bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance Analysis of a Closed-Loop Climate Control System Using Underground Air Tunnel
Authors: Goswami D. Y. , Ileslamlou S.
Published: June 2008

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.
Combined Solar and Internal Load Effects on Selection of Heat Reclaim-Economizer HVAC Systems
Authors: Sauer Harry J. , Howell Ronald H. , Wang Zijie
Published: June 2008

The concern for energy conservation has led to the development and use of heat recovery systems which reclaim the building internal heat before it is discarded in the exhaust air. On the other hand, economizer cycles have been widely used for many years in a variety of types of HVAC systems. Economizer cycles are widely accepted as a means to reduce operating time for chilling equipment when cool outside air is available. It has been suggested that heat reclaim systems should not be used in conjunction with an HVAC system which incorporates an economizer cycle because the economizer operation would result in heat being exhausted which might have been recovered. Others suggest that the economizer cycle can be used economically in a heat recovery system if properly controlled to maintain an overall building heat balance. This study looks at potential energy savings of such combined systems with particular emphasis on the effects of the solar load (amount of glass) and the internal load level (lights, people, appliances, etc.). For systems without thermal storage, annual energy savings of up to 60 percent are predicted with the use of heat reclaim systems in conjunction with economizers when the heat reclaim has priority. These results strongly demonstrate the necessity of complete engineering evaluations if proper selection and operation of combined heat recovery and economizer cycles are to be obtained. This paper includes the basic methodology for making such evaluations.
Dynamic Solar Domestic Hot Water Testing
Authors: Spirkl W.
Published: June 2008

A dynamic test procedure for SDHW systems is presented, which generalizes correlation models to the nonstationary case. It is shown that testing can be performed with an accuracy comparable to stationary models. The dynamic procedure is more flexible in testing, since any load and radiation profiles are explicitly taken into account including the ones naturally occurring. The parameters are identified using filter and least squares technique. Long-term performance is predicted with the same model used for testing. The method is applied to experimental data of four different systems.
Inflatable Concentrators for Solar Propulsion and Dynamic Space Power
Authors: Grossman G. , Williams G.
Published: June 2008

This paper describes the development of an inflatable concentrator for solar propulsion, providing the source of heat to a hydrogen engine aboard the Solar Rocket. The latter is a device designed to carry payloads from a low earth orbit (LEO) to a geosynchronous orbit (GEO) at significant mass savings in comparison to chemical propulsion; it involves two lightweight parabolic reflectors in an off-axis configuration focusing solar radiation into the absorbers of the engine, which causes the emission of a hot hydrogen jet. Each of the reflectors has an elliptical rim with a 40 m major axis, providing heat to the propellant sufficient to produce about 40 lbs. of thrust. The same concentrator concept is contemplated for space power application—to focus solar radiation on a conversion device, e.g., a photovoltaic array or the high temperature end of a dynamic engine. Under the present project, a one-fourth scale, 9×7 m off-axis inflatable concentrator has been under development as a pilot for the full-scale flight unit. The reflector component consists of a reflective membrane made of specially designed gores and a geometrically identical transparent canopy. The two form together an inflatable lens like structure which, upon inflation, assumes the accurate paraboloidal shape. This inflatable structure is supported along its rim by a strong, bending-resistant torus. The paper describes the development of this system including the analysis leading to determination of the gore shapes, the reflector membrane design and testing, the analysis of the supporting torus, and a discussion of the effects of the space environment.
Determination of the Distribution of Incident Solar Radiation in Cavity Receivers With Approximately Real Parabolic Dish Collectors
Authors: Bammert K. , Hegazy A. , Lange H.
Published: June 2008

The absorption of solar heat and the attendant thermal and mechanical loadings on the tubes of cavity receivers depend predominantly on the flux distribution of the incident solar radiation. For an axially symmetric cavity receiver with a parabolic dish collector, it is simple to determine the insulation pattern on the receiver internal surfaces if the system is ideal. In such a system the surface of the dish is perfectly parabolic (no contour flaws are present), and the sun’s central ray impinges on the dish surface parallel to the focal axis (no sun tracking flaws are present). These two conditions cannot be achieved in practice, and therefore the feasible parabolic dish system is referred to as a “real” system although, in actual fact, it is only an approximation to any actual system. The purpose of this paper is to devise calculation principles which permit analysis of a receiver designed for ideal conditions (Bammert and Seifert, 1983; Bammert and Hegazy, 1984; Johanning, 1987) to verify its structural adequacy under the nonideal conditions to be expected in reality. Of the many possible imperfections in real collectors, two were selected which increase the loadings sustained. The first case concerns flaws in the contour of the dish surface. These locally increase the radiation concentration on the receiver inside walls and tubing. In the second case, sun-tracking errors give rise to axially asymmetric radiation distributions. In both examples, greater than design basis loadings will occur in the receiver tubing. Both kinds of flaws considered in this paper are of a purely deterministic nature. Other flaws statistically distributed on the dish surface (Ko¨hne and Kleih, 1987, Gu¨ven, Bannerot, and Mistree, 1983; O’Neill and Hudson, 1978; Ratzel et al., 1987) do not cause structural overloading but must be taken into account in thermal performance analysis. The paper presents a method of analyzing the flux distribution on the internal surfaces of a cavity receiver with an approximately real parabolic dish collector. After a discussion of the theoretical principles, the effects of the collector contour and sun tracking errors on the insulation pattern are described with reference to an example.
The Effect of Component Efficiency and Operating Conditions on the 50-kW Dish Stirling System in Riyadh, Saudia Arabia
Authors: Noyes Gerold W.
Published: June 2008

This paper deals with the development of a weather data base and the performance prediction of a 50-kW dish Stirling system. An analysis of direct solar insolation data for 1985 from the site in Riyadh, Saudi Arabia was made to determine the available solar energy. A parameter study was done of the effects of component efficiencies and operating conditions on instantaneous and yearly average system efficiency using the prepared weather data. The system performance was found to be most affected by wind, mirror reflectivity, and exact placement of the receiver in the focal point of the mirror.
Optimization of a Closed-Cycle OTEC System
Authors: Uehara Haruo , Ikegami Yasuyuki
Published: June 2008

Optimization of an Ocean Thermal Energy Conversion (OTEC) system is carried out by the Powell Method (the method of steepest descent). The parameters in the objective function consist of the velocities of cold sea water and warm sea water passing through the heat exchangers, the phase change temperature, and turbine configuration (specific speed, specific diameter, ratio of blade to diameter). Numerical results are shown for a 100-MW OTEC plant with plate-type heat exchangers using ammonia as working fluid, and are compared with calculation results for the case when the turbine efficiency is fixed.
An Automation of Collector Testing and Modification of ANSI/ASHRAE 93-1986 Standard
Authors: Huang B. J. , Hsieh S. W.
Published: June 2008

The steady-state performance test of solar collectors using ANSI/ASHRAE 93-1986 Standard was revised and an automation for the testing was carried out in the present study in order that the test can be easily performed outdoors in areas with variable weather conditions. It was shown that the 95 percent settling time of the collector τ95 can be adopted as the time basis in the selection of steady-state period for the test. To make the best use of the time available for the testing, the steady-state period defined by ANSI/ASHRAE 93-1986 Standard was changed to the τ95 plus five minutes, or ten minutes, whichever is larger. To reduce scatter uncertainty in the test results, the test period for the efficiency calculation was chosen as the segment of the last five minutes in the steady-state period and a steadiness condition defined statistically was adopted. To shorten the time for each test run a PC-based expert testing system, which is completely automatic and requires no operator, was developed in the present study. Using this expert system associated with the modified ANSI/ASHRAE 93-1986 Standard, we can effectively carry out the collector test at variable weather conditions with small scatter uncertainty and can substantially shorten the duration of a test.
Development of an Outdoor Test Facility for Wind Convectors
Authors: Monaghan P. F. , Finn D. P. , Oosthuizen P. H.
Published: June 2008

This paper deals with measurement of heat transfer performance of wind convectors, an alternative air source evaporator system for heat pumps. An automatically controlled and monitored outdoor wind convector test facility that is capable of measuring heat transfer rates and overall heat-transfer coefficients to within ± 5 percent measurement uncertainty for up to three wind convectors has been designed, built, and tested. Data on air temperature and humidity, solar radiation, and wind speed and direction are simultaneously collected. The choice of measurement technique for each variable and an error analysis for each sensor is discussed. Typical graphical test results are presented.
Analysis of Combined Radiation and Convection in a Particulate-Laden Liquid Film
Authors: Kumar Sunil , Tien C. L.
Published: June 2008

This paper presents a simple but complete model of the interaction of radiative flux with convective transport in an absorbing, emitting, and scattering falling film. The method also accounts for the spectral and angular variations of the radiative emissions, reflections, refractions, scattering, and of the radiative properties and intensities. Such a model is required to design Direct Absorption Solar Receivers and to numerically evaluate their performance. Previous analyses in the literature have either neglected the flow field or have not considered scattering in the film and have assumed the properties to be spectrally constant. The present model has no such drawbacks, and is easy to understand and to numerically implement on the computer.
A Comparative Study of the Aerodynamics of Several Wind Turbines Using Flow Visualization
Authors: Eggleston D. M. , Starcher K.
Published: June 2008

Flow visualization techniques were used to study the flows over the Enertech 21-5, Carter 25, and Enertech 44-50. Despite centrifugal effects superimposed on the aerodynamics, tufting (gross aerodynamic behavior) and oil flow (average boundary layer behavior), tests reveal the nature and many of the details of the flows involved. Results were compared to expected flow patterns based on angles of attack calculated from the PROPPC code. Chord Reynolds numbers ranged between 75,000 (Enertech 21-5) to 1,340,000 (Enertech 44-50). The typical low Reynolds number flow characteristics of these airfoils, including laminar separation bubbles, turbulent reattachment, and complete separation were observed. Full or partial reattachment due to tower shadow was observed on each machine. Spanwise flow was observed near the leading edge of the Enertech 21-5. Cyclic radial flow from tower dam effect was also noted.
Yaw Loads and Motions of a Horizontal Axis Wind Turbine
Authors: Hansen A. C. , Butterfield C. P. , Cui X.
Published: June 2008

A computer model named YawDyn has been developed at the University of Utah for the analysis of yaw dynamics of horizontal axis wind turbines. This paper reports results of a validation study of YawDyn using full-scale test results from the Combined Experiment rotor developed and operated by the Solar Energy Research Institute (SERI). A primary finding of recent research is that stall hysteresis has a large influence on yaw moments on the Combined Experiment rotor. Instantaneous pressure distributions measured on the rotor provide conclusive evidence that stall hysteresis is present and calculations using YawDyn show that the hysteresis increases the mean yaw moment dramatically (typically over 100 percent) and the cyclic yaw moments to a lesser extent. This paper provides details of the effect of stall hysteresis and shows the results of representing hysteresis using a Gormont model for dynamic stall. Results of validation efforts using YawDyn to analyze both fixed and free-yaw behavior demonstrate the model is qualitatively correct. The quantitative accuracy of the model varies depending upon the test conditions and the rotor. In some situations the model is adequate while in other situations the accuracy must be improved. Consideration of stall hysteresis and skewed wake aerodynamics is vital to the success that has been obtained to date.
Evaluation of Prediction Methodology for Blade Loads on a Horizontal Axis Wind Turbine
Authors: Hartin John R.
Published: June 2008

The accurate prediction of horizontal axis, wind turbine cyclic blade loads is critical in the design of a machine that is fatigue life limited. A rotor code called LOADS has been developed that analyzes blade loads for rigid-hub rotors of simple geometry but includes blade flap-wise flexing and unsteady aerodynamics. The code is described and the results of its application to the SERI Combined Experiment Tests using turbulent wind simulation are presented with some initial conclusions regarding the accuracy of the results.
Effects of Trees on Wind Flow Variability and Turbulence
Authors: Elliott D. L. , Barnard J. C.
Published: June 2008

This paper describes the results of a field experiment at the Goodnoe Hills, Wash. site to examine the effects of trees on wind flow variability and turbulence. Although vegetation at the site consisted primarily of grass, scattered areas of trees that penetrated the site provided an excellent opportunity to evaluate the effects of surface roughness changes on the wind flow characteristics. Wind data collected at nine towers across the site revealed that surface roughness changes in the upwind fetch caused pronounced variations in the wind flow over site. At two towers that were frequently 200 m to 300 m downwind of a grove of 10-m to 18-m trees, 20–30 percent reductions in wind speed and a factor of two to three increase in turbulence were measured at a height of 32 m. A substantial increase in the magnitude of the wind gusts, as well as a considerable decrease in the mean wind speed, was observed when a tower was downwind of the trees. Implications for a wind turbine located downwind of the trees, with a hub height near 30 m, would be reduced power output, more variable power output, more start/stop cycles, and increased stress caused by the tree-induced turbulence. The effects of the trees on the wind flow characteristics were considerably reduced at heights of 60 m and at distances greater than 500 m.
Rotor Dynamics of Flywheel Energy Storage Systems
Authors: Jayaraman C. P. , Kirk J. A. , Anand D. K. , Anjanappa M.
Published: June 2008

This paper deals with the dynamic analysis of the magnetic bearing stack system. The stack consists of a single flywheel supported by two magnetic bearings. To model the system, the dynamic equations of a magnetically suspended flywheel are derived. Next, the four control systems controlling the four degrees-of-freedom of the stack are incorporated into the model. The resulting dynamic equations are represented as first-order differential equations in a matrix form. A computer simulation program was then used to simulate the working of the magnetic bearing stack. Real time plots from the simulation are used to show the effect of dynamic coupling on torque response. Frequency response is used to determine the resonance frequencies of the stack system. It is found that system stability depends on flywheel speed. On the basis of the above results suggestions are made to improve stability and allow the stack to be spun beyond 60,000 rpm.
An Optimal Open-Loop Flow Control for Double-Storage Solar Water Heaters
Authors: Dgany E. , Sokolov M.
Published: June 2008

A considerable increase in the available energy from a double-storage solar water heater (SWH) can be achieved by an open loop control of the transfer flow rate (TFR) between the main water storage tank, which is attached to the collector, and a secondary storage tank. The optimal TFR profile, which is based on a priori-known load profile, weather, and solar radiation patterns, ensures minimal auxiliary energy requirement to meet load demands. The optimization is performed by comparing the evaluated system performance under various TFR profiles which are described by a sequence of step functions. Typical computations indicate that the utilized energy of a typical system may be increased by 10 to 30 percent while similar thermal efficiency optimization yields only a few percent improvement.
Analysis and Comparison of Active Solar Desiccant and Absorption Cooling Systems: Part I—Model Description
Authors: Warren Mashuri L. , Wahlig Michael
Published: June 2008

A comparative analysis has been performed to compare the cooling and dehumidification performance of future ventilation-mode desiccant systems, proposed advanced absorption systems, and conventional vapor compression systems. A common framework has been developed for direct comparison of these different cooling technologies; this method is described in this paper. A companion paper presents the application of this method to specific cooling systems in five cities.
Thermodynamics of Phase-Change Energy Storage: The Effects of Liquid Superheating During Melting, and Irreversibility During Solidification
Authors: De Lucia M. , Bejan A.
Published: June 2008

This paper considers the question of whether the optimum phase-change temperature for maximum exergy storage is universally equal to the geometric mean of the heat source and environment temperature, Tm=(T∞Te)1/2. The study consists of three parts. The first deals with the conduction-melting process, and shows that the optimum melting temperature is generally greater than the geometric mean of the source and environment temperatures. The second part covers the conduction-solidification process, and concludes that the irreversibility of solidification decreases monotonically as the phase-change temperature increases. The third part treats the complete cycle of melting (storage) followed by solidification (retrieval), and demonstrates that the optimum phase-change temperature is greater than the optimum temperature of the melting process alone.
Analysis and Comparison of Active Solar Desiccant and Absorption Cooling Systems: Part II—Annual Simulation Results
Authors: Warren Marshuri L. , Wahlig Michael
Published: June 2008

A comparative analysis has been performed to compare the cooling and dehumidification performance of future ventilation mode desiccant systems, proposed advanced absorption systems, and conventional vapor compression systems. A common framework has been developed for direct comparison of these different cooling technologies; this method is described in a companion paper. This paper presents the application of this method to annual simulations of cooling system performance in five cities.
Estimation of Heat-Transfer Coefficients, the Upward Heat Flow, and Evaporation in a Solar Still
Authors: Sharma V. B. , Mullick S. C.
Published: June 2008

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.
Thermal Contact Electronic Packaging in Solar Pointing Space Environment
Authors: Lund Kurt O. , Colangelo Anthony M. , McKim Gregory S.
Published: June 2008

A thermal design for a solar pointing Space Shuttle mission is presented. The apparatus, which will measure solar flux intensity variations, contains sensors and data acquisition electronics which must be maintained within certain temperature constraints. The thermal design, which utilizes parallel heat flow paths and conduction fins to reject dissipated heat, is shown by finite difference thermal modeling to maintain component temperatures within these constraints. In the thermal modeling, arithmetic nodes are used to represent surface radiosity for radiation heat transfer. Also, the concept of mean fin conduction length and effective fin capacitance are introduced as means of simplifying the model representation of the conduction fins. An experiment was conducted to evaluate the chip/fin contact conductance.
Heat Transfer of Buried Pipe for Heat Pump Application
Authors: Mei Viung C.
Published: June 2008

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.
The p-h Diagram and the Vapor-Compression Cycle
Authors: Klausner J. F. , Mei R.
Published: June 2008

The representation of the p-h diagram for the thermodynamic vapor-compression cycle found in educational textbooks is misleading, and a suggestion for future presentation is made.
Frequency Domain Modeling of Free Yaw Response of Wind Turbines to Wind Turbulence
Authors: Madsen P. H. , McNerney G. M.
Published: June 2008

A frequency domain method has been developed in order to study the yaw dynamics and its effect on energy capture under various turbulence regimes for free yaw wind turbines. It is assumed that the primary forcing of the dynamic yaw originates from the variations from wind turbulence in the average wind direction and the horizontal gradient of the wind speed across the rotor. The horizontal components of the turbulence velocity are modeled as random processes with spectral characteristics calibrated from measurements. The corresponding statistics of the wind direction and the gradient are derived and used as input for an equivalent linear model of the system dynamics. The parameters in the simplified system model are identified from the results of a numerical test of a comprehensive yaw response and energy capture simulation model, developed at U. S. Windpower. The yaw response of the USW Model 56-100 machine to turbulence has been analyzed for a number of wind conditions and compared to test results with satisfactory agreement. The analysis shows that the effect on the yaw error from the horizontal wind gradient is twice the effect from wind direction changes.
Flux Density Distribution in the Focal Region of a Solar Concentrator System
Authors: Schubnell M. , Keller J. , Imhof A.
Published: June 2008

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.
Analytical Equation for the Top Heat Loss Factor of a Flat-Plate Collector With Double Glazing
Authors: Samdarshi S. K. , Mullick S. C.
Published: June 2008

An analytical equation for the top heat loss factor of a flat-plate collector with double glazing has been developed. The maximum computational errors resulting from the use of this equation are plus or minus three percent compared to numerical solution of the heat balance equations. The equation is considerably more accurate than the currently used semi-empirical equations over the entire range of variables covered. It is found that the computational errors resulting from simplification of the proposed equation by approximation of the individual heat-transfer coefficients are much lower than the errors resulting from the use of semi-empirical equations.
Parametric Analysis of Horizontal Air and Liquid Earth Loops
Authors: Froehlich Donell P. , Glawe Barbara J.
Published: June 2008

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.
A Predictive Model for the Migration of Double-Diffusive Interfaces
Authors: Zangrando F. , Fernando H. J. S.
Published: June 2008

A model is proposed to predict the rate of entrainment and detrainment and the conditions for equilibrium for diffusive boundaries of a double-diffusive fluid layer. The system consists of a stably stratified layer overlying a mixed, turbulently convecting layer, which is representative of solar pond situations. The results show that the entrainment/detrainment laws are strongly dependent on the operating parameter range. At equilibrium, the density stability ratio at the edge of the convection zone depends only on the ratio of molecular diffusivities. The predictions are in satisfactory agreement with the available experimental data at large values of salinity and temperature gradients.
Greenhouse Heating With a Fresh Water Floating Collectors Solar Pond: A Feasibility Study
Authors: Arbel A. , Sokolov M.
Published: June 2008

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.
Dynamic Analysis of a Closed-Cycle Solar Adsorption Refrigerator Using Two Adsorbent-Adsorbate Pairs
Authors: Hajji A. , Worek W. M. , Lavan Z.
Published: June 2008

A dynamic analysis of a closed-cycle, solar adsorption refrigerator is presented. The instantaneous and daily system performance are studied using two adsorbent-adsorbate pairs, Zeolite 13X-Water and Chabazite-Methanol. The effect of design and operating parameters, including inert material thermal capacitance, matrix porosity, and evaporator and condenser temperatures on the solar and cycle coefficients of performance are evaluated.
Numerical Simulation of a Desiccant Bed for Solar Air Conditioning Applications
Authors: Farooq S. , Ruthven D. M.
Published: June 2008

The cyclic operation of a desiccant bed used for solar air conditioning applications has been simulated numerically. The results suggest that the optimal choice of desiccant is not seriously limited by the shape of the isotherm since the effect of isotherm shape on the moisture removal rate can be adequately compensated by appropriate adjustment of the cycle time. The conditions required to maximize the moisture removal rate for any given degree of nonlinearity also satisfy the requirements for maximizing the thermal coefficient of performance.
Exergy Control for a Flat-Plate Collector/Rankine Cycle Solar Power System
Authors: Manfrida Giampaolo , Kawambwa Shukuru J. M.
Published: June 2008

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.
A Case Study of Electric Utility Demand Reduction With Commercial Solar Water Heaters
Authors: Ewert Michael , Hoffner John E. , Panico David
Published: June 2008

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.
Effect of Control Algorithms on Fixed Pitch Wind Turbine Generators
Authors: Rizzi M. P. , Auslander D. M.
Published: June 2008

The effect of control upon wind turbine performance is described using actual wind speed data from a wind farm in California. The algorithms studied are currently in use by some turbine manufacturers. The algorithms were studied using a computer model of a fixed pitch, utility connected wind turbine with an induction generator. The algorithm efficiency was found to be robust over a wide range of parameters. However, the number of start/stops varied greatly with different algorithms and parameters.
Analysis and Simulation of a Two-Phase Self-Pumping Water Heater
Authors: Walker H. A. , Davidson J. H.
Published: June 2008

The thermal performance of a two-accumulator self-pumping solar water heater is characterized in a daily simulation. The passive vapor transport system operates in cycles, alternating between run, pressurizing, and pump phases. Three isothermal closed-system thermodynamic models characterize the operational phases of the system. The applicable conservation of mass and energy equations of each model are combined in the numerical simulation. Instantaneous temperature and heat transfer rates, as well as integrated energy quantities and thermal efficiencies, are compared to experimental values. The qualitative behavior of the analytical model agrees with that of the physical system. Multiplying thermal loss coefficients by 2.5 and adjusting the theoretical solar model to correspond with measured insolation forces quantitative agreement of overall daily performance. The simulation reveals the impact of the duration of the pressurizing and pumping phases on overall performance. The volume and thermal capacitance of the components used during the pressurizing and pump phases should be minimized, while the insulation on those components should be maximized to optimize system performance. The validated model will be used in future work to optimize system design.
The Input-Output Model for Solar Water Heaters: Model Errors and Long-Term Performance Estimate
Authors: Kadi L. , Bourges B. , Adnot J.
Published: June 2008

Based on the experimentally observed linear relationship between the daily collected energy and the daily insolation, the development of a test method for the performance rating of solar water heaters is studied using both an analytical modeling of the daily performance and detailed simulation. This analysis has shown the existence of a general linear correlation between daily performance and external conditions (daily irradiation, ambient temperature, main’s water temperature, and storage initial temperature). Two approaches appeared possible for the long-term performance estimate and, for its simplicity, the statistical one was chosen for the definition of the European test method for solar water heaters, also known as the input-output test method.
Design Considerations for Heat-Pipe Solar Receivers
Authors: Adkins Douglas R.
Published: June 2008

Heat pipes are being developed to transfer solar energy from the focal point of a parabolic dish concentrator to the working fluid of Stirling engines. With these receivers, concentrated solar energy that is absorbed on the concave surface of a dome is removed by the evaporation of liquid sodium on the convex side of the dome. Vaporized sodium then condenses on an engine’s heater tubes and transfers energy to the working fluid of the engine. The condensed sodium returns to the absorber surface where it is redistributed across the dome by the capillary action of a wick. Issues concerning the flow of sodium in a heat-pipe solar receiver are investigated in this paper. A comparison is made between various wick options, and general issues concerning the design of heat-pipe receivers are also discussed.
Thermal Analysis and Fundamental Tests on a Heat Pipe Receiver for a Solar Dynamic Space Power System
Authors: Fujiwara Makoto , Sano Tamotsu , Suzuki Kiyoshi , Watanabe Shinya
Published: June 2008

Studies and fundamental tests were conducted on the heat pipe receiver. The advantages of this receiver are not only the improvement of reliability by the temperature uniformity, but the effective use of phase change thermal storage materials. The latter makes the receiver lighter and smaller. A performance analysis of the heat pipe receiver containing the thermal storage material was conducted using a computer program. Potassium was used as the working fluid of the heat pipe receiver. Results are compared with conventional receivers. The thermal storage ability of the phase change material (LiF-CaF2) was examined, and the influence of the volume change of the thermal storage material during its phase changes was also examined separately by an elementary test in a thermal storage tube. A prototype of the heat pipe receiver element was fabricated, and its performance was studied by simulating the insolation cycle with an electric heater.
Comparison of a Cavity Solar Receiver Numerical Model and Experimental Data
Authors: Hogan R. E. , Diver R. B. , Stine Wm. B.
Published: June 2008

Results from a numerical model of axisymmetric solar cavity receivers are compared with experimental data for tests of a novel test bed receiver in the Sandia National Laboratories solar furnace. The computed energy transfer rates and temperatures are compared with the experimental data for different receiver geometries, aperture sizes, and operating conditions. In general, the agreement between the numerical model and the experimental data is better for the small-to-midsized apertures than for the large apertures. The analysis indicates that for the larger apertures, the convective heat losses are over predicted. It also suggests that these losses could be better characterized. Sensitivity analyses show that both the total solar energy input rate and the convective heat-loss coefficient significantly affect the receiver thermal performance and that the distribution of the input solar flux significantly affects the temperature distribution in the receiver.
A Passive, Vapor Compression Refrigerator for Solar Cooling
Authors: Loehrke R. I.
Published: June 2008

A new, completely passive, vapor compression refrigerator is described in this paper. This refrigerator combines elements of the heat pipe and the vapor jet refrigerator and is referred to here as a heat pipe refrigerator. It may be driven with heat from low temperature solar collectors or with industrial waste heat and used to provide cooling. Compression work is provided by gas dynamic processes and liquid pumping may be obtained using gravitational or capillary forces. No power is required for operation. The device has no moving parts and may be externally similar to a heat pipe with three heat transfer zones. The working fluid is chosen to match the desired operating temperature range. Water, at subatmospheric pressure, is an appropriate fluid for operation around room temperature. Theoretical considerations indicate that the thermal coefficient of performance of the heat pipe refrigerator will depend strongly on the magnitude of the temperature differences over which it is designed to operate. Results from a laboratory test confirm the concept and demonstrate cooling down to the freezing point using water vapor at 51°C to drive the device and with heat rejection at 18°C.
Mathematical Modeling of a Thin Layer Solar Kiln
Authors: Elsayed Moustafa M.
Published: June 2008

Mathematical modeling of a thin layer solar kiln for drying of agricultural products is presented. Governing equations of the air temperature and air humidity ratio, material temperature and its moisture content, and the glass temperature are derived together with their initial conditions. Heat and mass transfer between the material and the air and the air and the glass cover are estimated using empirical relations for the coefficients of heat and mass transfer. The results of the calculations are analyzed to predict the effect of the following parameters on the transient performance of the kiln: (a) rate of absorbed solar energy in the kiln, (b) rate of air flow through the kiln, (c) volume of the dried material, (d) ambient temperature, and (e) humidity ratio of the ambient air.
Gas Desorption From Seawater in Open-Cycle Ocean Thermal Energy Conversion Barometric Upcomers
Authors: Ghiaasiaan S. M. , Wassel A. T. , Pesaran A. A.
Published: June 2008

Gas desorption from warm and cold seawater under open-cycle ocean thermal energy conversion (OC-OTEC) conditions is addressed in this paper. The desorption process of dissolved O2, N2, and CO2 in the barometric upcomers of an OTEC plant is simulated mathematically. The model considers the growth of bubbles originating in the ocean and bubbles formed in the upcomers. Bubble growth is induced by gas mass transfer and water evaporation at the bubble-liquid interface, as well as by the decreasing hydrostatic pressure. Heterogeneous nucleation at pipe wall crevices and on suspended particles in the water stream is also modeled. Bubble coalescence due to turbulent shear and differential buoyancy is simulated. The results generated show the deaeration efficiency as a function of flow and geometric parameters. The calculations show that gas desorption in the barometric upcomers can be appreciable. Such desorption is enhanced by increasing the concentration of the incoming and/or the heterogeneously formed bubbles. Results of existing experiments are discussed and predictions are shown for the selected test conditions.
Direct Contact Gas-Liquid Heat Exchange for Energy Recovery
Authors: Fair James R.
Published: June 2008

Energy from hot gas discharge streams can be recovered by transfer directly to a coolant liquid in one of several available gas-liquid contacting devices. The design of the device is central to the theme of this paper, and experimental work has verified that the analogy between heat transfer and mass transfer can be used for design purposes. This enables the large amount of available mass transfer data for spray, packed, and tray columns to be used for heat transfer calculations. Recommended methods for designing the several types of gas-liquid contacting device are summarized.
Durability Testing of an Elastomeric Foam for Use in Hydraulic Accumulators
Authors: Pourmovahed A.
Published: June 2008

When inserted in the gas side of a hydraulic accumulator, elastomeric foams can reduce the thermal loss and significantly improve the efficiency of this device. In this study, an open-cell polyester-urethane foam was placed in a 2.5-liter piston-type accumulator and cycled 36,000 times at 0.025 Hz. No significant chemical breakdown was measured in the foam, but a 25 percent compression set occurred due to foam cycling. This resulted in a reduction in accumulator efficiency from 84 percent with the new foam to 81 percent with the 36,000-cycle foam. To counteract the compression set, a quantity of new foam was added to the old foam. This increased the efficiency to 83 percent. The accumulator had an efficiency of 66 percent without the foam. The foam reduced the steady-state accumulator wall temperature significantly. It did not cause any problems in the hydraulic system.