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.     

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.     

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.     

Single-stage and advanced absorption heat transformers operating with lithium bromide mixtures used to increase solar pond's temperature

Mathematical models of single-stage and advanced absorption heat transformers operating with the water/lithium bromide and water/Carrol™ mixtures were 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. Plots of coefficients of performance and gross temperature lifts are shown against the temperatures of the heat supplied by the solar pond. The results showed that the single-stage and the double absorption heat transformer are the most promising configuration to be coupled to solar ponds. With single-stage heat transformers it is possible to increase solar pond's temperature until 50°C with coefficients of performance of about 0.48 and with double absorption heat transformers until 100°C with coefficients of performance of 0.33.     

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.     

Dependence of ground heat loss upon solar pond size and perimeter insulation calculated and experimental results

Ground heat losses from solar ponds are modelled numerically for various perimeter insulation strategies and several solar pond sizes. The numerical simulations are steady state calculations of heat loss from a circular or square pond to a heat sink at the outer boundaries of an earth volume that surrounds the pond on the bottom and sides. Simulation results indicate that insulation on top of the ground around the pond perimeter is rather ineffective in reducing heat loss, and that uninsulated sloping side walls are slightly more effective than insulated vertical side walls, except for very small ponds. The numerical results are used to derive coefficients for a semi-empirical equation describing ground heat loss as a function of pond area, pond perimeter and insulation strategy. Experimental results for ground heat loss and energy balance in the 400 m² solar pond at the Ohio State University are reported. Analysis of this data, along with data on solar energy input, heat gain by the pond, heat loss through the gradient zone, and heat extraction from the pond yields a good energy balance. Numerical simulation of ground heat loss from this pond shows good agreement with the results obtained from pond measurements. Loss turns out to be large because of unexpectedly high values of earth thermal conductivity in the region.     

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.     

Prospects and scopes of solar pond: A detailed review

Solar pond is an artificially constructed pond in which significant temperature rises are caused to occur in the lower regions by preventing convection. To prevent convection, salt water is used in the pond. Those ponds are called “salt gradient solar pond”. In the last 15 years, many salt gradient solar ponds varying in size from a few hundred to a few thousand square meters of surface area have been built in a number of countries. Nowadays, mini solar ponds are also being constructed for various thermal applications. In this work, various design of solar pond, prospects to improve performance, factors affecting performance, mode of heat extraction, theoretical simulation, measurement of parameters, economic analysis and its applications are reviewed.     

Solar ponds for space heating

Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft² house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.     

Field testing on nonsalt solar ponds

A new type of nonsalt solar pond was investigated by field testing. The roof of the solar pond was formed using a transparent double film. Three kinds of tests were carried out under the following conditions: (1) insulating pellets were packed between the layers of the transparent double film of the roof at sunset; (2) the water surface of the pond was insulated using only the two transparent films; (3) the water surface of the pond was covered by the double film with the top surface blackened on which solar energy can be collected, while pond water was circulated using a solar cell powered submerged water pump. The warm water stored in the solar pond by the above methods was utilized as a heat source for a gas engine powered heat pump used to heat a greenhouse. In this report, the results of the field tests on the above solar ponds and greenhouse heating system are discussed. Also the utility of a combination plant using a solar pond and underground borehole storage system is evaluated.Important conclusions on performance are as follows: (1) collection efficiencies of these solar ponds become 9–54% corresponding to the weather conditions and pond temperatures; (2) maximum temperature of the pond water under weather conditions at Osaka is about 80°C; (3) the solar pond can be effectively utilized for heating a greenhouse; (4) the combination plant using the solar pond and the underground storage layer can store heat of 1119 MJ m⁻² yr⁻¹.     

Heat extraction methods from salinity-gradient solar ponds and introduction of a novel system of heat extraction for improved efficiency

Heat has generally been successfully extracted from the lower convective zone (LCZ) of solar ponds by two main methods. In the first, hot brine from the LCZ is circulated through an external heat exchanger, as tested and demonstrated in El Paso and elsewhere. In the second method, a heat transfer fluid circulates in a closed cycle through an in-pond heat exchanger, as used in the Pyramid Hill solar pond, in Victoria, Australia. Based on the experiences at the El Paso and Pyramid Hill solar ponds, the technical specifications, material selection, stability control, clarity maintenance, salt management and operating strategies are presented. A novel method of extracting heat from a solar pond is to draw the heat from the gradient layer. This method is analysed theoretically and results of an experimental investigation at Bundoora East, RMIT, are presented. An in-pond heat exchanger made of polyethylene pipe has been used to extract heat for over 2 months. Results indicate that heat extraction from the gradient layer increases the overall energy efficiency of the solar pond by up to 55%, compared with conventional method of heat extraction solely from the LCZ. The experimental results are compared with the theoretical analysis. A close agreement has been found. From this small-scale experimental study, convection currents were found to be localised only and the density profiles were unaffected. An experimental study using an external heat exchanger for brine extraction and re-injection at different levels within the gradient layer still needs to be conducted to determine the effect of the heat extraction from the non-convective zone (NCZ) on the stability of the salinity gradient (both vertically and horizontally) and an economic analysis needs to be conducted to determine the economic gains from increased thermal efficiency.     

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.     

Salt gradient stabilized solar pond collector

This paper presents a theoretical analysis of a salt gradient solar pond as a steady state flat plate solar energy collector. 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 by solving the Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation as it passes through the pond water. These evaluations, in combination with energy balance considerations, enable the derivation of the expressions for solar pond efficiency of heat collection as well as the efficiency of heat removal. The efficiency expressions are Hottel-Whillier-Bliss type, prevalent for flat plate collectors. Numerical computations are made to investigate the optimization of geometrical and operational parameters of the solar pond. For given atmospheric air temperature, solar insolation and heat collection temperature, there is an optimum thickness of nonconvective zone for which the heat collection efficiency is maximum. The heat removal factor is also similar to that of a flat plate collector and the maximum efficiency of heat removal depends on both the flow rate and the temperature in the nonconvective zone.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

The shallow solar pond energy conversion system

The concept of a shallow solar pond energy conversion system is presented as an effective way to produce large-scale electric power from solar energy. Water is used both for heat collection and heat storage. Inexpensive layers of weatherable transparent plastic over the water suppress heat loss to the environment. The hot water is stored in an insulated reservoir at night. The stored hot water heats a thermodynamic fluid, probably Freon 11, which drives a turbine and an electric generator.A shallow solar pond system can be built using materials, fabrication techniques and geometries that are presently used on a large scale in U.S. insustry. A 10 MWe plant built in the Southwest would require a total area of about 2 km² and could provide power for a community or a manufacturing process. The estimated busbar cost of electricity (1975 dollars) for a shallow solar pond system, which could come on line in as short a time as 5–7 yr, is 56 mills/kWh. This cost could be reduced with the development of improved and cheaper plastics and more efficient turbines.Another potentially important use of shallow solar ponds is to provide process hot water, up to the boiling point, for industrial and commercial purposes. Also, a shallow solar pond could provide hot water for the space heating, air conditioning and hot water needs of a community of homes or apartments.     

Modeling and testing a salt gradient solar pond in northeast Ohio

A dynamic computer model of a salt gradient solar pond as an annual-cycle solar energy collection and storage system was developed. The model was validated using experimental results of a solar pond located at the Ohio Agricultural Research and Development Center (OARDC), Wooster, Ohio. The model was then used to analyze the transient energy phenomena which occurred within the storage zone of the pond. Generalized daily weather functions used were the incident solar radiation upon a horizontal surface, the daylight length and the daily maximum and minimum ambient air temperatures.Various simulations were performed to evaluate the OARDC solar pond and to improve its overall effective capacity of heat storage. It was found that 4–6 weeks variation in start-up time and 5–10°C variation in start-up temperature had no effect on late summer peak storage temperature. The pond operated at a 20 per cent collection efficiency with a 1.5-m deep gradient. Insulating the pond in the winter would be beneficial if no heat was removed during the fall. Reducing the gradient zone thickness to 1 m and enlarging the storage zone could improve the performance of a 3-m deep pond. The model could be used to predict and analyze the transient thermal response of large storages associated with solar heating system for a variety of purposes and climatic conditions.     

An accurate upper estimate for the transmission of solar radiation in salt gradient ponds

The objective of the present work is the estimation of maximum transmission of solar radiation within a body of natural water under the most favorable conditions, with special reference to salt gradient solar ponds.The study, based on recent data, includes two alternative analytical approaches based on the split of the solar spectrum into the appropriate number of spectral bands and numerical calculation of discrete values of integrals according to Beer's law. As far as radiation transmission properties of clear water are concerned, it was found that the broadly used absorption law, which is commonly referred to as an upper transmission limit, is derived from original work by Schmidt[23] and deficient data, employed at the time, are responsible for the appreciably lower theoretical maximum transmission then derived. The accurate upper transmission limit derived now also gives comparative higher heat collection efficiency. Comments have been made for the introduction of a water clarity dimensionless factor in terms of the upper transmission limit, describing the economical operation limits for solar pond water.     

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.     

A study on the transient behaviour of solar ponds

In this paper, the transient behaviour of solar ponds has been investigated using a finite difference formulation. The performance of solar ponds can be successfully analysed and the effects of various parameters studied. The thickness of the layer with density gradient has a profound effect on the performance of a solar pond and an increase of the thickness of this layer from 1 to 2m increases substantially the operating temperature for the same overall efficiency.

The effect of the pattern of heat extraction is discussed. Heat extraction at a constant rate will result in large temperature fluctuations from summer to winter. But, if the heat is extracted at a varying rate proportional to the monthly average solar radiation, then the temperature fluctuation is considerably decreased as compared to the case of constant heat removal for the same yearly efficiency. It is possible to operate a solar pond in the Melbourne area with a yearly efficiency of 15% having a high temperature of 67 °C in summer and a low of 40 °C in winter.

The performances of solar ponds in Alice Springs and Darwin have been studied. It is found that the maximum bottom temperature does not go beyond 80 °C for a pond efficiency of 20%. Since these two cities have the most favourable locations for solar ponds, the indicated maximum temperature casts doubt on the prevalent view that a solar pond can operate at a temperature above 90 °C with an efficiency of 20%.     

Benefit cost analysis of gel solar ponds

A benefit to cost (B/C) analysis was performed on gel ponds based on experimental data collected from the circular demonstration gel pond (5 m dia × 1.25 m depth) located at UNM. The measured transmissivity, predicted temperature profile, and calculated surface heat losses with volumetric heat generation were used as physical data in the coded design model (GPDM) used for sizing the solar pond. These data were used in the coded economical analysis model (GPEA) to calculate capital, operation, life cycle, and cost of delivered energy for a specific pond. A general case study was considered to demonstrate the potential and economical feasibility of gel ponds as a source of hot water (45°C) for domestic use in five regions in the United States. An optimized gel pond showed B/C values as high as 1.35 for high insolation areas (Southwest, Puerto Rico) and as low as 0.45 for low insolation areas (Great Lakes, Atlantic NE) compared to 0.96 and 0.48 for salt gradient ponds of the same size and location. In a special case study to demonstrate industrial applicability of gel ponds as a source of hot water (65°C) for a textile mill (Cairo, Egypt), the optimized pond had a B/C value of 1.07 compared to 0.93 for an optimized salt gradient pond of the same load output. In general, for the same size (400 m² × 4 m deep), location (southwest) and extraction temperature (45°C), in gel and salt gradient ponds, the gel has higher capital cost (19%), lower operating cost (53%), lower delivered energy cost (26.4%), and higher extraction efficiency (32.5%). While, for the same load output (150 kW thermal) and location (Cairo), a gel pond has higher capital cost (21.5%), lower operating cost (63.5%), smaller surface area (21%), shallower depth (28.6%), and lower delivered energy costs (13%). Using GPDM (Gel Pond Design Model) and GPEA (Gel Pond Economic Analysis) computer programs, a sufficient engineering and economic analysis can be performed for gel and salt gradient ponds, respectively.