Patent Description:
Many energy sources, particularly clean energy sources such as wind turbines and solar panels, generate energy that does not temporally match the load experienced. In much of the developed world, energy generation follows experienced load, such that energy is provided as needed. Under circumstances of high load, techniques such as the use of peaker generators and spinning and non-spinning reserves on thermal generators allow for generation that matches high and variable load. However, despite the availability of such techniques, there are often instances where energy storage is important for meeting energy load.

Existing energy storage systems all have drawbacks of one form or another. Size, price, storage efficiency, efficacy, and safety are all concerns when designing an energy storage system. Generally, smaller size, lower price, reduced loss in both inputting energy for storage and extracting it for distribution, reduced losses for continuous operation, and safe disposal are all preferred characteristics of energy storage systems.

A flywheel is one type of energy storage system that stores energy as rotational kinetic energy. An example of a flywheel for energy storage is disclosed in <CIT>. A flywheel rotor is a weighted, rotationally symmetric mass that spins while physically coupled, directly or indirectly, to a motor/alternator that itself is electrically coupled to a converter, such as a back-to-back inverter system, constituting an AC-AC conversion subsystem. When power is received for storage, the rotor is driven, increasing the rotational speed of the flywheel rotor. When power is to be extracted, the flywheel rotor drives the motor/alternator. The faster a flywheel rotor spins, the more energy it stores, but the faster it spins, the higher the rotational losses due to aerodynamic drag. To reduce aerodynamic drag, the flywheel is operated in a chamber which is evacuated, also referred to as a vacuum chamber, to operating pressures that equate to small fractions of an atmosphere. For example, in certain embodiments, the operating pressure range is. <NUM> Torr to. <NUM> Torr. (<NUM> Bar = <NUM> Torr).

Elastomeric seals, such as o-rings, are commonly used to seal flanges on vacuum chambers. One drawback to using an elastomeric material is that the permeation rate of gasses is higher for elastomers than for metal seals. Using metallic seals on smaller flanges is very common but becomes more difficult and expensive as the diameter or total length of the seal gets larger. Such seals have the further disadvantage that they are typically single use.

Greases are commonly applied to improve the sealing characteristics of elastomeric seals and help protect them from deterioration due to oxidation and hardening. Typically, grease is applied as a light coating all around the elastomeric seal to provide a thin layer that compensates for microscopic surface finish imperfections. Such a grease layer is typically thin relative to the seal cross-section, and thus does not contribute significantly to slowing permeation.

Manufacturing metallic vacuum chambers using stainless steel helps prevent corrosion of seal surfaces. However, stainless steel is relatively expensive compared to ordinary steel alloy. Thus, it would be desirable have a technique for preventing corrosion across seal surfaces that allows less expensive steel to be used for vacuum chambers.

The invention provides a vacuum chamber for a flywheel device according to the subject-matter of claim <NUM>. The subject invention uses a grease channel in conjunction with an o-ring to improve the seal between two adjacent planar surfaces. The use of a grease channel may reduce the permeation of gasses, especially water vapor, and thus reduces corrosion. The invention is typically used when at least one of the two planar surfaces is metallic and is thus subject to corrosion. However, it may also be used in other contexts.

In certain embodiments, the invention is used to improve a seal between adjacent elements in a flywheel device. It improves the seal by reducing permeation of gasses through the seal. Specifically, in certain embodiments, the invention is used to reduce gas permeation across the seal: (<NUM>) between a top plate and a vacuum cap, and/or (<NUM>) between the top plate and an enclosure.

Embodiments relate to a flywheel device that includes an enclosure, a top plate that fastens to the enclosure, where the top plate includes a first opening; and a cap, where the cap has a top side and a bottom side, which when fastened to the first opening forms a seal between the bottom side and the first opening, the bottom side includes an o-ring groove configured to hold an o-ring, a grease channel concentric with the o-ring groove; and an inlet port configured to enable grease to flow into the grease channel.

Non limiting and non exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified.

The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein, which are defined by the appended claims.

The invention now will be described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention, that is defined by the appended claims. The following detailed description is, therefore, not to be taken in a limiting sense.

As used herein the following terms have the meanings given below:.

Vacuum chamber or simply chamber - as used herein, refers to a sealed container enclosure, or vessel that is fully or partially evactuated of gasses. Essentially, the chamber interior is maintained at a lower atmospheric pressure than exists exterior to the chamber.

Grease - an oil-based semi-solid or solid material that is typically used to lubricate parts of mechanical equipment. Grease is often applied to the part being lubricated using a grease gun. As used herein, grease refers to oil-based greases, including vacuum oil, as well as any other corrosion inhibiting substance that can be injected, molded or otherwise inserted into a grease channel.

<FIG> is a block diagram of an exemplary flywheel energy storage system <NUM>, that includes a vacuum chamber, according to one embodiment. The energy storage system includes a flywheel rotor <NUM>, a motor/alternator <NUM>, a first inverter <NUM>, a capacitor <NUM>, a second inverter <NUM>, and an AC line <NUM>. Energy is drawn from, or delivered to, an AC line <NUM>, such as a three-phase <NUM> line. The first <NUM> and second <NUM> inverters as well as capacitor <NUM> illustrate an exemplary back-to-back converter system for converting the input alternating current into an alternating current acceptable to the motor/alternator <NUM>. The motor/alternator <NUM> converts between electrical and mechanical energy, so that energy can be stored in or drawn from the flywheel rotor <NUM>. The motor/alternator <NUM> is physically coupled to the flywheel rotor <NUM> either directly or indirectly using a shaft. The motor/alternator <NUM> is coupled to the remainder of the system <NUM> via wires or other electrical couplings. Generally, although only one of each component is shown, in practice a flywheel energy storage system <NUM> may include multiples of each individual component. <FIG> is one exemplary type of ac-to-ac conversion system. In general, the inventions described herein pertain to a broad range of ac-to-ac conversion topologies, as well as systems that interface directly to a direct current (dc) line. The latter are of especial relevance for dc microgrid and solar photovoltaic applications.

Flywheel energy storage system <NUM> includes a flywheel device <NUM>, illustrated in <FIG>, which has a vacuum chamber <NUM>, also referred to as chamber <NUM>, that is fully or partially evacuated of gas or air. Flywheel device <NUM> includes flywheel rotor <NUM> and may include other elements of system <NUM>. Chamber <NUM> is formed by a flywheel enclosure <NUM>, a top plate <NUM> and a vacuum cap <NUM>. Top plate <NUM> fastens to enclosure <NUM>, and vacuum cap <NUM> fastens to a central portion of top plate <NUM>. Typically the central portion is a cut-out, circular region, which is circular symmetric to a central, vertical axis <NUM> of flywheel device <NUM>.

In certain embodiments, flywheel device <NUM> also has a bottom plate and a bottom vacuum cap. Generally, the subject invention may be used to seal the bottom plate and bottom vacuum cap in the same or a similar way that it is used to seal the top plate and the top vacuum cap, as described hereinbelow.

In order to maintain a vacuum or lower atmospheric pressure in the interior of chamber <NUM> than exterior to it, it is desirable for the seal between adjacent elements of flywheel device <NUM> to block as much as possible the flow of gasses from the exterior of chamber <NUM> to its intererior. Thus, in various embodiments, the seal between top plate <NUM> and enclosure <NUM> are configured to block gas permeation to the extent possible in view of various limitations, suchas material properties, cost, manufacturing efficiency, and the like. Similarly, the seal between vacuum cap <NUM> and top plate <NUM> are configured to block gas permeation to the extent possible in view of the limitations. In certain embodiments, vacuum cap <NUM> has a top opening, illustrated hereinbelow in <FIG>; in this case the seal between the top opening of vacuum cap <NUM> and the top of vacuum cap <NUM> is also configured to form a seal that blocks gas permeation to the extent possible in view of the limitations.

The subject invention decreases the permeation of gasses, typically ambient air, from the outside to the inside of a vacuum chamber, which may be fully or partially evactuated of gasses. This is accomplished by introducing a grease channel, i.e. a channel, groove, duct or canal, that is filled,or substantially filled, with grease. When a grease channel is used to form a seal, external gasses must permeate the grease channel as well as an o-ring to reach the chamber.

<FIG> is a bottom isometric view of a vacuum cap <NUM> that includes an o-ring groove <NUM> but which does not include a grease channel, according to one example not forming part of the claimed invention. <FIG> illustrates a bottom isometric view of vacuum cap <NUM>, according to the invention, which includes an o-ring groove <NUM> and a grease channel <NUM>. In this embodiment, grease channel <NUM> is exterior to o-ring groove <NUM>. While locating grease channel <NUM> exterior to o-ring groove <NUM> has the advantage of preventing rust and corrosion in the o-ring groove, in certain embodiments it may be advantageous to locate grease channel <NUM> on the interior side of o-ring groove <NUM>. Generally, grease channel <NUM> is concentric with, in close proximity to, and adjacent to o-ring groove <NUM>, but this is not required.

Grease channel <NUM> is a channel or groove machined into the bottom circular portion of vacuum cap <NUM>. In certain embodiments, grease channel <NUM> is substantially less deep than o-ring groove <NUM>. For example, the grease channel <NUM> may have a depth in a range from <NUM>% to <NUM>% of the depth of the o-ring groove <NUM>, with <NUM>% being a preferred value. In one embodiment, o-ring groove <NUM> has a depth of. <NUM> inches and a width of. <NUM> inches. This size accommodates an o-ring that is. <NUM> inches high by. <NUM> inches wide. An o-ring groove is typically slightly shorter and wider than the o-ring itself so that when vacuum cap <NUM> is fastened to top plate <NUM> there is room for the o-ring to flatten somewhat, i.e. compress in the vertical direction and expand in the horizontal direction, due to the compressive force exerted against it. The compression of the o-ring forces the o-ring against the sides of o-ring groove <NUM>, creating a seal. In this embodiment, grease channel <NUM> is. <NUM> inches wide and. <NUM> inches deep.

In order to ensure that the entire channel, substantially all of the channel, can be filled with grease, two ports <NUM>, <NUM> are provided that allow grease to flow from the outside of flywheel device <NUM> into grease channel <NUM> and back. In this embodiment, ports <NUM> and <NUM> are positioned <NUM> degrees apart from one another. One port, referred to as inlet port <NUM>, is used as the grease inlet while outlet port <NUM> is used as an outlet to allow any overflow grease to escape.

In operation, once vacuum cap <NUM> is fitted onto and fastened to top plate <NUM> pressurized grease, from a grease gun for instance, is injected into inlet port <NUM>. The grease enters grease channel <NUM> through inlet port <NUM> and flows substantially evenly in both directions inside grease channel <NUM>. Typically, grease is injected until flows out of and emerges from outlet port <NUM>, signifying that grease channel <NUM> is completely full.

Vacuum cap <NUM> has an inner wall <NUM>. In certain embodiments, vacuum cap <NUM> also has a top opening <NUM> that may be used, for example, for service access or sensor circuits.

<FIG> is a top isometric view of the top of vacuum cap <NUM>, referred to as vacuum cap top <NUM>. In the embodiment shown, a ring of bolts <NUM> is used to fasten vacuum cap <NUM> to top plate <NUM>. Outlet port <NUM> is visible in this view.

<FIG> is a view of the bottom of vacuum cap <NUM>, referred to as vacuum cap bottom <NUM>. In the embodment shown, vacuum cap bottom <NUM> includes o-ring groove <NUM>, grease channel <NUM>, inlet port <NUM> and outlet port <NUM>. Each of ports <NUM> has one opening or hole in vacuum cap top <NUM> and one in vacuum cap bottom <NUM> through which grease may flow.

<FIG> is a cross-section of vacuum cap <NUM> that illustrates an o-ring <NUM> fitted into o-ring groove <NUM> and a grease channel <NUM>. To the left of o-ring groove <NUM> in the embodiment shown is a slight gap, and to the left of that is inner wall <NUM> of vacuum chamber <NUM>. To the right of o-ring <NUM> is grease channel <NUM> that in normal operation would be full of grease to block gas permeation across o-ring <NUM> and into chamber <NUM>.

<FIG> are illustrations that depict the progression of grease when grease channel <NUM> fills. In this example, the bottom of vacuum cap <NUM> is depicted to show the filling of grease channel <NUM>.

<FIG> illustrates grease channel <NUM> when a relatively small amount of grease has been injected. Since the grease flows into grease channel <NUM> through inlet port <NUM> that port is covered with grease and is not visible. The grease flows evenly in both directions from inlet port, given sufficient pressure, <NUM> upon entering grease channel <NUM>.

Next, <FIG> illustrates grease channel <NUM> when it is nearly full of grease. At this point, only outlet port <NUM> is visible, i.e. grease covers all other parts of grease channel <NUM>. Typically, grease channel <NUM> continues to fill until grease begins to extrude from outlet port <NUM> at which point the injection of grease would halt.

Generally, a grease channel functions as a barrier to corrosion. With this barrier in place, it allows the manufacturer to fabricate a vacuum chamber out of low cost steel. Grease channel <NUM> greatly limits corrosion on the interior side of the grease channel, including the surface of top plate <NUM> on the interior side of grease channel <NUM>. Grease channel <NUM> also reduces the flow of gases including water vapor which can have a damaging effect.

<FIG> illustrates a cross section of an example <NUM> that does not form part of the claimed invention, in which the principle of the invention is used to improve the seal between top plate <NUM> and enclosure <NUM>. In embodiment <NUM>, an o-ring groove <NUM> and a grease channel <NUM> are machined into the top of enclosure <NUM> where it contacts the bottom side of top plate <NUM>. An o-ring <NUM> is disposed inside of o-ring groove <NUM>. It may be appreciated that embodiment <NUM> may or may not include an inlet port or an outlet port. In this embodiment, it is possible to apply grease to the grease channel <NUM> before placing and fastening top plate <NUM>. An inlet port and/or an outlet port may be added in certain embodiments.

Claim 1:
A vacuum chamber (<NUM>) for a flywheel device (<NUM>) comprising:
an enclosure (<NUM>), with a vertical axis (<NUM>), having a top and a bottom;
a plate (<NUM>) fastened to the top of the enclosure (<NUM>), wherein the plate (<NUM>)
includes a first opening; and
a vacuum cap (<NUM>), comprising a top side (<NUM>) and a bottom side (<NUM>), which is fastened to the plate (<NUM>) and forms a seal between the bottom side (<NUM>) and
the first opening, characterised in that the vacuum cap bottom side (<NUM>) comprises:
an o-ring groove (<NUM>) in the vacuum cap bottom side (<NUM>) configured to hold an o-ring;
a grease channel (<NUM>) in the vacuum cap bottom side (<NUM>), concentric with the o-ring groove (<NUM>), configured to hold grease to reduce permeation of gas into the vacuum chamber (<NUM>);
an inlet port (<NUM>) connecting the top side of the vacuum cap (<NUM>) to the grease channel (<NUM>), which enables grease to be injected into the grease channel (<NUM>); and
an outlet port (<NUM>) connecting the top side of the vacuum cap (<NUM>) to the grease channel (<NUM>), which enables gases to escape from the grease channel (<NUM>).