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。     

Heat losses from a salt-gradient solar pond

A theoretical approach for the calculation of heat losses from a cylindrical flat-bottomed salt-gradient solar pond is discussed. Steady-state heat losses from the sides and bottom of the pond have been estimated, when the pond is uninsulated as well as insulated. The insulating materials considered in the present study are dry sand, mud powder, dry cement, marble dust and mica powder. The effective insulation is varied by varying the thickness and interstitial air pressure of the insulating materials. We find that the losses are reduced to a minimum with a marble dust wall of thickness 0·20 m at an interstitial air pressure of 0·5 mm of mercury.     

纳米颗粒对盐梯度太阳池热性能影响的实验研究

为了增强太阳池的储热温度以及热效率,采用了在太阳池下对流层添加纳米颗粒的方法.通过光照实验与沉降实验选取质量分数为0.010％的碳纳米管溶液添加到太阳池的下对流层,然后与普通盐梯度太阳池进行对比实验,并对实验数据进行分析、计算.实验结果表明:在相同模拟光源下,含纳米颗粒小型太阳池的下对流层平均温度提高了 1.7℃,效率...     

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%.     

Effect of thermal trap on the performance of an underground shallow solar pond water heater

A simple analysis of an underground shallow solar pond water heater has been presented. The effect of a thermal trap at the top of the system has also been incorporated in the analysis. Using the model, the effect of various system parameters, viz. thermal trap thickness, heat capacity of water mass, flow rate and duration of flow rate have been studied in detail. Numerical calculations have been made for a typical winter day at New Delhi (India). It is concluded that the system with thermal trap gives better performance in comparison with a system with a movable insulation system.     

Stability,effective insulation and heat loss of a salt-gradient solar pond

A relation between salt gradient G_c and temperature gradient G_T is derived. Heat losses are estimated for a natural solar pond in a steady state and a thickness of insulating material to achieve the required insulation is suggested. Enlargement of the non-convecting zone in an unsteady state is also discussed. Predicted values are compared with the reported results and good agreement is found.     

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.     

Significant depth of ground water table for thermal performance of salt gradient solar pond

In case of noninsulated salt gradient solar pond, heat losses through bottom and sides are significant. The magnitude of losses depends upon the location (depth) of water table, which act as a heat sink. Simulation analysis indicates that deeper the water table, lesser are the heat losses and higher is the temperature achieved by the pond. The present analysis, however, reveals two more significant conclusions – firstly; increase in depth of water table increases the maturation temperature and highest temperature of the pond, but does not affect the time of acquiring these temperatures. Secondly, there is a significant depth of water table, below which, further depression does not have significant impact on thermal performance of pond. This conclusion is of practical significance where efforts are done to depress the water table.     

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.     

Correlations for the yearly or seasonally optimum salt-gradient solar pond in Greece

Simple correlations and corresponding nomographs are presented, which express the maximum useful heat received from salt-gradient solar ponds throughout the year or during a specified season of the year, and the corresponding optimum depth of the nonconvective zone in terms of the thickness of the upper convective zone and the temperature under which the maximum useful heat is received. The correlations are valid for the Athens (Greece) area or for regions with a similar climate, because solar radiation and ambient temperature values for Athens have been employed, obtained by a statistical process of hourly measurements over a period of about 20 years. For other climates, it is easy to develop similar correlations using the same methodology. Development of the proposed correlations is based on a method, which simulates the transient operation of the salt-gradient pond using finite-differences, and calculates the useful heat received hourly along the typical year. Thus, the useful heat received during a period or throughout the year is calculated as a sum of hourly values. Calculations of the useful heat are performed for a great number of values of the parameters of the problem, and the combinations of values that maximize useful heat are selected and used for developing correlations and corresponding nomographs. The correlations presented may be employed in the design of the optimum solar pond under the specific requirements of each application.     

石蜡/泡沫铝复合相变材料对太阳池热性能的影响

传统盐梯度太阳池以显热储热,储热密度较低。提出了在储热层添加石蜡/泡沫铝复合多孔介质相变层的方法,构建了宽度为300mm,长度为400mm,深度为500mm的小型实验太阳池,分别进行了传统盐梯度太阳池和添加石蜡/泡沫铝复合多孔介质相变材料(PCM)的太阳池热性能对比实验。实验研究与理论分析表明:未添加石蜡/泡沫铝复合PCM的下对流层(LCZ)的储存(火用)和(火用)效率的最大值分别13.814 MJ/m³与9.38%;添加4块石蜡/泡沫铝复合PCM的LCZ的储存(火用)和(火用)效率的最大值分别为15.659 MJ/m³与12.05%,同前者对比分别提高了1.845 MJ/m³与2.67%,从而证明石蜡/泡沫铝复合PCM能够有效提高LCZ的储存(火用)与(火用)效率。此外,后者在太阳池LCZ的温度上升期可使其温度提高3℃左右,因此,添加石蜡/泡沫铝复合PCM能够提升太阳池LCZ的蓄热能力。     

非对流层数对太阳池性能的影响

盐梯度太阳池是一种兼具集热与蓄热功能的太阳能利用设施。在太阳池的工程应用中,非对流层(NCZ)层数对太阳池性能影响问题十分重要,然而以往对这方面的研究较少。文章通过构建小型太阳池,设置不同的NCZ层数,分析其对太阳能集热和储热性能的影响。研究结果显示:随着NCZ层数增加,太阳池集热性能减弱;无NCZ太阳池平均每小时的集热量分别是含有1,2,3层NCZ太阳池集热量的1.11倍、2.52倍和4.03倍,热效率分别是其1.39倍、2.86倍和5.21倍;NCZ层数增加,夜间散热量减小,无NCZ太阳池中,夜间平均散热量是含3层NCZ太阳池散热量的3.56倍。在构建太阳池时,应充分考虑当地的气候条件和实际用途,设置适当的NCZ层数,方可发挥太阳池的最大功效,达到最大经济效益。     

Applicability of steady state equations for solar pond thermal performance predictions

The steady state thermal performance of solar ponds is compared with hourly numerical simulation results and limitations of the steady-state equations are discussed. The approach to steady-state is described by the definition of a time constant. Based on numerical simulation results, a simple way of evaluating monthly performance is given. The concept of utilizability is extended to solar ponds and it is shown that the yearly utilizability curve is independent of location.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

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.     

Two-dimensional modeling of a salt-gradient solar pond with wall shading effect and thermo-physical properties dependent on temperature and concentration

In this paper,the behavior of a salt-gradient solar pond with the square cross-section has been studied experimentally and numerically.A small-scale solar pond were designed and built to provide quantitative data.A two-dimensional,transient heat and mass transfer model has been solved numerically by using finite-control-volume method.In this study,all the thermo-physical properties are variable as the function of temperature and salt concentration.Numerical results as obtained for the experimental pond have been satisfactorily compared and validated against measured data.Furthermore,the wall shading effect has been elaborated to improve the agreement between two sets of results.The temperature of the storage zone is predicted well by the model.It also can be observed that the initial concentration profile is preserved with time.The stability of the pond in time has been investigated in order to distinguish the critical zones.Finally,the application of an energy analysis gives an efficiency of about 12%for the pond.     

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.     

The effect of a baffle plate on the transient performance of a shallow solar pond water heater

A straightforward transient analysis of a shallow solar pond (SSP) water heater fitted with a baffle plate has been presented. In order to study the performance of the system, numerical calculations have been made for a typical cold day in Delhi, viz. 18 Dec. 1984. It is concluded that the SSP can also be used as a built-in storage water heater, with better performance being achieved with the use of a baffle plate.     

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 ( ).