ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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).

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: A performance study is presented of a Rankine organic cycle powered by a low temperature solar collector. In this work a two-phase collector is considered where the heat transfer fluid is vaporized and its saturated vapor expands in a turbine according to a Rankine cycle. The collector system is divided into a boiling and a nonboiling (subcooled) part: The limit between the two depends upon the value of flow rate and radiation. A modified form of the Bliss equation is used to model the thermal performance of the collector in terms of thermal efficiency versus DTI [DTI= (Absorber average temperature-Ambient temperature)/ Solar Radiation]. The system is analyzed by second-law analysis, and it includes several exergy losses of different types (heat transfer, heat loss, etc.) which determine the overall exergy balance. Different working fluids are considered, and optimization to a certain extent is demonstrated from this point of view. In order to minimize irreversibilities and guarantee the most efficient conversion processes, the most important point is the right selection of the collector operating pressure level, which depends on the instantaneous value of radiation and ambient temperature (as well as on the collector thermal performance). The choice of the optimal pressure level is done by means of second-law arguments; the flow rates across the collector, the turbine, and the condenser are consequently determined. A simulation over a typical sunny day in Florence, Italy allows the calculation of the expected daily performance.

Beschreibung: The City of Austin is studying the impact of solar water heaters on summer peak electric demand. One passive and two active solar water heating systems were installed on city-owned commercial buildings which had electric water heaters in 1985 and have been monitored for two years. A method has been developed to determine the peak demand reduction attributable to the solar systems. Results show that solar water heating systems are capable of large demand reductions as long as there is a large hot water demand to displace. The average noncoincident demand reduction (during the water heater’s peak output) ranged from 0.8 to 5.8 kilowatts per system. However, the coincident demand reduction during the utility peak demand period was 0.3 to 0.8 kilowatts per system. Thus, a critical factor when assessing the benefit to the electric utility is the time of hot water demand.

Beschreibung: The fresh water floating collectors solar pond was investigated both experimentally and theoretically in a previous work, and it is now matched, by simulation, with the heat load requirements of a greenhouse. Results of the simulation indicate that such a pond is a potential energy source for greenhouse heating. This is especially true when the material properties are such that solar absorption and storage are enhanced. To demonstrate this point, three sets of collectors constructed with materials of different physical (radiation) properties were tested. One set is constructed of common materials which are readily available and are normally used as covers for greenhouses. The second set is made of improved materials which are also available but have a smaller long-wave transmittance. The last set is made of “ideal” material which additionally possesses selective radiation absorption properties. Collectors made of “ideal” materials make a superior solar pond; thus, manufacturing films with improved properties should become a worthwhile challenge for the agricultural polyethylene-films industry. Preliminary economic studies indicate that even with the low oil (〈 $20/Bbl) prices which exist between 1986-1989, the fresh water floating collectors solar pond provides an economically attractive alternative to the conventional oil-burning heating system. This is especially true in mild climate areas and when the large initial investment is justified by long-term greenhouse utilization planning.

Beschreibung: Equations and parameter characteristics were examined for both closed and open-earth loops. Analysis and graphs are presented on the heat exchange effectiveness of air and liquid earth loops and how the heat transfer is affected by major parameters such as: pipe diameter, soil thermal conductivity, fluid velocity, and fluid type. The open loop will produce more heat per unit length of pipe while the closed produces more total system heat. However, final loop selection is based on many factors that are both site and system specific.

Beschreibung: In high temperature solar energy applications highly concentrating optical systems, such as, e.g., parabolic dishes, achieve typical radiation flux densities 〉2 MW/m2. In order to investigate thermo and photochemical reactions at temperatures 〉1500 K and radiation flux densities 〉2 MW/m2 a solar furnace was built at Paul Scherrer Institute (PSI). This furnace is a two-stage concentrator. The first stage is a prefocusing glass heliostat with a focal length of 100 m. The second stage is a highly concentrating parabolic dish with a focal length of 1.93 m. To design experiments to be carried out in the focal region of the parabolic dish, the radiation flux as well as its density distribution have to be known. This distribution is usually measured by radiometric methods. However, these methods are generally rather troublesome because of the high temperatures involved. In this paper we present a simple method to estimate the characteristic features of the radiation flux density distribution in the focal region of a concentrator system. It is well known from solar eclipses that the mean angular diameter of the moon is almost equal to that of the sun (9.1 mrad versus 9.3 mrad). Hence, the lunar disk is well suited to be used as a light source to investigate the flux distribution in a solar furnace. Compared to the sun the flux density is reduced by 4·105 and the flux density distribution can be inspected on a sheet of paper located in the plane of interest, e.g., the focal plane. This distribution was photographed and analyzed by means of an image processing system. The density distribution was also simulated using a Monte Carlo ray tracing program. Based on this comparison, and on further ray tracing computations, we show that the peak flux density decreases from 8.9 MW/m2 in December to values below 4 MW/m2 in June and the net radiation flux from 25 kW to 15 kW, respectively.

Beschreibung: The two-stage linear Fresnel reflector solar concentrator is analyzed via an in-depth study of an installed, nominally 220 KWt system. The concentrator includes: (1) a primary linear Fresnel reflector comprised of curved mirrors and (2) a secondary nonimaging CPC-type trough with a tubular receiver. The principal practical design options for the secondary concentrator are evaluated. Via a computer simulation which includes ray-tracing of the primary reflector, we evaluate the sensitivity of energy output to: concentrator optical errors, system geometry, tracking mode, and the option of using flat versus curved primary mirrors. The two-stage Fresnel concentrator can be considerably less expensive than the corresponding parabolic trough collector, but is found to deliver about one-fourth less yearly energy. However, much of this difference could be eliminated through the use of higher-quality CPC reflectors.

Beschreibung: Efficiency of binary power cycles can be improved by expanding the hydrocarbon working fluids through two-phase region in a turbine and exiting at saturated or superheated condition. This improvement can be achieved if there is no condensation during the expansion or if there is condensation, the droplet size is extremely small. In order to verify this, a particle sizing technique for extremely small particles in flow is needed. In this study, a laser-based technique is developed by which it is possible to detect particles as small at ten angstroms in size. The basis of the technique is that particles of size less than one third of the wavelength of the incident radiation will scatter according to Rayleigh scattering theory. According to this theory, the intensity of the scattered light will be the same in the forward as well as in the backward directions. Therefore, measurement of the scattered intensity at two or three different angles will confirm the presence of Rayleigh scattering. The size of the particles can, then, be calculated from the measured scattered intensity according to the Rayleigh scattering equation.

Beschreibung: The present work enables prediction of the performance of a solar still through simple calculations. Estimation of the temperature of the glass cover by an empirical relation developed in this work permits calculation of the heat-transfer coefficients, the upward heat flow, and evaporation. Since some of the heat-transfer coefficients vary substantially and nonlinearly with temperature, the empirical relation developed for glass cover temperature is based on an approximate solution of the heat balance equation. Hence, the overall upward heat flow factor is obtained with a maximum absolute error of three percent compared to the value obtained through a numerical solution of the heat balance equation along with the relations for vapor pressure and latent heat. The fraction of upward heat flow utilized for evaporation is determined with a maximum absolute error of 0.5 percent. The range of variables covered is 30°C to 80°C in water temperature, 5W/m2K to 40W/m2K in wind heat-transfer coefficient, and 5°C to 40°C in ambient temperature.