Power Generation from Low-Temperature Heat by Hydro-Osmotic Processes

Abstract

A system and method for generating power from waste heat, the system including (1) a forward osmosis module having an FO membrane a water inlet, a water outlet, a draw solution solute inlet and a diluted draw solution outlet; (2) a hydro-turbine using the diluted draw solution for generating power; (3) a CO.sub.2 absorption reactor to permit the introduction of compressed CO.sub.2 into the diluted draw solution to cause substantial separation of draw solution solute from the water, which water can be processed for subsequent recycling to the FO module, the CO.sub.2 absorption reactor configured to discharge a mixture of separate draw solution solute and absorbed CO.sub.2; and (4) a heat exchanger for transferring waste heat from an incoming heated fluid to the mixture of draw solution solute and CO.sub.2.

Claims

1. A system for generating power from waste heat using osmotic polymers that can be regenerated using CO.sub.2 absorption, the system comprising: a forward osmosis (FO) module comprising an FO membrane configured to permit the passage of an osmotic polymer draw solution solute along the membrane to draw water across the membrane, the FO module further comprising a water inlet connected to a water inlet line and a water outlet connected to a water outlet line, the FO module further comprising a draw solution solute inlet connected to a draw solution solute inlet line and diluted draw solution outlet connected to a diluted draw solution outlet line; a hydro-turbine connected to the diluted draw solution outlet line for generating power as diluted draw solution passes therethrough; a CO.sub.2 absorption reactor configured to permit the introduction of compressed CO.sub.2 into the diluted draw solution so as to cause substantial separation of draw solution solute from the water, which water can be processed for subsequent recycling to the FO module for continued power generation during the forward osmosis cycle, the CO.sub.2 absorption reactor configured to discharge a mixture of separate draw solution solute and absorbed CO.sub.2; and a heat exchanger for transferring waste heat from an incoming heated fluid to the mixture of draw solution solute and CO.sub.2.

2. The system of claim 1, further comprising a CO.sub.2 desorption reactor configured to separate the CO.sub.2 from the separate draw solution solute so as to regenerate the draw solution solute for recycling to the permeate side of the membrane in the FO module.

3. The system of claim 1, further comprising a pressure regulator in the water outlet line of the FO module configured such that the FO module can be operated as a pressure assisted FO module when in use.

4. The system of claim 1, wherein the draw solution solute comprises an amine-terminated branched polymer.

5. A method for generating power from waste heat using osmotic polymers that can be regenerated using CO.sub.2 absorption, the method comprising: directing water and an osmotic polymer draw solution solute into a forward osmosis (FO) module comprising an FO membrane configured to permit the passage of the osmotic polymer draw solution solute past the membrane to draw the water across the membrane, the FO module further comprising a water inlet connected to a water inlet line and a water outlet connected to a water outlet line, the FO module further comprising a draw solution solute inlet connected to a draw solution solute inlet line and diluted draw solution outlet connected to a diluted draw solution outlet line; directing diluted draw solution through a hydro-turbine connected to the diluted draw solution outlet line for generating power; directing compressed CO.sub.2 into a CO.sub.2 absorption reactor configured to permit the introduction of the compressed CO.sub.2 into the diluted draw solution so as to cause substantial separation of draw solution solute from the water, which water can be processed for transfer to the FO module, the CO.sub.2 absorption reactor configured to discharge a mixture of draw solution solute and CO.sub.2; and directing the mixture of draw solution solute and CO.sub.2 into a heat exchanger for transferring waste heat from an incoming heated fluid to the mixture of draw solution solute and CO.sub.2.

6. The method of claim 5, further comprising directing the heated mixture into a CO.sub.2 desorption reactor configured to separate the CO.sub.2 from the draw solution solute so as to regenerate the draw solution solute for transfer to the FO module.

7. The method of claim 5, further comprising pressuring the water inlet into the FO module such that the FO module can be operated as a pressure assisted FO module.

8. The method of claim 5, wherein the draw solution solute comprises an amine-terminated branched polymer.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0027] The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

[0028] FIG. 1 shows a schematic view of one embodiment of the present invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0029] Embodiments of the present invention employs an FO system for a power generation process capable of efficiency greater than 25% in the conversion of low-temperature waste heat to power, and economically cheap to exploit. Referring to FIG. 1, for example, system 10 reflects one embodiment of a system configured to convert waste heat into power. In one embodiment system 10 comprises an FO module 12 configured for an inlet line of water 14, an inlet line for draw solution solute 16 and an outlet line of diluted draw solution 18. The FO module also comprises an outlet line 22 of water and some residual draw solution (as explained further below).

[0030] The diluted draw solution 18 is directed to a hydro-turbine 24 for generation of power, with the diluted draw solution 18 further directed to a gas-liquid mixer 26 where compressed CO.sub.2 28 is injected into the diluted draw solution 18 to separate the water from the draw solution solute. An outlet line of water-rich solution 30 is then directed to a means for cooling 32, such as by example a cooling tower, and into water storage 34. Some residual draw solution solute may still reside in the water-rich solution 30, which is why some of the residual draw solution solute may pass through the FO module into outlet line 22.

[0031] The unabsorbed CO.sub.2 38 from the gas-liquid mixer 26 is then directed to a CO.sub.2 compressor 42 for delivery back to the gas-liquid mixer 26. The output of the gas-liquid mixer 26 is a mixture of CO.sub.2 and draw solution solute 44, which is then directed to a heat exchanger 46, into which waste heat 48 (in the form of, for example, hot gas) is directed for purposes of transferring heat to the CO.sub.2/draw solution solute mixture 44. The cooled gas 52 is then directed away from the heat exchanger 46, leaving a heated mixture of CO.sub.2 and draw solution solute 54, which is then directed into a CO.sub.2 desorption module 56 to separate the CO.sub.2 58 from the regenerated draw solution solute 62. The separated CO.sub.2 is then directed to the CO.sub.2 compressor 42, while the concentrated draw solution solute 62 is then directed to a means for cooling 64, which for example could be a cooling tower, before being introduced back into the FO module 12. A membrane 68 is provided in the FO module, which membrane is described further herein. In one embodiment, a pressure regulator 72, for example a controllable valve, is provided in the outlet line 22 of the FO module to maintain the elevated pressure of the incoming water line 14, as described further herein.

[0032] As reflected by example in FIG. 1, one embodiment of the process for power generation from low-temperature waste heat uses CO.sub.2-philic polymers as osmotic agents. For example, as described in co-pending applications identified above, acceptable osmotic solutes include amine-terminated branched polymers, including amine-terminated branched PEGs; for example, amine-terminated glycerol ethoxylate, trimethylolpropane ethoxylate, and/or amine-terminated pentaerithritol ethoxylate. Ionic draw solutes are also contemplated.

[0033] The main elements of the system function as follows: a concentrated osmotic draw solution is pumped under low pressure (for example, 1-2 atms) through the permeate side of a forward osmosis membrane module 12, while fresh water 14 is pumped at slightly higher pressures (for example, 1-3 atms) through the feed side of the FO membrane module 12. Based on the osmotic potential of the draw solution and the membranes used, water is pulled across the semi-permeable membrane at a high flux rate. Typical FO draw solutions have an osmotic potential of around 200-400 atms, but can be higher depending upon the solute, while fresh water has no discernible osmotic potential. This huge osmotic potential difference between the two liquid streams enables high flux rates across the membrane 68.

[0034] When power generation is needed, a concentrated/regenerated (preferably polymeric) draw solution is pumped at low pressure (15 psig) through the draw side of the FO module 12, preferably a pressure-assisted forward osmosis (PAFO) membrane system, while fresh water is routed through a low-pressure water pump (15-30 psig) to the feed side of the PAFO membrane 68, at a slightly higher pressure than the draw solution (DS) pump. The high difference in osmotic potentials between these two streamsdraw solution wateracross the FO membrane 68, assisted by the slightly higher hydraulic pressure from the water pump, enables large volumes of water at high flux rates (50-150 liters/m2/hr, LMH) to permeate across the membrane, resulting in a pressurized fluid flow from the increased volume of liquid after membrane permeation. This mixed stream is, in turn, routed through a high-flow hydro-turbine 24, to produce hydro-power as needed. The vastly increased flow of water and draw solution across the high-flow hydro-turbine results in efficient production of electrical energy, with efficiency levels reaching hydro-electric turbines (75-90%), unlike the use of heat engines (with their inherent Carnot cycle limitations for low-temperature heat streams).

[0035] The polymeric draw solutions preferably used are CO.sub.2-philic, and undergo a phase separation from water when the polymeric molecules absorb CO.sub.2. Thus, once CO.sub.2 is injected under pressure (preferably around 50-75 psig) into the water-polymer mixture, in a suitably engineered gas-liquid mixer 26, the polymer absorbs the injected CO.sub.2 and therein substantially phase-separates from its water solution. The separated water-rich stream 30, essentially consisting of most of the permeated water across the FO membrane 68, is directed to a cooling tower 32 and thence to water storage 34. Any water (which has not permeated through the FO membrane) and residual polymer still left in the water-rich stream 22, is directed to the polymer-rich stream 62.

[0036] The CO.sub.2-philic polymer 44, now substantially phase-separated from its water solution, is directed to a heat exchanger 46, wherein the temperature of the stream is raised to its CO.sub.2 desorption temperature (70-85 C.). The hot stream 54 is thereby directed to a gas desorption system 56, wherein most of the injected CO.sub.2 is recovered and directed to a CO.sub.2 compressor 42 for pressurization and the next cycle of gas injection. The CO.sub.2-free polymeric stream is directed to a cooling tower 64 before being used in the next cycle of power production in the FO system. Any un-absorbed CO.sub.2 38 in the gas-liquid mixer is directed to the compressor 42 inlet for a closed loop, minimizing an loss of CO.sub.2 in the process.

[0037] A main advantage of the above-described process is the lower volume of the liquid stream to be subjected to a temperature increase in the heat exchanger, since most of the water from the diluted draw solution has already been separated in the gas-liquid mixer, under the action of CO.sub.2, to cause phase separation between polymer-rich and water-rich phases. Given that the inherent specific heat of the preferred polymer (0.8) is lower than the specific heat of water, decreased heat supply is needed for raising its temperature for CO.sub.2 desorption and polymer regeneration, if minimal water is present in the mixture to be heated. The power needs for the CO.sub.2 compressor and the pumps needed in embodiments of the present invention, including the example shown in FIG. 1, are the main penalties in energy production and energy efficiency.

[0038] The efficiency of the hydro-turbine depends on the flow rate of the mixture of draw solution and water, which in turn, is dependent on the flux rate of water across the FO membrane 68, driven by the osmotic gradient between fresh water and the draw solution. The power generation cycle depends on high flux rates across the FO membranes. Earlier FO membranes had low flux rates (3-5 liters/m2/hr, LMH), leading to an energy density of less than 5 Watts/m2 or lower, thus needing very large membrane areas, in turn resulting in high capital costs. In addition, operation in the conventional pressure retarded (PRFO) mode caused a reduction in flux rates, since the applied hydraulic pressure worked against the osmotic pressure of the draw solution.

[0039] Embodiments of the present invention comprise pressure assisted forward osmosis for the forward osmosis module, which serves to increase flux across the membrane 68. The PAFO mode of operation is made feasible for this particular application, given that the feed solution is essentially fresh water, and hence membrane fouling, or salt migration through the membrane, are not operational issues. Pressurizing the feed water enables higher trans-membrane flux rate, while assisting the osmotic pressure on the draw side of the membrane. The PAFO mode, due to the applied pressure on the feed side, also reduces reverse flux of the osmotic agent to the feed side, an improvement over current FO practices. In addition, concentration polarization effects are minimized, maintaining the required flux rates. Forward osmosis membrane performance is critically dependent on the diffusion of the draw solute to the support layer of the membrane and its diffusion back to the bulk solution after osmotic dilutionthe PAFO mode helps in optimizing FO membrane performance. In the application of the invention described herewith, the draw solution (DS) is preferably on the active layer (AL) side of the membrane, while the water is on the porous support layer side of the membrane (the AL-DS mode), minimizing polarization effects.

[0040] The proposed power generation system embodiments are practical and feasible due to current availability of commercial high-flux membranes (>50 LMH) for the PAFO mode. Commercially available carbon nanotube FO membranes from, for example, Porifera (PFO-9S) have a membrane surface area of 67 m.sup.2. For a feed concentration of 30% PEG 400 (osmotic potential of 48 atm) against fresh water, the flux rate of water across the membrane was measured at 33 liters/m.sup.2/hr (LMH). For a 95% PEG 400 draw solution (osmotic potential>400 atm 12,842 ft of water head) against fresh water, a flux rate well in excess of 165 LMH is easily possible (an 8.33 increase in osmotic potential), especially since the feed side is fresh water, operated in the PAFO mode. Thus, across a PFO-9S FO membrane, the total water flux would be around 11,055 liters/hr (2,920.5 gallons/hr). This equates to 48.675 GPM, in excess of the 45 GPM used in power generation calculations.

[0041] Alternatively, and preferably, the use of nano-filtration (NF) membranes in the PAFO mode, with their higher pore sizes (Molecular Weight Cut-off, MWCO, of 200 Daltons, Da), enables the required high flow rates of 45 GPM needed for efficient generation of hydro-osmotic power. The larger pore diameter in the active layer of these membranes yields much higher water flux rates across these membranes, under low applied hydraulic pressures, as compared to traditional RO or FO membranes. Normally, NF membranes have lower salt rejection than traditional RO or FO membranes. However, given that substantially salt-free fresh water is used as the feed solution in the FO process, the increased benefit of higher flux rates and attendant higher power generation becomes the critical driver for membrane choice.

[0042] The equation for ideal hydro-dynamic calculations for the power generated by a hydro-turbine is: P=Q*H/k, where P=power in KW, Q=flow rate in GPM, H=static head in feet, and k=5,310 gal.ft/min.kW. Assuming a 200 atm differential in osmotic potential between the two solutions, the static head computes to almost 6,421 ft (1 atm=9.783 m; 1 m=3.281 ft).

[0043] If we assume an osmotic differential of 200 atm between the concentrated draw solution and the water feed solution, for a 45 GPM flow, the possible power rating of a hydroelectric turbo-generator is 54.415 KW, at 100% efficiency [P=(45 GPM)*(6421 ft/5310 gallt/min.KW)]. Hydropower is the most energy efficient power generator in industry. Currently, hydropower is capable of converting 90% of the available energy into electricity. Assuming an efficiency of 75% for hydro-osmotic power production, the net power generation capacity for the described invention (54.415*0.75), at the flow rates calculated above, is around 40.81 KW. Assuming a parasitic power need for the described system (for CO.sub.2 compressor, pumps etc) of 15 KW, the net power capacity is around 25.81 KW for a 45 GPM liquid flow through the hydro-osmotic turbine, produced from low-temperature waste heat. Thus, the energy produced in 1 hour is 25.81 kWh.

[0044] If the now diluted draw solution is brought back to its original concentration, using low-temperature waste heat or using renewable energy, (e.g., from solar thermal sources) for re-cycling back to the FO system, an efficient hydro-electric power generation system would be feasible, similar to conventional pumped hydro-electric storage. Calculating the thermal needs for an embodiment such as that shown in FIG. 1, a 45 GPM flow equates to 2,700 gallons of liquid flow in one hour. If we assume the post-FO liquid mixture consists of 200 gallons (1,832.6 lbs) of concentrated draw solution (specific gravity1.1; specific heat0.8), fed to the FO module, and 2500 gallons of water to the FO module, and the 200 gallons of the CO.sub.2-philic polymer is separated, the action of CO.sub.2 absorption by the engineered polymers slightly raises its temperature due to the heat of reaction. Assuming the separated polymer is at 72 F. at the outlet of the gas-liquid mixer, before being directed to the heat exchanger, the energy needed to heat the polymer to its CO.sub.2 desorption temperature of 200 F. would be around a maximum of 187,658.24 Btu (=1,832.6*128*0.8). Assuming a 75% efficiency of heat exchanger operations, the total waste heat energy required would be around 250,211 Btu. Transforming it to kWh (1 Btu=0.00029307 kWh), the approximate thermal energy needed to regenerate the osmotic polymer for the next cycle of power generation is 73.33 kWh.

[0045] Thus, the thermal efficiency for the process is 25.81 kWh/73.33 kWh=35.20%, for power generation from low-temperature waste heat (200-400 F.), using CO.sub.2-philic polymeric draw solutions with an inherent osmotic potential of 200 atms, and high flux-rate FO modules with NF-FO membranes, used in the PAFO mode. The power generation efficiency is enhanced in the described invention, since only a small volume of the water-polymer mixture needs to be heated from waste heat, after phase separation of the polymer from water, for desorption of the absorbed CO.sub.2.

[0046] This efficiency is well in excess for all current methods of power generation from low-temperature waste heat, including organic Rankine cycles (10-12% ), the Kalina cycle (12-15% ), thermo-electric generators (5% ) or other processes being developed. Such high efficiencies are possible, since the process does not rely on energy conversion by heat engines, and thus, is not limited by Carnot cycle constraints. The capital costs for the system are also low, in comparison to current methods for power generation from low-temperature waste heat. The draw solution agents are available fairly cheaply at industrial scales; the NF membrane modules are also readily available in industry; the heat exchangers do not have to fabricated from exotic alloys, since no salt solutions or corrosive agents are used in the system, and the temperatures are below the boiling point of water; and the pumps and other equipment can be made from reinforced plastics or stainless steels.

[0047] Persons of ordinary skill in the art may appreciate that numerous design configurations may be possible to enjoy the functional benefits of the inventive systems. For example, the embodiments of the present invention can be used on higher temperature and high temperature waste heat, where the efficiency of the system may change depending upon the temperature. Thus, given the wide variety of configurations and arrangements of embodiments of the present invention the scope of the invention is reflected by the breadth of the claims below rather than narrowed by the embodiments described above.