Theoretical and experimental comparison of conventional and advanced solar pond performance

Numerical and physical experiments were carried out to compare the performance of two solar pond systems: (a) a conventional salt gradient solar pond (CSP) and (b) a salt gradient pond operated as an “advanced solar pond” (ASP). The main differences in the ASP, as originally proposed by Osdor[1], are an increase in overall salinity and the introduction of a stratified flowing layer near the bottom of the gradient zone. The increased salinity is meant to reduce evaporative heat loss and make up water requirements, while the additional flowing layer allows extra heat extraction and possibly higher temperatures to develop in the lower convective zone. A numerical study was performed to evaluate the salinity effect and the results show only a minor effect of increased salinity on heat collection efficiency. However, slightly higher collection temperatures are obtained, which may provide some benefit for heat engine efficiency. Physical experiments were performed to test the feasibility of constructing and maintaining the necessary flow system for the ASP and also to compare the performance of the ASP and the CSP under similar laboratory conditions. These tests showed that a stable stratified flowing layer could be maintained and that the ASP configuration produced higher efficiencies.     

太阳池的研究与应用

自从1902年Kalecsinsky首次发现了天然太阳池现象以后，经过长期的研究和发展，太阳池技术已被广泛应用于发电、取暖、海水淡化．矿物加工等领域，太阳池成为近期内进行大规模太阳能热利用的最有前景的低温热源装置。主要综述了太阳池的集热原理及建造方法、太阳池中热量的贮存及提取方式、太阳池的应用以及研究动向等，并指出目前我国太阳池技术还处于实验研究的阶段，而我国具有丰富的太阳能和盐资源，大力开发太阳池技术将为发展地方经济起到重要的作用。     

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.     

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.     

An analysis of the non-convecting solar pond

The thermal behaviour of the non-convecting solar pond is examined by numerical solution of the dynamic equations, incorporating detailed representation of the losses from the surface and using hourly meteorological data for a site in southern England.Temperature histories for the first few years of operation are given, showing the influence of the leading physical characteristics of the pond. It is shown that the pond temperatures are strongly dependent on the effective extinction coefficient for solar radiation and the thermal losses from the pond bottom. The temperature history approaches a quasi-steady form within two to three years of operation, depending on the load demand. Using realistic assumptions for the main determinants of pond behaviour, it is shown that modest loads (around 10 per cent of the average insolation) can be served in this climate at temperatures appropriate for practical applications.     

The nonconvecting solar pond applied to building and process heating

Recent studies indicate that solar pond house heating systems could be competitive with some conventional ones, particularly if a pond were to be used to supply thermal energy to several buildings. It is appropriate and important, therefore, to extend these investigations to include industrial process heating and also the influence of variations in salt content and unit price of salt on pond cost. To do this, equations are derived which yield a single set of dimensions for a hypothetical pond satisfying a given heating requirement. Pond dimensions are determined for house heating, winter crop drying and paper processing in the Richland, Washington area; cost estimates for hypalon-lined ponds of these dimensions are then compared with costs attributed to conventional thermal energy sources used for these purposes. Such comparisons can help guide researchers in determining requirements necessary for the pond to be competitive, e.g. the maximum stabilizing salt content allowable for a competitive pond.     

Thermal analysis of honeycomb solar pond

A solar pond consisting of honeycomb panels placed on a thin layer (～ 1 cm) of silicone oil floating on the body of a hot water reservoir is considered and analysed for the heat transfer processes in the system. An explicit expression for the transient rate of heat extraction at constant temperature is derived to obtain the annual variation of retrieved heat flux. The year-round thermal performance of the system has been investigated. For a solar pond with a 10 cm high honeycomb structure, annual average efficiencies of 65, 48, 33 and 24% are predicted for retrieved heat flux at temperatures of 40, 60, 80 and 90°C, respectively. A comparison between honeycomb solar pond and salt-gradient solar pond is also presented.     

Numerical model of a solar pond

This paper points out an idealization of considerable significance in a recent numerical model of a solar pond due to Wang and Akbarzadeh [3] and outlines the refinements required in the formulation of the above model. Typical temperature calculations from teh resultant model are also presented. of the above model. Typical temperature calculations from the resultant model are also presented.     

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.     

Experimental study of the salt gradient solar pond stability

Many natural systems such as oceans, lakes, etc.…, are influenced by the effect of double-diffusive convection. This phenomenon, which is a combination of heat and mass transfer, can destroy the stability of system-flows.In the case of solar ponds the middle layer, that is linearly stratified, acts as a thermal and mass insulator for the lower layer. This middle layer, called the Non-Convective Zone (NCZ), needs special care to avoid convection and to maintain its stability. In fact, due to an excess of heat stored, a thermal gradient occurs within the NCZ. A convective movement appears at the bottom of the stratified-layers and then grows to a double-diffusive convection movement. This movement transforms the stratified-layers into a well mixed layer, reducing the storage capacity of the pond.Laboratory small-scale pond and middle-scale outdoor solar ponds were designed and built to provide both quantitative data and to study the dynamic processes in solar ponds, including the behavior of the gradient zone.Particle Image Velocimetry (PIV) visualization-experiments carried out in the mechanical and energetic laboratory in the engineering school of Tunisia and experiments in the field showed that the instability of solar ponds could be limited by using porous media placed in the lower layer of the stratification.     

Performance analysis of a bittern-based solar pond

An attempt has been made to analyze the performance of the bittern-based solar pond of a 1600-m² area located in Bhavnagar, India (latitude 21°, 45'N and longitude 72°, 12'E). Solar radiation transmission in the pond has been measured with a silicon solar cell module and the results are used in the calculation. Thermal efficiency of the pond is worked out by the correlations proposed by Kooi and Hull. Its value is very very low and reasons for this are discussed in the text. Theoretical temperature profiles in the NCZ and optimum thickness of the NCZ are calculated based on the correlations developed by Kooi. Calculated temperature profiles and observed profiles in NCZ match quite satisfactorily.     

Physics of shallow solar pond water heater

This communication presents a theoretical analysis of a shallow solar pond water heater, which is in good agreement with the experiments of Kudish and Wolf (1979) and the authors. the heater consists of an insulated metallic rectangular tank with black bottom and sides and a transparent cover at the top. After the collection of solar energy during sunshine hours the heater stores a substantial amount of heat because the top glass cover is covered by an adequate insulation in the night. Analytical expressions for the transient rise of temperature of water in the tank have been derived taking into account the appropriate heat transfer processes during day and night. These experimental results as well as those of Kudish and Wolf (1979) have been found to be in good agreement with the theory presented in this paper. the effects of one more glass cover on the top, and of the thickness of the bottom and side insulation and tank depth on the water temperature have also been studied.     

太阳池热能利用技术

回顾了太阳池的发展历史,介绍了太阳池的结构和工作原理,论述了从太阳池中提取热量的方法及太阳池的应用.太阳池发电可为不发达地区提供部分电能,是一种很有竞争力和发展前景的太阳能收集和储存系统.     

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.     

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 ( ).     

Thermal analysis of three zone solar pond

This paper presents a periodic analysis of a three zone solar pond as a solar energy collector and long term storage system. We explicitly take into account the convective heat and mass flux through the pond surface and evaluate the temperature and heat fluxes at various levels in the pond during its year round operation by solving the time dependent Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation in the pond water. Eventually, an expression, for the transient rate at which heat can be retrieved from the solar pond to keep the temperature of the zone of heat extraction as constant, is derived. Heat retrieval efficiencies of 40.0 per cent, 32.1 per cent, 28.3 per cent and 25.5 per cent are predicted at collection temperatures of 40, 60, 80 and 100°C, respectively. the retrieved heat flux exhibits a phase difference of about 30 to 45 days with the incident solar flux; the load levelling in the retrieved heat flux improves as the thickness of the non-convective zone increases. the efficiency of the solar pond system for conversion of solar energy into mechanical work is also studied. This efficiency is found to increase with collection temperature and it tends to level around 5 per cent at collection temperatures about 90°C.     

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.     

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。     

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.     

Optical analysis of the behaviour of a salt-gradient solar pond

The three-zone salt-gradient solar pond is a body of saline water that collects solar radiation and stores it in the water as thermal energy. The performance of solar ponds largely depends on the portion of solar radiation which reaches the bottom region (LCZ) and from which heat is extracted subsequently. An analysis is made to determine the form of the attenuation of the solar rays inside the pond as a function of wavelength and depth, taking into consideration that each zone has its extinction coefficient due to its salt concentration. Insertion of partitions between zones (between the UCZ and NCZ and between the NCZ and LCZ) has also been discussed. Equations describing the transmissions and reflection coefficients in the presence of partitions were derived. The portion of the solar energy that is absorbed by the different depths of NCZ has been calculated for Cairo. About 20% of the incident radiation is absorbed by the NCZ, and with the presence of transparent partitions this quantity decreases by about 20%.