cavity solar receivers. The solar collector efficiency, defined as the product of the optical efficiency of the collector and the thermal efficiency of the receiver, is optimized for comparing the performance of several solar concentrator configurations. Ten facet arrangements for the faceted stretched-membrane dish and the single-element stretched-membrane dish are modeled and their performances compared with that of a state-of-the-art glass-metal dish. Last, the initial designs of these two stretched-membrane dishes are described along with the results of preliminary performance measurements on their respective optical elements.

Description: Concentrated sunlight can be stored in the chemical bond by activating an endothermic reaction. This novel concept has been implemented recently with solar power captured in a central receiver equipped with chemical reactors. The related theory, presented in this report, singles out this interactive radiative-chemical system as distinctively stable thermodynamically, resilient to perturbations, internally regulatory and self-corrective. None of the thermochemical devices conceived so far bear all these attributes.

Description: The goal of this study is to determine and produce the thermal properties of solid-state phase-change materials appropriate for solar system space heating storage (transition temperatures in the 40 to 60°C range). A major effort is directed toward improving the overall heat storage characteristics of solid-state phase-change materials by increasing the materials’ thermal conductivities. The solid-state phase change materials focused on in this study are neopentyl glycol and pentaglycerine. The results from testing various materials are reported as thermophysical property values. The results from a constant heat flux, thermal storage charging experiment are reported for both the solid-state materials and the enhanced conductivity materials. The storage system modeled is a tube bank with hot fluid inside the tubes transferring heat to the solid-solid phase-change material outside the tubes.

Description: This paper analyzes velocity profiles for flow through circular tubes in laminar, turbulent, and transition region flows and how they affect measurement by flow-meters. Experimental measurements of velocity profiles across the cross-section of straight circular tubes were made using laser doppler velocimetry. In addition, flow visualization was done using the hydrogen bubble technique. Velocity profiles in the laminar and the turbulent flow are quite predictable which allow the determination of meter factors for accurate flow measurement. However, the profiles can not be predicted at all in the transition region. Therefore, for the accuracy of the flowmeter, it must be ensured that the flow is completely in the laminar regime or completely in the turbulent regime. In the laminar flow a bend, even at a large distance, affects the meter factor. The paper also discusses some strategies to restructure the flow to avoid the transition region.

Description: A method used to measure and validate the temperature field in an experimental downdraft biomass gasifier and reformulate the discrete test data into continuous mathematical functions is described. The reformulated temperature field is analyzed to identify different reaction zones within the gasifier. Distinct endothermic and exothermic regions, separated by rather complicated boundaries, are found.

Description: A numerical simulation model was employed to investigate the effects of ambient temperature and insolation of the efficiency of compound parabolic concentrating solar energy collectors. The limitations of presently used collector performance characterization curves were investigated and a new approach proposed. The major advantage of the new procedure over those employed previously is that different solar collector performance characteristics can now be readily normalized to a common set of environmental conditions. Thus, an equitable comparison may be made, in the context of the application conditions, of rating characteristics for disparate collectors which were obtained initially under different conditions.

Description: The mass weighted average storage temperature is a useful parameter for evaluating the performance of thermal storage systems. It is commonly used in system simulations since detailed temperature data in the storage medium are not known (or even considered). However, it is rarely used in the evaluation of operating systems because of the difficulty in determining it, especially in potentially multidimensional storage configurations, like a horizontal tank. This paper examines the issue of the evaluation of the average storage temperature, specifically from the point of view of reproducibility and how much data is required for a confident calculation. Unfortunately, this issue was not completely resolved and was shown to be strongly dependent on the locations of the measurements. Two examples are presented that demonstrate the usefulness of knowing the average storage temperature. The examples also give a quantitative demonstration of the effect of withdrawal flow rate on the amount of thermal energy that can be withdrawn from a given system. A reduction in flow rate by a factor of four increased the delivered energy by 70 percent.

Description: Space power technologies have undergone significant advances over the past few years, and great emphasis is being placed on the development of dynamic power systems at this time. A design study has been conducted to evaluate the applicability of a combined cycle concept—closed Brayton cycle and organic Rankine cycle coupling—for solar dynamic space power generation systems. In the concept presented here (solar dynamic combined cycle), the waste heat rejected by the closed Brayton cycle working fluid is utilized to heat the organic working fluid of an organic Rankine cycle system. This allows the solar dynamic combined cycle efficiency to be increased compared to the efficiencies of two subsystems (closed Brayton cycle and organic fluid cycle). Also, for small-size space power systems (up to 50 kW), the efficiency of the solar dynamic combined cycle can be comparable with Stirling engine performance. The closed Brayton cycle and organic Rankine cycle designs are based on a great deal of maturity assessed in much previous work on terrestrial and solar dynamic power systems. This is not yet true for the Stirling cycles. The purpose of this paper is to analyze the performance of the new space power generation system (solar dynamic combined cycle). The significant benefits of the solar dynamic combined cycle concept such as efficiency increase, mass reduction, specific area—collector and radiator—reduction, are presented and discussed for a low earth orbit space station application.

Description: A unified approach is developed for the analysis of one, two, or three-phase melting or solidification of a semi-infinite medium with or without subcooling or superheating and imposed with constant, monotonic, or cyclic temperature or flux conditions. A source and sink method is presented in which a sink front is used to characterize a melt front while a source front is used to characterize a freeze front. An integrodifferential equation is then derived for the interface position which is linearized locally for numerical solution. This position is, in turn, used as input for the determination of the temperature distribution and energy storage and release in different phases of the medium. The numerical solution presented in this paper has shown to be unique, convergent, stable, and accurate. The analysis has been applied to the study of phase change in a subcooled paraffin wax imposed with a cyclic temperature condition. Test results yield some interesting phenomena related to the merging of phase-change fronts and hysteresis of energy storage and release, among others, which have not previously been reported in the literature. Their relations to the energy storage and release are particularly stressed in the paper.

Description: Thermal modeling of packed bed, thermal energy storage systems has traditionally been limited to first-law considerations. The exceptions include a few second-law studies, noted in the Introduction, of sensible heat storage systems and the latent heat storage systems. The cited second-law studies treat the storage and removal processes essentially as “batch” heating and cooling. The approximation effectively ignores the significant temperature gradient, especially in the axial direction, in the storage medium over a substantial portion of both the storage and removal processes. The results presented in this paper are for a more comprehensive model of the packed bed storage system utilizing encapsulated phase-change materials. The fundamental equations for the system are similar to those of Schumann, except that a transient conduction equation is included for intraparticle conduction in each pellet. The equations are solved numerically, and the media temperatures obtained are used for the determination of the exergy (or availability) disposition in complete storage-removal cycles. One major conclusion of the study from both the first-law and second-law perspectives is that the principal advantage in the use of phase-change storage material is the enhanced storage capacity, compared with the same size of packed bed utilizing a sensible heat storage material. Thermodynamically, however, it does not appear that the system employing phase-change storage material will always, or necessarily, be superior to that using a sensible heat-storage material. The latter conclusion is reached only on the basis of the second-law evaluation.

Description: The effects of haze on the angular distribution of the diffuse sky radiance are investigated by examining and comparing data for individual clear skies with and without haze, and for other selected sky types, and by modeling and comparing data for such skies. It is found that haze appears to affect the radiance distributions only for clear, partly cloudy, and lightly overcast skies, and that for clear skies, increasing haze increases the absolute sky radiance values. In addition, the radiance distributions for clear skies with haze are found to exhibit characteristics generally similar to those for the nonopaque overcast skies. However, it is also found that the normalized radiance distributions for clear skies with haze, which generally provide clearer illustrations of distribution shape than the absolute radiance distributions, exhibit slightly less intense circumsolar and horizon brightening components than do the normalized distributions for clear skies without haze. In general, a systematic, but complex, relationship appears to exist between haze and the distribution of the diffuse sky radiance. The results may prove useful to several disciplines, including the design of solar technologies.