An investigation of rain and wind effects on thermal stability of large-area saltpan solar ponds

An investigation of the thermal stability of large area saltpan solar ponds under different climatic conditions is presented. The study focuses on time taken by the pond to reach its stable conditions with heavy rainfall and the effect of wind-mixing process for the stability of the pond. Investigations were carried out over a period of 60 days on a large-area solar pond of 90 cm deep. The temperature and density profiles obtained 34 days after filling showed that the pond had attained its stability with a bottom temperature of 63 °C. Results reveal that heavy rainfall is the prime cause for the pond to reach stability in a time period of about 30 days. Strong wind-induced mixing prevailed during the second half of the investigation, which contributed to the erosion of the nonconvecting zone is the cause for observed destabilization of the pond. The estimated critical wind speed for complete destruction of the nonconvecting zone is about 25 km/h.     

Development of the gel pond technology

A review of the development of the gel pond technology is presented. First, the emergence and growth of solar pond technology since the 1950's is described. The inherent problems encountered with the conventional salt gradient ponds are discussed, leading to the concept of the solar gel pond in which the salt gradient layer in the former is replaced by a transparent polymer gel. The major work in the first phase dealt with the experimental development of a polymer gel which met certain selection criteria. The criteria considered included transmissivity, stability of physical and chemical properties, high viscosity and other physical and optical properties. The gradual development of the polymer gel through an alternating process of testing and elimination and evaluation of relevant properties of the gel has been described. Modeling and optimization studies of the solar gel pond have been presented. Bansal and Kaushik's model for a salt gradient pond has been modified for a solar gel pond, and the results of the simulation are presented in a graphical form to serve as a quick reference for estimation of pond surface area, depth and flow rate for heat extraction depending on the extreme temperature required in the storage zone and the required heat load. Then, a cost-benefit economic analysis compares the economics of a solar gel pond with a conventional salt gradient pond. The construction of an experimental gel pond (18 m²) at The University of New Mexico is described, and the results of the study are summarized. Information on commercial scale ponds at Chamberino, New Mexico (110 m²), and in Albuquerque, New Mexico (400 m²), is provided. The review of the technology demonstrates the immense potential of the gel pond as a source of alternate energy for the years ahead.     

A parametric design study of solar ponds

The salt stratified solar pond is found to be a reliable solar collector and storage system. This paper discusses the effect of varying certain design parameters on pond steady-state temperatures. These significant parameters are sizing parameters—pond surface area and depth of the pond; operating parameters—storage volume and the heat extraction fraction; and geo-climatic parameters3s?olar radiation, water table depth and upper convective zone thickness. Studies indicate that there is an optimum depth and storage volume of the pond for each application in terms of temperature and heat load desired.     

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.     

A fully coupled,transient double-diffusive convective model for salt-gradient solar ponds

A fully coupled two-dimensional, numerical model that evaluates, for the first time, the effects of double-diffusive convection in the thermal performance and stability of a salt-gradient solar pond is presented. The inclusion of circulation in the upper and lower convective zone clearly shows that erosion of the non-convective zone occurs. Model results show that in a two-week period, the temperature in the bottom of the solar pond increased from 20 °C to approximately 52 °C and, even though the insulating layer is being eroded by double-diffusive convection, the solar pond remained stable. Results from previous models that neglect the effect of double-diffusive convection are shown to over-estimate the temperatures in the bottom of the solar pond. Incorporation of convective mixing is shown to have profound impacts on the overall stability of a solar pond, and demonstrates the need to actively manage the mixing and heat transfer to maintain stability and an insulating non-convective zone.     

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.     

Enhancing the thermal efficiency of solar ponds by extracting heat from the gradient layer

An alternative method of heat extraction from salinity-gradient solar ponds is investigated with the aim of increasing the overall energy efficiency of collecting solar radiation, storing heat and delivering this heat to an application. In this alternative method, heat is extracted from the non-convecting gradient layer of a solar pond as well as, or instead of, from the lower convective zone (LCZ). A theoretical analysis of combined gradient-layer and LCZ heat extraction is conducted to obtain expressions for the variation of temperature with depth in the pond, and the temperature gradient with depth. The dependence of the overall energy efficiency of the pond on thickness of the gradient-layer, temperature of delivered heat, and various combinations of gradient layer and LCZ heat extraction rates, including the limiting cases of gradient-layer heat extraction only, and LCZ heat extraction only, is then explored. This theoretical analysis suggests that heat extraction from the gradient layer has the potential to increase the overall energy efficiency of a solar pond delivering heat at a relatively high temperature by up to 50%, compared with the conventional method of heat extraction solely from the LCZ. The potential gain in efficiency using gradient-layer heat extraction is attributed to the lowering of heat losses by conduction to the upper convective (surface) zone that can be achieved with this method. Experimental investigations are proposed to test the predictions of the theoretical analysis in practice, and assess the impact of a number of idealized assumptions made on the findings reported here.     

Optimization of the gel solar pond parameters: Comparison of analytical models

A number of analytical models have been presented in the contemporary literature to describe and predict the thermal behavior of salt-gradient solar ponds under steady- and unsteady-state conditions. This paper presents a detailed theoretical comparison between three different analytical models proposed to simulate the thermal behavior of solar ponds. These models are slightly modified to represent the gel pond configuration. For experimental comparison, a gel pond constructed at the University of New Mexico, which has been in operation for several years, has been used as the reference pond. The gel pond differs from conventional salt-gradient ponds in that the nonconvective insulating layer is replaced by a transparent polymer gel. Numerical computations have been made to optimize the geometric and operational parameters of the pond using the three different models. The optimum thickness of the nonconvective layer (gel) and the thin upper convective layer (fresh water) and their effects on pond performance at a given ambient temperature, isolation and storage temperature have been calculated using all the models. The three models have also been used to calculate the absorptivity— transmissivity product (ατ), a parameter which represents the transparency of the nonconvective and the upper convective layers combined together. These calculated values have been compared with experimental values as measured through an actual gel layer to test the accuracy of the different models as applied to the gel pond.The results show that, under the same temperature difference between the storage zone and the ambient (20°C) and a yearly average insolation of 250 W/m², the model proposed by Wang and Akbarzadeh predicts an efficiency of approx. 32% as compared to the high values of 37.2 and 39% predicted by the Kooi model and Kaushik and Bansal model, respectively. Under the same conditions, the optimum gel thicknesses predicted by the three models are 62, 55, and 75 cm, respectively.     

Study on shallow solar pond applications at Tehran

In this study, a theoretical model which is validated experimentally is used to predict the performance of a shallow solar pond in Tehran. The theoretical and experimental results show good agreement. The maximum hourly water temperature of the shallow solar pond is found to lag behind the maximum hourly ambient temperature and solar radiation by 1–2 and 3.5 h, respectively.The maximum monthly daily-average water temperature follows the trend of the monthly daily-average solar radiation but leads the monthly daily-average ambient temperature in one month. The shallow solar pond, with 10-cm water depth, cannot be used as a thermal source in winter but can be used for many thermal applications in summer. With 5-cm water depth, the shallow solar pond can be used as a thermal source for low heat applications in most of the winter but can be used, even for moderate applications, where high temperature up to 95°C is obtained in summer. Using a reflector makes the 10-cm depth shallow solar pond useful for low heat applications and the 5-cm depth useful for moderate heat applications in most of the winter. Using a double cover top glazing is found to have no effect on improving the system performance.     

A study on the thermal storage of the ground beneath solar ponds by computer simulation

A salt gradient solar pond is a large scale solar collector having built-in heat storage capability. This is in part due to the mass of water in the pond and in part to the ground beneath the pond. Some scholars have already paid attention to the ground thermal storage. In this work, emphasis has been put upon the un-steady state performance and the transient behavior of SGSPs. A simple computer simulation method is adopted to study the ground heat loss and the heat recovery rate under varied combinations of the depth of the underground water table, the thickness of the lower convective zone, the heat withdrawal pattern, and the thermal properties of the soil. The effect of an insulation layer between the pond and the ground is also examined.     

The upper mixed zone of a salt gradient solar pond: Its dynamics, prediction and control

Parameterisation of the physical processes that occur at the surface of a salt gradient solar pond are used in an extended version of the numerical model DYRESM, to simulate the behaviour of such a pond. The model is run using meteorological data collected at the site of a 2000 m² solar pond at Alice Springs, Australia, and the comparison with actual pond data shows very good agreement. By breaking down the model output, the relative importance of each individual mixing mechanism on the depth of the surface mixed layer is calculated. For the case of Alice Springs it is shown that the evaporative effects completely dominate wind stirring effects. At another site at Halls Creek, Australia, wind stirring is seen to be dominant, however evaporative effects are still sufficient to cause an intolerable amount of deepening. A method of enhanced surface washing is proposed as a method of controlling surface layer deepening. Using the model, this is shown to be effective, irrespective of the cause of the deepening.     

The absorption chiller in large scale solar pond cooling design with condenser heat rejection in the upper convecting zone

The possibility of using solar ponds as low-cost solar collectors combined with commerical absorption chillers in large scale solar cooling design is investigated. The analysis is based on the combination of a steady-state solar pond mathematical model with the operational characteristics of a commercial absorption chiller, assuming condenser heat rejection in the upper convecting zone (U.C.Z.). The numerical solution of the nonlinear equations involved leads to results which relate the chiller capacity with pond design and environmental parameters, which are also employed for the investigation of the optimum pond size for a minimum capital cost. The derived cost per cooling kW for a 350 kW chiller ranges from about 300 to 500 $/kW cooling. This is almost an order of magnitude lower than using a solar collector field of evacuated tube type.     

Spectral calculation of the thermal performance of a solar pond and comparison of the results with experiments

This paper deals with a method and the result of the spectroscopic calculation on heat balance of a salt-gradient solar pond under the conditions of spectral solar radiation. Furthermore, reflection of the ray incident upon the surface of the pond water, refraction of the rays within the salt-water layer and diffusion of salt in the pond water are considered. On the other hand, in order to make a clear mechanism of the heat collection and heat storage of the solar pond, we conducted an indoor experiment and a numerical analysis on a small scale model of the salt-gradient solar pond with 2 m² surface area and 1.6 m depth, under the incident rays from a Xe-lamp solar simulator. According to the above experimental analysis, we made a simulation model of thermal performance for a solar pond and carried out the calculation from the heat balance. We found that the simulation calculations correspond well to the experimental results, so that our thermal simulation model and method might be correct. We also did the thermal calculation by changing the incident rays from a Xe-lamp into natural ray (Moon’s spectrum) and Halogen lamp. As a result, it was found that the temperature distributions in the solar pond were notably different due to spectral characteristics of the incident ray. Therefore, the spectroscopic consideration for thermal performance of any solar pond is necessary to obtain a correct solution under the spectral incidence with special wavelength distributions.     

Kinetics of diffusion of salt in solar ponds

In the present communication, the kinetics of diffusion of salt in a stacked layer solar pond has been investigated by using the step function for the initial state of salt distribution and a closed form solution for the salt concentration has been obtained with the boundary conditions of an operational solar pond. It has been predicted that the time required for a two layered solar pond with non-convective zone of about 1 m depth to reach equilibrium concentration gradient is about 585 days, whereas that required for a ten layered pond is only 96 days.     

Estimation of the thickness of the lower convective layer of solar ponds

In solar ponds, the lower convective layer plays a dual role. It provides a means for the extraction of energy from the pond. It acts also as an in-built seasonal energy store. The ground beneath the pond acts as an additional energy store. To enhance the ground energy storage, a method was earlier proposed by the authors employing trapezoidal-shaped trenches at the bottom of the pond. A rigorous method is presented for the determination of the thickness of the lower convective layer in the case of a flatbottomed pond and the trench depth for a pond with trapezoidal trenches. The energy storage to be provided depends on the magnitude and the pattern of energy extraction. For a constant extraction rate, the required thickness of the lower convective layer or the trench depth increases with an increase in the rate. For a sinusoidal extraction pattern, the thickness or depth increases with an increase in the phase lag of the extraction pattern from the insolation and exhibits a minimum for the amplitude of the extraction pattern for phase lags less than about 95 days. The results also indicate that, for a given heat load (total energy extracted in a year), there exists an area for which the cost of the pond and the associated system for energy utilization is a minimum.     

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.     

沼气、太阳能生态海岛

提出了沼气、太阳能生态海岛模型.利用太阳池淡化海水,为官兵提供饮用水,在太阳池底铺设换热器,为官兵提供生活用热水,并对沼气池进行加热.利用海岛官兵的生活废弃物生产的沼气进行炊事,大约可以满足51%左右的炊事用能需求.     

Towards the design of low maintenance salinity gradient solar ponds

Applications of simple methods to reduce the maintenance of a small solar pond are discussed in this paper. It was found that floating rings along with continuous surface flushing could effectively control and maintain a relatively thin upper convective layer. A novel system of salt replenishment (a salt charger) is introduced. It is shown that the application of the proposed system is capable of controlling the position of the lower interface. Criteria governing the design of a salt-charger for a salinity-gradient solar pond are developed theoretically and verified experimentally. The design procedures are presented. Experiences relating to the utilization of brine shrimps to improve the transparency of the pond are described.     

Thermal behavior of a small salinity-gradient solar pond with wall shading effect

The thermal behavior of a small-scale salinity-gradient solar pond has been studied in this paper. The model of heat conduction equation for the non-convective zone has been solved numerically with the boundary conditions of the upper and lower convective zones. The variation of the solar radiation, during a year, and its attenuation in the depth of the pond has been discussed. The wall shading area for a vertical wall square pond has been elaborated and its effect on the reduction of the sunny area has been included in the model. The temperature variation of the storage zone has been calculated theoretically and compared with the experimental results. The sensitivity analysis demonstrates the importance of the side and bottom insulation and the thickness of the non-convective zone, as well as the wall shading effect on the performance of the pond. The application of several loading patterns gives an overall efficiency of 10% for the small pond.     

Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.