SOLAR COLLECTOR

Given the imperative and growing need for fresh water, various desalination technologies have been developed and a great number of desalination plants has been erected.

The most important desalination methods that have been commercially adopted, regarding either sea water or brackish water desalination, could roughly be ranked in two major groups, depending on the relevant process adopted: The membranes group and the thermal (of thermal mimic) group. The first group includes the Reverse Osmosis (RO), the Forward Osmosis (FO) and the Electro-dialysis methods, while the second group mainly includes the Multi Stage Flash (MSF), the Multi Effect Distillation (MED) and the Vapor Compression (VC) methods.

Common base of all desalination technologies and of their commercial applications is the significant amount of energy, (either electrical or thermal, according to the technology applied), required for the operation of the desalination plant. Consequently, the energy cost is the main cost issue regarding the operational cost of any desalination plant, resulting to the relatively high price of the desalinated water produced.

Desalination process powered by renewable energy sources is a challenge for both economic and environmental reasons. Various such technologies have been developed and many research projects have been carried out, however, such methods have not been widely commercialized, mostly due to the high investment cost required. From the commercial point of view, the most popular solar desalination method is the reverse osmosis desalination powered by photovoltaic panels and is implemented mainly in small scale applications. Since the solar power is less than 1 KW/m2 and the efficiency of the solar panels is low, (approximately 18%), the panel surface required for such an application is high, resulting to high investment cost.

The desalination technology that commercially seems to be the most attractive in the recent years is the Reverse Osmosis (RO), mainly due to the remarkable reduction of the cost of the osmosis membranes used according to this technology. The main operational cost issue of this method is the cost of the electrical energy required by the high pressure pumps which pressurize sea water into the pressure vessels containing the semi - permeable membranes. Furthermore, in places not connected to the electrical grid (such as in some islands), the cost of electrical energy is much higher compared to the nominal one, resulting to very high RO desalination cost and discouraging such choices. The reverse osmosis desalination process according to the current invention is powered by renewable sources, and, therefore, it does not need any electrically driven high pressure pumps, cutting out the relevant energy cost. Briefly, it includes a reverse osmosis desalination unit and also a natural or manmade solar lake or pond containing high concentration salt solution. A high concentration salt solution stream leaves from the said lake or pond and returns to it, having lower salt concentration. The water evaporation from said lake, mainly due to the solar energy received by it, ensures that the salt concentration in said lake is maintained to the required level. Actually, said lake or pond operates as a large size and low cost solar collector. The required pressurization of the seawater, entering to the semi-permeable membranes of said desalination unit, is achieved by the displacement, due to volume increase under very high pressure, of a high concentration salt solution stream, coming form said lake or pond, as it absorbs water from a seawater stream, through semi - permeable membranes contained in the relevant pressure vessels of an additional forward osmosis unit incorporated for this purpose.

The design and operation of the desalination unit according to the current invention may be based on either a continuous flow process, or on a batch process. The continuous flow process has been selected and will be described below for practical purposes, however, the batch process approach is also valid and is based on similar principles and operating features as the continuous flow process.

The Reverse Osmosis Desalination process according to the current invention may be based on an only one step operation, however, a multi - step operation approach increases the efficiency to higher level and, therefore, it is the one adopted and described below.

Four liquid streams are included in said procedure according to the current invention:

- The high concentration salt solution stream, (from now on abbreviated as HCSS) in the pressurization unit (40).

- The brackish water or seawater that is going to be desalinated (from now on abbreviated as SWD) in the desalination unit (20).

- The brackish water or seawater (from now on abbreviated as SWP) in the pressurization unit (40).

- The desalinated water stream (from now on abbreviated as DW) in the desalination unit (20).

Regarding the flow direction of each one of the said four streams from one step of operation to the other, any combination may be considered, however in the following description (and without affecting the generality), the flow direction that has been adopted is: The SWD stream and the SWP stream enter in the last step of the desalination unit (20) or the pressurization unit (40) respectively and leave from the first step. The HCSS stream, been in counter flow, enters in the first step of the pressurization unit (40) and leaves from the last step.

Brief description of the drawings:

Figure 1 refers to said multi - step Reverse Osmosis Desalination process method according to the current invention and includes the following:

- The low cost solar collector, meaning the lake or pond containing high concentration salt solution. (10)

- The desalination unit (20) operating according to the Reverse Osmosis Principle, including its pressure vessels (22), its inlet (23) and outlet (27) piping of the seawater to be desalinated (SWD).

- The pressurization unit (40) operating according to the Forward Osmosis Principle including its pressure vessels (42), as well as the inlet (44) and outlet (47) piping of the high concentration salt solution (HCSS) stream. Also, the inlet (46) and outlet (45) piping respectively of the seawater which is used for the pressurization (SWP).

- The pressure / flow-rate conversion unit (60), including the cylinder pairs (61 ) / (70), the inlet (47) and outlet (44) piping for the high concentration solution stream (HCSS) as well as the inlet (27) and outlet (23) piping for the seawater to be desalinated (SWD).

- The filtering and treatment units for seawater (31) and high concentration salt solution (48) respectively.

- The storage tank for the desalinated water (28).

- The booster or circulation pumps for the seawater (32) and (49), for the high concentration salt solution (71) and the seawater brine to be rejected (72) and (51). Figure 2 refers to said pressurization unit (60), and includes the following:

- The pair of cylinders (61), which includes cylinders (611) and (612). Each one of these cylinders (611) and (612) consists of the cylinder body (62), the piston (63), the rod (64) and the inlet / outlet nozzles of the two parts of the internal volume of the cylinder that are separated by the piston (63).

- The flow control valves (65) and (66) for the high concentration salt solution stream (HCSS) from and to the vessels (42) of the pressurization unit and the flow control valves (67) and (68) for the seawater or brackish water that is going to be desalinated (SWD), to and from the vessels (22) of the desalination unit.

- The piping (47) for the HCSS stream from the pressure vessel (42) of one step of the pressurization unit to the flow control valves (65) / (66). Also, the piping (44) for the HCSS stream from said valves (65) / (66) to the pressure vessel (42) of the next step of the pressurization unit.

- The piping (27) of the SWD stream from the pressure vessel (22) of one step of the desalination unit to the flow control valves (67) / (68). Also, the piping (23) for the SWD stream from said valves (67) / (68) to the pressure vessel (22) of the previous step of the desalination unit.

Said units or parts included in said desalination procedure according to the current invention, are described below, referring to Figure 1 : The - at least one - low cost solar collector, which is actually a natural or manmade or appropriately modified lake or pond (10), which contains HCSS, feeds it to the pressurization unit (40) mentioned below, and then receives it back after it becomes thinner (meaning that its concentration is relatively lower), resulting to the reduction of the concentration of the salt solution in the lake (10). Said reduction is compensated mainly by the solar radiation which evaporates water from the lake, increasing the concentration of the salt solution it contains. Consequently, by adjusting the inlet and outlet HCSS solution flow-rates, to and from the lake, according to the size of the lake, the local climate and other local conditions, the concentration of the salt solution in the lake is maintained approximately at the required design level on a long time basis. Actually, this lake (10) operates as a solar collector as well as an energy storage device, storing energy in the form of high concentration salt solution.

This lake (10) should preferably be shallow. However, one or more less shallow areas may also be incorporated and relatively elevated temperatures may be developed to the HCSS solution which is near the bottom of the said less shallow areas. HCSS stream that feeds the pressurization unit (40) is preferably pumped out from the bottom of such less shallow areas.

Dark colored gravel, stones, or other suitable material should preferably be placed at the bottom of the lake, improving the absorption of the solar radiation and, therefore, increasing the evaporation rate in the lake and the concentration of the salt solution in it. Dark colored floating or semi-floating particles or devices could also be optionally incorporated.

Alternatively, slurry salt material, or salt pulp from salt pots, or even dry salt could be used in order to produce HCSS as they are mixed to the appropriate ratio with the lower concentration salt solution coming out from the said pressurization unit (40). For example, when one Kgr of 70% salt pulp is mixed with five Kgr of a 10% salt solution coming out from the pressurization unit, then six Kgr of 20% salt solution are produced in order to supply again the pressurization unit (40) with HCSS.

The desalination unit (20), is similar to the existing reverse osmosis desalination units, operates according to the Reverse Osmosis Principle and includes at least one desalination step. Each desalination step (for example step No "k"), includes at least one pressure vessel (22), containing semi-permeable membranes of any type, and:

The inlet pipe (23) supplying SWD stream (under high pressure) to the one side of the said semi-permeable membranes of pressure vessel (22). Said high pressure SWD stream, comes from the next desalination step (step No "(K+1)") through the pressure / flow-rate conversion unit (60). In case that step No "k" is the last step of the desalination unit (20), then said high pressure SWD stream, comes via SWD supply pipe (24) and the booster pump (32), through the pressure / flow rate conversion unit (60). Said high pressure of the SWD stream is achieved by the pressurization unit (40) and the pressure / flow-rate conversion unit (60) explained below and it exceeds the osmotic pressure value, developed for the certain conditions (temperature, salt concentration, etc) of the SWD stream. As a result, clear, desalinated water (DW) comes out from said SWD stream through the said semi-permeable membranes, and via the DW local outlet pipe (25), the DW central outlet pipe (26) and the booster pump (29) is stored in the desalinated water tank (28).

- The -SWD outlet pipe (27) which drains the SWD stream out of the pressure vessel (22), This SWD stream has higher concentration and lower flow-rate, compared to those of the supplying SWD stream entering the pressure vessel (22) through the inlet pipe (23) mentioned above. The pressurization unit (40), the purpose of which is to increase the flow-rate of the HCSS stream (under high pressure), and, via the pressure / flow-rate conversion unit (60), to transfer the pressure to the SWD stream, after adjusting it (in the pressure / flow rate conversion unit (60)), to the appropriate pressure level for the desalination process, and drive the said SWD stream (under pressure) into the pressure vessels (22) of the desalination unit (20) mentioned above. During the procedure mentioned above, the HCSS and the SWD steams are not mixed.

Said pressurization unit (40) operates according to the Forward Osmosis Principle and includes at least one pressurization step, while each pressurization step (for example step No "k") includes at least one pressure vessel (42), containing semi-permeable membrane of any type. The pressurization unit (40), in each one of its steps, also includes:

- The inlet pipe (44) providing HCSS stream under high pressure to the first side of the said semi-permeable membrane of said pressure vessel (42). Said HCSS stream comes from the previous pressurization step (step No "(K-1)"). In case that step No "k" is the first step of the unit, then said high pressure HCSS stream comes from the high concentration salt solution lake (10) via the filtering / treatment unit (48), the booster pump (71) and the pair of cylinders (70) of the pressure / flow rate conversion unit (60). Said high pressure HCSS stream leaves the pressure vessel (42) under high pressure, through the outlet pipe (47) and pressurizes the SWD of the same numbered step (step No "k") of the desalination unit (20), via the pressurization cylinders of the same step (step No "K") ofthe pressure / flow-rate conversion unit (60). In each step, said HCSS stream leaves the pressure vessel (42) through the outlet pipe (47) having lower concentration and higher flow-rate, compared to the relevant figures of the entering HCSS stream through the inlet pipe (44).

The inlet pipe (46) providing SWP stream to the second side of the said semipermeable membranes of said pressure vessel (42) of the same numbered step (step No "k") of the pressurization unit (40). Said SWP stream comes from the next pressurization step (step No "(K+1)"). In case that step No "k" is the last step of the unit, then said high pressure SWP stream, comes from the SWP supply pipe (46) via the booster pump (49). Said SWP stream leaves the pressure vessel (42) through the outlet pipe (45) to the previous pressurization step (step No "(K-1)") of the pressurization unit (40). In case that step No "K" is the first step of said pressurization unit (40), then said high pressure SWP stream leaves from the pressure vessel (42) back to the sea or the brackish water basin, via the return pipe (50) and the booster pump (51), having higher concentration and lower flow-rate, compared to the relevant figures of the of the supplying SWP entering the pressure vessel (42) through the inlet pipe (46) mentioned above.

Since, in any operation step, the concentration of the HCSS to the first side of the said semipermeable membranes of said pressure vessels (42) is much higher than the concentration of the SWP to the second side of the said semi-permeable membranes of said pressure vessel (42), water comes out from said SWP stream through the said semi-permeable membranes, entering into the HCSS stream and increasing its volume, under high pressure. In other words, the HCSS flow - rate increases from one step to the next. This flow-rate increase is used in order to pressurize the SWD in the relevant step of the desalination unit. The pressure / flow-rate conversion unit (60), purpose of which is to transfer the pressure developed to the HCSS stream in each step of the press urization unit (40), to the SWD stream entering in the adequate step of the desalination unit (20). Said pressure is not just transferred from the HCSS stream to the SWD stream entering into the desalination unit (20), but it is also adjusted to the level required for the desalination process in the desalination unit (20). Such pressure transfer (from HCSS stream to SWD stream) may be carried out in several ways / methods, such as using a turbine - pump pair of any type, or using a pressure vessel including a suitable flexible membrane which divides its volume into two separate / variable volume parts, etc. For the needs of the description of the current invention, the double cylinder method is adopted, without affecting the generality.

The pressure / flow-rate conversion unit (60) in each one of its steps and referring to Figure 2 includes:

- The pair of cylinders (61), including cylinder (611) and cylinder (612). Each one of cylinders (61 1) and (612) includes the cylinder vessel (or body) (62), the piston (63), the rod (64) and the inlet / outlet nozzles of two parts of the internal volume of the cylinder, separated by the piston (63). Optionally, more than one cylinder pairs may be included in each step.

- The flow control valves (65), (66), (67) and (68). Such valves may be of any type.

The type of valves seen on figure 2 is just indicative, and the design form has been borrowed from the power hydraulics design, without affecting the generality. Four valves are shown on Figure 2: Two for the HCSS stream flow control to and from the first cylinder and the second cylinder respectively, and another two for the SWD stream flow control from and to the first and the second cylinder respectively. However, depending on the design and the valve type, less (or more) than four valves could be also incorporated.

- Instrumentation and other control devices, not shown on Figure 2.

For each step of the pressure / flow-rate conversion unit (60), (for example step No "k"), referring to Figure 2: The HCSS stream leaves the pressure vessel (42) of the given step (step No "k") of the pressur ^' ization unit (40) and through the pipe (47) and the valve (65) enters into the right part of the first cylinder (611) and, been under high pressure, forces the piston (63) of the cylinder (611) to move to the left, displacing (also under pressure) the SWD contained in the left part of the cylinder (611). Such SWD stream, finds its way through valve (67) and the pipe (23) into the pressure vessel (22) of the said step (step No "k") of the desalination unit (20). Said HCSS stream is not possible to reach to the second cylinder (612), since it is blocked by valve (66). In the same way, the SWD stream is not possible to reach to the second cylinder (612), since it is blocked by valve (68). Meanwhile, The SWD stream leaves the pressure vessel (22) of the next step (step No "(k+1)") of the desalination unit (20) and through the pipe (27) and the valve (68) enters into the left part of the second cylinder (612) and, been under pressure, forces its piston (63) to move to the right, displacing (also under pressure) the HCSS contained in the right part of the cylinder (612). Such HCSS stream, finds its way through valve (66) and the pipe (44) into the pressure vessel (22) of the next step (step No "(k+1)") of the pressurization unit (40). Said SWD stream is not possible to reach to the first cylinder (611), since it is blocked by valve (67). In the same way, the HCSS stream is not possible to reach to the first cylinder (611), since it is blocked by valve (65). When the pistons of the cylinders reach to their limit point, the four valves change position, and, in this way, the first cylinder (611) and second cylinder (612) change roles, ensuring the operation continuity of the two cylinder block.

An additional cylinder pair (70) is incorporated in the said pressure / displacement conversion unit (60), before the first step, functioning in a similar way with the other cylinder pairs of said pressure / displacement conversion unit (60). Purpose of this additional cylinder pair is to use the SWD exiting from the first step of the desalination unit under high pressure, in order to pressurize the HCSS and force it to enter into the pressure vessel (42) of the first step of the pressurization unit (40). While one cylinder is operating in the way described above, the other cylinder is being filled up with HCSS under low pressure, coming from the filtering / treatment unit (48), via the booster pump (71) and SWD is displaced by the said other cylinder under low pressure and is rejected back to the sea, or brackish water pond, via the booster pump (72).

For each step and for each one of the two cylinders (611 / 612) of any step of the pressure / flow rate conversion unit (60) and also the supplementary pair of cylinders (70), the cross- section area of the two parts of the internal volume of the cylinder which are separated by the piston (63) may be the same, meaning that there is no rod and the piston is forced to move by the pressure applied to it. In this way, the flow-rate of the stream entering into one part of the cylinder is the same with the flow-rate of the stream leaving out from the other part of the cylinder. However, most preferably, said cross-section areas are not the same, because of the rod, meaning that the flow-rate of the stream entering into one part of the cylinder is not the same with the flow-rate of the stream leaving out from the other part of the cylinder. In this way, the pressure / flow rate unit (60) does not function simply as a pressure transmitter from the HCSS stream to the SWD stream, but it optimizes the pressure and flow rate ratios of said streams, targeting to the final cost minimization of the plant. The ratio of the flow rates of the inlet and outlet streams may be adjusted, by selecting the appropriate ratio of the rod diameter to the internal cylinder body diameter. When a cylinder of a step (for example step No "k") is in operation, it receives a HCSS stream under high pressure from the pressurization unit (40) into its part having the lower cross - section area, (which is the one with the rod) forcing its piston (63) to move and pressurize a SWD stream contained into the other part of the cylinder having the higher cross - section area, (the one without the rod), to the desalination unit (20). The following simple formula is applied: PHCSS * FHCSS ^* n = PSWD * FSWD, where:

- n is the efficiency factor, which is generally very high, (very proximate to the value of "1"), since the losses are extremely low.

- PHCSS and FHCSS are the pressure and the flow-rate respectively, of the HCSS stream entering into the cylinder. The following formula applies: PHCSS = (POSMHCSS - POSMSWP) - DPHCSS, where:

POS HCSS is the osmotic pressure of the HCSS stream at the first side of the semi-permeable membrane of the pressure vessel (42) of said step (step No "k") of the pressurization unit (40), for the given conditions (salinity, temperature, kind of salt dissolved, etc) of HCSS in said step (step No "k").

POSMSWP is the osmotic pressure of the SWP stream at the second side of the semi-permeable membrane of the pressure vessel (42) of said step (step No "k") of the pressurization unit (40), for the given conditions (salinity, temperature, kind of salt dissolved, etc) of SWD of said step (step No "k").

DPHCSS is the pressure drop of the HCSS stream in the piping, the valves, the cylinders etc of the pressurization unit (40) and the pressure / flow rate conversion unit (60), as well as of the water flowing through the semi-permeable membranes contained in the pressure vessels (42) of the pressurization unit (40). Said water flowing through the semi-permeable membranes, comes from the SWP stream (been at the one side of the semi - permeable membranes) and it is absorbed by the HCSS stream (been at the other side of the semi - permeable membrane), through said semi - permeable membranes. High value of DPHCSS results to low value of PHCSS.

- Pswo and FSWD are the pressure and the flow-rate respectively, of the SWD stream entering exiting from the cylinder. The following formula applies: PSWD = POSMSWD + DPSWD, where:

PoswiswD is the osmotic pressure of the SWD stream been at the first side of the semi-permeable membrane of the pressure vessel (22) of said step (step No "k") of the desalination unit (20), for the given conditions (salinity, temperature, kind of salt dissolved, etc) of SWD in said step (step No "k").

DPSWD is the pressure drop of the SWD stream in the piping, the valves, the cylinders etc of the desalination unit (20) and the pressure / flow rate conversion unit (60), as well as of the water flowing through the semi-permeable membranes contained in the pressure vessels (22) of the desalination unit (20) and is drained to the desalinated water tank (28). High value of DPSWD results to high value of

The above mentioned description applies to the operation cases usually met, meaning where the pressure of the HCSS stream is higher than the pressure of SWD stream, meaning that the difference between the HCSS stream concentration and the SWP stream concentration is relatively high and the concentration of the SWD stream relatively low. However, few cases may be met, where the HCSS stream pressure is lower than the SWD stream pressure. In this case, pressure / flow-rate conversion unit (60) operates in the reverse way (than the one described in the above), in order to increase the SWD stream pressure to the level required for the desalination process. More specifically, the HCSS stream is driven to the part of the cylinder (611) / (612) with the higher cross-section area (the one without the rod) and the SWD stream is driven to the part of the cylinder (611) / (612) with the lower cross-section area (the one with the rod), and the flow-rate of the SWD stream exiting from the cylinder is lower compared with the flow-rate of the HCSS stream entering into the cylinder.

The aforesaid process can be implemented not only using cylinder pairs (61), but in many other ways, such as using a turbine / pump pair (turbine for the high concentration salt solution stream (HCSS) and pump for the seawater stream (SWD)). In each step, the ratio of the f!ow-rate of the high concentration salt solution stream (HCSS) within the turbine to the flow-rate of seawater stream (SWD) into the pump is the one required for the operation of the particular step of the desalination unit (20) and is achieved either by selecting a turbine and pump with a suitable displacement ratio coupled to a common shaft, or by selecting a turbine and a pump with not necessarily associated displacements but coupled via a suitable transmission ratio gear box. Alternatively, any other transmission device may be used, such as pulleys and belts with suitable transmission ratio.

The DPHCSS and DPSWD figures as well as their possible variation in each step of the desalination (20) and the pressurization (40) units are related to the type and the properties of the semi-permeable membranes used, as well as to other design issues. Also, in the design of the installation, in each step of the desalination (20) and pressurization (40) units, the functional characteristics of the HCSS, SWD and SWP solutions (concentrations, flow- rates, etc.) are selected appropriately so that the final technical-economic results are optimized. It is possible that, in one or more steps of the desalination (20) and pressurization (40) units, the pressure of the SWD stream coming out from the next stage of the desalination unit (20) and entering into the left part of the cylinder (612) of a given step is not high enough to force the piston (63) to move and displace the HCSS contained in the right side of said cylinder (612), to the next stage of the pressurization unit (40). Additional power is required, for this piston movement. This additional required power is provided by an external source and is preferably applied directly to the rod (64) of said cylinder (612) by means of suitable equipment, such as a toothed rule mounted on the rod and a gear coupled to it. Said gear is connected to the shaft of a motor via a suitable transmission ratio gearbox. Said motor is powered by an external energy source. The above applies to the supplementary pair of cylinders (70).

Furthermore, It is possible that, in one or more steps of the desalination (20) and pressurization (40) units, the pressure of the SWD stream coming out from the next stage of the desalination unit (20) and entering into the left part of the cylinder (612) of a given step is much higher than the one required for the piston (63) movement and the displacement of the HCSS contained in the right side of said cylinder (612) to the next stage of the pressurization unit (40), meaning that a surplus of energy is observed. This surplus energy is extracted for any use, via a suitable equipment, such as toothed rules mounted on the cylinder rods and gears coupled to them. Said gears are connected to the relevant shafts of electric generators or hydraulic pumps via suitable transmission ratio gearboxes. The above apply also to the supplementary pair of cylinders (70).

What is usually expected (referring to the above mentioned) is to observe surplus power in some steps of the unit and shortfall power in some others. The preferred approach for arranging the above is the interconnection of all steps in any way, such as electrical, hydraulic, mechanical, etc, or a combination of the foregoing. If the electrical interface is selected, then the respective rod of each cylinder of each step of the pressure / flow rate conversion unit (60) is connected (for example by means of a toothed rule, a gear and the relevant rotation speed adjustment device) with a small generator / motor which is electrically connected to the respective generators / motors of the cylinders of the other steps of the pressure / flow rate conversion unit (60), using the suitable electrical equipment and adapters. In this way, the generators / motors of the steps with excess power act as generators giving power to the system, while the generators / motors of the steps with a power shortage act as motors, drawing power from the system. The above also applies to the additional pair of cylinders (70). Depending on the design parameters, the total power balance of all steps is usually not equal to zero, so, a (relatively small) total deficit or surplus is usually observed. In the case of a deficit, this is covered by an additional motor powered by external power. In the case of a surplus, this surplus is extracted and preferably used in order to cover auxiliary needs of the desalination plant. Instead of the above mentioned electrical interface, a hydraulic interface can be used and its operation is equivalent to that of electrical interface, where instead of generators / electric motors, hydraulic pumps / hydraulic motors are used at each cylinder of each step of the pressure / flow rate conversion unit (60), connected via appropriate piping and other relative equipment to a small hydraulic power unit including the necessary flow control equipment. If a mechanical interface is chosen, the corresponding rod of each cylinder of each step of the pressure / flow rate conversion unit (60) is provided with a toothed rule and a gear that is engaged to it and also it is mechanically connected by means of one or more gears of a suitable gear ratio to the corresponding axles of the gears the other cylinders of the pressure / flow rate conversion unit (60). The combination of two or more of the aforementioned modes can be applied, for example in the following way: The two toothed rules of the rods of the two cylinders (611), (612) of a pair of cylinders (61) are bridged with a gear which rotates around a fixed axis and is engaged to said toothed rods, so that the movement of the one rod is equal and opposite to the movement of the other rod. The foregoing applies to all the pairs of cylinders (61) of all steps and all of said gears of all steps are connected to each other, either electrically, or hydraulically or mechanically, as described above. The aforesaid additional power required to cover the total shortfall power of the interconnected cylinder rods (64) of the pairs of cylinders (61), (or of the turbine / pump pairs), may be conventional, such as electrical power, or preferably derived from an additional equipment provided for this purpose and not shown in figures 1 and 2. This additional equipment includes a pressurization sub-unit which is supplied by both a HCSS stream, coming either from the lake (10) or from anywhere else, and a SWP stream coming from the sea or from anywhere else. Said pressurization sub-unit operates in a way similar to that of the pressurization unit (40), providing pressurized HCSS stream under pressure, which is driven to a turbine coupled to a generator or a hydraulic pump, or a mechanical power transmission device and produces energy which is appropriately distributed to the cylinders (61) of the pressure / flow rate conversion unit (60) (or the turbines / pumps) in order to cover the corresponding power shortage.

An additional HCSS stream may be supplied not only to the first step of the pressurization unit (40) but also to one or more of its other steps, being mixed with the HCSS stream which enters in each given step coming from the upstream step, possibly after a portion of the HCSS stream is removed before said mixture. In this way, in each given step the HCSS stream concentration is increased, and therefore, the osmotic pressure of the HCSS stream is also increased not only in this given step but also in the downstream steps, compared with the relevant figures in case that said additional HCSS stream was not supplied.

Regardless of whether an additional high HCSS stream may be fed to subsequent steps of the pressurization unit (40) as mentioned above or not, an additional stream of seawater may be fed not only to the latter step but also to one or more of the upstream steps of either the pressurization unit (40) or of the desalination unit (20) respectively, or of both of them, being mixed with the seawater SWP stream or the SWD stream entering in the relevant steps of the pressurization unit (40) and / or the desalination unit (20) respectively. A portion of the SWD stream and / or the SWP stream may be removed prior to the respective mixing. This reduces the concentration and hence the osmotic pressure of the SWD stream and / or the SWP stream in the particular step, but also in the downstream steps, compared to the figures that would apply if there was no such addition to the SWD and / or the SWP stream.