FIELD OF THE INVENTION

[0001] The present invention relates to an osmotic heat engine for converting thermal energy into mechanical work that uses a semi -permeable membrane to convert osmotic pressure into electrical power.

BACKGROUND OF THE INVENTION

[0002] Increased global demand for energy, along with widening regulation of carbon dioxide emissions, have expanded interest in renewable energy sources and improved efficiencies in fuel use. However, an important restriction regarding the adoption of new fuels and energy technologies is the cost of power produced by those means. While subsidies and other forms of artificial support may assist in the introduction of these renewable energy sources, successful displacement of traditional fuels must necessarily be driven by total energy costs.

[0003] Pressure retarded osmosis (PRO), or "salinity power" as it is often referred to, is a membrane-based osmotic pressure energy conversion process. PRO utilizes osmotic flow across a semi-permeable membrane to generate electricity. PRO processes are discussed, for example, in U.S. Patent No. 3,906,250 to Loeb, U.S. Patent No. 3,587,227 to Weingarten et al., and U.S. Patent No. 3,978,344 to Jellinek, the disclosures of which are hereby incorporated by reference herein in their entireties.

[0004] At first, locations considered suitable for using PRO technology focused on river deltas at saline water bodies such as the ocean, Dead Sea or the Great Salt Lake. At these locations, an osmotic pressure gradient exists where freshwater from a river freely mixes with seawater. The PRO process utilizes this chemical energy and converts it into electricity. In the prior art PRO processes, saline water is pressurized and placed opposite of the freshwater across a semi-permeable membrane. The osmotic pressure difference between the seawater and the freshwater (which is greater than the hydraulic pressure induced on the seawater) causes osmotic flux to occur across the membranes. As flux occurs into the pressurized seawater, the pressure is relieved by expansion through a hydroturbine (or other means), which generates electricity.

[0005] PRO processes at river deltas, also known as "open loop" PRO, have several operational and design limitations. First is the need for extensive pretreatment of feed and draw streams, similar to that required in desalination processes, to prevent fouling of process membranes and components.

[0006] Another difficulty arises from the low differential osmotic pressures found between many natural feed waters. That is, the available osmotic pressure difference is not extraordinarily high unless the saline water body is hyper-saline, such as the Dead Sea or the Great Salt Lake. Unfortunately, the volumetric flow of water into these water bodies is somewhat small and hence will yield limited power for even a well-designed PRO process. Sea water, for example, has an osmotic pressure of approximately 2.53 MPa (25 atm), which does not allow for the high hydraulic pressures that are desirable for efficient power production. In cases where higher concentration streams are considered, higher hydraulic pressures may be used, but the process efficiency will suffer significantly from internal concentration polarization (ICP), which occurs in the support structure of the membrane used for the process. This phenomenon is particularly exacerbated by the increased support layer thickness required to resist the increased hydraulic pressures enabled by more concentrated streams. A final consideration is the need to place power facilities at the interface between natural streams, often areas of considerable environmental importance, such as estuaries, wetlands, and bays.

[0007] River delta PRO also runs in an open-loop configuration. This means that the feed and the draw solutions are returned to the ocean after the PRO process is complete. When the seawater and river water are brought into the PRO system, they must be filtered and disinfected to prevent fouling and biofilm formulation, respectively. In addition to adding to the overall cost of the project, any chemicals that are added to these waters must either be flushed out to sea or be removed through physical or chemical means. Disposal of disinfection chemicals and disinfection byproducts can have unforeseen environmental impacts. Diversion of river water may also have an environmental impact on delicate river delta ecology.

[0008] Thus, in order to create a viable pressure -retarded osmosis process, the use of closed cycle PRO systems, which are intended to use low temperature heat to recycle an osmotic agent, have been proposed. This approach does not capitalize on natural salinity gradients, but instead explores the use of osmotic pressure as a medium for the production of work, enabling the conversion of environmentally benign low temperature heat sources to electrical power. In several processes, the draw solution is a solution of an ionic salt, such as sodium chloride, as described for example in U.S. Patent No. 3,906,250 to Loeb. Heat applied to the OHE would re-concentrate the draw solution by vaporizing a portion of the water into steam, which would then be condensed to form the deionized working fluid.

Other processes involve the removal of a volatile organic solute, or the chemical precipitation of solutes followed by their re-dissolution.

[0009] A primary difficulty faced by these OHEs is poor thermal efficiency due to high heat input requirements for water and organic solute vaporization. In the case of chemically precipitable solutes, chemical feed stock consumption can pose difficulties to economic operation. An additional challenge is the difficulty of obtaining solute separation complete enough to avoid concentration polarization (CP) effects in the feed water. This is not a problem when water is vaporized and re-condensed as distilled working fluid, but could pose a significant problem when using removable draw solutes that are difficult to remove completely.

[0010] This points to an additional, reoccurring challenge in osmotically driven membrane processes: the difficulty of identifying a solute that may both create high osmotic pressures and be highly removable for reuse. Near complete removability is very important, because internal concentration polarization effects in the working fluid (feed solution) can drastically reduce membrane water flux. Thus, the ideal osmotic heat engine would use a draw solute that has the following features: (1) highly soluble; (2) completely removable; (3) has a high diffusivity for effective mass transfer in the membrane system, and (4) requires less heat for solute removal than that required for the vaporization of water or highly soluble organic solutes.

[0011] The invention described herein attempts to overcome some of the noted problems of the prior art by proposing an alternative means of power production that uses osmotic pressure to generate electrical power from sources of low-grade heat. While several prior investigations of the use of osmotic phenomena to produce power have been conducted, such as those used to convert "salinity power" from the mixing of natural saline and fresh water streams, relatively few studies have focused on the use of osmotic phenomena to produce power through the conversion of heat.

SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide an osmotic heat engine that includes dilute (nearly deionized) water as a working fluid and a membrane that is configured such that internal concentration polarization does not hinder the osmotic flow of water across the membrane.

[0013] It is another object of the present invention to provide an osmotic heat engine having a draw solute that is fully compatible with all system components. [0014] It is still another object of the present invention to provide an osmotic heat engine that uses a draw solute that is highly soluble and completely removable.

[0015] It is still another object of the present invention to provide an osmotic heat engine having a draw solute that provides for a large osmotic pressure gradient.

[0016] It is yet another object of the present invention to provide an osmotic heat engine that mitigates the environmental impacts of the PRO process. To that end, the present invention relates generally to a closed loop PRO process that utilizes a recyclable draw solute.

[0017] In one aspect, the present invention relates to a method of generating power using an ammonia-carbon dioxide osmotic heat engine. The method includes the steps of pressurizing a concentrated draw solution to a hydraulic pressure lower than its osmotic pressure on a first side of a semi-permeable membrane, introducing a dilute (nearly deionized) working fluid on an opposite side of the semi-permeable membrane, causing a portion of the dilute working fluid to flow through the semi-permeable membrane into the pressurized draw solution to create a water flux that expands the volume of the draw solution, inducing flow of the expanded volume of the draw solution through a turbine to produce power, and processing the expanded volume of the draw solution through a distillation column at a suitable temperature and pressure to separate the solutes from the draw solution, thereby producing new draw solution and working fluid streams for reuse in the system.

[0018] In another aspect, the invention relates to a method of generating power using an osmotic heat engine. The method includes the steps of introducing a concentrated draw solution to a first side of a semi-permeable membrane, introducing a working fluid to a second side of the semi -permeable membrane, causing a portion of the working fluid to flow through the semi-permeable membrane into the pressurized draw solution to create a water flux that expands the volume of the draw solution, inducing flow of the expanded volume of the draw solution to a first side of a first energy recovery device and to a first side of a second energy recovery device to depressurize the expanded volume of the draw solution, introducing a turbine driving fluid to a second side of the first energy recovery device to pressurize the turbine driving fluid and induce the flow thereof through a turbine to produce power, introducing the concentrated draw solution to a second side of the second energy recovery device to pressurize the concentrated draw solution and induce flow thereof to the first side of the semi-permeable membrane, and processing the depressurized draw solution through a recycling system to separate the solutes from the draw solution, thereby producing new concentrated draw solution for reuse in the system. In one embodiment, the first and second energy recovery devices are connected in parallel; however, other quantities and arrangements of the energy recovery devices are contemplated and considered within the scope of the invention. For example, one or more energy recovery devices could be used with the working fluid introduced to the membrane system and/or between any combinations of fluid sources.

[0019] In various embodiments of the foregoing aspect, the second energy recovery device pressurizes the concentrated draw solution to a hydraulic pressure lower than its osmotic pressure on the first side of the semi-permeable membrane. In one embodiment, the concentrated draw solution includes ammonia and carbon dioxide in ratio of greater than 1 : 1 and in one or more embodiments at a ratio between about 1: 1 to 2.5: 1. In another embodiment, the concentrated draw solution comprises a non-aqueous solvent. Other draw solutions, such as amine-based solutions are contemplated and considered within the scope of the invention.

[0020] In another aspect, the invention relates to an osmotic heat engine. The osmotic heat engine includes a pressure retarded osmosis unit having a semi-permeable membrane, a source of concentrated draw solution in fluid communication with the pressure retarded osmosis unit, a source of working fluid in fluid communication with the pressure retarded osmosis unit, a first energy recovery device in fluid communication with the pressure retarded osmosis unit for receiving a first portion of an expanded volume of a dilute draw solution therefrom, a source of turbine driving fluid in fluid communication with the first energy recovery device, a turbine in fluid communication with the first energy recovery device and the source of turbine driving fluid; wherein the first energy recovery device transfers the pressure from the expanded dilute draw solution to the turbine driving fluid to rotate the turbine, a second energy recovery device in fluid communication with the pressure retarded osmosis unit for receiving a second portion of the expanded volume of the dilute draw solution therefrom, and a solute recycling system in fluid communication with the first and second energy recovery devices for receiving the expanded dilute draw solution; wherein the recycling system provides the source of concentrated draw solution to the second energy recovery device where the concentrated draw solution is pressurized prior to delivery to the pressure retarded osmosis unit.

[0021] These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention and are not intended as a definition of the limits of the invention. For purposes of clarity, not every component may be labeled in every drawing. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

[0023] FIG. 1 depicts an osmotic heat engine system in accordance with one or more embodiments of the invention;

[0024] FIG. 2 depicts an alternative osmotic heat engine system in accordance with one or more embodiments of the invention;

[0025] FIG. 3 depicts flux data and demonstrates the relationship between water flux and draw solution concentration for the membrane;

[0026] FIG. 4 depicts the membrane power density relative to hydraulic and osmotic pressures in the osmotic heat engine of the invention; and

[0027] FIG. 5 depicts the osmotic heat engine efficiency as a percentage of Carnot engine efficiency, relative to the difference between the hydraulic and osmotic pressures of the draw solution.

DETAILED DESCRIPTION

[0028] The present invention relates generally to a method of generating power using an ammonia-carbon dioxide osmotic heat engine. The method includes the steps of

pressurizing a concentrated draw solution to a hydraulic pressure lower than its osmotic pressure on a first side of a semi-permeable membrane, introducing a dilute (nearly deionized) working fluid on an opposite side of the semi-permeable membrane, causing a portion of the dilute working fluid to flow through the semi-permeable membrane into the pressurized draw solution to create a water flux that expands the volume of the draw solution, inducing flow of the expanded volume of the draw solution through a turbine to produce power, and processing the expanded volume of the draw solution through a distillation column at a suitable temperature and pressure to separate the solutes from the draw solution, thereby producing new draw solution and working fluid streams for reuse in the system. [0029] The osmotic heat engine described herein is designed to compete with other types of heat engines including gas turbines (Bray ton Cycle), steam turbines (Rankine Cycle), internal combustion engines (gasoline, diesel), and external combustion engine (Stirling engines).

[0030] The present invention relates to a closed cycle osmotic heat engine. The system uses an ammonia-carbon dioxide draw solution and a deionized working fluid. The deionized working fluid may include water that is substantially (or nearly) deionized. What is meant by nearly deionized is that the deionized working fluid contains less than 1 ppm ammonia and carbon dioxide (or alternative draw solutes) and no other solutes. The draw solution is highly soluble, osmotically efficient and contains entirely removable and recyclable solutes. The use of deionized water as a working fluid maximizes membrane mass transfer by eliminating internal concentration polarization effects.

[0031] In one embodiment, the draw solution includes ammonium salts formed by the introduction of ammonia and carbon dioxide into water and is used in the OHE of the invention to generate electrical power; however, other thermolytic solutes are contemplated and considered within the scope of the invention. The draw solution is formulated by mixing ammonium bicarbonate salt with ammonium hydroxide to form a complex solution of ammonium salts including ammonium bicarbonate, ammonium carbonate, and ammonium carbamate. The amount of ammonium hydroxide added is minimized to minimize the concentration of unionized ammonia in the draw solution. In one embodiment, the concentrated draw solution has an ammonia to carbon dioxide ratio of between about 1 : 1 to 2.5: 1. In addition, the draw solution has a concentration of between 0.1 and 12 molar, preferably between about 3 to about 6 molar.

[0032] This draw solution has several desirable characteristics, including: (1) high solubility of the ammonium salts; (2) relatively low molecular weight and high diffusivity of the chemical species leading to high osmotic pressures and moderate external concentration polarization effects; (3) solutes that are almost completely removable in that the ammonium salts, upon heating with the draw solution at an appropriate temperature and pressure (for example 60° C at 101.3 kPa (1 atm), decompose to ammonia and carbon dioxide gases that may be readily removed to levels of less than 1 ppm; and (4) the thermal energy required for the removal and recycling of these solutes from a quantity of water is significantly less than that required to vaporize the water itself.

[0033] In the ammonia-carbon dioxide osmotic heat engine of the invention, the concentrated draw solution is pressurized to a hydraulic pressure lower than its osmotic pressure, a dilute working fluid (e.g., deionized water containing less than 1 ppm ammonia and carbon dioxide) flows- through the semi-permeable membrane into the pressurized solution, and this water flux expands the volume of the draw solution, inducing flow through a turbine, producing power. Heat is introduced to the osmotic heat engine to drive a separation of the solutes from the draw solution, resulting in renewed draw solution and working fluid streams. A pressure exchanger similar to those used in reverse osmosis (RO) desalination may be used to maintain the pressure of the draw solution side of the membrane in steady state operation.

[0034] The present invention uses a recyclable draw solute in PRO, where heat is input to the system which serves to regenerate the draw solute and excess heat is rejected to the environment in some way. The system is known as an "osmotic heat engine" because heat is absorbed and rejected and work is produced. While different conceptions of this type of system have previously been configured, ether poor membrane performance and/or inefficient use of heat limited further development, due in part to the inadequate performance of the selected draw solution agents and severe internal concentration polarization effects. [0035] In order to overcome the deficiencies of the prior art, the present invention proposes the use of ammonia-carbon dioxide (NH ₃ -CO2) osmotic heat engine. This heat engine 10 is illustrated in FIG. 1 and uses a mixture of ammonia and carbon dioxide gases mixed in solution. These gases form highly soluble ammonium salts in solution, which can generate osmotic pressures of over 250 atmospheres, more than 10 times that of seawater. The draw solution is highly soluble, osmotically efficient, and contains highly removable and recyclable solutes. The use of deionized water as a working fluid maximizes membrane mass transfer by eliminating internal concentration polarization effects. The results provided herein demonstrate the feasibility of the osmotic heat engine for the practical conversion of low temperature heat sources to power.

[0036] Generally, the OHE 10 includes a membrane system 12 in fluid communication with a source of the draw solution 11 and a source of the feed or working fluid 13. The membrane system 12 is also in fluid communication with one or more pressure exchangers 16, a turbine 20, and a system for recycling the draw solutes 22. Various systems of this type are described in U.S. Patent Publication Nos. 2010/0024423 and 2010/0183903, the disclosures of which are hereby incorporated by reference herein in their entireties.

Additional membrane systems are disclosed in U.S. Patent No. 7,560,029, the disclosure of which is also hereby incorporated by reference herein in its entirety. A pressure exchanger or combination of pressure exchangers may be used to, for example, pressurize the draw solution side of the membrane system 12 and/or as an interface between the dilute draw solution and a turbine.

[0037] The osmotic heat engine of the invention relies on the use of a deionized (i.e., containing little or no dissolved solutes) working fluid. The use of this fluid as a feed to the membrane is advantageous, because no ICP occurs. While salt leakage from the draw solution through the membrane may cause ICP, the membrane is chosen to reject salt to a high degree, which will serve to counteract this tendency. The membrane is a semipermeable membrane that has an active layer oriented toward the draw solution and a backing layer oriented toward the feed solution. The water flux that expands the volume of the draw solution is typically at least about 25 x 10 ^~6 m3/m2 -s.

[0038] One of the keys to the efficient osmotic heat engine process of the invention is the heat required to separate pure water from the diluted draw solution. This is where the benefit of using an ammonia and carbon dioxide draw solution (or similar) becomes apparent, because these gases may be successfully stripped from water using low temperature steam. Modeling of gas removal using Aspen HYSYS ^® (available from Aspen Technology, Burlington, MA) has shown that steam with temperatures as low as 40°C can be utilized under a vacuum gas stripping process. This allows for the utilization of a variety of heat sources that have typically little utility and very low to no cost.

[0039] The NH ₃ -CO2 osmotic heat engine's utilization of low grade heat is critical to its viable implementation as an electricity generating alternative. Low grade heat sources come from a variety of industries including, for example, metal manufacturing (steel mills), glass manufacturing, oil refining, and thermoelectric power generation. All of these industries use elaborate methods of reclaiming their waste heat, but low grade heat is always lost to the environment through water cooling or flue gases.

[0040] Renewable sources of heat may also be used. Geothermal heat sources are abundant, but are rarely of high enough quality to directly generate electricity. Typically, these sources may be used to heat and cool homes, but can also be used in a binary cycle configuration that utilizes the heat to vaporize a secondary liquid, such as ammonia, and expand that vapor through a turbine. The vapor can then by condensed by rejecting heat to the air or surface water. A similar concept of using warm water in the ocean is ocean thermal energy conversion (OTEC). This system includes an engine that utilizes the warm surface ocean water as a heat source and the cold deep ocean water as a heat sink. Similar to the geothermal binary cycle, OTEC uses warm water to vaporize a liquid, like ammonia, which then expands through a turbine. The gas is then condensed with the cold deep ocean water and recycled. For both of these processes, a gas is being used as the working fluid and hence a large turbine must be used (i.e., at least about 10 meters in diameter for steam turbine OTC). This is a design limitation that can be alleviated replacing the ammonia vapor system commonly used with the osmotic heat engine of the invention. By using the warm water to strip the NH ₃ -CO2 draw solution and the cold water to condense these gases, the working fluid directed through the turbine to generate power is instead a liquid. This is of significant benefit, as hydroturbines are much smaller than turbines designed to use lower density gases, and are very efficient at converting work into electricity.

[0041] A benefit of the osmotic heat engine of the present invention is the ability to successfully convert low grade heat sources into electrical energy. The configuration of the heat engine of the invention solves many of the previous economic and environmental issues of river outlet PRO due to its closed loop configuration and recyclable draw solute.

Utilizing low grade heat sources also provides an essentially cost-free energy source, because the cost of the energy is related only to the capital cost of the equipment amortized over the life of the equipment and maintenance. The heat required for separating the draw solutes from solution is typically introduced at a temperature of between about 35 and 250°C.

Furthermore, the temperature required for separating the draw solutes from solution is proportional to pressure and pressure is typically introduced at about 0.05 to about 10 atm.

[0042] FIG. 2 depicts an alternative OHE 100 that uses isobaric energy recovery devices in the PRO process to replace the use of a high-pressure draw solution that drives the turbine in the earlier described embodiments of the invention with high-pressure pure water or other solution chosen for its particular compatibility and/or operating characteristics. The use of the pure water (or other non-aqueous solutions) may reduce or eliminate the need for specialized materials for the turbine and related equipment, as may be required with certain draw solutions. Similar to the OHE 10 of FIG. 1, the OHE 100 of FIG. 2 includes a membrane system 112, a source of a feed or working fluid 113, a source of a draw solution 111, a turbine 120, and a draw solute recycling system 122. The OHE 100 also includes two isobaric energy recovery (i.e., pressure transfer) devices 116, 118, a pump 110, and a source of turbine driving fluid 124.

[0043] In operation, the working fluid 113 is introduced to a first side of the membrane system 112 and the concentrated draw solution 111 is introduced to a second side of the membrane system 112, via pump 110, and provides an osmotic pressure differential. The pump 110 assists in pressurizing the concentrated draw solution. A solvent generally moves across the membrane via osmosis, thus increasing the volume on the pressurized draw solution side of the membrane system 112. Generally, the increased volume of the pressurized draw solution may be decreased by flow through the turbine 120, which reduces the solution pressure and produces power. The depressurized solution may then be treated via the solute recycling system 122, as previously described.

[0044] In particular, the pressurized dilute draw solution is directed to one side of each of the two energy recovery devices 116, 118. A second side of one device 116 is in fluid communication with the solute recycling system 122, in particular a source of concentrated draw solution therefrom, and the pump 110. The first side of device 116 is also in fluid communication with the solute recycling system 122 to return the depressurized dilute draw solution to the recycling system 122. The pressured dilute draw solution passes through the device 116, pressurizing the concentrated draw solution from the recycling system 122 to the pump 110, thereby reducing the need for additional energy for transferring and pressurizing the concentrated draw solution. [0045] A second side of the other device 118 is in fluid communication with the turbine 120 and the source of turbine driving fluid 124. The first side of device 118 is also in fluid communication with the solute recycling system 122 to return the depressurized dilute draw solution to the recycling system 122. The pressurized dilute draw solution directed to the second device 118 pressurizes the turbine driving fluid 124, which may be pure water or other fluid compatible with the turbine 120, to spin the turbine and generate electricity. The reduced pressure dilute draw solution is directed to the recycling system 122. Essentially, the pressurized dilute draw solution passes through the two energy recovery devices 116, 118 in parallel. Depending on the nature of the turbine driving fluid, the source 124 may also be in fluid communication with the recycling system 122.

[0046] The foregoing OHEs were described with respect to using aqueous solutions; however, the systems described herein can also be used with non-aqueous solutions. The non-aqueous solvent could have the following properties: high surface tension, a high ability to dissolve a volatile solute, and low volatility. The solutes used could have the following properties: high solubility, capable of generating a high osmotic pressure, volatile when heated, and low enthalpy of vaporization. One possible example of a desirable solvent is dimethyl sulfoxide (DMSO). In a particular embodiment, the OHE would use a polar non-aqueous solvent with ammonium salts. The use of various polar and non-polar solvents and various salts are contemplated and considered within the scope of the invention.

[0047] The use of a non-aqueous draw solution would work similar to that of afore-mentioned water/ammonium salt systems, but would be much more energy efficient, because the vaporization in the solute recycling system would be largely of the solute only, with minimal vaporization of the solvent. Typically, in water based systems, 8-10% of the water is also vaporized, which represents a system inefficiency.

Examples: [0048] Flux experiments were conducted in the laboratory to determine the viability of the osmotic heat engine process of the invention. Water flux must be high if the generation of power is to be efficient. Previous tests on flux using reverse osmosis membranes showed that the flux rarely exceeded minimum values (no more than 2-3 gallons per foot squared membrane area per day (gfd) and often much less than 1 gfd).

[0049] The inventors investigated a commercially available membrane which is tailored for osmotic processes and found flux to be much better. The data was taken with the NH ₃ -CO2 draw solution on the active layer of the membrane. A deionized water feed was used to simulate osmotic heat engine conditions. Two temperatures were evaluated: 20°C and 40°C, and the feed and draw solutions were maintained at identical temperatures for both series of tests. The results are shown in FIG. 3.

[0050] Two temperatures were tested over a range of osmotic pressures. Fluxes over 50 gfd were obtained in some tests, suggesting that this particular membrane used is 50 times better than some previously tested membranes, which has a significant impact on the amount of membrane needed to produce a given amount of electricity. Higher fluxes yield smaller requirements for membrane area. Note that the permeate stream was not pressurized in these tests (and was not pressurized in previous investigations either).

[0051] From this data power generation data can be estimated by modeling the process in Aspen HYSYS ^® (available from Aspen Technology, Burlington, MA). Using various draw solution concentrations over a range of permeate pressurization, the amount of energy generation can be calculated by using the following equation:

Work = (Turbine efficiency) x (Hydraulic pressure) x (Volume flux) (1) Turbine efficiency often exceeds 90% and overall driving force causes flux across the membrane. As hydraulic power in the permeate stream is increased, flux decreases, but a maximum power generation point is established. FIG. 4 illustrates this feature for a range of draw solution concentrations and demonstrates how various draw solution concentrations perform in the osmotic heat engine of the invention over a range of permeate side hydraulic pressure. The energy production was modeled using Aspen HYSYS® (available from Aspen Technology, Burlington, MA).

[0052] It is important to note that the expected energy production from natural salinity gradients, such as those present at river/seawater interfaces, would be far lower than these modeled energy outputs. Using previously investigated membranes, the power output per membrane area in open fresh/seawater PRO was at most 1.4 W/m2. This data demonstrates that the NH ₃ -CO2 osmotic heat engine of the invention, using this tailored osmotic membrane can exceed that output by 200 times under certain configurations. Since membrane area is also used as the metric for capital cost and because the heat input to the system is essentially free, higher energy production per membrane area values have a direct bearing on the total cost of the electricity being produced.

[0053] In addition, projections of the performance of the ammonia-carbon dioxide OHE of the invention were also based on experimental data for water flux, calculations of power conversion efficiency in the turbine and pressure recover systems, and modeling of the energy requirements for the removal and recycling of the OHE draw solutes.

[0054] Measurements of water flux through semi-permeable membranes oriented in the PRO configuration (backing layer toward feed, active layer toward draw solution) provide data for estimations of engine performance. Membrane water flux data was obtained using a cross flow membrane cell and associated system components. The dimensions of the channel were 77 mm long by 26 mm wide by 3 mm dep. Mesh spacers were inserted within both channels to improve support of the membrane as well as to promote turbulence and mass transfer. A viable speed peristaltic pump (available from Masterflex of Vernon Hills, IL) with a dual pump head was used to pump both the feed and draw solutions in a closed loop. A constant temperature water bath (available from Neslab of Newington, NH) was used to maintain both the feed and draw solution temperatures. Heat transfer took place within the water bath through inline stainless steel heat exchanger coils which were submerged in the stirred bath. The draw solution rested on a scale (available from Denver Instruments of Denver, CO) and weight changes were measured over time to determine the permeate water flux. The membrane was placed in the cell such that the draw solution was against the active layer arid the feed solution was against the support layer. The membrane used to collect flux data was designed for forward osmosis desalination and was obtained from Hydration Technologies, Inc. (Albany, OR). The chemical makeup of the membrane is proprietary, but is believed to contain cellulose acetate polymers. The structure is asymmetrical with a separating layer supported by a relatively thin (i.e., less than about 50 /lm) support structure. Further support is provided by a polyester mesh embedded within the polymer support layer.

[0055] Osmotic water flux was determined for a range of draw solution concentrations. The draw solution was made by mixing ammonium bicarbonate salt (NH ₄ HCO ₃ ) with ammonium hydroxide (NH ₄ OH), forming a complex solution of ammonium salts, comprised of ammonium bicarbonate, ammonium carbonate and ammonium carbamate, with the latter being the most abundant in concentrated solutions. The amount of NH ₄ OH added was varied depending on the concentration of the draw solution and the temperature at which it was to be used. The amount of NH ₄ OH was minimized to minimize the concentration of unionized ammonia in the draw solution. Properties of the draw solutions used in modeling of the OHE, including osmotic pressure, density, viscosity, and pH, were obtained with Aspen HYSYS ^® , in conjunction with an electrolyte property package from OLI Systems, Inc.

(Morris Plains, NJ).

[0056] Experimental membrane water flux data were used to calculate fitted (apparent) mass transfer coefficients for predictions of external concentration polarization (ECP) at the interface between the membrane and the concentrated draw solution used to drive engine water flux. ECP effects for a concentrated draw solution in an OHE membrane system are predicted with the fitted mass transfer coefficient and based on film theory with highly concentrated ECP effects calculated based on extrapolation form the experimental data. This model fitting and extrapolation is considered necessary in light of the expected significance of ECP effects with the OHE's membrane system and the inadequacy of traditional film theory to describe mass transfer phenomena in highly concentrated non-ideal solution flows. The predicted membrane fluxes using the fitted efficiency were found to correlate well with observed water flux performance, within the range of experimental data.

[0057] Water flux measurements under unpressurized conditions are assumed to predict flux in the pressurized OHE system of the invention, following the governing equation for PRO under differing osmotic and hydraulic pressure conditions:

J _w = Α(σ An _m - Δ P) (2)

Here, A is the water permeability coefficient, σ the reflection coefficient, ~ltm the difference in osmotic pressures across the membrane between the draw and feed solution at the separation interface (i.e., the membrane active layer surface), and ΔΡ is the hydraulic pressure difference between the draw solution side and the working fluid. Δπιη is calculated from the bulk osmotic pressure of the draw solution after accounting for ECP effects as discussed above.

[0058] It was assumed that σ = 1 in all calculations because of the relatively high rejection of the FO membrane used. Furthermore, the water permeability coefficient A is assumed to be independent of the applied hydraulic pressure, implying negligible membrane compaction. The selection of the membrane, operating pressures, and temperature of the system includes the accuracy of predictions based on these assumptions. [0059] The power produced by the OHE (W) is a function of the quantity of water moving through its turbine per unit of time (V), the drop in pressure in that turbine which is equal to the applied hydraulic pressure on the draw solution side (~P), and the turbine efficiency (E):

W=EVAP (3)

The turbine efficiency E is typically greater than 90%. The efficiency of the pressure exchanger used to maintain steady state pressurization of the draw solution is typically greater than 95%. The combined efficiency of these two components is approximated, in the modeling effort described herein, to an overall efficiency of 90% for projections of power production, captured in the value of 0.90 for E in Equation 2 above. The volume flowing through the turbine per unit time (V) is equal to the product of the water flux through the membranes of the OHE (Jw) and the total membrane surface area. This flux is a function of both the hydraulic and osmotic pressures of the system, as shown by Equation 1 above. Increasing the hydraulic pressure relative to the osmotic pressure increases the power output per unit volume of water through the turbine, but will also reduce the total volume of water by reducing membrane water flux. Reducing hydraulic pressure will have the inverse effect.

[0060] Thermal efficiency is calculated by measuring the quantity of power produced relative to the quantity of heat used (for the separation and recovery of the draw solution). There are two measures of efficiency that may be considered in evaluating an engine's performance: thermal efficiency and Carnot efficiency. Thermal efficiency is simply the ratio of engine power output over heat input. Carnot efficiency is a measure of the efficiency of an engine relative to that of a Carnot engine, one which produces the maximum theoretical quantity of work from a given heat flow, based on a perfectly reversible process.

[0061] The "quantity of heat" component of engine efficiency can be calculated based on the heat duty of the distillation column used to separate the ammonia and carbon dioxide from the dilute draw solution, producing a re-concentrated draw solution and deionized working fluid. The column heat duty was modeled with Aspen HYSYS ^® in conjunction with an electrolyte property package from OLI Systems, Inc. (Morris Plains, NJ), following the procedures used in estimating the energy demands of forward osmosis desalination.

[0062] The efficiency of a Carnot engine (ή) is given by:

ή = 1 - Τ _Ι 7Τ _Η (4)

where T _H is the absolute temperature of heat delivered to the engine (from fuel combustion, for example) and TL is the absolute temperature at which heat is rejected to the environment. Measuring OHE efficiency against the efficiency of a Carnot engine establishes how effective the OHE is relative to the quantity of heat it uses. A geothermal power plant using 200°C heat, for example, obtaining a thermal efficiency of 20%, would not, by the thermal efficiency measure, seem to be a very efficient plant. The Carnot efficiency of such an engine, however, would be 55%, approximately equally to the Carnot efficiency of a coal fired power plant operation at 537°C. This is a particularly useful method of comparison between heat engine technologies when considering heat sources as low as 20°C above ambient temperatures, where maximum theoretical thermal efficiencies are quite low.

As the difference is osmotic pressure between two solutions increases, the flux through a semi-permeable membrane separating the two will increase as well. This relationship is not linear, due to concentration polarization effects at the surface of the membrane. In PRO mode (draw solution on active layer side of membrane) with deionized water as the feed, only external concentration polarization is expected to occur, assuming very high rejection of salts by the membrane. FIG. 3 illustrates the relationship between water flux and draw solution concentration for the membrane.

[0063] Osmotic water flux performance of the membrane was evaluated using an unpressurized NH ₃ -CO ₂ draw solution with deionized water as the feed stream, with feed and draw solutions isothermal. The driving force is calculated based on the bulk osmotic pressure of the draw solution. Dashed lines indicated pure water hydraulic permeability determined from reverse osmosis tests with the same membrane. Differences between these lines and experimental data are due to external concentration polarization.

[0064] Data are shown for 20°C and 40°C, with the feed and draw solutions in each case isothermal. Flux is shown relative to the osmotic pressures of the draw solutions. Higher temperatures lead to higher water fluxes due to the effects of temperature on the membrane water permeability and the diffusivity of the draw solutes. With the FO membrane operated in the PRO mode with a deionized water feed, water flux exceeds 25 x 10 ^~6 m ³ /m ² -s (or 50 gallons per square foot of membrane per day, GFD). The nonlinear relationship shown is due to ECP, caused by dilution of the draw solution at the membrane surface on the permeate side of the membrane. These experimental flux data are used to calculate the power output of the OHE as described below.

[0065] One criterion for optimizing the OHE is to select hydraulic and osmotic pressures which produce the highest power output per membrane area, or highest membrane "power density." The power density is calculated based on membrane water flux, draw solution hydraulic pressure, and anticipated ECP effects in the OHE membrane system. The ECP effects were calculated using a fitted mass transfer coefficient of 1.78 x 10-5 m/s, determined through experimental flux measurements in the PRO mode. The combined efficiency of the hydroturbine and pressure recovery device was assumed to be 90%. The relationship between the osmotic and hydraulic pressures in the OHE, relative to the membrane power density is shown in FIG. 4. Each curve corresponds to a fixed ammonia-carbon dioxide draw solution concentration.

[0066] The modeling indicated that the maximum membrane power density is achieved when the hydraulic pressure is approximately 50% of the osmotic pressure. For an OHE with a hydraulic pressure of 10.13 MPa (100 atm), the power density provided by use of a 4.6 M draw solution producing 19.16 MPa (197 atm) of osmotic pressure is approximately 170 W/m ² . This is quite high as compared to the power densities expected of river/seawater salinity PRO power plants, which are typically in the under 4 W/m ² range.

[0067] The power density may be further increased by increasing the cross flow velocity of the draw solution stream (to reduce ECP effects) or the hydraulic pressure of the OHE membrane system. Modeling of an OHE with significantly increased crossflow velocities (5 m/s in a 0.05 cm high flow channel), indicates that OHE power densities would be increased by approximately 61 % over those of a membrane system with the fluid dynamics of the test cell used in the study described herein (0.46 m/s in an 0.3 cm high channel). For an OHE operating at 10.13 MPa (100 atm) hydraulic pressure, the maximum power density would be about 274 W/m ² in this scenario.

[0068] Modeling of an OHE with a 20.26 MPa (200 atm) hydraulic operating pressure indicates that power densities would be increased by an additional 47% over those of a 10.13 MPa (100 atm) system. Increased crossflow velocity, however, will result in additional power consumption, and increased hydraulic pressure will require more expensive process components. These operating conditions will necessarily be factors in process optimization, balanced against correlating factors of process fluid pump power consumption and equipment capital and replacement costs.

[0069] The Carnot efficiency of the OHE was modeled over a range of osmotic and hydraulic pressures. In the calculation of engine thermal efficiency, the heat and electrical duties of the draw solution separation and recycling process are compared to the electrical production of the OHE power generating turbine for the combination of osmotic and hydraulic pressures examined. Because the electrical energy needed for the draw solute separation and recycling is negligible, the thermal efficiency is practically the ratio between the electrical energy produced by the OHE and the thermal energy required for the draw solute separation. This efficiency is compared to the theoretical efficiency of a Carnot engine operating with the same high and low temperature heat streams, giving a "percentage of Carnot efficiency" measure of OHE performance.

[0070] To determine the heat and electrical duty of the draw solute removal and recycling process, a draw solution of sufficient concentration to produce the osmotic pressure desired is specified in a HYSYS ^® chemical simulation model. This solution stream is directed to a distillation column with characteristics appropriate for the removal. One example of such a model specifies a single distillation column, effecting the separation of draw solutes from a 6 M (C0 ₂ basis) draw solution stream (which generates 31.94 MPa (315.26 atm) osmotic pressure in the OHE membrane system, containing structured packing 2.35 m (7.7 ft.) in height (30 theoretical stages) supplied with heat at 50°C. A column of this type operates at a bottom pressure and temperature of 10.62 kPa (0.1 048 atm) and 46.96°C (given a 3°C ΔΤ in the reboiler heat exchanger), and a top pressure and temperature of 10.54 kPa (0.1040 atm) and 35.55°C. The stream fed to the top of the column is preheated to 32°C with an energy requirement of 3196.8 MJ/m ³ (per m working fluid produced). The column heat duty is 3454.6 MJ/m ³ , supplied to the reboiler. Supplementary heating required to maintain all streams at specified temperatures is 385.7 MJ/m ³ , for a total heat duty of 7037.1 MJ/m ³ . The electrical duty for fluid pumping in the separation process is relatively negligible (0.48 MJ/m) A summary of the heat and electrical duties required for the separation of the draw solute at typical concentrations used in the simulations is provided in Table 1. This table also provides some of the properties of the draw solution that are relevant to modeling OHE performance. 1 43.7 1.1 358.0 0.12

2 84.4 1.2 593.4 0.13

3 120.1 1.2 865.7 0.16

4 157.8 1.3 1319.0 0.19

5 229.6 1.6 2847.7 0.26

6 319.7 1.8 7037.1 0.48

[0071] The overall Carnot efficiency of the OHE was calculated based on modeling as described above, over a range of supplied heat temperatures. Over a variety of temperatures the efficiencies were remarkably consistent. In FIG. 5, the Carnot efficiencies for OHEs operating with 50°C heat are shown, over a range of osmotic and hydraulic pressures in the OHE membrane system. For each combination of pressures, the temperatures were held constant, with a high temperature, or temperature of energy supplied of 50°C and a low temperature, or temperature of the ambient environment of 25 °C. [0072] FIG. 5 represents OHE engine efficiency as a percentage of Carnot engine efficiency, relative to the difference between the hydraulic and osmotic pressures of the draw solution. For a high temperature of 50°C and a low temperature of 25°C, the percentage of maximum theoretical engine efficiency (Carnot) reaches a maximum of approximately 16% as the net driving force (Δπ-ΔΡ) approaches zero. The osmotic pressure Δπ is based on the bulk osmotic pressure of the draw solution.

[0073] The results indicate that the highest engine efficiency is obtained when the difference between osmotic and hydraulic pressures approaches zero. Given equal osmotic and hydraulic pressure at equilibrium in a zero-flux condition, increases in osmotic pressure will increase membrane water flux, thereby increasing the amount of power produced by the OHE turbine. Increases in osmotic pressure are achieved by increasing the draw solution concentration. Higher concentration draw solutions require more energy for solute removal and recycling in the form of supplied heat. Therefore, as osmotic pressure is increased, power production, membrane water flux, and heat duty required by the solute recycling system all increase concurrently.

[0074] The distillation column used for the solute recycling system is however inefficient in its removal of N¾ and C0 ₂ from the dilute draw solution. Some water vapor is also removed, requiring heat which may not be converted to power production. As the concentration of the draw solution is increased, the amount of water vapor created in the distillation column increases as well, and this inherent inefficiency of separation results in decreasing OHE efficiency overall. However, as previously discussed, using a non-aqueous draw solution can eliminate this inefficiency. This increase in osmotic pressure does, however, result in increased water flux, which benefits OHE operation through increased membrane power density. Higher membrane power densities require less membrane area for a given energy capacity and thus less membrane cost. This represents a tradeoff between membrane capital cost and engine efficiency, which must be optimized in the design of an OHE system.

[0075] As is seen from the above discussion, although overall engine efficiency is fairly low, approaching a maximum of 16% Carnot efficiency, and likely operating at an efficiency of 5-10%, the power output per membrane area can be quite high, in excess of 250 W/m ² of membrane area. If the OHE uses thermal energy sources in the range of 40- 100°C, the cost of energy input to the engine may approach negligibility. An important consideration would be the capital and labor costs of the process and their impacts on the cost of electricity produced.

[0076] The use of the ammonia-carbon dioxide osmotic heat engine of the invention allows for power production from diverse energy sources such as heat from the reject streams of existing power plants, otherwise unproductive low temperature geothermal heat sources, low-concentration solar thermal energy, biomass heat (non-combustion) and ocean thermal energy conversion, among others. In all of these cases the process of the invention produces power that is renewable and carbon- free.

[0077] Having now described some illustrative embodiments of the invention, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention. In particular, although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives.

[0078] It is to be appreciated that embodiments of the devices, systems and methods discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The devices, systems, and methods are capable of implementation in other embodiments and of being practiced or of being carried out in various ways.

Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements, and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiments.

[0079] Those skilled in the art should appreciate that the parameters and configurations described herein are exemplary and that actual parameters and/or configurations will depend on the specific application in which the systems and techniques of the invention are used. Those skilled in the art should also recognize or be able to ascertain, using no more than routine experimentation, equivalents to the specific embodiments of the invention. It is therefore to be understood that the embodiments described herein are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described.

[0080] What is claimed is: