WO2009024933A2

WO2009024933A2 - Aquatic system for energy storage in the form of compressed air

Info

Publication number: WO2009024933A2
Application number: PCT/IB2008/053337
Authority: WO
Inventors: Pedro Domingues De Almeida; Pedro Nuno Dinho Da Silva
Original assignee: Universidade Da Beira Interior
Priority date: 2007-08-22
Filing date: 2008-08-20
Publication date: 2009-02-26
Also published as: WO2009024933A3; PT103812A

Abstract

The present invention concerns an underwater system for energy storage, which resorts to compressed air, directed to adjust in time the generation of electrical energy by large generation systems to the consumption curves. For this purpose, the system comprises a compressed air reservoir of variable-volume (3), a system anchorage component (4), and a union tube (2) connecting the system to a means of energy generation and air compression (1). Therefore, the present invention finds its main application in the adjustment of energy generation associated with floating oceanic wind parks, to be installed far away from coastal regions, or with other energy generation oceanic systems that are subjected to irregular schemes of generation or consumption.

Description

DESCRIPTION

"AQUATIC SYSTEM FOR ENERGY STORAGE IN THE FORM OF

COMPRESSED AIR"

Technical domain of -the invention

The present invention consists of an energy storage underwater system, which works through compressed air, composed of a compressed air reservoir of variable-volume, a pressure tube that connects the external system of energy generation to the reservoir, and a system anchorage component .

Hence, the present invention finds its application in the energy industry, aiming at adjusting in time the generation of electrical energy by large aquatic systems to the curves of electrical energy consumption.

Precedents for the invention

Electric distribution systems of similar dimension to the Portuguese national system present an important issue concerning the daily consumption cycles. These cycles imply that consumption peaks during some parts of the day, whereas at other times the consumption levels presented are several times inferior. Traditionally speaking, these cycles were known in advance and foreseeable and the generation of electrical energy (at thermal power stations and dams) could be adjusted to the daily consumption curves. However, the recent appearance of generation systems that rely on new sources of renewable energy has raised new issues on the subject, for systems such as these add amounts of uncertainty and unpredictability to the production levels. In quantitative terms, among the new procedures of generation of electrical energy, wind energy is by far the most important one. It is also this which raises more issues in terms of predictability and relation between the maximum and minimum values of power generation.

There are several methods available to lessen this problem of balance between the values of generation and the values of demand. The exchange of electrical energy with adjacent exterior networks and the direct adjustment of the generation in the electro-generator systems where the power generated is more easily adjusted (this adjustment is more easily and rapidly done in a dam than in a big thermal power station, for instance) are amongst those methods. Nonetheless, these methodologies present significant limitations. For example, consider a summer scenario where the storage of artificial dams is already nearing the minimum elevations, limiting the capacity of adjustment through the power generated in dams and the fact that adjacent electric distribution systems usually have coincident daily consumption cycles.

The storage of electrical energy would undoubtedly be the most effective way of adjusting the instantaneous values of generation to the demands of consumption but, given the huge amounts of energy that are involved, at present there are no technological solutions available that allow for such storage to be made in conditions of economic viability.

Therefore, there is no other option but to use indirect methods for storage through which the energy is stored in non-electrical forms, but may then be transformed into electricity with small amounts of energy loss, allowing to quickly change the energy that has been converted into electricity at each given moment.

Amongst these methods, at present, it is most common to store in the form of water drifting between two reservoirs at different elevations (different altitudes) , where the energy is stored in the form of gravitational potential energy and the amount of stored energy depends on the volume of the reservoirs and the difference of levels between them.

Another storage method with capacity to store large amounts of energy makes use of compressed air and is usually known by the abbreviation CAES (Compressed Air Energy Storage) . At present, there are few practical examples of this type of large dimension systems operating, considering that, up -to this moment, the practice that has been followed has been that of making the most of the pre-existing reservoirs, such as mines or natural caves. Given that the volume of air storage required for balancing systems of significant electricity generation is very large, the option of building reservoirs from scratch, intended for that purpose, has not been considered viable.

Compressed air may, therefore, be used for energy storage. When recovering energy from compressed air, most often mechanisms are used to convert into electrical energy the mechanical energy that results from the compressed air expansion till it reaches the atmospheric pressure. There are, however, relatively rare cases when the final purpose is to use the mechanical energy, hence, the process of conversion into electrical energy is not needed. One such example is the direct use of compressed air for the propulsion of vehicles, and industrial equipments. Nevertheless, in small dimension systems such as these, it is necessary to use very high pressure containers, which become extremely heavy, in order to store a beneficial amount of energy in a reduced volume.

Due to this factor, compressed air energy storage presents more economical and effective results in large dimension fixed systems. Systems aiming at storing energy to balance generation and consumption curves in electric distribution systems are practical examples to this type of large dimension systems.

For these purposes, in particular, large rigid containers or natural storage systems, such as mines and natural caves located at great depths, are more commonly used. These storage systems present a fixed-volume and the pressure of the air that has been stored varies during the usage cycle, a progressive reduction being observed as the air is consumed. This progressive pressure reduction that occurs while the stored energy is being recovered has a negative impact on the efficiency of the compressed air/electricity conversion systems.

A thorough research of this technique's situation led to the discovery of several documents that reveal compressed air energy storage technology. It must be noted that, among these, only a small subclass deals with compressed air underwater storage and, among them, some disclose the resource to rigid containers (usually made of steel or other metals) where the volume of air storage is constant and, consequently, the stored air pressure is variable. Document GB1356488 presents an underground system and document JP2071053 presents a surface system that uses an altitude differential between two containers. Essentially, the two documents describe compressed air storage systems that operate on a constant air pressure basis, the two systems being found ashore. Both of them use the hydrostatic pressure of a water column but employ rigid reservoirs of constant volume for storage. These systems are essentially different to the one that is now being proposed, for, since they are ground-based instead of underwater-based, they do not use the external pressure that is naturally produced by the surrounding aqueous environment to balance the pressure that is internally produced by the compressed air on the container walls.

When it comes to underwater systems, which depict combustible gases or compressed air storage systems, the document US6347910 is worthy of note. It refers to an energy storage system, fully sited at the bottom of a body of water. This system is completely different from the one that is now being proposed, given that, apart from having its foundation laid on the water bed instead of being sited at the intermediate depth that produces the desired working pressure, it relies on rigid containers of variable- pressure instead of using the concept of containers of variable-volume working at a constant pressure, and integrates the compression and decompression systems together with these containers at the bottom of the body of water.

Documents JP57008363 and JP2119638 describe a system in which a rigid container has its foundation laid at the bottom of a body of water. This container is employed to store compressed air at the hydrostatic pressure of the bottom of the body of water, the pressure balancing being performed through the water inlet and outlet motion in the container. It must be noted that the main difference between the subjects presented on both documents is that document JP2119638 does not mention a control valve regulating the water inlet and outlet in the container.

These systems are different to the one that is now being proposed, given that, apart from having their foundations laid at the bottom of the water body instead of being sited at the intermediate depth that produces the desired working pressure, they rely on rigid containers of constant volume, where the water inlet is allowed to fill up the volume that has not been taken by the compressed air, instead of using the concept of containers of variable-volume working at a constant pressure.

Document US4402632 also describes an underwater system of compressed, cooled, liquefied gas storage. However, it does not refer to a compressed air storage system, but to a combustible gases storage system, requiring thermal insulation. Hence, unlike the system that is now being proposed, it relies on a rigid container, thermally insulated, having its foundation laid at the bottom of the ocean, where the volume variation and the consequent pressure adjustment are achieved through a plunger motion. Basically, this system is entirely different from the present invention, both in its goal and technical solutions .

Document US4433940 presents the underwater system of combustible liquids or gases storage, which has the capacity to place the container at pre-selected depths, somewhere half-way of the water column, and requires thermal insulation. This system denotes some resemblances to the present proposal. Nonetheless, the container depicted in this document (US4433940) differs from the one that is proposed by the present invention, for, since it is intended for combustible liquids and gases storage, it uses a thermally insulated rigid reservoir, the volume being adjusted through a plunger motion. On the contrary, the present invention relies on a flexible container, "balloon"-type, which is not thermally insulated, all volume being naturally adjusted through the swing between the stored air pressure and the local hydrostatic pressure.

Document JP58214608 divulges a system that relies on a compressed air container of variable-volume, which is submerged to the bottom of the body of water, where its foundation lies. In this document, the working pressure of the compressed air is directly dependent on the local depth of the body of water, and it cannot be adjusted according to efficiency criteria of the systems of compression and recovery of stored energy. Furthermore, the described system is not adequate for oceanic systems whose depths involved exceed the optimum working pressures. It must also be noted that, in the case of energy generation systems that comprise significant surfaces (a floating oceanic wind park, for instance) , the system that has been described in the document would imply the use of reservoirs with several working pressures, as a result of the different depths of each plant location.

Within this ambit, none of the previously described inventions mentions the contents of the invention that is now being proposed, considering that the object is an energy storage system in the form of compressed air where the reservoir, of variable-volume, is suspended at any desired depth. It must be noted that the system of the present invention is adequate for oceanic systems whose depths involved exceed the optimum working pressures, for it allows to freely adjust the depth of each individual reservoir. This enables that they be all located at the same depth, making it possible to use identical air compression and energy reuse systems for all containers.

Brief description of the figures

Figure 1 presents a layout of the present invention operating principle. Thus, (1) refers to an energy generation system, (2) represents a pressure tube, which directs the compressed air produced by the energy generation system, (3) corresponds to a container of variable-volume and (4) characterizes a mechanism for anchorage to the bottom of the body of water.

Description of the invention

Essentially, this invention aims at storing energy in the form of compressed air in a reservoir of variable-volume (3), immersed in a deep body of water, in oceanic conditions, for instance, and making use of the water column's hydrostatic pressure, produced over the external part of the container surface, to accurately compensate the pressure produced by the compressed air over the internal surface of the container. This balance between internal and external pressures avoids the need to use in the container a coating that is resistant to significant pressure differentials . For this purpose, the system that is proposed by the present invention comprises a compressed air reservoir of variable-volume (3), a union tube (2) connecting the system to an external means of energy generation and air compression (1) and an anchorage mechanism (4) whose purpose is to establish the system on the desired site and depth, preventing it from undergoing horizontal or vertical displacements of consequence.

The storage process takes place at a constant pressure on the inside of a reservoir of variable-volume (3), which may take on several types, preferably being a deformable balloon (with a flexible, yet not elastic, surface and a behaviour that is similar to that of an inflatable buoy) . A way of implementing this concept consists in the previous definition of folding lines of the reservoir' s flexible surface, similarly to what happens with conventional structures of the bellows type. In this way, the reservoir changes its storage volume in order to be able to enclose variable amounts of compressed air. Given the fact that this flexible reservoir is immersed in an aqueous environment, the pressure produced by this environment is directly transmitted to the air that is enclosed inside the reservoir, reason why it stays compressed in accordance with the external hydrostatic pressure. The latter is only conditioned by the depth that the reservoir is located at. Therefore, as long as the reservoir does not undergo vertical displacements, it can be assumed to work at a constant pressure.

When this reservoir contains compressed air, it shall be subjected to a buoyancy force on account of the volume of displaced water. This buoyancy would oblige the reservoir to ascend were it not held by an anchorage system.

Therefore, apart from being composed by the deformable surface of air containment, the reservoir needs to include a restrain system connected to the anchorage mechanism. To avoid efforts that would oblige a very resistant surface being used in the reservoir (particularly in the areas where the anchorage system cables are fixed to the wall of the reservoir) , it is proposed that the anchorage system be fixed to a resistant pellicle or wire netting that covers the upper part of the reservoir instead of being fixed directly to its surface.

On account of the natural equalisation between the external hydrostatic pressure and the pressure of the air that is enclosed inside the reservoir, the walls of the reservoir do not need to endure major pressure differentials. The proposed containment system, composed of a wire netting or a pellicle connected to the anchorage mechanism, shall contribute to attenuate local efforts performed over the reservoir's surface, such as those that may result from the action of oceanic currents.

The approach proposed by this invention requires that the reservoir enables the pressure' s direct transmission between the surrounding aqueous environment and the stored air. Therefore, this reservoir cannot be entirely rigid and synclastic, for it would not enable the mentioned pressure transmission. However, the above-mentioned flexible and deformable surface model is not the only possible solution for the implementation of this invention. In fact, to have the necessary conditions to enclose air and transmit the pressure, the reservoir's surface may be: plastically deformable (as described by the preferential solution proposed above) ; elastic (as the common air balloons used at parties are) ; a combination of one or more rigid elements with deformable, plastic or elastic surfaces (for instance, a reservoir composed of a rigid bell-shaped cover in the upper part and a flexible surface in the lower part); composed of one or more rigid elements that may slide, some in opposition to the others, to allow the volume to be changed.

For the pressure of the stored air to be as desired for the systems of compression and energy reuse, the reservoir ought to be located at an intermediate depth between the free surface and the bottom of the body of water, chosen so that the hydrostatic pressure corresponds to the desired working pressure. Increasing the system installation depth has a direct influence upon the increase of the pressure produced on the external surface of the reservoir. In contrast, on account of the capacity of volume variation of the reservoir, increasing the external pressure shall result in a larger compression ratio of the air enclosed in it, the pressure of the compressed air tending to equal the external pressure produced by the water on the surface of the reservoir.

The installation depth of the storage system should, therefore, be selected so that the hydrostatic pressure produced on the reservoir corresponds to the working pressure that optimises the performance of the systems of air compression and energy recovery from the compressed air . Given the fact that this invention concerns the energy storage system and, therefore, is not an integral part of the air compression and expansion component, the specification of the working pressure (and, consequently, of the working depth) shall be established in accordance with the specific characteristics of the air compression and decompression/energy recovery units used in the external system of generation. However, in view of the typical characteristics of the units that are used at present, the amounts of energy to be stored to regularise electrical generations of large scale oceanic industrial systems, and the need to incorporate an anchorage system, the water column available (depth of elevation between the free surface of the body of water and its water bed) is expected to be in the order of a hundred meters or more. Hence, typically speaking, this invention shall be applied when the mentioned _.depths occur on a natural basis or when it is possible to achieve them artificially through dredging or similar methods.

The compressed air to be used for storing energy shall originate from an energy generation system (1), which includes the air compression and decompression units. Among the typical examples of these energy systems are platforms, either floating or with a rigid structure having its foundation laid at the bottom of the body of water, that comprise wind turbines or wave energy reuse systems. The compressed air produced by the energy generation system (1) is directed through a pressure tube (2) into the container of variable-volume (3) . The pressure tube shall be able to support the working pressure, for it is subjected to external pressures that differ between the atmospheric pressure, on the platform area, and the working pressure of the system, on the reservoir area.

Moreover, this tube shall be able to bear the oceanic environment (salty water) without undergoing external deterioration. Pressure tubes, with similar characteristics, are already in use in oceanic applications associated with scientific and industrial activities, such as oceanographic research or fossil fuels exploitation.

It must be noted that the transport of air between the energy external system and the compressed air reservoir may^¬ be accomplished through a single pressure tube, which shall be employed to direct the air from the energy system (air compression unit) into the reservoir during the stages of energy storage, and from the reservoir into the energy- system (air decompression and energy recovery unit) during the stages when the intention is to generate electrical energy from the compressed air. Nonetheless, it shall also be possible to use a conduit with two pressure tubes, one intended to direct the air from the external system into the reservoir and the other from the reservoir into the energy recovery system. The second configuration enables to direct the air in both directions simultaneously and, therefore, it shall enable the energy system to also simultaneously generate compressed air and electrical energy from the compressed air coming from the reservoir. This type of solution may, for instance, be employed when the air compression system directly uses mechanical energy from the capture systems (for example, wind turbines or wave energy recovery systems) without intermediate conversion into electricity. In this case, the power generation system would always be related to the recovery of compressed air from the reservoir. Hence, intermediate stages of power generation and power usage for air compression would be avoided, and so would the related conversion losses.

The compressed air container of variable-volume is lighter than the volume of displaced water, reason why it shall be sustained at the required depth, for the pressure produced by the water column to correspond to the operating pressure, through a mechanism of anchorage to the bottom of the body of water (4), which prevents it from ascending and undergoing significant lateral displacements.

In order to fix the mechanism of anchorage (4) to the bottom of the body of water, it is possible to rely on weights that succeed in duly fixing themselves to the bottom ^'of the body of water, preventing lateral displacements, and compensating the involved buoyancies. This type of anchorage solutions may be implemented through metal mechanisms, such as the conventional ship anchors, or other resources, for instance, reinforced concrete objects aground with configurations that prevent sliding along the water bed.

Another way to fix the reservoir anchorage system shall be that of employing foundations already laid in the water bed that may eventually be shared with the fixation system of the energy generation platform.

Besides the mechanisms of fixation to the water bed, the anchorage system needs to include elements connecting these mechanisms and the compressed air reservoir. As usual, in this kind of oceanic fixation solutions, these elements shall preferably be cables that are resistant to the environment and adequate to the efforts that they are subjected to. The connection of these cables to the reservoir may be established either through a direct connection to fixing points on the reservoir's surface, either through their connection to a wire netting or a pellicle that envelops at least the upper part of the reservoir (which shall be subjected to a buoyancy force), and is adequate for its fixation. The main advantage offered by the second solution of connection to the anchorage system shall be that of avoiding the need to greatly reinforce the waterproof pellicle, which constitutes the reservoir's surface itself, given that, by lodging the local efforts that are related to the fixing points of the anchorage cables, the mentioned cover shall prevent local efforts produced on the reservoir surface.

The compressed air storage system, which is the object of the present invention, aims at accumulating large amounts of energy for posterior conversion into electrical energy, enabling it to balance the power generation and consumption levels when it comes to large generator systems working at a constant pressure. Hence, the compressed air/electricity efficiency is favoured. The air compression and the decompression and energy recovery units shall be an integral part of the external system of energy generation, and, as such, they do not integrate the system of the present invention. The air compression process naturally results in a considerable heat release. Given the fact that these systems operate in an oceanic environment, the dissipation of the heat generated by the compression system may conveniently be established through dissipators that are in contact with the aqueous environment. Nevertheless, following the compression process, the air shall still present a higher temperature than that of the environment. This excessive temperature shall be reduced along the trajectory that leads to the storage reservoir, since the pressure tube that directs the air to the reservoir is immersed in the oceanic environment and, therefore, works as a natural heat dissipator. In case the specific conditions of implementation of the system recommend it, the heat dissipation along the air trajectory in the tube may easily be stimulated by connecting dissipation surfaces to the external part of the tube, which increase the area of contact with the aqueous environment. The residual heat that the air may present following the trajectory in the union tube, connecting the surface and reservoir, shall naturally be dissipated during its stay in the reservoir, since the latter finds itself equally immersed in an aqueous environment of extremely vast dimensions.

Contrary to what happens during the compression process, where the resultant heat could eventually interfere with the air storage process that is the object of this invention, the decompression process of the compressed air does not exercise any influence over the storage system, given the fact that the cooling occurs during the air expansion in the electrical energy generation system. Thus, this problem is external to the storage system, which is the object of this invention. However, considering that the operating conditions of the unit of decompression and energy generation occur in an oceanic environment, the concerned heat transfers may be accomplished in a much more convenient way than it would be possible in systems located on dry land, through the resource of either the atmospheric environment, or, and in particular, the vast aqueous environment that is available.

The energy storage system that is the object of this invention is preferentially applicable to energy generation systems in the deep sea or other bodies of water with depths in the order of 100 meters or more.

The implementation of floating oceanic wind parks to be installed far away from coastal regions, with resource to large dimension aerogenerator groups, seems extremely promising when compared to the current ones, located in shallow waters and usually near the coasts. Among the most important advantages presented by these parks located in the deep sea, what stands out is the increase in productivity, for the reasons that they are located in areas where the wind tends to blow stronger and more regularly, decrease in the environmental concerns related to visual impacts or damages to the avifauna, non-existence of space limitations that narrow the parks dimension, and decrease in the impact on the coastal traffic that parks located nearer to the coast may present.

Other energy generation systems located in the deep sea that may gain importance in the short run include wave energy reuse systems, solar energy collection floating systems or even oceanic thermal differentials reuse systems .

The system of energy storage through compressed air, object of the present invention, is preferably aimed at this type of oceanic systems of energy generation. This is to clarify that the completions of the present aquatic system for storing energy in the form of compressed air, previously described, are mere possible examples for implementation, simply established in view of a clear understanding of the invention principles. The previously mentioned completions may undergo variations and modifications provided that they do not divert substantially from the spirit and principle of the invention. Every variation and modification shall be included within the ambit of the present invention and protected by the following claims.

Claims

1. Aquatic system for energy storage in the form of compressed air comprising: one or more underwater reservoirs of variable-volume (3), wherein the air storage occurs at a constant internal pressure; one or more union tubes (2) connecting the energy storage system to an external means of energy generation and air compression (1); and anchorage components (4), which fix the aquatic system to the desired site and depth, hence preventing horizontal or vertical displacements.

2. Aquatic system for energy storage according to the previous claim, characterised in that the reservoir of variable-volume is of flexible, elastic or plastic nature .

3. Aquatic system for energy storage according to the previous claim, characterised in that the reservoir of variable-volume consists of a structure of the bellows type.

4. Aquatic system for energy storage according to claim 1, characterised in that the reservoir of variable-volume consists of a combination of one or more rigid elements with deformable, plastic or elastic surfaces, which constitutes .

5. Aquatic system for energy storage according to claim 1, characterised in that the reservoir of variable-volume consists of one or more rigid elements that may slide, some in opposition to the others, to allow the volume to be changed.

6. Aquatic system for energy storage, according to claim 1, comprising a combination of one or more reservoirs according to claims 2 to 5.

7. Aquatic system for energy storage, according to claim 1, characterised in that the anchorage components (4) consist of weights, metal mechanisms of anchor type, or reinforced concrete objects.

8. Aquatic system for energy storage, according to claim 1, characterised in that the elements connecting the anchorage components (4) and the compressed air reservoir are cables that are resistant to the environment and adequate to the efforts that they are subjected to and in that the connection between these cables and the reservoir is established by means of either a direct connection to fixing points on the reservoir surface, or the connection to a wire netting or a pellicle that envelops at least the upper part of the reservoir.

9. Use of the aquatic system for energy storage, according to the previous claims, characterised in that it is employed for storing energy, in the form of compressed air in a reservoir of variable-volume immersed in a deep body of water, which uses the water column's hydrostatic pressure, produced over the external part of the container's surface, to compensate the pressure produced by the compressed air over the internal surface of the container .