The Dead Sea Works builds brine ponds for chemicals production in the south of the Dead Sea. Bloch observed the halocline phenomenon in some of the ponds where it frequently occurs. He considered the possibility of using the phenomenon for the production of chemicals from the ponds. He planned an artificial solar pond, which was later built by H. Tabor in collaboration with scientists from the Dead Sea Works. Tabor made radiation measurements and temperature readings of the storage zone and found that the maximum temperature reached 103 °C. Based on this information, he concluded that solar pond energy collection efficiency was about 15%. During the 1960s, the Dead Sea Works constructed a larger pond. However, the project was abandoned due to budget problems.
The third solar pond of 1500 m2 was built in 1977 in Yavne by Ormat Industries Ltd. A 6-kW organic Rankine cycle (ORC) energy-conversion unit was installed for demonstration of both the pond technique as well as the energy-extraction method. The surface water of the pond was used to cool the ORC condenser. This was the first time that electricity was continuously generated day and night from a solar pond, thus demonstrating energy conversion as well as the storage capability of the pond. (4)
In 1977, Ormat built the fourth solar pond at Ein-Boqek on the shore of the Dead Sea. The energy produced at the pond, with a surface area of 6250 m2, operated a 150-kW turbogenerator with bottom-layer temperatures that reached 93 °C. In 1979 it was coupled to the grid and operated until 1983, when it was dismantled (Bronicki et al., Bronicki et al., 1980). This plant was the first used for the demonstration of water desalination with a solar pond when an IDE low temperature evaporator was combined with the plant, as reported by Doron (1986) (see Fig. 1).
Tabor, who is one of the pioneers of solar energy research in Israel, managed, led, and collaborated in many of the solar projects in Israel. He has accumulated an enormous amount of scientific work, a summary of which was published in 1980 (see Tabor and Weinberger, 1980).
Encouraged by the success of the Ein-Boqek demonstration, the Israeli government sponsored Ormat to construct a 5-MW solar-pond power plant (SPPP) near Beit-Ha'arava at the north of the Dead Sea. A 250,000-m2 pond area was used (actually there were two ponds, one of 210,000 m2 and another of 40,000 m2). The plant was connected to the grid in 1984 and it operated continuously for 1 year. It was later operated intermittently for demonstration purposes and for the extraction of heat accumulation in the pond (Tabor and Doron, 1981). This arrangement continued until 1989 when, due to lack of maintenance funds, the plant was shut down, but preserved. The wind screens were removed, the top layer slowly evaporated, and the brine in the bottom and gradient layers mixed and the pond became a simple brine pond. Figure 2shows the 5-MW solar power plant with the pond in front of it. Figure 3 shows the back of the building, where organic liquidused in the power plant evaporators resides, near the wall behind which the turbine is located.
Assessment of the marine power potential in Colombia
A.F. Osorio, ... Santiago Arango-Aramburo, in Renewable and Sustainable Energy Reviews, 2016
In the context of the mentioned framework project, it was simulated the hydrodynamic features of the Colombian Caribbean in river mouths using ELCOM (the Estuary, Lake and Coastal Ocean Model). The ELCOM model was developed by the “Centre for Water Research” (CWR) of the University of Western Australia [29]. This three-dimensional model works on the hydrodynamic and thermal processes on stratified bodies of water under external environmental forcing, thereby simulating the temporal and spatial behaviour of variables such as speed, temperature and salinity, using a semi-implicit finite differences scheme.
In practice, not all of this potential power can be used because of technical limitations in the energy conversion process. The technical potential can be calculated using the coefficients estimated by Stenzel and Wagner [28] for Pressure-Retarded-Osmotic power plants.
https://www.sciencedirect.com/topics/engineering/halocline
Related terms:
Nitrogen in Inland Seas
Edna Granéli, Wilhelm Granéli, in Nitrogen in the Marine Environment (Second Edition), 2008
2.1 Nitrogen in surface and deep water—Concentrations and pools
Concentrations of nitrogen and phosphorus have increased markedly in the Baltic Proper since measurements started 30–40 years ago (Fig. 15.3, see also Nausch et al., 1999; Perttilä et al., 1995; Rahm et al., 1996). Compared to conditions before 1950, concentrations of N an
In deep water, below the halocline (>60–80m in the Baltic Proper), dramatic and rather rapid changes in nutrient concentrations may take place. This is caused by intermittent inflow of oxygenated, saltier deep-water, which will replace oxygen-deficient or anoxic (H2S-containing), stagnant deep water. Complete renewal of deep-water only occurs with intervals of several years and stagnation periods may be more than decade-long.
Geoscience
Kenneth S. Schmitz, in Physical Chemistry, 2018
3-24.2 The Zones of the Ocean
Since air and water are both fluids, the zones of the atmosphere have counterparts in the zones for oceans. Likewise, both fluids have similar physical properties when it comes to function and dynamics.
The ocean can be divided into zones according to salinity. The surface zone refers to the upper region of the ocean. It is also called the mixed layer because this is the location of ocean surface currents that are driven by the winds. The surface currents tend to maintain constant temperature and salt concentration. The surface zone therefore ranges in depths from 150 to 1000 m. The next zone is characterized by a rapid change in temperature with ocean depth. This is the thermocline zone. If the salinity changes rapidly with depth, the zone is called the halocline zone. If a zone has a strong chemical gradient, it is called a chemocline. The halocline and thermocline often coincide, in which case this zone is referred to as the pycnocline zone. Temperature and ocean density are given as a function of depth in Figure 3-24.2.
Concentrations and pools
Concentrations of nitrogen and phosphorus have increased markedly in the Baltic Proper since measurements started 30–40 years ago (Fig. 15.3, see also Nausch et al., 1999; Perttilä et al., 1995; Rahm et al., 1996). Compared to conditions before 1950, concentrations of N and P have probably increased even more, but there are no data to verify this. In deep water, below the halocline (>60–80m in the Baltic Proper), dramatic and rather rapid changes in nutrient concentrations may take place. This is caused by intermittent inflow of oxygenated, saltier deep-water, which will replace oxygen-deficient or anoxic (H2S-containing), stagnant deep water. Complete renewal of deep-water only occurs with intervals of several years and stagnation periods may be more than decade-long. As a consequence there may be an initial build-up of nitrate in oxygenated deep-water, which is subsequently denitrified and instead ammonia will accumulate under anoxia.
Most of the water masses in the Baltic Sea are found above the halocline, e.g., in the Baltic Proper only 20% of the volume is below the halocline. Deep-water entrainment and especially internal nutrient turnover processes (phytoplankton uptake, N2fixation, sedimentation, denitrification) have a profound influence on concentrations in surface water. Decreases in nutrient concentrations in surface waters of the Baltic Sea have been interpreted as consequences of reduced external loading. However, the total amount of N and P in the water mass and active sediments, as well as the amounts turned over annually, are several times larger than the annual external addition. This means that changes in the balance between internal nutrient sources and sinks will influence concentrations. Nutrients accumulated in deep water will at least partly be transported to surface water. This means that the length of “stagnation” periods, as well as the magnitude of irregular intrusions of deep water, are important for nutrient conditions also in surface waters (Larsson and Andersson, 2004; Pitkänen et al., 2001).
Oscar-Andres Alvarez-Silva, in Pressure Retarded Osmosis, 2017
6 Effects of the Salinity Structure on the Potential
Additionally to the variability of the river flow, the thermohaline salinity structure of the river mouths and its variability are also important factors to be taken into account for harnessing SGE in those systems. River mouths are brackish water systems with a spatiotemporal varying salinity structure that depends on river discharge, tides, winds, and waves, among others, rather than strictly separated freshwater and saline water systems. Therefore, the available SGE resources from river mouths are not equivalent to the TP estimated from the salinity difference between freshwater and seawater as has been commonly assumed. Rather, it is determined by the salinity difference between the intake zones of diluted and concentrated waters, as well as on the distance L between these zones (Fig. 5.3); that is, the potential depends on the steepness and stability of the salinity gradient, which defines the practically available energy, but also the required energy to bring the water to the power plant.
The steady availability of freshwater and seawater in the minimum distance is the major prerequisite to determine the suitability of river mouths for harnessing SGE [3,11]. In river mouths with extensive mixing zones, large and costly pipeline systems are required to bring the water from beyond brackish zones toward the power plants; in this case, the frictional losses in the transport system may considerably reduce the net power output of the plant [11].
Depending on the salinity structure, river mouths can be classified as salt-wedge, strongly stratified, partially mixed, or vertically mixed [32]; this classification considers the trade-off between the buoyancy forcing by river discharge and the mixing forcing by tides. Salt-wedge and strongly stratified systems result from high to medium river discharges and low to medium tidal range; the average salinity structure of these systems has a well-developed halocline with small vertical salinity variations above and below the halocline. On the other hand, partially mixed and vertically mixed systems result from low to medium river discharges and high to medium tidal range; the mean salinity profile here either has a weak halocline or is practically uniform from the surface to the bottom [32]. Salt-wedge and strongly stratified river mouths offer more suitable conditions for SGE generation, since shorter transport systems are feasible because of the presence of higher and more stable salinity gradients.
The effect of the salinity structure on the potential is taken into account in the SSP, which refers to the net energy potential, considering the energy losses caused by the transport of water toward the power plant [12]. It can be expressed as
where H represents the longitudinal energy losses for water transport toward the power plant; this term considers the effect of the steepness of the salinity gradient in terms of the distance that the freshwater and seawater must be transported to bring them together in the power plant. These losses are calculated from the energy conservation equation in pipes as
where Qd is the design flow of the power plant (m3/s), ρ is the water density (kg/m3) for both solutions, D is the pipe diameter(m), f is the friction factor, and L is the distance between intake zones of diluted and concentrated solutions (m). The optimal distance between intake points is a trade-off between the maximal expected salinity difference and energy losses [12]. The SSP is the most robust estimator of the usable SGE resources from a river mouth, as it considers all factors discussed in this chapter.
The SSP is shown in Fig. 5.4 for two extreme conditions of the salinity structure of several river mouths (high and low river discharge conditions), as a function of the distance between intake points. To make it comparable and considering only the effect of the stratification and distance between intake points in the SGE potential, the efficiency and CF were set to η = 100% and CF = 1, and the SPP is presented by unit volume of freshwater.
THERMAL ANALYSIS OF A SMALL-SCALE SOLAR POND WITH A PLANAR REFLECTOR
W.A. Kamal, M.A. Hassab, in Energy Developments: New Forms, Renewables, Conservation, 1984
INTRODUCTION
General
Although the concept of collecting and storing solar energy in solar ponds has been known since the beginning of this century, only in the last two decades have the ponds made the transition from scientific curiosities to practical devices. To date, the physics and technology of large scale solar ponds as well as their use for power production, industrial process heating, space heating, desalination, salt production and greenhouse heating have been the subject of extensive research, reviewed in references (1), (2)&(3). However, the use of small scale non-convecting solar ponds (of area less than 50 square meters) for providing low-grade heat to residential and small industrial applications does not seem to have been investigated. Particularly attractive is the idea of using solar ponds for air conditioning, i.e. winter heating in higher latitudes and summer cooling in tropical & equatorial climates. The latter can be achieved by coupling a solar pond to an absorption chiller, while the former uses the pond's long term storage capability.
What Is A Solar Pond ?
A salt-gradient solar pond is a body of saline water in which the concentration increases with depth, from a very low value at the surface to near saturation at a depth of 1–2 m. This density gradient inhibits free convection with the result that solar radiation reaching the pond's lower region (storage layer) is trapped, temperatures at the bottom approaching the boiling point of the solution. This means that in the non-convecting pond, water acts as its own insulator. In real large ponds, three distinct regions can be recognised:
- 1.
surface mixing layer: caused by wind action and heat transfer to the ambient air.
- 2.
Insulating layer : the region of both concentration and temperature gradients (haloclines & thermoclines)
- 3.
Storage layer : the bottom convecting (mixed) layer is the zone for storage and extraction of energy.
Therefore, in a stable pond heat can be lost only by conduction which is rather slow (water is opaque to low frequency infra-red radiation).
Pond versus Flat Plate Collector
The harnessing of solar energy on a large scale is confronted with two intrinsic difficulties arising from two of the characteristics of solar radiation namely, the low energy densityand its intermittency and irregularity. The solar pond however, attacks four of the major technical problems pertaining to conventional collector technology (4) :
- 1)
low energy density, which requires large areas
- 2)
effect of intermittency, which requires storage
- 3)
negative influence of dust collecting on windows and needing removal
- 4)
transport of energy to central zones.
Thus, a large scale solar pond allows collection over large areas with negligible energy transport losses, there are no windows to be kept clean and there is built-in storage adequate to smoothe out diurnal and weekly fluctuations of output and which makes even seasonal storage possible.
However, for small scale applications the inherent storage capability is capitalised on while partial reversion to the classical collector in the form of covering the pond with a transparent cover is possible, as will be seen in the next section.
The Proposed Design
A solar water heating system that combines both the greenhouse and solar pond principles for collection and storage of solar energy is proposed
In general, the heater shown schematically in Figure 1 is composed of a metal or concrete tank properly insulated on its sides and bottom with the inner walls painted black. The top is made of transparent material. The tank is constructed either above or under ground. It is filled with a brine solution whose concentration increases from zero at the top to near saturation at a depth of, say, 1 meter, then a layer of constant salinity at the bottom furnishes the heater's storage zone. This means that the tank will operate as a limited size (3–D) solar pond with minimum ground and edge conduction losses and minimum top evaporative losses. To minimize top convection losses even further, an insulated door is hinged to the top of the tank which can be closed overnight. This cover offers the following advantages:
Figure 1. THE PROPOSED DESIGN
- –
it can be closed from sunset to sunrise, thus minimizing top losses against the cool night temperatures.
- –
it can be closed on rainy, cloudy and dusty days
- –
it works as a wind shield against northerly winds
- –
a reflecting surface can be affixed to its inside surface, thus augmenting the amount of energy intercepted by the pond and increasing its output.
Once stability is achieved, through salt replenishment, and after a warming up period of few months, a heat exchanger placed in the bottom (storage) layer will supply fresh hot water for domestic use, space heating or process heating. If the thermal load is properly controlled, a continuous supply of low-grade heat (60–80° C) is procured.
It is worth mentioning that the transparent cover, which prevents evaporation and protects against wind, with the result that a negligibly thin -if any- surface layer is formed, also keeps in some of the heat resulting from absorption of IR radiation in the top few centimeters, thus modifying the pond's daytime thermoclines and yielding higher temperatures at the bottom. The existence and the thickness of the top convective zone were found to have a profound negative effect on the yield of solar ponds (5).
The system – a nearly perfect solar trap – is proposed as a competitor to the conventional thermosyphonic flat plate collector system in domestic water heating and also in thermal environmental control (space heating and cooling).
Background
Analytical solutions of salt gradient pond thermal behaviourhave been given in references (6)&(7). Weinberger's model (6) was for the case of totally non-convecting pond, i.e. no upper or bottom convecting zones (no storage layer), and the analytical solution was obtained by super-imposing the effects of radiation absorption at the surface, in the body of the water and at the bottom, each considered separately. Rabl & Nielsen (7) considered the more general case in which a convecting heat storage layer is below a non-convecting insulating layer. An analytical solution was obtained by assuming sinusoidal heat load, insolation and ambient temp, variations and ideal ground loss conditions. Later, both Hull (8) and Hawlader & Brinkworth(9) solved the basic energy equation numerically using small time increments to accurately follow the external driving functions and load demand and to incorporate appropriate initial and boundary conditions, including the effects of ground conduction losses. This is the approach adopted in the present study.
From the results presented in (9), (10) & (5) it is evident that the pond temperatures and heat output are strongly dependent on the on the thermal losses from its bottom. In reference (10) the dependence of ground and edge losses on pond size and insulation is examined. It is shown that insulation placed at strategic locations around the pond can significantly decrease the perimeter thermal losses, especially for smaller ponds. Beldam (11) reported on experience with small scale (2.9m x 5.9 m x 0.86 m) solar pond which was constructed above ground, insulated on sides and bottom and covered by clear polyethylene sheet 1.4 m above the pond surface. However, the main objective was to evaluate the behaviour of the salt gradient with emphasis on the effects of heat extraction and of winter freeze in a northern climate.
In a previous paper (12) the authors developed a 3–D radiation model of the SSSP and examined the variation of energy reaching the bottom as well as the inside walls with variation of pond's proportions, in this paper, the thermal performance of the SSSP (heating up and load extraction) is studied.
Evaluation of solar thermal system configurations for thermoelectric generator applications: A critical review
Krishnadass Karthick, ... C.S. Sujith Kumar, in Solar Energy, 2019
5.2.2 Solar pond
Solar ponds collect and store solar energy quite economically (Velmurugan and Srithar, 2008). A solar pond traps and collects solar radiation by suppressing natural circulation to its upper layers using halocline or some other methods (Velmurugan and Srithar, 2008; Sayer et al., 2018). Salinity-gradient solar ponds can collect and store incident energy at temperatures up to 80 °C. A salinity-gradient solar pond consists of three regions namely lower convective zone (LCZ), a non-convective middle zone (NCZ) that exists due to the existence of a strong salinity-gradient and an upper convective zone (UCZ) (Akbarzadeh et al., 2005). Temperature difference across the lower and upper zones can be utilized to generate electricity through TEGs (Ding et al., 2016b). Two recent reviews were published in the field of solar ponds by Ding et al. (2018) and by Kasaeian et al. (2018). This sub-section, however, focuses solely on the thermal system configurations that have been used to bring about power generation using TEGs.
Singh et al. (2011) designed a system which utilized gravity assisted thermosyphon with water as the working fluid to transfer heat from the hot LCZ to the cold UCZ of the solar pond. The top end of the thermosyphon, which lied in the UCZ, had TECs attached to it. The schematic of the system is shown in Fig. 31. The experimental conditions were made similar to the solar pond using a test rig which used sixteen TECs as shown in Fig. 32. The module provided a maximum power of 3.200 W which was obtained at 0.24 A and 13.4 V for a maintained temperature difference of 27 °C across the TEG. The energy conversion efficiency was about 1%.
version efficiency was about 1%.
Fig. 31. Schematic showing the concept of generating electric power from salinity-gradient solar pond using a combination of thermosyphon and TECs.
Fig. 32. Schematic of the test rig (left) and actual test rig (right).
Singh et al. (2012a) used a different test rig to conceive energy from the TEGs. Sixteen TEGs were used with eight of them arranged on aluminium tablets. The two tablets were arranged with the hot water flowing through the central region while the cold water flowed through the outer region in counter flow arrangement. For a temperature difference of 30 °C, 60 °C and 75 °C, the maximum power obtained were 0.850 W, 3.460 W and 5.30 W respectively for an external load of 25 Ω. The extended study on the same setup by Singh et al. (2012b)reported maximum powers of 0.730 W, 5.920 W, 7.300 W and 9.560 W for a temperature difference of 40 °C, 60 °C, 80 °C and 100 °C respectively for an external load of 27.5 Ω.
Tundee et al. (2014) used an insulated solar pond of surface area 7 m2 and depth 1.3 m to conduct performance experiments on power generation using TEG and a thermosyphon. Tests were conducted using water and R134a as the working fluid. When water was used the LCZ temperature was 50 °C and the output voltage was 36.25 mV. On the other hand, when R134a was used as the working fluid, the LCZ temperature was 41 °C and output voltage of 234.25 mV was observed.
Singh et al. (2015) designed and tested a counter flow heat exchanger for TEG power generation from salinity-gradient solar pond. The counter flow heat exchanger consisted of a steel square channel of 50x50 mm2 area and 0.6 m length, forty TEGs and an acrylic tube. The hot fluid was supplied from an adjustable hot urn with water temperature varying from 40 °C to 90 °C. The highest power obtained was 7.02 W for 90 °C hot water temperature.
Singh et al. (2016) proposed an in-pond nonagon shaped heat exchanger which housed 14 TEGs on each side, totalling to a 126 TEGs on the heat exchanger. The heat was obtained from the LCZ of the solar pond. The schematic of the in-pond heat exchanger is shown in Fig. 33. The system was tested for cold water flow rates of 0.16 kg/s and 0.28 kg/s. For a flow rate of 0.16 kg/s, the maximum electrical efficiency, power output and open circuit voltage were1.2%, 35 W and 90 V respectively. The corresponding values for 0.28 kg/s were 1.50%, 48 W and 126 V respectively.
Fig. 33. Schematic of an in-pond nonagon shaped heat exchanger.
A small-scale passive electric power generation unit (PGU) was proposed by Ding et al. (2016c). The system involved no moving parts and operated without the use of a pump. 120 TECs were positioned in the outer and inner layers of this unit. The PGU is shown in Fig. 34. The unit produced a maximum power of 40.8 W for the hot side temperature of 99 °C. For normal operation, the LCZ temperature lies between 40 °C and 80 °C. The corresponding range of maximum power was in the range of 19.500 W to 27.400 W with the electric efficiency ranging between 0.37% and 0.68%.
Fig. 34. Schematic and experimental set-up for small-scale passive electric power generation unit.
Ding et al. (2016a) employed a plate type PGU to test the capability of solar pond in generating electricity. The PTGU was tested with different flow rates and hot water temperatures. 35.9 W of electricity was generated at a flow rate of 5.1 L/min at a hot water temperature of 81 °C.
In a simulation work by Ziapour et al. (2017), TEG was used instead of condenser of ORC to improve the system performance. Two models were presented with and without a heat exchanger as shown in Fig. 35. For an LCZ temperature of 90 °C, both models performed better than the ORC without TEG. The model without the heat exchanger had a higher thermal efficiency by 0.21% while the one with the heat exchanger had a higher thermal efficiency by 0.20%. The optimal pond layer thicknesses were found to be 0.15 m, 1.2 m and 1.4 m for UCZ, NCZ and LCZ respectively.
Fig. 35. Models of TEG based condenser with (top) and without (bottom) heat exchangers in Organic Rankine Cycle (ORC). All numerical numbers are denoting that the connecting pipes between the major components.
The experimental works of this section have been summarized in Table 4.
Table 4. Summary of experimental works on power generation using solar pond.
Ref. Equipment for heat transfer Location of TEG/TEC Th (°C) Tc (°C) ΔT (°C) Power obtained (W) Comments Singh et al. (2011) Copper tube thermosyphon charged with distilled and degassed water TEC in UCZ 907 63 27 3.2 16 TECs were used in series. The reported maximum is obtained at 13.4 V and 0.24 A. Singh et al. (2012a) Aluminium tablets arranged such that hot water flows centrally while cold water flows in its outer channel in a counter flow arrangement. N/A 5,585,100 252,525 306,075 0.85
3.46
5.3016 TEGs were used with sets of 8 on the tablets. Reported values are for a matched load of 25 Ω. The maximum 5.30 W was obtained at 13.4 V and 0.24 A. Singh et al. (2012b) Aluminium tablets arranged such that hot water flows centrally while cold water flows in its outer channel in a counter flow arrangement. N/A 406,080,100 0
0
0
0406,080,100 0.73
5.92
7.30
9.5616 TEGs were used with sets of 8 on the tablets. The reported values are for the flow rates of 0.021 kg/s and 0.14 kg/s of hot water and cold water respectively. The matched load is 27.5 Ω. The highest value of 9.56 W is obtained at 15.67 V and 0.61 A. Tundee et al. (2014) Copper tube heat pipe with R134a or water as working fluid TEG in UCZ 36.6 31.6 5 N/A 16 TEGs were used in series. When water was used as the working fluid, an output of 36.25 mV was observed. On the other hand, when R134a was used as the working fluid, 234.25 mV was received as output. Singh et al (2015) Acrylic tubed heat exchanger N/A 90 20 70 7.02 The output is reported for 40 TEGs connected in series. Hot water was supplied by a hot water urn. Singh et al. (2016) Nonagon shaped in-pond heat exchanger with PVC pipes. TEG in LCZ N/A
N/AN/A
N/A∼20.5
∼ 3535
48126 TEGs were used with 14 TEGs on each side of the nonagon heat exchanger. Cooling water was fed in the gap of the PVC pipes. An open circuit voltage of 90 V and an electrical efficiency of 1.2% was achieved for a cold water flow rate of 0.16 kg/s. The corresponding values for a flow rate of 0.28 kg/s are 126 V and 1.5%. Ding, et al. (2016c) PGU thermosyphon TEC in LCZ 99 20.8 79.2 40.8 The reported value is for the maximum temperature of LCZ. More realistic values lying in the range of 40 °C to 80 °C would give an output in the range of 19.5 W to 27.4 W with an electrical efficiency lying in the range 0.37% – 0.68% Ding, et al. (2016a) Plate type PGU Unit kept outside the pond 81 25 56 35.9 The reported value is for the flow rates of 5.1 L/min and 18.5 L/min of hot and cold water respectively.
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