Patent Application
20190136832
The present invention relates to energy storage and recovery, and finds particular, although not exclusive, utility in offshore wind power energy storage.
The input torque from a relatively low-speed wind turbine is comparatively high, therefore the gear box required is both very complex and very large, heavy and costly. For example, for a 6 MW turbine the operating speed at rated power is typically 12 RPM which is much lower than the 50 HZ synchronous four pole generator speed of 1500 RPM required by the grid. This very large difference in rotational speed means that a gear box of very high ratio is required (usually three stages, one of epicyclic configuration, and two of parallel shaft helical configuration and of overall ratio 70 to 100). Such gear boxes because of their complexity, and because of the fluctuating speed and load imposed by a wind turbine have been a notorious source of the many breakdowns in wind turbine systems, and have established a reputation for unreliability in these systems.
Moreover, the gear box is a source of significant power loss because of the frictional heat developed within the many individual gear trains, and therefore a large, heavy, complex and costly lube oil system is required not only for lubrication, but principally to remove heat from the system.
Furthermore, a wind turbine does not operate at constant speed so there is also a need for a large, heavy and costly solid state frequency control system, to convert the fluctuating electrical output from the wind turbine generator to the stable output required by the grid.
The gear box, lube oil system and frequency control system have all to be incorporated in the wind turbine nacelle. Wind turbines are therefore top heavy and require strong and costly support systems. Having such equipment in the nacelle also means that wind turbines need to be provided with a permanent means of internal access to the nacelle for maintenance and overhaul.
A partial solution to these problems has been identified in the art by eliminating the requirement for a gear box and lube oil system by the invention of direct drive generators which generate at 50 Hz frequency at wind turbine speed. Such generators have a very large number of poles. Instead of a conventional four poles they may have as many as 300 poles in the stator to achieve generating frequencies of 50 Hz. Having such a large number of poles means that the generator needs to be of extremely large diameter relative to a four pole design. The poles also need to be of permanent magnet type as there is not enough space to accommodate wound poles in a machine of acceptable diameter to fit within a turbine nacelle.
Current direct drive generators eliminate a number of the traditional wind turbine problems but they are much larger and heavier, than the traditional four pole equivalents, and a large, heavy and costly frequency control system is still required in the nacelle.
According to a first aspect of the present invention, there is provided an energy storage and recovery system, comprising: a wind turbine having a wind turbine generator configured to generate electricity at a predetermined frequency below 45 Hz (in particular below 30 Hz); a compressor for compressing gas, the compressor configured to receive electricity generated by the wind turbine, the compressor having a drive motor configured to operate at the predetermined frequency; and a pressure vessel for storing gas compressed by the compressor, the gas suitable for use in an expander for driving an expander generator.
The predetermined frequency may be below 15 Hz. The wind turbine generator may be configured to generate direct-current electricity, and the drive motor may be configured to operate on direct-current electricity, direct-current being a special case of alternating current in which the frequency is substantially 0 Hz. A direct current generator would be particularly suited to the very high torques and very slow speeds of wind turbines between 5 and 20 MW rating.
The predetermined frequency may correspond to a predetermined rotational frequency of the wind turbine; that is, the predetermined frequency may be determined as a product of a predetermined rotational frequency of the wind turbine and number of pole pairs present in the wind turbine generator. For instance, if the turbine were rotating at 20 rpm, and the wind turbine generator comprised 30 pole pairs, then the predetermined frequency would be given by 30×20 rpm/60=10 Hz. Alternatively, if the turbine were rotating at 12 rpm, and the wind turbine generator comprised 75 pole pairs, then the predetermined frequency would be given by 75×12 rpm/60=15 Hz. There may be, for instance between approximately 40 and 100 pole pairs, in particular 50 and 90, more particularly 60 and 80.
The wind turbine generator is configured to be incompatible with the grid. In this way, the system may be configured to use all the energy generated by the wind turbine (e.g. in its entirety) to produce compressed air for the storage system vessel. Similarly, energy recovered from the compressed air may be used solely for (and or may be entirely dedicated to) supplying electricity to the grid (e.g. the national grid).
The energy storage and recovery system may further comprise: an expander arranged to receive compressed gas from the pressure vessel; and/or an expander generator configured to generate electricity having a frequency in the range 50-60 Hz; wherein, the expander may be configured for driving the expander generator in response to expansion of the compressed gas.
In this way, avoiding any electrical connection between the wind turbine generator and the grid allows a free choice of wind turbine generator frequency and voltage. This enables the wind turbine generator and the associated compressor drive motor to be of a lower frequency than the 50 Hz grid frequency, (for example 5, Hz, 12.5 Hz, 25 Hz or 30 Hz). This low frequency would avoid the need for high ratio gear boxes and their associated complex lube oil systems which are needed to drive high speed or medium speed 50 or 60 Hz grid connected generators. At the same time it would facilitate the use of direct drive generators of much smaller size and weight than current types of direct drive generators, which because of the need to operate at 50 or 60 Hz have many more electrical poles than would be required by a low frequency system. The compressor drive motor would operate at the same frequency as the wind turbine generator.
The expander would drive the grid generator at normal 50, or 60 Hz grid frequency, which since it is driven only by the compressed stored air is never subject to the fluctuations in shaft power and speed which arise with a generator linked to the wind turbine. Hence the complexity of the generator control system, which has been a significant cost element in wind turbine systems to date, might be significantly reduced. With the system proposed therefore, at some loss of energy efficiency, capital cost savings and significant reductions in system complexity should accrue.
This invention is for a compressed air energy storage system applicable to offshore wind turbines in which all the power generated by the wind turbine is delivered to the storage system and none directly to the grid, so that the wind turbine generator and compressor motor can be designed, independently of the grid requirements, as low frequency machines or even direct drive machines. The low frequency wind turbine generator may then be directly driven from the wind turbine without any need for complex multi stage gears and their associated lube oil systems. Such a low speed, low frequency, direct drive generator will be of substantially lower size, weight and cost than a direct drive generator of 50 or 60 Hz design.
At least one of: the expander generator; the expander; an expander-generator set/system (comprising the expander generator and the expander); the pressure vessel; and/or the compressor; may be spaced from the turbine.
In this way, complex equipment may be placed away from the environment in which the turbine is located, which is often exposed. In some embodiments, the pressure vessel may take the form of a pipe for conveying pressurised air from the compressor to the expander; however, in other embodiments, a pipe is provided simply for transmitting the air, and does not form part of the pressure vessel. In some embodiments, electricity from the wind turbine generator may be conveyed at the predetermined frequency to compressors spaced away from the turbines.
In this way, the invention allows for the single compressed air storage vessel to be extended from offshore to land, acting as pipeline for the delivery of compressed storage air as well as a vessel. This consequently allows an expander-generator set to be shore based and connected directly to the land based grid without the need for any undersea transmission cables. The pipe may have a diameter of at least 3m, 4m, 5m or 6m. In this way, friction/pressure losses can be mitigated.
The pressure vessel may be configured to hold air at a pressure of up to 0.5 MPa, 1 MPa, 1.5 MPa, 2 MPa, 2.5 Mpa or 3 MPa.
A plurality of turbines may be used in a turbine array.
The pressure vessel may comprise a buoyancy chamber of an off-shore wind turbine. For instance, the buoyancy chamber may be a buoyancy column of an articulated wind column.
The pressure vessel may comprise an air-tight container configured to be located underwater, for instance on a sea bed.
This vessel may be of lightweight and/or low-cost construction, for instance of steel or concrete construction, since being located on the sea bed, the sea water pressure around the reservoir would counteract the pressure of the stored compressed air within the reservoir. The vessel may be spherical or cylindrical in shaped, or may be any other convenient shape for the environment into which it is to be placed. The vessel may be configured to operate as a displacement vessel; that is, when empty the vessel may be flooded with water, which would then be displaced when compressed air is supplied to the vessel. In this way, the pressure of air extracted from the vessel may remain constant.
The compressor and expander may be a single compressor-expander system/set. The expander may comprise the compressor being run in reverse. The drive motor and generator may be a combined motor-generator system. The generator may be the drive motor run in reverse. The compressors may be gear type compressors.
The compressor and expander may be combined into a single machine such that a subset of the impellers are dedicated to compressing air and a further distinct subset are dedicated to expanding the air. In any specific compression or expansion step, the unused impellers may be operated against closed inlet valves, or may be decoupled.
The compressor and/or expander may be single- or multi-stage, and the expander may be a turbo expander. In multi-stage compressors/expanders, valves in the inlet of each stage may be provided such that various combinations of stages may be bypassed consecutively as required, to mitigate for any decline/increase in storage vessel pressure, so that each expander stage can always be operated at near the optimum pressure ratio, and consequently at the best efficiency.
The system may further comprise an air drier to dry stored air, thereby preventing internal condensation and icing of the expander system during the energy recovery phase.
The expander may comprise a combustion system for increase the power available from the expander. The combustion system may be provided with a fuel supply. Alternatively or additionally, a heating system may be provided to heat the stored compressed air prior to submission into the expander; for instance via electrical heating or via the (or another separate) combustion system. However, it is preferred that such a combustion system is absent.
The energy storage and recovery system may further comprise: an expander arranged to receive compressed gas (e.g. air) from the pressure vessel, and configured for driving an expander generator in response to expansion of the compressed gas; and an expander heat exchanger connected to the expander such that heat is transferred from sea water within the expander heat exchanger to the gas used in the expander.
The energy storage and recovery system may further comprise: an expander arranged to receive compressed gas from the pressure vessel, and configured for driving an expander generator in response to expansion of the compressed gas; a compressor heat exchanger connected to the compressor such that heat is transferred from the gas compressed in the compressor to working fluid within the compressor heat exchanger; a working fluid reserve, for holding the working fluid from the heat exchanger; and/or an expander heat exchanger connected to the expander such that heat is transferred from the working fluid held within the working fluid reserve to the gas used in the expander.
In some embodiments, compressed air within the storage vessel may be supplied from an air compressor driven by an electric motor, for which the power is provided by means of the wind turbine generator of the wind turbine itself or from the electrical grid to which the expander generator is connected, thereby providing a means of storage capacity for the grid itself, for the benefit of other renewable energy sources such as solar or tidal energy etc. and also for the base load generating plant.
According to a second aspect of the present invention, there is provided a method of storing energy for subsequent recovery, the method comprising the steps of: providing an energy storage and recovery system according to any preceding claim; generate electricity at a predetermined frequency below 30Hz using a wind turbine generator of a wind turbine; receiving electricity generated by the wind turbine at a compressor; operating the drive motor of the compressor at the predetermined frequency to compress gas; and storing the compressed gas in a pressure vessel.
For most envisaged applications, the stored air will be at ambient temperature, since over the storage period any residual heat of compression will be lost to the atmosphere. Alternatively or additionally, during the compression phase the heat of compression may be extracted via intercoolers and aftercoolers and stored (for instance as hot water in an insulated vessel), thus making it available for later use in the energy recovery phase.
According to a third aspect of the present invention, there is provided an energy storage and recovery system, comprising: a compressor for compressing gas; a wind turbine arranged to drive the compressor; a pressure vessel for storing gas compressed by the compressor; an expander arranged to receive compressed gas from the pressure vessel, and configured for driving an expander generator in response to expansion of the compressed gas; and an expander heat exchanger connected to the expander such that heat is transferred from sea water within the expander heat exchanger to the gas used in the expander.
According to a fourth aspect of the present invention, there is provided an energy storage and recovery system, comprising: a compressor for compressing gas; a wind turbine arranged to drive the compressor; a pressure vessel for storing gas compressed by the compressor; an expander arranged to receive compressed gas from the pressure vessel, and configured for driving an expander generator in response to expansion of the compressed gas; a compressor heat exchanger connected to the compressor such that heat is transferred from the gas compressed in the compressor to working fluid within the compressor heat exchanger; a working fluid reserve, for holding the working fluid from the heat exchanger; and an expander heat exchanger connected to the expander such that heat is transferred from the working fluid held within the working fluid reserve to the gas used in the expander.
The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.
The present invention will be described with respect to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. Each drawing may not include all of the features of the invention and therefore should not necessarily be considered to be an embodiment of the invention. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that operation is capable in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that operation is capable in other orientations than described or illustrated herein.
It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term “connected”, used in the description, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression “a device A connected to a device B” should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. “Connected” may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other. For instance, wireless connectivity is contemplated.
Reference throughout this specification to “an embodiment” or “an aspect” means that a particular feature, structure or characteristic described in connection with the embodiment or aspect is included in at least one embodiment or aspect of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, or “in an aspect” in various places throughout this specification are not necessarily all referring to the same embodiment or aspect, but may refer to different embodiments or aspects. Furthermore, the particular features, structures or characteristics of any embodiment or aspect of the invention may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments or aspects.
Similarly, it should be appreciated that in the description various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Moreover, the description of any individual drawing or aspect should not necessarily be considered to be an embodiment of the invention. Rather, as the following claims reflect, inventive aspects lie in fewer than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.
Furthermore, while some embodiments described herein include some features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form yet further embodiments, as will be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practised without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
In the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, coupled with an indication that one of said values is more highly preferred than the other, is to be construed as an implied statement that each intermediate value of said parameter, lying between the more preferred and the less preferred of said alternatives, is itself preferred to said less preferred value and also to each value lying between said less preferred value and said intermediate value.
The use of the term “at least one” may mean only one in certain circumstances. The principles of the invention will now be described by a detailed description of at least one drawing relating to exemplary features of the invention. It is clear that other arrangements can be configured according to the knowledge of persons skilled in the art without departing from the underlying concept or technical teaching of the invention, the invention being limited only by the terms of the appended claims.
The vessel 5 is configured to store compressed atmospheric air and can thus act as a means of energy storage. The internal volume of the buoyancy chamber for the support system shown in
Combinations of each of the features discussed above are also considered.
(C) shows how using a hot water reservoir could increase the power and energy recovery in a single stage expander by preheating of the compressed air before expansion. The heated water may be provided by supplying water to compressor intercoolers during compression (i.e. during energy storage). This heated water may be stored in an insulated vessel 109.
(D) shows a two stage expander in which hot water is used in a preheater as in (C), and also in an interstage heater similar to (B) for even greater power and energy recovery. With additional expander stages and interstage heaters the energy recovered could be further increased.
In any system for storage of compressed air, in which the reservoir pressure reduces in the energy recovery phase of operation, there is a difficulty in achieving a high efficiency in the expander because the expansion ratio reduces significantly as the storage reservoir is depleted.
In the compressor mode, air is received and passed to impeller c1 where it is compressed due to operation of the motor 137, then cooled in heat exchanger h1 before being passed to impeller c2, whereupon it is cooled in heat exchanger h2, passed to impeller c3, returned to heat exchanger h2 where it is cooled still further, sent to impeller c4 and finally cooled in heat exchanger h3 before being sent to the pressure vessel. The working fluid in the heat exchangers h1, h2, h3 that has been heated up by compression of the air is stored for further use.
In the expansion mode, air is heated in heat exchanger h3 on its way to be taken from the pressure vessel to impeller c4, where expansion drives the motor 137. It is heated in heat exchanger h2, expanded by impeller c3, heated again in heat exchanger h2, expanded in impeller c2, heated in heat exchanger h1 and expanded in impeller c1 before being exhausted.
While various embodiments of devices, systems, and methods of using the same the same have been described in considerable detail herein, the embodiments are merely offered as non-limiting examples of the disclosure described herein. It will therefore be understood that various changes and modifications may be made, and equivalents may be substituted for elements thereof, without departing from the scope of the present disclosure. The present disclosure is not intended to be exhaustive or limiting with respect to the content thereof.
Further, in describing representative embodiments, the present disclosure may have presented a method and/or a process as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth therein, the method or process should not be limited to the particular sequence of steps described, as other sequences of steps may be possible. Therefore, the particular order of the steps disclosed herein should not be construed as limitations of the present disclosure. In addition, disclosure directed to a method and/or process should not be limited to the performance of their steps in the order written. Such sequences may be varied and still remain within the scope of the present disclosure.
