Experimental evaluation of a single-stage heat transformer used to increase solar pond’s temperature

A heat source at temperatures not higher than 80°C was used to simulate the heat input to an absorption heat transformer from a solar pond. An experimental absorption heat transformer operated with the water/Carrol mixture was used to demonstrate the feasibility of these systems to increase the temperature of the heat obtained from the solar ponds. Carrol™ is a mixture of LiBr and ethylene glycol [(CH₂OH)₂] in the ratio 1:4.5 by weight. Flow ratios, gross temperature, useful heat, and coefficients of performance are plotted for the heat transformer versus temperature and solution concentration. Gross temperature as high as 50°C were obtained. The maximum temperature of the useful heat produced by the heat transformer was 132°C. The COP for the unit was in the range 0.14–0.36.     

Transient behaviour of salt gradient stabilised shallow solar ponds

This paper presents the results of experimental and analytical investigations of the transient thermal processes in a salt gradient stabilised pond of shallow (10 cm) depth, useful for short-term storage applications. A small convective sublayer (about 3 cm thick) of uniform temperature tends to form at the bottom of the pond. The convective-non-convective zone boundary below the gradient zone exhibits movements with time of day as well as from day to day. This suggests that only local properties and local gradients are relevant to the stability condition. A thin oil layer cover at the pond surface considerably enhances the temperature in the pond and aids its stability. A simple transient thermal model of the pond is developed. The observed temperatures and depths of the zones are in close agreement with theory.     

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 ( ).     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

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.     

Optimum size of non-convective zone for improved thermal performance of salt gradient solar pond

The salt gradient solar pond is a long-term heat storage system with a considerable warm-up time. A pond is efficient when it reaches the desired temperature quickly and maximum heat is subsequently retrieved at steady state. This requires optimum sizing of the non-convective zone. In the present work, the optimum size of the non-convective zone for fast warm-up is determined. This is found to differ considerably from the optimum size of the steady state criterion. The possibility of achieving both performance parameters, i.e. fast warm-up and maximum heat collection later on, is analyzed. It is suggested that when commissioning a pond, the size of the non-convective zone should at first be the optimum value from the warm-up rate criterion, but may later be changed to the optimum size from the steady state criterion.     

The steady state salt gradient solar pond

The three-zone salt gradient solar pond is analyzed as a steady-state flat-plate solar energy collector. The resultant efficiency equation is of the Hottel-Whillier-Bliss type commonly used for flat-plate collectors. The quantities that occur in this equation—the effective absorptivity-transmissivity product ατ, the loss factor U_L, the heat removal factor F_R, and the incident angle modifier θ(i)—are related to the physical properties and dimensions of the pond. For a given ΔT/H [(fluid inlet temperature—surface temperature)/insolation], the thickness of the nonconvective zone can be adjusted for maximum efficiency. U_L and ατ are smaller than the equivalent quantities for flat-plate collectors, while θ(i) and F_R are close to unity. As a consequence, steady-state salt-gradient solar ponds are less efficient than common flat-plate collectors at low ΔT/H, but they are more efficient at high ΔT/H.     

Large scale solar cooling design using salt gradient solar ponds

The future of large scale cooling is closely linked to the long term economically viable component development for collection and storage of solar energy at relatively high temperatures. As such, solar ponds at the present state of their development are undoubtedly considered as the only promising large scale solar collection and heat storage device for such applications. The present analysis, based on numerical calculations, allows a parametric investigation of solar pond design and operational characteristics to the capacity of a conventionally designed, commercially available, absorption chiller. The results can prove very useful for the rough design and pond size selection for operation of chillers of a substantial capacity, directly from solar ponds.     

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.     

Meteorologically efficient commissioning of salt gradient solar pond

Salt gradient solar pond has thermal performance parameters as rate of warm-up, highest achievable temperature, and cumulative heat collection. All these are strongly influenced by the meteorology. Consequently, specific to the meteorology of a geographic location, there is a best starting day for the as pond defined by Singh et al. [1]. The present work has done rigorous analysis of influence of meteorology on pond??s thermal performance. It is found that the starting day has strong influence in initial stage of pond warm-up; however the effect diminishes in long-term. Finally pond started on any day of the year acquires same highest temperature. It is also found that in order to retrieve maximum heat, waiting for the best starting day to commission a pond is not judicious, rather it is always more beneficial commission the pond at the earliest possible day. This finding is of practical significance while planning to put a pond in operation.     

Multiple reflection phenomenon in salt gradient solar ponds

This paper investigates the performance of a 3-zone solar pond with a diffusely reflecting bottom under two modes of heat extraction. The study includes the effects of the absorption of solar radiation in pond water and the multiple reflections arising from the reflection of radiation at the bottom and the rereflection of light at the surface of the pond system. The calculations have indicated that the efficiency of a solar pond system decreases with increasing values of bottom reflectivity. For the case of heat extraction at constant temperature, optimal efficiencies obtained by considering multiple reflections are approximately 2 per cent higher than those obtained for single reflection.     

The stability of an unsustained salt gradient solar pond

A solar pond of area 5.712 m² was constructed. It was filled with water to a height of 87.5 cm. The stability of density and temperature profile, variation of salt flux due to temperature and depth, temperature loss during night time, and the evaporation losses at the surface were analyzed.     

Thermal characteristics and performance of salt gradient solar ponds

A mathematical model with various parameters such as effective absorptivity-transmitivity product and total heat loss factor, including ground losses and angle of refraction, which are related to the physical properties and dimensions of the pond, is developed to study the thermal behaviour of salt gradient solar ponds at different operational conditions. A linear relation is found between the efficiency of the solar pond and the function (ΔT/H ). The convective heat loss, the heat loss to the atmosphere due to evaporation through the surface of the pond and ground heat losses have been accounted for in finding out the efficiency of the pond. The dependence of the thermal performance of the solar pond on the ground heat losses is investigated and minimized using low cost loose and insulating building materials such as dry dunes and, Mica powder and loose asbestos at the bottom of the pond. The ground heat losses are considerably reduced with the asbestos (loose) and the retention power of solar thermal energy of the pond increases.     

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.     

Experimental study about erosion in salt gradient solar pond

An experimental study was executed using a small model pond to examine the erosion phenomenon on the gradient zone of a solar pond. By means of observing the flow pattern of the heat reserve zone, it was made clear that the erosion was caused by naturally occurring convection, which is based on the vertical temperature difference in the heat reserve zone. In other words, the ruling factor in erosion is the vertical temperature difference in the heat reserve zone. With regards to a real solar pond, a policy was proposed which suppresses erosion velocity by controlling the pond's operating conditions while keeping the temperature gradient of the non-convection zone (NCZ) as large as possible and making the vertical temperature difference in the heat reserve zone small. On analysis, the rational modification was effected by the Grashof number Gr, which is a dimensionless number and shows the influence of the flotage on the speed field and the temperature field. As a result, it was found that the modified dimensionless Grashof number explains the erosion velocity of the gradient zone well. Moreover, the function of the correlation between the erosion velocity and the modified Grashof number was obtained by regression calculation and could estimate the erosion condition of the gradient zone quantitatively.     

Conductive heat transfer in salt gradient stabilized solar ponds

This communication deals with heat transfer in salt gradient solar ponds. Spatial variations in thermal properties have been considered and the resulting one dimensional heat conduction equation with a source term is solved explicitly to obtain a closed form mathematical expression for temperature distribution in the non-convecting zone of the solar pond. The present analysis is not restricted to any one typical variation but rather applicable to any variation profile in thermal properties.