WO2010088919A1 - Osmotic energy reservoir - Google Patents

Abstract

A method and apparatus for storing energy, during periods of availability of excess energy of a power generation plant utilizing renewable energy source. The excess energy is exploited for pressurizing a medium concentration solution (1), the static pressure of said solution is higher than its osmotic pressure, therefore when introduced to semipermeable membrane, the solution splits, by reverse osmosis, into two portions: high concentration solution (10); and the permeate (11), each of the two portions is stored in a reservoir. During periods of low availability of the energy generating source, the solution (10) is pressurized to a pressure less than its osmotic pressure and is introduced to semipermeable membrane where its volume increases, by osmosis, with the permeate (11), the resultant solution is introduced to inlet of a turbine producing energy, part of produced energy is used for pressurizing solution (10), whilst the rest of produced energy feeds the load.

Description

Osmotic Energy Reservoir Technical Field

The present invention relates to the technical field of energy storage and, in particular, to a method and apparatus for converting and storing electrical and/or mechanical energy in the form of osmotic energy. The stored osmotic energy is converted into electrical and/or mechanical energy when required. The source of the energy is the excess available energy, such as solar energy and wind energy, during the periods of low demand.

Background Art Although The methods of exploiting renewable energy in large scale— such as photovoltaic arrays, wind farms and solar towers— have a great potential for generating energy, its share in generating energy is much less than its capability. The intermittency of these methods limits their share which can be contributed in a power grid. Sometimes this intermittency continues for days, for example, power supplied by solar energy plants can be interrupted or considerably reduced in very cloudy days.

Storing energy in quantities high enough to compensate the shortage of energy source for several days is a perfect solution to increase the reliability of renewable energy, such as solar energy and wind energy, so increases its capability for high penetration in a power grid. Storing energy means storing electrical and/or mechanical energy during the periods of low demand, and at the same time, high availability of the intermittent energy source, then converting stored energy back into electrical energy during periods of no or low availability of the intermittent energy source.

Several methods have been used for storing energy, but there is no method, as far as known to the inventor of the present invention, stores energy in the form of osmotic energy.

Storing energy in batteries in the form of electrochemical energy is one of the used methods for storing energy. During low demand periods the excess energy is used to charge the batteries that will supply energy when required. Batteries are not convenient for large scale storage systems because of its high cost. Hereafter is an example to estimate the number of batteries required for each kilowatt of the installed power in a photovoltaic power plant, assuming the following: the capacity factor of the power plant is 25%, number of days that the batteries will supply the load without additional energy input is 10 days, batteries used are led-acid batteries 200 Ah 12 Volt, the minimum allowable state of charge of the batteries is 25%, the actual capacity of the battery is 80% of the nominal capacity, this reduction of capacity happens along the life of the battery. According to the above data, the amount of energy required to be stored for each KW installed power:

1 KW x 25% capacity factor x 24 hours x 10 days = 60 KWH Stored energy per battery: 200 Ah x 75% x 80% x 12 volts /1000 = 1.44 KWH/battery

Number of batteries required:

60 KWH/1.44 KWH/Battery = 41.666 battery

This number of batteries will considerably increase the cost per KW of the plant. Another impact, also, comes from the short lifecycle of the battery. Excluding the method disclosed in the present invention, gravitational potential energy is the only energy form which can be exploited in large scale energy storage systems. This form of energy is employed in systems called Pumped Storage Reservoirs. In these systems, during low demand periods the excess energy is used to pump water from a low elevation reservoir to a high elevation reservoir, the reservoirs should be horizontally close to each other to reduce friction losses in the conveying pipelines. When required, the water flows from the high reservoir to the low reservoir through turbines to generate electricity. The disadvantage of pumped storage reservoirs is the rarity of the locations which can accommodate such systems. The head at which the pumps and the turbines operate depends on the geography of the location of the system. Some locations have as low as 100 meters head; others have more than 700 meters head. As the energy density is in linear relationship with the head, locations with low head are inconvenient for several days' storage capacity; this makes the convenient locations rarer. Therefore, a need exists for a novel method that uses cheap and common materials as energy storage medium and, also, this novel method can be implemented in locations where there are no such requirements for the site characteristic.

Disclosure of Invention

It is a general object of the present invention to provide a method and apparatus to store the excess available energy of a power plant, particularly, a power plant in which the control over the input energy is limited, such as solar power plants and wind farms. The method disclosed in the present invention stores the energy in the form of osmotic energy, and then when required, converts it into electric energy. The method comprises two phases: "reverse osmosis phase", during this phase the energy storage process takes place, and the other phase is "osmosis phase", during this phase process of converting stored energy into electric energy takes place.

The main components of reverse osmosis phase are illustrated in Figure 1, these components are: medium concentration reservoir Rl, low concentration reservoir R2, high concentration reservoir R3, high pressure pump HP, two booster pumps BPl and BP2, membrane array M and pressure exchanger array PX. Pressure exchanger array comprises a large number of pressure exchanger units. Each pressure exchanger unit is a positive displacement device; its function is to transfer pressure from a high pressure fluid to a low pressure fluid. In Figure 1 the high pressure solution enters the pressure exchanger at point 9 and leaves at point 10 where its pressure drops. The low pressure solution enters the pressure exchanger unit at point 4 and leaves at point 6 where it gains its pressure. As there are pressure losses in the pressure exchanger, the pressure at point 6 - P6 is less than P9 and PlO is less than P4.

During low demand periods, a medium concentration aqueous solution exits reservoir Rl at point 1 where it splits into two portions, one is withdrawn at point 2 by the high pressure pump HP and pressurized to point 5 and the other is withdrawn at point 3 by the booster pump BPl then by the pressure exchanger array PX and pressurized at point 6, the pressure at point 6 - P6 is equal to P5. The function of BPl is to compensate part of the pressure losses in the pressure exchanger array PX. The solution delivered by pressure exchanger array PX and the solution delivered by the high pressure pump HP are mixed again together at point 7, which has the same pressure as points 6 and 5, where the solution enters the membrane array M. A reverse osmosis process takes place in the membrane array and due to this process the entering solution at point 7 is split into two portions, one exits at point 8, the pressure at point 8 P8 is slightly less than P7— this pressure drop is due to the pressure losses along the membrane surface— while the concentration of the solution at point 8 is high, therefore the osmotic pressure at point 8 is also high— higher than that at point 7. The other portion of the solution, called the permeate, passes through the membrane then it exits the membrane array at point 11. The pressure at point 11 is atmospheric pressure, while the concentration of the solution at point 11 is low, therefore the osmotic pressure at point 11 is also low. This low osmotic pressure solution flows to reservoir R2 where it is stored. The high osmotic pressure solution at point 8, which is also high static pressure solution, gains more pressure by the booster pump BP2 and leaves the pump at point 9, this slight pressure increase in BP2 is required to compensate the pressure losses along the membrane surface and part of the pressure losses in the pressure exchanger array. The high osmotic pressure solution loses its static pressure and leaves the pressure exchanger array, at atmospheric pressure, and flows to reservoir R3, at point 10, where it is stored.

In the membrane array, the permeate flows from the high osmotic pressure side to the low osmotic pressure side if the following condition is satisfied: (P8 - P11) - (π8 - 7ill) > 0 Or P8 > (π8 - πll) + Pll (1)

Where P is the static pressure and π is the osmotic pressure— which increases with the increase of the solution concentration. The above relation determines the minimum output pressure of the high pressure pump P5, as P5 equals to P7 and P7 can be calculated by adding the pressure drop along the membrane array M to P8.

Briefly, in the reverse osmosis phase described above, during low demand periods, the excess available energy of a power plant, which is the input of this phase, is employed., mainly, to operate the high pressure pump HP and the booster pumps BPl and BP2. The output of this phase is that two bodies of different concentration solution have been provided. Thus, osmotic potential, which is capable of producing energy in a later phase, has been created. When the apparatus is in full storage state, reservoirs R2 and R3 are full while reservoir Rl is empty. And when the apparatus is in zero storage state, R2 and R3 are empty while Rl is full.

The main components of osmosis phase are illustrated in Fig. 2, these components are: medium concentration reservoir Rl, low concentration reservoir R2, high concentration reservoir R3, hydraulic turbine T, three booster pumps BP3, BP4 and BP5, membrane array M and pressure exchanger array PX.

During periods of no or low availability of the energy source, osmosis phase takes place to generate required electric energy to feed the load. A high concentration solution— therefore it is also a high osmotic pressure solution— exits reservoir R3 at point 10 where its static pressure increases by means of three devices. First device is booster pump BP3 where the solution gains a slight static pressure increase, at point 12, to compensate part of the pressure losses in the pressure exchanger array PX. Second device is pressure exchanger PX where it gains most of its pressure at point 13. Third device is booster pump BP4 where the solution gains a slight pressure increase, at point 14, to compensate the pressure losses along the membrane surface and part of the pressure losses in the pressure exchanger array. Then the solution enters the membrane M at point 14. When the high concentration solution starts to flow from reservoir R3 at point 10, a low concentration solution— therefore it is also a low osmotic pressure solution —exits reservoir R2 at point 11 where its static pressure is atmospheric. The solution is withdrawn by booster pump BP5 where the solution gains a slight pressure increase, at point 21, to compensate the pressure losses in the membrane array. Now, the low static pressure solution, which is also low osmotic pressure solution, enters the membrane array M at point 21 where osmosis process takes place, and due to this process most of the entering solution at point 21— the permeate— passes through the membrane to the other side of the membrane where it mixes with the solution entering the membrane at point 14, then the solution exits the membrane array at point 15. The osmotic pressure of the solution at point 15 is less than that at point 14, and that is because it mixes with the permeate which has a very low osmotic pressure. The static pressure at point 15 is slightly less than that at point 14 due to pressure losses in the membrane array.

In the membrane array, the permeate flows from the low osmotic pressure side to the high osmotic pressure side if the following condition is satisfied:

(P15 - P21) - (πl5 - π21) < 0 Or P15 < (πl5 - π21) + P21 (2)

The above relation determines the maximum input pressure of the turbine P16, as P16 equals P15.

The solution at point 15 splits into two portions, one enters the pressure exchanger array at point 17 and exits at point 19 at atmospheric pressure, the other portion enters the turbine T at point 16 and exits at point 18 at atmospheric pressure. Thus, the energy contained in the solution due to its high static pressure is converted, in the turbine, into mechanical energy, which usually will convert into electric energy. Most of the energy generated by the turbine will supply the load with the required electric energy, while a small portion is used to operate BP3, BP4 and BP5. As mentioned above most of the solution at point 21 passes through the membrane to the other side of the membrane, while the rest of it exits the membrane at point 22. The solution at point 22 is called the bleed, and it is important for keeping the concentration inside the membrane, at the low concentration side, at low limit. The solution exiting the turbine and the solution exiting the pressure exchanger mix again at point 20 where it mix with the bleed where it returns to its concentration at point 1 and stored again in Rl.

Briefly, in the osmosis phase described above, During periods of no or low availability of the energy source, the osmotic potential created by the availability of two solutions with deferent concentration— which is the input of this phase— is employed to operate the turbine T. The output of this phase, and also the output of the whole method, is the net electric energy generated by the turbine T. Brief Description of Drawings

Figure 1 is a schematic diagram illustrating the reverse osmosis phase of a single stage energy storage apparatus according to one embodiment of the present invention.

Figure 2 is a schematic diagram illustrating the osmosis phase of a single stage energy storage apparatus according to one embodiment of the present invention.

Figures 3A and 3B illustrate the operation of a pressure exchanger.

Figures 4A and 4B illustrate the usage of the pressure exchanger array in reverse osmosis phase and osmosis phase.

Figure 5 is a section in a pressure vessel of a membrane array. Figures 6A and 6B are schematic diagrams illustrating an efficient use of the storage reservoirs in both reverse osmosis phase and osmosis phase.

Figure 7 shows a proposal for implementing the method of the present invention in a solar power plant.

Figures 8A and 8B are schematic diagrams illustrating a multistage energy storage apparatus according to the best mode of the invention in both reverse osmosis phase and osmosis phase.

Figures 9 and 10 are schematic diagrams illustrating the usage of medium pressure membranes with high osmotic pressure solution.

Best Mode for Carrying Out the Invention The above described method is one embodiment of the invention. Hereafter the best mode is disclosed. Several ideas and modifications are disclosed hereafter in order to increase the efficiency and lower the cost of implementing the method of the present invention.

Pressure exchanger array As mentioned above, the pressure exchanger array comprises a large number of pressure exchanger units arranged in parallel; the pressure exchanger unit is a positive displacement device. Although the rotary pressure exchanger is efficient and durable, but as its operation involves leakage between the two different concentration solutions, which causes a reduction in the osmotic potential, the preferred type is the reciprocation pressure exchanger.

As shown in figure 3A and figure 3B, each pressure exchanger unit consists of a cylinder 301, inside the cylinder a free piston 302 moving along the axis of the cylinder. The flow of the liquids is controlled by electrically operated valves: the inlet valves 303a and 303d and outlet valves 303b and 303c.

In figure 3A, the liquid A, which is at high static pressure, enters cylinder 301 at point (a) through the open valve 303a. As valves 303c and 303d are closed, the flow of the liquid A pushes the piston 302 in the direction shown in figure 3a, and the piston pushes the liquid B to exit the cylinder through the open valve 303b. Liquid B exits the cylinder, at point (b), at a pressure slightly less than the pressure of liquid A at point (a). The difference between the pressure at point (a) and the pressure at point (b) is equal to the pressure losses due to the flow from point (a) to (b) and due to the friction between the piston and the cylinder. The piston moves until it reaches the sensing element 304b— the sensing element might be photocell or limit switch. As shown in figure 3B the sensing element sends signal to valves 303a and 303b to close and to valves 303c and 303d to open. Now, the liquids inside the piston are isolated from the high pressure at points (a) and (b), and pressure equalization takes place between the liquids inside the cylinder and the points (c) and (d). The pressure at point (d) is low and it is slightly lower at point (c), the fluid B enters cylinder 301 at point (d) through the open valve 303d. As valves 303a and 303b are closed, the flow of the liquid B pushes the piston 302 in the direction shown in figure 3B, and the piston pushes the liquid A to exit the cylinder through the open valve 303c. Liquid A exits the cylinder, at point (c), at a pressure slightly less than the pressure of liquid B at point (d). the difference between the pressure at point (d) and the pressure at point (c) is equal to the pressure losses due to the flow from point (d) to (c) and due to the friction between the piston and the cylinder. The piston moves until it reaches the sensing element 304a, as shown in figure 3A the sensing element sends signal to valves 303c and 303d to close and to valves 303a and 303b to open to start new cycle.

As there is no leakage between liquids A and B, within a long time that the volume created by one piston displacement can be neglected, the volume of liquid enters through valve 303a V_a equals the volume of liquid exits valve 303c V_c and the volume of liquid enters through valve 303d V_d equals the volume of liquid exits valve 303b V_b. Also it is obvious that the displacement of liquid A equals to that of liquid B. So:

V_a = V_b = V_c = V_d (3)

The pressure exchanger array is a common component between both phases of the invention. In other words, there is no need to add a pressure exchanger array in each phase, as both phases will not work simultaneously. The designated maximum flow rate of the pressure exchanger array is equal to that of the highest maximum flow rate of the two phases. Using the same pressure exchanger array with the phase of the lower maximum flow rate does not affect the performance, in contrary, the losses are less as the liquids' velocities are less.

In figures 4A and 4B, the points 4, 10, 9, 6, 12, 19, 17, and 13 represents the same points with the same numbers in figures 1 and 2. Figure 4A illustrates the pressure exchanger array when it is used in the reverse osmosis phase, when the valves al, bl, cl, and dl are open and valves a2, b2, c2, and d2 are closed, the direction of flow will be as it is in figure 1. (reverse osmosis phase). In figure 4B, which illustrates the pressure exchanger array when used in the reverse osmosis phase, when the valves al, bl, cl, and dl are closed and valves a2, b2, c2, and d2 are open, the direction of flow will be as it is in figure 2. (osmosis phase).

Membrane array

The membrane array comprises a large number of pressure vessels arranged in parallel. The pressure vessel PV in fig. 5 is a long cylindrical shaped vessel inside which a number of membrane elements are arranged in series. The membrane element ME is of cylindrical shape which fits inside the pressure vessel. In the center of the membrane element a perforated tube PT.

An aqueous solution enters the pressure vessel at point (e) and passes along the membrane elements where it exits at point (f). As the solution moves from point (e) to point (f), part of it passes across membrane layer and collects in the perforated tube PT. the membrane layer allows only water to pass across the membrane layer, whereas it is impermeable to the solute of the solution. Although the membrane is impermeable to the solute, a small portion of the solute can pass across the membrane layer. The measure of the ability of the membrane to reject the solute is the rejection coefficient. As the main constituent of the solution that passes across the membrane is water, this solution is a low concentration solution and it is called the permeate. The permeate collects in the perforated tube PT where it mixes with another solution entering the perforated tube at point (g), the solution that results from this mixing exits the perforated tube at point (h).

During the travel of the solution from point (e) to point (f), the permeate splits from the solution driving away the solvent (water) whereas most of the solutes remains, therefore the concentration of the solution at point (f) is higher than that at point (e). Also the solution at point (g) mixes with a lower concentration solution (the permeate), therefore the concentration of the resultant solution at point (h) is less than that at point (g). Within a specific time, the volume of solution enters the membrane at point (e) equals to the sum of the volumes exit the membrane (the solution that exits at point (f) and the volume of the permeate), So:

V_e = Vp_errneate + V_f (4) Also, the volume of solution exits the perforated tube at point (h) equals to the sum of the volumes enter the perforated tube (the solution that enters at point (g) and the volume of the permeate), So:

V_h = V_permeate + V_g (5)

In the reverse osmosis phase, the concentration of the entering solution at (e) is higher than the concentration of the entering solution at (g). According to relation (1), the reverse osmosis process takes place when the following condition is satisfied:

Pt ≥ (JI_f - H_g),+, P_g (6) .... _

In the osmosis phase, the concentration of the entering solution at (e) is less than the concentration of the entering solution at (g). According to relation (2), the osmosis process takes place when the following condition is satisfied:

P_h < (π_h - π_e) + P_e (7)

Similarly to the pressure exchanger array, the membrane array is a common component between both phases of the invention. Two valves at each of the four points (e), (f), (g), and (h) are connected. Each valve is connected, from the other side, to the proper component of the apparatus. One valve is open during the reverse osmosis phase and the other is closed, and vice versa.

Efficiency

Improving the overall efficiency of the method is one of the main purposes of implementing the best mode of the invention, the overall efficiency of the method η is defined as follows: η = E_o/E_ro (8)

Where E_ro is the total input energy in the reverse osmosis phase, and E₀ is the net output energy produced in the osmosis phase. More specifically, and referring back to figuresl and 2, E_ro is the energy required to split a solution of volume Vl that occupies the reservoir Rl in figure 1, into two solutions of volumes VIl and VlO that occupy the reservoirs R2 and R3 respectively, and E₀ is the energy produced by joining two solutions of volumes VIl and VlO that occupy the reservoirs R2 and R3 respectively in figure 2, into one solution of volume Vl that occupies the reservoir Rl. When estimating these energies, we are dealing with energy and volume— not power and volume flow rate.

The input energy in reverse osmosis phase is consumed by the high pressure pump and the booster pumps, so the total input energy: E_ro = E_HPin + E_Bpiin + E_Bp2in (9)

The output energy in the osmosis phase is produced by the turbine, but part of it is consumed by the booster pumps, so the net output energy:

E₀ ⁼ EjOut ^' EβP3in ^' EβP4in ^" EβPSin (10)

From equations (8), (9), (10) : T| = ( E_TOut - E_Bp3j_n - EβP4in - E_BP5in) / ( Eπpin + Eβpiin + EβP2in) (H)

Where E_BPljn, E_BP2i_n , E_BP3in, E_BP4in , E_Bp_5in are the energy consumed by booster pumps BPl, BP2, BP3, BP4, BP5, E_HPi_n is the energy consumed by high pressure pump HP and E_To_ut is the energy produced by hydraulic turbine T. It is obvious that the efficiency increases with the term E_TOut and it decreases with the increase of all other terms of equation (11). The terms E_BPljn, E_Bp₂j_n , E_BP3in, E_Bp_4in , E_BP5in are not the main terms which affect the efficiency of the method, as their values are small. They just represent the energy consumed to compensate the pressure losses in the process — losses arising from the solution flow in the membrane array M, pressure exchanger array PX, piping, and fittings— plus the hydraulic and mechanical losses of the booster pumps and their electric losses, if they are driven by electric motors. The sum of these losses is small compared with the energy consumed by the High pressure pump and the energy produced by the turbine. So, the main two terms of equation (11) that affect the efficiency of the method are E_Hpj_n and E_Tout. Therefore, these two terms are analyzed hereafter: In figure 2, E_Tout = (P16 - P18) . V16 . η_τ (12)

In figure 1, E_HPin = ( P5 - P2 ) . V5 / η_P (13)

Where η_τ and η_P are the turbine efficiency and the high pressure pump efficiency.

It is understood that η_τ and η_P are required to be as high as possible and they are not part of the subject of the present invention, therefore we will focus on the following two terms of equations (12) and (13):

(P16 - P18) . V16 from equation (12),

( P5 - P2 ) . V5 from equation (13)

In order to maximize the efficiency, it is required to minimize the difference between the values of the two terms. In more details, it is required to minimize the difference between the volumes (V16 and V5) and to minimize the difference between the pressures (P16 and P5)— P18 and P2 are equal and constant at atmospheric pressure, so they have no effect on the value of the above terms.

Starting with the volumes (V5 and V16): In figure 1, applying equation (4) we get: V7 = V_permeatel + V8 But V8 = V9_; and Applying equation (3) we get V9 = V6, so V8 = V6, therefore

V7 = V_permeatei + V6: V_permeatel = V7 - V6 But V5 = V7 - V6, so: V5 = Vpermeatel (14) Where V_permea_tei is the permeate volume in the reverse osmosis phase. Equation (14) means that the volume handled by the high pressure pump HP (V5) is equal to the volume of the permeate in the reverse osmosis phase.

Similarly, in figure 2, applying equation (5) we get: V15 = V_permeate2 + V14 But V14 = V13, and Applying equation (3) we get V13 = V17, so V14 = V17, so V15 = V_permeate2 + V17 : V_permeate2 = V15 - V17 But V16 = V15 - V17, so:

V16 = V_permeate2 (15)

Where V_permeate2 is the permeate volume in the osmosis phase. Equation (15) means that the volume handled by the turbine T (V16) is equal to the volume of the permeate in the osmosis phase.

From figure 1, VlO = V4 = V3 and from figure 2, VlO = V12 = V19, so: V3 = V19 but, from figure 1, V3 = Vl - V2 and from figure 2 V19 = Vl - V18 - V22, so: Vl - V2 = Vl - V18 - V22 or: V2 = V18 + V22

But V2 = V5 and V18 = V16 so:

V5 = V16 + V22 (16)

V5 is the volume handled by the high pressure pump, V16 is the volume handled by the turbine, and V22 is the volume of the bleed. As mentioned in the invention disclosure, the bleed, in the osmosis phase, is the necessary portion of the low concentration solution which should be allowed to exit the membrane in order to keep the concentration inside the membrane, in the low concentration side, at low limit. Equation (16) means that the volume handled by the high pressure pump HP is larger than the volume handled by the turbine T, and the difference equals to the volume of the bleed. In the best mode of the invention, instead of mixing the bleed (represented by point 22) with the solution at point 20, the bleed is stored in a separate reservoir and will mix with the rest of the solution in a different way, thus the volume V5 will be reduced. This will be detailed when describing figures 8A and 8B which illustrates the best mode.

Above, the volume of the liquids handled by HP and T are estimated, now it is required to estimate the pressure at which they operate (P5 and P16):

Referring back to figure 5, and as mentioned above, the concentration of the solution at point (f) is higher than that at point (e). The increase of the concentration depends on the ratio of the volume of the permeate to the volume entering at point (e). In the reverse osmosis desalination plants, this ratio is called Recovery Ratio and this expression will be used hereafter. Applying this to figure 1, the difference between the concentration at point 8 and the concentration at point 7 increases with the increase of the recovery ratio of the reverse osmosis phase. The solution at point f flows to point 7 without mixing with solutions of other concentration, therefore the concentration at point 7 equals to the concentration at point 1. Similarly in figure 2, the concentration at point 15 is equal to the concentration at point 20, the solution at point 20 mixes with the bleed (point 22) and composes the solution at point 1 but as the volume of the bleed is small compared to the volume of the solution at point 20, therefore the concentration at point 20 is slightly higher than that at point 1. So the concentration at point 15 is slightly higher than that at point 7. So, we can conclude that the difference between the concentration at point 8 and the concentration at point 15 increases and decreases with the increase and decrease of the recovery ratio of the reverse osmosis phase.

Taking into consideration that the osmotic pressure increases with the increase of the concentration. We can reformulate the above statement as follows:

The difference between the osmotic pressure at point 8 and the osmotic pressure at point 15 (τι8 - πl5) increases and decreases with the increase and decrease of the recovery ratio of the reverse osmosis phase.

Now from relation (1) we can say that P8_min = (π8 - πll) + PIl And from relation (2) we can say that P15 _max = (πl5 - π21) + P21 Therefore P8_min - P15 _max = (π8 - πll) + PIl - ((πl5 - π21) + P21) P7_min - Pioss - P15 _max = (π8 - πll) + PIl - ((πl5 - π21) + P21) P7_min - P|_0SS - P15_max = π8 - πll + PIl - πl5 + π21 - P21 P7_min - Pioss - P15_maχ = π8 - πl5 - πll + π21 - (P21 - PIl)

Where P|_0SS is the pressure losses in the membrane which is, approximately, equal to the pressure rise by the booster pump BP2. Also from figure 2 (P21 - PIl) is equal to the pressure rise by the booster pump BP5. The terms P|_0SS and (P21 - PIl) are small compared to P7_min and P15_max, besides they are in opposite sides of the equation and have the same sign and there values are approximately equal, therefore the two terms can be neglected. Also nil is equal to π21 because they have the same concentration, so: P7_min - P15_max = π8 - πl5, but P5_min = P7_min and P16_max = P15_max so: P5_min - P16_max = π8 - πl5 (17)

As mentioned above, maximizing the efficiency requires minimizing the difference between P16 and P5. Also, as mentioned above, the term (π8 - πl5) decreases with the decrease of the recovery ratio of the osmotic phase. Therefore and referring to equation 17:

A high overall efficiency of the method requires a low recovery ratio of the reverse osmosis phase.

Using low recovery ratio requires modifications to the method described in figures 1 and 2 in order to have high energy density. ^a" ^* Energy density

Energy density is the amount of the net energy produced during osmosis phase per unit volume of the storage medium. The storage medium is the solution which is stored in the reservoirs of the apparatus. The preferred solution: A) should be ionic solution with high solubility in order to have high osmotic potential; B) the viscosity of the solution should be low when it is in its highest concentration, to reduce the pressure losses arising from the flow of the solution in the different components of the apparatus; C) it should be common and cheap, therefore it should be aqueous solution and the solute is a common salt such as sodium chloride, but the preferred solution is concentrated sea water. The source of the raw water from which the concentrated sea water is produced is a near sea, and it is produced in the last few months before the start up of a system constructed according to the present invention, and during the construction period. The concentrated sea water is produced by the means of a multistage reverse osmosis plant, this plant is not part of the system, but it is part of the construction equipments. As the production of the solution can take several months, the size of this temporary RO plant is small. Still another smaller plant is required to be a permanent part of the system for the purposes of maintenance and to compensate any leakage might happen.

In order to have high Energy Density without affecting the efficiency, modifications are required to the method described in figure 1 and 2. Understanding of the relationship between the energy density and recovery ratio is required to suggest these modifications: As per the definition of the energy density above, we can get the energy density equation as follows: e ⁼ ( EjOut ^" EβP3in ^" EβP4in " Eβpsin ) / Vl

To conclude an indication to the way the energy density varies with the variation of the recovery ratio, an accurate estimation for the energy density is not needed, therefore some assumptions are suggested to simplify the previous equation: as the values of terms E_BP3in, E_BP4in, and E_BP5in are small compared to the value of the term E_Tout, so they can be neglected, therefore: e = E_TOut / Vl From equation 12 and as P18 is atmospheric pressure: e = P16 . V16 . η_τ / Vl

Assuming that rejection coefficient is 100 % (it is more than 99% for commercial membranes), in such case the solution at point 11— which is stored in reservoir R2 in the reverse osmosis phase— in figure 1 is pure water ( the concentration is zero ). So, the concentration of the solution at point 21— which is coming from reservoir R2 in the osmosis phase— in figure R2 is zero, therefore there is no need to bleed some of the solution entering the membrane at point 21, so V22 = 0 and V_perm_eatei = V_permeate2 = V16, so, from the previous equation : e = P16 . Vp_ermeatei- Ατ I Vl , but Vl = V7 and the recovery ratio = V_permeatei/ V7, so: r = V_permeatel/ Vl (18) where (r) is the recovery ratio of the reverse osmosis phase. Therefore: e = P16 . r . η_τ (19)

It is understood that using high concentration solution increases the osmotic potential which results in high energy density. The limitations for increasing the concentration of the solution is that the highest concentration in the system should not exceed a safety margin, this margin is close to the saturation point of the solution, and insures that there is no salt deposition at any point in the system. So, the highest possible concentration depends, only, on the type of the solution. In other words, the highest concentration in a system— also the highest osmotic pressure— using a specific solution is constant. A point of the highest concentration in figure 2 is point 14. Applying the mass continuity equation to the solute in the membrane in figure 2 and assuming V22 =0, V21 = V_permeatei and the concentration of the permeate is zero (100% rejection coefficient is assumed): V15 . C15 = V21 . C21 + V14 . C14, where C is the concentration in mass of solute per unit volume of the solution. C_permeati = CIl = C21 = 0 therefore:

V15 . C15 = V14 . C14 Also, V14 = V15 - V21 V15 . C15 = (V15 - V21) . C14

C15 = C14 . (1 - (V21/V15)) , but V21 = V_permeatei, V15 = Vl because V22 = 0 (from figure 2), so:

C15 = C14 . (1 - (Vp_ermeatei/Vl)) so from equation 18:

C15 = C14 . (1 - r), assuming that the osmotic pressure is in linear relationship with the concentration: πl5 = πl4 . (l - r) (20)

Applying equation 7 to the membrane in figure 2, we get: P15_max = (πl5 - π21) + P21 , but π21 = 0 , P21 = 0 and P16_max = P15_max , so: P16_max = πl5 (21) From equations 19, 20 and 21 =, we get: e = πl4 . (1 - r) . r . η_τ, by differentiation we get: de/dr = πl4 . η_τ .(1- 2r), at the maximum e, de/dr = 0, so: r = ^ΛA for maximum energy density.

Therefore the recovery ratio is approximately equal to 50% for maximum energy density, whilst it should be low for maximum efficiency (10% to 15% is a convenient recovery ratio). Modifying the method described in figures 1 and 2, so that the method takes place in several stages is required. This will be detailed when describing figures 8A and 8B which illustrates the best mode.

High energy density reduces the required volume of the storage medium, accordingly it also reduces the required volumetric capacity of the reservoirs that contain the storage medium. As the reservoirs contain a large volume of the storage medium, the structure of the reservoirs is of reinforced concrete.

As mentioned above, when the system is in full storage state, reservoirs R2 and R3 are full while reservoir Rl is empty. And when the system is in zero storage state, R2 and R3 are empty while Rl is full. This requires that the volume of reservoir Rl is equal to the sum of the volumes of reservoirs R2 and R3. Also, this means that the sum of the volumetric capacity of the reservoirs is twice the volume of the storage medium i.e. the volume of the reservoirs is 200% the volume of the storage medium. Figures 6A and 6B illustrate an arrangement of the reservoirs where the volume of the reservoirs is only 120% of the volume of the storage medium i.e. the excess volume is 20%. In this arrangement the reservoir R2 is divided into 12 equal sections (from R2-1 to R2-12), the volume of each section is 10% of the maximum volume of the solution at point 11 in figures 1 and 2 i.e. when the system is in full storage state. Each of these sections is connected to the system through two valves: VIvIl and VIvI. Also the reservoir R3 is divided into 12 equal sections (from R3-1 to R3-12), the volume of each section is 10% of the maximum volume of the solution at state 10 in figures 1 and 2 i.e. when the system is in full storage state. Each of these sections is connected to the system through two valves: VIvIO and VIvI.

In figure 6A, during reverse osmosis phase, all the valves are closed except, for example, VIvI of R2-2, VIvI of R3-2, VIvIl of R2-6 and VIvIO of R3-6. The sections R2-2 and R2-3 contains solution at point 1 and they are connected together because valves VIvI of both of them are open. As the process of reverse osmosis goes on, the solution flows from sections R2-2 and R3-2 to sections R2-6 and R3-6 until they are full and R2-2 and R3-2 are empty. Then, VIvIl of section R2-6 and VIvIO of section R3-6 close. Now, this pair of sections is ready to work as a source of the solutions in the osmosis phase. Also, VIvI of sections R2-2 and R3-2 close, while VIvIl of section R2-2 and VIvIO of section R3-2 open, now this pair of sections is ready to work as a receiver of the solutions in the reverse osmosis phase.

Similarly, figure 6B illustrates the status of the valves and the directions of flow during the osmosis phase assuming that R2-12 and R3-12 are the source of the solutions at state 11 and 10 respectively and that R2-10 and R3-10 are the receivers of the solution at state 1.

If Rl and R2 are divided into 22 sections the excess volume will be 10% only.

A related issue to the energy density is the land area requirement. The reservoirs are by far the most bulky component of a system constructed according to the present invention, but it does not add more area requirement to the whole power plant that comprises such energy storage method. The perfect exploitation for the present invention is with the renewable energy power plants, such as large photovoltaic arrays, wind farms and solar towers. All of these sorts of power plants require a large area. Figure 7 shows an example of a large photovoltaic array where the photovoltaic array is installed on the upper surface of the concrete reservoirs. In the middle of the reservoirs is the power plant. The power plant comprises all other components of both power generation system and energy storage system. The design in figure 7 does not require any additional area requirements for the energy storage system.

Membrane productivity

Membrane productivity is the power produced in the osmosis phase per unit area of the membrane. As the membrane array is a major component of the present invention, it is very important to have a high Membrane productivity, thus reducing the required size of the membrane array.

As mentioned above, the solution used in the system is obtained through artificial process. The purpose is to have the highest possible concentration— The high concentration provides a high osmotic pressure— at point 14 in figure 2. From equation 20 and as the recovery ratio r is low, the osmotic pressure at point 15 is also high. Also from equation 21 the static pressure at point 16 is high. The pressure at point 16 is the inlet pressure of the turbine. Also the flow rate of the turbine is equal to the flow rate of the permeate. Therefore the power generated by the turbine is higher than that the power generated in other methods using pressure lower than that at point 16 and having the same permeate flow. Therefore, the present method has membrane productivity higher than that used in other methods having the same permeate flow rate. To demonstrate the significance of this point:

In methods exploiting the osmotic potential of the natural sea water to generate power— these methods are for power generation and they are not for energy storage— the osmotic pressure of a point equivalent to that at point 14 is 26 bar (for natural sea water), assuming that the difference between the osmotic pressure at this point and the static pressure at the point equivalent to point 16 is 13 bar, so the inlet pressure of the turbine is 26 - 13 = 13 bar. In the present method πl4 can be higher than 90 bar, assuming the same difference between τtl4 and P16— so that both cases have approximately the same permeate flow rate— the inlet pressure is 90 - 13 = 77 bar i.e. about 6 times the other example. If using high pressure membrane πl4 will be about 200 bar.

Besides the advantage of having high membrane productivity, the present invention has another advantage regarding to the membrane maintenance. Usually the methods employing reverse osmosis or osmosis process use an open cycle i.e. the source of the solution is open such as seawater. The solution of these sources has a high level of impurities and requires a costly process of filtration and treatment to maintain the membrane in good condition. The present method is a closed cycle where the system will be charged once during the start up of the system, so this costly filtration and treatment is performed once at the start up. The ongoing filtration and treatment requirement is much less than the open cycle.

Illustration of the best mode

Figures 8A and 8B illustrate the schematic diagrams of the best mode. In the example illustrated in these figures, the method comprises three stages— the number of stages is illustrative and the present invention is not limited to this number of stages. Also, reservoir R4 is added to store the bleed generated in the osmosis phases instead of mixing it with the rest of the solution in Rl, then in the osmosis phase it enters the membrane separately— the bleed has a small volume compared to the total volume of the solution.

The reservoirs R2 and R3 are divided into sections, a section of reservoir Rl is composed by connecting two sections from R2 and R3.

A number of valves are added to the system in two groups, when the valves of one group are open the valves of the other group are closed, so there are two positions, in one position the system works in the reverse osmosis phase and in the other it works in the osmosis phase.

In the reverse osmosis phase, a medium concentration solution (at point 1 figure 8A) stored in the reservoirs R2-1 and R3-1 (which are connected together and acting as one section of reservoir Rl through two open valves) flows, exactly as previously described, through booster pump PBl-I, pressure exchanger PXl, high pressure pump HPl, and membrane array Ml where it splits into the permeate and a solution at point 23. The solution at point 23 starts the second stage through PB1-2, HP2, PX2, and M2 where it splits into anther mount of the permeate and a solution at point 24. Again the solution at point 24 starts the third stage through PB1-3, HP3, PX3, and M3 where it splits into anther mount of the permeate and a solution at point 25. The solution at point 25 has a high osmotic pressure and a high static pressure; it gains slight pressure by booster pump BP2, then it flows to the reservoir R3-3 through the series of pressure exchangers where it exchanges its pressure. When the solution at point 1 starts to flow from the reservoirs R2-1 and R3-1, another solution— the permeate— starts to flow from reservoir R4 at point 22 and enters the perforated tube of the membrane of the third stage where it mixes with the permeate of this stage and the resultant solution enters the second stage then the third stage where its volume increases by mixing with the permeate of every stage, then the resultant solution flows to reservoir R2- 3 where it is stored. The booster pumps BP6-1, BP6-2 and BP6-3 supply the required pressure for the flow from reservoir R4 to reservoir R2-3. To start the osmosis phase, all valves in figure 8A switch from open to close and from close to open as in figure 8B. In the osmosis phase, a high concentration solution (at point 10 figure 8B) stored in the reservoirs R3-5 gains a high pressure through a series of booster pumps and pressure exchangers BP3-1, PXl, BP3-2, PX2, BP3-3, PX3, and BP4, then it enters the perforated tube of the membrane M3 of the third stage— at point 26— where it increases its volume by mixing with the permeate of this stage, then it flows through turbine T3 and pressure exchanger PX3 and collects at point 27. The solution at point 27 enters the second stage through M2, T2 and PX2 and collects at point 28. The solution at point 28 enters the third stage through M3, T3 and PX3 and collects at point 1 where it is stored in the reservoirs R2-7 and R3-7 (which are connected together and acting as one section of reservoir Rl through two open valves). When the solution at point 10 starts to flow from the reservoirs R3-5, another solution starts to flow from reservoir R2-5 at point 11 and enter the first stage through booster pump BP5-1 and the membrane array Ml. After the separation of the permeate, the rest of the solution at point 29 starts second stage through BP5-2 and M2 where the permeate splits and the rest of the solution at point 30 starts third stage through BP5-3 and M3 where the permeate splits and the rest of the solution at point 22 (the bleed) is stored in reservoir R4.

In figure 8A the solution at point 25 flows to reservoir R3-5 without mixing with other solutions, also in figure 8b the same solution flows from reservoir R3-5 to point 26 without mixing with other solutions, therefore we can say that the following volumes, in figures 8A and 8B, are equal V25, V31, V32, V33, VlO, V34, V35, V36, V26. Applying equation 3, we can say that these volumes are also equal to the following volumes V37, V38, V39, V40, V41, V42, V43, V44, V45, V46, V47, V48 . From figure 8A: V49 = Vl - V37 and from figure 8B: V52 = V44 - Vl, but V37 = V44, therefore

V49 = V52. Similarly, we can prove that V50 = V53 and V51 = V54, so volumes handled by high pressure pumps HPl, HP2 and HP3 are equal to the volumes handled by turbine Tl, T2 and T3 respectively, while in figures 1 and 2, the volume handled by pressure pump HP is larger than that handled by turbine T. So, the volume handled by the high pressure pumps is reduced and accordingly the power consumed by the high pressure pumps is reduced, thus the efficiency improved.

The previous paragraph shows the positive effect of holding the bleed in a separate reservoir R4. Hereafter, the effect of using multistage in the method, when using a low recovery ratio, on the energy density is clarified. As the efficiency increases with low recovery ratio, we assume that the recovery ratio r = 10%. In figure 8A:

V23 = Vl . (1 - r) = 0.9Vl, similarly we get V24 = 0.81V1 and V25 = 0.729V1. Also: V37 = V39 = V41 = V25 = 0.729V1, so V49 = Vl - V37 = 0.271V1. Similarly: V50 = 0.171V1 and V51 = 0.081V1. In figure 8A and 8B and as mentioned above V52 = V49, so V52 = 0.271V1. Similarly, V53 = 0.171V1 and V54 = 0.081V1. The energy generated by turbine: Tl = P52 . V52 . η_τ = 0.271 . P52 . Vl . η_τ. Similarly we can estimate the energy produced by T2 and T3. Therefore the energy produced according to the multistage option is:

Eτou_tM = (0.271 . P52 + 0.171 . (P53 - P52) + 0.081 . (P54 - P53)) . Vl . η_τ (22)

To compare with the case of single stage with the same recovery ratio, from figure 2 Eτo_uts = P16 . V16 . η_τ.

But the osmotic pressure at point 14 is equal to the osmotic pressure at point 26 in figure 8B and both are equal to the maximum osmotic pressure we can reach, so ττl4 = π26, but the recovery ratio is the same, so πl5 = π54. Assuming that the difference between osmotic pressure and static pressure at point 15 is the same as that at point 54 (this assumption makes the permeate flow rate in both cases equal), so P15 = P54, but P15 = P16, therefore E_TOuts = P54 . V16 . η_τ.

But V16 = V_permeate = 0.1Vl , so E_TOuts = 0.1 . P54 . Vl . η_τ.

P54 = P52 + (P53 - P52) + (P54 - P53), so;

Eτo_uts = (0.1 .P52 + 0.1 . (P53 - P52) + 0.1 . (P54 - P53)) . Vl . η_τ (23)

Dividing equations 22 and 23 by Vl and comparing the remaining three terms we find that first term of equation 22 (multistage example) is 2.7 times greater than the equivalent term in equation 23, and the second term is 1.7 times greater, while the third term is 0.2 times smaller. The first term is the major term as P52 is more than 70% of the whole pressure range. Therefore we conclude that the energy density in the example illustrated in figures 8A and 8B is more than two times that in the single stage. Membrane maximum pressure

It is important in the present invention to use high osmotic pressure solution. But this requires that the static pressure in the reverse osmosis phase to be higher than that high osmotic pressure. There are two ways to achieve that. The first is to use high pressure membrane which can withstand the high static pressure. The other way is to use two membrane arrays connected in parallel. There are two arrangements for using two membranes in parallel. First arrangement is shown in figure 9. A high osmotic pressure solution enters the membrane MH at point 61 and exits at point 62, and a low osmotic pressure solution enters the membrane ML at point 63 at atmospheric pressure. If one membrane is used— not two in parallel— the static pressure required will be P62 where P62 > (π62 - π63), but instead of that an intermediate solution enters the membrane MH at point 65 with atmospheric pressure and an intermediate osmotic pressure π65 where, for example, π65 = (π62 + π63)/2. So the required static pressure at point 62 is reduced: P62 > (π62 - π65), so P62 > (π62 - π63)/2

The solution at point 65 enters at point 65 and exits the membrane at point 66 where it enters the pump P which delivers the solution at point 67 at a higher pressure. The solution enters the membrane ML and exits at point 68 with the same osmotic pressure of point 65. The pressure at point 68— which is, approximately, equals to the pressure delivered by the pump P— is:

P68 > (π68 - π63)

Because π68 is medium P68 is also medium. Then the solution enters with its pressure to the turbine, and the turbine generates the largest part of the energy required for the pump P, the rest of the energy required for the pump is supplied by the electric motor EM. A bypass valve BV is required to control the concentration of the intermediate solution. The process can be reversed to osmosis phase by lowering P62 and P68 to the pressures that satisfy the conditions in which osmosis process takes place. This arrangement involves losses due to the operation of the pump and the turbine The other arrangement for the parallel membrane is shown in figure 10. And it is simpler than the other one and more efficient. There is no turbine and the circulating pump CP is just to compensate the pressure losses in the membranes MH and ML. the static pressure at points 63 and 64 is atmospheric while it is high at points 61 and 62. The intermediate solution at points 65, 66, 67 and 68 is kept at intermediate static pressure. The process in membrane ML is the same as the previous arrangement, whilst the static pressure at point 62 is very high— P62 > (τι62 - π63)— so that the reverse osmosis process takes place; This high static pressure applies high stresses on the membrane. But the intermediate static pressure of the intermediate solution— " within the travel of the solution from point 65 to 66— works as equalizing pressure in the other side of the membrane and reduces the stresses on the membrane.

Also a bypass valve BV is required to control the concentration of the intermediate solution. The losses in this arrangement are low.

Temperature effect

The ambient temperature has small effect on the efficiency of the method. As the ambient temperature varies along the hours of the day, the temperature of the solution varies. But the osmotic pressure of a solution is in linear relationship with its absolute temperature, therefore the required pressure to operate the high pressure pumps, when the solution is warmer, is higher. While the operating pressure of the turbines, when the solution is colder, is lower. But the effect of temperature on efficiency is very small for the following reasons: A) the fluctuation of the ambient temperature along the day is small compared to the average absolute temperature, B) the fluctuation of the temperature of the solution is less than the fluctuation of the ambient temperature, C) when using photovoltaic arrays as the source of energy, the difference between the average temperature of the solution during reverse osmosis phase (energy consuming phase) and the average temperature during osmosis phase (energy generating phase) is less than the fluctuation of the temperature of the solution. The effect of the temperature is less when using wind farms as the source of energy.

Industrial Applicability The present invention is perfectly exploited when used as energy storage method for large scale renewable energy power plants that requires large storage capacity enough to supply the load for days. The perfect storage system comprises several modules. Increasing the number of modules increases the average efficiency of the system as the pumps and turbines will work close to their optimum point on their performance curve. Each module comprises the multistage apparatus described above, for example six stages. Therefore the system comprises a large number of turbines, high pressure pumps and booster pumps. The capacity of these turbines and pumps should be high, as small turbines and pumps have insufficient efficiency. So, this large number of high capacity equipments requires large scale power plants. Excluding the reservoirs and storage medium, the cost of all other components of the system, such as membrane array, pressure exchangers array, turbines and pumps are not related to the storage capacity of the system, but they are related to the power plant capacity and the maximum demand load. Therefore the disclosed energy storage method becomes more feasible when it is required to have large storage capacity for a specific power plant capacity.

Having described the present invention, it will be understood that the above described modes and drawings are to be considered in all respects only as illustrative and not restrictive. Therefore various modifications may be made without departing from the scope and spirit of the invention.

Claims

Claims

Claim 1. A method for storing electrical and/or mechanical energy in the form of osmotic energy and for converting said stored energy into electrical and/or mechanical energy when required, said method comprising: A) reverse osmosis phase, said phase comprising: withdrawing a medium concentration solution (1) from a reservoir, said solution is under atmospheric pressure; pressurizing said solution to a pressure higher than the difference between the osmotic pressure of solution (8) and the osmotic pressure of solution(ll), a portion of the required energy to pressurize the said solution is an excess available energy of a power plant; introducing the pressurized solution (7) to the inlet of a membrane,, where a reverse osmosis process takes place; storing the permeate solution (11), resulting from the reverse osmosis process, in a reservoir at atmospheric pressure; introducing the high concentration solution (8), resulting from the reverse osmosis phase, to an energy recovery device, the purpose is to exploit the high static pressure of said solution to pressurize portion of solution (1); storing the high concentration solution (10) in a reservoir at atmospheric pressure; B) osmosis phase, said phase comprising: withdrawing a high concentration solution (10) from a reservoir; pressurizing the thus withdrawn solution to a pressure less than the difference between the osmotic pressure of solution (15) and the osmotic pressure of solution(21); introducing the pressurized solution (14) to the inlet of the high concentration side of a membrane; withdrawing a low concentration solution— the permeate— (10) from a reservoir; introducing the thus withdrawn solution to the inlet of the low concentration side of a membrane, where an osmosis process takes place between the said solution and solution (14); splitting the medium concentration solution (15) into two portions; introducing one portion of the split solution (17) to an energy recovery device, the purpose is to exploit the high static pressure of the said solution to pressurize solution (10); introducing the rest of the split solution (16) to an energy generating device; allowing small portion of the solution introduced to the inlet of the low concentration side of a membrane (21) to exit the membrane, said portion is the bleed (22); mixing the bleed (22) with the solution leaving the energy recovery device (19) and with the solution leaving the energy generating device (18) to form solution (1); and storing solution (1) in a reservoir at atmospheric pressure.

Claim 2. A method as claimed in claim 1, wherein, in the osmosis phase, the bleed (22) is stored in a separate reservoir, and in the reverse osmosis phase, the bleed is introduced to the inlet of the low concentration side of a membrane.. _ . .

Claim 3. A method as claimed in claim 2, further comprises several stages, said stages comprises: A) in the reverse osmosis phase, introducing solutions (23), (24), and similar solutions, if any, to the second stage, third stage, and subsequent stages, if any, as solution (1) is introduced to the first stage; introducing solution (22) to the inlet of low concentration side of the membrane of the last stage, where it mixes with the permeate of the last stage, the resultant solution exits at the outlet of the low concentration side of the membrane; introducing the thus resultant solution to the inlet of the low concentration side of a membrane of the stage before the last stage, and similarly, introducing the resultant solution of each stage to the inlet of the low concentration side of the membrane of the stage before, until solution (11) exits the outlet of the low concentration side of the membrane of the first stage; introducing solution (31) to the inlet of an energy recovery device of the last stage, then to the energy recovery device of the stage before the last stage, and subsequently to the rest of stages until solution (10) exits the energy recovery device of the first stage; B) in the osmosis phase, introducing solutions (29), (30), and similar solutions, if any, to the second stage, third stage, and subsequent stages, if any, as solution (11) is introduced in the first stage; introducing solutions (35), (36), and similar solutions, if any, to the second stage, third stage, and subsequent stages, if any, as solution (10) is introduced in the first stage; introducing solution (26) to the inlet of high concentration solution side of the membrane of the last stage, where it mixes with the permeate of the last stage, the resultant solution exits at the outlet of the high concentration side of the membrane; and introducing the thus resultant solution to the inlet of the high concentration side of a membrane of the stage before the last stage, and similarly, introducing the resultant solution of each stage to the inlet of the high concentration side of the membrane of the stage before, until the resultant solution exits the outlet of the high concentration side of the membrane of the first stage.

Claim 4. A method as claimed in claim 3, wherein the cycle of the solution is a closed cycle i.e. in the normal operation conditions, there is no adding of the solution to the system and there is no removing of the solution from the system.

Claim 5. A method as claimed in claim 4, wherein the concentration of solution (10)— the highest concentration in the cycle— is the highest possible concentration without a possibility of salt deposition.

Claim 6. A method as claimed in claim 5, wherein the solution is an aqueous solution.

Claim 7. A method as claimed in claim 6, wherein the solution is a concentrated seawater solution.

Claim 8. A method as claimed in claim 6 or claim 7, wherein the static pressure of solution (25) is higher than its osmotic pressure.

Claim 9. A method for using a membrane of a designated working static pressure less than the osmotic pressure of the high concentration solution (62), said method comprising:

A) in a reverse osmosis process, introducing a high concentration solution (61) to the inlet of the high concentration side of a membrane at a static pressure less than the difference between the osmotic pressure of solution (62) and the osmotic pressure of solution (63), said static pressure is close to but higher than the half of the said difference; introducing a low concentration solution (63) to the inlet of the low concentration side of a membrane— said membrane is not the one mentioned in the preceding step— at atmospheric pressure; introducing an intermediate solution (65) to the inlet of a low concentration side of a membrane, said membrane is the membrane to which solution (61) is introduced, the value of the osmotic pressure of solution (65) is in the middle between the value of the osmotic pressure of solution (62) and that of solution (63), whilst the said solution is at atmospheric pressure; pressurizing solution (66) to a pressure higher than the difference between the osmotic pressure of solution (68) and the osmotic pressure of solution (63); introducing solution (67) to the inlet of the high concentration side of a membrane, said membrane is the membrane to which solution (63) is introduced; introducing solution (68) to the inlet of an energy recovery device, the recovered energy is to provide part of the energy required to pressurize solution (66); providing energy to compensate the difference between the energy required to pressurize solution (66) and that recovered from solution (68); and allowing a small portion of solution (67) to bypass the membrane; B) in an osmosis process as claimed in A) wherein the static pressure of solution (61) and the static pressure of the solution (67) are reduced to the pressures that satisfy the conditions in which osmosis process takes place.

Claim 10. A method as claimed in claim 9, wherein, A) in a reverse osmosis process, the static pressure of the intermediate solutions (65), (66),(67) and (68) is kept constant at intermediate static pressure, and the differences of the values of the static pressures of the said four solutions are small, said pressure differences are just to compensate the friction losses arising from the flow of the intermediate solution in the two membranes, whilst the static pressure of solution (62) is higher than the difference between the osmotic pressure of solution (62) and the osmotic pressure of solution (63), B) in an osmosis process as claimed in A) wherein the static pressure of solution (61) and the static pressure of the intermediate solution are reduced to the pressures that satisfy the conditions in which osmosis process takes place.

Claim 11. A method as claimed in claim 6 or claim I₁ wherein the static pressure of solution (25) is less than its osmotic pressure, and wherein the reverse osmosis process and osmosis process are as in claim 9.

Claim 12. A method as claimed in claim 6 or claim 7, wherein the static pressure of solution (25) is less than its osmotic pressure, and wherein the reverse osmosis process and osmosis process are as in claim 10.

Claim 13. An apparatus for storing electrical and/or mechanical energy in the form of osmotic energy and for converting said stored energy into electrical and/or mechanical energy when required, said apparatus comprising:

A) common components for both reverse osmosis phase and osmosis phase comprising: a reservoir for storing a medium concentration solution (Rl); a reservoir for storing a low concentration solution (R2); a reservoir for storing a high concentration solution (R3);

B) components for reverse osmosis phase comprising: a booster pump (BPl) for withdrawing and for contributing in pressurizing a portion of a medium concentration solution stored in reservoir (Rl); an energy recovery device (pressure exchanger array) (PX) for pressurizing said portion of the solution after it is delivered by said booster pump (BPl), and for recovering energy of a high static pressure solution; a high pressure pump (HP) for pressurizing the rest of said solution— the source of energy required to operate said high pressure pump is the excess available energy of a power plant— the delivery pressure of said high pressure pump is equal to the delivery pressure of said pressure exchanger array, and the delivered solution by said high pressure pump mixes with the delivered solution by said pressure exchanger; a membrane array (M) for receiving the solution delivered by said high pressure pump and said pressure exchanger array, the outputs of the membrane are high concentration solution which is stored in said high concentration solution reservoir (R3) and low concentration solution (permeate) which is stored in said low concentration solution reservoir (R2); a booster pump (BP2) for withdrawing a high concentration solution produced by said membrane array, and for delivering said high concentration and high static pressure solution to said pressure exchanger array;

C) components for osmosis phase comprising: a high pressure pump (BP3) for withdrawing and for contributing in pressurizing said high concentration solution stored in reservoir (R3); a pressure exchanger array (PX) for pressurizing said solution after it is delivered by said booster pump, and for recovering energy of a high static pressure solution; a booster pump (BP4) for pressurizing said solution after it is delivered by said pressure exchanger array; a booster pump (BP5) for withdrawing of a low concentration solution stored in reservoir (R2); a membrane array (M) for receiving the low concentration solution delivered by booster pump (BP5), and for receiving the high concentration solution delivered by booster pump (BP4), the outputs of the said membrane are a medium concentration and a high static pressure solution and a relatively low concentration solution (the bleed); and a turbine (T) for generating energy, wherein the liquid introduced to said turbine is a portion of said medium concentration and high pressure solution that is an output of said membrane.

Claim 14. An apparatus as claimed in claim 13, further comprises: a reservoir (R4) for storing, in an osmosis phase, said relatively low concentration solution (the bleed); and, for reverse osmosis phase, a booster pump for withdrawing the bleed from said reservoir and delivering said solution to the inlet of low concentration side of said membrane.

Claim 15. An apparatus as claimed in claim 14, further comprises several stages, said stages comprises: A) for reverse osmosis phase: booster pumps (BPl-I), (BP1-2), (BP1-3), and other booster pumps, if any, according to number of stages; pressure exchanger arrays (PXl), (PX2), (PX3), and other pressure exchanger arrays, if any, according to number of stages; high pressure pumps (HPl), (HP2), (HP3), and other high pressure pumps, if any, according to number of stages; membrane arrays (Ml), (M2), (M3), and other membrane arrays, if any, according to number of stages; booster pumps (BP6-1), (BP6-2), (BP6-3), and other booster pumps, if any, according to number of stages;

B) for reverse osmosis phase: booster pumps (BP3-1), (BP3-2), (BP3-3), and other booster pumps, if any, according to number of stages; pressure exchanger arrays (PXl), (PX2), (PX3), and other pressure exchanger arrays, if any, according to number of stages; turbines (Tl), (T2), (T3), and other turbines, if any, according to number of stages; membrane arrays (Ml), (M2), (M3), and other membrane arrays, if any, according to number of stages; and booster pumps (BP5-1), (BP5-2), (BP5-3), and other booster pumps, if any, according to number of stages.

Claim 16. An apparatus as claimed in claim 15, wherein, in each stage, there is only one membrane array and one pressure exchanger array for both reverse osmosis phase and osmosis phase, the apparatus further comprises two groups of valves, when one group is closed the other group is open so that the apparatus is in a position where the direction of flow— flow of the solution in the different components of the apparatus- is as it should be in reverse osmosis phase, and when the positions of valves are switched from open to close and from close to open the direction of said flow is as it should be in osmosis phase.

Claim 17. An apparatus as claimed in claim 16, wherein the high concentration solution reservoir (R3) is divided into several equal sections, and the low concentration reservoir (R2) is divided into the same number of equal sections as high solution concentration reservoir, a space required for storing the medium concentration solution is created by connecting one section of reservoir (R3) and one section of reservoir (R2).

Claim 18. An apparatus as claimed in claim 17, wherein the solution stored in reservoir (R3) is the highest possible concentration without a possibility of salt deposition.

Claim 19. An apparatus as claimed in claim 18, wherein the solution is an aqueous solution.

Claim 20. An apparatus as claimed in claim 19, wherein the solution is a concentrated seawater solution.

Claim 21. An apparatus as claimed in claim 19 or claim 20, wherein the designated working static pressure of any membrane of the membranes of said apparatus is higher than the osmotic pressure of the solution of the highest concentration which said membrane handles.

Claim 22. An apparatus as claimed in claim 19 or claim 20, wherein the designated working static pressure of some or all membranes of said apparatus is less than the osmotic pressure of the solution of the highest concentration which said membranes handle, each of said membranes is replaced by a set of components comprising: a membrane array (MH) for receiving a high concentration solution, said solution enters said membrane at the inlet of the high concentration side of said membrane— the designated working static pressure of said membrane is less than the osmotic pressure of the solution that exits said membrane at the outlet of the high concentration side of said membrane; a high pressure pump (P) for withdrawing an intermediate solution that exits membrane array (MH) at the outlet of the low concentration side of said membrane, and pressurizing said solution— which its osmotic pressure is in the middle between the osmotic pressure of the high concentration solution and an osmotic pressure of a low concentration solution— to a static pressure higher than the osmotic pressure of said intermediate solution; a membrane array (ML) for receiving said low concentration solution, said solution enters said membrane at the inlet of the low concentration side of said membrane— the designated working static pressure of said membrane is equal to that of membrane (MH); a turbine (RT) for recovering the energy of said intermediate solution when exits the membrane (ML) at its high concentration solution outlet, said energy is because of the pressure of said solution and it is used for providing part of the energy required to operate said pump (P); and a bypass valve for controlling the concentration of said intermediate solution.

Claim 23. An apparatus as claimed in claim 22, wherein the turbine and the high pressure pump are replaced by a circulating pump (CP), said circulating pump is for circulating the intermediate solution between membrane (MH) and membrane (ML), the pressure of the said intermediate solution is kept constant at intermediate static pressure.