IL44799A - Method and apparatus for generating power - Google Patents

Description

, METHOD AND APPARATUS FOR GENERATING POWER The present application is directed to improvements -in or modifications to the method and apparatus for generating power described in our Israel Patent Application 42658 filed July 3 , 1973 , and is for a Patent of Addition to that application.

Our Israel Patent Application No. 42658 describes a number of arrangements employing Pressure-Retarded-Osmosis (PRO) for the production of useful energy from certain naturally available energy sources including solar energy.

The present invention is directed to improving or modifying such PRO power plants to enable them to be generally applicable for the production of economical power from solar energy or other energy sources.

The basio Pressure-Retarded-Osmosis (PRO) method and apparatus for generating power described in Patent Application 42658 is generally characterized by the steps of applying a hydraulic pressure to a first, liquid of a first osmotic pressure and introducing same into a first pathway which is at least partially defined by one face of a semipermeable' membrane; introducing a second liquid having a lower hydraulic pressure and a lower osmotic pressure into a second pathway which is at least partially defined by the opposite face of the membrane; maintaining the hydraulic pressure difference between liquids on the opposite faces of the membrane at a pressure difference which is less than the osmotic pressure difference between the liquids, at every point in the two pathways, thws effecting by Pressure-Retarded-Osmosis a passage of at least part of the second liquid through the semipermeable membrane, forming a pressurized mixed solution of greater volume than said first liquid introduced into said first pathway; and converting the potential energy-stored in the pressurized mixed solution to ^useful energy such as electrical or mechanical energy.

The basic PRO apparatus as described in Patent Application 42658 is illustrated in Pig. 3 of the present application.

This apparatus is used in the various systems illustrated in the present application and is hereinafter referred to as the PRO section.

In the previous Application No. 42658, a number of power generating systems were described illustrating the PRO approach. However, the described systems have a number of limitations.

One limitation is that such systems could be used, as a practical matter, only in certain areas of the world.

The only osmotic power plant scheme in the previous application which could be considered to approach general applicability for production of economic power was that of Figure 9 in that application, in which water evaporation ponds were used for reooncentration of the mixed solution, i.e.. brine diluted in the Pressure-Retarded-Osmosis (PRO) unit. However, as ^ will be discussed later, the area required for evaporation or distillation would be in the order of 1,000 square kilometers for a 1,000 megawatt power plant utilizing evaporation ponds. This area is so large that it would restrict the use of osmotic power plants to areas which in effect already process evaporation ponds, such as the Dead Sea and Great Salt Lake.

Another limitation on the general use of osmotic power plants according to the previous patent application is that an expendable source of a low-osmotic pressure aqueous solution, such as ocean water or a river, must be available in the region of the evaporation ponds. Large quantities of this solution are expended. For example, in the aforementioned 1,000 megawatt plant using evaporation ponSs, a daily quantity of 4,300,000 cubic meters of, water of low osmotic pressure aqueous solution must be expended. Even if large quantities of such solutions are available such as in sea water, charges for pumping, filtering, etc., may be exorbitant. Some of these costs may be partially recuperated if the water from the evaporation ponds is recovaed, as indicated in the previous patent application. It is clear that such water would be fresh and would be useful for disposal in areas requirin fresh water supplies if the low osmotic pressure solution were sea water or other ¾sol ion too saline to drink.

However, the forms of the invention therein disclosed would still be restricted, i.e. would not be of general applicability for the production of economical power from solar energy or other sources.

The present application is directed- to making the technique of Patent Application 42658 of general applicability by eliminating both of the above limitations, specifically ■ by enabling the solar energy collecting area to be greatly ' reduced,, and by obviating the need of expendable material altogether.

According to one aspect of the invention, therefore, there is provided.a method of generating power compris|ngi applying a hydraulic pressure to a first liquid of a first osmotic pressure and introducing same Anto a first pathway which is at least partially defined by one face of a semipermeable membrane; introducing a second liquid having a lower hydraulic pressure and a lower osmotic pressure into a seoond pathway which is a least partially defined by the opposite face o the membrane; maintaining the hydraulic pressure difference between liquids on the opposite, faces, of the membrane at a pressure difference which is less than the osmotic pressure difference between the liquids,, at every point in the two pathways, thus Effecting by fressure-Retarded-Qsmosis a passage of at least part of the second liquid through the semipermeable membrane, forming a pressurized mixed solution of greater volume than said first liquid introduced into said first' pathway; convertin the potential energy stared in the pressurized mixed solution to useful energy such as electrical or mechanical energy; recovering the first and second liquids by separating from said mixed solution a quantity of second liquid substantially equal to the quantity which passed through the membrane and mixed with the first liquid; restoring substantially the original temperatures to the recovered, first and second liquids; reapplying the above mentioned hydraulic pressure difference between the recovered first and second liquids; and recycling the recovered first and second liquids through the first and second pathways respectively.

According to another aspect, there is provided a heat engine for generating power from heat comprising: a semi-permeable membrane; means for applying a hydraulic pressure to a first liquid of a first osmotic pressure and introducing same into a first pathway which is at least partially defined by one. face of the semipermeable membrane; means for introducing a second liquid having a lower hydraulic pressure and a lower osmotic pressure into a second pathway which is at. least partially defined by the opposite face of the membrane; means for maintaining the hydraulic pressure difference between liquids on the opposite faces of: the membrane at a* pressure difference hich is less than the osmotic pressure, difference between the liquids,; at every point in the two pathways, thus effecting by Pressure-Retarded-Osmosis a passage of at least part of the seoond liquid through the semipermeable membrane, forming a pressurized mixed solution of greater volume than said first liquid introduced into said first pathway;' means for converting the potential energy stored in the -pressurized mixed solution to useful energy such as electrical or mechanical energy; separating; means for recovering the first and second liquids by separating from said mixed solution a quantity of second liquid substantial equal to the quantity which passed through the membrane arid mixed with the first liquid; temperature restoral means for restoring substantially the original temperatures to the recovered first and second liquids; means for reapplying the above mentioned hydraulic pressure difference between the recovered first and second for reoycling the recovered firsthand second liquids through the first and second pathways respectively.

The invention is herein described, somewhat diagrammatically, and by way of example only, with reference to the accompanying drawings, whereinί Fig. 1 is a block diagram illustrating the generalized concept of a PRO heat engine for generating power.

Fig. 2 is a block diagram of a PRO heat engine constructed in accordance with the Carnot 6ycie> utilizing solar energy as a heat source; Fig, 3 is a block, diagram illustrating-la^portionvof a PRO heat engine constructed in accordance with the invention, Fig. 3 being essentially the Pressure-Retarded-Osmosis (PRO) section described in Application 42658; Pig, 4 is a block diagram illustrating a PRO heat engine using distillation in the thermal unmixin section; Pig, 5 is a block: diagram illustrating a PRO heat engine with a' distillation plant usin dichlorodifluoromethane as the solvent; F'Ug. 6a illustrates a methanol-hexane binary liquid system whose miscibility, is a function of temperature , this system being useful in effecting the thermal separation of a mixed solution containing these two .species into, a diluted and a concentrated solution respectively; Pig. 6b illustrates a trlethylamine-water binary liquid system whose miscibility is also a function of temperature, and could therefore be useful in effecting the thermal . separation .,. Pig. 7 is a block diagram illustrating a PRO heat engine using a Pig. 6*¾?Sary liquid circulation system (methanol-hexane) ; and .

Pig, 8 is a block diagram of a PRO heat engine' using a solution containing a solute whose solubility is a function of temperature.

The new system is shown in its most generalized form in Pig, 1, It consists of two sections. First, there is the pressure-retarded osmosis (PRO) section 102, which delivers work available from the free energy decrease occurring during mixing of a diluted and a concentrated solution. Second, there is the thermal unmixing section 104, in which heat absorption and rejection supplies energy for free energy- recovery, i.e. unmixing and temperature restoral to the two solutions after which they, are returned via lines 106, 108 to the PRO section 102, thus' completing the cycle. The thermal unmixing process can involve intermediate gaseous or solid phases in the course of restoring the original solutions or can be conducted so that the original liquid solutions are direotly produced. . 1 The above description fits in every respect the definition of a heat engine, i.e. a man-made device which makes it possible for a working substance to undergo a cyclic process in the conversion of heat into work. Thus the invention is believed to represent the first use of any osmotically-operated power process as the work producing tfomponent of a heat engine. The invention is also believed to represent the first use of pressure-retarded osmosis as the workfproducing component of a heat engine.

The heat source, diagrammatically indicated by arrow 110 in Fig. 1, for effecting the thermal unmixing in section 104, could theoretically be any/ energy source such as fossil fuels or nuclear energy. However, because of the dwindling energy resources from fossil fuel, only solar energy will hereinafter be mentioned.

With regard to solar energy collectors, ' several promising types exist., These include solar ponds, i.e. ponds in* which the solar energy maintains a gradation in temperature by means of a gradation in salt concentration, and solar heating devices such as those incorporating selective coatings for trapping solar energy in the form of heat. With both of these types the heat can be extracted by the passage through the unit of a. heat-transfer fluid. Solar distillation plants ■ are also energy collectors. In addition to this service they also perform the unmixing functions.

The heat sink into which the heat from the thermal unmixing section 104 is rejected as Indicated by arrow 112, is preferably of the liquid type such as,.sea water or river, water. However ambient air can also serve as an appropriate heat sink. It is assumed Eierein that any of these potential sinks can provide a heat rejecting temperature not exceeding 25°C.

In Pig. 1, the net work output from the PRO section 102 is indioated by arrow 114 and block 116, the mechanical and hydraulic losses from the PRO section being passed to the .heat sink as indioated by arrow 118. . ; i Pig. 2, in which the parts corresponding to Pig. 1 are correspondingly numbered, shows an idealized PRO heat engine operating between temperature limits of 100° and 25° centigrade. ' It is assumed that its heat source is a solar energy collector 120.which loses half the splar energy inoident upon it. This PRO engine is subject to the maximum efficiency imposed by the law of Carnot: Emax β 00 < ° (1-·8* ( 00) a 209δ where 273° is absolute temperature at 0° centigrade.

Based on this limitation and the above assumptions it is seen that, of 100 units of energy incident on the solar energy colleotor , a maximum of θ units can be obtained as useful work. The useful work obtained from an actual PRO engine must be measured against this criterion; i.e. the useful work from the PRO engine will* always be less than, but should approach as.closely as possible to, 10 percent of the energy incident o the solar energy collector.

Because the PRO heat engine obtains its work from the free energ decrease during mixing of the solutions, the maximum work obtainable, i.e. the Garnot work, is equal in- magnitude to this decrease in free energy.

Following are a number of important advantages of the PRO heat engine: ( 1 ) By making pressure retarded osmosis the work-producing process of a heat engine,' the working liquids undergo a cyclic prcQBBs and are therefore not expended at all. The production of useful energy by PRO becomes of general applicability, requiring only the availability of solar or other energy source and no material resource expenditure.

This extends the usefulness of the technique of the above-cited patent specification in which applicability of the systems therein described as examples was limited to locations having an expendable source of low osmotic pressure solution such as river water, brackish water, or sea water. . ' (2) The PRO heat engine can be ef icient in the conversion of energy to work. As will be shown, the thermal efficiency of the PRO engine can be in the order of at least 60 percent of that thermodynamically possible. This means that when operating between the limitations. of the previous section 100° to 25° centigrade, and with 50{8 losses in the solar energy,, collector, the PRO heat engine will convert (.6) (10) = 6 percent of the incident solar energy to $useful$work.

This thermal efficiency will enable the solar collector area to be drastically reduced in¾yc6mparison to what is required with the evaporation ponds described as examples . in the above-cited patent application. For a 1,000 megawatt PRO heat engine this cycle need be only in the order of 60-70 kilometers as compared with about 1,000 sqiiiare kilometers for' evaporation po&ds. (3) The PRO heat engine possesses practical advantages over existing heat engines. Most existin heat engines utilize the vapor power cycle. In such a cycle, a working fluid under pressure is vaporized by addition of heat from a high temperature heat source. It then does work of expansion in a turbine or engine, af eisgwhich it is condensed by heat removal to a low temperature heat sink. The liquid is compressed to the original pressure, thus completing the cycle.

Because of the phase changes occurring in the vapor, power cyole, it must be conducted within limits which add to the cost of necessar equipment. For example the expansion of saturated steam in a heat engine is accompanied by partial condensation to liquid water. However, this liquid content cannot exceed 10 or 12 percent because of excessive wear on turbine blades or engine pistons. Thus the expansion of the steam is limited. This limitation can be minimized by super-heating the tearn prior to expansion, but this adds to the cost of the equipment, and for the low heat source temperatures considered herein of 100°C, no additional efficiency is gained.

An additional limitation of the vapor power cycle, using steam lies in the fact that, all of the vapor must be finally condensed. This means that at the heat rejection temperature of 25°C, a vacuum must be maintained in the condenser, including means for continuous removal of air which might leak in through pump seals, etc.

The PRO heat engine overcomes the above limitations of the vapor power cycle because no vapor exists during the power production part of the cycle (and indeed need not exist in, any part of the cycle, as will, be seen). (4) Another very important advantage of the PRO heat engine is that it provides, a liquid under high hydraulic pressure as the energy produqing fluid. By this means the engine can utilize a high pressure hydroturbine. The word is used here to mean any liquid-driven turbine , in contrast to a steam turbine or other turbine driven by gas or vapors other than a steam turbine. Hydroturbines are more efficient thattft steaw turbines or other vapor turbines. Furthermore, a hydroturbine is inherently safer since at high pressures much less energy is stored in a liquid than in a gas.

Pig, 3 illustrates; the . PRO section 102 of the heat engine, and Pigs. 4-8 illustrate different arrangements which may be used for the thermal unmixing section 104.

The pressure-retarded osmosis (PRO) section, consisting of a pump 22J. the membrane unit 124, and a hydroturbine 126, is siiown in Pig. 3 as it would operate under ideal conditions. A concentrated solution* by which is meant one having a high osmotic pressure ^igh^' and having a volume of V cubic meters by .fspump 122 to a hydraulic pressure P atmospheres (atm) requiring a work input of PV cubic meter atmospheres (rn^ atm), after, which it is pumped via line 127 into the high pressure side of the membrane unit 124.

Simultaneously a diluted solution, by which is meant one having a. low osmotic , pressure, (#"low).> and having a volume of AVfmll is pumped (by a pump not shown) via line 128 into the low hydraulic pressure side of the membrane unit 124. The diluted solution permeates through the membranes against the hydraulic pressure P because it is arranged that everywhere in the unit P< 'ilP where ΔΡ is the osmotic pressure difference (atm) between the solutions on each side of the membrane.

This is the fundamental principle of pressure-retarded osmosis, - as described in the above-cited patent application..

A volume (V + AV) ra3 of mixed solution is sent to hydroturbine 126 at the pressure P atm. Thus the hydroturbine delivers P (V + LV) m^.atm of work in the course of reducin the pressure of the mixed solution to zero. The net outpu -of work is equal to the difference between the output from the hydroturbine and the input to the pump, i.e. the net work is ΡΔ» f B3 atm.

It is important to understand that net work is obtained only from AV, the volume of permeat liquid passing through the membranes. In order to minimize the size of the memfcrarie unit it may be stated as a. first guideline: Guic&inel. the ratio should be maximized of net work delivered to volume of liquid passed through the membranes.

This is accomplished by using a, high hydraulic pressure. However^P must be less tha ΔΡ everywhere in the unit, as described above, and the minimum ΔΡ occurs between the diluted solution and the mixed solution. Therefore it follows as a corollary to Guideline 1 that the osmotic pressure difference between the mixed solution and the diluted solution should be high.

It should be realized that Guid¾Line 1 is also appropriate for the thermal , unmixing section (104, Pigs. 1 and 2). If · the ratio is high of net work delivered to viume of permeate passed through the membranes, then less mixed solution must be separated by the thermal unmixing techniques.

Thermal unmixing echniques may he divided into several categories depending on the nature of the intermediate phases employed in the unmixing. Thus it is., possible to utilize 1 vapor and solid intermediate phase as well as to divide the mixed solution directl into diluted and concentrated solutions.

The best thermal unmixing technique would seem to be that which minimizes the thermal energy input requirement.

A guideline f'or carrying out this requirement can be gleaned from Fig. 2. Since this is a Carnot cycle, the thermal efficiency is maximized. Therefore the ratio of thermal energy input to net work is minimized. Since the net work is equal to the free energy decrease, it is possible^ to say that the ratio of thermal energy input to free energy decrease is also minimized. This value is 50/10 = 5 for the temperature chosen. In any actual plant, the ratio will be higher ¾>ut this value of five may be considered as a target at which to aim.

Therefore it may be stated as a second guideline: Guideline 2 : within thermodynamic limitations, the ratio of thermal energy input to free energy decrease should be minim zed in the PRO heat engine. · ' The use of distillation as the thermal unmixing process is shown in general terms in the heat engine of Fig. 4 , parts corresponding to those in Fig. 3 being correspondingly numbered. The distillation plant 130 , divides the unpressurised mixed solution, from input 1 32 into a first output «134 of V m^ unpressurised concentrated solution having a high value of Ejosmotic pressure (ΙΓ^^)» ,and a second output "stream 136 of a diluted solution in the form of a vapor. The concentrated solution in|istream 134 is joined to the PRO section of Big. 3 at the pump 122. Meanwhile, the diluted solution vapor in .stream 136 is condensed in condenser I38 and coo&ed, thus rejecting part of the incoming thermal energy to the heat sink as shown by arrow 140, after which the diluted solution, having a volume AV, is joined via line ,128 to the PRO section at the low pressure side of the membrane unit. The remainder of the cycle is as described above in the discussion on the PRO section, as a result of which the unpressurized mixed solution of Fig. 3 is sent to the distillation plant for thermal unmixing, and the cycle completed.; The heat for .the distillation plant is supplied from a solar energy collector 142.

Water is a poor choice as a solvent in a distillation plant; in the thermal unmixing section of a PRO heat engine.

Specifically is fails badly to meet Guidi¾ine..2 because of its very high volumetric cbatent of vaporization of 580 cal/cm^.

Even with concentrated salt solutions for which the free energy of separation will be in the- order of 5 cal/cm-', the ratio of heat input to free energy of. separation will be in the order of <*TJ°- = 120.. This factor would increase the solar collector area requirement for a 1000 megawatt plant from the theoretical n nimum&value of about 4 km (assuming an average isolation flux of 520 calories per day on each square centimeter) to 500 km , ,if water were used in a simple distillation process such as solar distillation. If it i& ore realistically assumed that the solar collector efficiency is about 50 percent, 1000 km 2 of solar collector area would be required. Other losses might increase the area requirement further.' The second guideline may in principle he. approached more closely inidistillation plant b employing the use of "heat multiplying" distillation plants such as.multiple effect or multistage flash distillation plants. In these the . arrangement is such that one kilogram of steam (or its thermal equivalent) entering .from the heat source is capable of vaporising, say 10.pounds of steam, thus increasing the efficiency of the process 10-fold. However there is a limitation on the heat multiplying capability. In each effect or stage' of the plant, the vapor condenses at the boiling water of pure water characteristic of the pressure in the effect. However, the concentrated brine in the same effect boils at a higher temperature because of its salt content. This phenomenon is known as boiling ^jaSint elevation (BPE). Its, effect is cumulative from effect-to-effect and the possible heat multiplying capability of the plant is limited b . the, available source-to-sink temperature such that: Max. dist. plant heat multiplying « Source_temp. ¾ Sink temp. capability ! BPE Now we know that boiling pcfeit, elevation increases directly with osmotic pressure difference such that we may say: Max. dist. plant heat (Source temp. - sink temp.) multiplying capability osmotic pressure difference However it was stated as a coroi ary to the first design guideline that the osmotio pressure difference between the mixed solution and the diluted solution should be as high as possible. The complete implementation of this guideline would reduce the heat multiplying capability of the plant drastically. This capability is further reduced by the fact that for solar energy heat sources discussed herein, the difference between source and sink emperature is low.

Therefore a distillation plant to be described subsequently is not of the heat multiplying type, and it is explained why such a plant would not be feasible with solar collectors as the energy source. However it is recognized that heat-multiplying distillation plants might be efficacio.us under other conditions* The disadvantage of water as the solvent phase in a PRO heat ^engine may be overcome by using a solvent with a low value of volumetric latent heat of vaporization.

Virtually all liquids have a volumetric latent heat of vaporization lower than water. Some of these such^as the halogenated organic compounds, and especially those containing fluorine, are exceptional in this regard. For example dicHIorodifluoromethanei C ClgFg, a commercially available refrigerant ("Freon-12") has a volumetric lament heat of vaporization of only 44 cal/cm^. By the use of such materials Guideline 2 is approached much more closely than with water. From a practical standpoint the use of Freon-12 means that the solar energy collecting area can be an order of magnitude less in area than if water is used, ail other things being the same.

Fig. 5 (parts corresponding to Fig. 4 being correspondingly numbered) illustrates a PRO heat engine with a distillation plant using C C?. Fg» dichlorodifluoromethane , , as the solvent in the dilute solution. For this calculation the following conditions and/or assumptions were utilized: ( 1 ) A relatively non-volatile solute of molecular weight 46 was used; e.g. Ethanol meets,this requirement. (2) The osmotic' pressure of the mixed/solution was 275 atmospheres. This value is sufficiently high that a high hydraulic pressure in the order of 255 atm can be used, according to Guideline 1. (3) The efficiencies of the solar collector, the-pressurizing pump, and the hydroturbine were assumed to be 0$ (thermal), 9 $ (mechanical), and 95 e (mechanical), respectively. (4) It was assumed that' there was a 2 atmosphere pressure drop due to hydraulic friction in any unit of the apparatus. (5) Raoult*3 Law was assumed to apply to all solutions. (6) The system was operated so that 1 volume of concentrated solution was diluted with 0*4 volume of solvent, C C12P2. · . . (7) The density of all solutions was assumed to be one. (8) The temperature of the condensate leaving the condenser was 25°C.

Based on these assumptions the operating conditions of Pig. 5 !gere obtained; Of special interest is the requirement that the concentrated solution have a temperature of 94°C, as compared with the condensate temperature of 25°C.

The temperature of 94°C is due chiefly to two requirements first, the mpl fraction of CC12F2 must be low in the. concentrated solution in order to provide a high enough osmotic pressure to the concentrated solution^ second, the partial pressure of CCTgPg in the concentrated solution must be at least equal to the partial pressure of pure CC12F2 in the condensate at 25°C in order to condense this solvent.

Even with the use of a single-effect distillation plant, the temperature of 94°C is approaching the assumed, upper temperature limit from a solar collector. For this reason it is clear why heat multiplying plants may not be feasible with solar collectors. The total temperature difference must be at least the sum of the minimum permissible temperature differences (boiling point elevation) in each stage, and this sum might exceed the temperature difference possible with a solar energy collector.

Of most interest are the figures showing the distribution of energy, based on 100 units of solar energy entering the solar collector, it is seen that the net work is 4.5 units, i.e. the overall' efficiency is 4.5 percent. This means that a PRO heat engine Using CC12F2 in raster would require 2 4/.045 = 90 km area for a 1000 megawatt plant, as compared 2 with about 1000 km for a plant using water as the solvent.

As stated above, Freon 12 is an exceptional material.

It may be one of the relatively few solvents with a latent heat of vaporization sufficiently low to overcome the basic , deficiency when distillation is used as the thermal unmixing te'fchnique, namely that the lata* heat of vaporization is usually too high in comparison to the free energy of separation.

There may also be used a thermal unmixing system involving separation into two. liquid phases, a technique which should be inherently more efficient than distillation since no latent heat of vaporization is involved. ■ .' ■ A number of binary liquid systems are distinguished by the fac that their mutual solubility is a strong function of temperature. Pig. 6a shows such a system, metha ol-hexane.

- ■ A¾ove a temperature of 42. 6°C^ ; taiSwn as the upper cona e temperature, the two species are miscible in all proportions. Below this temperature, say at 25°C , the liquids are only partially miscible, and two liquid phases exist in equilibrium, a 5f° solution (by wiight) of the methanol and a 95$ solution of methanol. These two solutions are called conjugate solutions. ■ ' . · ' ■ ' "'' ' '· This behavior immediately suggests means of thermal unmixing in a PRO heat engine. Assume that a methanol-hexane solution is, 22$ methanol and at a temperature appreciably higher than 42, 6°C . The solution is coo&ed down to 25°G at which temperature the 5 and 95$ methanol solutions form, The amount of each solution will be determined by the lever arms Y and X such that: Y a weight of 5fo solution . = 100 . weight of 9??& solution These two solutions can be reheated Sbove 42. 6° to a temperature region where they are again naturally and completel miscible. At this temperature the total free energy of these two separated solutions is higher than that of the solution obtained by mixing them. (The free energy of a system decreases when it undergoes a natural process.)., Therefore the decrease in free energy upon mixing can be utilized to , produce useful energy by means. of the PRO section. After passage through the hydroturbine , the mixed solution is ready for cooling, and the cycle is completed.

The tility of this method is not limited to liquids having an upper consolute temperature. Binary systems such as triethylamine - HgO exhibit a lower consolute temperature, as can be seen in Fig. 6B. With such systems unmixing is accomplished by a temperature rise, followed by cooling of the separated liquids to bring them into the miscible range. · Next, mixing occurs in the PRO section to produce useful energy; the mixed ' solution is heated t,o separate" the concentrated and diluted solutions; and the cycle is 'completed.

The advantage over distillation of either type of such binary systems in the PRO heat engine is that the phase ¾ changes are liquid-liquid and not liquid-vapor. Thus the thermal energy input need tnot include values required by the latent heat of vaporization, but can approach much more closely to the free energy of separation in accordance with Guideline 2.

Fig. 7 (parts corresponding to those of Pig. 5 being correspondingly numbered) shows a PRO heat engine including a liquid separator, generally designated 156 which employs methanol-hexane as the liquid-system whose misibility is a function of temperature. The methanol-hexane system was chosen to ' lustrate this method because its upper consolute temperature of 4?.'6°C is between: the maximum temperature of 100°C, assumed to be the heat source temperature with a solar energy collector 142 and 25°C, assumed to be the lowest temperature to which the system can be conveniently cooled.

The following other conditions and assumptions were utilized: (1) The concentrated, solution, diluted solution, and mixed solutions have the compositions of Pig. 6a. The weight ratio of concentrated to diluted solution is also as shown in Pig. 6a, i.e. 100/40 = 2.5, and this is equivalent, for reasons of density difference, to a volume ratio of 1/0· 36 = 2.8. (2) Raoti.lt · s Law applies to all solutions in the PRO section. (3) Efficiencies of the solar- collector 142, the pump 122 and the hydroturbine 126 are as before 50, 95 and 95 percent respectively.

Based on these conditions and assumptions the conditions of Pig. 7 were obtained. The mixed solution temperature, after passing through the hydroturbine 126, is 95°C. It passes through a heat exchanger 15 where it is partially cooled to 48°C in the course of preheating the separated solutions. The mixed solution is then cooledtVto 25°C by heat rejection to the cooler (arrow 154). This causes the concentrated and diluted solutions to form. These are divided in the separator 156, after which they pass through the heat exchanger 152 where the are heated to 72°C. They then pass through the solar energy collector 142 where they are heated to 100°C. The concentrated solution is compressed, by pump 122 to 360 atm, which warms it to 124°C after which it passes into the membrane unit 124 where it absorbs the diluted ©solution, so that the temperature is reduced to 117°C. In the membrane unit, the pressure drops to 358 atm on the mixed solution after which it is depressurized through the hydroturbine 126,'(producing work, and reducing the temperature of the solution to 95°C. This completes the cycle.

Also shown in Pig* 7 are the energy distributions. We consider the solar energy incident on the collector as 100$. Half of this is lost in the solar collector, and thus 50$ goes forward to the heat engine. If this PRO engine were ideal, i.e. employed the Carnot cycle, then 10 percent of the solar energy incident on the solar collector would be available for work as shown in Pig. 2. However the cycle is not a Carnot cycle since temperature absorption and rejection are not all accomplished at the .maximum and minimum temperature respectively. Therefore it is assumed that only.9. percent of the solar, energy is availableoto supply the free energy of separation.. This.is a higher Vale than.was obtained with distillation and follows from the fac that the separation into the two solutions is accomplished Without an intermediate vapor phase, and thus no energy must be utilized to supply the latent heat of vaporization.

Of this 9 percent, 6 percent will be available for useful work and 3 percent will be unavailable due to mechanical and hydraulic losses in the circulating streams, and to the fact that some fraction of, the available free energy is lost in providing an adequate driving force in the PRO section for permeant transfer..

It is well know that , solubility of salts and other solutes is a function of temperature. This behavior in which an intermediate solid phase is produced, can be used as a basis for operation of the thermal unmixing section of a PRO heat engine. The technique can be used for solutes whose solubility either decreases or increases with temperature. This is . illustrated in Fig. 8 for a solute whose solubility increases with temperature.. The solubility characteristics of potassium nitrate are such that it could meet the solution and . precipitation requirements shown.

As shown in Pig. 8 (parts corresponding to those of Pig, 7 being correspondingly numbered),,, the unpressured mixed solution, containing 60 parts salt and 80 parts water, is 95°C after passing through the hydroturbine 126. It passes through a . . heat exchanger 152 where it is cooled to 48°C in the course oil preheating the filtrate (to be discussed) and the unpressurized concentrated solution. The mixed solution, which is now almost saturated with regard to the salts, is then cooled to 25°C by means of the cooler 160. This causes salt to precipitate. The slurry of salt and solution is sent to a filter 162 which separates the slurry into a solid phase containing 30 parts of salt and a filtrate containing 30 parts of salt and 80 parts of water. The filtrate is warmed to 72°C by passing through the heat exchanger via line 164. The hot filtrate is divided into two parts. One part, the diluted solution, containing 11 parts salt and 29 parts water, is sent via line 166 to the solar energy collector 142 for final heating. The remainder, ;I9 parts salt and 51 parts water,, is sent via line 168 to the dissolver 170 where it dissolves the' 30 parts of salt from the filter.

The solution emerging from the dissolver is the unpressurized concentrated solution, and it contains 49 parts salt and 51 parts water. It is assumed that in the dissolving process the solution temperature drops to 55°C. (The actual temperature attained will depend on the heat of solution, which can be positive or negative). The concentrated solution is passed via line 172 through the heat exohanger 152 where its temperature is raised to 72°C, the same ae that of the diluted solution, These solutions then go through the same processes in the PRO section as desoribed for partially ' miscible liquids, the cycle being competed with the unpressurized mixed solution emerging from the hydroturbine 126.

This technique for thermal unmixing has the same basic limitation as distillation, namely the changing of one of the components into a phase other than a liquid phase. The energy * required for this may be high compared to the free energy of separation. However it appears that- by a judicious choice of solute this process can be made efficient.

Many other variations and applications of the invention will be apparent.

Claims (17)

WHAT IS CLAIMED IS:,

1. A method of generating power comprising: applying a hydraulic pressure to a first liquid of a first osmotic pressure and introducing same into a first pathway which is at least partially defined by one face of a semipermeable membrane; introducing a second liquid having a lower hydrauli pressure and a lower osmotic pressure into a second pathway which is at least partially defined by the opposite face of the membrane; maintaining the hydraulic pressure difference between liquids on the opposite faces of the membrane at a pressure difference which is less than the osmotic pressure V difference between the liquids, at every point in the two pathways, thus effecting by Pressure-Retarded-Osmosis a passage of at least part of the second liquid through the semipermeable membrane, forming a pressurized mixed solution of greater volume than said first liquid introduced into said first pathway; converting the potential energy stored in the pressurized mixed solution to useful energy suoh as electrical ¾r mechanical energy; recovering the first and second liquids by separating from said mixed solution a quantity of second liquid substantially equal to the quantity which passed through the membrane and mixed with the first liquid; restoring substantially the original temperatures to the recovered first and second liquids; reapplying the above mentioned hydraulic pressure difference between the recovered first and second liquids; and recyoling the reoovered first and second liquids through the first and second pathways respectively.

2. The method according to Claim 1 wherein the reoovering of said first and second, liquids is effected by thermal separation.

3. The method according to Claim 2, wherein said thermal separation is effected by distillation.

4. The method according to Claim 2, wherein said thermal separation is effected by using as said first and second liquids two liquid species whose miscibility is a function of temperature.

5. The method according to Claim 2, wherein the thermal separation is effected by using as said first Liquid a solution of a solvent and a solute whose solubility is a function of temperature.

6. The method according to any one of Claims 2-5, wherei said thermal separation is effected by using solar energy as the energy source.

7. A heat engine for generating power from heat comprising: a semipermeable membrane; means for applying a hydraulic pressure la>,ija;jSirst liquid of a first osmotic pressure and introducin same into a first pathway which is at least partially defined by one face of the semipermeable membrane; means for introducing a second liquid having a lower hydraulic pressure and a lower osmotic pressure into a second pathway which is a least partially defined by the opposite face of the membrane means for maintaining the hydraulic,' pressure difference between liquids on the opposite faces of the membrane at a pressure difference which is less than the osmotfi1! pressure difference between the liquids, at every, point in the two pathways, thus effecting by Pressure- Retarded-Osmosis a passage of at1 least part of the second liquid through the semipermeable membrane, forming a pressurized mixed solution of greater volume than said first liquid introduced into said first pathway; means for converting the, potential energy stored i the pressurized mixed solution to, usefu energy such as electrical or mahanical energy;, separating means for recovering the first and second liquids by separating from said mixed solution a quantity of second liquid substantial equal to the, quantity which passed through the membrane and mixed with the first liquid; temperature restoral means for restoring substantially the original temperatures to the. recovered first and second liquids; means for reapplying the above mentioned hydraulic pressure difference betwee the recovered first and^ second: liquids; and means for recycling the recovered first and second liquids through the first and second pathways respectively. '

8. ; A heat engine according to Claim 7, wherein said separating means and temperature restoral means are . . thermal means.

9. A heat engine according to Claim 8, wherein said thermal separating means comprises a distillation device.

10. · A heat engine according to Claim 9, wherein both said first and second liquids include . diohlorodifluoromethane as the solvent, and the solute is a low molecular weight compound such as ethanol.

11. A he.at engine according to Claim 8, wherein the two liquids are liquid species Whose miscibility is a. function of temperature , , and wherein said thermal separating means .. comprises means for changing the temperature of the mixed solutiai r

12. A heat engine according to Claim 11, wherein one of the said liquid species is methanol and the other is hekane.

13. · A heat engine according to Claim 11, wherein one of the said liquid species is triethylamine, and the other is" water. (

14. A heat engine according to Claim 8, wherein the first liquid is a solution of a solvent and a solute whose solubility is a function of temperature, and wherein seid thermal' separating means including means utilizing said latter property to precipitate the solute.

15. The method according to Claim 14, wherein said first liquid is a solution of water and potassium nitrate-, and said second liquid is water.

16. The method of generating power substantially as described with reference to and as illustrated in any of the accompanying drawings. »

17. The heat engine for generating power substantially as described with reference to andaas illustrated in any of the accompanying drawings.