GB2593732A

GB2593732A - Ocean heat power plant

Info

Publication number: GB2593732A
Application number: GB2004734.6A
Authority: GB
Inventors: Wasa Tverlid Steinar
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2020-03-31
Filing date: 2020-03-31
Publication date: 2021-10-06
Anticipated expiration: 2040-03-31
Also published as: WO2021201692A1; GB202004734D0; GB2593732B

Abstract

An assembly and cyclic method for generating electricity from a temperature difference e.g. OTEC, comprising, a reaction chamber 401 for forming a hydrate from a carrier gas e.g. nitrogen, argon, ethane, methane, and a water-based fluid e.g. seawater, a first column 403 for dissociating the hydrates back into the carrier gas comprising first and second portions at upper and lower temperatures, a mechanical device 402 e.g. a one/two way displacement pump and/or a flow regulating device, connecting the first column to the chamber, a generator 405 connected to the first portion (optionally via an expansion valve), and a second column 406 connected to the outlet of the generator and the inlet of the reaction chamber. The columns may be symmetric about a longitudinal axis and comprise a 6000 series aluminium. The first column may comprise radially extending members and/or a flexible portion connecting the first and second portions. The assembly may comprise a metallic filter. The second column may be at a pressure greater than the chamber but lower than the first column. The first column may comprise first, and second longitudinal section connected to upper and lower reservoirs.

Description

Ocean Heat Power Plant

Field of the invention

The present invention relates to systems and methods for converting thermal energy from water into electrical energy. In one form, the system utilises the hydration formation and dissociation of a carrier gas in a closed loop cycle.

Backqround of invention Ocean thermal energy (OTE) conversion is a process for generating electricity from temperature differences present in tropical regions of the ocean. In these tropical, typically equatorial, locations, the power density emitted by the sun does not vary substantially over the year. As such, the surface of the water maintains a temperature of 25 to 30°C by absorption of infrared radiation emitted from the sun. This absorption means that infrared radiation from the sun fails to penetrate deeper than a few hundred metres into the water, resulting in considerably cooler deep waters. This sets up a temperature difference between the surface and the deeper waters. Typically, this temperature difference may be around 15 to 25°C, but it depends on the current patterns carrying cooler waters from different locations, as well as the latitudinal location (average power intensity from the sun). Based on this temperature difference, the theoretical Carnot efficiency is relatively low at around 7 to 8%. As this efficiency is low, it is especially important to minimise internal and external losses in the system to enable economic viability of the method.

The theory for the operation of a conventional closed loop OTE conversion is provided by the Rankine cycle. Generally speaking, the cycle comprises four stages. In the first stage, the working fluid, that is to say, the fluid that is used to generate work, is pumped from a lower to a higher pressure. Typically, a pump is used for this purpose. In the second stage, the fluid evaporates via heat transfer from the warmer surface. This phase change is accompanied by a large volume increase because the evaporated gas expands. In the third stage, the expanding gas is passed through a turbine and does work on the turbine, therein generating power. The exiting gas has a lower temperature and pressure, and the pressure difference across the turbine provides the work done. In the final step, the gas is cooled by the surrounding cold seawater, and condenses. In this step, latent heat is released, and the energy is transferred to the seawater. In order for the cycle to generate net energy, the expanding gas must provide more energy to the turbine than is required to pump the working fluid through the system. In effect, the working fluid transfers heat received from the warmer water to the cooler portion while converting a portion of the heat into mechanical work.

It has reported that in some systems the working fluid may not comprise a pure fluid. Typically, in these cases, the working fluid is an ammonia-water mixture, as is adopted in the so-called Kalina cycle. It is known that by using a mixture comprising components with differing boiling and/or condensing points, more heat can be supplied and extracted from the surrounding sea water. For example, by using a mixture, the phase transition occurs across a range of temperatures, and this can lead to a larger average temperature range across the heat cycle, thereby increasing the Carnot efficiency of the cycle. In some examples, Kalina cycles may vary the proportion of the ammonia-water mixture around the cycle to provide further optimisation. However, such a cycle requires additional equipment, for example separators, which increases the upfront cost of the build, as well as ongoing maintenance costs.

Conventional systems use water, which is not well-suited for an especially optimised method as it requires pressure reduction for the evaporation stage, and ammonia-water mixtures are more popular to increase the volatility of the working fluid. Although this mixture offers favourable efficiency, it has large environmental concerns if leaks occur. In particular, ammonia is toxic at moderate concentrations.

The present invention at least partially solves the aforementioned problems.

Summary of the invention

In general terms, the disclosure is directed to a heat power plant for generating net output power from a temperature difference using a gas-water hydrate reaction cycle.

A problem of conventional ocean heat power plants is that the Carnot efficiency is low, and therefore irreversible energy losses significantly affect the overall heat cycle efficiency.

A problem with other renewable energy sources is that they do not offer the same appealing efficiency and economic benefits as non-renewable energy sources do. In particular, its inherent intermittency, lack of predictability and lack of flexibility to match end user-demand. A problem with non-renewable sources is that they contribute to global warming, which is a threat currently faced by humanity.

The methods and assemblies herewith disclosed at least partially solve these problems.

In accordance with a first aspect of the present invention there is provided a cyclic method of generating electricity from a temperature difference, wherein the temperature difference is defined as the difference between an upper temperature within an upper portion of a first column, and a lower temperature at a reaction chamber, the cyclic method comprising: mixing a carrier gas with a water-based fluid in the reaction chamber at a first pressure to form a hydrate; moving said hydrate to a lower portion of the first column, wherein the first column is at a second pressure, wherein the second pressure is higher than the first pressure, and wherein the first column is at least partially filled with water; dissociating the hydrates back into the carrier gas at an upper portion of the first column and providing the carrier gas from the end of the first column at the upper temperature to a second column through a generator, wherein said carrier gas is formed by hydrate dissociation, wherein said generator is arranged to generate electrical power, and transporting the carrier gas through the second column back to the reaction chamber, wherein the second column is at a third pressure.

Providing said hydrate to the lower portion of the first column further may comprise: using a two-way displacement pump disposed between the reaction chamber and the first column to move a volume of hydrates into the first column, and an equal volume of water into the reaction chamber; and using a one-way pump disposed between the reaction chamber and the first column to move a further volume of hydrates from the reaction chamber into the first column.

The volume provided by the one-way pump per unit time may be greater than the volume provided by the two-way displacement pump per unit time multiplied by the volume fraction of the carrier gas in the hydrate Providing said hydrate to the lower portion of the first column further may comprise: using a one-way displacement pump disposed between the reaction chamber and the first column to move a volume of hydrates into the first column; and using a flow-regulating device to move water from the first column into the reaction chamber to maintain the volume of water in the reaction chamber.

The second pressure may be larger than the third pressure, and the third pressure may be larger than the first pressure.

The carrier gas may comprise substantially pure nitrogen or argon. Alternatively, the carrier gas may comprise an argon-nitrogen mixture or an ethane-methane mixture.

The water-based fluid in the reaction chamber may comprise seawater, and the seawater may be provided from an external source radially proximal to the reaction chamber.

The first pressure may be greater than the pressure of the external source radially proximal to the reaction chamber, which defines pressure difference, and the seawater may be provided into the reaction chamber with a pump, and the unreacted seawater may be dumped out from the reaction chamber via the pressure difference.

The method may further comprise: collecting water in the carrier gas downstream of the generator with a separator, and the method may comprise generating desalinated water.

The generator may comprise a plurality of turboexpanders arranged in series. Each adjacent turboexpander in the series may be coupled with a connecting channel, and the carrier gas may be passively heated in the connecting channel before passing through the next turboexpander in the series.

A closed expansion valve may be disposed between the generator and the first end of the first column, and the method may further comprise: opening the expansion valve to provide the carrier gas to the generator; and the opening of the expansion valve may occur when a predetermined criteria is met.

The predetermined criteria may be whether the carrier gas pressure is larger than a predetermined pressure.

The predetermined criteria may be given by a predetermined time of day, and said time of day may correspond to end user demand.

The first pressure may be around 30 bar, the second pressure may be around 1000 bar and the third pressure may be around 40 bar.

The upper temperature may be between 283 to 300 Kelvin and the lower temperature may be between 273 to 283 Kelvin.

In accordance with a second aspect of the present invention there is provided an assembly for generating electricity from a heat cycle, wherein the assembly comprises: a reaction chamber, wherein the reaction chamber is configured to contain a first pressure, a water-based fluid and a hydration reaction between a carrier gas and the water-based fluid; a first column, wherein the first column is configured to contain a second pressure and water, and the first column comprises a first portion at an upper temperature, and a second portion at a lower temperature; a mechanical device connecting the reaction chamber to the first column, wherein the mechanical device is configured to move hydrates from the reaction chamber to the second portion of the first column; a generator configured to generate power and connected to the first portion of the first column; a second column, connected an outlet of the generator to an inlet of the reaction chamber, the second column configured to contain a third pressure and move the carrier gas into the reaction chamber.

The assembly may further comprise: a two-way displacement pump configured to move a volume of hydrates from the reaction chamber into the first column and an equal volume of water into the reaction chamber; and a one-way pump configured to move a further volume of hydrates from the reaction chamber to the first column.

The assembly may further comprise: a one-way displacement pump configured to move a volume of hydrates from the reaction chamber into the first column; and a flow-regulating device configured to move water from the first column into the reaction chamber.

The water-based fluid may comprise seawater, and the reaction chamber may comprise a pump configured to move seawater from an external source radially proximal to the reaction chamber into the reaction chamber.

The assembly may further comprise: a separator, disposed downstream of the generator, configured to collect water in the carrier gas to generate desalinated water.

The first and second column may be axially symmetric about a longitudinal axis of said columns.

The first column may comprise one or more radially extending members along at least a portion of the longitudinal length of said column.

The first column, second column and the reaction chamber may comprise a 5000 or 6000 series aluminium alloy.

The assembly may further comprise a metallic filter, wherein the metallic filter is configured to mix the carrier gas with the water-based fluid.

The first column may comprise a flexible tubing connecting the first portion of the first column with the second portion of the first column.

The first column may comprise: a first longitudinal section arranged to transport water only; a second longitudinal section arranged to transport hydrates only; an upper reservoir disposed at the upper temperature; a lower reservoir disposed at the lower temperature; wherein each longitudinal section is connected to each reservoir; wherein the radial extent of the first and second longitudinal sections is smaller than the radial extent of the upper and lower reservoir.

Brief description of the drawings

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure la shows an exemplary pressure-volume diagram and Figure lb shows an exemplary temperature-entropy diagram for a heat cycle.

Figure 2a shows an exemplary pressure-temperature hydrate phase diagram for some common gases and Figure 2b shows an exemplary energy-pressure phase diagram.

Figure 3 shows a schematic of an exemplary ocean heat power plant.

Figure 4 shows a schematic of an exemplary closed-loop ocean heat power plant.

Figure 5 shows a schematic of an exemplary open-loop ocean heat power plant. Figure 6 shows an exemplary two-way displacement pump.

Figure 7a and 7b show an exemplary closed-loop ocean heat power plant.

Figure 8 shows an exemplary series array of turbochargers.

Figure 9 shows an exemplary ocean heat power plant farm.

Figure 10a-c show exemplary first and second columns of the ocean heat power plant, and Figure 11 is a flow diagram.

Figure 12 shows a schematic of an exemplary ocean heat power plant.

Figure 13 shows a schematic of an exemplary closed-loop ocean heat power plant.

Detailed description

Global warming is a large threat facing humanity. Renewable energy sources provide a plausible route for reducing greenhouse gas emissions, but often do not offer the same appealing efficiency and economic benefits as non-renewable energy sources do. As such, it is imperative to generate new methods and systems, which are capable of generating clean electricity in an appropriate economic manner. In particular, a fundamental problem with the renewable energy industry is its inherent intermittency and lack of predictability. Energy storage systems which are capable of delivering energy to end-consumers when they need it most is therefore of genuine value.

In Ocean Thermal Energy (0Th) conversion systems, the Carnot efficiency defined by the temperature difference from the surface of the sea and deeper area of the sea is inherently low. Therefore, to produce an economically viable OTE heat engine the inventors have realised that it may be beneficial to achieve one or more of: i) increase the flowrate within the cycle; ii) increase the net energy released per cycle; iii) reduce thermodynamic irreversibility in the cycle; and iv) encourage thermodynamic equilibrium to be reached. At the same time, it is important to consider the longevity and environmental impact of the system. The terms 'sea' or 'ocean' are used interchangeably, and the concepts discussed herein may be used in any larger body of water in which a temperature difference occurs. Another example is a lake.

(i) Increasing the flowrate of the working fluid in the cycle One proposed route to increase the net output power of a heat system is increasing the flowrate of the working fluid in the cycle. That is to say, in any given complete cycle a certain amount of energy is generated by the system, and by increasing the frequency of the cycle, the total generated energy per unit time (power) is increased. However, such a system requires more expensive equipment because it requires higher power pumps which reduce the Carnot efficiency, and only provide diminishing returns. The main concern with increasing the frequency of the cycle is that the temperature differences between the system and surrounding water are small, and so the heat flux is also small. In other words, the heat flux limits the maximum practical flow rate. Other problems may include the operation of alternative corrosion mechanisms, such as erosion corrosion a higher flowrates. For these reasons, the main aspect of this invention considers other means to increase the power output of the OTE conversion system to solve the problem of an economically viable system.

00 Increasing the net work done per cycle Figure la shows a pressure-volume diagram for a typical Rankine cycle. Figure lb shows a temperature-entropy diagram for a Rankine cycle. In a heat cycle, the area enclosed in the P-V diagram of Fig. la defines the net work done by the system. That is to say, it defines the power output in the cycle. The exact shape of the enclosed area depends on the system as would be appreciated by the skilled person and the figure is in no way limiting. In Figure lb, if the process is reversible, then the area enclosed in the temperature-entropy diagram is equal to the net heat transferred into the system. Figure la teaches that a larger change in volume increases the net work done by the system.

Figure lb teaches that a larger entropy change in the phase transition, in the Rankine cycle, increases the amount of heat that can be transferred to the system (and that therefore that may be converted to work later).

Equation 1: dU = dQ -dW = dQ -pdV; Equation 1 states the first law of thermodynamics, wherein U is the internal energy of the system; Q is the heat flow into the system; and W is the work done by the system. In the gas phase, the work done is given by the pressure (p) multiplied by the change in volume (V). As such, to increase the net work done by the system per cycle, one can increase the pressure, or increase the change of volume.

(iii) Reducing thermodynamic irreversibility in the cycle Equation 2: dU = dQ -dW -actuat -TdSsys -dWrev Equation 2 states the relationship between the entropy of the system and the internal energy of the system, wherein S is the entropy of the system, and T is the temperature. Combining this with the general teaching of the second law of thermodynamics in that the change of entropy of the universe can never be negative leads to the result that, if total work done by the system is not reversible, heat is lost to the surroundings (to enable the entropy of the universe to increase), thereby reducing the amount of work that can be done.

(iv) Encouraging thermodynamic equilibrium Although thermodynamics provides the theoretical efficiency of the heat cycle, it is important to highlight that the Carnot efficiency is only true at thermal equilibrium, which practically speaking is not possible. In short, during operation of the cycle, the temperature of the working fluid in an upper temperature region and a lower temperature region of the system is not the same as the temperature of the body of fluid that is heating or cooling it. If a condensation-evaporation cycle is used, it is preferable to maximise the efficiency of the system by selecting a working fluid that exhibits a phase change as close to these temperatures as possible at the given design pressure. In practice, a pure fluid, such as water, may not effectively maximise this thermodynamic efficiency and therefore mixtures of fluids may be a route to optimise the process somewhat. Furthermore, during operation the working fluid is in motion and as previously mentioned the heat flux between the surroundings and the system effectively limits the maximum flow rate. For example, if the heat flux is low, and the flow rate is too large, then the temperature difference across the heat cycle is reduced, therein reducing the Carnot efficiency. It is therefore advantageous to configure the system to maximise the heat flux into the system. The improvement is two-fold, it increases the overall temperature difference, and increases the maximum achievable flowrate in the system i.e. the net power can be increased (more complete cycles per unit time).

Fig. 2a is a pressure-temperature phase diagram showing phase transitions, which occur in a water and gas mixture. The phase stability of a number of common gases are shown. The area of the diagram defined above the phase line (i.e, higher pressure and lower temperature) corresponds to stable hydrates for a particular type of gas, while the area below the phase line (i.e., lower pressure and higher temperature) corresponds to separate gas and water phases.

Based on typical temperature conditions in, for example, tropical oceans, the upper temperature of the water is likely to be around 30 degrees Celsius, and the lower temperature is likely to be around 5 degrees Celsius. As such, for tropical oceans, the ideal phase stability line for the working fluid of the heat cycle comprises a vertical line at 30 degrees Celsius extending to zero pressure, as labelled in the Figure. Such a curve: i) enables a larger pressure difference across a turbine such as a turboexpander, increasing the total power output; ii) reduces the temperature difference between the upper temperature of the body of water and the temperature of the hydrate melting (which increases the Carnot efficiency); hi) reducing the temperature difference between the lower temperature of the body of water and the temperature of the hydrate formation (which increases the Carnot efficiency).

From the pure gases phase lines, it is apparent that no single pure gas is especially well suited for this purpose. However, the inventors have discovered that by mixing gases, the phase stability curve is modified to more closely match the idealised curve.

For the consideration of the choice of gas, it is preferable to consider other factors than thermodynamic efficiency of the system. For example, it is preferred that the gas is cheap, so that an economically viable device can be made. The use of nitrogen is especially suited as it is both readily abundant and cheap. The proposed window of operation is shown for Nitrogen in Figure 2a. Argon is also well suited and the operation window is also shown in Figure 2a. The mixture of argon and nitrogen may be a particularly favourable mixture at the appropriate proportions. Another consideration is the density of the hydrate, in a preferred example, the density of the hydrate is less than water so that the hydrates are able to float water.

Fig. 2b shows schematically the four transitions, which occur during a reversible process of hydrate formation and decomposition/dissociation employed in the embodiments of the invention described below. A first horizontal axis of the diagram represents temperature. The vertical axis represents the energy which is contained in a mixture of the water and hydrate-forming gas during the process. The second horizontal axis illustrates schematically the state of the mixture, i.e. the phase change between solid state (i.e. hydrates have been formed) and melted state (i.e. the hydrates have decomposed).

Consider for example, the state marked A as a starting state. In this state, the water and hydrate-forming gas are present together in a chamber (typically with the gas in a layer above the water), and the temperature and pressure of the system are slightly below a phase transition line. Accordingly the state A is unstable, and a transition occurs (a process marked as 1), in which hydrate crystals are formed. Significant energy is expelled in this process, as latent heat of fusion, and this energy must be removed from the system for process 1 to be completed. It is emphasised here that the latent heat of fusion is very large, and can cause local heating. With reference to the phase diagram of Figure 2a, it is important to prevent this, as it may lead to hydrate dissociation. The embodiments of the invention at least partially solve this problem. The resulting state is shown as state B, where the hydrate formation process is complete. In process 2, the hydrate crystals are very slightly heated to a temperature above the phase transition temperature (a process marked as 2), where again the hydrates (now in state C) are unstable. In process 3, the hydrate crystals melt, regenerating the hydrate-forming gas and separately the water. Significant energy must be input to the system during process 3, resulting in decomposed gas and water (state D). Finally, slight cooling of the system returns the mixture to state A (process 4).

Note that the small amounts of energy respectively absorbed and released in processes 2 and 4 cancel each other, as do the much larger amounts of energy respectively released and absorbed in processes 1 and 3. In the process, the total energy of the gas -hydrate mixture is unchanged. Energy is conserved under ideal conditions. However, in the present invention, the surroundings are used to provide thermal energy for processes 1-4 and therefore the energetic cost of melting is not required. As such, the energetic states as shown may be used to generate energy. For example, by allowing the gas formed on dissociation to carry out work. If only the system is considered, then energy is not conserved, as thermal energy is exchanged from the surroundings.

However, the surroundings also lose thermal energy (the temperature difference between the upper temperature and lower temperature of the body of water is reduced) and therefore the total energy is conserved. It is emphasised that the energy stored in a temperature difference of water is very large due to water's considerable heat capacity. For overall net power to be extracted from the system, it is required that the energy needed to move the gas and hydrate mixture across the area traversing the temperature difference is less than the energy extracted as work. The present invention at least partially optimises this process.

As mentioned previously, the total thermal energy that can be stored in the system as a hydrate is determined by the T-S diagram and indicates that a larger change in entropy increases the total thermal energy that can be stored in the system. As such, the use of the solid-gas hydration reaction is particularly effective at increasing the net power output of the heat cycle.

Figure 3 shows an exemplary ocean heat power plant. The heat plant comprises a first column 301 and a second column 302. The first and second columns traverse across a region with a temperature difference. In this example, the temperature difference is defined between the temperature at or near the surface of a water body and the temperature at or near the bottom of the water body. As would be appreciated by the skilled person, the water body may correspond to any water body, be that the sea, a lake or anything equivalent.

The first 301 and second column 302 define an end distal and an opposite end proximal to the surface of the water body. The end proximal to the surface of the water body defining a top-side, and the other end defining a bottom-side. Subsequent references to top and bottom refer to this meaning, unless explicitly stated otherwise. In the example of Figure 3, the top side of the first column is coupled to the top side of the second column via a connecting channel. A generator 305 is disposed in the connecting channel connecting the first 301 and second column 302 together. The bottom side of the second channel is connected to a bottom side of the crystallisation chamber 303 via a further connecting channel. The top side of the crystallisation chamber 303 is connected to the bottom side of the first column via one or more mechanical devices. As such, Figure 3 illustrates a closed cycle.

Figure 4 shows schematically a closed cycle heat power plant as shown in Figure 3. The cycle is defined by the flow of a working fluid, as depicted by the arrows. The process is cyclic and, at steady state, there is no first or last step, as such. For simplicity, the cycle will be described from the bottom-up. In a first step, the working fluid is passed from the bottom of the second column into the bottom of the crystallisation chamber 401 via a pressure difference. The pressure difference being defined between the higher pressure second column 406, 407 regions and the crystallisation chamber 401. In an example, the working fluid comprises a so-called carrier gas. In some examples, the carrier gas may be essentially pure, but, in other cases, the carrier gas may be a mixture of gases of predetermined proportions. As mentioned previously, the carrier gas is preferably chosen to optimise the efficiency of the cycle by generating the idealised curve shown in Figure 2a.

In the example, the crystallisation chamber 401 is filled with water. When the carrier gas is provided into the crystallisation chamber, the carrier gas is dispersed via a filter disposed in the connecting channel 407 at the entrance into the bottom of the crystallisation chamber. This filter increases the effective surface area of the carrier gas-water interface by decreasing the average size of carrier gas bubble, thereby increasing the rate of hydration reaction between the carrier gas and the water. In an example, this filter may comprise a partially sintered metallic component, i.e. the filter comprises a plurality of interconnected channel of pores on the size scale of the metallic powder. As the filter increases the number of bubbles, it also controls the absolute size of the generated hydrate crystals, i.e. by increasing the number of nucleation sites for hydrate crystal growth. In effect, the size of the bubbles generated by bubbling the carrier gas through the filter sets the size of the hydrate crystal. The crystallisation chamber 401 may also comprise anti-agglomerates. These anti-agglomerates prevent agglomeration of the generated hydrates. In this way, the size of the hydrate crystals remain small (the same order of magnitude as the carrier gas bubbles) and a workable hydrate slurry is formed. The exact composition of these anti-agglomerates is dependent on the composition of the carrier gas. In some examples, the anti-agglomerate may comprise a surfactant. The surfactant may be dissolved into the water in the crystallisation chamber 401 at a suitable concentration to ensure the stability of the hydrate crystals. As given by Figure 2a, the hydration process can only occur below a particular temperature for a given pressure.

In the chemical hydration process, the carrier gas bubbles react with water to form a solid hydrate. This reduces the volume of the gas significantly, as the carrier gas is stored within a solid. However, it is preferable if the density of the hydrate is less than that of the water inside the crystallisation chamber 401. In this case, the hydrate floats inside the crystallisation chamber 401 to the top side of the crystallisation chamber. It is emphasised that in this reaction, the overall volume change is large and negative, but this volume reduction is only associated with the carrier gas, which is being input into the crystallisation chamber and transformed into the hydrate. The hydrate itself actually has a larger volume than the water that it is comprised from since the hydrate is configured to be less dense than water, i.e. float in water. As such, the volume change inside the crystallisation chamber 401 is actually positive. The inventors anticipate this problem and provide an efficient method to accommodate for this volume increase.

The use of gas hydrates, which exhibit a lower density compared to water, is particularly advantageous to the efficiency of the ocean heat power plant. In particular, it means that the operation of ocean heat power plants is no longer restricted to tropical areas.

Therefore, the ocean heat power plant is also envisaged to operate with net positive energy output in cooler water regions. In the following explanations, temperature and pressure conditions are suited to tropical waters. However, as would be appreciated by the skilled person, the temperature and pressure conditions can be determined with reference to Figure 2a and suited to the particular water temperature conditions. For these cooler waters, the ideal phase boundary between dissociation and hydrate formation may comprise a vertical line, extending to zero bar, at, for example, 16 degrees Celsius In the next step, the hydrates are provided into the first column 403 via a mechanical device 402. The mechanical device 402 preferably comprises any form of two-way positive displacement rotary pump in combination with an additional one-way pump. In an example the additional pump may comprise a centrifugal pump, but may also comprise a one-way displacement pump. The main difference between a two-way positive displacement pump and the additional pump is that fluid only passes in a single direction for a one-way pump but in both directions for a two-way positive displacement pump. A further difference between displacement pump and a centrifugal pump, which may optionally be used as the additional one-way pump, is that fluid passes via a pressure difference in a centrifugal pump, but no such pressure difference is required in a two-way positive displacement pump.

The two-way positive displacement pump may be a peristaltic, lobe, progressive cavity type pump or a form of sluice.

The positive displacement pump of the mechanical device 402 displaces water from inside the first column 403 to the crystallisation chamber 401 and at the same time displaces the same volume of hydrates from the crystallisation chamber 401 to the first column 403. The pressure of the water inside the first column 403 is much larger than the pressure in the crystallisation chamber, and therefore, this process occurs without much external power input. In some examples, the positive displacement pump may comprise a mechanical break to limit the rate at which the pump operates. Since the pump operates via displacement, the pressure difference across the pump can be maintained.

This displacement process also ensures that the water used up in the hydration reaction is replenished. For certain carrier gas mixes, the gas to water ratio in the generated hydrates may be around 1:4. Therefore, during the displacement process, more water passes from the first column 403 into the crystallisation chamber 401 than water passes in the opposite direction. For example, if the volume of displaced per cycle of the displacement pump is V', then the volume of water passing into the crystallisation chamber 401 is 'V' but the volume of water passing into the first column 403 is only 0.8V. This is problematic as it depletes water from the first column 403.

Another challenge is that in the hydration reaction over time the combined volume level of water and hydrates increases, since gas is pumped into the crystallisation chamber 401 externally, and the density of hydrates is less than the density of water. If the cycle continues indefinitely, then the level of hydrate and water in the crystallisation chamber 401 continues to rise until the chamber 401 is completely filled. Further hydration formation therefore leads to an increase in pressure inside the crystallisation chamber 401. A sufficiently large increase in pressure may prevent gas from passing to the crystallisation chamber 401 from the second column 406, 407. This means that a further pump would be required upstream of the crystallisation chamber 401 to provide the carrier gas into the crystallisation chamber 401. As mentioned, pumps are a source of irreversibility and therefore it is preferable to avoid the use of additional pumps. By way of example, the minimum pressure of the crystallisation may be greater than around 150 bar at 5 degrees Celsius. However, this minimum pressure is defined by the physical properties of the given carrier gas/mixture as outlined in Figure 2a.

For these reasons, it is preferable to use a two-way positive displacement pump in combination with an additional one-way pump. In an example, the one-way pump may be a centrifugal pump such as a rotary impellor system, and is configured to pump hydrates and water from the crystallisation chamber 401 to the first column 403. The general working principle of a centrifugal pump is to generate a pressure difference by some form of cyclic motion. This pumping requires significant energy as the first column 403 is at a much larger pressure than the crystallisation chamber 401. In some examples, the first column may be 1000 bar or more. Therefore, to reduce the total energy consumed in pumping, it is preferable to only pump the minimum volume of hydrates and/or water into the first column required to match the increase in volume associated with hydration formation. In other examples, the one-way pump may comprise a one-way displacement pump. For example, the pump may be a screw pump. As mentioned, the large pressure difference between the first column 403 and the crystallisation chamber 401 means that considerable power is required in this process. As such, it is preferable to only displace the minimum volume required.

In practice, the depletion of water from the first column 403 may be more likely to set the required rate of the additional one-way pump. For example, if the fraction of carrier gas to water in the hydrates is 1:4, then if the volume of water displaced per cycle in the displacement pump is 'V' then 0.2V of water is lost from the first column 403 per cycle. To avoid water depletion in the first column, it is therefore necessary to pump more water into the first column 403. In one example, it is preferable for cycle efficiency to only pump hydrates. In this example, the rate of pumping hydrates is greater than 20% of the rate of pumping in the displacement pump, thereby maintaining the water level in the first column. In general, the rate of pumping in the one-way pump is greater than the volume fraction of the carrier gas multiplied by the rate of pumping in the displacement pump.

In other examples, the mechanical device 402, 502 may comprise a one-way displacement pump, such as a screw pump. In this case hydrates are passed one-way into the first column against a pressure gradient. In these examples, the volume increase of the water and hydrate level is advantageous, as it enables hydrates to be forced into the one-way displacement pump. It is emphasised that when the mechanical device comprises a two-way displacement pump, the hydrates are provided into the two-way displacement pump by gravity, i.e. they float into the water contained in the volume. The large pressure difference between the first column 403 and the crystallisation chamber 401 means that an external power source is required to drive the one-way pump. If the mechanical device comprises only a one-way displacement pump then the level of water and hydrates in the first column 403 rises, but water is depleted from the crystallisation chamber 401. To solve this problem, it is envisaged that the mechanical device 402 may comprise a further device configured to control the flow of higher pressure water from the first column 403 to the crystallisation chamber 401 (flow regulating device). This component would preferably be essentially passive, and may also comprise a mechanical break. In a preferred example, the mechanical break is regenerative. By way of example, the component may be a choke, or any other device capable of controlling fluid flow from an area of high pressure to an area of lower pressure as would be appreciated by the skilled person. The flow regulating device is configured to provide water into the crystallisation chamber 401 to ensure that water is not depleted. That is to say, water is at least provided into the crystallisation chamber 401 at substantially the rate in which it is being used up in the hydration reaction. Or, water is provided into the crystallisation chamber 401 to maintain the water and hydrate level.

In the next step, the hydrates pass from the bottom of the first column to the top of the first column. As previously mentioned, in these examples, the hydrates are less dense than water, and therefore rise inside the first column by gravity. With reference to Figure 2, the top side of the first column is exposed to warmer conditions, and therefore as the hydrates rise they increase in temperature. The hydrates may increase in temperature via heat transfer from the surrounding warmer body of water to the first column by any mixture of convection, conduction and radiation. Above a certain temperature and the pressure of the first column, the hydrates spontaneously dissociate. The threshold temperature defines an imaginary line intersecting the first column along the isothermal line of this temperature. It is emphasised that this is only strictly true at equilibrium, and the line may be shifted in position based on the dynamic conditions of the cycle. Hydrates above this line dissociate, hydrates below this line do not. The level of water inside the first column 403 is preferably above this imaginary isothermal line so that when the hydrates float to the surface of the water in the first column 403, they are able to undergo the dissociation process.

In the hydrate dissociation process, there is a considerable increase in volume of gas as the phase change is from solid to a gas. In an example, the connecting channel between the top of the first column and the generator/turboexpander 405 contains a valve. This valve is initially closed. The closed valve therefore defines a fixed volume of space at the top of the first column. As more gas is released in the dissociation process, this space is filled up with gas. Eventually, the volume is completely filled and then the pressure inside the top region of the first column 403 increases. Conventionally in heat cycle processes, compression is seen an energy cost step (work must be done to compress the gas), and therefore not advantageous. However, in this case, the stored chemical energy in the gas hydrate is used to generate the compression. Therefore, no mechanical energy needs to be input for this compression to occur. However, it is still true that compressive heating may lead to thermal energy being lost from the gas, and in this sense, compressive heating needs is still a disadvantage.

Therefore, in another example, no valve is disposed between the top of the first column and the generator/turboexpander 405. In this way, the released gas from hydrate dissociation is allowed to expand and do transfer internal energy as work to the generator. When a valve is used, the first column acts effectively as an energy storage unit. This may be advantageous as energy storage is a fundamental problem that clean renewable energy sources need to address. Having a method for generating power, which may also be used as an energy storage unit is therefore of genuine value.

By way of example, solar energy only provides energy during the daylight. In order to provide energy to a consumer during non-daylight periods, therefore requires storage.

Similarly, wind power only provides energy when there is wind. The periodicity of wind does not necessarily match with the consumer need for energy. Conversely, the temperature difference across the ocean in tropical waters is approximately constant over the course of a year (by virtue of the equatorial climate) and over the course of a day (the heat capacity of water is large). Therefore, the untapped energy supply derived from the temperature difference in the ocean can be released at any given time.

When a valve is used, the pressurised carrier gas may be allowed to pass through the connecting channel once a predetermined pressure is reached. Or, alternatively, when a predetermined time of day is reached, such as a time of day corresponding to peak power consumption, or, a time of day where the energy supply falls below that of the energy demand. That is to say, the time of day corresponds to an end user requirement for the power. For example, this be during hours of the day where there is no daylight and other renewable sources such a wind are not operating reliably. Once the valve is opened, the pressurised gas is allowed to expand and pass through the connecting channel from the top of the connecting channel from the top of the first column through to the second column 406 via the generator/turboexpander 405 down the pressure gradient. In this expansion, there is a large expansion in the gas, and the expanding gas is able to do work to turn a turbine and generate electricity. The work done is given by the equation 1. Both the volume increase and pressure are very large, and so the work done is correspondingly large. As a first approximation, it can be assumed that the process is adiabatic, that is the expansion is sufficiently rapid to neglect heat transfer in or out of the gas. Therefore, referring to Equation 1, as the work is done, the internal energy of the gas drops and this corresponds with a significant temperature drop. This drop in temperature may lead to hydration formation at certain pressure conditions. In general, there is a critical pressure at which hydration formation will occur, but as Figure 2a illustrates, depending on the properties of the carrier gas. At the same time, it is preferable if the pressure in the second column 406 is greater than the crystallisation chamber 401, so that the carrier gas flows from the second column to the crystallisation chamber without a pump, which is a source of irreversibility. It is therefore envisaged that the pressure inside the second column is monitored, and if the level is below the pressure inside the crystallisation chamber, then the expanding carrier gas is bypassed around the turboexpander through another connecting channel to the second column 406 to increase the pressure. At the same time, the temperature is monitored in the connecting channel immediately downstream of the turboexpander to ensure that hydration formation is stable. The carrier gas is then transported down to the bottom side of the second column by the flow of the carrier gas. It is envisaged that initially, the second column 406 may be installed essentially containing a vacuum or a temporary buffer gas, and then, after installation, the carrier gas is provided into the column until the second column operating pressure is reached. The similar process may be applied to the first column.

In an example, the pressure in the crystallisation chamber is around 30bar, the pressure in the first column 403 is around 1000 bar, and the pressure in the second column is 40 bar.

Although it is envisaged that a favourable carrier gas is one capable of forming a hydrate which is less dense than water. The inventors anticipate that it is equally possible to engineer the fluid inside the crystallisation chamber 401 and first column 403 to be denser than the formed gas hydrate. That is to say, it is not essential that the gas hydrate is less dense than water, as the water may equally be replaced by fluid which is denser than the gas hydrate. In other examples, the water may be pressurised sufficiently to increase the density of the water to levels above the density of the hydrate. Although water is, to a first approximation, incompressible, the hydrates being solids are less compressible, and therefore provided that the difference in density between the hydrate and water is small, pressurisation of the water (and hydrate) is a plausible route to ensure that the hydrates float within the water at that pressure.

Figure 5 shows another example of an open-cycle ocean heat power plant. The operation of this example is similar to that shown in Figure 4, but the main difference is that sea water is input into the system at the bottom of the crystallisation chamber 501. In the hydration reaction, the sea water reacts with the gas to form a gas hydrate. It is emphasised that only water reacts with the gas, and therefore, the salt content of any excess sea water is raised. The enriched salt water is then released back into the ocean after the reaction. It is preferable if the operating pressure of the crystallisation chamber 501 is less than the pressure inside the second column 506, 507, but also less than the hydrostatic pressure of the water outside the system and surrounding the crystallisation chamber 501. In this case, a pump is not needed to provide sea water into the crystallisation chamber 501. The hydrates that form are preferably configured to be less dense than the sea water. It is noted that, compared to pure water, this constraint is easier to achieve as the density of sea water is larger than that of water by virtue of the salt content. The salt rich sea water may then be pumped back into the ocean. In another example, it may be advantageous to run the pressure of the crystallisation chamber 501 at the hydrostatic pressure of the surrounding water around the crystallisation chamber 501. In this case, pumps are required to provide seawater both in and out of the chamber 501, but the pressure difference is greatly reduced.

The hydrates then pass into the first column via the mechanical device 502 in the same way as described with reference to Figure 4. In this example, the first column is filled with water not seawater. As previously described, the mechanical device may comprise a two-way positive displacement pump in combination with a further one-way pump. Or, alternatively, the mechanical device 502 may comprise a one-way displacement pump in combination with a flow-control device, such as a choke.

The other main difference between the example of Figure 4 is in the work extraction part of the cycle 505. In this example, when the expanding gas is allowed to expand through a connecting channel, any water dispersed in the expanding gas is also passed through the turboexpander 505. Immediately downstream of the turboexpander 505 is a separating device to separate condensed water. This separating device may comprise a container which is able to collect the condensed water produced during the sudden temperature drop. The collected water may then be collected and transported away. In this way, the cycle also generates clean, de-salinated water from sea water. Again, this is advantageous, as conventional de-salination systems require large amounts of energy to operate. In effect, de-salination occurs Tree-of-charge' in this system.

A valve may also be disposed between the top of the first column 504 and the connecting channel to the turboexpander 505. In the same way as previously described, this can be used as an energy storage unit, but also as a water-generation storage unit. For example, as the temperature of the carrier gas rises, dispersed water may vaporise. Vaporised water vapour means that the amount of water that can be collected can be increased, as water vapour is much more compressible than liquid water.

Referring back to Figure 4, in the closed loop cycle, it is preferable if the quantity of water passing into and through the turboexpander is small. As the loop is closed, water that passes into the turboexpander is not easily recovered, and this accounts towards water depletion in the first column. Therefore, it may be preferable to remove water before the gas passes through the turboexpander. In an example, a water absorbing chemical may be disposed in the connecting channel capable of leaching the water from the gas. It is envisaged that such a mechanism would be better suited to the continuous mode of operation without the use of a valve. In another example, a water separator may also be disposed upstream from the turboexpander. In another example, the separator may be disposed downstream from the turboexpander, and the loss of water may be replenished periodically by external means. In some examples, water may be provided into the second column 506. The water would then collect at the bottom of the second column 506. In these examples, the gas outlet 507 connecting the second column and the crystallisation chamber 501 is disposed above the bottom of the second column. The collected water may then, periodically, be provided back into the crystallisation chamber such that the collected water level in the second column 506 remains below the level of the gas outlet 507. In some examples, overpressure in the second column 506 may be used to pump the collected water back into the crystallisation chamber 501. In other examples, a pump may be disposed at the bottom of the second column 506 to pump the water into the crystallisation chamber intermittently.

In an example, the pressure in the crystallisation chamber 501 is around 30 bar, the pressure in the first column 503 is around 1000 bar, and the pressure in the second column is 40 bar.

It is appreciated that although in some examples the hydrates are less dense than water (or less dense than the fluid in the first column 403, 503), in some examples, the hydrates may be denser. In these examples, a further pump may be used to provide the hydrates from the bottom of the first column 503 to the top of the first column 503 and as a result of this pumping, it is necessary for there to be a pressure difference across the longitudinal length of the first column 403, 503. As previously mentioned, during the hydration formation, there is a significant volume change during hydration formation as gas is stored in a solid. However, in these examples, the hydrates that form are denser than water, but the volume change in the crystallisation chamber is still positive as gas is supplied into the crystallisation chamber 401, 501. Since the hydrates are denser than water, the mechanical device 402, 502 will be largely the same as previously described, except that additional pumps (in the crystallisation chamber 401, 501 and in the first column 403, 503) are required to feed the hydrates into and out from the mechanical device.

Figure 6 shows a schematic of an exemplary two-way positive displacement pump as shown in less detail in Figures 4 and 5. The pump 600 comprises a rotating element 601 housed inside an external casing. The rotating element 601 defines two volumes 602, 603 and is flush against the external casing of the housing to form a liquid-tight seal. It is appreciated that two volumes is given purely for illustrative purposes and, in general, the rotating element may define a plurality of volumes. In some examples, the two-way displacement pump may comprise a gear type wheel, defining greater than two volumes, to serve the same purpose. As previously described, the two-way displacement pump is disposed between the crystallisation chamber 301, 401 and the first column 403, 503.

The first column 403, 503 is at a higher pressure than the crystallisation chamber 401, 501 and so there is a pressure difference across the two-way displacement pump.

During a rotation cycle of the rotating element 601, the volumes 602, 603 empty and fill at each of the first column 403, 503 and the recrystallization chamber 401, 501. That is to say, the volumes 602, 603 fill and empty at the same time. For example, at the top of the pump 600, water from the first column 403, 503 passes into the volume 602, 603, but at the same time, gas hydrates pass from the volume 602, 603 to the first column. The gas hydrates pass into the first column 403, 503 as they are less dense than water and therefore float via gravity. The rate in which they rise is determined to a large extent by the difference in density of water and the gas hydrate. It is envisaged that this floating of gas hydrates may limit the rate at which the pump 600 can operate. As such, a larger density difference between the gas hydrate and water is preferable. At the same time as when the gas hydrates pass into the first column, water from the first column 403, 503 passes from the first column into the volume 602, 603 by gravity.

It is noted that the incompressibility of water means that although the pressure of the water inside the first column 403, 503 is much greater than that of water in the crystallisation chamber 401, 501, the densities are largely the same. The inventors appreciate this fact, and use a displacement pump 600 for this reason.

Since there is a pressure difference across the pump 600, it is envisaged that the pump may only require power to start the rotation of the rotating element 601. For example, during operation, the inertia of the pump may be sufficient to pass the vertical arrangement (as depicted in Figure 6) such that the weight/pressure of the water can continue to drive the rotating element around over half a cycle. In some examples, a mechanical break may be required to limit the rotation of the pump to ensure that all hydrates are able to pass out from the volumes 602, 603 into the first column 403, 503 and out of the crystallisation chamber 401, 501. In some examples, a power supply may be provided for the pump 600. If a mechanical break is used, it is preferable that the mechanical break is a regenerative one. That is the break is able to recover some energy from the breaking mechanism.

Figures 7a and 7b illustrates an exemplary ocean heat power plant. In both Figures, a third column is introduced into the system. Figure 7a shows the initial conditions, and Figure 7b shows the system after a certain time has elapsed. In Figure 7a, the water level 703 in the first column 705 and the third volume 708 is the same (as marked by the dotted line). The first column 705 is connected to the crystallisation chamber of the third column 710, and the third column 708 is connected to the crystallisation chamber of the first column 711 via a corresponding connecting pipe 704. The first column 705 and third column 708 are also connected to a second column 709 via a further corresponding connecting pipe, which provides gas from the bottom of the second column 709 to the bottom of the first 705 or third column 708 via the respective crystallisation chamber 710, 711. Initially, the carrier gas is pumped into crystallisation chamber of the first column 711. As mentioned previously, the gas reacts with water inside the crystallisation chamber 711 to produce a solid gas hydrate. This results in a volume increase in the crystallisation chamber 711 as gas is provided externally into the first column (and in the preferred examples, the hydrate is less dense than water). The gas hydrates then float in the first column 705 by gravity, and at the top of the column 70Th dissociate into gas and water. This dissociation reaction is associated with a large volume increase, as gas is released from a solid. The dissociated gas is channelled through a turboexpander 706 to generate power. The carrier gas then passes back down the second column and into the first column to complete the cycle.

As mentioned previously, when the gas hydrates are provided into the first column 403, 503, 705 with a two-way positive displacement pump, the water level in the first column 403, 503, 705 is depleted. In Figures 4 and 5, a further one-way pump was required to provide further hydrates and/or water into the first column 403, 503 to maintain the water level in the first column 403, 503. In these examples, no additional one-way pump is required. This is advantageous, as the pressure difference between the first column 403, 503, the crystallisation chamber 401, 501 is large, and therefore the pump requires a significant mechanical energy input to operate, which reduces the overall efficiency of the cycle. In this example, the water level in the first column is allowed to fall and this leads to a volume increase in the crystallisation chamber 711. In this example, the volume of the crystallisation chamber 711 is not constrained, and so this does not lead to pressurisation, instead the excess water is provided into the third column 708 via the respective connecting pipe 704. The water level in the third column 708 therefore rises.

After a certain number of cycles, the water levels may be as shown in Figure 7b. The water level in the first column 705 is then on, or below an imaginary isothermal line 707.

This imaginary isothermal line 707 defines the minimum temperature, at the given operating pressure, in which hydrate dissociation can spontaneously occur. When this water level is reached, a valve between the third column and the second column is opened, and a valve between the second column and first column is closed. It is appreciated that this isothermal line 707 is not real as such, but can be defined theoretically by calculations based on the operating conditions of the heat power plant. As such, in practice, this means that only when the water level reaches a predetermined level (corresponding to this calculated imaginary isothermal line 707) are the valves switched. After the values are switched, the carrier gas from the second column is provided into the crystallisation chamber of the third column 710 and the process repeats. In effect, the third column 708 and first column 705 'switch roles'. In this way, the water levels in the first column 705 and third column 708 oscillate about the isothermal imaginary line 707 and an additional one way pump is not required to regulate the water levels in the first 705 and third column 708.

Figure 8 shows a preferred example of the generator 405, 505 as depicted in Figure 3 and 4. In this example, the generator 800 comprises a plurality of turboexpanders 801 arranged in series. As such, the total pressure difference across the first column 303, 403 to the second column 306, 406 is split up across the plurality of turboexpanders 801.

As shown, between each generator 801 the carrier gas is allowed to reheat 802. This reheating is a passive heating element, that is to say, the heating is provided by the surrounding water body. In this example, there may be one or more closed valves disposed between each turboexpander. Therefore, as carrier gas is channelled through one turboexpander 801 to the adjacent one in the series 801, the carrier gas is contained in the connecting channel portion 802 between said turboexpanders 801. In an example, the connecting channel 802 may comprises a meandering structure to minimise the total space required and to increase the surface area of said connecting channel 802. It is also envisaged that the connecting channel is configured to operate as an efficient heat exchanger.

The one or more valves may then be opened after a predetermined time, or after a predetermined temperature is reached in the connecting channel portion 802, releasing the carrier gas from the connection channel portion 802 through the adjacent turboexpander 801. This arrangement is particularly preferable for ensuring optimal efficiency.

In other examples, the gas may flow continously between each turboexpander.

It is appreciated that the exact number of the turboexpanders 801 and turboexpander stages depends on certain design conditions such as total pressure difference, temperature, as well as more practical issues such as available space and cost.

Figure 9 depicts schematically an ocean heat power plant farm, which comprises a plurality of ocean heat power plants 901 arranged in a vertical fashion. These heat power devices may be any of the examples described in the description. It is appreciated that the ocean heat power plants 902 may also be located on a shoreline. As would be appreciated by the skilled person, it is only necessary for the first and second column (or third column) to traverse a body of water with a temperature difference. The first and second column may be fully submerged in one example, but in another example, they may protrude from the surface of the body of water.

The inventors appreciate that the columns 403, 503, 406, 506, 705, 708, 709 in some examples may be buoyant. In an example, the ocean heat power plants may be attached to conventional oil and gas equipment, such as risers. In other examples, the columns may be attached to the seabed by a form of suction device, or by other mechanical means. It is envisaged that any temperature differences may be used to generate electricity. For example, any temperature differences that originate from conventional oil and gas processes may also be used for extracting energy.

Figure 10a shows an exemplary section of the second column 406, 506, 709. In some examples, efficient heat exchange is not required in the second column 406, 506, 709, and the second column 406, 506, 709 comprises a cylindrical tube. That is, it is not required to cool or heat the gas in the second column 406, 506, 709. Preferably, the tube is a 6000 series Al alloy, such as 6082 or 6061. In some examples, the tube may be several hundred of metres in longitudinal length and preferably manufactured by way of hot or cold extrusion. This manufacturing process and choice of material is especially cost-effective.

It is envisaged that the aluminium alloys, in particular the 5000 and 6000 series alloys, are especially suited for this purpose as they have excellent saline corrosion properties; high thermal conductivity and are readily extruded.

Figure 10b shows an exemplary section 1000 of the first 403, 503 or third column 708. It is appreciated that the following examples of Figure 10b may be applied to all previous examples. In these columns 403, 503, 708, efficient heat exchange at the top and bottom of said columns 403, 503, 708 is preferable. As mentioned previously, the Carnot efficiency of any cycle is given by the temperature difference across the cycle. In the examples given, this temperature difference corresponds to the temperature difference across the column 403, 503, 708. To maximise the thermodynamic efficiency, it is necessary that the temperature at the top and bottom of the columns 403, 503, 709 reaches that of the surroundings. The use of efficient heat exchangers reduces the time for thermal equilibrium to be reached. This has two effects: i) the Carnot efficiency increases; ii) the flowrate of the carrier gas /hydrate in the heat cycle can increase. Therefore the heat flux in effect limits the productivity of the cycle.

In Figure 10b, the heat exchanger 1001 may comprise a plurality of radially extending members 1002. Preferably, the heat exchanger 1001 has longitudinal axial symmetry so that it may be easily extruded. The heat exchanger 1001 is preferably a 6000 series Al alloy, such as 6061 or 6082. In some examples, each of the plurality of radially extending members 1002 may comprise one or more branching nodes 1003. That is, at a given radial distance from the central tubing of the heat exchanger 1004, the radially extending members 1002 may branch into two or more further radially extending members at a branching node 1003. In this configuration, the total surface area to volume ratio of the heat exchanger 1001 increases, and therefore increases the total heat flux into or out of the first/third column 403, 503, 708.

In Figure 10c, a side profile of an alternate first and/or third column 403, 503, 708 is depicted, which comprises longitudinal portions 1005, 1006 of different radius. In an example, the longitudinal portion 1005 of smaller radius is the tubing 1000 of Figure 10a.

That is, the central longitudinal portion 1005 of the column 403, 503, 708 is not configured to be an efficient heat exchanger. Rather, poor heat exchange is preferred. In this way, the temperature inside the column 1005 is not the same as the surrounding water body, and the location in which the temperature is high enough (at the given operating pressure) for hydrates to dissociate is closer to the top of the column 403, 503, 708. Preferably, the hydrates rise in the water inside the column up to the wider portion of the column 1006 before dissociating. That is, hydrate dissociation does not occur in the central portion of the column 1005. Such an arrangement increases the total volume of hydrates, which are able to dissociate at any given time, therein increasing the total volume of expanding gas, i.e. more work can be done per cycle. It is preferable that the longitudinal length of these wider portions of the column 1006 are relatively short such that the heat flux in or out of these regions is large enough to heat/cool the volume effectively.

In some examples, and as shown in Figure 10c it is also preferable to include the radially extending members 1002, 1003 of Figure 10b at the wider longitudinal sections 1006.

This increases the total heat flux, which increases the productivity of the heat cycle.

It is envisaged that the diameter of the central longitudinal section 1004, 1005 may be optimised for specific operation conditions. For example, if the first column 403, 503 is operating as a energy storage unit, then a wider section may be preferable, as thermal equilibrium is less important, and it is more important that the energy storage unit is able to store a sufficient amount of energy. In continuous operation, it is preferred that the diameter of the columns 1004, 1005 may be smaller so that thermal equilibrium is reached at the working fluid flow rate. This size range means that the total cost of each energy producing ocean heat plant is relatively small, and it is envisaged that a large number of these ocean heat plants may be disposed instead of a single larger one.

In some examples, the power to operate the ocean heat plant may be supplied by renewable sources, such as wind, solar or tidal power. As such, the heat plant is capable of producing net energy without any carbon emissions.

In some examples, a potential may be applied to the ocean heat plant to reduce corrosion rates. In one example, a negative potential may be applied to the ocean heat plant (cathodic protection) and in another example, a positive potential may be applied to the ocean heat plant (anodic protection) to ensure passivation. This process is known as electro-chlorination.

Figure 11 is a flow diagram of the general method steps, comprising: mixing carrier gas with a water-based fluid to form hydrates (Si), moving hydrates to first column (S2), dissociating hydrates and providing gas to turbine (S3) and moving the gas back to the reaction chamber for forming the hydrates (S4, leading to Si).

Figure 12 discloses an exemplary ocean heat power plant equivalent to that shown in Figure 7, except the first and third columns 1203, 1208 comprise two longitudinal sections 1203a, 1203b, 1208a, 1208b disposed between an upper 1203c, 1208c and lower 1203d, 1208d reservoir and the connecting channel between the second column 1206 and the third column 1208 is not shown for clarity. The configuration of the first and third column 1203, 1208 can also be applied to the heat power plants examples illustrated in Figures 4 and 5, and the exemplary schematics of the columns shown in Figure 10a to 10c. The lower 1203d, 1208d, upper 1203c, 1208c and the longitudinal section 1203a, 1208b are at least partially filled with water.

In this arrangement, each longitudinal section 1203a, 1203b, 1208a, 1208b of the respective column is configured to transport either water or the hydrate slurry. By separating the downward flow of water and upward flow of the hydrate slurry into separate sections, internal frictional losses caused by their interference are eliminated, thus leading to an improvement in cycle efficiency. Furthermore, the imaginary isothermal line 1207 defining the temperature (for the particular operating conditions) in which the hydrate slurry dissociates is well below the upper reservoir 1203d, 1208d of the respective column. In this way, it is possible to have a much larger volume of dissociating hydrates (i.e., increase energy storage capability and energy output per cycle) without introducing problems with heat transfer and transport losses. As previously mentioned with reference to Figure 10c, it is preferable if the radially thinner sections of the column 1203a, 1203b, 1208a, 1208b do not act as efficient heat exchangers, such that hydrate dissociation does not occur within these regions. In this way, a larger volume of solid hydrates can be disposed closer to the turboexpander to generate power. This means that the distance in which the expanding gas needs to travel to the turboexpander is smaller, and thus, less energy is consumed prior to the generator.

In some examples, the lower reservoir 1203d, 1208d may also comprise a portion of larger radius (equivalent to that shown for the upper reservoir 1203c, 1208c to ensure that there is a constant supply of water the mechanical device 1202, 1209. The lower and upper reservoir portions of the columns 1203c, 1203d, 1208c, 1208d may comprise the heat exchanger arrangement as illustrated in Figure 10b -similar to that shown in Figure 10c. The function of these heat exchanger arrangements is to ensure that the temperature inside the reservoir is as close as possible to the external temperature of the water so that the maximum Carnot efficiency possible is attainable.

The hydrates enter the lower reservoir of the column 1203d, 1208d via the mechanical device 1202, 1209. In a first case, the hydrate is less dense than water. In this case, the hydrate floats in the water inside the lower reservoir 1203d, 1208d and pass into the longitudinal section 1203b, 1208b arranged to transport hydrates by connecting said section 1203b, 1208b with the lower reservoir 1203d, 1208d directly above the mechanical device 1202, 1209. In some examples, the hydrates may be directed towards this section 1203b, 1208b with mechanical guides. The guides may comprise a meshed element defining a channel from where the hydrates enter the lower reservoir 1203d 1208d to the longitudinal section 1203b, 1208b.

In a second case, the hydrate is denser than water. In these cases, the hydrates do not float out from the mechanical device 1202, 1209 and into the longitudinal section 1203b, 1208b as the hydrates sink in water. Instead, it is preferable if the longitudinal section 1203b, 1208b connects with the lower reservoir 1203d, 1208d via the bottom side of the reservoir 1203d, 1208d and the mechanical device is disposed such that hydrates are able to sink to enter into the lower reservoir 1203d, 1208d. That is to say, the mechanical device is positioned above the lower reservoir 1203d, 1208d and the hydrates sink in and fill up the longitudinal section 1203b, 1208b. In some examples, a pump is used to assist with the filling of the longitudinal section 1203b, 1208b. In other examples, the arrangement of the mechanical device and longitudinal section 1203b, 1208b is equivalent to the first case (hydrates are less dense than water) but a pump is required to provide the hydrates into said section 1203b, 1208b.

In both the first and second cases described above, the longitudinal section 1203b, 1208b preferably transports only a hydrate slurry and the hydrates are provided up into the upper reservoir 1203c, 1208c by filling. That is to say, in every cycle of the mechanical device 1202, 1209 a further volume of hydrates enter into the longitudinal section 1203b, 1208b and each successive volume of hydrates forces the hydrates further up toward the upper reservoir 1203c, 1208c.

The longitudinal section 1203b, 1208b is configured to connect to the upper reservoir 1203c, 1208c such that when hydrates are provided into the upper reservoir 1203c, 1208c, water is not able to flow back down this section 1203b, 1208b.

In the first case, where the hydrates are less dense than water, it is preferable that the longitudinal section 1203b, 1208b enters the upper reservoir 1203c, 1208c above the water level in the reservoir 1203c, 1208c. In this way, as the hydrates enter the upper reservoir, water is not able to flow back down this section 1203b, 1208b.

In the second case, where the hydrate is more dense than water, it is preferable that the longitudinal section 1203b, 1208b enters the upper reservoir 1203c, 1208c below the water level in the reservoir 1203c, 1208c. In these examples, a pump would then be required to provide the denser hydrates into the upper reservoir 1203c, 1208c. However, as the hydrates enter the upper reservoir, the water in the upper reservoir 1203c, 1208d is not able to flow back down this section 1203b, 1208b (it is also pumped in the same direction).

Generally speaking, the longitudinal section 1203a, 1208a arranged to carry water from the upper reservoir 1203c, 1208c to the lower reservoir 1203d, 1208d has a connecting entry point in the upper reservoir 1203c, 1208c at a level in which water is present.

In the first case, where the hydrates are less dense than water and float, then the entry point is below the water level, so that water can be provided back down the section 1203a, 1208a and into the lower reservoir 1203d, 1208d where water can be supplied back into the crystallisation chamber 1201, 1210 via the mechanical device 1202, 1209.

In the second case, where the hydrates are more dense than water and sink, the entry point is below the 'upper' water level, but above the "lower" water level in the reservoir 1203c, 1208c.

In Figure 12, the exemplary mechanical devices 1202, 1209, and crystallisation chamber 1201, 1210 as shown may be any of the mechanical device or crystallisation chamber examples as provided previously in the description as would be appreciated by the skilled person.

It is appreciated that references to toplcabovetupper' and tottomit belowIlower' are used for clarity throughout the description, unless otherwise stated, top and bottom and their equivalents may be interpreted in the following way. Top' refers to a region of the heat plant that is in a warmer region. Bottom' refers to a region of the heat plant that is in a cooler region. In other words, the 'upwards' sense is defined in the direction of increasing temperature. That is to say, regions of the heat plant closer to the surface of the water are closer to the 'top' side of the heat plant as they are in a region of greater temperature. Equally, regions of the heat plant closer to the seabed are closer to the 'bottom' side of the heat plant as they are in a region of lower temperature.

Figure 13a shows an alternative closed loop exemplary heat power plant. The exemplary heat power plant comprises: a reaction chamber 1301 at a first pressure and a first 1302 column at a second pressure, wherein the reaction chamber 1301 is disposed with a volume encompassed by the first column 1302; a mechanical device 1303 connecting the reaction chamber 1301 to the first column 1302, wherein the mechanical device 1303 is configured to move hydrates from the reaction chamber 1301 to the first column 1302; a generator 1305 configured to generate power and connected to the first column; and a second column 1306 connected to an outlet of the generator and to an inlet of the reaction chamber 1301, and configured to contain a third pressure and transport the carrier gas into the reaction chamber 1301.

In the exemplary heat plant, the first pressure is less than the third pressure, which in turn is less than the second pressure. In an example, the first pressure is around 10 bar, the third pressure is around 30 bar and the second pressure is around 1000 bar.

In the exemplary heat plant, the reaction chamber 1301 and the first column 1302 is at a first temperature, and the second column 1306 is at a second temperature. The first temperature is greater than the second temperature. This temperature difference defines the Carnot efficiency. In an example, the first temperature is 278K, and the second temperature is 275K.

The exemplary ocean heat plant is configured to perform a cyclic method of generating electricity, wherein a pressure difference is defined as the difference between a reaction chamber 1301 at a first pressure and a first column 1302 at a second pressure, the cyclic method comprising, at least some, of the following steps: mixing a carrier gas with a water-based fluid in the reaction chamber 1301 at the first pressure to form a hydrate; transporting the hydrate to the first column 1302, wherein the first column 1302 is at a second pressure, and the second pressure is greater than the first pressure, and wherein the first column is at least partially filled with water; dissociating the hydrates into carrier gas and water at the second pressure and providing the carrier gas from the first column 1302 to the second column 1306 via a generator 1305, wherein said carrier gas is formed by hydrate dissociation, wherein said generator is arranged to generate electrical power; and transporting the carrier gas through the second column 1306 back to the reaction chamber 1301, wherein the second column 1306 is at a third pressure.

In the heat cycle, carrier gas is provided into the reaction chamber 1301. The carrier gas may be provided through a filter to reduce the size of the carrier gas bubbles. In an example, the filter may comprise a metal powder sintered filter. The reaction chamber 1301 is at least partially filled with water, and as the carrier gas is bubbled through the reaction chamber 1301 the carrier gas reacts with water to form a solid hydrate. With reference to Figure 13b, an exemplary temperature and pressure for the reaction chamber 1301 is 278K at 10bar. Under these conditions, the pressure-temperature phase diagram states that the thermodynamically stable phase is the solid hydrate phase.

In some examples, the hydrate which forms may be more or less dense than water. The hydrate may then either float in the reaction chamber 1301 and into a mechanical device 1303, which operates as a the two-way positive displacement pump previously described in that a volume of water is provided from the first column 1302 into the reaction chamber in exchange for the same volume of hydrates from the reaction chamber 1301 to the first column 1302. If the hydrate is denser than water, then an additional one-way pump may be disposed to pump hydrates (and water) from the reaction chamber 1301 into the first column 1302, or, to pump the hydrates into the mechanical device 1303.

As the hydrates enter the first column 1302, they are then exposed to the higher pressure conditions inside the first column 1302. With reference to Figure 13b, at the exemplary pressure 1000 bar and temperature 278K, the thermodynamically stable phase for the hydrates is to dissociate into the carrier gas and water. As such, even if the hydrates are denser than water, and do not float out from the mechanical device 1303, the hydrates will still spontaneously dissociate to release the much lower density carrier gas, which is much less dense than water, and therefore rises out from the mechanical device and into the first column 1302. Alternatively, if the hydrate is less dense than water, it is able to float and rise to the top of the first column 1302. It is emphasised that the top and bottom side of the first column are substantially at the same temperature.

The arrangement of the reaction chamber 1301 inside the first column 1302 is particularly preferential when operating at a constant temperature, as the latent heat released during the hydration formation reaction in the reaction chamber can be transferred internally into the first column 1302. In the first column 1302, latent heat is required for the dissociation reaction, and this heat can be therefore supplied by both internal heat transfer from the reaction chamber 1301 and external heat transfer from the surroundings. This arrangement is therefore kinetically advantageous, as it ensures that the hydration formation/dissociation reaction continues to operate, when otherwise the reaction may have been kinetically limited by the generation or consumption of thermal energy.

In the closed loop system, the volume in the first column 1302 is finite, and the gas released by the expanding gas is constrained by the volume of the first column 1302. As such, the pressure increases inside the first column 1302. In these examples, a closed valve is disposed between the turboexpander 1305 and the first column to allow the pressure to increase. After a pre-determined pressure is reached, the valve may be opened, and the dissociated carrier gas is allowed to expand through and into the turboexpander to turn the generator in order to produce electricity. The carrier gas can then pass through a connecting channel and back into the reaction chamber 1301.

Preferably, the pressure of the connecting channel is greater than the reaction chamber 1301. For example, the pressure of the connecting channel may be around 30 bar. In this way, a pump is not required to provide the carrier gas from the connecting channel back into the reaction chamber 1301.

In some examples, the gas is allowed to continuously pass through the connecting channel into the turboexpander 1305 to generate power. The turboexpander may preferably be a turboexpander series as shown in Figure 8.

The inventors have invented a method to extract energy from ambient temperature using a pressure difference. That is, the expanding gas is allowed to pass through the turboexpander 1305 and expand adiabatically, which results in a temperature drop. This temperature drop generates a temperature difference and it is this temperature difference, which defines the Carnot efficiency. In the example given in Figure 13, energy is extracted by generating a temperature difference from a pressure difference. The inventors anticipate that it is not desirable for adiabatic expansion to lead to freezing of the expanded carrier gas. As such, the expansion process is configured such that the pressure drop (and therefore temperature drop) across the turboexpander is low enough to prevent this freezing. Generally speaking, this pressure and temperature drop threshold depends on the physical properties of the carrier gas, for example, its freezing point.

The inventors have realised that by using a gas mixture, it is possible to produce a negative slope on the pressure-temperature phase diagram where both the latent heat and volume change in the dissociation reaction are positive. This special feature enables energy to be extracted from the pressure difference.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: 1 A cyclic method of generating electricity from a temperature difference, wherein the temperature difference is defined as the difference between an upper temperature within an upper portion of a first column, and a lower temperature at a reaction chamber, the cyclic method comprising: mixing a carrier gas with a water-based fluid in the reaction chamber at a first pressure to form a hydrate; transporting said hydrate to a lower portion of the first column, wherein the first column is at a second pressure, wherein the second pressure is higher than the first pressure, and wherein the first column is at least partially filled with water; dissociating the hydrates back into the carrier gas at an upper portion of the first column and providing the carrier gas from the end of the first column at the upper temperature to a second column through a generator, wherein said carrier gas is formed by hydrate dissociation, wherein said generator is arranged to generate electrical power, and transporting the carrier gas through the second column back to the reaction chamber, wherein the second column is at a third pressure.
2 The method according to claim 1, wherein providing said hydrate to the lower portion of the first column further comprises: using a two-way displacement pump disposed between the reaction chamber and the first column to move a volume of hydrates into the first column, and an equal volume of water into the reaction chamber; and using a one-way pump disposed between the reaction chamber and the first column to move a further volume of hydrates from the reaction chamber into the first column.
3 The method according to claim 2, wherein the volume provided by the one-way pump per unit time is greater than the volume provided by the two-way displacement pump per unit time multiplied by the volume fraction of the carrier gas in the hydrate.
4 The method according to claim 1, wherein providing said hydrate to the lower portion of the first column further comprises: using a one-way displacement pump disposed between the reaction chamber and the first column to move a volume of hydrates into the first column; and using a flow-regulating device to move water from the first column into the reaction chamber to maintain the volume of water in the reaction chamber.
The method according to any preceding claim, wherein the second pressure is larger than the third pressure, and the third pressure is larger than the first pressure.
6. The method according to any preceding claim, wherein the carrier gas comprises substantially pure nitrogen or argon.
7. The method according to any preceding claim, wherein the carrier gas comprises an argon-nitrogen mixture or an ethane methane mixture.
8 The method according to any preceding claim, wherein the water based fluid in the reaction chamber comprises seawater, and wherein the seawater is provided from an external source radially proximal to the reaction chamber.
9 The method according to claim 8, wherein the first pressure is greater than the pressure of the external source radially proximal to the reaction chamber, defining a pressure difference, and the seawater is provided into the reaction chamber with a pump, and unreacted seawater is dumped out from the reaction chamber via the pressure difference.
10. The method according to any one of claims 8 to 9, wherein the method further comprises: collecting water in the carrier gas downstream of the generator with a separator, and therein the method comprises generating desalinated water.
11. The method according to any preceding claim, wherein the generator comprises a plurality of turboexpanders arranged in series.
12. The method according to claim 11, wherein each adjacent turboexpander in the series is coupled with a connecting channel, and wherein the carrier gas is passively heated in the connecting channel before passing through the next turboexpander in the series.
13. The method according to any preceding claim, wherein a closed expansion valve is disposed between the generator and the first end of the first column, and wherein the method further comprises: opening the expansion valve to provide the carrier gas to the generator; and wherein the opening of the expansion valve occurs when a predetermined criteria is met.
14. The method according to claim 13, wherein the predetermined criteria is whether the carrier gas pressure is larger than a predetermined pressure.
15. The method according to claim 13, wherein the predetermined criteria is given by a predetermined time of day, and wherein said time of day corresponds to end user demand.
16. The method according to any preceding claim, wherein the first pressure is around 30 bar, the second pressure is 1000 bar and the third pressure is 40 bar.
17. The method according to any preceding claim, wherein the upper temperature is between 283 to 300 Kelvin and the lower temperature is between 273 to 283 Kelvin.
18 An assembly for generating electricity from a heat cycle, wherein the assembly comprises: a reaction chamber, wherein the reaction chamber is configured to contain a first pressure, a water-based fluid and a hydration reaction between a carrier gas and the water-based fluid; a first column, wherein the first column is configured to contain a second pressure and water, and the first column comprises a first portion at an upper temperature, and a second portion at a lower temperature; a mechanical device connecting the reaction chamber to the first column, wherein the mechanical device is configured to move hydrates from the reaction chamber to the second portion of the first column; a generator configured to generate power and connected to the first portion of the first column; a second column, connected to an outlet of the generator to an inlet of the reaction chamber, the second column configured to contain a third pressure and move the carrier gas into the reaction chamber.
19 The apparatus according to claim 18, wherein the assembly further comprises: a two-way displacement pump configured to move a volume of hydrates from the reaction chamber into the first column and an equal volume of water into the reaction chamber; and a one-way pump configured to move a further volume of hydrates from the reaction chamber to the first column.
20. The apparatus according to claim 18, wherein the assembly further comprises: a one-way displacement pump configured to move a volume of hydrates from the reaction chamber into the first column; and a flow-regulating device configured to move water from the first column into the reaction chamber.
21. The assembly according to any one of claims 18 to 20, wherein the water-based fluid comprises seawater, and wherein the reaction chamber comprises a pump configured to move seawater from an external source radially proximal to the reaction chamber into the reaction chamber.
22. The assembly according to claim 21, further comprising: a separator, disposed downstream of the generator, configured to collect water in the carrier gas to generate desalinated water.
23. The assembly according to any one of claims 18 to 22, wherein the first and second column are axially symmetric about a longitudinal axis of said columns.
24. The assembly according to any of claims 18 to 22, wherein the first column comprises one or more radially extending members along at least a portion of the longitudinal length of said column.
25. The assembly according to any one of claims 18 to 23, wherein the first column, second column and the reaction chamber comprise a 6000 series aluminium alloy.
26. The assembly according to any one of claims 18 to 25, further comprising a metallic filter, wherein the metallic filter is configured to mix the carrier gas with the water-based fluid.
27. The assembly according to any one of claims 18 to 22, wherein the first column comprises a flexible tubing connecting the first portion of the first column with the second portion of the first column.
28 The assembly according to any of claims 18 to 22, wherein the first column comprises: a first longitudinal section arranged to transport water only; a second longitudinal section arranged to transport hydrates only; an upper reservoir disposed at the upper temperature; a lower reservoir disposed at the lower temperature wherein each longitudinal section is connected to each reservoir; wherein the radial extent of the first and second longitudinal sections is smaller than the radial extent of the upper and lower reservoir.