Computer simulation of solar pond thermal behavior

A computer model of salt gradient solar pond thermal behavior has been developed and used to verify the validity of assuming constant salt solution physical parameters and long term averaging schemes for ambient temperature and insolation in previous solar pond analytical models. A theoretical limit for pond transparency is calculated which is significantly higher than that previously assumed. It is suggested that a transparent membrane be placed just below the air/water interface of solar ponds to maintain pond solution purity and approach the theoretical limit for transparency. A means of estimating the diffuse insolation input into a solar pond is given which utilizes sky color temperatures for different values of the clearness index (K_T). A single sky color temperature is calculated for each average clearness index value ( ).     

On the performance of a salt gradient solar pond

The performance of a laboratory-scale salt-gradient solar pond is described in this paper. Different methods of saline injection to the bottom layer and corresponding temperature and concentration profiles as a function of depth are reviewed and compared with experimental results. A time history of the development of temperatures, salinities and elevations of the lower and upper layers at various climatological situations is reported. The ‘dynamic stability’ and ‘equilibrium boundary criterion’ are discussed and verified experimentally for the lower and upper gradient interfaces.     

The daily performance of a solar pond integrated with solar collectors

The present study deals with heat storage performance investigation of integrated solar pond and collector system. In the experimental work, a cylindrical solar pond system (CSPS) with a radius of 0.80 m and a depth of 2.0 m and four flat plate collectors dimensions of 1.90 m × 0.90 m was built in Cukurova University in Adana, Turkey. The CSPS was filled with salty water of various densities to form three salty water zones (Upper Convective Zone, Non-Convective Zone and Heat Storage Zone). Heat energy collected by collectors was transferred to the solar pond storage zone by using a heat exchanger system which is connected to the solar collectors. Several temperature sensors connected to a data acquisition system were placed vertically inside the CSPS and at the inlet and outlet of the heat exchanger. Experimental studies were performed using 1, 2, 3 and 4 collectors integrated with the CSPS under approximately the same condition. The integrated solar pond efficiencies were calculated experimentally and theoretically according to the number of collectors. As a result, the experimental efficiencies are found to be 21.30%, 23.60%, 24.28% and 26.52%; the theoretical efficiencies to be 23.42%, 25.48%, 26.55% and 27.70% for 1, 2, 3 and 4 collectors, respectively. Theoretical efficiencies were compared with the experimental results and hence a good agreement is found between experimental and theoretical efficiency profiles.     

Modeling the performance of a solar pond as a source of thermal energy

The use of solar ponds is becoming more attractive in today's energy scene. A major advantage of solar ponds over other collectors is the ability to store thermal energy for long periods of time. The solar pond comprises a hydraulic system subject to processes of heat and mass transfer. The design of this system and the related equipment requires a thorough knowledge of the pond heating-up process and expected thermohaline structure within the pond. The current study considers that convection currents in the pond are inhibited by the salinity distribution, and applies a finite difference implicit model in order to investigate the interaction among physical variables represented by various dimensionless parameters. Variables which are included in the analysis comprise the solar radiation input and absorption as it passes through the pond; diffusion and dispersion of heat within the pond; absorption of heat at the bottom of the pond; and withdrawal of heat from layers within the pond. The physical variables generate 3 dimensionless variables associated with the pond's heating-up process. A 4 dimensionless variable is associated with the heat utilization. The analysis represented in this paper concerns the interaction between these dimensionless parameters and its implications.     

The effect of bottom reflectivity on the performance of a solar pond

The reflectivity of the bottom of a solar pond increases on account of the accumulation of dirt or the presence of undissolved salt. The effect of the reflection of the solar radiation at the bottom of the pond on the seasonal performance of the pond has been studied using a three zone model. The spectral reflectivity of dirt and common salt were measured in the laboratory and used in the analysis. The results obtained from the analysis show that the presence of dirt at the bottom of the pond does not affect the performance of the pond substantially. On the other hand, the presence of undissolved salt at the bottom of the pond results in substantial deterioration of the pond performance.     

Design and performance characteristics of honeycomb solar pond

This paper presents the design and performance characteristics of a honeycomb solar pond. It considers natural convection suppression in an air layer incorporating a cellular array and points out that a cell of size 1.25 cm × 1.25 cm matches quite well with temperatures operational in a solar pond. Honeycomb transmittance to incident radiation is calculated by taking into account the refraction, scattering and absorption by vertical walls. Results corresponding to a wide range of angles of incidence are presented. Honeycomb effectiveness on heat loss reduction and solar collection efficiency is investigated. Explicit results on optimization of system efficiency are presented. It is found that a honeycomb depth of 12–17 cm is optimum. An efficiency of 40–60% is predicted at a collector temperature of 90°C. The results of earlier workers are discussed.     

Mathematical modelling of salt gradient solar pond performance

This paper presents a mathematical model of the performance of the salt gradient solar pond. A lumped parameter model of the upper convective zone, non-convective zone and lower convective zone is used. This model enables the temperatures of the upper-convective zone and the lower convective zone of the solar pond to be predicted. The experimental results agree well with theoretically predicted values. The major error in the theoretical results is due to the difference between the theoretical value of the solar radiation inside the water and that observed experimentally. It is found that the experimental value of the solar radiation at a depth of 90 cm is approximately 26 per cent of the total solar radiation falling on the solar pond surface, whereas the corresponding theoretical value is found to be 33 per cent. The results conclude that the lumped parameter model can be used as a simple model to predict the performance of the solar pond.     

Modeling and simulation of solar pond floor heating system

This paper presents the development of modeling, simulation and analysis of a solar pond floor heating system. The developed computer simulation has been used to study the potential of using such a system under climatic conditions in Jordan. It was found that the solar pond heating system could meet most of the winter season in Jordan with Solar fraction in the range 80–100% for at least 2 months of the season. It must be emphasized that the feasibility of such a system is its utilization in district heating and not for individual households due to the limiting economical factors of high capital cost of the solar pond for small domestic applications.     

Thermal performance modeling of the reverse absorber shallow solar pond

Reverse absorber type shallow solar ponds are proposed as being capable of attaining higher temperatures and still higher efficiencies than the conventional type due to convection suppression and elimination of top radiative losses. The theoretical thermal analysis and simulation of the performance of two configurations of the reverse absorber shallow solar pond (RASSP); one with the top insulated and the other with top exposed, are presented. The ensuing model equations were solved to obtain the desired performance parameters. For a pond depth of 0.10 m, results of the simulations show that water temperatures up to 70°C could be obtained in a RASSP with double glass covers, higher than could be gotten in either an RASSP with top insulation or a conventional SSP of equal depth. The effect of pond depth on the proportions of the radiation incident on the RASSP that is either collected as thermal energy or lost was studied. The average transmissivity-absorptivity products, (τα), overall heat loss coefficients, UL and optimal pond depths were also computed.     

Control of a solar pond

Solar ponds hold the promise of providing an alternative to diesel generation of electricity at remote locations in Australia where fuel costs are high. However, to reliably generate electricity with a solar pond requires high temperatures to be maintained throughout the year; this goal had eluded the Alice Springs solar pond prior to 1989 because of double-diffusive convection within the gradient zone. This paper presents control strategies designed to provide successful high temperature operation of a solar pond year-round. The strategies, which consist mainly of manipulating upper surface layer salinity and extracting heat from the storage zone are well suited to automation. They were tested at the Alice Springs solar pond during the summer of 1989 and maintained temperatures in excess of 85°C for several months without any gradient stability problems.     

Effect of the gap spacing on the shallow solar pond performance

It is stated by Garg et al. [Energy Convers. Mgmt 22, 117 (1981)] that, for a shallow solar pond where the gap spacing is large (300 mm), the convective heat loss is somewhat lower than for a conventional collector with a shorter gap spacing (20 mm). How much the total heat transfer coefficient is lowered, and what is the effect of the gap spacing on the performance of the shallow solar pond were not mentioned. In this study, the effect of gap spacing on shallow solar pond performance is studied. For this purpose, a computer program is constructed and is experimentally tested using a shallow solar pond of 6.6 m² area. It is concluded from the results that the effect of gap spacing on the top loss coefficient is independent of the temperature difference between the upper film of the water bag and the glazing of the shallow solar pond. The results also show that the change in gap spacing of the shallow solar pond has not an important effect on the pond performance. Hence, the large gap spacing in the conventional design shallow solar pond (300 mm) cannot be considered as a major difference with the flat plate collector design (20 mm), as Garg et al. stated.     

Effect of water turbidity on thermal performance of a salt-gradient solar pond

The effect of water turbidity on the thermal performance of a salt-gradient solar pond is studied using a one-dimensional theoretical model. The theoretical model uses an empirical correlation that includes the effect of water turbidity on solar radiation penetration in water. The correlation is based on a uniform turbidity distribution in water; however, the correlation is extended to include a non-uniform turbidity distribution with respect to depth of water. The results indicate that water clarity plays a significant role on thermal performance for salt gradient solar ponds.     

Interferometric study of mixing layer development in a laboratory simulation of solar pond conditions

Events that induce or inhibit mixing layer development in a thermohaline system are simulated in laboratory experiments involving salt-stratified solutions heated from below, cooled from above and/or irradiated from above. A Mach-Zehnder interferometer is used to visualize mixing layer development and to infer salt concentration and mass density distributions in stable regions of the solution. In experiments involving cooling from above by means of an isothermal boundary, the development of a top mixing layer is strongly influenced by the cooling rate. While the formation of a secondary mixed layer is associated with large cooling rates ( 10³ W/m²), no such layer is associated with lower rates (≈100 W/m²). Secondary mixed layer development is also observed to occur in experiments involving heating from below. Experiments involving irradiation of the solution reveal the strong effect which infrared radiation absorption in the upper layers can have on inhibiting the development of both top and bottom mixed layers.     

Effect of heat exchanger on the performance of a shallow solar pond water heater

In this communication, a mathematical model has been developed to predict the performance of a shallow solar pond water heater with a heat exchanger. Explicit heat balance equations are written for the plate temperature and water tank temperature, as well as for the heat extracting fluid temperature, by properly taking into account the absorption of solar radiation in the body of pond water. It is seen that efficiencies may be achieved as high as 60% at water flow rates of 0.1 – 0.2 kg/s m². Thereafter, the efficiency becomes almost constant at higher flow rates.     

Sensitivity analysis of performance of crop growth simulation models to daily solar radiation estimation methods in Iran

Solar radiation is the single most important environmental factor driving canopy photosynthesis and transpiration. This weather characteristic is measured only in a limited number of weather stations. Hence, in many situations it has to be estimated from other weather characteristics such as sunshine duration and temperature using empirical relations. In this study, the Ångstrom and Hargreaves formulas have been used for solar radiation estimation, based on monthly and annual weather data for three weather stations in Esfahan province, Iran. Deviations of estimated solar radiation from measured values (both absolute and relative) varied with month of the year and with estimation method. Estimated and measured radiation values were used in a crop growth simulation model to explore sensitivity of simulated production with respect to radiation estimation method. Maximum deviation for winter barley and silage maize was around 9%.     

Computer simulation of solar cooled buildings in Khartoum

The transient state heat transfer formulation has been used to determine the air conditioning cooling load for two selected one room buildings in Khartoum: one made from brickwork and the other from wood. This formulation has been achieved by applying an explicit finite difference numerical techniques and adopting thermal network of electrical analogy to solve the transient conduction heat transfer equations. A computer program was developed to analyse the thermal network and to determine the cooling load for the wall, roof, window, ventilation and the total cooling load for each building for a typical hot summer day in Khartoum.A solar cooling system comprising mainly of a flat plate solar collector, a Lithium Bromide-Water absorption air conditioner, a storage and auxiliary tanks was integrated in the building and the energy equation for each component was obtained. A general simulation program for the solar cooled buildings has been developed and it was found that about 65% of the total cooling load demanded by the brick building could be supplied by energy from the solar collector, the other 35% portion had to be met by the auxiliary tank compared with 70% of the total cooling load demanded by the wood building which could be supplied by energy from the solar collector.     

Analysis of the exergetic performance of the reverse-side absorber-plate shallow solar pond

Results of exergetic performance analysis of three shallow solar pond (SSP) types – the CSSP, the RASSPgc, and RASSPins – are presented for the first time. The study shows that the highest irreversibilities are encountered in the components of the RASSPgc and that better exergetic performances in SSPs may be obtained by improving the surface properties: absorptivity, reflectivity, and transmissivity. Steady-state analysis also shows that exergy ‘losses’ in the SSPs due to irreversibilities in their water masses are significant and result from the direct absorption of solar radiation. Transient analysis reveals that the RASSPgc achieves the highest overall exergetic efficiency (4.37%), followed by the RASSPins (3.96%) and then the CSSP (2.87%). At the end of a 24 h operation, the exergy content of the water masses in the RASSPgc and the CSSP is negligible, whereas the water mass in the RASSPins retains 0.057 MJ of the exergy accumulated during daytime heating.     

配合集热器的盐梯度太阳池热性能实验与分析

构建表面积为1.50 m×1.50 m的小型实验用盐梯度太阳池,并与平板太阳能集热器配合使用,分别对普通太阳池和集热增强型太阳池进行了储热、放热实验。实验研究与理论分析表明:单独盐梯度太阳池的放热量为3.5×103k J,热效率为13.6%;集热增强型太阳池放热量可以达到4.8×103k J,且热效率增至28.1%。另外后者下对流层温度最高可提升10℃以上,从而证明太阳能集热器可以有效提高太阳池热效率,增加下对流层储热量。此外,考虑了放热过程换热器对太阳池下对流层的扰动,对比实验前后的溶液浓度,可以看出实验后太阳池盐度曲线合理,非对流层呈良好梯度分布,太阳池稳定性并未遭到破坏。     

Computer simulation of amorphous MIS solar cells

A computer model to simulate amorphous MIS solar cells is developed. The model is based on the self-consistent solution of the electron and hole continuity equations, together with the Poisson equation under proper boundary conditions. The program developed is used to investigate the cell performance characteristics in terms of its physical and structural parameters. The current-voltage characteristics of the solar cell are obtained under AM1 solar illumination. The dependences of the short-circuit current, open-circuit voltage, fill factor and cell conversion efficiency on localized gap state density, carrier lifetime, cell thickness and surface recombination velocity are obtained and discussed. The results presented show how cell parameters can be varied to improve the cell performance characteristics.