WO1980001301A1 - Energy conversion system for deriving useful power from sources of low level heat - Google Patents

Abstract

An energy conversion system for deriving useful power from the thermal gradients in the ocean (12), or from solar, geothermal, or other sources of low level heat, by using warm water to heat a confined working gas such as air whereby a pressure increase results due to warming the gas, arranging so that the expansion moves a piston (14) or other device to extract power, and then cooling the gas and compressing it back to initial conditions while directly or indirectly contacting it with cooler water to thereby decrease the work needed for recompression. Net useful work results from the difference between the work of expansion at higher temperature and the work of recompression at lower temperature.

Description

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Description

Energy Conversion System For Deriving Useful Power From Sources Of Low Level Heat

Technical Field

The present invention relates to energy conversion systems for deriving useful power from sources of low level heat such as thermal gradients in the ocean, or from solar, geothermal or other sources of low level heat and more particularly by using warm water to heat a gas, expanding the gas to extract power and then cooling and compressing the gas with cooler water back to initial conditions.

Description of the Prior Art

Designs now being proposed for Ocean Thermal Energy Conversion (OTEC) plants nearly all use conventional tubular heat exchangers to vaporize a working fluid, such as ammonia, using heat from a warm water source, then employing the ammonia vapor to drive a turbine, and thereafter condensing the ammonia vapors by indirect exchange with cold water brought up from the bottom. An early "open cycle" design used water vapor from the warm water as the working fluid in order to avoid the very expensive and troublesome indirect heat exchangers, but this system requires an extremely large turbine. Designs using ammonia or other working fluid are now receiving intensive attention and study, and now appear to be generally preferred.

However, these designs all require enormous surface areas tubular or other indirect heat exchangers because the operation inherently has a very low conversion efficiency, typically 2 to 3%, and so enormous amounts of heat must be transferred from water to ammonia or other working fluid.

Moreover, this heat must be transferred at very small temper¬ ature differences, such as 2 to 5°F. to conserve the meager difference available between warm water at perhaps 80° and '

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cold water at about 40°. For comparison, a conventional power plant will have 2000° or more on the heat source and roughly 100° on the cold receiver. For an OTEC plant many times more surface area will be required, presenting major problems with regard to cost, fouling/cleaning, and corrosi in an ocean environment, as well as the possibility of ser¬ ious leaks. Titanium tubing has been proposed, using a wal thickness of only 0.03 inches to avoid intolerable cost, bu this poses a major risk as regards mechanical integrity of equipment in such a hostile environment as the ocean.

While the present OTEC designs promise to provide pow from an inexhaustible source at competitive cost and with minimum harm to the environment, there is considerable room for improvement in the areas of cost, reliability, fouling and maintenance, and materials of construction problems. Plastics are of great interest in that they are low-cost, easy to fabricate, and not attacked by salt water. However, they have not been suitable for heat exchangers because of poor heat transfer properties. Heat exchangers are the largest, most costly, and most critical part of conventiona systems.

Theoretically, the maximum efficiency possible for suc energy conversion is given by the relationship:

^Tl ^" 2 E = _χ

where E is fractional efficiency, T, is temperature of the warm source, and ₂ is temperature of the cold receiver. Fo a typical OTEC case having 80° warm water and 40° cold water the theoretical maximum efficiency is 7.4%. Actual designs can achieve only 2.5% to 3% efficiency, allowing for practi¬ cal heat exchangers and "parasitic" power losses for water pumps and other auxiliaries.

In general, the designs being pursued are based on evaporating a liquid at about constant pressure, with the result that all of the heat input takes place at nearly - 3 -

constant temperature, thus preventing the application of countercurrent or crossflow heat exchange over a maximum practical temperature range. Moreover, the latent heat of condensation is finally rejected to the cold receiver, again at nearly constant temperature.

Summary of the Invention

The present invention provides a practical method to obtain useful energy from the warm and cold ocean water by ^• directly contacting warm water with a gas such as air in a confined zone to cause an increase in pressure, then expand¬ ing the gas while continuing to supply heat from the warm water, and extracting power from the expansion by means of a piston or other device. The gas is then cooled and recompressed in direct contact with cold water through part or all of the compression cycle, so that the cycle can be repeated. One piston can serve two chambers of confined working gas connecting to opposite sides of the piston, whereby much of the work for compression is supplied directly from expansion so as to minimize the amount of power that is handled via auxiliary facilities. At pressure levels above about 10-20 atmospheres, it becomes desirable to control solubility of gas in water, so instead of directly contacting the air with seawater, warm water is first used to warm a heat absorbing packing, then the water flow is discontinued while air is passed through the packing to warm the gas.

Similar operations are used in the cooling part of the cycle. The invention is also useful to derive energy from solar collectors, geothermal sources, etc., and for "bottoming" to make use of low level heat rejected from power stations using nuclear or fossil fuel. It is an object of this invention to provide improvements by making basic modifications to the system design which eliminate the use of indirect exchangers, and allow using air as the working fluid instead of ammonia, propane, or fluorinated compounds, while at the same time providing a more effective and practical way to recover

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energy from low level heat sources, and at lower cost.

One application of my invention employs direct contact of water with air as the working gas, to provide heat exchange together with effective countercurrent staging. As a result, the temperature range over which the working fluid operates is increased along with efficiency. Thus, with 80° warm water and 40° cool water, it becomes practical to operate over a range of 30° - 40° (75° to 45°) giving a max¬ imum efficiency of 5.7% according to the previous equation, compared to the range of only 20° and an efficiency of 3.7% in some comparable designs using indirect heat exchange with ammonia.

A further advantage for the new system is that it becomes practical to superheat the warm water by solar radia- tion in a confined zone at the surface. This may not be practical in alternative systems such as the ammonia type.

By using gas as the working fluid, my invention avoids losses due to latent heat, and introduces the capability for effective use of countercurrent and crossflow heat exchange and staging where applicable. Direct contact between air and water provides surface for heat transfer at relatively low cost, so a large surface area can be justified. This can allow heating or cooling the water over a greater temperature range, e.g., 5° to 10°F. compared to the usual 2° to 5°, to thereby decrease the water flow rate required. Since flow rates of both warm and cool water are very large, the resulting savings can be considerable in costs and pumping power.

Brief Description of the Drawings

The foregoing and other objects, advantages and feature of the present invention will be apparent from the following description of several embodiments of the present invention taken in connection with the accompanying drawing in which: - 5 -

FIGURE 1 is a schematic side view of an OTEC plant constructed in accordance with the present invention and designed for mid-depth ocean operation;

FIGURE 2 is a schematic side view of a modification of the OTEC plant of FIGURE 1 adapted to employ superheating by using solar collectors on the surface;

FIGURE 3 is a schematic side view of an OTEC plant constructed in accordance with the present invention and designed for ocean bottom location operation; FIGURE 4 is a schematic side view of an OTEC plant constructed in accordance with the present invention and designed for ocean surface location operation;

FIGURE 5 is a schematic side view of a modification of the OTEC plant of FIGURE 4 in which the free piston system of FIGURE 4 is replaced with a floating piston.

FIGURE 6 is a schematic side view of an OTEC plant constructed in accordance with the present invention in which a compressible packing is used in conjunction with a floating piston. FIGURE 7 is a schematic side view of a novel design of a cold water pipe used in the present invention; and

FIGURE 8 is a block diagram of a system combining an OTEC system of the present invention with a mariculture system.

Description of the Preferred Embodiments

Basically, OTEC plants may be classed into three broad groups: an ocean surface or near ocean surface location, mid-depth ocean location at 200 to 600 feet, for example, and an ocean bottom location that may be 1000 feet deep or more. The surface location is easily accessible to ships and lends itself to industrial manufacturing and a "floating city" as part of the OTEC complex. Solar collectors can be incorpor¬ ated to superheat the warm water. Mid-depth location avoids the major wave action associated with the surface, and the design pressures are moderate, while access to the surface

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can be maintained by using a vertical tower of reasonable size. The bottom location eliminates the long heavy moori cables, or alternative powered position-holding system nee in the two preceding cases, but is subject to very high pressures and decreased accessibility. It is also subject fouling due to benthic marine life. The surface location also subject to fouling from algae, barnacles, etc., while the mid-depth location should be less susceptible to fouli Therefore, the primary description that follows will be gi for a mid-depth ocean location by way of example, but it should be clearly understood that the new improved system intended equally for use at the surface or on the bottom, with suitable modifications in design for adaption to the considerable change in environment. The specific elements of my system include a tank or vessel containing a working fluid such as a gas or air. T air is alternately warmed and cooled, whereby the expansio and contractions move a piston, bellows, diaphragm, or oth device to develop power output. The power can appear as shaft work driving a flywheel, while the net work output c be used as such or to drive an electrical generator, or fo other purposes.

Work is taken out during expansion, but considerable work must be put back in during compression at the lower temperature. This difference, or net work output, is only perhaps 5% of the gross power operating, so it is apparent that the usual turbine expanders and compressors are not suitable in that their efficiency is not much more than 90 and they would leave little or no net power output. My design uses a low friction piston operating in a cylinder to handle the work output and input. Preferably, one cylinder and piston serve two tanks (or more) connecti to opposite ends of the cylinder so that one side is in th expansion part of the cycle while the other side is in the compression part of its cycle. Then the work available fr expansion provides most of the work for compression, minim ing the forces on the piston rod, and the shaft work that

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handled externally. . While the amount of work taken out dur¬ ing expansion exceeds that required for compression, it is not completely in the proper balance timewise in this simple system, so a flywheel is used to store power and supply it as 5 needed. More than two cylinders can be connected to the same flywheel to give a more even power balance.

Preferably, heat is supplied during the warm part of the cycle by direct contact of the gas with warm water, for example by spraying the water through the tank or cylinder.

10 Spraying down through the tank tends to give countercurrent heat transfer as well as staging, thereby minimizing the water flow rate required, and increasing the temperature change on the water flowing through. Open mesh packing, low pressure drop trays, etc., can be used to improve contacting

15 and staging. In simple terms, the ideal is to first heat the gas to warm water temperature at constant volume to generate maximum pressure, and then maintain this temperature during expansion. In practical systems this ideal can be approached but not attained.

20 Next the gas is cooled, again by direct contact with a spray using cool water, and the gas is then compressed while cold water sprays maintain the gas at minimum practical temperature. As will be discussed later, the water sprays may be shut off part way through the expansion or contraction,

2.5 resulting in decreased net power output but a higher thermal efficiency that can approach the ideal Carnot efficiency.

The work for isothermal expansion or compression of an ideal gas is given by:

V,

30 W = RT In V,

where W is the work in Btu/mol of gas, R is the gas constant 1.99, and In V₂/V, is the natural logarithm of the ratio of final to initial gas volume. Net work output is the work of expansion minus that of compression, which values are in the ratio of the corresponding absolute temperatures. In a

_OjWPI practical system there are significant debits due to ineffi¬ ciencies and "parasitic" losses such as water pumps, etc., that may consume 20% of the net power.

Heat input during expansion ideally is equal to the wor output plus the sensible heat added to warm the gas up to the higher temperature. Efficiency is then the net work output divided by the heat input. For various cases of interest on OTEC this ideal efficiency is 4 to 8% without superheat, and 5 to 15% or more when providing solar collectors to superheat the warm water. Practical systems can have about 80% of these ideal efficiencies.

Various types of equipment and arrangements thereof will be described for the purpose of illustration, but are not intended in any way to limit the broad scope and potential applications of the basic invention. To mention only a few uses other than OTEC, one use is for "bottoming" on conven¬ tional power plants to generate additional power from low level heat. Nuclear plants have especially large amounts of such waste heat that can cause thermal pollution. Moreover, the invention can be adapted to use heat directly from the nuclear reactor and eliminate the steam cycle. Efficiency of the plant can thereby be improved considerably, to perhaps 40% versus the present 30% or less.

An excellent prospect is to use the system in combina- tion with solar collectors to provide shaft work or electri¬ city. A warm fluid temperature of 190°F. can be obtained, which is much higher than available from warm ocean water and more than compensates for the loss of a low temperature ocean heat sink. One of the limitations of solar energy today is the need for a new engine to use low level heat effectively, and it appears that OTEC technology will be useful here. Geothermal heat is another good application, since it usually provides heat at 150° to 300°F.

While the concepts have been put in simple terms for greater clarity and understanding, the task of combining and integrating these essential operations into a workable and practical system has been neither simple nor straightforward. - 9 -

In part, this results from the low theoretical efficiencies and the high gross work loads. Thus, the best turbines with 90% efficiency, operating on the working gas would leave no net output. Moreover, for an OTEC plant, the buoyancy of the system and tanks is a profound concern which presented formid¬ able obstacles before the problems were solved. Also, contacting between gas and water is easy to visualize, but countercurrent action with staging is desired, together with tolerable pressure drop on the water and air flows, all with a cyclic operation imposed.

Before going into specific design details, a general alternative will be described that allows extracting the net work from the system by transferring it to a liquid, such as water, that can then be run. through a turbine in continuous steady state operation. This turbine operates on only the ^• net power output; consequently, a turbine efficiency of 90% is quite acceptable. In this alternative, a "free" piston is used in a vertical cylinder, with water above the free piston and working gas below in communication with the gas being heated and cooled.

Starting with the free piston at the top of the cylind¬ er, the working gas is cooled to pull the free piston down¬ ward. At the same time, a valve opens to allow water from a low pressure source to flow in above the piston to fill the space and generate a hydraulic head or pressure which helps to compress the cooled gas as the piston moves down to its lowermost position. Next, the working gas is heated using warm water so that the pressure increases and it tends to expand, moving the piston upward. Water valves are switched at this time so as to discharge the water above the piston to a high pressure reservoir as the piston moves up. Water from the high pressure reservoir flows through a turbine and is returned to the low pressure reservoir that supplies water as described above. This arrangement provides a thermodynam- ically "reversible" operation, and is therefore highly efficient. A temperature change of 30°F. can provide 37 feet water head on the turbine at a nominal 20 atm. operating pressure. ^BURE^_/

OMPI l Vy, wipo- #NATlO The preceding explanations together with subsequent discussion and specific examples will allow designing and evaluating a broad spectrum of applications and cases to optimize each system with regard to cost, engineering aspects, and other considerations. As the first specific example, an OTEC plant will be described for mid-depth ocean location at 400 feet deep and a nominal operating pressure of 12 atmospheres on the working gas to minimize stress on the vessels. Two cases will be shown, one with, and the other without, superheating by using solar collectors on the surface. These examples are shown in FIGURES 1 and 2 respectively.

OMpj y. ϊpδ— - 11 -

TABLE 1

Superheat No Superheat

Warm Water °F. 95 80 Cold Water °F. 40 40 Gas Temperature Max. °F. 90 75 Gas Temperature Min. °F. 45 45 Gas Pressure Max. Atm. 18.2 14.9 Gas Pressure Min. Atm. 10.0 10.0 Gas Volume Max. CF/mol 36.8 36.8 Gas Volume Min. CF/mol 22.0 26.2

Piston Displacement CF/mol 14.8 10.6

Work of Expansion Btu/mol gas 562 363 Work Compression Btu/mol 516 343

Net work 46 20

Base Efficiency % 5.3 3.5 Btu from Warm Water Per Cycle 877 573 Temperature change on Water °F, 15 10

Water Flow/Cycle: lb./mol gas

Warm 58.5 57.3

Cold 55.4 55.4

Parasitic Losses, % of Net Work

Water flow at 20 feet head 9 19 Gas circulation at 40 feet head 4 10

13 29

OMPI

A Vbm WIP In the first case (Column 1) , water is superheated to 95°, which is well within the capability of solar collectors, but also below the lethal temperature of warm water life. Net work per mol of gas and thermal efficiency are both higher with superheating than without. Also, it becomes practical to use a greater temperature change on the warm and cold water flows. Parasitic power losses are lower with superheat, and as a percent on net power output the advantag is even more significant. Both cases use the same equipment volume at expanded conditions. This example illustrates the large advantage obtained by moderate superheat. In this example, the solar collection can be simply a confined layer of surface water that is drawn in for use. A depth of 10 feet supplies day to day storage. It should not be concluded that superheat is necessary to have an attractive system, as will be shown next for two cases without superheat. The second column in Table 2 is the same case as the second column in Table 1, while the first column is for a new case wherein the water sprays are cut off part way through the expansion or compression in order to conserve water.

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TABLE 2

Short Spray Full Spray

Warm Water °F. 80 80 Cold Water °F. 40 40 Gas Temperature Max. ° 75 75 Gas Temperature Min. °F 45 45 Gas Pressure Max. Atm. 14.9 14.9 Gas Pressure Min. Atm. 10.0 10.0 Gas Volume Max. CF 36.8 36.8 Gas Volume Min. CF 26.2 26.2

Piston Displacement CF 10.6 10.6

Work of Expansion, Btu/mol gas 360 363 Work Compression Btu/mol Gas 348 343

Net Work Btu/mol gas 12 20

Base efficiency % 5.6 3.5

Heat from Warm Water Btu/mol gas 214 573 Temperature Change on Water °F 10 10

Water Flow/Cycle lb./mol gas

Warm 21.5 57.3 Cold 19.7 55.4

Parasitic Losses - % of Net Work

Water Flow at 20 feet head 12 19

Gas Circulation at 40 feet head 8 10

20 29

OJrtPI WIPO » Modifying the spray pattern has increased efficiency consid¬ erably and cut the relative proportion of parasitic losses. These cases both use the same size vessels and the same operating pressure. Net work output is lower at the higher efficiency, and a choice between the two will depend on engineering, economic factors, etc.

Having presented some of the major features and advantages of the new system, a more detailed description of its character and operation will now be given, together with specific illustrations and design conditions which will define the system and point out modifications, combinations, and optimizations that can be applied by those skilled in the art.

By way of example, a preferred embodiment of the inven¬ tion will be described for application at mid-depth in the ocean at about 400 feet deep - below the level of wave disturbance but still with access to the surface by way of a vertical conduit. The system, operation, and parameters are as described in Table 1. Cycle length is about .002 hours (7.6 seconds) and water flows are maintained throughout most of the cycle but are shut off 1/2 to 1 second before expan¬ sion or compression is completed at the extremes of volume, in order to allow water drainage.

Contacting of water and air is carried out using an open mesh or pads as packing to improve contacting and also to provide very desirable staging which minimizes water con¬ sumption. Water flows downward and air upward through the packing which may, for example, extend through a height of 10 to 20 feet. Work for pumping the water is minimized by filling warm or cold water reservoirs while they are at a convenient low pressure level, and then pressure balancing the reservoir to the working chamber while the water is flow¬ ing to the appropriate contacting zone. A single contacting zone can be used to which warm or cold water is supplied alternatel .

A double-acting piston is used, with gas expanded on one side, while gas is being compressed on the other side of the piston. Net work output is that from expansion at the higher temperature, minus the work needed for compression at the lower temperature. A flywheel or other device is used to balance the amount and timing of wor input and output, connected to multiple cylinders. A 10° increase in the warm water temperatuer allows a corresponding 10° increase in the operating temperature for the warm part of the cycle. As a result, efficiency increases to 7.4% instead of 5.7%, according to the previous equation.

A second advantageous factor results from the water vapor pressure, which increases rapidly with temperature. Since direct contacting is used, the air will be approximate¬ ly saturated with water at the higher temperature, thereby increasing its volume. Upon cooling, water vapor will condense. The effect then is to accentuate the change in volume and increase the net work output, especially at low operating pressures. For example, at 90°f. the water vapor will add about 5% to the gas volume. ,

Solar collectors have been developed that can generate water temperatures considerably higher than the 90° used in this example, and appear to offer great promise for use with the new OTEC system. Thus, at a reasonably warm water temperature of 150°, the system can realize 14.7% efficiency. It will be seen that the very large exchangers required in alternative designs are a major cost item, and incur serious problems due to fouling and corrosion. A large conventional power plant burning coal may generate 1000 megawatts of electricity, and an OTEC plant of comparable size using exchangers would require about 90 million square feet of surface. Keeping this enormous surface area clean and free of leaks is a formidable undertaking in the ocean environment, but is no longer a problem in the new design which operates in a different manner so as to avoid the need for such exchangers.

It will now be shown how to increase the work output, still without using superheat. The change, illustrated in Table 3 is to raise the operating pressure to 20 atmospheres maximum, from the previous 14.9 atm.

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TABLE 3

Pressure Base

Warm Water °F. 80 80 Cold Water °F. 40 40 Gas Temperature Max. °F. 75 75 Gas Temperature Min. °F. 45 45 Gas Pressure Max. Atm. 20. ,0 14. ,9 Gas Pressure Min. Atm. 10. ,0 10. ,0 Gas Volume Max. CF 36. 8 36. 8 Gas Volume Min. CF 19. ,5 26. 2

17.3 10.6

Work of Expansion Btu/mol gas 681 363 Work of Compression Btu/mol gas 643 343

Net Work Btu/mol gas 38 20

Efficiency % 4.3 3.5

Heat from Warm Water Btu/mol gas 891 573

Temperature Change, on Water °F. 10 10

Water Flow/Cycle: lb./mol gas

Warm 89.1 57.3 Cold 85.3 55.4

Parasitic Losses, %J of Net Work

Water Flow at 20 ft. head 16 19

Gas Circulation at 40 ft. head 5 10

21 29 - 17 -

The first column in Table 3 shows the new case at higher pressure, while the second column is the same case as in the second column of Table 2 using full sprays. Again, the volume of tanks at maximum expansion has been kept unchanged. Although the losses- are somewhat higher than the best case with partial sprays or with superheat, they can be further decreased by optimizing the design.

Increasing the maximum pressure (to more than 20 atmospheres) and compression ratio would give further improvement, for example, 4.24 compression ratio versus the above 2.12 would give 4.88% thermal efficiency, and the losses would be only 18.7% of net work output. A somewhat deeper location would minimize stresses on the vessel walls. However, at the higher pressure, air solubility in the water becomes significant, so consideration must be given to controlling loss of air, or replacing it. One way around the problem is to drop the minimum pressure to 5 atmospheres while maintaining the same 21.2 maximum pressure, giving a compression ratio of 4.24. This is a realistic basis for design, although in making comparisons and evaluations, allowance should be made for the fact that the 5 atmosphere case has a maximum gas volume about double that of the preceding two cases.

The preceding three comparisons have thus shown: 1. The beneficial application of solar collectors to provide warm water. In fact, they can be used to supply all of the heat input, as by using closed circuit recirculation of the warm water supply. This is a most promising approach in that it releases OTEC from the need for a warm water source, and the plants can then be considered for many other locations provided that a suitable cold receiver can be made available.

2. The application of partial use of water sprays in order to decrease water consumption and increase system efficiency. ^■ The sprays or other devices used for contacting gas and water can be discontinued during the later part of the expansion or compression cycle, but in general will be operated in such a manner that the gas temperatures remain within a range bounded at the upper end by the warm water supply and at the lower end by the cold water temperature.

3. Increasing compression ratio on the working gas results in greater net work output per mol of gas, together with increased thermal efficiency and lower parasitic losses. This is to be balanced against the accompanying increased cost of equipment, and the complication that may result from significant solubility of air in water at pressures above 10-20 atmospheres.

Various alternatives can be combined to increase efficiency and work output beyond the specific examples given above, and design studies must be integrated with cost and economics in order to optimize an application. Such factors depend on cost of fuel, available technology., etc., and can change with time.

While the description so far has given a broad picture of the invention, with some modifications and applications, it will be found in making detailed designs that alternatives arise when selecting process design or equipment which can make a large difference in the economics. One such case is the means for introducing water into the zone for contacting with the working gas, when the latter is undergoing pressure cycles. A convenient way to handle this is to have a water reservoir which is filled at a time when the working gas is at low pressure, displacing gas back into the main vessel. Then the water reservoir is "pressure balanced" : to the working gas zone or tank by opening a valve in connecting piping, whereby the water can now be fed to the contacting zone while requiring a pumping pressure build-up to overcome only the differential pressure across the sprays or other contacting devices, rather than the full system pressure. Preferably the reservoir is filled while connected to the main tank to thereby return the gas to the working zone. Of course, the reservoir can be a suitable confined zone within the main tank, since the volume of water used per cycle is only a small fraction of the tank volume.

A specific example is given below for an OTEC module developing 50 megawatts of gross power output, representing an attractive but not necessarily optimum case. The basis will be as shown in column one of the preceding Table 3 for mid-depth location operating between 10 to 20 atmospheres. Warm water is available at 80°F. Cycle time is an important variable since the work output increases with the number of cycles per hour. A cycle time of 0.002 hours or 7.6 seconds will be used.

Results are shown in Table 4.

TABLE 4

OTEC at Mid-Depth Location

Gross Electric Power Generated, megawatts 50

Amount of Working Gas, mols 8,950 Combined Tank Volume, CF 329,000

Buoyancy, short tons 10,500

Buoyancy, tonx/KW gross 0.2

Warm Water Flow, CFS 1,780

Parasitic Losses MW

Water Flow 8.0

Gas Circulation 2.5

10.5

Net Power Output, MW 39.5

Net Thermal Efficiency % 3.4

Buoyance in tons per kilowatt output is a major consid- eration in that it reflects the relative size of equipment, as well as to some extent the cost. For comparison with the above 0.21 tons/KW, a representative value for alternative designs is about 1.0 tons/KW when using indirect heat exchangers and ammonia as the working fluid. Moreover, cost per ton for the latter can be expected to be higher since the equipment is more complicated and requires more fabrication, and more labor to assemble. For example, the heat exchangers on warm and cold water may each have 120,000 tubes to be mounted in tube sheets and carefully sealed against leaks. Now, referring to FIGURE 1, reference numeral 10 desig¬ nates an OTEC plant disposed at a mid-depth location below ocean surface 12. OTEC plant 10 includes a double acting piston 14 operatively disposed in cylinder 16. Piston 16 is

-BU connected to piston rods 18 and 20 which in turn are opera- tively connected to flywheel 22 disposed in housing 24. Tower 26 communicates with OTEC plant 10 via housing 24 to provide access to the surface. Cylinder 16 communicates at its left end with tank 28 which contains fan 30 and baffles 32 mounted in cylindrical housing 34. Tank 28 is provided with a bottom drain line 36 which has a check valve 38 disposed therein. Cylinder 16 also communicates at its right end with tank 40 which con- tains fan 42 and baffles 44 mounted in cylindrical housing 46. Tank 40 is provided with a bottom drain line 48 which has a check valve 50 disposed therein.

Each of tanks 28 and 40 is equipped with means to spray alternately warm water and cold water therein as follows. Tank 28 is provided with warm water supply line 52 which has a valve 54 and spray nozzle 56 therein. Tank 28 is also provided with cold water supply line 58 which has a valve 60 and spray nozzle 62 therein. Tank 40 is provided with warm water supply line 64 which has valve 66 and spray nozzle 68 therein. Tank 40 is also provided with cold water supply line 70 which has a valve 72 and a spray nozzle 74 therein. Flywheel 22 is operatively connected to a power input device, turbine 76, by means of a common drive shaft 78.

The method of operation of OTEC plant 10 is as follows, selecting as a starting point a condition wherein the piston 14 is at the extreme right in cylinder 16 and wherein tank 28 is in its maximum warm condition and tank 40 is at its maxi¬ mum cold condition. At this point, warm water obtained by pumping from an upper location in the ocean is sprayed into tank 40 through v/arm water supply line 64 and spray nozzle 68 by opening valve 66 (which had been closed) . Fan 42 recircu¬ lates air upward through baffles 44 countercurrent to the warm water, and downward around the outside of cylindrical housing 46. Simultaneously, cold water obtained by pumping from a lower location in the ocean is sprayed into tank 28 through cold water supply line 58 and spray nozzle 62 by opening valve 60 (which had been closed). Fan 30 recirulates - 22 -

air upward through, ba fles 32 countercurrent. to the warm water and downward around the outside of^•.^'•housing 34, As a result of the air being warmed in tank 40 increasing" its pressure ^'and air being cooled in tank 28 reducing its 5 pressure/ piston 14 is moved to the left in cylinder 16, driving flywheel^* 22 by means of connecting rods 18 and 20. A part of the energy so produced is stored in^' flywheel 22 with a portion of. the energy being taken out in the form o useful work by driving turbine ^'76 by means of drive shaft

10 When piston 14 reaches its extreme ^"left position, war water^" supply' line^'"^'64 is shut off by closing valve "66 and c water supply 'line ^"58 is shut off by closing valve^{' '}60. At this point, cold water obtained from a lower location in t ocean is sprayed..into tank^' 40 through cold water supply li

15 70 and spray nozzle 74 by opening valve 72 Cwhich ^"had been closed) . Fan^' 42 ^'recirculates air upward through baffles 4 .countercurrent: to- he ^'cold water, and downward around the outside ^"of cylindrical housing 46. " Simultaneous warm wate obtained from an upper location "in the^{" '}ocean is sprayed in

20 tank 28 through ^'warm water supply line 52 and spray nozzle by opening valve ^'54 (which bad been closed) .. Fan 30 recir culates air upward through baffles 32 countercurrent to th warm water and downward and around the^" outside of cylindri housing-34. As a result of the air being warmed in tank 2

25 increasing the ^"pressure ^"and air.'bei^'ng cooled in tank 40 reducing its pressure,^" piston 14 is moved to the right in cylinder^' 16, driving flywheel 22 by ^"means of connecting_ ro 18 and 20. Again,^"a part of the energy is stored in flywh 22 with a portion of the energy being taken in the form of -30 useful work by driving turbine 76 by means of drive shaft The system^" is then back to .its starting point describ above.^" " The above process is then continued until flywheel reaches^' its designed speed, at which point the energy outp of the OTEC plant 10 is taken^" essentially completely by

35 turbine 76.. Water is drained from tank ^'28 through bottom drainline 36 and check ^"valve 38 when the pressure of tank 2 is above ^'ambient-.water^"-pressure ^"and is controlled to preve escape of air. Likewise,^" water^" is drained from tank 40

^ UR through bottom drainline 48 and check valve 50 when the pressure of tank 40 is above ambient water pressure and is controlled to prevent escape of air.

Now referring to FIGURE 2, reference numeral 12 again refers to the ocean surface and reference numeral 26 refers to access tower 26 for OTEC plant 10. Reference numerals 80 designate a series of solar collectors which may be con¬ structed of plastic, having an upper transparent portion 82 serving as a solar window and a lower portion 84 serving as a trough. The solar collectors 80 are connected together in pairs by means of inlet manifold 86 and in alternating pairs by means of outlet manifold 88 as shown in FIGURE 2. Inlet manifold 86 is connected to inlet line 90 and outlet manifold 88 is connected to outlet line 92. The solar collectors 80 are operated in combination with OTEC plant 10 as follows. Used warm water discharged alter¬ natively from tanks 28 to 40 via drain lines 36 and 48 is fed to inlet line 90 and then via inlet manifold 86 to the individual solar collectors 80 wherein the water is warmed as a result of sunlight through upper transparent portion 82 while the water passes through troughs 84. The warmed water is then collected from the solar collectors 80 through outlet manifold 88 and returned to OTEC plant 10 where it is fed alternatively into tanks 28 and 40 via warm water supply lines 52 and 64 respectively as described in detail in connection with FIGURE 1.

To further show the broad application of the new system, a somewhat different design will now be defined for an OTEC plant located on the ocean bottom, at a depth of 2000-2500 feet. Results are summarized in Table 5, and the design is shown in FIGURE 3. The second column of Table 5 shows the effect of increasing compression ratio and piston displace¬ ment.

0MPI - 24

TABLE 5

OTEC at Bottom Location - ca. 2000 feet depth

Warm Water °F. 80 80

Cold Water °F. 40 40

Gas Maximum °F. 75 75

Gas Minimum °F. 45 45

Gas Pressure Maximum Atm. 72 80

Gas Pressure Min. Atm 48 40

Gas Volume Maximum CF/mol 8. 12 9.7

Gas Volume Minimum CF/mol 5. 11 4.6

Piston Displacement, CF/mol 3.01 5.1

Work of Expansion, Btu/mol gas 433.8 741.7 Work Compression, Btu/mol gas 409.5 700.1

Net Work, Btu/mol gas 24.3 41.6

Heat from Warm Water, Btu/mol gas 644 952

For 50 Megawatts Gross Power

Amount of Working Gas, mols 13970 8170

Combined Tank Volume, CF 113400 79660

Buoyancy, short tons 3630 2550

Buoyancy tons/KW 0.073 0.0

Warm Water Flow at 10° ΔT, CFS 2002 1730

Parasitic Losses MW

Water Flow 8.9 7.7

Gas Circulation 4.1 2.4

13.0 10.1

Net Power Output MW 37.0 39.9

Net Thermal Efficiency % 2.80 3.5 Buoyancy is quite low as a result of the relatively high pressure, meaning that the equipment size is small. It will be designed to stand the differential pressure encountered, but not the full operating pressure as would be necessary for surface equipment at the same high pressure. A cycle length of 0.002 hours has been used to allow comparison with pre¬ vious examples, but this could easily be increased to 0.004 or more while still maintaining a very favorable low buoy¬ ancy. Increased pressure level, and higher pressure ratios are thus useful ways to minimize buoyancy and equipment size. As mentioned before, air solubility in water at these high pressures should be taken into consideration. For the example case, air solubility is controlled by using a form of indirect heat exchange, but one that does not transfer heat through a costly metal surface, as in a tubular exchange, which would be seriously affected by fouling. Instead, water is flowed through a packed bed to warm the packing. Water flow is then stopped, and after draining, the working gas is circulated through the warm bed. In this particular design, separate beds are provided for warm and cold operations. A large surface area can be provided at low cost, and fouling does not seriously impair heat transfer to the gas. Solu¬ bility of air in the water is not a problem since there is only minor direct contact between water and air. Now referring to FIGURE 3, reference numeral 100 desig¬ nates an OTEC plant constructed in accordance with the present invention and designed to operate on the ocean bottom 112. OTEC plant 100 is provided with a double acting piston 114 which is operatively disposed in cylinder 116. Piston 114 is operatively connected by means of connecting rods 118 and 120 to flywheel 122 disposed in housing 124. Flywheel 122 is operatively connected to a turbine 126 by means of common drive shaft 128.

Cylinder 116 communicates at its right end with the top of tank 130 which is provided with a plurality of baffles 132 designed to give staging and limit mixing between water and air. Tank 130 is supported on the ocean bottom 112. Cylinder 116 communicates at its left end with the top tank 134 which is provided with a plurality of baffles 136 designed similar to baffles 132. Tank 134 also is supported on oceam bottom 112.

OTEC plant 100 also includes a cold water contacting chamber 140 which contains therein a bed of conventional packing material 142 and a circulating fan 144 in its upper, inner portion. Chamber 140 is provided at its upper portion with a cold water inlet line 146 and with a cold water outle line 148 at its bottom containing check valve 149. OTEC plant 100 also .includes a warm water contacting chamber 150 which contains therein a bed of conventional packing materia 152, similar to that contained in chamber 140, and a circu¬ lating fan 154 in its upper, inner portion. Chamber 150 is provided at its upper portion with a warm water inlet line 156 and with a warm water outlet line 158 at its bottom containing check valve 159.

Chamber 140 is provided at its top with a gas outlet line 160 and its bottom with a gas inlet line 162. Chamber 150 is provided at its top with a gas outlet line 164 and at its bottom with a gas inlet line 166. Tank 130 is provided at its top with a gas inlet line 168 and at its bottom with gas outlet line 170 having valve 171 therein. Tank 134 is provided at its top with a gas inlet line 172 and at its bottom with a gas outlet line 174 having valve 175 therein. Gas outlet line 160 is provided with a three-way valve 176 which communicates with gas outlet line 168 and with lin 178 which in turn communicates with gas inlet line 172 havin a valve 173 therein.. Gas outlet line 164 is provided with a three-way valve 180 which communicates with gas inlet line 172 and also with line 182 which in turn communicates with gas inlet line 168 having valve 169 therein. Gas outlet lin 170 is provided with three-way valve 184 which communicates with gas inlet line 162 and with line 186 which in turn communicates with gas inlet line 166. Gas outlet line 174 i provided with three-way valve 188 which communicates with ga inlet line 166 and with line 190 which in turn communicates with gas inlet line 162.

Oirt OTEC plant 100 operates as follows, commencing with piston 114 at its far right position in cylinder 116. Tank 130 and chamber 140 at this point are each in their cold condition and tank 134 and chamber 150 at this point are each in their warm condition. In this example of the present invention, chamber 140 will always be in a cold condition by reason of cold water being introduced through inlet line 146 to the top of the bed of packing material 142 and withdrawn through outlet line 148. Also, chamber 150 will always be in a warm condition by reason of warm water being introduced through inlet line 156 to the top of the bed of packing material 152 and withdrawn through outlet line 158. The valves 169 and 171 are open and three-way valves 176 and 184 are set to circulate air through chamber 140 and tank 130 by means of fan 144. Likewise, valves 173 and 175 are open and three-way valves 180 and 188 are set to circulate air through chamber 150 and tank 134.

Now the valves 169, 171, 173. and 175 are shut off and warm water is passed through chamber 150 to warm the bed 152 to substantially warm water temperature. Similarly, cold water is passed through chamber 140 to cool the bed 142 to substantially cold water temperature.^" The valves 169, 171, 173 and 175 are now opened and three-way valves 176 and 188 are set to circulate air through chamber 140 and tank 134 by means of fan 144 to warm tank 130. Likewise, three-way valves 180 and 184 are set to circulate air through chamber 150 and tank 130 by means of fan 154 to cool tank 134.

As a result of heating the air in tank 130 and its expansion and cooling the air in tank 134, and its contrac- tion, piston 114 moves to the left in chamber 116, driving flywheel 122 by means of connecting rods 118 and 120. A part of the energy so produced is stored in flywheel 122 with a portion of the energy being taken out in the form of useful work by driving turbine 126 by means of drive shaft 128. When piston 114 reaches its position at the far left of cylinder 116, the valves 169, 171, 173 and 175 are shut off to stop circulation of air through the beds. Then cold water is circulated by pumping through chamber 140 by means of lines 146 and 148 to cool bed 142 and warm water is circula¬ ted by pumping through chamber 150 by means of lines 156 and 158 to warm bed 152. Bed 142 is then drained through line 148 and check valve 149 and bed 152 is drained through line 158 and check valve 159.

Now valves 169, 171, 173 and 175 are opened and three- way valves 176 and 184 are set to circulate the warm air in tank 130 through chamber 140 and tank 130 by means of fan 14 to thereby cool the air. Likewise, three-way valves 180 and 188 are set to circulate the cold air in tank 134 through chamber 150 and tank 134 by means of fan 154 to thereby heat the air. As a result of heating the air in tank 134 and its expansion and cooling the air in tank 130 and its contractio piston 114 moves to the right in chamber 116, driving flywhe 122 by means of connecting rods 118 and 120. Again, a part the energy so produced is stored in flywheel 122 with a por¬ tion of the energy being taken out in the form of useful wor by driving turbine 126 by means of drive shaft 128. Multipl of chambers 150 and 140 can be provided to allow more time i the cycle for water to flow through the packing.

At full travel, piston 114 is at the far right and is a the starting position described above. The above process is then continued until flywheel 122 reaches its designed speed at which point the energy output of the otec plant 100 is taken essentially completely by turbine 126.

One additional example will be given, for a 50 megawatt plant at an ocean surface location, using the "free piston"/ water turbine system mentioned earlier. In this case, high buoyancy can be accommodated simply by having the system float, and hence does not introduce major design problems, although cost will tend to increase at higher buoyancy in that it reflects larger equipment. Minimum pressure is set at two atmospheres, corresponding to a submergence of only 3 feet, at which pressure the working gas is contacted directl • with water for heat exchange. Parameters for this case are given in Table 6 for alternative pressure levels, both using a 0.002 hour cycle. TABLE 6

OTEC at Surface Location

Gas Pressure, Maximum, Atm. Gas Pressure, Minimum, Atm. 2 2 Warm Water in °F. 80 80 Cold Water in °F. 40 40 Gas Temperature Max. °F. 75 75 Gas Temperature Min. °F. 45 45 Gas Volume Maximum CF 195. 0 195.0 Gas Volume Minimum CF 122. 7 92.0

Piston Displacement CF 72.3 103.0

Work of Expansion Btu/mol gas 433.9 743.7 Work of Comresssion Btu/mol gas ^: 409.5 700.1

Net Work Btu/mol 24.4 41.6

Heat from Warm Water Btu/mol gas 644 952

For 50 WM Gross Electric Power

Amount of Working Gas, mols 13,900 8,200 Combined Tank Volume, CF 2,710,000 1,600,000 Buoyance, Short Tons 86,700 51,200 Buoyancy, Tons/KW 1.73 1.02 Warm Water Flow CFS 1,940 1,690

Parasitic Losses MW

Water Flow 8.9 7.7

Gas Circulation 4.1 2.4

13.0 10.1 Net Power Output MW 37.0 39.9

Net Thermal Efficiency % 2.80 3.49

^"BUREA QM - 30 -

The system arrangement using a "free piston"/water turbine is illustrated in FIGURE 4. The lower side of the piston connects to the air working chamber, while the zone above the piston is kept filled with water supplied from a low pressure reservoir. The piston moves through the cylind er and is sealed at its edges to prevent substantially leak¬ age.^' At the top of the cylinder are connections to low and high pressure water reservoirs or manifolds, each connection provided with a suitable check valve which allows flow in only one direction.

During expansion of the gas in the working chamber, the piston moves up, forcing water into the high pressure mani¬ fold, while connection to the ^"low pressure manifold is close off by the check valve. In the next part of the cycle, gas in the working chamber is cooled, causing the piston to move downward, during which access to the high pressure manifold is closed off by the check valve and the other check valve opens to allow water to flow into the cylinder from the low pressure manifold. This arrangement then transforms energy into the form o a high pressure water source, and a low pressure receiver. Multiple systems and reservoirs will allow a continuous smooth water flow, which can be passed through a turbine to generate electricity or to provide shaft work as desired. While described as an alternative for simplicity, this system can be combined with a single or double-acting gas piston. Thus, the gas piston can be^' used to drive a water piston of smaller diameter to give increased pressure differential and thereby permit using a smaller and less costly water turbine-.^'

Now referring to FIGURE 4, reference numeral 300 desig¬ nates an OTEC plant designed to operate at a location at the ocean surface 312. OTEC plant 300 includes a free piston 314 which is operatively disposed in vertical cylinder 316. At its upper end cylinder 316 communicates with a high pressure water line 318 provided with a check valve 320 and with a lo 'pressure water line 322 provided with a check valve 324. High pressure water line 318 communicates with the high pressure side of turbine- 326 and low pressure wat r line 322 communicates with the ^'low pres^'sure ^'side ^'of turbine.326.

At its lower end cylinder 316 ^'communicates via conduit 327 with ^'a. tank .328 which^{' '}is provided in its upper portion 5 with a plurality of vertically spaced baffles^' 330. In its lower portion, tank 328 is provided' ith a^" contacting chamber .332 which contains a plurality o .vertically spaced gas- liquid contacting trays 334. In its upper portion, chamber 332 is provided with ^"a fan 336 and communicates with a

10. vertically and. centrally aligned conduit 338 which is dis¬ posed in the' middle"of baffle 330..^" At^' its bottom/ tank 328 s provided with a drain' line 340.^' Chamber^" 332 is provided with a cold water^" inlet^" line ^'3.46 having a valve. ^'348 therein, which communicates^" with spray line. ^"350 which enter^'s^' the upper

15 portion of chamber^" 332.^"

The^" operation of OTEC plant 300 is as follows, starting at the point where "piston 314 is at its lowermost position in cylinder 316 and where cylinder 316 is. filled with water above ^"piston 314.. At this condition, tank 328 is at its

2Q lowest temperature ^"with cold water being sprayed into chamber^" 332 ^"through ^"cold water^" inlet line 346 with valve 348 open^". Fan 336 is operating continuously and is circulating air up through conduit 338 to the top of tank 328 and then downwardly around the^" outside ^'of conduit 338 and chamber 332

25 and then upwardly through the nterior of chamber 332. At this point the ^"cold water^" spray is stopped" by^' shutting valve 348 and the^" water^" is allowed to drain from^' chamber 332 through drain line 340; . . . ^■ . -, .. , ....

At this point, warm water is introduced into chamber 332

30 via conduits- 342 and 350 by opening valve.344 to thereby warm the ^'air in"tank- 328. As a result, the air expands moving piston 314" upward which in turn increases the pressure of the water in cylinder 316 above piston 314, so that the water flows -through ^"high- pressure conduit 318 and check valve 320

35 which allows flow only in one ^"direction (upward in FIGURE 4).

The^{" '}spent warm water drains downward from chamber^" 332 and out through drain line ^"340. At this point, piston 314 reaches its highest point with he^" air in tank 328 at its highest temperature.^' The warm water^" supply is then shut off closing valve 344 in, line 342 and allowing the warm water to drain.

At this point, cold water is introduced into chamber 33 via conduits 346 and 350 by opening valve 348. This results in cooling the air in tank 328 which causes it to contract pulling piston 314 downward. In this operation, check valve 320 automatically closes while check valve 324 in low pressure water line 322 opens to fill the cylinder 316 above piston 314. While the average gas pressure in tank 328 decreases, piston 314 moves downwardly reaching its lowest point which is the starting point in the operation referred to above. As a result of water flowing from high to low pressures through conduits 318 and 322, power is generated i turbine 326 by the passage of water therethrough. Instead of the free piston system shown in FIGURE 4, useful power can be extracted by the method shown in FIGURE 5. In this alternative, gas within the tank is alternately warmed and cooled by water sprays causing the gas to expand and then contract, thereby moving a floating piston at the bottom of the tank. The arrangement allows the piston to drive the flywheel while minimizing friction. This particu¬ lar example operates with a relatively low pressure ratio between maximum and minimum pressures, resulting in a relatively high proportion of the heat load being as sensibl heat for warming or for cooling the gas. However, there is an accompanying advantage in that the water streams can then be cooled or warmed over a greater temperature range since greater countercurrent contacting becomes practical. Flow rates of water are thereby decreased. A variation is to flow the gas from a tank at higher pressure (warm) to one at lower pressure (and cold) through a turbine which recovers energy. Continuous operation of the turbine can be achieved using multiple tanks. Thus, each tank will be depressured through a series of steps, while a similar series of tanks will be going through stepwise increases in pressure. Connections are then made to appro- ^• priate tanks via switching valves and manifolds in order to hold constant pressure in or out of each specific turbi - 33 -

These turbines will then operate at several different levels of pressure. At any given time, the tank at highest pressure will connect through the turbine to a tank which is at the highest pressure step in the cold part of the cycle. Now referring to FIGURE 5, reference numeral 400 desig¬ nates an OTEC plant designed in accordance with the present invention for operating at a location at the ocean surface. OTEC plant 400 is provided with a floating piston 414 opera¬ tionally disposed in vertically aligned cylinder 416. Piston 414 is operatively connected by means of connecting rods 418 and 420 to flywheel 422 which in turn is operatively connected to turbine 424 by means of drive shaft 426.

Flywheel 422 and turbine 424 are disposed in the bottom portion of tank 430 which is provided in its upper portion with a plurality of vertically spaced horizontal contacting devices 432 such as baffles, trays or the like. Flywheel' 422 and turbine 424 have a shield 426 disposed above them to pro¬ tect them against water passing downwardly in tank 430. At the uppermost part of tank 430, there is provided a manifold 434 having a plurality of water sprays 436. Manifold 434 communicates with conduit 438 which in turn communicates with cold water supply line 440 having valve 442 and warm water supply line 444 having valve 446.

OTEC plant 400 operates as follows starting with piston 414 at its top position. At this point, tank 430 is in its cold condition with valve 442 open and cold water being sprayed into the top of tank 430 through lines 440 and 438, manifold 434 and sprays 436. Valve 442 is then closed to shut off the supply of cold water and the water is drained out of tank 430 and cylinder 416 around piston 414 which is loose fitting.

At this point warm water is introduced by pumping into the top of tank 430 through lines 444 and 438, manifold 434 and sprays 436. This results in the air in tank 430 warming up and expanding, which in turn pushes the water level down in cylinder 416 and piston 414 moves downwardly driving fly¬ wheel 422 and turbine 424 via drive shaft 426, in a manner similar to the operation of OTEC plants 10,100 and During this operation, the spent warm water drains around piston 414 and piston 414^' moves to its lowermost position a the^{" '}air warms up.

At this point, the ^"warm water supply is shut off by closing valve 446 in ~line". 44 and cold water^" is introduced into tank 430 by opening valve ^"442 in cold water^" supply lin 440. The cold water cools the air in-tank 430 and it con¬ tracts, resulting in a rise in the water level around pisto 414 which in turn rises^" in cylinder 416, driving flywheel 42 and turbine 424.. As cold water continues to be sprayed int tank-^"430, the^" air is cooled until piston 414 reaches it^'s uppermost position, which brings it to the starting point referred to above.^"

FIGURE 6 illustrates an OTEC plant comprising a U-shape OTEC plant vessel^' 500 having legs 502 and 504. Floating pistons 505 and 506- are disposed respectively in legs 502 an 504 of the vessel 500 and are supported by water^" therein. Compressible packing 508 is disposed in the upper part of le 502 above floating piston 504 and compressible packing 510 i disposed in the upper part of leg 504 above floating piston 506. The ^"compressible packing may comprise a foamed plastic having a .density of 1 to 10 lb./cubic ft.^", for example, or may comprise horizontal layers of screens spring mounted. A water spray head 512 is disposed in leg 502 in the upper portion thereof above compressible packing 508. Simi¬ larly, a water spray head is disposed in leg 504 in the uppe portion thereof above compressible packing 510.. Vessel 500 is provided with sources of cold and warm water which may be alternatively introduced into leg 502 through spray head 512 by means of conduit 514 containing valve 516 and into leg 50 through"spray head 518 by means of conduit 520 containing valve 522.

Leg 502 is provided at its top with a conduit 524 havin a valve _.526, which ^'communicates with a turbine (not shown). Similarly, leg 504 is provided at its top with a conduit 528 having a valve 530, which communicates^" with said turbine.

OTEC plant 500 operates in the same basic manner as do the ^"other^" OTEC plants described hereinbefore.^" Thus, - 35 -

example," warm water may be introduced into leg.502 and cold water^' may be introduced into leg^" 504 to move-the^" OTEC system into the position shown in FIGURE 7 wherein packing 508 is in its expanded position and packing 510 is in its compressed 5 position.' "The spongelike^" packing is added within the cylin¬ der to fill the entire ^'cylinder^'' and provide temperature control and good heat transfer so that the gas can be kept • ' captive. Thus, the sponge ^"is wet with cold water before

■ the^" compression stroke so that it takes up the heat released

10 during compression and holds the temperature down. Then, before expansion in the ^"next step, thie packing is rinsed with warm-water to thereby supply heat to the gas^' during^" expansion, Additional water may or may not be added while the piston is moving. No fan is needed. Between^" steps, the 'packing is

15 squeezed to remove ^"most of the ^"water, using suitable ^"mechan¬ ical means (not shown) .. .- ;.. . . .

A preferred" alternative is to have both warm and cold packings always within the ^' cylinder, each maintained at its temperature ^"so that the^" packing does^' not have ^"to be heated or

20 cooled. During compression the cold packing is in^• an expanded condition to fill most of the cylinder, while the^" warm packing is kept compressed to a small volume by.suitable mechanical means. Then during expansion the warm packing is expanded to fill most of the ^'cylinder while the cold packing is

25 compressed to small volume.

Net power is withdrawn as follows. At the point of maximum pressure a" small portion of high pressure gas is taken^" out (e:g., 5%) and passed through^' the turbine and then returned to another cylinder that is at its minimum pressure.

30 With surge chambers and multiple tanks the turbine can operate ^•continuously. Regarding the flywheel in FIGS. 1, 3 or 5, a good arrangement is to have it stop completely at the end of each stroke so that more time can be taken^'"as desired to adjust temperature ^'of the packing and drain-out water. The flywheel

35 can be ^"held by a brake^" when^' it stops, with no loss in efficiency. It then^' reverses^' direction each time it stops.

OTEC plant 500 may be provided optionally with valve 532 which can be closed at the end of each stroke to slow down the cycle and allow more time to adjust the temperature of the packing when switching between hot and cold. Normally the valve 532 will be wide open. Also, the length of the liquid legs may be set to control the cycle length. Thus, for example, a longer length gives more inertia and therefore a longer cycle. With regard to the water used in the process¬ ing cycle, excess water may be drained out at a proper point in the cycle through conduit 534 having valve 536.

FIGURE 7 shows an improvement on the cold water pipe design for OTEC plants. Such plants have manageable mechan¬ ical design problems in still water but in most practical cases there are variable water currents at different levels that impact on the pipe. Designing for these forces results in very high costs. The present invention overcomes these forces by placing "thrusters" at spaced points vertically to offset the currents. The thrusters can use propellors, moto driven, or a jet of water directed downstream. The water ca come from OTEC discharge.

Construction of the cold water pipe then becomes rather straightforward technology and light plastic can be used. Buoyancy elements may be added to offset the weight of pipe so that all this weight does not hang from the top connectio

More particularly, in FIGURE 7, OTEC plant 600 which is disposed toward the surface 602 of the ocean is connected to cold water pipe 604 having an upper end 606 and a lower end 608 which is disposed above ocean bottom 610. Cold water pipe 604 is at least partially supported by a plurality of buoyancy rings 612 and 614 disposed about the outside surfac thereof at spaced points. Thruster 616 is also disposed on the outersurface of cold water pipe 604 and includes rotatable rings 618 and 620 motor 622 which drives propellor 624, and rudder 626. Cold water pipe 604 is also provided with a second propellor 628 driven by motor 630 so as to maintain a slight positive pres sure inside cold water pipe 604 to thereby maintain the wall thereof in tension rather than compression to avoid their collapse due to water pressure.

In the operation of the thruster system, the propellor 624 pulls the cold water pipe 604 upstream and the rudder 626 provides a self-aligning mechanism to maintain the thruster system in the proper orientation. Instead of using' the combination of the motor- 622 and propellor 624, other 5 equivalent mechanisms may be used such as a water jet, for example.

FIGURE 8 shows a combination of an OTEC plant 700 and a mariculture system 702 which includes a plurality of culture vessels 704, 706" and 708 designed to produce different types of sea food at different' temperatures. For example, culture vessel 704 may be ^"a solar pond designed to operate at a tem¬ perature of 80° to 90°F. to raise shrimp, kelp, mullet and other fish. Similarly, culture vessel 706 may be designed to operate at a temperature of 80°F. to raise shrimp at an optimum temperature for their culture. Also," culture vessel 708 may be designed to operate at a temperature of 60 to 65° to raise lobster at an optimum temperature for their culture. In this operation of the system shown in FIGURE 7, water at a temperature of about 90°F. is introduced into OTEC plant 700 by means of warm water conduit 710. Water from the OTEC plant may be withdrawn through conduit 712 at a^' temperature of about 80°F. and introduced into culture ^"vessel 706 through conduit 714. The water from culture vessel 706 (including waste products and .unused food) exits therefore through con- duit '716 and is then passed through conduit 718 to culture vessel 704. As ^"indicated above,^' vessel 702 may be a solar pond which is designed to increase the temperature of the water as it passes therethrough from 80° to 90°F.

Water removed from OTEC plant 700 (including waste products and unused food) may also be passed through conduit 720 into culture vessel 708. Cold water from the ocean may then be introduced through conduit 722 into vessel 708 in a - proportion sufficient to reduce the overall temperature to about 60 ° to 65°F. Water exiting from vessel 708 may then be passed through conduits 724 and 718 to culture vessel 704. Combinations of an OTEC plant and a mariculture system have a number of important advantages such as: 1. the combination saves pumping on warm and cold water,

2. the cold water has been warmed up to usable temperature,

3. the water has absorbed oxygen from the high pressure gas to several times that of normal seawater (dissolved oxygen is a major limiting factor in setting the growing capacity of mariculture systems) , and

4. solar heating is..particularly useful in both OTEC and mariculture.

In mariculture of animals, it is essential to avoid water that is supersaturated in nitrogen as it causes the

"bends" due to gas bubbles^'. Therefore; the mariculture tank ^• are located at sufficient depth to avoid this; water from OTEC may or may be fully saturated. Wastes are returned to surface pond in sunlight where algae grow to feed oysters, etc., along with kelp which calms the water and is harvested Wastes provide nutrients along with other sources such as nutrients in water from the bottom. In case of storm, the •facilities may be lowered to 30-100 feet for protection. It will be attractive ^"to operate mariculture, or more broadl aquaculture, at pressures such as 2-20 atmospheres and add air or oxygen^" to increase solubility of oxygen in the water. This will allow higher production rate or yield per acre per year, which is needed for best economics. It also applies t all OTEC systems but is especially desirable in the case of the present invention as loss of air due to solubility is inherent in the present invention and is a debit for OTEC, but is more than offset by the credit in aquaculture.

Nitrogen also dissolves in the water and if aquaculture is at too low a pressure then nitrogen bubbles are released within the animals, causing the well-known divers "bends", as referred to above. Therefore, the pressure should not be much below that corresponding to nitrogen saturation. Nitro gen is less soluble in water than oxygen, and the contacting may not achieve saturation. The main point is to avoid supersaturation of water with nitrogen.

Part or all of the facilities may be located on land, and solar heating is particularly effective for heating whil using cool ocean water for cooling. The following three ^zr types of solar heating may be considered for use in^' these systems of the present invention:

(i) . closed loop circuit with warm water discharged from OTEC at above ambient ocean tem^'perature- (ii) once-through water flow, entering' at above ^'ambient but leaving OTEC at ambient temperature or less, and (iii) solar OTEC system^" on land using closed loop on warm water or oil, etc. (e.g., at 200-300 F.). Cooling is by ocean or a cooling tower.

The solar collectors can concentrate or focus the_. sun to give a temperature of up to 200-500°F. Vapor pressure of water adds to the work output, but oil or other liquid with low vapor pressure improves efficiency and may be preferred to cut the area of solar collectors which may take 100-500 acres for a plant sized to produce 30 megawatts of electri¬ city.

Fresh water can be recovered in the cooling part of the cycle by having a closed system on the cool water. This requires indirect cooling of this water as by heat exchange with cool ocean water. The warm water can be ocean water heated in solar collectors, and directly contacts the gas.

The system is also good, for pumping water. By flowing more water in at low pressure and out at high, the system gives pumping without additional mechanical pumps.

An improved use of solar collectors is to operate them in reverse at night to further cool the cold stream, by radiation to the sky. Normally, they are used only in the daytime and not at night. Some storage of cold and/or warm water may be desirable. The principle is similar to frost forming on a still night when the temperature is well above freezing.

Mariculture can operate once-through on the water so that it does not have to be cleaned up, or it can be recycled in a closed circuit, in which case heat must be added to maintain the temperature. Solar heating then cooperates well with mariculture in the solar pond. If a conventional OTEC

^"BUREA - 4 0 -

plant is ursed with ammonia boiling at say 100°F. then the warm water must leave the OTEC ^• plant, at above 100°F.. and this is fatal for animals such as shrimp and fish..^'' However^", in the ^'case of the^{" "}present inven tion, a much ^"grea^'ter^" temperature ^'drop on the water can be used effectively, to drop the discharge temperature to a • level suitable ^"for mariculture.^" Thus, the same^"100°F. may achieved with ^"water entering at say 120°F. and leaving at 85^CF. which is very suitable.^' Similarly, on the cold.water a .conventional OTEC plant will raise it by only about 5°F. to say 45*-50°F. which ^"is too low even^' for northern lobsters. In contrast, the presen invention may be ^'designed to give 60°-65°F.. on the ^'cold wate discharge,^' which ^'is suitable for growing lobsters, salmon, etc.

Solar heating of the warm water^" is very desirable ^"to improve output and economies^', but"a closed circuit on the warm water is then^' almost essential to avoid an enormous solar area^". ^• Then^" the^{" "}water^" in this solar pond, is too hot fo aquaculture.^" The solution to this problem is to have a temperature gradient in the pond from^' top to bottom ..- e.g., 9Q°-100°F. at the^" top, and 8Q°-85°F. at the bottom. Mixing from top to bottom must be ^"restricted b devices^" such as natural kelp or artificial seaweed or plastic, sheet^'s or ribbons." Warmer.water has lower density which also helps.

The final stages^' of solar superheat may be ^"enclosed as shown in FIGURE 2 ^'of the ^'drawings.

Of course,^" the ^'alternative methods and equipment disclosed can be ^'combined in arrangements different from those ^"shown in the ^'figures^' and examples^' to'satisfy specific needs. Thus, while ^'I have ^"described certain preferred forms of my invention and certai .methods of its use, it will be understood that this invention may be otherwise embodied within the- ^'scope ^'of the^' following claims.

'

Claims

1. An energy conversion system which comprises means for providing a source of relatively warm liquid, means for providing a source of relatively cool liquid, means for alternately contacting a confined gas with said warm liquid to expand said gas and with said cool liquid to contract said gas in a cycle to thereby produce a pressure differential, means for putting in work in the expansion portion of said cycle and for taking out work in the contraction portion of said cycle under conditions wherein the working gas is expanded to a volume greater than that which would result from the change in temperature alone, and means for producing work by use of the pressure differen¬ tial of said gas.

2. An energy conversion system according to Claim 1 wherein said liquid is water and the sources of relatively warm and cool water is the ocean and said gas is air.

3. An energy conversion system according to Claim 2 includ¬ ing a plurality of solar collectors adapted to warm water and provide the source of relatively warm water.

4. An energy conversion system according to Claim 3 including means for returning the warm water used to contact the air to said solar collectors for reheating and subsequent use in heating said air.

5. A system according to Claim 1 wherein said warm and cold liquids are directly obtained from and returned to the environment without substantial heat being transferred thereto or therefrom for use in said system.

6. An energy conversion system which comprises means for providing a source of relatively warm water, means for providing a source of relatively cool water, means for contacting a confined gas with said relatively 5 warm water to thereby expand said gas, means for contacting said gas with said relatively cool water to thereby contract said gas, means for alternately contacting said gas with said warm water and said cool water to thereby alternately warm an

10 cool said gas in a cycle to thereby produce a pressure differential, means for putting in work in the expansion portion of sa cycle and for taking out work in the contraction portion of said cycle under conditions wherein the working gas i

15 expanded to a volume greater than that which would resul from the change in temperature alone, and means for providing work by means of the pressure differ ential of said gas.

7. An energy conversion system according to Claim 6 wherein 20 the source of relatively warm and cool water is the ocean and said gas is air.

8. An energy conversion system according to Claim 7 wherein the contacting means and work producing means are located at substantially the ocean surface and the average air pressure is in the range of about 1 to 5 atmospheres.

25 9. An energy conversion system according to Claim 7 wherein the contacting means and work producing means are located at an ocean depth of about 200 to 600 feet and the average air pressure is in the range of about 5 to 10 atmospheres.

30.

10. An energy conversion system according to Claim 7 wherein the contacting means and work producing means are locat¬ ed at substantially the ocean bottom and the average air pressure is greater than about 10 atmospheres. - -

11. An energy conversion system according to Claim 7 including a plurality of solar collectors adapted to warm water and to provide the source of relatively warm water.

12. A system according to Claim 6 wherein said warm and cold water are directly obtained from and returned to the environment without substantial heat being trans¬ ferred thereto or therefrom for use in said system.

13. An energy conversion system according to Claim 6 in combination with at least one mariculture growing

. means.

14. An energy conversion system according to Claim 6 wherein said means for providing a source of relatively cool water is provided with at least one thruster designed to offset water currents.

15. An energy conversion system according to Claim 6 wherein said means for contacting said gas with said relatively warm water comprises a resilient sponglike material.

16. An energy conversion system according to Claim 6 wherein said means for contacting said gas with, said relatively cool water comprises a resilient sponglike material.

17. A method of energy conversion which comprises alter¬ nately contacting a confined gas with relatively warm liquid to expand said gas and with relatively cool liquid to contract said gas to thereby produce a pres¬ sure differential under conditions wherein the working gas is expanded to a volume greater than that which would result from the change in temperature alone and producing work by means of said pressure differential.