Method and Apparatus for Converting Thermal Energy to Mechanical Energy

The present invention relates to mechanical energy generating systems and, more particularly, to a method and apparatus for converting thermal energy to mechanical energy.

As is universally appreciated, the world's supply of conventional fuels, such as natural gas, oil, coal, and the like, is being rapidly consumed and the continued availability of such fuels has, in recent years, been seriously questioned. Although these fuels are utilized for many purposes in our society, perhaps no uses are more important than as a source of thermal energy convertible to mechanical energy for furnishing motive power for vehicles and boats or to electrical energy for powering our households and industries. Of course, numerous alternative energy sources are available and under development, e.g., solar energy, nuclear energy, and the like, and to the extent that these alternative sources are utilized they will partially alleviate the present and prospective fuel crisis. However, nuclear energy, for example, is expensive and its use creates monumental environmental problems which remain unresolved. Solar energy as a source of power is still in the developmental stage and, at least at present, is not totally practical for use in all climates, particularly in areas where cloud, fog or smog cover is frequent. Indeed, there is not yet available a viable alternative to the ever increasing consumption of conventional fuels to supply the power necessary to operate today's society.

There have been however, numerous suggestions for systems which more efficiently utilize energy available from conventional sources by minimizing thermodynamic and fluid dynamic losses. Unfortunately, even these systems fail to significantly improve energy conversion efficiency and, generally, omit even to make maximum use of available energy conversion opportunities. For example, U.S. Pat. No. 3,358,451 discloses a system for converting the energy of a liquid stream to power by heating a liquid working fluid to form a two-phase fluid, accelerating the fluid, separating the liquid and vapor phases, converting the kinetic energy of the liquid phase to work, condensing the vapor phase to liquid and reuniting the liquid streams prior to heating again. This system, however, neglects to extract the maximum work potential from the available energy of the working fluid and, therefore, relinquishes, instead of using, valuable thermal energy to a heat transfer medium in a condenser.

It is therefore an object of the present invention to provide a system for producing mechanical energy which extracts more of the available work from a working fluid than has heretofore been possible.

It is another object of the present invention to provide a method for producing mechanical energy which need not consume or utilize the world's conventional fuel supply.

It is yet another object of the invention to provide an environmentally safe method for producing mechanical energy.

It is still another object of the invention to provide a method for producing mechanical energy from thermal energy where the thermal energy is derived from available ambient energy sources, such as the atmosphere, rivers, oceans, waste heat sources, etc.

It is another object of the invention to provide a method and system which is particularly useful in refrigeration and air conditioning applications and which substantially reduces the energy requirements for such applications.

Other objects and advantages will become apparent from the following description and appended claims, taken together with the accompanying drawings in which:

FIG. 1 illustrates, in schematic form, a preferred embodiment of the method and system of the present invention which utilizes only a single working fluid stream.

FIG. 2 illustrates an embodiment of an energy conversion tube useful in the system of the present invention.

FIG. 3 illustrates a preferred form of energy conversion tube useful in the system of the present invention.

Referring to the drawings, and particularly to FIGS. 1, there is shown a continuous closed cycle system for converting the energy potential of an appropriately selected pressurized working fluid into mechanical shaft energy with system energy losses, including useful shaft work, made up by drawing energy, in the form of heat, from an available thermal energy source, such as radioisotopes, nuclear reactors, combustion heat (particularly from burning of non-conventional fuel sources such as garbage), solar energy and from ambient thermal sources where available in sufficient quantity (e.g., the atmosphere, rivers, oceans, waste heat sources, etc.). The system illustrated utilizes two working fluid streams or paths, a motive stream and a secondary stream. These streams may be of the same or of different working fluids. In another form, the system utilizes only one working fluid stream. Although there are numerous working fluids which may be used, as a general matter any liquid is suitable which is useful in an expansion work cycle taking into account the maximum and minimum temperatures and pressures of the selected cycle and the need for vaporization and condensation therebetween. For a system operating at or slightly above or below normal ambient temperatures those working fluids are most advantageous which are low boiling, and preferably those which boil substantially below the freezing point of water. Typical of these kinds of working fluids are carbon dioxide, liquid nitrogen and the fluorocarbons. Exemplary of useful fluorocarbons are difluoromonochloromethane, pentafluoromonochloroethane, difluorodichloromethane, and the mixtures and azeotropes thereof. For higher temperature, higher pressure systems the fluids may include water or other well known coolants, even including the liquid metals, e.g., sodium, potassium, mercury, and the like.

After passage through the tube 14 of the heat exchanger in the practice of the invention, the working fluid stream is directed to a means for converting the static pressure or potential energy of the stream to velocity or kinetic energy, such means hereinafter referred to as an energy conversion tube (ECT), as will be more fully described. In the ECT the velocity of the stream is caused to increase while at the same time causing the static pressure and temperature of the stream to substantially decrease. As the pressure decreases, some of the thermal energy contained in the liquid is liberated and a portion of the liquid is vaporized. The resulting stream contains an increased proportion, by volume, of vapor.

In a preferred form of the invention, the energy conversion tube includes a plurality of longitudinally spaced apart nozzle sections (see FIGS. 2 and 3) interconnected by a plurality of recovery sections. The liquid working fluid, having high potential or static energy (high static pressure), and being substantially saturated, as defined hereinafter, enters the first nozzle section and is accelerated therein to convert it to a high velocity, relatively lower static pressure stream or jet of fluid flowing axially through the tube. The velocity of the fluid increases, as does the kinetic energy, due to the decreasing cross-sectional flow area as the fluid moves through the nozzle section. As the fluid accelerates and the static pressure thereon decreases, the saturated liquid begin to vaporize and, in so doing, consumes some of its thermal energy. The result is an increased kinetic energy, decreased static pressure, decreased temperature and increased vapor content stream exiting the nozzle section. By "substantially saturated" as used herein, it is meant that the liquid is either saturated or so nearly saturated that under the flow conditions experienced in the first nozzle section, the liquid will vaporize at least in part. Most preferred is the condition wherein the liquid is in fact at saturation at the entrance to the first nozzle section of the ECT.

The high velocity fluid stream, consisting of a relatively high velocity liquid fraction and a substantially higher velocity vapor fraction, exits the nozzle section and enters a pumping and recovery section wherein the momentum of the vapor fraction is converted to additional velocity and increased temperature and static pressure of the liquid fraction. This is accomplished by transferring a portion of the kinetic and thermal energy of the vapor fraction to the liquid fraction whereby the liquid fraction is energized or regenerated for another expansion cycle through the next nozzle section. It is believed that in the pumping and recovery section, consistent with the conservation of momentum, the relatively fast moving vapor impacts with the relatively slow moving liquid resulting in a momentum exchange between liquid and vapor and a reduction in vapor fraction velocity. The velocity reduction is accompanied by a static pressure increase without a net expense of work (compression process) causing at least a portion or the vapor to condense and to transfer its latent heat of condensation to the liquid fraction. The net effect on the motive stream working fluid is to further increase the velocity and kinetic energy of the liquid, to condense most of the vapor, to recover a portion of the static pressure which was converted to kinetic energy in the nozzle section, and to increase working fluid temperature (to a value higher than at the nozzle section exit but lower than at the nozzle section inlet) so that the increased static pressure, increased temperature liquid is ready for further acceleration and expansion in the next nozzle section. The progressive temperature increase of the liquid through the pumping and recovery section is due to the vapor continuously applying its stagnation pressure to the liquid during the pumping process. This repeated vaporization-condensation sequence, occuring within the energy conversion tube, directs the work of vaporization downstream toward the low pressure area rather than more or less uniformly dissipating it in all directions. The effect is to create a pumping action upon the liquid stream with the result that a higher velocity, and thus a higher kinetic energy, is imparted to the liquid stream exiting the tube than would be observed in simple nozzling devices. Moreover, the use of a plurality of spaced nozzle sections permits a portion of both the latent and kinetic energy content of the vapor following initial nozzling to be transferred back to the liquid where its thermal portion is susceptible of reuse and conversion to additional kinetic energy. By contrast, in conventional systems, for example where only a single nozzle section is used, the latent thermal energy of the vapor is largely relinquished to the heat transfer medium in a condenser rather than being further used to do additional work. This recovery and reuse of the latent heat lies in the heart of the improved performance obtainable with the system of the present invention.

An energy conversion tube in its simplest form is shown in FIG. 2 at 100 and includes a plurality of spaced apart nozzle sections 102, 104, 106, 108 interconnected by a plurality of generally cylindrical recovery sections 110, 112, 114. In the embodiment illustrated in FIG. 3, energy conversion tube 200 comprises a plurality of longitudinally diverging diffuser sections 210, 212, 214. It is generally preferable to utilize diffuser sections as the recovery sections since they act as a controller preventing the static pressure front within the tube from being so low at any point that radial, rather than downstream directed, expansion of a liquid droplet occurs.

The energy conversion tube also desirably includes means for disrupting metastable flow conditions therein. Any of the numerous well known methods for disrupting metastable flow are suitably used. For example, a secondary flow stream, such as a mercury stream, may be added to the working fluid stream. Alternatively, mechanical means may be used, such as interposing diverters or turning vanes in the working stream. Still other methods for disrupting metastable flow involve use of non-mechanical means, e.g., using sound or radio waves. By disrupting metastable flow while at the same time utilizing spaced apart nozzle sections wherein the flow area of each nozzle section is smaller than the flow area of nozzle sections upstream thereof, the desired pumping action in the tube and increased liquid flow velocity can be achieved.

Referring to FIG. 1 it will be appreciated that the fluid employed can be any of the working fluids described hereinbefore. Since the system of FIG. 1 is continuous and closed, for descriptive purposes the inlet to ECT 50 has arbitrarily been selected as the system starting point. The fluid enters ECT 50 in liquid form, preferably saturated, at temperatures and pressures corresponding to an enthalpy content at least as high as the anticipated energy losses, including useful shaft work, in the system. In passing through ECT 50, which is a device similar to the energy conversion tube hereinbefore described, the potential energy of the fluid stream is partially converted to kinetic energy and the velocity of the stream is considerably increased while the static pressure and temperature of the stream is considerably decreased. Since kinetic energy is a function of both mass and velocity, where it is desirable to decrease flow velocity without decreasing stream kinetic energy the mass of the stream can be increased by adding a secondary flow stream, e.g., mercury. The addition of the secondary stream decreases the overall flow velocity while the mass increase keeps the kinetic energy substantially constant. The secondary stream also functions as an aid in disrupting metastable flow in the ECT. Vaporization of a portion of the liquid stream occurs within ECT 50 changing the physical nature of the stream from substantially all liquid to largely vapor, e.g., about 70-90% vapor by volume although only about 10-30% vapor by weight.

The relatively high velocity stream exiting ECT 50 impinges directly upon the blade portion of turbine 52 to convert a portion of the kinetic stream energy to mechanical shaft energy of the turbine. If desired, an optional throttle valve (not shown) can be inserted downstream of ECT 50 and upstream of turbine 52 to control the amount of kinetic energy converted in the turbine.

The stream exiting the turbine is spent and, if the system is to be continuous and closed and the spent stream is to be recycled, the energy content of the stream must be raised. This is preferably achieved by separating the liquid and vapor fractions of the turbine exhaust stream, increasing the static energy content of each fraction, and then recombining the fractions. One means of accomplishing the separation is by gravity separation, for example by vertically stacking the liquid and vapor fraction removal lines 54 and 56, respectively, with liquid line 54 below vapor line 56. In addition the diameter of vapor line 56 is made considerably larger (for example by a factor of 10) than the diameter of liquid line 54 to encourage the vapor-liquid separation. Vapor compressor or vapor pump 58 in line 56 and liquid pump 60 in line 54 operated by the shaft energy produced by turbine 52 increase the static pressure and energy content of the vapor and liquid fraction streams. At the same time the vapor compression in pump 58 increases the vapor temperature to a value considerably in excess of the liquid fraction temperature. The liquid and vapor streams are reunited downstream of pumps 58 and 60. The cold liquid serves as a heat sink for the vapor, for example by passing the vapor onto a thin film of the liquid in conventional fashion, causing substantially immediate vapor condensation so that the combined streams at the inlet to pump 62, which is also operated by the shaft energy produced by expansion engine 52, is substantially all liquid. Pump 62 is a liquid pump which increases the static pressure of the liquid stream to a pressure sufficient to handle, without vaporization, the thermal energy increase which the stream experiences in absorber section 64. At the same time, the pump increases the energy content of the stream.

Pumps 58, 60 and 62 also preform the necessary function of rapidly removing the turbine exhaust stream to prevent back pressure build-up at the turbine which could adversely affect its efficient operation. It will, of course, be appreciated that it is not necessary to employ a three pump arrangement as shown in FIG. 2. Instead, for example, a single centrifugal pump can be used in lieu of pumps 58-60, or indeed, any pump configuration is suitable which will perform the two basic functions of pumps 58, 60 and 62, i.e., to remove the turbine exhaust stream and to increase the static head and energy level thereof.

The liquid stream leaving pump 62 is directed through an absorber section 64 thermal energy is added to the system to make up for energy converted to mechanical shaft energy in the turbine 52 and for energy losses due to friction and other thermodynamic inefficiencies elsewhere in the system. The absorber section 64 should be of sufficient length or area to permit absorption of some predetermined quantity of energy and, to this end, the absorber section 64 includes control means (not shown) whereby the quantity of energy absorbed can be closely controlled. In passing through absorber section 64, the stream is re-energized to the desired extent by thermal absorption. The temperature of the stream leaving pump 62 should be considerably below the temperature of the thermal source in order that the stream may be re-energized to the desired extent by drawing upon the thermal energy available from the source. The amount of thermal energy added to the stream must be sufficient to make up for the energy converted to mechanical shaft energy in turbine 52 which is not added back into the stream in pumps 58, 60 and 62 and for energy losses due to friction and other thermodynamic inefficiencies elsewhere in the system. It will be appreciated, however, that whatever the thermal source employed, thermal energy must flow from it to the system. For this reason, until the thermal source is selected and its thermal conditions are defined, an appropriate working fluid and the pressure and temperature parameters of any particular system cannot be finally selected. The stream leaving absorber section 64 is substantially saturated and has a sufficient energy content to start the cycle over again at the inlet to nozzle 50. Alternatively, if desired to control the net power output, some thermal energy could be added to the fluid prior to adding the static energy thereto.

The following example is intended to illustrate one set of operating parameters for a system in which the thermal energy source is the ambient (postulated to be in the range 85.degree.-100.degree. F) and in which Freon 22, commercially available from E. I. duPont de Nemours & Company, Inc., is utilized as the working fluid.

Freon, difluoromonochloromethane, was employed as the working fluid in the system illustrated in FIG. 1. The fluid energy content parameters in BTU per pound of working fluid at the indicated locations in the system as well as energy additions (+) to and energy losses (-) from the system are set forth below:

Assuming the working fluid to be a liquid having no substantial velocity, all of the fluid energy is potential, i.e. static and thermal (hereinafter "PE") and none is kinetic (hereinafter "KE"):

Pe = 33.7 btu/lb.

Ke = 0

pe = 23.7 btu/lb

Ke = 10.0 btu/lb

Pe = 27.1 btu/lb.

Work done by fluid in turbine, assuming a turbine inefficiency loss of 33% = -6.6 BTU/lb.

Thermal energy transferred from source = +4.2 BTU/lb.

As hereinabove indicated, once thermal source temperature conditions are known and the desired turbine mechanical energy output is selected, the turbine exhaust stream temperature can be determined and from this value an appropriate working fluid and minimum pressure and temperature operating parameters can be identified. In this connection, while it is appreciated that there is considerable latitude, from the standpoint of operability, in selecting temperature and pressure operating parameters, the size and cost of the system components are closely related to the operating temperatures and pressures. Thus, as a practical matter, the ultimate use of the system, e.g., for vehicle motive power wherein size may be critical or as a home power source wherein size may not be very important, may influence the physical size of the system and thereby limit the choice of operating temperature and pressure parameters.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications can be made by those skilled in the art without actually departing from the scope of the invention. Accordingly, all modifications and equivalents may be resorted to which fall within the scope of the invention as claimed.