GB2466480A

GB2466480A - Extracting energy from waves in the form of compressed air

Info

Publication number: GB2466480A
Application number: GB0822591A
Authority: GB
Inventors: Seamus Dominic Garvey
Original assignee: University of Nottingham
Current assignee: University of Nottingham
Priority date: 2008-12-11
Filing date: 2008-12-11
Publication date: 2010-06-30
Also published as: GB0822591D0

Abstract

This invention relates to a Wave Energy Converter (WEC) system intended for compressing air. The system comprises a number of air compressor units (9 figure 2) arrayed in a flotilla such that although each individual device compresses by only a small ratio, the combined effect is to raise the air pressure from atmospheric conditions by a factor of several tens. The devices are arranged as a series of banks 2 & 4 interleaved with intermediate air reservoirs 1, 3, 5 & 6 between the banks and with each bank comprising a parallel combination of individual devices. The individual devices each use buoyancy to cause movement of one part relative to another part which includes a "heave-plate" or "drag-anchor" (22 figure 2) to resist vertical motion. A linear magnetic coupling (20 & 21 figure 2) may be used to transmit motion across the wall of a sealed chamber (9 figure 2) and by using a deformable divider within that sealed chamber, air compression is achieved without the use of any moving seal. Options are described using either a bellows-structure or a rolling-membrane (10 figure 2) as the deformable divider.

Description

Wave Energy Converter Flotilla to Compress Air Directly

Field of the Invention:

This invention describes a class of system for extracting energy from waves in the form of compressed air. The abbreviation WEC will be applied for a Wave-Energy Converter.

Background.

It is recognised that wave energy will play an important role in the future energy mix of virtually every country with a coastline. At the present time, wave energy is far less developed than wind-energy and the ratios of device costs to net value of the energy recovered from them are believed to be substantially higher for wave energy harvesters than they are for wind turbines.

Most designs of wave energy converter (WEC) seek to harness the power of the waves as directly as possible into the form of electrical energy. Although electrical energy is unquestionably the most versatile of all energy forms and the most easily transmitted continuously, it is virtually impossible to store substantial amounts of electrical energy directly. This is increasingly problematic as the penetration of renewable energy harvesters grows. As this happens, the instantaneous mismatches between supply and demand grow concomitantly and the requirement for more energy storage also grows.

There is a steady enlargement of the area of large-scale energy storage and the thinking in most cases is that an effective means is required to take electrical energy, convert it into some form for which long-term large-scale storage is appropriate and finally transform it back again at the time when the electrical energy is required again.

In view of the role that energy storage will play in future energy systems, it is logical to question whether, perhaps, it might be better to extract the energy in a storable form in the first instance and then convert that storable form to electricity at a time where electrical power is in relatively high demand.

The present patent is motivated by the idea of extracting renewable energy from sea waves in the form of compressed air. A key motivation is the possibility of storing energy directly and to do this in a cost effective way, it is necessary to achieve relatively high-pressure air -in the order of several tens of bar (several MPa). *..S

Prior Art.

* Several designs of direct-compression WEC have already been presented. Most of these involve a * direction connection to the seabed and so they cannot be used in deep seas. The present invention is : motivated mainly by the conversion of waves in deep sea. All of the previous designs involve a moving * seal to compress the air. I. * ** * * * * * S. * S

S S S **

A WEC is described in W0200807615 comprising two floating bodies moving relative to each other.

This WEC does compress air directly but the compression action involves two sliding seals and, most importantly, it is a single-stage device. As such, it is quite different from the present invention.

In US2008260548, a structure mounted to the ocean floor is proposed. The structure includes a floatation body that moves with the waves -driving a sealing plate in a compression chamber. The design is intended for use in isolation. By contrast, the devices proposed in the present invention are intended for use in a flotilla. Individual devices here have no direct connection to the sea-floor.

In GR1005790, a system of parallel wave-driven compressors is described where each one comprises a downward-facing cup and where the compression is achieved by the rising of water level in the cup.

Induction of new air occurs when the water levels fall again. Here the pressure achieved directly by the wave-driven compressors is very low -being limited by the head that is available in a single wave.

Significantly, there is no series-configuration of the compression and the open-bottomed cup objects are not compatible with achieving series-compression in any case. This patent proposes that the low-pressure air gathered from a number of parallel cup devices be used to drive a rotary machine which then drives a single compressor capable of generating high pressure air.

Patent US4613287 describes a two-stage compressed-air apparatus having one low pressure compression section and one higher pressure compression section. The lower pressure stage acts as a feed for the higher pressure stage. The higher pressure stage involves a set of mechanical linkages and a piston travelling in a cylinder. Because it has a moving seal, this device is fundamentally different from the content of the present application.

The present design involves no moving seal. The use of a magnetic coupling provides a mechanical linkage without the need for a moving seal. Moreover, a major premise of the invention is that a flotilla of devices will be deployed with each one compressing air by only a relatively small pressure ratio. Each device uses a drag anchor to react the movement of the wave. The flotilla is configured as a series of compressor banks -each bank comprising a parallel interconnection of wave-driven compressors.

Summary of the Invention.

The present invention is characterised by a number of features: (1) The devices described here operate together in a flotilla with some devices inducting atmospheric-pressure air and raising its pressure by a small multiple -typically around 1.2.

Then subsequent banks of devices raise the pressure further by a multiple of around 1.2. The multiplication goes on until, at some stage, instead of multiplying pressure by a fixed amount, relatively constant increments of pressure are achieved instead. The final outlet pressure is * ..** intended to be -6MPa to allow for very cost-effective storage of energy. **.*

* . * There are two key advantages of this feature (low pressure-ratios). One is that the energy required simply to increase the pressure of the air within one device to discharge pressure is * :" only a small proportion of the total energy required to compress and discharge the bulk of the air present. By contrast, if the pressure ratio is high, it requires a relatively large amount of energy to raise the pressure compared with the amount required simply to discharge the high pressure air. If an individual WEC attempts to compress air by a substantial ratio, then the fact *. : that the wave heights are not consistent will lead to many waves saturating the device (in the * *: sense that they compress and expel all air present during one stroke) and many other waves failing to achieve the discharge pressure. However, if an individual device attempts to compress air by only a small ratio, it will be capable of extracting amounts of energy proportionate to the wave energy over a fairly wide range of wave heights. The second key advantage of the low pressure ratio is that the compression will occur in many small stages with opportunities to cool the air in between. The result is a nearly-isothermal total compression. If the heat of compression is going to be lost anyway (as it is in most cases where air is compressed), then it is better to lose that heat over many stages between compression stages than to lose it in one single stage at the end.

(2) Within the flotilla described above, all (or most) devices are similar in nature but dimensional changes are introduced between those at the lower-pressure end and those at the higher-pressure end.

(3) The individual devices in the flotilla are separated into "banks". Each individual "bank" comprises a parallel interconnection of a number of similar devices. Between successive banks, floating air-reservoirs are located. The capacity of these reservoirs does not need to be large. A guideline capacity would be that one reservoir holds several times more volume than the total volume of air that could be delivered into it by one single stroke of all of the individual units in the bank on the input side and several times more volume than the total volume of air that could be extracted from it by one single stroke of all of the individual units in the bank on the output side.

At any one time, all of the units in a single bank are operating to raise pressure by the same amount. The actual amount is determined at that time by the pressures in the reservoirs on the input and output sides.

(4) A special air-treatment "reservoir" may be included before the first bank whose function is to allow salt and moisture to settle out of the air before that air is inducted into the flotilla.

(5) Each individual device features a closed chamber sub-divided into two distinct internal volumes. The induction-side volume contains air at the lower of two pressures handled by the device, PL. The expulsion-side volume contains air at pressures between PL and PH inclusive where PH represents the pressure required for air to begin to be expelled. In the first-stage devices from a given flotilla, PL is atmospheric pressure, PA. In all other cases (to a good approximation), P11 �= P �= PA. Because the total volume enclosed within the closed volume is a constant and because it is always occupied predominantly by air, the closed-volume itself has substantial buoyancy and this is used directly.

(6) The two distinct volumes are separated by a deformable surface and deformations of this surface are controlled by a disc fixed to a rod which is movable relative to the closed-chamber.

The action of the waves directly causes the rod to move relative to the closed-chamber and this *.... intrinsically causes the total volume of the closed-chamber to be split in different proportions between the induction-side volume and the expulsion-side volume. A one-way valve is in place * : * between the expulsion-side volume and the induction-side volume (normally accommodated in the disc) and it is oriented such that Pg cannot fall significantly below PL. Similarly, another * : one-way valve is in place on the expulsion volume preventing any high pressure air from re-entering the expulsion-side volume after it has been discharged and also preventing PH from * * becoming significantly larger than the pressure downstream of the WEC. In some configurations of this invention, the deformable surface comprises a bellows construction. In :: * other configurations, the deformable surface takes the form of a rolling membrane.

(7) The inequalities PH �= PL �= PA mentioned above in (3) are slightly inaccurate in the sense that PH does fall very slightly below PL while the expulsion side volume is filled through a one-way valve. Similarly, in the first stage of compression, PL necessarily falls slightly below PA in order that atmospheric air flows into the chamber. S... * * S...

*..... * * S.

S S. * *. S * . S * S. *. I * SI I *S

GENERIC EMBODIMENT.

Figure 1 shows, schematically, how the general system is configured. Air is inducted naturally into an initial reservoir (1) for air-treatment and allowing water and salt to settle out. Physically, this object might be largely submerged (having close-to-neutral buoyancy) in order that it did not obscure the individual wave energy converters from their source of energy. Additional buoyant elements might be located at the ends of the initial reservoir to maintain it in a stable vertical location without raising it too close to surface.

The air from the initial reservoir, (I), is drawn into individual wave energy converters in a first bank of individual wave-energy converters (2) where it is compressed by a small ratio. This air is discharged into a second reservoir (3) which acts as the inlet store for a second stage of compression in the form of another bank of individual wave-energy converters (4). The output from that bank feeds into yet another reservoir (5) ... and so on until finally the pressure at the final reservoir (6) is at a useful level.

This final reservoir could be connected to air-storage provisions (7) and/or to direct expansion units (8) from which electricity could be derived.

Being only schematic, Figure 1 does not attempt to represent all of the stages (banks) that would typically exist in a particular implementation. This would typically be in the order of 20. Moreover, the devices in any one bank might not be arrayed in a straight formation -quite likely an arc or a number of nested arcs. The numbers of devices in the individual stages would taper such that the net (input) volume flow through each device is expected to be similar. S... * . **5S

S * S *.* * * S.. * *5 S * .. * .* *5 * * . S *5

SPECIFIC EMBODIMENT A OF AN INDIVIDUAL WAVE ENERGY

CONVERTER.

Figure 2 shows one preferred embodiment of an individual wave energy converter. In this, a closed chamber (9) contains a rolling-membrane (10) which divides the internal volume of the chamber into two parts -a low-pressure part (11) and a higher-pressure part (12). The closed-volume is buoyant in its own right because of the relatively high volume of air contained within it but in normal designs, an additional thick layer of expanded polystyrene or similar solidified foam (not shown in Figure 2) would be used to enhance the buoyancy of this element.

Within the closed-chamber (9), a pair of plates (13) clamps a portion of the surface of the rolling membrane and this pair of plates acts very much like a piston. A one-way valve is incorporated within this pair of plates such that the pressure on the higher-pressure part-volume (12) can never fall significantly below the pressure on the lower-pressure part-volume (11) by allowing air to pass relatively freely from (11) to (12) when the "piston" (13) is rising, some air-flow in this direction tends to occur. When the "piston" (14) is falling, the reverse air-flow is blocked. A further one-way valve (15) is incorporated into the lower-pressure part-volume (11) such that the pressure in this part-volume is never significantly below the pressure of the intake-side reservoir. This valve prevents any outward flow of air from the closed volume. A third one-way valve (16) is incorporated into the higher-pressure part-volume (12) such that the pressure in this part-volume never significantly exceeds the pressure in the reservoir on the output-side. This valve allows air to be expelled into the output-side reservoir from the closed-volume but it does not permit any reverse flow.

The "piston" sits at the top of a driving-tube, (17). A slender central rod (18) is fitted into this tube in order to assist in guiding the "piston" (13) and driving-tube when the piston is in a relatively high position. The outer diameter of the driving-tube, (17), is a reasonable fit to the inside diameter of a tube-shaped part of the closed-volume (19). At the end of the driving-tube, (17), a set of permanent magnets (20) is located and this set of permanent magnets is partly responsible for ensure that the axial forces required to move the "piston" against the pressure-difference can be sourced from outside the tube where a second set of permanent magnets (21) completes a linear magnetic coupling with the first set of permanent magnets (20). The second set of permanent magnets is coupled to a "drag anchor" (22) via a set of cables (23). In other contexts, this "drag-anchor" has been termed a "heave plate". The drag-anchor (22) presents a resistance to motion such that when buoyancy forces on the (coated) closed-chamber (9) are inclined to pull the entire assembly upwards, the drag-anchor (22) acts substantially to react those buoyancy forces. The drag-anchor can be almost any shape but simple discs andlor cones are most likely to produce acceptable response at minimal cost.

A mechanical stop, (24), prevents the external (second) set of permanent magnets (21) from ever sliding off the end of the tube-shaped portion of the closed-volume (19). A set of external guide-rods * *. (25) run parallel to the tube-shaped part of the closed-volume (19) and serves to guide the external * * (second) set of permanent magnets (21).

S.. The driving-tube is engineered to have axial relief grooves (not shown in Figure 2) such that air has a : reasonable flow-path between the remaining small volume left beneath the driving path at any one : * instant and the remaining residual volume under the driving-tube (17) and within the "tube-shaped part" (19) of the closed-volume (9). In this way, the net residual volume of the higher-pressure partial volume (12) can be made to be very small -thereby minimising the risk of bounce-back of the "piston" : on any one occasion where the wave-height fails slightly to achieve some minimum level. S. S

S S S S S*

SPECIFIC EMBODIMENT B OF AN INDIVIDUAL WAVE ENERGY

CONVERTER.

Figure 3 shows an alternative embodiment of an individual wave energy converter. In this, there is, once again, a closed chamber (9) containing a rolling-membrane (10) which divides the internal volume of the chamber into two parts -a low-pressure part (11) and a higher-pressure part (12). The closed-volume is buoyant in its own right because of the relatively high volume of air contained within it. However, in this embodiment, the closed-volume is normally completely submerged. A further buoyant element (26) is in place in this embodiment (rigidly fixed to the exterior of the closed-volume as Figure 3 shows) to ensure that the centre of buoyancy of the combination of the further buoyant element (26) and the closed-volume (9) is relatively high.

The "piston" in this embodiment is located at the bottom of a driving-tube, (17). A slender central rod (18) is fitted into this tube in order to assist in guiding the "piston" (13) and driving-tube when the piston is in a low position relative to the closed-volume (9). The outer diameter of the driving-tube, (17), is a reasonable fit to the inside diameter of a tube-shaped part of the closed-volume (19). At the upper end of the driving-tube, (17), a set of permanent magnets (20) is located and this set of permanent magnets is partly responsible for ensure that the axial forces required to move the "piston" against the pressure-difference can be sourced from outside the tube where a second set of permanent magnets (21) completes a linear magnetic coupling with the first set of permanent magnets (20). The second set of permanent magnets is coupled to a "drag anchor" (or heave-plate) (22) via a set of cables (23). The drag-anchor (22) presents a resistance to motion such that when buoyancy forces on the (coated) closed-chamber (9) are inclined to pull the entire assembly upwards, the drag-anchor (22) acts substantially to react those buoyancy forces. The drag-anchor can be almost any shape but simple discs and/or cones are most likely to produce acceptable response at minimal cost.

* . . A mechanical stop, (24), prevents the external (second) set of permanent magnets (21) from ever *....: sliding off the top end of the tube-shaped portion of the closed-volume (19). A set of external guide- * rods (25) run parallel to the tube-shaped part of the closed-volume (19) and serves to guide the external (second) set of permanent magnets (21).

* The driving-tube in this case is not a close fit within the tube-shaped part (19). However, the set of * permanent magnets would be a reasonably close fit and the outer-diameter of the tube would be guided : : : : positively at the location where it enters the main volume of the "closed-volume", (9).

The primary difference between Embodiments A and B is that in one case (A), the transmission of relative motion is done underneath the main buoyant elements whereas in the case of (B) the transmission of relative motion is done above the main buoyant elements. * .

*..*.* * * S... S. S * SS * .* * S S *S

SPECIFIC EMBODIMENT C OF AN INDIVIDUAL WAVE ENERGY

CONVERTER.

Figure 4 shows a minor modification of embodiment B in which the rolling-membrane (10) is replaced by a bellows structure (1 Oa). The embodiment is similar in all other respects.

SPECIFIC EMBODIMENT D OF AN INDIVIDUAL WAVE ENERGY

CONVERTER.

Just as Figure 4 extends the idea of a rolling-membrane structure (of Figure 3) to a bellows structure, a similar extension is possible with Figure 2. Figure 5 describes this embodiment.

OTHER EMBODIMENTS.

The generic embodiment of this invention is described in the section above entitled "generic embodiments". It might be inferred from the description here that the devices in each "bank" are identical. In fact, it is sensible to make the devices in any one bank of wave-driven compressors identical to each other but the devices used for compression at the higher-pressures might be different from those used for devices operating at lower pressures. In particular, a simple inverted-cup (27) coupled to a drag-anchor (22) and having one-way valves for its input and output sides might sometimes be attractive for the first stage of compression. (This is shown in Figure 6.) The wave passes by and acts as a piston (28) inside the inverted cup. *... * * S...

S..... * S 5.. * S... * S.. SI * * . S * S. *5 S 5*