THE THERMAL CHARACTERISTICS AND ECONOMIC ANALYSIS OF A SOLAR POND COUPLED LOW TEMPERATURE MULTI STAGE DESALINATION PLANT PART II: ECONOMIC ANALYSIS

This paper presents the thermal performance and economic feasibility of matching the SGSP with the MSF destilation plant with a daily product water output of 1000 m³/day. The analysis are based on the assumption that the solar pond is to be used as the sole heal source (thermal energy) for the distillation plant. The thermal simulation of the MSF desalination process was predicted by using a mathematical model based on stage by stage calculations taking into account the variations in fluid properties and flow conditions. The generated simultaneous equations of the mass and energy balances were combined and arranged in a matrix form and then translated into algorithm to predict process variables such as temperature and flash evaporation rates.

The paper discusses optimisation of the size of the pond and the number of stages for three different storage zone temperatures taking into account the large variation in quantity of energy supplied by the pond between summer and winter. One result is that oversizing the pond, leading to some rejection of the heat collected during the summer (which is referred to as peak clipping), will result in a higher utilisation factor of the desalination plant and a reduction in the summer/winter yield ratio. Optimum peak clipping days, leading to the minimum product water cost, for each storage zone temperature and performance ratio is presented.

The sensitivity analysis of the various factors affecting the overall water costs show that the capital costs comprise about two thirds (2/3) of the total desalinated water costs. This demonstrates and re-emphasises the inherent and basic fact that solar desalination is a capital intensive enterprise. Each 1% increase in interest rate increases solar pond thermal energy costs by about 13–15% and desalinated water costs from SP/MSF combination by about 10–13%.     

DESIGN AND PERFORMANCE EVALUATION OF A SOLAR POND FOR INDUSTRIAL PROCESS HEATING

This paper describes the design of a solar pond for delivering 54 m³/day of hot water at 60°C to a catering facility in Singapore. The design of the pond was carried out in two steps. First, the depths of different layers of the pond were determined by considering the maximum temperature of the storage zone and the useful energy gain. For the given load and the local meteorological conditions, the optimum depths of various layers of the pond were found as follows:

Depth of surface-mixed layer:0.32 m

Depth of the insulation zone :1.00 m

Depth of storage zone:1.00 m

Total depth of the pond:2.32 m

The minimum payback period was used as the economic figure of merit to determine the optimum area of the pond. The optimum area of the pond is 6000 m². The payback period depends on the transparency of the pond, and for the conditions considered in the study, it varies between 3 and 4.5 years, The solar fraction varies from 65% for extinction coefficient, ( μ = 1.0m^?1 to 94% for μ = 0.55 m^?1, An experimental pond with an area of 14 m² and a depth of 1.5 m was built and tested over a period of time under the meteorological condition of Singapore. These results are used to validate the mathematical equations used in the design of the solar pond. A good agreement was found to exist between experimental and analytical results.     

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.     

Experimental study of a solar desalination pond as second stage in proposed zero discharge desalination process

This work represents the efficiency of a solar desalination pond as a second stage of proposed zero discharge desalination processes to reach fresh water and also concentrated brine from the effluent wastewater of the desalination unit of Mobin petrochemical complex. So a solar desalination pond is constructed after a pretreatment unit to concentrate the softened wastewater to about 20 wt%. The concentrated wastewater is as a suited feed for a forced circulation crystallizer. During one year, the effects of major parameters such as ambient temperature and solar insolation rate are investigated, experimentally. specific gravity in each layer of concentrated brine wastewater is evaluated. Also, evaporation rates are calculated theoretically and are verified by experimental data. Theoretical values predict evaporation rate accurately. Results show good agreement with experimental data. According to results, maximum evaporation rate is 5 l/m² day when the insolation rate is about 24,602 kJ/m² day Solar energy absorption factor on June is max. Also, experimental results show the best proposed time to gain highest thermal energy is on spring therefore performance efficiency of solar desalination pond promote on spring comparing with the other months. Extracted data for specific gravity prove the bottom of solar desalination pond, layer 1, is best zone for energy saving and energy utilization.Also, theoretical values of evaporation rate are calculated according to measured temperatures and related mass conservation equation. Comparison between theoretical and experimental values shows dusty weather, humidity and wind velocity affects on heat transfer coefficients approximately. So, deviations between theoretical data and measured values can be explained. Results show good agreements with experimental data.     

Economic assessment of a two-stage solar organic Rankine cycle for reverse osmosis desalination

The current paper presents the economic evaluation of a two-stage Solar Organic Rankine Cycle (SORC) for using the mechanical energy produced during the thermodynamic process to drive a Reverse Osmosis (RO) desalination unit. The developed integrated system is briefly analysed and the specific fresh water cost, as well as the cost of energy is calculated. The economic assessment results are compared with those obtained from a low-temperature SORC-RO and two alternative variants of PhotoVoltaic RO (PV–RO) systems (with and without batteries). It is found that the critical fresh water cost for the system under consideration is 7.48 €/m³ of permeate water and the cost of energy equals to 2.74 €/kWh, when the water cost is slightly higher than the critical one (meaning 8 €/m³). These values are considered satisfactory enough, in comparison to the other autonomous desalination technologies. Additionally, the specific fresh water cost of the developed technology was calculated to be 6.85 €/m³, being very close to the values of the PV–RO systems. The variant of two-stage SORC significantly improves the efficiency and reduces the cost of the already developed prototype system (single-stage low-temperature SORC for RO desalination), because the specific cost is found to be much lower and taking into consideration its reliability, this technology can constitute an alternative desalination method competitive to the PV–RO on the basis of techno-economic feasibility.     

Comparison between solar distillers with and without solar concentrator

In order to desalinate sea water by only solar energy using they were designed and built three distillers in which experimental measurements were taken to calculate production, performance and cost. Distillers by solar concentration are: a parabolic trough distiller (PTD) and a Fresnel linear distiller (FLD), and a stepped basin distiller (SBD). As a result, fresh water production under similar climatic conditions was 990 cm³/m²/day with an efficiency of 45.8% for the PTD, 855 cm³/m²/day with 38% of efficiency for the FLD and 5910 cm³/m²/day with an efficiency of 4.4% for the SBD. That is, although the SBD has 10 times lower efficiency than the PTD, it produces almost 6 times more fresh water per m² of distiller’s surface. Regarding the cost of production of each liter of desalinated water, it was calculated in € 0.086, € 0.103 and € 0.034 for the PTD, FLD and SBD, respectively.     

THE THERMAL CHARACTERISTICS AND ECONOMIC ANALYSIS OF A SOLAR POND COUPLED LOW TEMPERATURE MULTI STAGE DESALINATION PLANT PART I: THERMAL CHARACTERISTICS

Salt Gradient Solar Ponds (SGSP) have the potential of providing low grade energy with the advantage of an annual thermal energy storage cycle. The development of Multi-Stage Flash (MSF) distillation plants operating below 100°C allows SGSP to be considered as the heat source for these systems.

In this paper, two schemes of matching the SGSP with the MSF distillation plant are presented. The first scheme is based on the assumption that the solar pond is to be used as the sole heat source for the distillation plant (i.e. all the plant's thermal energy requirements are provided by the solar pond). The second scheme considers a hybrid system (solar + fuel), where a 20,000 m² solar pond is linked to an otherwise stand alone, fuel driven desalination plant. Both options are simulated with the same daily product water output of 1000m³/day. The thermal simulation of the MSF desalination process was predicted by using a mathematical model based on stage by stage calculations taking into account the variations in fluid properties and flow conditions. The generated simultaneous equations of the mass and energy balances were combined and arranged in a matrix form and then translated into algorithm to predict process variables such as temperature and flash evaporation rates.     

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.     

The ice pond—production and seasonal storage of ice for cooling

The ice pond is a scheme of making ice in winter and storing it for cooling in summer. During winter a large excavated reservoir is filled with a snow/ice mixture produced by a snow machine of the type used in ski resorts. At the end of the cold season the ice is covered with insulation. When cooling is needed, chilled water is pumped from the pond, and, after being warmed by the load, the water is returned to the pond. Since the ice in the ice pond is porous, it forms a very effective heat exchanger. Water can be extracted at temperatures between 0 and 1°C, thus providing excellent potential for dehumidification. Experimental ice ponds have been built and operated from 1980 to 1982, and a full scale ice pond (with 10,000 m³ capacity) has been built to supply cooling for a 12,000 m² office building. This paper describes the operating experience gained so far. The basic processes of ice making, ice preservation and ice utilization are described, and the economic prospects for ice ponds are analysed.     

Thermal analysis for system uses solar energy as a pressure source for reverse osmosis (RO) water desalination

As Natural resources are becoming limited and energy price dramatically increased, energy utilization with efficient systems is essentially required to be used in desalination technologies. The use of solar energy in desalination processes is one of the most promising applications of renewable energies. The primary focus on desalination by solar energy is suitable for use in remote areas. A proposed desalination system uses solar radiation, which concentrated by parabolic dish to heat up the working fluid in a closed space. Then the generated pressure in this space used to push salt water into RO module.Daily production rate of fresh water quantity for suggested system compared with other solar techniques is a promising rate for each m² of solar radiation collecting surface. The production rate for one operation cycle could reach to 1800 L/cycle of fresh water at low water salinity (Brackish water with 5000 ppm) and 55 L/cycle at highest water salinity (sea water salinity with 42,000 ppm). The required energy needed to produce 1 kg of fresh water is also promising even when in case of using another type of energy, also operating cycle has ability of repetition according to salinity concentration through sunny hours.     

Hydrostatic pressure plants for desalination via reverse osmosis

Renewable energies (solar and wind energies) associated to reverse osmosis (RO) are gaining renewed interest for brackish and seawater desalination. Another potential source of energy is the hydrostatic pressure at a sufficient operative depth or height to perform the RO process. This article provides a comparison of the energy requirement of various hydrostatic pressure-RO plants. For submarine and underground plants, the required energy is equal to 2.98 and 3.54 kWh, respectively, for 1 m³ of produced fresh water. In case of hydrostatic pressure generated by a column of water due to a head difference between the sea level and an adjacent mountain, the energy required is equal to 1.4 kWh. These energy requirements compare well with the usual energy requirement for desalination, between 3 and 10 kWh for 1 m³ of produced fresh water. However, the main drawback associated with hydrostatic pressure plants relates to their construction and their maintenance, which are expected to be more complicated and costly than for a ground plant.     

Solar energy desalination for arid coastal regions: development of a humidification–dehumidification seawater greenhouse

The long-term aim of our research is to develop humidification–dehumidification desalination technology for farms in arid coastal regions that are suffering from salt-infected soils and shortages of potable groundwater. The specific aim of our current study was to determine the influence of greenhouse-related parameters on a process, called Seawater Greenhouse, which combines fresh water production with growth of crops in a greenhouse system. A thermodynamic model was used based on heat and mass balances. The dimension of the greenhouse had the greatest overall effect on the water production and energy consumption. A wide shallow greenhouse, 200 m wide by 50 m deep gave 125 m³ d⁻¹ of fresh water. This was greater than a factor of two compared to the worst-case scenario with the same area (50 m wide by 200 m deep), which gave 58 m³ d⁻¹. Low power consumption went hand-in-hand with high efficiency. The wide shallow greenhouse consumed 1.16 kW h m⁻³, while the narrow deep structure consumed 5.02 kW h m⁻³. Analysis of the local climate indicated that the structure should be built facing the NE direction. We are also in the process of building a commercial size Seawater Greenhouse at a site by the sea. The aim is to demonstrate the technology to local farmers and companies in the Arabian Gulf. The system will allow for the reclamation of salt-infected land by not relying, at all, on groundwater resources.     

Assessment of electricity and hydrogen production performance of evacuated tube solar collectors

In this work, a unified renewable energy system has designed to assess the electricity and hydrogen production. This system consists of the evacuated tube solar collectors (ETSCs) which have the total surface area of 300 m², a salt gradient solar pond (SGSP) which has the surface area of 217 m², an Organic Rankine Cycle (ORC) and an electrolysis system. The stored heat in the heat storage zone (HSZ) transferred to the input water of the ETSCs by means of an exchanger and thereby ETSCs increase the temperature of preheated water to higher level as much as possible that primarily affects the performance of the ORC. The balance equations of the designed system were written and analyzed by utilizing the Engineering Equations Solver (EES) software. Hence, the energy and exergy efficiencies of the overall system were calculated as to be 5.92% and 18.21%, respectively. It was also found that hydrogen generation of the system can reach up to ratio 3204 g/day.     

Energetic and exergetic performance evaluation of natural circulation solar water heating systems

This study deals with the energy and exergy analyses of natural circulation solar water heating (SWH) systems. The system comprises of a single glazed flat plate solar collector (FPSC) with absorber plate of 2 m², and a separate insulated well-mixed vertical water storage tank (WST) of 125 liters. The variable heat transfer coefficients, water inlet and outlet temperatures of the FPSC; and temperature of heated water stored in the WST are predicted theoretically for each interval. The daily energy and exergy efficiency of the FPSC, WST and SWH system are estimated to be about 39 and 4.36%, 67 and 38.55%, 27 and 1.01%, respectively. It is found that the water inlet temperature, optical efficiency and the solar radiation strongly influence the performance of the FPSC both energetically and exergetically. It is observed that change in the mass flow rate of water improves the exergy efficiency of the FPSC significantly. FPSC has been identified as a critical component of the system where exergy destruction of 308 W/m² takes place daily as compared to 24 W/m² in the WST against available solar exergy of about 663 W/m².     

Performance of a concentrated photovoltaic energy system with static linear Fresnel lenses

A new type of greenhouse with linear Fresnel lenses in the cover performing as a concentrated photovoltaic (CPV) system is presented. The CPV system retains all direct solar radiation, while diffuse solar radiation passes through and enters into the greenhouse cultivation system. The removal of all direct radiation will block up to 77% of the solar energy from entering the greenhouse in summer, reducing the required cooling capacity by about a factor 4. This drastically reduce the need for cooling in the summer and reduce the use of screens or lime coating to reflect or block radiation.All of the direct radiation is concentrated by a factor of 25 on a photovoltaic/thermal (PV/T) module and converted to electrical and thermal (hot water) energy. The PV/T module is kept in position by a tracking system based on two electric motors and steel cables. The energy consumption of the tracking system, ca. 0.51 W m⁻², is less than 2% of the generated electric power yield. A peak power of 38 W m⁻² electrical output was measured at 792 W m⁻² incoming radiation and a peak power of 170 W m⁻² thermal output was measured at 630 W m⁻² incoming radiation of. Incoming direct radiation resulted in a thermal yield of 56% and an electric yield of 11%: a combined efficiency of 67%. The annual electrical energy production of the prototype system is estimated to be 29 kW h m⁻² and the thermal yield at 518 MJ m⁻². The collected thermal energy can be stored and used for winter heating. The generated electrical energy can be supplied to the grid, extra cooling with a pad and fan system and/or a desalination system. The obtained results show a promising system for the lighting and temperature control of a greenhouse system and building roofs, providing simultaneous electricity and heat. It is shown that the energy contribution is sufficient for the heating demand of well-isolated greenhouses located in north European countries.     

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.     

CONSTRUCTION AND OPERATIONAL EXPERIENCE OF A 6000 m SOLAR POND AT KUTCH, INDIA

A 6000 m² solar pond was constructed at Bhuj in India in the premises of a milk processing dairy plant to supply process heat and demonstrate the technical and economic viability of solar pond technology in the Indian context. An inexpensive lining scheme, consisting of alternating layers of clay and LDPE (low density polyethylene) combination was used for lining the pond. The pond attained a maximum temperature of 99.8°C under stagnation in May 1991 but developed leakage soon after. A failure analysis that was carried out subsequently indicated that the leakage was caused by the combination of high stagnation temperature and large air pockets below the liner. The lining scheme was re-designed and the pond re-established in June 1993. Hot water supply to the dairy started in September 1993 and continued until April 1995. After an interruption of nearly one year, hot water was resumed in August 1996. The total cost of construction of the Bhuj Solar Pond was US$90 000 (1997 prices), including heat exchanger and piping etc., corresponding to a unit cost of US$15 m⁻².     

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.     

Optimized solar-powered liquid desiccant system to supply building fresh water and cooling needs

This paper studies the feasibility of using a solar-powered liquid desiccant system to meet both building cooling and fresh water needs in Beirut humid climate using parabolic solar concentrators as a heat source for regenerating the liquid desiccant. The water condensate is captured from the air leaving the regenerator. An integrated model of solar-powered calcium chloride liquid desiccant system for air dehumidification/humidification is developed. The LDS model predicted the amount of condensate obtained from the humid air leaving the regenerator bed when directed through a coil submerged in cold sea water. An optimization problem is formulated for selection and operation of a LDS to meet fresh water requirement and air conditioning load at minimal energy cost for a typical residential space in the Lebanon coastal climate with conditioned area of 80 m² with the objective of producing 15 l of fresh drinking water a day and meet air conditioning need of residence at minimum energy cost. The optimal regeneration temperature increases with decreased heat sink temperature with values of 50.5 °C and 52 °C corresponding to sink temperatures of 19 °C and 16 °C.     

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.