Chapter 14
Solar Ponds
César Valderrama*
Abstract
The chapter introduces the fundamentals of solar pond technology as a thermal storage process. It includes a brief description of solar pond technology in the context of energy storage, history, fundamentals; a classification of the different ponds used for capture and storage of solar energy; the fundamentals of the salinity gradient solar pond, describing the research efforts over recent decades; the design, construction, and the establishment of the salinity gradient, operation, and control; and finally the heat extraction systems. A discussion of investment, construction, and operational cost is also presented together with a review of the different applications in which solar ponds have the potential to supply low-grade thermal energy.
Keywords
thermal energy storage
nonconvective solar storage
salt gradient solar pond
low-heat thermal application
renewable energy
1. Introduction
Climate change and depletion of fossil fuels are of major concern. The world community has prompted a broad discussion of technical and financial resources for promoting increased energy efficiency in the use of renewable energy resources. Solar energy is regarded as one of the most promising substitutes for traditional energy resources; however, its intermittent and unstable nature is a major drawback, which leads to a disparity between supply and demand [1]. To enhance the fraction of energy utilization and make solar energy products more practical and attractive, thermal storage systems today are perceived as crucial components in solar energy applications. Thermal energy storage systems utilize either thermochemical reactions or the sensible or latent heat capacity of materials to provide a heating or cooling resource, which can be replenished as required [2]. In sensible thermal storage, energy is stored by changing the temperature of the storage material. The amount of heat stored is proportional to the density, specific heat, volume, and variation of temperature of the storage material. The performance of a storage system depends mainly on the density and specific heat of the substance used, which determine the necessary storage volume [3]. Water is one of the best storage liquids for low-temperature heat storage [4]. It has a higher specific heat than any other material and is cheap and widely available. Surface water bodies (ponds or lakes) can be used to collect and store solar heat [5]. In a pond the creation of a salinity gradient results in a higher salt concentration and density at the bottom, and the heat absorbed remains trapped there because the salinity gradient inhibits natural convection and the cooler water at the surface acts as an insulator as it does not mix with the saline water. Darkening the bottom surface of the pond also results in more solar radiation being absorbed [2]. There are many factors which affect the economical and operational size of the heat storage solar pond for a particular application. These factors include (1) the purpose of the solar energy system (load), (2) the area of the collector, (3) the meteorological conditions at the location, and (4) the operational characteristics of the system [6].
Solar ponds are an old natural phenomenon that was first documented by Von Kalecsinsky (1902) for Medve Lake in Transylvania (Hungary) where temperatures up to 70 °C at a depth of 1.32 m were recorded at the end of the summer. Similar observations were reported by Anderson (1958) and Wilson and Wellman (1962) for several other lakes as well as by other authors [7–9].
The concept of an artificial solar pond as a possible means of collection and storage of solar energy was proposed in the middle of the last century [7]. The convection currents that normally develop due to the presence of hot water at the bottom and cold water at the top are diminished or minimized by the presence of a strong density gradient from bottom to top. Thus, the water in the lower zones can be warmer than the water above without, simultaneously decreasing density and causing convection to the surface [9,10]. The most attractive characteristics of solar ponds are, first, the capacity of long-term storage which can supply sufficient heat for the entire year and, second, the annual collection efficiency in the range (15–25)% for all locations and the capacity to supply adequate heat even at higher latitudes. It has been found that solar ponds of areas of the order of 1000 m2 or higher are more cost effective than flat plate collectors with higher efficiency as their cost per square meter is much lower than those for flat plate collectors [11].
2. Types of solar ponds
There are several types of solar ponds and most are based on the use of a salinity gradient. Their designs attempt to overcome some of the disadvantages observed in these types of ponds, for example, the need to establish a salinity gradient and the intensive level of maintenance and operation procedures that is required. Several authors have proposed different classifications of solar ponds; among them are the formulations by Kaushika [12], El-Sebaii et al. [13], and Ranjan [11]. Solar ponds can be classified according to four basic factors: (a) convecting or nonconvecting, (b) partitioned (multilayered) or nonpartitioned, (c) gelled or nongelled, and (d) separate collector and storage or in-pond storage [2]. However, most research efforts are presently focused on the nonconvecting salinity gradient solar pond [14] which will be discussed in detail. They are easier to operate and cheaper to construct. Other types will also be briefly discussed based on the most representative studies reported in the literature.
It is observed that salinity gradient solar ponds (see Section 2.1) have the advantage of long-term energy storage over nonsalinity solar ponds such as membrane-stratified ponds and shallow solar ponds, which are more suitable for short-term energy storage because of the higher rate of increase of the temperature of the pond water.
2.1. Salinity Gradient Solar Pond
A solar pond consists of three distinct zones as can be seen in Fig. 14.1 [8,15]. The first zone, which is located at the top of the pond and contains the less dense salt/water mixture, is the absorption and transmission region, also known as the upper convective zone (UCZ), which has the function of protecting the salinity gradient layer. Its stability is controlled by the addition of water onto the solar pond surface and the prevention of wind agitation. The second zone, which contains a variation of salt/water density, increasing with depth, is the gradient zone or nonconvective zone (NCZ), also called the salinity gradient layer. The main purpose of this zone is to act as an insulator to prevent heat from escaping to the UCZ, thus maintaining higher temperatures at the deeper zones. The last zone is the lower convective zone (LCZ) also called the energy storage zone, which consists of saturated brine with almost homogeneous salinity and density. The salts used include sodium chloride, magnesium chloride, or sodium nitrate which are dissolved in water with concentrations varying from (20 to 30)% at the bottom (LCZ) to almost zero at the top (UCZ) [11].
Figure 14.1 Salinity gradient solar pond scheme.
When solar radiation is incident on the solar pond, part of the radiation is reflected away from the top surface while most of the incident sunlight is transmitted down through the top surface of the UCZ. A fraction of the transmitted radiation is rapidly absorbed in the surface layer. However, this absorbed heat is lost to the atmosphere by convection and radiation heat transfer. Some of the remaining radiation is absorbed in the middle NCZ before the rest reaches the bottom of the pond. In the LCZ the absorbed solar energy is converted to heat and stored as sensible heat in the high-concentration brine. Since there are no heat losses by convection from the bottom layer the temperature of this layer can rise substantially. A double-diffusion process occurs where the temperature and salinity fields make opposing contributions to the fluid density [16]. The temperature difference between the top and the bottom can be as high as 60 °C [17]. The thermal energy stored can be used for the heating of buildings, power production, and industrial processing [18]. It has been studied around the world for more than half a century and successful case studies have been reported in Israel, USA, India, China, Australia, and Spain [19–24].
2.1.1. Design and Construction
The design objective is to meet the energy requirements of the application which integrates the solar pond as a heat source at low temperature. The medium to be heated (water, air, other fluids) and the temperature to be supplied are critical to the design specifications. Economic viability is critically affected both by the location of the facility and the maximum use of local resources. The availability of a salt source (salt or brine), freshwater, and enough land area (flat terrain) are the key requirements in the assessment of potential locations [23]. The next step is sizing the solar pond to meet energy requirements. The most important parameter is solar radiation; the higher the radiation on the site selected the higher the energy efficiency and operating temperature of the pond. A typical solar pond of depth 3 m and a storage zone of 1 m thickness would receive approximately (20–25)% of the radiation incident upon the pond surface. Heat losses to the ground must also be taken into account and, in practice, approximately (15–20)% of the incoming radiation is available for extraction to an application, resulting in a rise of (40–50) °C above the daily average temperature of the location. A simple way to estimate the surface area needed to meet any particular average thermal load was proposed by Akbarzadeh et al. [23]. First, the annual solar energy incident on a square meter of horizontal surface at the location of the pond (e.g., 7 GJ m–2 a–1, where “a” refers to annum) is calculated. Then, an estimate of the horizontal surface area on which the incoming solar radiation over a year is required to meet the annual load. If, for instance, the annual load is 2800 GJ, this area would be 400 m2. Finally, multiply this surface area by a factor of 5–10 to estimate the surface area of a solar pond to meet this annual load. In this example the solar pond would need to have an area between (2000 and 4000) m2 to supply an annual thermal load of 2800 GJ. The thermal performance of a solar pond is also affected by the pond depth; the pond is usually contained in earth excavated to a depth of (3 or 4) m. The deeper the storage zone the larger the storage capacity and the larger the quantity of heat available for an extended time as a result of the reduction in heat losses and and increased thermal efficiency of the pond.
Solar pond construction involves consideration of several strategies related to the location of the pond, earth excavation (land chosen should be as flat as possible), lining, insulation, and pond shape. Local geology is critical in site selection and care must be taken to avoid connections with aquifers [12]. To minimize heat losses to the ground, it is desirable that the groundwater table should be at least 5 m below the ground surface. For large ponds construction the best option involves the use of soil excavated from the periphery of the pond for the pond walls. The bottom of the pond will thus be below the surrounding ground level. This approach can represent a significant reduction in the construction costs of the pond. A lining is necessary, for both environmental as well as performance reasons. The liner material should be able to withstand the anticipated maximum pond temperature, be resistant to UV radiation, and should not react with the salt. Above all, it should be mechanically strong. Thermal insulation at the base of the pond and the sides may be used to reduce heat losses; this may enhance pond heating but may represent excessive cost, especially for larger ponds.
2.1.2. Settling the Salinity Gradient
The stratification of layers is artificially done. Before the pond is heated the profile set when filling the pond may remain constant provided there are no external disturbances such as wind. It is crucial that when the bottom is heated the density at the bottom is greater than that of the cooler brine on top, otherwise mixing takes place. Three methods have been adopted for the establishment of the initial salt density gradient: natural diffusion, stacking, and redistribution. The first method relies on the natural diffusion between a freshwater layer and the layer of saturated salt solution [12]. In the natural diffusion method the upper half is filled with water; top and bottom concentrations are maintained constant by regularly washing the surface and by adding salt in the bottom. Due to the upward diffusion of salt a salinity gradient will be established [13]. Stacking involves filling the pond with a storage layer of high-concentrated salt solution and several other layers of salt solutions at different concentrations. The concentration of salt in successive layers is changed in steps from near saturation at the bottom to freshwater at the top. The practical approach for stacking used in most solar ponds is that the bottom layer is filled first and successively lighter layers are floated upon the lower denser layers [25]. The redistribution method is considered to be most convenient and recommended for large-area solar ponds. The first step is to fill the pond up to half the depth of the planned gradient zone with high-concentration brine. Freshwater or low-salinity water is then injected horizontally into the brine through a diffuser. The water will stir and uniformly dilute the brine above the diffuser. Injection starts from the desired level of the boundary between the NCZ and the LCZ. As injection proceeds and the pond surface level of the water rises the diffuser is simultaneously raised in increments from its position within the brine solution toward the surface of the pond. The speed of raising the diffuser is twice the speed of the rise of the pond water; that is, after each 50 mm rise of water the diffuser is raised by 100 mm. The timing of diffuser movement is adjusted so that the diffuser and the water surface reach the final level at the same line, which is the boundary between the UCZ and NCZ. Finally, the UCZ is formed by adding freshwater above the current water surface, and at the end of this process the pond is full and the desired salinity gradient created as can be seen in Fig. 14.2 [12,13,23]).
Figure 14.2 (a) Scheme of the redistribution method for establishing the salinity gradient and (b) the density profile obtained during salinity gradient settling in the 500 m2 Escuzar Solar Pond, Granada (Spain) in June 2014.
2.1.3. Control and Maintenance
A solar pond should be maintained by periodical addition of a saturated salt solution at the bottom, washing the surface with freshwater, and monitoring and controlling disturbances such as treatments of algae blooms. Salt chargers and flushing systems have been designed and used recently for salt addition and to compensate the losses caused by evaporation and to renovate the surface water, respectively [24]. Moreover, the addition of freshwater (or even seawater) to keep both the pond depth and the surface concentration constant is a critical parameter in water scarcity areas. Indeed, it is important to point out that salt concentration in the UCZ can be fixed at approximately 4% and water only added when the concentration is higher than this value, thus avoiding high consumption of freshwater. A more complex situation is the case of inland solar pond applications where water is scarce. In such cases the availability of makeup water is a major issue and may limit the construction of a pond [26].
Maintaining a high transparency of water is one of the most important elements in achieving high efficiency and delivering the collected heat at high temperature. The growth of algae and deposition of tree leaves, flying debris such as dust or larger agglomerates that can contribute to an increase of pond water turbidity, and increased resistance and absorption of radiation above the storage zone are the main concerns. Acidification of the pond provides a simple and reliable maintenance procedure for preventing or inhibiting algal blooms and maintaining high transparency. In this case, the pH in the salt gradient zone should be maintained by the addition of hydrochloric acid to keep it below 4 [23,24,26].
Finally, a parameter to be considered is surface mixing, especially in locations with strong winds and for larger solar ponds. Surface turbulence depletes the salt concentration gradient and can promote complete convection in the surface region, causing a reduction in thermal performance. Windbreakers are used to reduce the possibility of wind-induced surface turbulence and most of the windbreaks used in pilot and larger solar ponds are floating rings [23,24].
2.1.4. Heat Extraction
There are two methods of extracting heat from the lower convective zone of a solar pond. The first, which is the most commonly used method, is pumping hot brine from the LCZ through a diffuser to prevent excessive velocity and motion within the pond and thereby minimize erosion of the gradient zone. An external heat exchanger is used to extract heat from brine and the cooled brine is returned to the bottom of the pond (Fig. 14.3a). This method was successfully used at the 5 MW solar pond power plant in Beit Ha’arava (Israel) near the Dead Sea; heat was extracted from the top of the storage zone and the cold and heavier brine was returned to the bottom of the pond at the same side of the pond. Being cooler than the stored hot brine, it remained on the bottom of the pond and spread over the entire area before rising up when filling the volume and/or being heated during the next charging period [26,27].
Figure 14.3 Heat extraction methods in solar pond technology.
(a) External heat exchanger, (b) internal heat exchangers, and (c) internal heat exchanger at the gradient zone.
(a) External heat exchanger, (b) internal heat exchangers, and (c) internal heat exchanger at the gradient zone.
The second method involves a heat exchanger that is placed in the lower convective zone of the pond. Its most appropriate position is just below the gradient zone, so that the heat removal can stimulate convection throughout the lower convective zone and remove heat from its entire volume (Fig. 14.3b). Since the working fluid is typically freshwater the heat exchanger can be constructed from low-cost materials such as plastics. The low thermal conductivity of the plastic pipes is compensated by increasing the heat transfer area on the in-pond pipes; that is, by installing more pipes and/or increasing the diameter of the pipes. The disadvantages of this method are related to the large quantity of tubes required, difficulties in locating the heat exchanger, and the difficulty in effecting repairs. This method was used at a 3000 m2 demonstration solar pond in Pyramid Hill (Australia) by using plastic pipes connected to weights on the pond bottom by ropes to overcome buoyancy forces and has proved to be a simple and reliable method of heat extraction [13,23]. These two heat extraction methods are shown in Figs. 14.3(a,b). Recently, a novel system of heat extraction has been proposed and assessed both theoretically and experimentally with the aim of improving overall energy efficiency. In this method, heat is extracted from the NCZ as well as, or instead of, the LCZ (Fig. 14.3c). Theoretical and experimental investigation showed that heat extraction from the NCZ increases overall thermal efficiency by up to 55% compared with the conventional method of heat extraction solely from the LCZ [28,29].
2.2. Saturated Solar Ponds
This type of pond is designed to improve or reduce the level of maintenance of the salinity gradient by making the pond saturated at all levels, with a salt whose solubility increases with temperature. A number of salts besides KNO3 have been used and are found to be appropriate; these include Na2B4O7 (borax), KAl(SO4)2, CaCl2, MgCl2, and NH4NO3 [11]. Such saturated ponds have no apparent diffusion problems and the gradients are self-sustaining depending on local temperature. This gives these ponds the advantage of inherent stability [13]. Harel et al. [30] developed the equilibrium solar pond concept as a generalization of the saturated solar pond and the anticipated advantages over the saturated solar pond, which relies on the fact that the fluid in the equilibrium solar pond is unsaturated. This advantage has two important consequences: crystallization is only possible under significant cooling of the brine; and the absence of solid salt at the bottom of the pond. Both consequences lead to an increase in the absorption of energy in the LCZ and thus higher thermal performance of the pond.
Two main advantages can be identified in comparison to the conventional solar pond: first, the zero salt flux throughout the pond eliminates the need for addition of salt after the pond is set up and the need for disposal of water from the pond. Second, due to the high concentration in the bottom region a higher bottom temperature can be achieved before the onset of boiling of the salt solution, thus increasing the thermal efficiency of the pond [11].
2.3. Solar Gel and Membrane Ponds
A solar gel or viscosity-stabilized pond is a nonconvective and nonsalt solar pond and was proposed to minimize or eliminate evaporation losses from the surface by reduction of heat losses. These ponds use a transparent polymer gel as a nonconvecting layer. The polymer gel has low thermal conductivity and is used at a near solid state, so that it will not convect [31]. The polymer gel has good optical and thermal insulating properties but the cost of the gel is high.
Materials suitable for viscosity-stabilized solar ponds should have high transmittance for solar radiation, high efficiency of the chosen thickness, and should be capable of performing at temperatures up to 60 °C. Polymers such as gum arabic, locust bean gum, starch, and gelatin are all potentially useful materials for this configuration [13]. Wilkins [32] reported the design and construction of two solar gel ponds of (110 and 400) m2 (5 m deep with a gel thickness of 0.60 m) in New Mexico to provide process heat for a food company. It was observed that solar gel ponds are superior to salt gradient solar ponds from the point of view of efficiency, ease of operation, and economics [33]. The potential and economic feasibility of gel ponds as a source of hot water (45 °C) for domestic use in the United States was demonstrated. Industrial applicability of gel ponds as a source of hot water (65 °C) for a textile mill in Cairo (Egypt) has also been shown [11].
Membrane-stratified solar ponds belong to the group of nonconvective solar ponds, having a body of liquid between closely spaced transparent membranes to minimize convective heat transfer. The disadvantage of adding a physical layer to a system where solar radiation is the only input is reduction of total transmission of sunlight to the bottom of the pond. Thus, the membrane space for suppressing convection should be very small and a large number of highly transparent films is required [31]. Three types of membranes are suggested for the membrane-stratified solar pond, which are horizontal sheets, vertical tubes, and vertical sheets [13].
2.4. Shallow Solar Pond
The term shallow solar pond (SSP) has been derived from that of the solar still. The name implies that the depth of water in the SSP is relatively small, typically (4–15) cm, which is like a conventional solar still consisting of a blackened tray holding some water [34]. An SSP is essentially a large water bag or pillow placed within an enclosure with a clear upper glazing. Water is placed within the bag, which is generally constructed from clear upper plastic film and a black lower plastic film, in such a way that the film is in contact with the top surface of the water, and thus prevents the cooling effect due to evaporation [35]. The black bottom of the pond absorbs solar radiation; as a result, the water gets heated.
Total solar energy absorbed by system cannot all be used as useful energy. There are losses due to conduction, convection, and radiation. Using a suitable insulation material, conduction losses can be reduced. To reduce the thermal losses by convection and radiation, one or two transparent sheets are used over the pond [36]. Solar energy collection efficiency is directly proportional to water depth, whereas water temperature is inversely proportional to water depth. Solar energy is converted to thermal energy by heating the water during the day. The water is withdrawn from the SSP before sunset (or more precisely when the collection efficiency approaches zero) for utilization or storage. Different studies have been carried out with the aim of assessing their performance. The Solar Energy Group at Lawrence Livermore Laboratory (USA) developed an SSP to supply heated water (25–60) °C for industrial and commercial uses around 1973 [37]. Later, in the late 1980s, the Erel Company developed a shallow water pond on top of which float thermal diodes composed of an array of translucent honeycombs made of plastic material. The water heated in the pond could reach temperatures of about 85 °C. This type of pond is suitable for supplying warm water for household use and other low-temperature applications, for example, laundries, textile factories, canned food factories, greenhouses. More recently, a pilot system was installed in the Kibbutz Maoz-Haim (Israel) to supply hot water for a housing project of 42 units [38]. El-Sebaii [39] studied theoretically and experimentally an SSP integrated with a baffle plate to suit prevailing weather conditions at Tanta (Egypt). The system could provide 88 L of hot water at a maximum temperature of 71 °C at 3:00 pm with a daily efficiency of 64% when the baffle plate was used without vents. The pond could retain hot water until 7:00 am the next day at a temperature of 43 °C, which represents a benefit for most domestic applications.
3. Investment and operational cost
The costs of a solar pond vary widely according to location and application. The major cost factors are the earthwork (excavation, leveling, and compaction), the lining and isolation, the salt source, the freshwater or low-density water, the land, and monitoring and control equipment. Furthermore, economic feasibility analysis of the specific design, the site, and integration of the solar pond with the application is essential. Indeed, economy of scale affects solar pond technology, thus small ponds are considerably more expensive than large ponds, and there is wide variability in per unit area cost estimates of solar ponds operated at different locations throughout the world. This variability was reported by Kaushika [12] for two solar ponds of approximately 2,000 m2 constructed between 1981 and 1982 in Alice Springs (Australia) and Miamisburg, Ohio (USA). The total costs reported were (15 and 32) $ m–2, respectively. The relative 2015 cost of these both ponds taking into account inflation amounts to c. (39 and 83) $ m–2, respectively. More recently, in 2014 a 500 m2 solar pond was constructed in Granada (Spain) with the purpose of delivering heated water (>60 °C) to replace or minimize fuel oil consumption at a mineral flotation processing facility. The total cost of construction was 190 $ m–2 with 50% accounting for earth and civil engineering work. The site location was difficult to access, penalizing the cost of excavation, levelling, and civil works. Cost analysis was performed for this solar pond by increasing the size to 5 000 m2. The cost of the expanded pond was estimated to be 90 $ m–2, which was in line with the figure quoted by Kaushika [12] and confirming that economy of scale has a huge impact on solar pond viability. Another cost factor with wide variability is the lining material. The cost of the lining of a solar pond in El Paso, Texas (USA) in 1991 was reported to be 4 $ m–2 [40], while liners for solar ponds in Alice Springs, Miamisburg, and Granada were quoted as (3, 11, and 9) $ m–2, respectively. The lower cost was for cases in which one layer of plastic lining was sufficient, while the higher cost applies to intensive earth moving, sophisticated linings, underground leak detection, and monitoring facilities. Furthermore, the cost of manpower involved in the construction also impacts total cost [22].
Annual operation and maintenance costs for large ponds are approximately (3–5)% of the investment. This includes all previously mentioned tasks, including gradient control, monitoring, clarity control, and makeup water for evaporation compensation. A 5% figure can be used for small ponds, while 3% is more appropriate for large ponds [22]. The annual operation and maintenance cost for the Granada solar pond was estimated to be around 3%.
4. Applications of solar ponds
Solar ponds are ideal for storing energy for applications needing low-grade thermal energy.
4.1. Industrial Process Heating
Heating applications particularly suited to solar ponds include provision of warm air for commercial salt production [23]; grain, fruit, and wood drying; hot water for the dairy industry in rural locations [21]; remote mining operations [41]; and any industrial process in a rural environment requiring low-grade heat (at temperatures up to 80 °C). This form of heating is particularly important in saving fossil fuel consumption and thus reducing the emissions of greenhouse gases [13].
4.2. Desalination
Solar pond–powered desalination is a promising renewable energy system for producing significant quantities of freshwater. Research undertaken during the El Paso Solar Pond Project from 1987 to 1992 mainly focused on the technical feasibility of thermal desalination coupled with solar ponds [42]. Thermal desalination processes such as multistage flash and multiple effect evaporation may use solar ponds to heat incoming salty water with zero greenhouse emissions [29,43]. Saleh et al. [44] reported that a 3000 m2 solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min–1 of distilled water pointing out that the solar pond appears to be a feasible and appropriate technology for water desalination. Suarez et al. [45] evaluates the utilization of direct contact membrane distillation (DCMD) coupled to a salt gradient solar pond for sustainable freshwater production at terminal lakes. Terminal lakes are water bodies that are located in closed watersheds and, therefore, the only output of water occurs through evaporation and infiltration. The majority of these lakes, which are commonly located in the desert and influenced by human activities, are increasing in salinity. Water production of the order of 2.7 L d–1 m–2 from a solar pond was reported to be possible if the pond is constructed inside a terminal lake.
Economic and technical assessment for solar ponds combined with a multistage flash (MSF) desalination system [43] indicating that large land areas of c. (73–185) m2 are required to produce desalinated water at the rate of 1000 L d–1 assuming that the storage zone temperature ranged between (70–90) °C. Recently, Salata et al. [46] studied the feasibility of integrating a solar pond with an absorption heat transformer, the latter being a thermal machine that extracts heat from a source (at an available temperature) and enables a portion of the heat collected/obtained, to be available at higher temperatures. To produce 1 m3 d–1 of desalinated water a solar pond area ranging from (1000 to 4000) m2 is needed, together with a thermal flux drawn of between (40 and 20) W m–2, respectively. Thus allowing the absorption heat transformer to increase the temperature of part of the stored energy (about 50%) to reach typical temperatures of up to 130 °C needed for the traditional desalination of seawater by distillation. A further benefit is that the more concentrated brine issuing from the desalination process may be used either to maintain solar ponds or in an integrated salt production system. This approach, called zero discharge desalination, proposes concentrating the rejected streams of brine solutions to near saturation point and using NaCI solutions to fill additional solar ponds. This system will be suitable at places where potable water is in short supply, but brackish water is available.
4.3. Electrical Power Production
Extensive research has also been carried out to utilize the thermal energy produced by solar ponds to produce electrical power. The best showcase for such power generation was a project near the Dead Sea (Israel) involving a large solar pond of 210 000 m2 having a depth of 4.5 m linked to a Rankine cycle heat engine which produced 5 MW electrical power. In El Paso a 100 kW Rankine cycle turbine was used to generate electrical power from a 3700 m2 solar pond and the power was fed to the local electricity grid. In Alice Springs (Australia), a 1600 m2 solar pond supplied heat to run a 20 kW organic vapor screw expander Rankine cycle engine and generator [47,48]. Organic Rankine cycle (ORC) engines developed specifically to produce electric power from lower temperature heat sources (80–90) °C have been used in these applications. An ORC was developed by Ormat’s solar pond power plants in Ein-Boqek and in Beit Ha’arava (Israel) [26]. The organic liquid, which absorbs the heat from the hot brine, vaporizes under relatively high–pressure conditions, expands through a special vapor turbine, then condenses at near atmospheric pressure and is pumped back into the vaporizer. Because of low temperature the solar pond power plant requires organic working fluids that have low boiling points such as halocarbons, e.g., freon, or hydrocarbons (such as propane) [13]. The characteristics of the low boiling point organic fluid simplifies the design of the turbine and the overall heat exchange system. However, conversion efficiency is limited due to the low operating temperature of (70–100) °C and the low thermodynamic performance, resulting in low net thermal-to-electric energy conversion efficiencies which are of the order of 7%. The overall efficiency for electric energy production was measured to be in the range (0.8–2) %, and this is due to low Carnot efficiency and low turbine efficiency which adversely affect their economic viability.
In recent times the concept of combining a chimney with a salt gradient solar pond for generation of power has been assessed through several demonstration projects. Incorporation of an air turbine (carrying out a trilateral flash cycle) into a solar chimney for desalination and power production has also been examined for salt-affected areas [49]. The results indicated that the system was able to produce power with the potential benefit of being able to generate power intermittently at any time (day or night) and at times of peak demand (or high cost for electricity) [50]. It is shown that for conditions in northern Victoria (Australia) (with daily annual solar radiation of 19 MJ m–2) 60 kW of power can be generated for the case where air in the chimney has been heated from an initial (20–50) °C [11].
New trends in power generation by solar pond technology involve the application of thermoelectric concepts avoiding low conversion of thermal energy to power and offer remarkable influence on medium- and small-size solar ponds. Recent studies have been carried out by coupling solar pond with thermosyphon and thermoelectric modules for electric power generation at the laboratory scale [51]. A thermoelectric generator is a device which converts heat directly into electrical energy. The process is based on the Seebeck effect [52,53]. The thermoelectric generator system is designed to be powered by the hot and cold water from a salinity gradient solar pond with a temperature difference in the range (40–60) °C between the LCZ and UCZ. The system is capable of producing electricity even on cloudy days or at night as the salinity gradient solar pond acts as a thermal storage system. Preliminary results indicated that these systems have promising potential to produce electricity from low-grade heat for power supply in remote areas [11].
4.4. Salinity Mitigation
The integration of solar ponds with salinity mitigation or interception schemes is a particularly attractive potential application. Many areas of formerly productive land are suffering from rising salinity levels around the world as a result mainly of tree clearing and irrigation. Many salinity mitigation interception schemes involve the use of evaporation basins, into which saline groundwater is pumped. Solar radiation vaporizes the water, leaving the salt behind [18].
Solar ponds could be incorporated in such evaporation basins to produce heat and/or electricity from otherwise unproductive land. If evaporation ponds are established in a chain the first few ponds in the chain provide ideal opportunities for creating salt gradient solar ponds. While the surface of a solar pond acts as an evaporation surface, heat can be withdrawn from the bottom of the pond for industrial process heating or other applications.
4.5. Production of Chemicals
Solar ponds can be used to produce sodium sulfate, chloride salts, fertilizer, and other common industrial chemicals, either in situ or simply by making use of the heat provided by the pond. The most closely related commercial technologies are the evaporation ponds at salt production facilities [23]. This application is undoubtedly, at present, one of the most important uses of solar energy for processing heat [18]. A good example is the 2500 m2 solar pond constructed and successfully operated in the production of lithium carbonate from the Zabuye salt lake in the Tibet plateau [54–56].
4.6. Aquaculture and Biotechnology
Applications requiring relative low-temperature water heating such as aquaculture and biotechnology farming, including fish, brine shrimps, and various algae, are ideal candidates for solar pond operations. In many cases, the solar pond can be used to control the environment for growth as well as providing desired thermal energy upon demand. This dual benefit may help to make solar ponds an even more attractive economic proposition [18]. China has been very successful in studying and applying solar pond technology in different applications. Such technology has been widely applied in the production of Glauber’s salt and in aquaculture during winter periods [57,58].
4.7. Buildings and Domestic Heating
Because of the large heat storage capability in the LCZ of a solar pond, it has ideal use for house heating even over several cloudy days [13]. A 2000 m2 solar pond at Miamisburg in 1978 was used to supply heat to a municipal swimming pool. It supplied all of the heat needed for the pool during the swimming season and also supplied heat to a bath house during other parts of the year [59]. Styris et al. [60] proposed a formula to determine pond dimensions for the heating requirements of projects involving house heating, winter crop drying, and paper processing in the Richland area, Washington (USA).
More recently, the integration of solar ponds with heat pumps had been proposed. The heat pump could serve as an air conditioner in summer and the freshwater layer above the top partition (in a partitioned solar pond) could be designed to serve as a heat sink to increase the coefficient of performance of the air conditioner [61]. Indeed, gas engine powered heat pumps can greatly increase the effectiveness of a solar pond that is attached to a heating load requiring temperatures above 40 °C such as greenhouse heating during the winter season [62]. At the Ohio Agricultural Research and Development Center (Ohio, USA), a solar pond, (18.5 × 8.5 × 3.0) m, was constructed to supply the winter heat requirements of a single-family residence and partial needs of an adjacent greenhouse. In this system, hot brine was extracted from the pond and passed through shell and tube heat exchangers. When the pond temperature was higher than 40 °C the heat extracted was supplied to a water-to-air discharge heat exchanger in the greenhouse; when the pond temperature was less than 40 °C the heat extracted was first upgraded by a heat pump and then used for heating a greenhouse [12].
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