Showing papers in "Journal of Solar Energy Engineering-transactions of The Asme in 1982"

Effect of Circumsolar Radiation on Performance of Focusing Collectors

Abstract: In this paper the Lawrence Berkeley Laboratory (LBL) circumsolar data are used to calculate the effect of circumsolar radiation on both the instantaneous and the long-term average performance of focusing collectors. To speed up the computations, the optical properties of the concentrator are expressed in terms of an angular acceptance function. The angular acceptance function and the brightness distribution of the sun are then convoluted, a procedure which requires, for each circumsolar scan, only 56 multiplications and additions (one for each of the angular intervals at which the brightness has been measured by the LBL circumsolar telescope). While the formalism is applicable to all focusing collectors, only parabolic troughs and dishes are evaluated explicitly in this paper. The instantaneous performance corresponding to particular circumsolar scans is of interest for the analysis of test results of collectors with high concentration. For most predictions of long-term average performance a far simpler approach will, however, be adequate. For this purpose a standard synthetic circumsolar scan has been developed that describes the brightness distribution of the solar disk (''limb darkening'') and of the circumsolar region. The radiation intercepted by a receiver is calculated once for the solar portion and once for the circumsolar portionmore » of this standard sun shape. The long-term average radiation intercepted by the receiver is then obtained by averaging the solar and the circumsolar intercept factors, weighted according to the long-term average circumsolar ratio for the location and period under study. Tested against hour-by-hour simulations, this simple approach was found to have an rms error of only 0.2 percent, negligible compared to typical uncertainties in system performance calculations. This analysis points out the usefulness of the long-term average circumsolar ratio as a simple measure of circumsolar radiation.« less

An Overview of Open-Cycle Desiccant-Cooling Systems and Materials

Abstract: This paper has presented a review of the thermodynamics of three desiccant cooling cycles: the ventilation cycle, the recirculation cycle, and the Dunkle cycle For the ventilation cycle the qualitative effects of changes in the effectiveness of individual components were analyzed There are two possible paths to improved dehumidifier performance: changing the design of dehumidifiers using currently available desiccants so as to increase effectiveness without increasing parasitic losses, or developing new desiccants specifically tailored for solar cooling applications The later part of this paper has considered the second option A list of desirable desiccant properties was defined, properties of currently used solid and liquid desiccants were compared to this list, and a hypothetical desiccant type that would give improved system performance was discussed

Second Law Analysis of Desiccant Cooling Systems

Abstract: A second law analysis of constant pressure open cycle cooling systems was performed in order to assess the potential performance of desiccant cooling systems. The results yield upper bounds which are independent of the nature of the desiccant. Calculations carried out for a wide range of operating conditions show that the performance of these open cycle systems strongly depends on the humidity ratio. For ARI conditions, the reversible COP is about 4.66, and it approaches infinity at very low outdoor humidity ratios.

Performance of a Cross-Cooled Desiccant Dehumidifier Prototype

Abstract: A cross-cooled desiccant dehumidifier prototype was constructed and tested. The cross-cooled dehumidifier prototype was constructed in the form of a cube with a side dimension of 0.6 m. The cooling was achieved by passing air through rectangular channels perpendicular to the process channels which are lined with desiccant sheets, consisting of 9 ..mu..m Syloid 63 silica gel held in a Teflon web. The process for the manufacture of the silica gel sheets was developed at the Illinois Institute of Technology. The performance of the dehumidifier prototype was parametrically studied in a test bed that simulates the performance of a cross-cooled desiccant cooling system. The system was tested over a range of operating conditions typically encountered in field installations of such systems. The performance parameters studied are moisture cycle, cooling capacity, and coefficient of performance.

Analysis of two-phase flow solar collectors with application to heat pumps

Abstract: A thermodynamic model is developed to analyze the thermal performance of twophase solar collectors The well-known equilibrium homogeneous theory is used to model the two-phase flow in the solar collectors The resultant set of coupled ordinary differential equations for saturated pressure and quality of working fluid in the collector tubes are solved by an iterative procedure using a fourth-order RungeKutta method The results are then applied to determine the thermal performance of a solar assisted heat pump which uses two-phase flow collectors as the evaporator The results indicate that even with the use of less expensive bare solar collectors as evaporator for the heat pump, the heating coefficient of performance (COP /SUB H/) as high as 6 can be obtained under realistic ambient conditions provided a proper matching exists between the collector's evaporative capacity and the compressor's pumping capacity

The Use of Ground-Coupled Tanks in Solar-Assisted Heat-Pump Systems

Abstract: A research program at Brookhaven National Laboratory (BNL) has studied ground coupling, i.e., the use of the earth as a heat source/sink or storage element, for solar-assisted heat-pump systems. As part of this research program, four buried tank experiments were operated between December 1978 and March 1981 in order to determine the feasibility of ground-coupled tanks in these systems. Heat was added to or removed from the tanks according to a weekly schedule derived from computer simulations of solar heat-pump systems in the local (New York) climate. Each tank was operated according to a different control strategy. This paper presents experimental results from these tank experiments for this period, and compares these results to those generated by a computer model, GROCS, developed at BNL. The model is found to be valid, for the most part, using undisturbed soil thermal properties which provide the best fit to the data most of the time. Its results are very sensitive to soil thermal conductivity during periods of large heat addition to the tanks. A ground coupled tank is found to be desirable in series solar-assisted heat-pump systems. However, no important carry-over of summer-collected heat to winter was observed.

The solar transmittance of some convection suppression devices for solar energy applications: an experimental study

Abstract: The solar transmittance of seven convection suppression devices (CDSs) has been measured as a function of both angle of incidence and azimuth angle using a large integrating sphere test facility. The CSD designs tested include a honeycomb and various slat geometries made from FEP Teflon film, and a tubular glass honeycomb. All FEP Teflon CSDs tested had solar transmittances not less than 0.94 for angles of incidence up to 45 deg, whereas for the glass tubular CSD, the solar transmittance was not less than 0.87 over the same range. The results for FEP Teflon CSDs compare well with previous theoretical and experimental studies of similar CSDs. Empirical solar transmittance correlation equations have been derived, based on a simple CSD solar transmittance model, and they match the measured performance of the CSDs to within 2 percent. The transmittance, reflectance, and absorptance of each CSD to isotropic diffuse radiation have been determined. The radiation properties data presented provides extensive information on some alternative CSD designs, some of which have not been analysed previously.

High-temperature ceramic heat exchanger element for a solar thermal receiver

Abstract: The development of a high-temperature ceramic heat exchanger element to be integrated into a solar receiver producing heated air was studied. A number of conceptual designs were developed for heat exchanger elements of differing configuration. These were evaluated with respect to thermal performance, pressure drop, structural integrity, and fabricability. The final design selection identified a finned ceramic shell as the most favorable concept. The shell is surrounded by a larger metallic shell. The flanges of the two shells are sealed to provide a leak-tight pressure vessel. The ceramic shell is to be fabricated by a innovative combination of slip casting the receiver walls and precision casting the heat transfer finned plates. The fins are bonded to the shell during firing. The unit is sized to produce 2150 F air at 2.7 atm pressure, with a pressure drop of about 2 percent of the inlet pressure. This size is compatible with a solar collector providing a receiver input of 85 kw(th). Fabrication of a one-half scale demonstrator ceramic receiver was completed.

Transient Response of Solar Collectors

Abstract: A transient response model is developed for collectors whose dynamic behavior is controlled predominantly by the absorber surface and heat-transfer fluid. Such collectors can be described adequately by two differential equations for which exact solutions are possible. The equations are solved for the case of evacuated tube collectors, and the solutions are found to exhibit qualitatively new transient phenomena. In particular, it is found that fluid flow rates, thermal masses, and heat-transfer coefficients are important factors in determining the overall response of a collector, and that the functional form of the response depends on how these parameters are related. The model is compared with test results for four evacuated-tube collectors whose heat-transfer configurations and fluid flow rates encompass a broad range of transient response regimes. Excellent agreement is obtained in all cases. In addition to providing a detailed insight into the problem of collector dynamics, the model is a potentially useful tool for obtaining information about heat-transfer coefficients and fluid flow distribution effects.

Selection of a Working Fluid for an Organic Rankine Cycle Coupled to a Salt-Gradient Solar Pond by Direct-Contact Heat Exchange

Abstract: An analysis was conducted of power production by an organic Rankine cycle engine coupled to a solar pond with direct-contact heat exchange. Use of a direct-contact boiler reduced the projected plant cost by 25 percent. The choice of a working fluid affects plant efficiency, turbine size, and working fluid losses. Pentane was shown to be the working fluid best suited to this application.

Comparative Productivity of Distillation and Reverse Osmosis Desalination Using Energy From Solar Ponds

Abstract: This paper presents comparative analyses of two methods for producing desalted water using the heat collected by a solar pond - the first by distillation, and the second by reverse osmosis. The distillation scheme uses a multiple-effect distiller supplied with steam generated in a flash boiler using heat from a solar pond. Solar pond water passes through a heat exchanger in the water system ahead of the flash boiler. The second scheme uses a similar arrangement to generate hydrocarbon vapor which drives a Rankine cycle engine. This engine produces mechanical/ electrical power for the RO plant. The analyses use two pond water temperatures -82.2/sup 0/C (180/sup 0/F) and 71.1/sup 0/C (160/sup 0/F) -- which seem to cover the range expected from salt-gradient ponds. In each case, the pond water temperature drops by 5.56/sup 0/C (10/sup 0/F) while passing through the vapor generator system. Results of these analyses show that, based on the assumptions made, desalted water could be produced by distillation at productivity rates much greater than those estimated for the RO plant.

Transient Simulation of Absorption Machines

Abstract: This paper presents a model for a water-cooled Lithium-Bromide/water absorption chiller and predicts its transient response both during the start-up phase and during the shutoff period. The simulation model incorporates such influencing factors as the thermodynamic properties of the working fluid, the absorbent, the heat-transfer configuration of different components of the chiller and related physical data. The time constants of different components are controlled by a set of key parameters that have been identified in this study. The results show a variable but at times significant amount of time delay before the chiller capacity gets close to its steady-state value. The model is intended to provide an insight into the mechanism of build-up to steady-state performance. By recognizing the significant factors contributing to transient degradation, steps can be taken to reduce such degradation. The evaluation of the residual capacity in the shut-off period will yield more realistic estimates of chiller COP for a chiller satisfying dynamic space cooling load.

Systematic Rotation and Receiver Location Error Effects on Parabolic Trough Annual Performance

Abstract: This report deals with the effects of certain systematic errors on performance and, therefore, their influence on the design of troughs. Systematic rotation error is the angle between the reflector vertex-focus axis and the vertex-sun axis; systematic receiver location error is the vectorial deviation of a receiver from focus. The existence of systematic rotation errors and systematic receiver location errors can have a significant effect on the annual performance of parabolic trough collectors. These systematic errors can exist in addition to errors which are random in nature and which, therefore, can be treated statistically. Systematic rotation errors of 16 milliradians result in annual performance degradation of greater than 30 percent. Systematic receiver location errors can have a similar effect depending upon magnitude. The technique for calculating the influence of systematic errors on performance is outlined and methods of identifying and minimizing these errors are suggested.

A Methodology for Assessment of Wind Turbine Noise Generation

Abstract: The detailed analysis of a series of acoustic measurements taken near several large wind turbines (100 kWand above) has identified the maximum acoustic energy as being concentrated in the low-frequency audible and subaudible ranges, usually less than 100 Hz. These measurements have also shown any reported community an­ noyance associated with turbine operations has often been related to the degree of coherent impulsiveness present and the subsequent harmonic coupling of acoustic energy to residential structures. Thus, one technique to assess the annoyance potential of a given wind turbine design is to develop a method which quantifies this degree of impulsiveness or coherency in the radiated acoustic energy spectrum under a wide range of operating conditions. Experience has also shown the presence of annoying conditions is highly time dependent and nonstationary, and, therefore, any attempts to quantify or at least classify wind turbine designs in terms of their noise annoyance potential must be handled within the proper probabilistic framework. A technique is described which employs multidimensional, joint probability analysis to establish the expected coincidence of acoustic energy levels in a contiguous sequence of octave frequency bands which have been chosen because of their relationship to common structural resonant frequencies in residential buildings. Evidence is presented to justify the choice of these particular bands. Comparisons of the acoustic performance and an estimate of the annoyance potential of several large wind turbine designs using this technique is also discussed.

Design of a Packed Bed Thermal Storage Unit for a Solar System

Abstract: A method for the design and sizing of a packed bed thermal storage unit for a hot air solar heating system is presented. A controlled bypass of the heat storage unit during the retrieval process is used in order to maintain the fluid temperature leaving the system at a constant value. The use of a bypass arrangement with a closed air circulation loop for the collector enables the storage unit to operate at a higher mean temperature and the system to utilize the maximum allowable pressure drop. This results in a greater heat storage per unit volume of storage material. The complete solar system is simulated numerically during both the heating and retrieval modes of operation to obtain a set of design charts for sizing the thermal storage unit. In the simulation, consideration is given to geographical location, composition of the storage bed and the maximum allowable pressure drop across the bed. A set of design curves for State College, Pennsylvania, is presented.

Modeling of the Surface Convective Layer of Salt-Gradient Solar Ponds

Abstract: A one-dimensional numerical model is developed to predict the diurnal variations of vertical temperature and concentration profiles in salt-gradient solar ponds. The model employs augmented thermal and mass diffusivities due to turbulent wind mixing and double-diffusive convection. Numerical results indicate that the thickness of the surface convective layer increases with wind speed (or the wavelength of the surface wave). Double-diffusive convection is, in the absence of wind, capable of sustaining a mixed layer at the surface when strong cooling occurs over the pond. In field ponds, the surface convective layer is probably maintained as a result of the combined action of wind-generated turbulent mixing and double-diffusive convection.

Modeling and analysis of an all-plastic flat-plate solar collector

Abstract: The concept of an all-plastic liquid flat-plate solar collector was studied. Such a collector eliminates the need for coatings, metals, and all other materials except plastics. The concept appears to offer possibilities for the mass production of a lowcost, lightweight solar collector which is reasonably efficient. Material considerations for the concept are discussed, and thermal analysis is presented. Mathematical models were developed for study of this concept. Convection and nonlinear radiation between the covers was computed in all of the models. In addition absorptance and conduction in the cover(s) and the fluid cover plate were studied in order to determine their effects on the panel performance. A model panel was fabricated, and computer results were compared with test data for the efficiency of the panel.

Gradient Zone Constraints in a Salt-Stratified Solar Pond

Abstract: An operational condition is defined for a salt-stratified solar pond. A criterion based on empirical evidence is formulated which shows a limit is approached as steady operating conditions are attained. For sodium chloride ponds, the criterion indicates a dependence between salt and temperature gradients during steady-state conditions. At the present time, appropriate data for explaining the physical basis of this relationship are not available. Significant results are found as a result of the gradient zone criterion. First, the gradient zone criterion leads to an explanation for the formation of an upper convecting zone. Results obtained with the gradient zone criterion relate to salt maintenance in a pond. Depending on the temperature operating conditions of a pond, a specific amount of salt must be in the pond. Also, salt concentration levels maintained in the upper and lower convecting zones are shown to significantly affect a pond's thermal energy performance.

A regional comparison of solar, heat pump, and solar-heat pump systems

Abstract: A comparative study of the thermal and economic performance of the parallel and series solar-heat pump systems, stand-alone solar, and stand-alone heat pump systems for residential space and domestic hot water heating has been undertaken for the United States using F CHART 4.0. The results are useful for a regional assessment of the viability of the different systems, and for assessing policies that will encourage the implementation of the most energy efficient system. The magnitude of the potential energy savings was determined for each system on the basis of an equal system cost in the case of the series, parallel, and solar systems. The cost was governed by current federal tax credits, and found to be 10,000 dollars. The size and cost of the heat pump are the same in the series, parallel, and stand-alone heat pump systems. A line can be drawn across the United States north of which the parallel heat pump system saves the most energy, and south of which the solar system saves the most. The better of either the solar or the parallel systems consistently used less energy than either stand-alone heat pump or series systems for all locations. The conventional oil or gas furnacemore » seasonal efficiency which would be required to save as much primary energy as the better alternative system was identified regionally. In all but the northern portions of the United States, conventional furnaces would use more primary energy than the better alternative system. The price that the solar collector in the series heat pump system would have to be so that a larger collection system could be installed and the series system would match the energy savings of the preferred system, whether solar or parallel heat pump, was calculated. This price was one-half to two-thirds of current collector prices. The break-even electricity price was determined which is the price below which the life cycle savings of the alternative system are positive.« less

OTEC Plant Response and Control Analysis

Abstract: An analysis is presented which allows prediction of closed-cycle OTEC power plant system response and control. Two basic operational control schemes are presented, which are primarily related to the type of seawater pumps employed. Variable flow seawater pumps allow optimization of the OTEC thermal-cycle state points for maximization of net generated power. Constant flow pumps are cheaper and simpler, but do not allow direct control over the evaporator and condenser operating temperatures. A system of nonlinear differential equations representing the basic elements of a constant seawater flow OTEC plant with turbine bypass flow control has been formulated for computer solution. Typical normalized response curves are presented for pressures, temperatures, mass flow rates, and generator speed for a small-scale, 50-kW OTEC plant design.

Heliostat Operation at the Central Receiver Test Facility, 1978–1980

Abstract: The operations and maintenance data and conclusions presented in this are for the 222 heliostats that have been in use at the Central Receiver Test Facility (CRTF) from 1978 through 1980. The CRTF beam produces a total power of 5.5 MWth and a peak intensity of 2250 kW/m/sup 2/ near solar noon. Improvements in the targeting accuracy have been made. The mirror reflectance is maintained near 80 percent by cleaning with rain or snow. The CRTF heliostats logged almost 300,000 operating hrs by the end of 1980.

An Experimental Investigation of a Stationary Reflector/Tracking Absorber Solar Collector at Intermediate Temperatures

Abstract: This paper describes the design, construction, and testing of a concentrating solar collector based on the Stationary Reflector/Tracking Absorber (SRTA) principle. The system consists of a fixed, 2.5-m dia, spherical mirror which focuses solar radiation on a movable, cylindrical absorber tracking the sun. This work is an extension of earlier tests with a similar collector in which output temperatures up to 150/sup 0/C were obtained with water. The present system, using a mineral-oil-based heat transfer fluid, was capable of heating the fluid up to 300/sup 0/C under steady-state conditions. The direct radiation efficiency ranged from 50 percent at low temperatures up to 30 percent near 300/sup 0/C, showing the SRTA to be an effective concentrating collector for the intermediate temperature range which can be constructed at relatively low cost.

Predictions of Low-Frequency and Impulsive Sound Radiation From Horizontal-Axis Wind Turbines

Abstract: This paper develops theoretical models to predict the radiation of low-frequency and impulsive sound from horizontal-axis wind turbines due to three sources: (i) steady blade loads, (ii) unsteady blade loads due to operation in a ground shear, (iii) unsteady loads felt by the blades as they cross the tower wake. These models are then used to predict the acoustic output of MOD-I, the large wind turbine operated near Boone, N. C. Predicted acoustic time signals are compared to those actually measured near MOD-I; good agreement is obtained.