Osmotic Heat Engine

Abstract

A method of converting thermal energy into mechanical work that uses a semi-permeable membrane to convert osmotic pressure into electrical power. A closed cycle pressure-retarded osmosis (PRO) process known as an osmotic heat engine (OHE) uses a concentrated ammonia-carbon dioxide draw solution to create high osmotic pressures which generate water flux through a semi-permeable membrane against a hydraulic pressure gradient. The depressurization of the increased draw solution volume in a turbine produces electrical power. The process is maintained in steady state operation through the separation of the diluted draw solution into a re-concentrated draw solution and deionized water working fluid, both for reuse in the osmotic heat engine.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Application No. 60 / 858,245, filed Nov. 9, 2006, the subject matter of which is herein incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]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[0003]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 these 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 dri...

AI Technical Summary

Benefits of technology

[0019]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.

[0023]It is yet another object of the present invention to provide an osmotic heat engine that mitigates the environmental impacts of the PRO process.

[0028]c) 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;

Problems solved by technology

However, an important restriction regarding the adoption of new fuels and energy technologies is the cost of power produced by these 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.

PRO processes at river deltas, also known as “open loop” PRO, have several operational and design limitations.

Another difficulty arises from the low differential osmotic pressures found between many natural feed waters.

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.

However, the primary obstacle to a viable PRO process is poor membrane performance.

Previous investigations into PRO have found that membrane flux performance was too poor to make power generation a viable option.

Unfortunately, membranes currently in use are asymmetric in structure.

The porous support layer plays a significant, and often hindering, role in osmotic flux performance.

This problem, coupled with ICP and fouling phenomena, make the available osmotic pressure even smaller.

Other issues associated with the draw solute include compatibility with the system components and the membrane.

Seawater may be corrosive to metal parts and both freshwater and seawater may contain biological components that cause biofouling to system components, including the membrane.

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.

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 which are difficult to remove completely.

This points to an additional, reoccurring challenge in osmotically driven membrane processes—the difficulty of identifying a solute which may both create high osmotic pressures and be highly removable for reuse.

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