GB2324120A

GB2324120A - Converting thermal energy of a natural water source into useful power

Info

Publication number: GB2324120A
Application number: GB9707209A
Authority: GB
Inventors: Ian Robert Fothergill
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-04-09
Filing date: 1997-04-09
Publication date: 1998-10-14
Also published as: WO1998045598A1; GB9707209D0; AU6844298A

Abstract

A water transfer vessel 3, travels between a station 2, on the surface of the ocean, and the ocean depths, where the vessel collects cool water and subsequently brings it to the surface. The station has reservoirs 4,16 from which cool and warm water is supplied respectively to a condenser 5, and an evaporator 8, of a low boiling point fluid circuit incorporating a heat engine (turbine) 11. Warm exhaust water is discharged via the vessel 3 at the ocean depths. A single transfer vessel 3, may be used (figure 1) and the or each vessel may be propelled between levels by a variety of means including propellers, water jets, cables or chains, buoyancy control, and impulse drive (piston or catapult). Alternatively the vessel may be propelled along the surface between an offshore location and a land based energy conversion facility (figure 20).

Description

METHOD AND APPARATUS FOR CONVERTING THERMAL ENERGY OF A NATURAL WATER SOURCE This invention relates to a method and apparatus for the conversion of thermal energy of a natural water source into useful power. and more particularly to the conversion of ocean thermal energy.

The concept of ocean thermal energy conversion (often referred to as OTEC) is down. The surface temperature of oceans in tropical regions is typically about 0 25 C but at great depths for example 1,000 metres below the surface. the temperature is typically 50C. This thermal gradient can be exploited by using the temperature difference to drive a thermodynamic heat engine with a low boiling point fluid such as ammonia, propane or a refrigerant. Such fluids can be made to boil at ocean surface temperatures and condense at the cooler ocean depth temperatures. The temperature difference between the two sources of water thus enables a heat engine to convert ocean thermal energy to useful energy.

Known proposals and prototype systems for exploiting ocean thermal energy in the above way utilise a surface vessel containing a heat engine. typically a turbo generator, driven from evaporated low boiling point fluid. The fluid is heated by the surface water and the resulting vapour emerging from a turbine outlet is condensed by heat transfer with cool water before being pumped back to an evaporator to continue the thermodynamic cycle. Large volumes of cool water from the ocean depths are required to supply the condenser and ensure effective operation of the cycle. The proposed systems use a very large cold water pipe typically about 30 metres diameter and 1,000 metres long, suspended from the surface vessel. The engineering demands to provide a cold water pipe of such a size and with the structural integrity to withstand not only its own weight but the mechanical loading from ocean currents and surface waves are formidable.

It is an object of the present invention to obviate or mitigate the aforesaid disadvantage.

According to a first aspect of the present invention there is provided a method for converting thermal energy of a natural source of water into useful power, the water source comprising water at a first temperature at a first level and water at a second temperature at a second level, the method comprising the steps of at least partially filling an at least partially hollow vessel with water from said first level, propelling said vessel to said second level and using the difference between the first and second temperature of the water to drive a thermodynamic heat engine and thereby produce useful power.

The vessel may then be filled with water from the second level and propelled in the opposite direction to the first level.

The natural source of water may be an ocean or the like.

Preferably one of the levels is at or near the water surface. the other being at a significant depth below. Because of the small temperature difference between water at the ocean surface and large depths energy conversion efficiencies are low necessitating very high water volume flow rates to create large thermal capacity hot and cold reservoirs. To reduce the engineering problems associated with very large cold water transfer pipes in current system proposals or prototypes one or more vessels are used to transfer water between the ocean surface and its depths. Such vessels would advantageously have a high volume capacity and be streamlined such under propulsion large volumes of water may be transferred between ocean depths and the surface at high rates.

According to a second aspect of the present invention there is provided apparatus for converting thermal energy of a natural source of water into useful power, the water source comprising water at a first temperature at a first level and water at a second temperature at a second level the apparatus comprising an at least partially hollow vessel with propulsion means for propelling it between said first and second levels, the vessel having means for collecting water from either of said levels and means for dispensing the collected water at the other level. and a thermodynamic heat engine for producing useful power from the difference between the first and second temperatures of the water.

A surface station may be provided at the water surface with the vessel docking there when it is at the surface and descending therefrom to the first level below. The surface station may be a floating vessel. or alternatively. may be located on land immediately adjacent the water. The thermodynamic heat engine may be located at the station or on the vessel. Reservoirs for water at the first and second temperatures may be provided in order to supply the thermodynamic heat engine.

The thermodynamic heat engine preferably comprises a turbine generator connected between first and second heat exchangers. the first heat exchanger being connected to supply means for supplying warm surface water at said second temperature to it and containing a liquid that vaporises at the surface water temperature, the turbine generator being driven by the vapour and the second heat exchanger having means for supplying cool water from the depths at said first temperature to it in order to condense the vapour. and means for returning the liquid to the first heat exchanger.

The vessel conveniently receives cool water from the depths and ascends to the surface where it may discharge the cool water into a cool water reservoir at the surface station. It is then filled with warm water either from exhausts of the condenser and/or evaporator or from the water surface. and descends to the first level where it discharges the warm water prior to refilling with cool water.

Advantageously discharge of warm water and recharging with cooler water may be carried out whilst the vessel is descending in cool water regions in order to minimise vessel transfer periods and avoid thermal hot spots at cool water depths by gradual release of warm water.

Power generated by the turbine generator may conveniently be converted into electrical power using a generator and stored for subsequent use on shore using batteries, electrolytic conversion to hydrogen or compressed air. The stored energy may be located on the surface station or at ocean depths.

A further alternative means for storing turbine generated energy involves its conversion to heat and transference to a high thermal capacity reservoir or substance preferably involving a solid to liquid phase change with a large associated latent heat of fusion. In some cases storage of converted energy via batteries. compressed air, hydrogen or heat on a surface vessel or floating platform may prove advantageous in order to supply and fuel passing shipping or aircraft such platforms possessing suitable marine and or aircraft landing facilities.

The thermodynamic heat engine could be located at relatively cool ocean depths, possibly though not essentially. on the ocean bed and warm water transferred from the surface to the engine with the vessel. In this case such deep underwater storage reservoirs could be located close to or associated with the power generating facility thus avoiding the need for transportation of compressed air or hydrogen from a surface vessel to deep underwater reservoirs.

The vessel may be propelled by known propeller or jet propulsion methods.

Alternatively the propulsion means may comprise a cable or chain that connects the vessel to a winch preferably mounted on a surface station. In such cases the vessel would possess negative buoyancy so that it will descend under its own weight to a prescribed cool water depth with a charge of warm surface water before gradually changing the charge during the descent period in cool water regions to a cool water charge and being winched to the surface for replenishment of the surface vessels cold reservoir.

In an alternative embodiment there are two counterbalanced vessels interconnected by a cable or chain that passes over one or two wheels on the surface station such that under the same water charge conditions they would each possess equal negative buoyancy creating equal but opposite turning moments at the wheel thus remaining in equilibrium. Each of the counterbalanced vessels may be driven between the surface and the required cool water depth (equal to the free length of the cable or chain) by appropriate rotation of the wheel drive, the vertical movement of each vessel being always in the opposite sense to the other. The above opposite ascent and descent cycles for the two counterbalanced vessels may be continued indefinitely providing a continual replenishment of cool water for thermal energy conversion on the surface vessel.

Alternatively a drive wheel may be located on the vessel itself. In this case the cable or chain is suspended from the surface station and is preferably weighted or anchored at its lower end enabling the water transfer vessel to drive up and down the cable or chain by its on board drive.

Vertical propulsion of the vessel may alternatively be induced by creating successive states of negative and positive buoyancy causing respective descent and ascent of a charged water transfer vessel. Such controlled changes in the buoyancy state may be induced by discharging water from the charged vessel such that whilst charged it will have negative buoyancy and sink but on discharging water whilst at depth a void within the water transfer vessel will result of volume equal to the discharged water volume large enough to induce a state of positive buoyancy causing the vessel to rise. Means for producing water free volumes or voids in a water transfer vessel include: pumping; pressurisation from high pressure gas reservoirs; high pressure vapour formation from an on board boiler or evaporator driving a piston or bellows; a mechanically driven piston or bellows system.

With the vessel at its cool water depth (i.e. its descent limit) the state of buoyancy is reversed from a negative value to a positive value causing the vessel to rise. If the vessel geometry remains constant then the above void creation cannot be achieved without discharging an equal volume of water. However the buoyancy may be adjusted by changing the volume of the vessel. A volume increase or indeed decrease may be induced by a source of pressure that acts to displace a first portion of the vessel relative to a second portion. Such a source of pressure may be a supply of compressed fluid, pressurised vapour or a piston driven by a mechanical drive.

Alternatively a first portion of the vessel may be connected to an elongate flexible member such as a cable and a tensile force in the flexible member causes the first portion to move relative to a second portion thereby forming a void.

In an alternative embodiment the buoyancy of the vessel is controlled by using a removable weight. When the weight is in contact with and supported by the vessel it sinks under negative buoyancy and when it is removed it rises under positive buoyancy.

Instead of propelling the vessel using on board facilities such as propellers or an external drive using a chain or cable. propulsion may be produced by means of an external impulse applied to the vessel. such an impulse being hydraulically.

pneumatically or mechanically based. A vessel possessing positive buoyancy may be propelled downwards to cool water depths using an impulse drive supported by the surface vessel and after eventually coming to rest as a result of opposing buoyancy and drag forces, the vessel will ascend under its state of positive buoyancy with a cool water charge until it docks with the surface vessel to discharge its contents and repeat the cool water extraction cycle.

Alternatively if the vessel possesses neutral buoyancy it may be propelled downwards with a lower magnitude impulse than would be necessary for a vessel with positive buoyancy such that it will dock with a submerged impulse generating facility advantageously anchored to or supported on the ocean bed at the cool water depth.

This impulse generator would impart upward propulsion to the vessel. Such a facility may be used in conjunction with a vessel having positive buoyancy if necessary.

In all cases considered the energy expended in propelling a water transfer vessel between surface and deep ocean regions must be less than the energy derived from the ocean temperature differential.

It will be noted that in methods involving the creation of positive buoyancy in order to propel the water transfer vessel to the surface, energy will be expended through for example gas pressurisation or mechanical means for creating voids. Some of this energy may be recovered during the ascent period using for example on board water turbines and stored for later use.

In circumstances where the vessel has negative buoyancy thus making it sink, hydraulic turbines may be used to generate power during the descent period for later use.

Generation of hydraulic power during an ascent or descent period will increase vessel drag and slow it down. However drag forces increase with the square of the relative velocity between a vessel and the water through which it is propelled thus yielding greater energy losses through drag from higher velocities than lower ones.

In order to decelerate a water transfer vessel to zero velocity prior to docking or reversal of direction at cool water depths various means may be exploited and will depend to a large extent upon the means of propulsion. In general the propulsive force applicable will be discontinued for the deceleration period or reversed.

In this latter case this reversal in propulsive force direction would. for a propeller drive, involve a reversal in its angular direction. For buoyancy propulsive methods the state of buoyancy would be reversed and for chain or cable drives the driving winch would be slowed down and stopped thus allowing the negative buoyancy state of the water transfer to oppose continued upward motion. In addition to the above water vessel deceleration may be enhanced by introducing hydrodynamic braking which will increase the existing opposing drag forces such braking being created for example by the release of a hydrodynamic parachute. More productive hydrodynamic braking may be effected by exposing water turbines to oncoming water during the deceleration period the turbines otherwise being concealed behind streamlined vessel casing members during non-braking periods. Alternatively the pitch of turbine blades may be adjusted and the drive shaft to a generator disengaged when turbine power is not required.

Specific embodiments will now be described. by way of example only. with reference to the accompanying drawings in which: Fig 1 is a schematic representation of a floating ocean thermal energy conversion station using the method and apparatus of the present invention; Figs 2a - b show scrap schematic views of a water transfer vessel of the present invention with a propeller and jet propulsion drive respectivel; Fig 3 is a schematic illustration of a floating surface station and water transfer vessel with chain or cable drive; Fig 4 is a schematic illustration of dual, counterbalanced water transfer vessels with chain or cable propulsion and a surface station with drive unit; Fig 5 is a schematic illustration of a water transfer vessel with a chain or cable supported by a surface station with a drive unit on board the water transfer vessel: Fig 6 is a schematic illustration of a water transfer vessel with positive buoyancy propulsion created by a pumped discharge of water; Fig 7 is a schematic illustration of a water transfer vessel with positive buoyancy propulsion created by compressed air discharge of water: Figs 8a - b are schematic illustrations of water transfer vessel with positive buoyancy propulsion produced by water discharge from a saturated vapour driven piston; Figs 9a - b are schematic illustrations of a water transfer vessel with positive buoyancy propulsion produced by a motorised piston; Figs 1 0a - 10b and 11 are schematic illustrations of a water transfer vessel containing on board thermal energy conversion equipment; Figs 1 2a - b are schematic illustrations of a water transfer vessel with buoyancy propulsion produced by an external compressed air supply fed from a floating surface station; Figs 1 3a - b are schematic illustrations of a water transfer vessel with impulse propulsion from launch tubes on the ocean surface and bed; Figs 1 4a - c are schematic illustrations of a water transfer vessel propelled by an adjustable ballast device; Figs 1 spa - b are schematic scrap diagrams of water transfer vessels having an extensible portion; Figs 1 6a - b are schematic scrap views of alternative embodiments of a water transfer vessel having an extensible portion with bellows; Figs 17a - d are schematic scrap views of further alternative embodiments of a water transfer vessel with extensible portion: Figs 1 8a - b are schematic scrap views of a water transfer vessel having bellows; Fig 19 is a schematic scrap view of a water transfer vessel having an external propeller water turbine.

Figs 20a - b are schematic diagrams of water transfer to a land-based station using the method of the present invention; and Fig 21 is a schematic diagram of an alternative embodiment of transportation of cool and warm water to a land based station using the method of the present invention.

Referring now to the drawings. Fig 1 shows an ocean thermal energy conversion station 2 floating on the surface water of an ocean. A large volume capacity water transfer vessel 3 shown docked with the station 2. is designed to move between the station and deeper water below where it collects cool water.

The plant 2 contains a cool water storage reservoir 4 which supplies a condenser 5 via a delivery pipe 6 and pump 7. The relatively warm surrounding surface ocean waters provide a warm reservoir and warm water from here is pumped into an evaporator 8 via a pump 9 and delivery tube 10. The evaporator 8 is connected to a turbine 11 which in turn is connected to an electrical generator 12. A liquid with a low boiling point such as, for example, ammonia. propane or a suitable refrigerant is heated in a evaporator 8 by the warm water and the resulting vapour is forced through blades of the turbine 11 causing it to rotate and drive the electrical generator 12. The vapour emerging from the turbine 11 is fed to the condenser 5 where it liquefies through heat exchange with the cool water (not shown) and is subsequently fed back to the evaporator 8 using a pump 13. This cycle is continuous and the resultant electrical energy from the generator 12 may be stored in batteries (not shown) in an energy conversion and storage unit 14 for subsequent utilisation.

The energy conversion and storage unit 14 may alternatively convert the generated electrical power into hydrogen, using a known electrolytic process, which is compressed and stored in a gaseous state. Alternatively hydrogen storage methods include absorption of hydrogen by an iron - titanium metal alloy or liquification by cooling it below its critical temperature. Other alternative means for energy storage include the production and storage of compressed air using on board compressors driven from the turbine and supplying one or more high pressure storage cylinders; the direct conversion of turbine generated power into heat and transference to a thermally insulted high thermal capacity thermal reservoir involving. for example. a solid to liquid phase change of a heat storage media with a large latent heat of fusion.

General control equipment, system maintenance facilities and any additional energy storage facilities are accommodated in units or compartments 14 and 15.

As will be explained in more detail below. the water transfer vessel 3 travels between the surface station 2 and deep water. The vessel 3 is hollow and collects cool water from the depths and brings it to the surface where it is discharged into the cool water reservoir 4 as described below.

The evaporator 8 discharges warm exhaust water to a water reservoir 16 via an outlet pipe 17 whilst continuous replenishment of surrounding sea water is provided by the delivery pipe 10 and pump 9. The cool water reservoir 4 supplying the condenser 5 is continuously replenished with cold water 18 from the water transfer vessel 3 by means of a lower large fast-acting valve 19 connected to the vessel 3 and a delivery pipe 20 and a large pump 21 connected to the reservoir 4. When the vessel 3 is docked the fast-acting valve 19 is releasable coupled to the delivery pipe 20 by a fast-acting slideable connecting tubular member 22. The valve 19 has an associated on-board large reversible pump 23 to enable high cool water discharge rates to the reservoir 4 and high cool water charging rates at cool water depths.

Exhaust water from the condenser 5 is discharged to the reservoir 16 via an exhaust pipe 24. As the cool water in the water transfer vessel 3 depletes slightly warmer exhaust water from the reservoir 16 is pumped into the vessel 3 using a large pump 25, a delivery pipe 26 and an upper large fast-acting valve 27 on the vessel 3.

The warm exhaust water is thermally insulated from the lower cooler water by means of a piston 28 that is slideably fitted within the vessel 3. On completing its cool water discharge operation the water transfer vessel 3 will contain a warmer exhaust water charge from the reservoir 16 with the piston 28 at its lowest position. Under these conditions the water transfer vessel 3 is propelled downwards and as it approaches the cool water depth (typically 1,000 metres) the warm water in the vessel is discharged through upper valve 27 by operating reversible pump 23 to force the piston 28 upwards. During this process the vessel 3 recharges with cool water that enters through valve 19, the pump 23 operating in the reverse direction to that when discharging cool water to the cool water reservoir 4 of the floating surface station 2.

Rapid discharging of cool water from the water transfer vessel 3 to the surface station cool water reservoir 4 and rapid re-charging of vessel 3 at depth is advantageous to optimise generator 12 output power levels and for this reason cool water recharging at depth takes placed whilst the vessel 3 is in motion as well as during reversal in direction on reaching its descent limit. Although only two valves 19 and 27 are shown in Fig 1 additional ones together with associated pumps where appropriate may be used to increase cool water charging and discharging rates.

Furthermore, the piston 28 may be motorised by means not shown to increase further the cool water charging and discharging rates by driving it downwards in the vessel 3 during a cool water discharging operation and upwards during a charging operation.

As an alternative to the lower fast-acting valve 19 and the associated pump 23 which are located on the vertical wall of a vessel 3, a valve (not shown) may be located axially or close to the axis on the underside of the water transfer vessel 3 such that while descending at cool water depths this underside valve is opened to allow cool water to flood in under dynamic water pressure forcing the piston 28 upwards and the residual warm water out of the upper valve 27 thereby charging the vessel with cool water without the need for an associated pump. When opened this underside valve may have an aperture approaching the cross sectional area of the vessel 3. To maximise water flow rates all valves may advantageously possess streamlined geometries to minimise impedance to flow. The cool water delivery pipe 20 on the surface station 2 would in this case be configured to couple with the underside valve (not shown), rather than as shown in figure 1, to feed upwards from the underside valve to the underside of the cool water reservoir 4. If the reservoir 4 is deeper than that shown in Fig 1 such that the level of cool water within it always lies below the lowest point of the underside valve (when the vessel 3 is docked) then delivery of cool water from vessel 3 though a modified delivery tube 20 (not shown) may take place under gravity alone without the need for the pump 21, so long as the hydrostatic pressure at the modified delivery tube 20 inlet is greater than that at its outlet, this condition being achieved, for example, depressurisation of the reservoir 4 which otherwise together with the vessel reservoir 16 would be vented to atmosphere (not shown). To enhance delivery flow rates the pump 21 may still be used.

To optimise energy production rates or power generation it is desirable that as well as minimising cool water discharging and charging periods the water transfer vessel 3 descends from the surface 1 to the prescribed ocean depth and back again with the maximum possible velocity. To enable this the water transfer vessel 3 possesses a highly streamlined geometry in order to minimise the retarding effects of drag forces from the water medium through which it travels. The movement of the water transfer vessel 3 is in a vertical direction and to assist such movement stability ballast 29 is provided at a lower end together with stabilising fins and guidance devices (not shown) to accommodate the effects of transverse ocean current and ensure accurate docking with and hydraulic coupling to the floating station 2.

Figs 2a - b show alternative means for propelling the water transfer vessel.

Fig 2a shows a propeller drive 31 and Fig 2b a water jet drive 32. Preferably. though not essentially, the water transfer vessel 3 has neutral buoyancy for these types of propulsion and will thus remain stationary unless momentum is imparted to it by a reactionary or external force.

The water transfer vessel 3 has at an upper end a streamlined nose cone 30 containing an energy supply (not shown) to power the propeller 31 or jet 32 drive.

The energy supply may be derived from an energy conversion unit (not shown) in the surface station 2 and may take the form of electric batteries electrolytically generated hydrogen, compressed air heat or some other suitable energy form. The nose cone 30 also contains control equipment for propulsion and guidance purposes.

The guidance devices may include vanes to steer or guide a water transfer vessel 3 or means (not shown) for orientating the propellers or jets. In an alternative embodiment the vessel 3 is equipped with more than one jet or propeller offset from longitudinal axis of the vessel 3 providing a steering capability by inducing differential thrusts. Steering or guidance may also be achieved with the aid of side thrusters giving appropriate transverse momentum to the vessel. For reasons of clarity the stabilising fins and guidance means are not shown in the followillO embodiments.

As an alternative to the propeller or jet propulsion of the water transfer vessel embodiments shown in figures 2a and 2b, propulsion is provided by means of an externally driven cable or chain. Fig 3 shows the surface station 2 supporting a winch mechanism 33 which drives a cable or chain 34 connected to the water transfer vessel 3.

For this form of propulsion the water transfer vessel ; possesses negative buoyancy and will thus sink in the absence of a vertical upward opposing force equal to or greater than the submerged weight of the water transfer vessel 3. Such an opposing force is generated in the cable or chain 34 by tension of a magnitude dependent upon the winding speed of the winch mechanism 33 and the submerged weight of the vessel 3.

In operation the water transfer vessel 3 is fully charged with warm water from the condenser and evaporator exhaust reservoir 16 or from the warm surface water. In the latter case excess water from the reservoir 16 is discharged to the ocean. The water transfer vessel 3 is allowed to sink under its own weight by allowing the cable or chain winch mechanism 33 to free wheel resulting in relatively negligible tension in the cable or chain 34. On approaching the requisite cool water depth typically 1,000 metres, brakes (not shown) are applied to the winch mechanism to decelerate and eventually stop the water transfer vessel 3. During the deceleration stationary and early acceleration periods of the vessel 3 valves 19 and 27 are open to allow rapid change of water in the vessel 3 from warm to cool water using the pump 23. Ascent of the vessel 3 is provided by winding the supporting cable or chain 34 over the winch mechanism 33. On reaching the surface l the water transfer vessel 3 discharges the cold water 18 via the valve 19 to the cold water reservoir 4 of the station 2 for power generation and or temporary energy storage as described above. The energy required to drive the winch mechanism 33 is derived directly from the energised turbine 11 or from the energy conversion and storage unit 14.

In an alternative embodiment shown in figure 4 a pair of identical counterbalanced water transfer vessels 3 are provided. In this embodiment the surface station supports two spaced winch or sprocke and drive a common cable or chain 37 interconnecting the two counterbalanced water transfer vessels 3 which dock at ports 39 and 40. As for the single cable or chain drive embodiment shown in Fig 3 each water transfer vessel 3 has negative buoyancy creating tension in the common connecting cable or chain 37. The counterbalanced configuration means that torque required from the winch or sprocket wheels 35 and 36 is less than that for the single vessel 3 arrangement in Fig 3 and is required only to overcome drag forces as the water transfer vessels 3 move through the water and not to support the weight of either vessel 3. The water transfer vessels 3 move vertically in opposite directions, the length of the connecting cable or chain 37 being approximately equal to the maximum prescribed cool water depth neglecting the separation of the winches or sprocket wheels 35 and 36. Either of the two winch or sprocket wheels 35 or 36 is used to propel the water transfer vessels 3 with the one not driving being an idle wheel. Alternativels both drive wheels 35. 36 may be operated together for propulsion of the water transfer vessels 3 but means (not shown) are provided to ensure appropriate synchronisation of rotational movement to ensure comparable and preferably equal load sharing.

To provide a continuous cool water supply for the condenser 5 continuous vertical cycling of the counterbalanced water transfer vessels 3 is maintained between the surface I where cool water is discharged via the docking ports 39 and 40 to the cool water reservoir 4 and the ocean depths from where cool water is derived.

However, because two water transfer vessels 3 are employed the effective cool water supply rate and the attendant turbine power generating potential is approximately twice that for a single water transfer vessel 3 having the same capacity as either of the counterbalanced water transfer vessels 3 of figure 4.

The power generation and energy storage performed on the station 2 described in relation to the embodiment of figure 1 are applicable to the cable or chain drive propulsion embodiments described here. The embodiments of Figs 1 and 4 share many common features and are therefore given the same reference numerals. One significant difference is that the water delivery pipe 9 of the evaporator 5 is disposed vertically so that it passes through the cool water reservoir 4 but is thermally isolated from it using an insulator 41.

The warm water and cool water transfer operations described for propeller 3 1 or jet propulsion 32 methods are also applicable to cable or chain drive systems.

The water transfer vessel 3 propulsion means using cable or chain drive mechanisms may be modified as shown in the embodiment of Fig 5 by providing a drive mechanism 42 in the water transfer vessel 3 itself rather than on the surface station 2. In this embodiment one end of the cable or chain 43 is supported from a secure anchorage 44 on the station 2. the other end 45 extending downwards and being anchored or weighted as indicated as 46. The cable or chain 43 passes axially through the water transfer vessel 3 and is isolated from the water charge 18 of the vessel 3 by a protective tube 47 thus preventing any possible contamination of the charge 18 from cable or chain lubricants and rust protectors such as grease or thick oil.

Energisation of the cable or chain drive mechanism 42 is conveniently derived from a stored energy supply on the surface station using for example electrical power cables 48 fed from a battery unit 49. Alternatively energisation may be derived from an energy supply (not shown) on the water transfer vessel 3 itself such as batteries or compressed air which may have been energised earlier from energy conversion in the unit 14 on the surface station 2. In a further alternative embodiment the cable or chain 42 drive mechanism is powered directly from an ocean thermal energy conversion located in the water transfer vessel 3 itself with associated local energy storage facilities such as batteries.

In operation the cable or chain drive mechanism 42 drives the water transfer vessel 3 between the surface 1 and maximum accessible depth if the cable or chain 43 is sufficiently weighted or anchored 46 with the vessel 3 having neutral buoyancy.

Alternatively, for a vessel with negative buoyancy and an unweighted or unanchored cable or chain 43 the vessel 3 is driven upwards by the drive 42 and downwards under its own weight. In this latter case the drive mechanisms 42 fi-ee wheels or is idle during the water transfer vessel 3 descent mode. On reaching its prescribed cool water depth the vessel 3 warm charge is replaced by a cool water charge 18 and ascends to the surface to discharge its cool water contents to the cool water reservoir 4 of the surface station 2 for thermal energy conversion as described above. As the cool water charge 18 in the water transfer vessel 3 is depleted it may be replaced by relatively warm water from the condenser and evaporator exhaust reservoir 16 (see Fig 1) in order to maintain its charged neutral or negative buoyancy state as appropriate. Alternatively, the warm water may be obtained directly from surrounding ocean waters with the excess exhaust water in the reservoir 16 being discharged to the ocean.

In a further alternative embodiment vertical propulsion of the water transfer vessel or vessels 3 is provided by creating a change in vessel buoyancy from a state of negative buoyancy (causing the vessel to sink) to a state of positive buoyancy (causing the vessel to rise). Such a buoyancy change is produced by creating a low pressure void in the water transfer vessel 3 when it is has reached or is approaching its greatest depth to provide positive buoyancy in the vessel 3 for upward propulsion.

Removal or reduction of the void volume to create a negative buoyancy state after cool water discharge to the cool water reservoir 4 will cause the water transfer vessel 3 to sink. The energy required to induce voids may be provided in various ways as described below.

The propulsive potential of a void will increase with its volume because larger volumes will produce greater buoyancy and hence higher velocities of the water transfer vessel 3. However the larger the void volume of the vessel 3 the lower will be the available volume for carrying water. Furthermore. in order to create a void work must be done to overcome the effects of hydrostatic pressure which at 1000 metres water depth is about 100 bars. The energy consumed by void creation must be such that the converted available energy from ocean thermal gradients is sufficiently greater than the void induction energy requirements to allow useful power levels to be generated for subsequent land based or ocean based utilisation.

Figure 6 shows a schematic view of an alternative embodiment of a water transfer vessel 3 with buoyancy propulsion provided by generation of a low pressure void volume 51 by the pumping of water from the water transfer vessel 3 using a high pressure high capacity pump 50. The void jl is shown immediately below upper conical nose 30 of the vessel 3 and above a piston 28. The discharge of water from the vessel 3 through the valve 19 induced by the pump 50 will cause the piston 28 to move downwards creating an increase in void volume. Necessary pump and vessel guidance control systems and energy storage facilities may conveniently be accommodated within the upper conical region 30 isolated from the void volume 51 to avoid any detrimental effects of pressure cycling in cone 30. The vessel's stability and buoyancy are enhanced by ballast 29 housed at its lower end. Void creation takes place at cool water depths after the vessel 3 has been charged with cool water and the resultant positive buoyancy propels the vessel 3 upwards to station 2 for cool water discharging and recharging with evaporator and condenser exhaust water or environmental ocean water prior to removal of the void 51 and consequential negative buoyancy to repeat the descent.

Another water transfer vessel embodiment uses a high pressure void 51 for propulsion purposes and is shown in figure 7. A compressed air container 52 is housed in the upper nose cone region 30. The container 52 is at a pressure significantly higher than the hydrostatic pressure at the level of the thermally insulated piston 28 under maximum void volume conditions. The void 51 is produced by opening an outlet valve 53 in the compressed air container 52 therebv allowing compressed air to emerge and force the piston 28 downwards. Water 18 is forced out of the water transfer vessel 3 through a discharge outlet valve 54 thereby lowering the level of container water under the piston 28. A pneumatic pump 55 fed from the compressed air outlet valve 53 may be used to produce greater void 51 volumes for a given mass of compressed air by increasing the void air pressure to a value greater than the final pressure within the compressed air container 59.

Instead of using compressed air in order to produce voids in the water transfer vessel 3 a saturated vapour generator may be employed. An embodiment of a water transfer vessel having such a generator is shown schematically in Figs 8a - b. Figure 8a shows the water transfer vessel ; with a warm water charge 65 during its descent prior to void creation and Fig 8b shows the vessel 3 during ascent with a predominantly cool water charge 18 and after void creation.

In this embodiment a saturated vapour generator 56 in the form of a boiler having an (optional) internal heater 66 is provided. The boiler 56 generates a vapour 57 of sufficient pressure to overcome the prevailing hydrostatic pressure corresponding to the depth of the water transfer vessel 3. To prevent unwanted condensation of generated vapour a thermally insulated differential piston 58 is provided above the boiler 56 to thermally isolate the vapour 57 from a water reservoir 59 above the piston 58. The differential piston 58 is of inverted 'T' shape with a horizontal head section 62 and a vertical stem section 63. Thermal insulation is also advantageous in order to minimise heat loss through walls of the water transfer vessel 3 around the void and vapour generation region.

The nose cone 30 of the water transfer vessel 3 is recessed at 67 to receive a stem of the inverted T-shaped piston 58. At the bottom corners of the cone 30 there are provided outlet channels 60 which provide communication between the water reservoir 59 and the surrounding ocean.

In operation, saturated vapour produced by the boiler 56 acts on the underside of the head section 62 and because the surface area on which it bears is greater than that acting on the water reservoir 59 the piston 58 moves upwards and water is forced out of the reservoir 59 through outlet channels 60 against prevailing hydrostatic pressure greater than the saturated vapour pressure, thereby reducing the overall weight of the vessel 3 and increasing its buoyancy from a negative buoyancy state (which will cause the vessel 3 to sink) to a positive buoyancy state (which will cause it to rise). As with previous embodiments described the water transfer vessel 3 changes its warm water charge whilst at cool water depths to a cool water charge. In this embodiment the vessel 3 is caused to ascend by controlled vapour void 51 production to the surface station 2 where it is discharges its cool water contents to the cool water reservoir 4. The void volume is then reduced or removed to provide the vessel 3 (now with a warm water charge) with negative buoyancy. The water transfer vessel 3 is charged with warm water from the surface 1 environment rather than from the exhaust water 16 of the evaporator and condenser since the former is at a higher temperature than the latter and will more readily vaporise a low boiling point liquid in the evaporator as described below.

The energy required for the boiler 56 and the heater 66 is provided from the surface station 2 via, for example. power cables. Alternatively the water transfer vessel 3 possesses energy storage facilities on board in the form of, for example.

electric batteries or high thermal capacity substances. These may be continually replenished or re-energised from the surface station 2 each time a cool water discharge to the surface station 2 is carried out. All necessary control systems for water transfer vessel 3 guidance and vapour generation are accommodated in a control unit in a part of the conical nose 30 not occupied by the piston 58 and isolated from any vapour.

If the vapour generated by the boiler 56 is derived from a low boiling point liquid within the boiler 56 then the heat associated with the temperature of the warm water charge 65 within the vessel 3 (prior to exchanging it for a cool water charge at cool water depths) may be adequate to generate sufficiently high saturated vapour pressures 57 to displace the water 59 through the outlet channels 60 thereby creating a void 51. In this case warm water 65 is drawn into the boiler 56 through an inlet tube 63 using a pump (not shown) and discharged to the environment through an outlet tube 64. Depleted warm water 65 from the vessel 3 is replaced by cool water 18 using the pump 23. The cool water 18 is thermally isolated from the warm water by the piston 28. The internal heater 66 may be optionally used to generate higher saturated vapour pressures which enables the use of a smaller cross section area stem 63 of the differential piston 58 yielding proportionately greater water discharge volumes hence greater buoyancy. Air trapped within the recess 67 conveniently flows into the surrounding nose cone 30 during void creation thus minimizing any resultant opposing pressures from compressed air in the recess 67.

With the vapour filled void 51 the water transfer vessel 3 rises under positive buoyancy forces and at the surface 1 discharges its cool water contents 1 8 to the surface station 2 as described before. On production of the vapour void the piston 58 may be locked into the maximum void volume position to facilitate vessel 3 ascent and for subsequent descent from the surface. The piston 58 may be released and the void volume reduced to create negative buoyancy by pumping cool water 18 through the boiler 56 to condense the vapour. The saturated vapour pressure may be used to drive an on-board turbine (not shown) prior to this to make use of the available energy.

As a further alternative to the water transfer vessel 3 embodiments described above voids may be generated by mechanical means rather than hydraulic pumping, compressed air or high pressure vapour. Fig 9 shows a schematic representation of such an embodiment of the water transfer vessel 3 in which a piston 68 is fitted below the nose cone 30. The piston 68 is driven by a screw drive assembly comprising an upper motor drive 70 with a rotatable depending threaded drive rod 71 with a protective cover 72 supported by a threaded bush 73 integral and axial with the piston 68. The threaded rod 71 is mechanically coupled to the motor drive 70 and is rotatable via an associated gear box (not shown) in response to motor energisation thereby causing the piston to move up and down according to the direction of rotation of the motor drive unit. The power supply to the motor is supplied by for example on board electric batteries 74 in the nose cone 30 although pneumatic or hydraulic supplies are alternative options. In an alternative embodiment (not shown) motor power is transmitted down power cables from the surface station 2. When the piston 68 is driven downwards relative to the vessel 3 water 18 is forced out through a discharge outlet 27 in the wall of the vessel 3. This creates a low pressure air void between the piston 68 and nose cone 30.

When the piston 68 is at its uppermost position immediately adjacent the nose cone 30 there is no void and the vessel 3 has negative buoyancy allowing it to sink.

When the piston 68 is moved to its lowest position a maximum void volume is created resulting in positive buoyancy.

Figs 10a - b show embodiments of water transfer vessels 3 with an on board thermal energy conversion system. Propulsion of such vessels may be any of the means described above in relation to previous embodiments and are not shown in the figures for reasons of clarity.

The water transfer vessel 3 shown in Fig 10a has two water chambers 75 76 that extend axially along the vessel 3 and are separated by a vertical thermally insulating wall 77. A first chamber 75 holds warm water from surface or near surface ocean regions and a second chamber 76 holds cool water obtained from deep ocean regions.

An evaporator 78 is received in the warm water chamber 75 and a condenser 79 in the cool water chamber 76. The evaporator 78 and condenser 79 contain the liquid and vapour of a low boiling point fluid. Coupled to the evaporator 78 is a turbine 80 driven from vapour generated in the evaporator 78 by the heat transferred from warm water in the warm water chamber 75. Emergent vapour from the turbine is fed to the condenser 79 for condensation by heat transfer to cold water in the cold water chamber 76 prior to being returned to the evaporator with a pump 81 for continued cycling. Rotational energy from the turbine 80 drives a generator or compressor 82 which supplies power directly to the appropriate propulsion unit (not shown) with excess power being transmitted via power cables (for electrical power generation) or pipes (for compressed air generation) to an energy storage unit 83 located in the upper nose cone region 30. The stored energy is subsequently transferred or delivered to a main energy storage unit (not shown) either undersea or surface based prior to ocean or land based utilisation.

The water chamber 75 continuously feeds warm water to the evaporator 78 via an inlet valve 88 and the second chamber 76 continuously feeds cool water to the condenser 79 via an inlet valve 89. Pumps (not shown) are provided to feed water through the valves 88, 89. Exhaust water from the evaporator 78 is rejected from the water transfer vessel 3 into the ocean via an outlet valve 90 positioned above the ballast 29. Similarly exhaust water from the condenser 79 is discharged to the ocean via an outlet valve 91 that is diametrically opposite the evaporator outlet valve 90. To replace the discharged warm water. water from the ocean surface is admitted through an inlet valve 92 positioned below the nose cone 30 using pump 96. Similarly cold water replenishment from the ocean depths is achieved by opening inlet valve 95 positioned diametrically opposite the warm water inlet valve 92 and using pump 97.

The interior of each of the first and second chambers 75. 76 is fitted with a thermally insulated piston 93 which prevents heat transfer between the residual and fresh water.

Fig 1 0b shows a similar embodiment to the water transfer vessel of Fig 1 0a and features in common with the embodiment of figure 1 0a are given the same reference numerals. The vessel interior is divided laterally into an upper chamber 84 containing warm water and a lower chamber 85 containing cool water. Between the two chambers there is an evaporator 86 associated with the upper chamber 84 and a condenser 87 associated with the lower chamber 85. As with the embodiment of Fig 1 0a the evaporator 86 drives a turbine 80 which generates power by means of a generator 82 for direct vessel drive with excess power fed to an on-board energy facility 83 for subsequent utilisation.

Thermal insulation between residual and fresh water in each of the chambers 84, 85 is provided by a thermally insulating piston 94.

Fig 11 shows water transfer vessel 3 having a single chamber 98 with an evaporator 99, condenser 100. turbine 80 and generator 82 providing thermal energy conversion similar to that used in a double chamber embodiment described in relation to Figs 10a and 10b. The chamber 98 is charged with cool water using a pump 101 when the water transfer vessel 3 is at cool water depths but not necessarily at its maximum depth. The vessel 3 will then be propelled upwards by propulsion means powered by an on-board stored energy supply unit 102. Any suitable propulsion means described above may be used. When the vessel 3 reaches warmer water the evaporator 99 is fed with warm water to enable thermal energy conversion. When it approaches the surface 1 the water transfer vessel 3 will recharge the chamber 98 with warm water using the pump 101.

The evaporator 99 has a first inlet valve 103 and pump (not shown) for admitting warm water when the vessel 3 is at the surface of the ocean. Exhaust water is discharged to the ocean through an outlet valve 104. A second inlet valve 88 allows water to be drawn from the chamber 98 when it contains warm water.

The condenser 100 has a first inlet valve 89 for receiving cool water from the chamber 98 and an outlet valve 108 through which exhaust water is discharged to the ocean. A second inlet valve 107 is provided to allow cool water to be drawn from the ocean when the vessel 3 is at the ocean depth. As the ocean surface 1 is approached by the vessel 3 the surrounding water temperature will continue to increase resulting in a progressive increase in energy conversion efficiency. At the ocean 1 surface cold water discharged from the condenser outlet 108 is replaced in the chamber 98 by environmental warm water via an inlet valve 105. A thermally insulated piston 106 thermally isolates the fresh water from the residual cool water charge in chamber 98.

The vessel 3 is then propelled downwards initially using stored energy from the unit 102 to the cooler depths and power generated by drawing cool water via the inlet valve 107 and a pump (not shown) from the surrounding cool water. The condenser 100 exhaust water is discharged to the environment via the valve 108. For the above descent the evaporator 99 gains its heat from the warm water in the chamber 98 via valve 88 this water having been derived from the upper warmer ocean region.

As indicated above a fraction of on board converted energy is used to power on board propulsion units. However all locally (on board) converted energy may be temporarily stored in the water transfer vessel 3 for conveyance to the surface station 2 and energisation of an external surface based water vessel propulsion system such as the cable winch or chain sprocket drive mechanism described above. The residual energy not used for water vessel 3 propulsion is accumulated and stored in the surface vessel for subsequent ocean or shore based utilisation. Alternatively energy may be transmitted for shore based utilisation via power cables routed down to the ocean bed and along to the shore. Should any cold water remain in the water transfer vessel after an ascent it may be discharged to the surface station 2 for use in a thermal energy conversion system.

Systems control guidance and energy storage facilities of the water transfer vessel 3 are located within the nose cone 30 the vessel 3 itself possessing a streamlined geometry to maximise power generation rates.

Fig 1 2a shows schematically. an embodiment of a water transfer vessel 3 that is propelled by compressed air. The surface station 2 has a compressed air supply 109 with a compressed air flexible delivery tube 110 pneumatically coupled to the water transfer vessel 3. The tube 110 is coiled around a drum 111 on the surface station 2 and extends to the cool water depth by uncoiling it as the water transfer vessel 3 descends. The water transfer vessel 3 has a water chamber 114 and at its lower end a water discharge valve 112. The vessel 3 has negative buoyancy and thus sinks under its own weight. On reaching the prescribed cool water depth compressed air is fed by the supply 109 through the tube 110 to the water transfer vessel 3. This forces water out of the vessel 3 via the water discharge valve 112 (a pneumatic pump may be used for this purpose) thereby creating a void 113 at the upper end of the vessel 3, the void 113 having a pressure equal to the prevailing hydrostatic pressure and being of sufficient volume to change the vessels negative buoyancy to positive buoyancy.

The vessel 3 is then caused to return to the surface from the ocean depths with the chamber 114 full of cool water. The cool water is used in a thermal energy conversion system on the surface station 2. The compressed air void 113 for propulsion may be volume limited and remain with a fixed volume at a constant high pressure throughout its ascent by employing a fixed volume void chamber 11 5 at the upper region of the water transfer vessel 3 as shown in Fig 12b. A valve 116 is provided through which water is discharged from the void chamber 115 until this chamber is empty at which point the valve 116 will close. On reaching the surface 1 the high pressure air in the void 115 is fed back to the compressed air supply 109 on the surface station 2 for-re-use in order to minimise energy consumption.

Alternatively compressed air in the air delivery line 110 is fed back to the supply as the feed line 110 is recoiled during the vessel's ascent. In an alternative embodiment (not shown) the compressed air is used to drive a turbine and generate useful power rather than returning it to the supply.

In a further alternative embodiment (not shown) a rigid compressed air delivery tube is used instead of a flexible version. The tube passes axially through the water transfer vessel 3 with dynamic seals at the top and bottom ends of the vessel to avoid loss of the vessel's contact as it is propelled over the delivery tube. The vessel has a fast operating release valve at its free (cool water) end. When the vessel reaches the free end of the delivery tube the valve is received by the vessel 3 at the bottom seal and the valve rapidly opens releasing compressed air into the vessel 3. The compressed air pressure and flow rate from the delivery tube will be such that a void will rapidly be created in the upper part of the vessel 3 reversing its buoyancy from a negative state to a positive one. After having exchanged the contents of its water chamber for cool water during the vessel's 3 occupancy of the cool water depths, the vessel will rise to the surface and discharge its contents to the surface vessel for thermal energy conversion. The pneumatic energy void may be used productively by, for example, driving a turbine or pumping it back to a compressed air storage supply.

As an alternative to propelling a water transfer vessel 3 to deep ocean regions and to the surface again using the relatively steady drive conditions as described above a vessel 3 may be induced to travel to cool ocean depths and return to the surface by means of a impulsive or short period force imparted to the vessel. In this case the vessel 3 is accelerated to a relatively high downward velocity by the impulse force causing it to penetrate to deep cool regions where its initial warm water charge may be changed to a cool water charge. The vessel buoyancy is positive throughout the descent and ascent period. At the end of the vessels descent it decelerates to zero velocity as a result of opposing buoyancy and drag forces.

At the end of its descent the vessel ascends by virtue of its positive buoyancy.

Thermal energy conversion is, as before, out on the surface station 2 which also contains the impulse drive facilities. Alternatively the vessels may have an on-board thermal energy conversion unit as described before.

The embodiment of Fig 1 3a shows a water transfer vessel driven by an impulse drive. The surface station 2 has a vertical cylindrical launch tube 117 in which the water transfer vessel 3 docks whilst at the surface 1. Hydraulically coupled to the launch tube 117 is a hydraulic pump 118 with an associated hydraulic accumulator 119 which enhances hydraulic delivery rates by storing large amounts of energy for rapid release when required. The hydraulic supply to the pump 118 and accumulator 119 is derived from the surrounding water via an inlet 120 with a large capacity compressed air supply 121 providing pneumatic energy. In operation. with the water transfer vessel 3 docked within th hydraulically pressurised from the pump 118 and accumulator 119 thereby accelerating the water transfer vessel 3 downwards to initiate the cool water transfer operation. Compressed air from the supply 121 may be replenished by means of a compressor 122. Docking at the launch position as the vessel 3 returns to the surface to discharge its cool water is achieved with guidance means (not shown) incorporated in the vessel 3.

During the descent and ascent of the vessel the hydraulic accumulator 119 is charged using the hydraulic pump 118 and is ready to release its energy again to the launch tube 117 by the time the vessel 3 returns for docking.

Other impulse drives may be used for propelling the water transfer vessel to cool ocean depths. These include air or mechanical spring mechanisms. Downward acceleration with or without external impulse mechanisms may also be produced by lowering the water level in the launch tube 117 with the vessel 3 mechanically supported in an air environment and allowing the water transfer vessel 3 to accelerate downwards under its own weight. In this case the hydraulic impulse generator described above may not be appropriate to enhance the free fall acceleration in view of the air surrounding the upper regions of the vessel 3 within the launch tube 117.

However, a pneumatic impulse generator may be employed in place of a hydraulic one to induce downward momentum and enhance free fall acceleration.

In an alternative embodiment a mechanical or air spring direct contact launch mechanism may be provided in the form, for example, of a driven piston or catapult.

Such a mechanism is applicable with or without the vessel free fall acceleration. In order to lower the launch tube water level for free fall inducted momentum the launch tube may be sealed at the upper region and pressurised with air to a pressure equal to the hydrostatic pressure corresponding to the depth of the depressed water level below the surrounding surface level 1.

During the ascent or descent of the vessel 3 the tube 117 is vented allowing the water to rise to the surface level 1 and enable the vessel 3 on re-entering the launch tube 117 to rise to an elevated support position within the tube 117 and discharge its cool water contents. During this discharge period the water level within the launch tube 117 may be lowered again by pressurisation of the air space so that the vessel is ready for descent.

Figure 1 3b shows an impulse generator station anchored to the ocean bed using ballast 123. The generator comprises a vertical inverted launch tube 125 that is hydraulically impulsively pressurised from a hydraulic accumulator 125 in combination with an associated hydraulic pump 126, a compressed air supply 127 and a large capacity auxiliary compressed air supply 128 for replenishing any air loss from the supply 127 using a compressor pump 129.

During docking within the launch tube 124 the water transfer vessel 3 discharges its warm water charge and admits cool water from the surroundings. The vessel 3 is then propelled back to the upper launch tube 117 by a hydraulic impulse delivered to the lower launch tube 124. It will be understood that, as before, other types of impulse generator may be used. This embodiment has the advantage that positive buoyancy of its vessel is not required. Instead the vessels may be provided with neutral buoyancy enabling it to reach greater depths from the surface impulse generators.

Figs 1 4a to 1 4c shows an alternative embodiment of a water transfer vessel 3 in which propulsion is provided by a detachable ballast weight. The surface station 2 has a winch 130 for lowering or raising a cylindrical ballast weight 131 with a cable or chain 132, the weight 131 being received within the transfer vessel 3.

The water transfer vessel 3 has an axial closed cavity 133 surrounded by a water chamber 134 and an upper void 135. An axial tube 136 extends from the cavity 133 to an upper end 137 of the vessel 3. The cavity 133 receives the cylindrical ballast weight 131, the weight 131 being shorter in length than the cavity 133.

Connected to the ballast weight 131 is the cable or chain 132 which passes upwards through the tube 136 emerging at the upper end 137 of the vessel 3 through a flexible seal 139. At the base of the cavity 133 there is a resilient member 140 which supports the ballast weight 131 when the cable or chain 132 is not in tension (as shown in Fig 14b). This creates a state of negative buoyancy in the water transfer vessel 3. The vessel 3 is designed such that in the absence of the ballast weight 1 3 1 the vessel 3 will have positive buoyancy. If tension in the cable or chain is generated by the winch 130 on the surface plant 2 such that the ballast weight 13 1 is lifted off the resilient member 140 (as shown in Fig 14c) the vessel 3 will rise under positive buoyancy. During vessel ascent the ballast weight 131 remains out of contact with the support member 140 by appropriate winding of the winch which is controlled by. for example, monitoring of the cable tension or sensors on the vessel 3. After recharging with warm water (from the condenser or evaporator exhaust or the surrounding seawater) at the surface the weight 131 is lowered onto the resilient support member 140 yielding a negative buoyancy state in the vessel 2 and thus downward propulsion.

Fig 15a shows a schematic embodiment of an extensible water transfer vessel 141 consisting of an upper chamber 142 slidably connected to a lower chamber 143, the lower chamber 143 having a cool water reservoir 144. Dynamic seals 145 between the two chambers 142. 143 are provided to prevent leakage of liquid or gaseous internal contents out of the environment or external ocean water into the vessel chamber 142, 143.

Sliding movement of the upper chamber 142 relative to the lower chamber 143 increases the axial length of the vessel and provides a void 146 of adjustable volume.

Increasing the volume of the void 146 yields a corresponding increase in the vessel's buoyancy and results in an upward propulsive force once the buoyancy state has become positive. Conversely sliding the chamber 142 downward with respect to the lower chamber 143 reduces the void volume and the buoyancy until the vessel 141 possesses a state of negative buoyancy and sinks under its own weight.

Figs 1 6a and 1 6b show an alternative embodiment of an extensible vessel 141 in which bellows 147 connect the upper edge 148 of the lower chamber 143 and lower edge of the upper chamber 142. Extension of the bellows by separating the upper and lower chambers 142 and 143 will increase the void volume 146 and hence the vessel buoyancy as before.

Fig 1 7a shows an extensible water transfer vessel with a motorised screw drive. A drive motor 150 is supported in an upper cone region 151 of the upper chamber 142 on a support plate 152. A threaded drive rod 153 passes axially through the plate 152, a void region 154. a threaded bush 155 in a supporting roof plate 156 in the lower chamber 143 and into an isolating protective cover 157. The roof plate 156 as well as providing mechanical support also isolates the contents of lower chamber 143 from the upper void region 154 region.

In operation, when the vessel 141 is at the surface of the ocean. the upper 142 and lower 143 chambers are driven axially together reducing the void 154 volume and creating a state of negative buoyancy under which the vessel 141 will sink. On reaching the necessary cool water regions the vessel 141 discharges its warm water charge contained in the lower chamber 143 and derived whilst on the ocean surface, and fills with a cool water charge. During this time the void volume 154 is increased by reversing the motor 150 drive direction and forcing the upper 142 and lower 143 chamber apart. This will create a state of positive buoyancy in the vessel 141 producing upward propulsion and with the aid of guidance facilities the vessel will rise and dock with a surface station (not shown in this figure) where it will discharge its cool water contents for thermal energy conversion on the station.

Energisation for the motor drive may be provided from batteries on the vessel 141 which may be charge intermittently from a main charging supply on the surface plant or alternatively drive power may be transmitted to the drive motor 150 on the vessel 141 from the surface plant supply via power cables. All guidance. motor drive and other control and where appropriate energy storage systems are contained in an upper containment cone 151.

It will be appreciated that other mechanical devices may be employed to create or increase and control void volume, for example, hydraulic pistons. motorised cam based or lever systems.

An important feature of an extensible water transfer vessel 141 is that the cool water chamber 143, once charged. remains isolated from the rest of the vessel 141 and the environment since void creation does not require any discharge of water. Other methods may be used for creating voids 154 in an extensible water transfer vessel 141 and examples are described below with reference to Figs 1 7b - 17d.

Fig 1 7b shows a water transfer vessel with on board compressed air containers 157 located in the upper nose cone 151. a valve 158 and an associated pump 159 for controlled release of compressed air into a void space 154 for creating positive buoyancy conditions in order to propel the vessel 141 from ocean depths to the surface. To prevent the upper 142 and lower 143 chambers from separating mechanical stops or axial displacement limiters (not shown) are provided.

Downward propulsion under negative buoyancy conditions in the vessel 141 is created by, for example pumping the high pressure air in void 154 back to the on board compressed air container 157 or to similar containers in the surface station.

Alternatively, the pneumatic energy associated with a high pressure void may be used productively by driving a turbine on a surface vessel and storing generated energy on board.

As an alternative to compressed air high pressure vapour may be generated in an evaporator or boiler (not shown) located in the nose cone 151 and fed into the void volume via a control valve equivalent to the compressed air valve 158. By this means controlled buoyancy changes in the vessel 141 may be provided by appropriate thermal control of the evaporator. Negative buoyancy conditions are induced by reducing the saturated vapour temperature and hence pressure and positive buoyancy produced by increasing it. Good thermal insulation of the vapour generation and vapour void regions 154 is advantageous in order to reduce heat loss and hence thermal energy conversion efficiencies.

In the water transfer vessel shown in Fig 17c compressed air is fed from surface station via an airline 160. This enters the cone 151 of the vessel 141 via a valve 161 and passes through the support plate 152 via a valve 162. Compressed air flow into the void volume 154 forces the plates 152 and 156 apart thus increasing void volume and therefore vessel buoyancy. Reduction in void volume and vessel buoyancy will result from withdrawing compressed air from the void 154 via the feed line 160 and pumping it back to the surface station storage supply.

For all high pressure vapour or compressed air void creation methods using either fixed geometry or extendible water transfer vessels provision may be made to allow a void to increase in volume up to a limit in response to decreasing opposing hydrostatic pressures from vessel ascent thus increasing buoyancy forces and upward velocity. However for fixed geometry vessels this void volume increase will be at the expense of the vessel's water charge.

The water transfer vessel embodied in Fig 1 7d represents a variation of the design shown in Fig 17a. The motor is replaced by gear box 1 64 and rotation is produced in the threaded drive rod 153 by means of an external cable 163 mechanically coupled to the gear box 164 at one end. The cable 163 passes through a dynamically sealed entry point at the upper end of the vessel 141 and extends to a winch mechanism (not shown) on the surface station at the other end. In operation inertial forces associated with the vessel during descent will be opposed when the vessel enters cool water depths by engaging the winch drive (preferably through a clutch mechanism) on the surface station. This will generate tensile forces in the cable 163 and produce rotation of the threaded drive rod 153 through appropriate gearing in the gear box. The chambers 142 and 143 are driven apart thereby creating a void and upward propulsion of the vessel. The void volume is maintained during the ascent period by a suitable locking mechanism (not shown). During ascent tension will remain in the cable or chain because of drag forces opposing the winch winding forces as the cable 163 is wound in. During descent. with the void volume reduced.

the winch free runs thereby yielding negligible tension in the cable 163.

In alternative embodiments other mechanisms may be employed in the vessel 141 to produce voids, such mechanisms being energised by the above cable or chain tensile forces. Further the threaded rod and other void producing mechanisms driven from an external cable bv means of a surface based winch are applicable to void production in both fixed geometry and extensible geometry water transfer vessels using either pistons or bellows.

Figure 1 8a shows an embodiment of a water transfer vessel with high pressure storage cylinders 166 for supplying air to a void 165 defined by bellows 167. The compressed air is supplied via a valve 168 and a reversible pump 169 thereby creating the void 165 by extending the bellows. The reversible pump 169 is used to return compressed air in the bellows to the storage cylinders 166 thus reducing the void 165 volume and vessel buoyancy when required. Energy to pump the compressed air 165 in the bellows back into the storage cylinders is derived from an on board energy storage facility 171 which is charged up intermittently from converted ocean thermal energy. Although the bellows are not essential for this operation they serve to isolate the compressed air within from water contained in a chamber 170 of the vessel, thereby reducing corrosion effects and air loss through dissolution in the water. The propulsion of the water transfer vessel is otherwise identical to the piston system described in relation to Fig 7.

Figure 1 8b shows the water transfer vessel of figures 9a and 9b fitted with a bellows 172. A drive motor 173 is supported on a plate 174 and is connected to a threaded drive rod 175 which passes through the plate 174 and a lower plate 177. The emergent length of the threaded drive rod 175 is protected by an elongate cap 178 isolating the drive rod 175 from the water charge 170 below the plate 1 77. In operation a void is created by driving the bellows 172 open against the hydrostatic pressure using the motorised screw drive rod 175. The void is reduced in volume or eliminated by reversing the motor drive direction.

In cases where upward propulsion of a water transfer vessel is produced by creating voids to increase buoyancy a large proportion of the energy consumed to produce these voids may be recovered by incorporating within the water transfer vessel design hydraulic turbines that are driven by the water transfer vessels vertical motion through the water. The rotational power of the turbine may be converted to a suitable form such as electrical power or compressed air by using an associated electrical generator or compressor driven by the turbine and the resultant energy is stored for subsequent use. Turbines for energy recovery processes may be axial with the water transfer vessel and typically supported in the region of the nose cone.

Alternatively or additionally axially offset turbines may be mounted on the periphery of a vessel 3.

Figure 19 shows a schematic embodiment of part of a water transfer vessel with an outboard propeller type turbine 179 which has variable pitch blades 180 and is driven by movement of the water transfer vessel 3 through the water. The turbine 179 is contained within an outer streamlined upper compartment 181 shown in broken line with means (not shown) to expose fully. when required. the turbine blades 180 to the water. Such means may be, for example, slidable or hingable panels in the compartment 181 with provision to allow unimpeded escape of water driving the blades in order to avoid water stagnation leading to inefficiencies in power production. The turbine 179 is connected to an electrical generator 182 and both are supported on a support panel 183 within the nose cone 30.

Electrical power generated by a thermal energy conversion system on a surface station may be transmitted directly down power cables extending to and along the ocean bed to a land based utilisation or distribution centre for use on land.

Alternatively, generated energy may be converted into a suitable transportable form for local or remote based utilisation. Storage may for example be in the form of heat preferably involving a high thermal capacity substance which has undergone a phase change with an associated large latent heat of fusion. Alternatively electrical power generated may be used to charge batteries or electrolytically produce hydrogen from water with transportation being carried out via compressed hydrogen cylinders or via pipe lines preferably routed along the ocean depth to land based utilities. In this case compressed hydrogen may be conveyed to a deep ocean storage reservoir possibly with neutral buoyancy or preferably to the sea bed with piping systems coupled to such a reservoir. Energy storage may also be in the form of compressed air and be distributed to utilisation sites via compressed air lines along the ocean bed as for hydrogen or by compressed air cylinders.

Instead of generating useful power off shore as described ocean thermal energy conversion may be carried out on land by conveying cool water derived off shore in a water vessel towards the shore where it discharges its cool water charge to an on shore thermal energy conversion system. Warm water for vapour generation may be derived in a similar manner except from ocean surface regions rather than large depths. The transportation of warm water is carried out by water transfer vessels propelled along the ocean surface.

The discharged water from a land based thermal energy conversion facility may be transported in a water transfer vessel back along the original cool or warm water extraction routes and discharged gradually en route to avoid local heating of deep cool water and cooling of surface warm water.

Fig 20a illustrates an alternative method of extracting warm water from the ocean. A surface water transfer vessel 184 has an on board cable or sprocket drive mechanism 185 for propelling the vessel 184 along a cable or chain 186. This is supported at one end by a land based anchorage 187 close to a thermal energy conversion facility 188 on land including a large warm and cool water storage facility 189. The other end of the cable or chain 186 is supported by an off shore floating platform or vessel 190 permanently anchored 191 to the sea bed.

The surface vessel 184 extracts warm water from anywhere along its cable or chain route prior to discharging it to the land based reservoir 189 and returning for dispersal of condenser and evaporator exhaust water charge from the thermal energy conversion facility. Because of the finite weight of the cable or chain 1 86 it will curve downwards with sufficient depth over most of its length to avoid interference from passing marine craft. The vessel 184 will lift the cable or chain towards the surface as it propels itself along to or from the shore.

Fig. 20b illustrates a method for extracting and transporting cool water from off shore ocean depths to a land based thermal energy conversion plant 188. This may be used in conjunction with the method of extracting warm water as described above.

A cable or chain 192 is laid along the ocean bed 193 and supported at one end by a land based anchorage 194 and at the other end by ocean bed anchorage 195. A cool water transfer vessel 196 contains an on board cable or sprocket drive mechanism 197 for driving the vessel along the cable or chain 192. The vessel 196 possesses positive buoyancy and thus lift the cable or chain off the sea bed as it proceeds along to transfer its cool water charge to the water storage reservoir 1 89. The reservoir 1 89 possesses two large thermally insulated storage chambers (not shown) one for warm water and the other for cool water. These are charged intermittently with warm and cool water as appropriate for continuous use by the plant 1 88. The exhaust water from an evaporator and condenser (not shown) and the thermal energy conversion plant 188 is fed to the cool water transfer vessel 196 and is discharged gradually as the vessel descends. Discharged water may also be fed to the surface warm water transfer vessel. Surrounding water is taken on to the vessel to compensate for dispersed water loss in order to preserve vessel buoyancy conditions.

Instead of using cable propulsion for cool and warm water transfer to a shore based reservoir 189, propeller or jet propulsion methods could be used.

A further alternative to ocean thermal energy conversion on a floating surface plant or submersible water transfer vessel 3 is shown in figure 21. A floating surface based vessel or water tanker 198 has a very large water carrying capacity significantly larger than the water charge capacity of a typical water transfer vessel. The surface vessel or water tanker 198 will accept many charges of cool water derived from ocean depths by a water transfer vessel 3 and warm water derived from the surface 1. These accumulated cool and warm water charges will be stored in the vessel 198 in separate thermally insulated chambers and on reaching their capacity the vessel 198 will be propelled by, for example a propeller or cable winch propulsion system to a shore based thermal energy conversion facility 188 where it will discharge its water contents to an associated reservoir 189 with large thermally insulated warm and cool water storage chambers prior to returning for recharging. A continual supply of cool and warm water charges to the shore based reservoir 189 for use by the thermal energy conversion facility 188 may be assured by employing a succession of water tankers 198 discharging their contents to the reservoir 189 in turn before returning to appropriate ocean sites for recharging and subsequent discharging to the reservoir 190.

It will be understood that the surface station could take many forms. For example, it could be a floating vessel. a platform supported on legs in the water, or a land-based construction.

Moreover, it will be understood that the surface station could be replaced by a station located at relatively cool ocean depths with warm water being delivered from the surface to an underwater storage reservoir.

Claims

1. Apparatus for converting thermal energy of a natural source of water into useful power, the water source comprising water at a first temperature at a first level and water at a second temperature at a second level. the apparatus comprising an at least partially hollow vessel with propulsion means for propelling it between said first and second levels, the vessel having means for collecting water from either of said levels and means for dispensing the collected water at the other level, and a thermodynamic heat engine for producing useful power from the difference between the first and second temperatures of the water.

2. Apparatus according to claim 1 wherein the natural source of water is an ocean or the like.

3. Apparatus according to claim 2. wherein one of the levels is at or near the water surface, the other being at a significant depth below.

4. Apparatus according to claim 3. wherein the second level is at or near the water surface and the first level is at a significant depth below. the first temperature being lower than the second temperature.

5. Apparatus according to claim 4, wherein there is provided a station at the water surface with which the vessel docks at said second level and from which the vessel descends to the first level.

6. Apparatus according to claim 5, wherein the station floats on the surface of the water.

7. Apparatus according to claim 5. wherein the station is located on land adjacent the water.

8. Apparatus according to claim 5, 6 or 7, wherein the thermodynamic heat engine is provided on the station. the station having water reservoirs for water at said first and second temperatures.

9. Apparatus according to claim 5. 6 or 7, wherein the thermodynamic heat engine is provided on the vessel, the vessel having reservoirs for water at said first and second temperatures.

10. Apparatus according to claim 9. wherein the vessel has a moveable thermally insulating piston between the water reservoirs.

11. Apparatus according to any one of claims 5 to 10, wherein the thermodynamic engine comprises a turbine generator connected between first and second heat exchangers, the first heat exchanger being connected to supply means for supplying warm surface water at said second temperature to it and containing a liquid that vaporises at the surface water temperature, the turbine generator being driven by the vapour and the second heat exchanger having means for supplying cool water from the depths at said first temperature to it in order to condense the vapour, and means for returning the liquid to the first heat exchanger.

12. Apparatus according to claim 11! wherein the vessel has an inlet for receiving cool water at the depths or warm water at the surface and an outlet for discharging warm water at the depths or cool water at the surface.

13. Apparatus according to any one of claims 5 to 12, wherein the vessel has guidance and direction control equipment for docking with the surface station

14. Apparatus according to any preceding claim wherein the propulsion means is one or more propellers.

15. Apparatus according to any one of claims 1 to 13, wherein the propulsion means is one or more water jets.

16. Apparatus according to any one of claims 5 to 13, wherein the propulsion means is provided by an elongate flexible member attached between the vessel and the surface station, the elongate flexible member being extensible from the surface station to the ocean depths.

17. Apparatus according to claim 16. wherein the elongate flexible member is a cable or chain.

18. Apparatus according to claim 17. wherein the cable or chain is driven by a winch on the surface station.

19. Apparatus according to claim 16 or 17, wherein there is provided a drive wheel on the vessel for engagement with the elongate flexible member, the vessel being moveable relative to the member by rotation of the wheel.

20. Apparatus according to claim 19, wherein the wheel is a sprocket.

21. Apparatus according to any preceding claim, wherein there are provided two counterbalanced vessels, the propulsion means being provided by an elongate flexible member interconnecting the vessels and at least one drive wheel for driving the elongate flexible member.

22. Apparatus according to claim 21. wherein there are provided two spaced drive wheels for the elongate flexible member.

23. Apparatus according to claim 22. wherein one of the drive wheels is idle and the other drives the elongate flexible member.

24. Apparatus according to claim 22. wherein the drive wheels each drive the elongate flexible member and a synchronisation device is provided for synchronising rotation of the wheels.

25. Apparatus according to any one of claims 16 to 24 wherein the vessel has negative buoyancy.

26. Apparatus according to any one of claims 1 to 24 wherein the means for propulsion is provided by an adjustable buoyancy control in the vessel.

27. Apparatus according to claim 26 wherein the buoyancy is adjustable between a negative state to permit the vessel to sink and a positive state to cause the vessel to ascend.

28. Apparatus according to claim 27. wherein the adjustable buoyancy control comprises means for discharging water from the vessel to create a low pressure void and means for charging the vessel with water.

29. Apparatus according to claim 28 wherein the means for discharging water is a fluid pump.

30. Apparatus according to claim 29. wherein the fluid pump is mounted on the vessel.

3 1. Apparatus according to claim 29. wherein the fluid pump is mounted on the surface station and connected to the vessel by a fluid delivery line.

32. Apparatus according to any one of claims 28 to 31. wherein the means for discharging water comprises an evaporator and a piston. the evaporator creating vapour that drives the piston to discharge water from the vessel.

33. Apparatus according to claim 32. wherein the piston is a differential piston.

34. Apparatus according to claim 27, wherein the adjustable buoyancy control is provided by means for changing the volume of the vessel.

35. Apparatus according to claim 34. wherein the means for changing the volume of the vessel comprises a source of pressure that acts to displace a first portion of the vessel relative to a second portion of the vessel.

36. Apparatus according to claim 35 wherein the source of pressure is a supply of compressed fluid or pressurised vapour.

37. Apparatus according to claim 35, wherein the source of pressure is a piston moveable by a mechanical drive.

38. Apparatus according to any one of claims 35 to 37 wherein said first and second portions are interconnected by a bellows.

39. Apparatus according to claim 27 wherein the adjustable buoyancy control is provided by removable weight.

40. Apparatus according to claim 39 wherein the weight is in contact with the vessel to provide negative buoyancy and is removable to provide positive buoyancy.

41. Apparatus according to claim 40, wherein the weight is removable by a winch.

42. Apparatus according to claim 40 or 41. wherein the weight is received in a cavity in the vessel, the cavity having a bottom wall on which the weight may rest.

43. Apparatus according to claim 42 wherein resilient means are provided between the weight and the bottom wall.

44. Apparatus according to any one of claims 5 to 12, wherein the propulsion means comprises at least one impulse drive.

45. Apparatus according to claim 43 wherein an impulse drive is provided at the surface station and the vessel has positive buoyancy.

46. Apparatus according to claim 45. wherein an impulse drive is also provided at the first level.

47. Apparatus according to claim 44. wherein an impulse drive is provided at the surface station and the first level the vessel having neutral buoyancy.

48. Apparatus according to any one of claims 43 to 47, wherein the impulse drive comprises a launch tube within which the vessel may be received. and means for supplying an impulsive force to a launch tube.

49. Apparatus according to claim 48 wherein the means for supplying an impulsive force is a hydraulic impulse generator.

50. Apparatus according to claim 49. the hydraulic impulse generator comprising a hydraulic accumulator, hydraulic pump and a source of compressed air.

51. Apparatus according to claim 48. wherein the means for supplying an impulsive force is a mechanical piston or catapult.

52. Apparatus according to claim 11 or any one of claims 12 to 50 when dependent from claim 11, wherein an output of the turbine generator is converted into electrical power and stored.

53. Apparatus according to claim 11 or any one of claims 12 to 50 when dependent from claim 11 wherein an output of the turbine generator is converted into compressed air and stored.

54. Apparatus according to claim 11 or any one of claims 12 to 51 when dependent from claim 11. wherein an output of the turbine generator is converted to hydrogen.

55. Apparatus according to claim 11 or any one of claims 12 to 51 when dependent from claim 11, wherein at least part of the turbine generator output is used to drive the propulsion means of the vessel.

56. A method for converting thermal energy of a natural source of water into useful power, the water source comprising water at a first temperature at a first level and water at a second temperature at a second level. the method comprising the steps of at least partially filling an at least partially hollow vessel with water from said first level, propelling said vessel to said second level and using the difference between the first and second temperature of the water to drive a thermodynamic heat engine and thereby produce useful power.

57. A method according to claim 56 further comprising the step of filling the vessel with water from the second level and propelling it in the opposite direction towards the first level.

58. A method according to claim 56 or 57. wherein the second level is the surface of an ocean or the like where the second temperature is relatively warm and the first level is at a depth significantly deeper than the second level where the first temperature is relatively cool.

59. A method according to claim 58, wherein the vessel docks with the station at the surface.

60. A method according to claim 59. wherein the vessel discharges collected cool water to a cool water reservoir of the station and discharges warm water at the first level.

61. A method according to claim 59, wherein the vessel discharges warm water gradually between the first and second levels.

62. A method according to claim 60 or 61. wherein the warm water is collected from an exhaust of the thermodynamic heat engine.

63. A method according to any one of claims 56 to 60 wherein the vessel is propelled by controlling its buoyancy, the vessel having negative buoyancy when descending to the first level and positive buoyancy when ascending to the surface.

64. A method according to claim 63, wherein the buoyancy of the vessel is adjusted by creating or removing a low pressure void therein.

65. A method according to claim 64. wherein the low pressure void is created by discharging water from the vessel.

66. A method according to claim 64, wherein the water is discharged by introducing a pressurised vapour or fluid to the vessel.

67. A method according to claim 64. wherein the low pressure void is created by changing the volume of the vessel.

68. A method according to claim 63. wherein the buoyancy is controlled by applying or removing a weight on the vessel.

69. A method according to any one of claims 56 to 62, wherein the vessel is propelled by applying an impulse force.

70. A method according to claim 68. wherein the impulse force is provided by a hydraulic drive.

71. A method according to any one of claims 56 to 69. wherein the output of the thermodynamic heat engine drives a generator and the energy is stored.

72. A method according to claim 70, wherein the energy is stored electrically.

73. A method according to claim 70. wherein the energy is stored as compressed air.

74. A method according to claim 70, wherein the energy is electrolytically converted to hydrogen and stored.

75. A method according to any one of claims 70 to 73. wherein at least part of the generator output is used to propel the vessel.

76. Apparatus substantially as hereinbefore described with reference to any one of the accompanying drawings.

77. A method substantially as hereinbefore described with reference to any one of the accompanying drawings.