GB2532250A

GB2532250A - Power generation

Info

Publication number: GB2532250A
Application number: GB1420177.6A
Authority: GB
Inventors: Amundsen Lene; Barfod Schuller Reidar
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-11-13
Filing date: 2014-11-13
Publication date: 2016-05-18
Also published as: GB201420177D0

Abstract

An osmotic power generator 1 comprises a first chamber 2 containing a first salt solution e.g. filtered seawater, and a second chamber 3 containing a second salt solution, with a higher salinity than the first salt solution. The second chamber 3 is separated from the first chamber 2 by a semi-permeable membrane 4. An electrical power generator e.g. turbine 5 is connected to the second chamber 3 and driven by pressurized fluid from the second chamber. Solid salt from a reservoir 8 may be added to the second chamber 3 to maintain the salinity of the second solution. The system may be used to provide power to an underwater installation, e.g. for charging a battery or capacitor.

Description

Power Generation

TECHNICAL FIELD

The present invention relates to the field of power generation, and in particular to power generation by osmosis.

BACKGROUND

There are many subsea applications where electrical power is required, for example in the exploration for and production of oil/gas. In order to provide power, a remote source at the surface may be used, in which case power cables must be connected between a subsea installation and a surface facility. Such power cables are costly to install and maintain.

An alternative is to provide a local power source, such as a rechargeable battery or capacitor. In this this case, power must be generated locally, for example, by generating power from sea currents or waves. A problem with this type of power generation is that it relies on a regular and predictable source of energy to generate the power, and usually requires an expensive installation with many moving parts.

Electrical power may be generated by osmosis. This relies on the osmotic pressure difference between a source of salt water (the sea) and a source of fresh water (typically a river). A typical pressure difference is of the order of 25 bars. The pressure difference forces water through a turbine which generates electrical power. However, power generated in this way can only be generated at a river estuary, and so is not suitable for subsea installations remote from the river estuary as it would still require power cables to connect the turbine to the remote subsea installation.

SUMMARY

It is an object to provide a mechanism for generating power, and in particular a means for generating power usable by a subsea installation.

According to a first aspect, there is provided an osmotic power generator. A first chamber contains a first salt solution. A second chamber contains a second salt solution, the second salt solution having a higher salinity than the first salt solution.

The second chamber is separated from the first chamber by a semi-permeable membrane. An electrical power generator is operatively connected to the second chamber and configured to be driven by pressurized second slat solution from the second chamber.

As an option, the first salt solution is sea water. In this case, the osmotic power generator optionally further comprises a filter disposed upstream of the semipermeable membrane.

The osmotic power generator optionally comprises a salt reservoir containing solid salt and operatively connected to the second chamber, thereby allowing a salinity of the second salt solution to be maintained. As a further option, the osmotic power further comprises a flow control device between the salt reservoir and the second chamber, thereby allowing an operator to control a flow of salt into the second chamber.

As an option, the osmotic power generator further comprises a valve located between the second chamber and the electrical power generator, thereby allowing an operator to control a flow of the second salt solution to the electrical power generator.

The electrical power generator is optionally electrically connected to any of a capacitor, a rechargeable battery, a Peltier element, a condenser and a subsea installation.

As an option, the electrical power generator comprises a turbine.

According to a second aspect, there is provided a subsea installation comprising the osmotic power generator as described above in the first aspect.

According to a second aspect, there is provided a method of generating electrical power. A first chamber containing a first salt solution is provided. A second chamber containing a second salt solution is provided. The second salt solution has a higher salinity than the first salt solution, and is separated from the first chamber by a semipermeable membrane. The second fluid is allowed to flow from the second chamber and through an electrical power generator configured to generate electrical power from a pressure difference.

As an option, the first salt solution is sea water. As a further option, the method comprises the step of filtering the sea water at a location upstream of the semipermeable membrane.

The method optionally comprises adding salt from a salt reservoir to the second chamber. The flow of salt is optionally controlled from the salt reservoir to the second chamber using a flow control device.

As an option, the method further comprises using a valve located between the second chamber and the electrical power generator to control a flow of the second salt solution to the electrical power generator.

The method optionally further comprises connecting the electrical power generator to any of a capacitor, a rechargeable battery, a Peltier element, a condenser and a subsea installation.

An optional example of the electrical power generator is a turbine. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates schematically in a block diagram an exemplary osmotic power generator; and Figure 2 is a flow diagram showing exemplary steps.

DETAILED DESCRIPTION

It has been recognised that osmotic power can be generated using two liquids having different salt concentrations; unlike existing osmotic power generators, it is not necessary for one of the liquids to contain no salt at all.

Turning to Figure 1, there is illustrated an osmotic power generator 1. The osmotic power generator is provided with a first chamber 2 and a second chamber 3 separated by a semi-permeable membrane 4. The semi-permeable membrane 4 allows H2O molecules to pass through it, but does not allow Na or CI ions to pass through it.

The first chamber 2 holds sea water and the second chamber 3 holds liquid having a higher salinity than the sea water. The liquid in the second chamber is typically a saturated NaCI solution (termed brine). Sea water typically contains around 3.5 wt% NaCI, whereas brine typically contains around 26 wt% NaCI. In order to equalise the NaCI concentration across the semi-permeable membrane, H2O molecules will move from the first chamber 2 to the second chamber 3. This creates a differential pressure across the semi-permeable membrane, which can be used to drive an electrical power generator 5. An example of an electrical power generator 5 is a turbine, but it will be appreciated that any other type of device that generates a power using a pressure difference or a fluid flow caused by a pressure difference may be used. . The first chamber has an inlet 6 for sea water. The second chamber has an outlet to the electrical power generator 5, and a further outlet 7 to discharge liquid into the sea.

The operation consumes salt from the second chamber 3, and so the salinity of liquid in the second chamber 3 will reduce over time. In order to address this, the second chamber 3 is connected to a salt reservoir 8 of solid salt (NaCI) crystals in order to maintain the saturated solution in the second chamber. A valve may be provided between the second chamber 3 and the salt reservoir 8 in order to allow an operator to control the addition of salt to the second chamber 3.

The flow of liquid is from the sea (at a pressure determined by the depth of the osmotic power generator) into the first chamber 2. The liquid then flows through the semipermeable membrane into the second chamber 3. This liquid is at a higher pressure than the surrounding sea water, and so is forced through the electrical power generator to generate electrical power which can be used by a subsea installation 9 to which the electrical power generator 5 is connected. Typically, this power may be used to trickle charge a battery or capacitor at the subsea installation 9. The brine is then discharged into the sea through outlet 7.

A valve (not shown) may be provided between the second chamber 3 and the electrical power generator 5 in order to control power generation.

Note that the first chamber may simply be an inlet to the semi-permeable membrane 4, but it is important that the sea water is filtered before reaching the semi-permeable membrane 4 in order to prevent particulates in the sea water from 'blinding' the semipermeable membrane 4.

Figure 2 is a flow diagram illustrating exemplary steps. The following numbering corresponds to that of Figure 2: S1. Sea water flows into the first chamber 2.

S2. Owing to the salinity difference between the sea water and the salt solution in the second chamber, water passes from through the semi-permeable membrane 4 from the first chamber 2 to the second chamber 3. This increases the pressure in the second chamber 3.

S3. Salt solution in the second chamber is allowed to flow out of the second chamber through the electrical power generator 5.

S4. As salt solution flows out of the second chamber 3, the salinity of the salt solution in the second chamber is reduced. In order to address this, salt is added from the salt reservoir 8 to the second chamber.

S5. The electrical power generator 5 generates electricity as the pressurised salt solution flows through it and is discharged into the sea.

S6. The generated electrical power is used to power a subsea installation. There are many ways in which this can be achieved. A subsea installation may be powered directly. However, it is more controllable to use the generated electrical power to charge a battery or a capacity, or to provide power to a condenser or Peltier element.

Example:

Normal sea water, containing 3.5 w% NaCI, has an osmotic pressure of approximately bar, while saturated brine has an osmotic pressure of approximately 500 bar. A pressure difference approaching 475 bars should therefore be available for power generation.

The maximum possible power generated from fluid flowing with a pressure drop is given by the following equation: PH= Ap[Pcd* V[m Where P is power, Ap is pressure difference and V is volumetric flow rate.

The required continuous flow rate of brine to generate a power of 300 W is given by equation 2: 3 (2) V = 300Wen - = 0.0227 in 475 -105Pa s hr This leads to a salt consumption from the reservoir 8 of approximately 8.3 kg/hr. (Saturated brine at 0°C contains approximately 26 w% NaCI.) The efficiency of system and membranes is obviously less than 100%, so the flow rate must be somewhat larger to produce a power of 300 W. Note that where power generation is required for a limited time, the salt reservoir 8 may contain enough salt for the expected lifetime of operation. Alternatively, the salt reservoir 8 may be provided with an inlet to allow it to be recharged with salt when 20 required.

The concept of using an osmotic power generator 1 where the low salinity chamber contains sea water (rather than fresh water) allows local subsea power generation to be used.

It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention. For example, while the above description refers to power generation using a turbine, any other type of generator that generates electrical power from a fluid pressure difference may be used. (1)

Claims

CLAIMS: 1. An osmotic power generator comprising: a first chamber containing a first salt solution; a second chamber containing a second salt solution, the second salt solution having a higher salinity than the first salt solution, the second chamber being separated from the first chamber by a semi-permeable membrane; an electrical power generator operatively connected to the second chamber and configured to be driven by pressurized second slat solution from the second chamber.
2. The osmotic power generator according to claim 1, wherein the first salt solution is sea water.
3. The osmotic power generator according to claim 2, further comprising a filter disposed upstream of the semi-permeable membrane.
4. The osmotic power generator according to claim 1, 2 or 3, further comprising a salt reservoir containing solid salt and operatively connected to the second chamber, thereby allowing a salinity of the second salt solution to be maintained.
5. The osmotic power generator according to claim 4, further comprising a flow control device between the salt reservoir and the second chamber, thereby allowing an operator to control a flow of salt into the second chamber.
6. The osmotic power generator according to any one of claims 1 to 5, further comprising a valve located between the second chamber and the electrical power generator, thereby allowing an operator to control a flow of the second salt solution to the electrical power generator.
7. The osmotic power generator according to any one of claims 1 to 6, wherein the electrical power generator is electrically connected to any of a capacitor, a rechargeable battery, a Peltier element, a condenser and a subsea installation.
8. The osmotic power generator according to any one of claims 1 to 6, wherein the electrical power generator comprises a turbine.
9. A subsea installation comprising the osmotic power generator according to any one of claims 1 to 8.
10. A method of generating electrical power, the method comprising: providing a first chamber containing a first salt solution; providing a second chamber containing a second salt solution, the second salt solution having a higher salinity than the first salt solution, the second chamber being separated from the first chamber by a semi-permeable membrane; allowing the second fluid to flow from the second chamber and through an electrical power generator configured to generate electrical power from a pressure difference.
11. The method according to claim 10, wherein the first salt solution is sea water.
12. The method according to claim 11, further comprising filtering the sea water at a location upstream of the semi-permeable membrane.
13. The method according to claims 10, 11 or 12, further adding salt from a salt reservoir to the second chamber.
14. The method according to claim 13, further comprising controlling a flow of salt from the salt reservoir to the second chamber using a flow control device.
15. The method according to any one of claims 10 to 14, further comprising using a valve located between the second chamber and the electrical power generator to control a flow of the second salt solution to the electrical power generator.
16. The method according to any one of claims 10 to 13, further comprising connecting the electrical power generator to any of a capacitor, a rechargeable battery, a Peltier element, a condenser and a subsea installation.
17. The method according to any one of claims 10 to 16, wherein the electrical power generator comprises a turbine.