Supercritical Thermal Power System Using Combustion Gases For Working Fluid

Abstract

A supercritical thermal power system including components conventionally luded in a Rankine cycle, uses a portion its own combustion gases as its only working fluid. The system recirculates all the combustion gases, cools them, and purges the excess amounts from the system. The cooled remainder portion is reheated to conserve energy and mixed with oxygen and fuel in the combustion chamber to lower the temperature of the burning gases to pass cooler combustion gases to a turbine for minimizing failure otherwise due to excessive heat in the system. By using a portion of the system's own combustion gases as the only working fluid, the system's overall efficiency is significantly increased over contempory systems.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

A wide variety of thermal power systems operating generally within a Carnot cycle have been developed throughout the years. One notable example is the Walter engine which called for the direct combustion of hydrogen peroxide and diesel oil in a combustion chamber and feeding the combustion gases through a prime mover to effect an energy conversion. A disadvantage of the Walter engine resided in its having to tolerate the hot combustion gases being directly fed into the prime mover. Since the gases by being so hot tended to damage the prime mover, water was sprayed into the combustion chamber to bring the combustion temperature within a workable range; however, the heat required to evaporate the cooling water seriously degraded the system's efficiency. In addition, the Walter engine, an "open-cycle" system required that the exhaust gases be pumped out of the system. This creates considerable design difficulties, especially where the ambient pressure is high, as is the case at great depths in the ocean. An alternate system approach is to design a "closed-cycle" power system using a secondary recirculating fluid for effecting a power transfer when it is heated by combustion gases in a boiler-like chamber. An immediate disadvantage of this approach is apparent since a relatively inefficient heat transfer occurs as the gases heat the secondary recirculating fluid to seriously lower this system's overall efficiency. An attempt to raise the efficiency in a "closed-cycle" system has been proposed by Ernest G. Feher in U.S. Pat. No. 3,237,403 issued Mar. 1, 1966 in his "Supercritical Cycle Heat Engine." This heat engine used CO.sub.2 at supercritical temperatures and pressures as the secondary recirculating fluid to function as the working fluid in a modified Rankine cycle. The advantages of employing carbon dioxide as the working fluid at its supercritical temperature and pressure levels are disclosed and thoroughly explained in the Feher Patent and this publication provides a nearly complete background for a thorough understanding of the present invention. However, with the Feher approach, a serious loss of efficiency burdens the engine which is created at the heat transfer interface between the heater element and the recirculating CO.sub.2. The Feher engine does not directly use the combustion gases to drive the turbine but rather relies on a conventional heat exchange in a heater for system power. Here again, direct employment of uncooled combustion gases as the working fluid in a turbine has been avoided due to the combustion gases high temperature which causes heat damage to the turbine.

SUMMARY OF THE INVENTION

The present invention is directed to providing a supercritical thermal power system using its combustion gas as its only working fluid and includes a turbine receiving the combustion gas to provide an energy conversion. A regenerator and condenser receive the combustion gas expelled from the turbine and condense the excess amounts of water vapor and CO.sub.2 to either expel or to store them. While passing through the regenerator in the opposite direction, the residue of the carbon dioxide is raised above its critical temperature and assumes the gaseous state. The gaseous residue is fed to a combustion chamber where oxygen and a distillate fuel are being burned to form more combustion gas. In the chamber the gaseous residue is mixed with the burning oxygen and fuel to cool the resultant combustion gas which is vented to drive the turbine. Directly employing the combustion gases as the working fluid results in a higher overall system efficiency and by mixing the gaseous residue with the burning oxygen and fuel, the temperature of the combustion gases is lowered to prevent heat damage to the turbine.

A prime object of the invention is to provide a thermal power system having a higher overall system efficiency.

Another object is to provide a thermal power system using its combustion gases as its only working fluid.

Still another object is to provide a system using recirculating combustion gases comprised mostly of CO.sub.2 at supercritical temperatures as its working fluid.

Yet another object is to provide a closed thermal power system designed to store its combustion by-products.

Still another object is to provide a closed thermal power system having a high discharge pressure to overcome ambient pressures found at extreme ocean depths.

A further object is to provide a thermal power system burning low-cost hydrocarbon fuel and oxygen at low temperatures not requiring elaborate heat exchangers or feed water injectors.

Still another object is to provide a thermal power system of small size yet having a high overall efficiency.

These and other objects of the invention will become more readily apparent from the ensuing description when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of the invention.

FIG. 2 is a schematic representation of another embodiment of the invention .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, two representative thermal power systems are depicted each embracing the heart of the present invention, that being, using the combustion gas as its only working fluid to effect an energy conversion. In both of these systems the specific temperatures and pressures discussed in the specification and appearing on the drawings are for purposes of demonstrating specific embodiments. If different efficiencies are tolerable, then variation from the identified temperatures and pressures is permissible within the teachings of the invention.

A turbine 10, preferably a single stage turbine, is assembled to allow a low gas expansion ratio and is capable of operating at high efficiency over a wide operating range to convert heat energy to rotary motion. The turbine receives combustion gases at 3000 PSIA and 1600.degree. F., exacts an energy conversion, and vents or exhausts them at 1500 PSIA and at 1445.degree. F. A mechanically interconnected load 11 converts the turbine's rotary motion to electrical power, or whatever other energy conversion desired, and a mechanical linkage 11a extends to another system component, the purpose of which will be elaborated on below.

Exhausted combustion gases are received by a regenerator 12 which cools the exhausted gases to approximately 125.degree. F and vents them to its "downstream" side. Within a cross duct 12a in the regenerator the exhausted combustion gases are maintained above the critical 87.degree. F. temperature of CO.sub.2 to retain the CO.sub.2 portion of the combustion gases in its gaseous state. However, within the cross duct, water vapor, formed as a by-product of burning and diffused throughout the combustion gases, is condensed and flows along with the other combustion gases at the duct outlet.

The physical construction of the regenerator optionally assumes one of many of a variety of different configurations. A typical regenerator is no more than an elongate cylinder having a pair of concentrically, coaxially disposed ducts the inner one 12a passing the exhaust gases in one direction and the outer one 12b having a cooler fluid traversing in the other direction, as will be explained later, to cause the desired temperature drop in the exhaust gases.

Following the regenerator, a coil 13a in a condenser 13 receives the cooled combustion gases, now lowered to the 125.degree. F. temperature at approximately 1450 PSIA pressure. A sufficient quantity of sea water is pumped through another coil 13b in the condenser to perform a direct conductional cooling of the combustion gases bringing the carbon dioxide below its critical temperature of 87.degree. F. Because CO.sub.2 pressurized to 1100 PSIA, or greater, undergoes a direct change of state from gaseous form to liquid form, or vice versa, when brought to its critical temperature of 87.degree. F., the carbon dioxide in the present system, being under a pressure of approximately 1450 PSIA, undergoes a direct transition to change from the gaseous state to the liquid state. Such an immediate change in state takes place in the condenser by the heat transfer occurring between the carbon dioxide in condenser coil 13a and the sea water in condenser coil 13b.

Having liquid CO.sub.2 and liquid water in the system "downstream" from coil 13a permits the purging of carbon monoxide or other noncondenseable gases also created as by-products of the burning process. A simple valve arrangement 14 allows purging and disposal of these by-products to increase the system's overall efficiencies. In addition, when the amounts of water vapor and carbon dioxide created during burning are beyond the quantity needed for maintaining a suitable level of working fluid this excess amount of the liquid form of the combustion gas is vented at this point. For more efficient operation, most of the water is purged to retain carbon dioxide as the system's major working fluid.

With the remainder of the combustion gases, that portion not being vented off at valve 14, still in the system, a pump 15 draws it in at 1450 PSIA pressure at 68.degree. F. The pump increases the remainder's pressure to 3050 PSIA and slightly raises its temperature to 85.degree. F. Although the temperature of the remaining gases, optionally, is raised above the critical 87.degree. F. temperature in the pump to convert it to the gaseous state, system efficiency was enhanced by feeding it back through regenerator 12 via the above mentioned longitudinal duct 12b.

The heat transfer between the combustion gases vented from the turbine through longitudinal duct 12a and the remainder of combustion gas passing through longitudinal duct 12b is mutually beneficial to both fluids. The hot gases in duct 12a are cooled by the liquid remainder in duct 12b to condense water vapor and to lower their temperature. Simultaneously, the liquid remainder in duct 12b is heated above and beyond its critical temperature to change to the gaseous state and to raise the remainder's temperature to approximately 1315.degree. F. at 3020 PSIA. Thusly, the heated remainder has been appropriately preprocessed to be recirculated and recycled back to a combustion chamber 16.

The combustion chamber is connected to receive oxygen and a distillate fuel from a source of oxygen 17 and a source of fuel 18. Suitable valving is provided to ensure that the proper volumes of each are fed to the combustion chamber for complete burning.

Since the stoichiometric burning of diesel oil, for example, in oxygen produces a burning temperature of approximately 6000.degree. F., most combustion chambers are unable to contain such a high temperature, with the exception of heavy, massive chambers made of substantial volumes of high refractory materials. In the present invention, to lower the internal temperature of the combustion chamber, the remainder of the combustion gases of the previous cycle are recycled back into the combustion chamber to mix with the burning oxygen and fuel. Further protection of the chamber from overheating is ensured by directing the flow of recycled gas to line the chamber inner walls insulating them from the flames. The temperature of combustion is significantly reduced by the recycled remainder and the temperature of the burning combustion gases is brought considerably below that temperature which causes damage to the chamber and the following turbine. Since temperatures in excess of 3000.degree. F. may cause heat damage to a stainless steel coaxial boiler or a conventional turbine, bringing the gases ducted to the turbine to a temperature of 1600.degree. F. at 3000 PSIA, leaves an adequate margin of safety. Thus, by the mixing action of the remainder of the combustion gases of the preceding cycle with the burning oxygen and fuel, the overall thermal efficiency of this system is measured to approach 42 percent, far above the efficiencies reached by contemporary systems calling for the injection of water coolants in the combustion chamber.

By prepressurizing the system with carbon dioxide at approximately 1450 PSIA and including a mechanical linkage 11a, shown in phantom between load 11 and pump 15, the system is readied for immediate operation.

A modification of the aforedescribed system is shown in FIG. 2 which, in addition to having the components already identified, includes a water separator 19 connected on the "downstream" side of longitudinal duct 12a. The water separator passes condensed water at 1475 PSIA and 125.degree. F. to a water storage compartment separated from the fuel oil in source 18 by a flexible wall 18a. The pressurized condensed water acting on the flexible wall tends to force the remaining volume of fuel oil into the combustion chamber at a high flow rate. A metering pump 18b is desirably included to precisely regulate the fuel flow to the combustion chamber.

In a similar manner, a feeder line on the "downstream" side of pump 15 feeds excess liquified CO.sub.2 to a CO.sub.2 storage tank 20 and also to a bank of bottles representing oxygen source 17. As oxygen is used and the bottles become empty, excess CO.sub.2 is stored by merely opening an ingress valve 17a and shutting an egress valve 17b. Here again, a metering pump 17c allowing the selective change from one oxygen bottle to the other and for regulating the bottle's flow rate is highly desirable to ensure reliable system operation.

An advantage of storing the recovered products of combustion in, for example, a submersible where precise bouyancy and trim control are important, becomes apparent when it is noted that the energy-transfer process imparts no gain or loss of weight making weight or trim compensation unnecessary.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings, and, it is therefore understood that within the scope of the disclosed inventive concept, the invention may be practiced otherwise than specifically described.

Claims

What is claimed is:

1. A supercritical thermal power system producing and using a portion of its composite combustion gas as its only working fluid comprising:

a regenerator having a first duct and a second duct receiving said combustion gas in said first duct at a first pressure and a first temperature for venting said combustion gas at a reduced second pressure and second temperature while providing an energy conversion said first pressure and said second pressure being in excess of 1100 PSIA;

a condenser coupled to said first duct for lowering the temperature of said combustion gas below a critical temperature for effecting an immediate change of state of the carbon dioxide in said combustion gas to a liquid form;

means connected to the said condenser for purging an excess quantity of said liquid form of said carbon dioxide from said system while retaining a remainder of said liquid form in said system;

means joining said condenser to said second duct for increasing the pressure and temperature of the remainder of said liquid form of said carbon dioxide to a value substantially corresponding to said first pressure and to a temperature level above the critical temperature of said liquid form of said carbon dioxide to change the state of said remainder to a gaseous fluid at the output of said second duct;

a source of oxygen;

a source of distillate fuel; and

a combustion chamber connected to the oxygen source, the distillate fuel source, and the output of said second duct, upon feeding said gaseous fluid to said combustion chamber during combustion of said oxygen and said distillate fuel, resultant combustion gases are cooled to a magnitude which prevents heat damage to said system and which ensures higher system efficiency.

2. A system according to claim 1 in which said regenerater passes said combustion gas at said second pressure and said second temperature to effect a partial reduction of temperature of said combustion gas and said condenser is serially connected and water cooled to further reduce the temperature of said combustion gas below the critical temperature of carbon dioxide ensuring its change of state to said liquid form.

3. A system according to claim 2 in which said increasing means includes a pump joined to said second duct, said remainder quantity is changed in state to said gaseous fluid by the heat of said combustion gas at said second pressure and said second temperature, thereby ensuring the raising of the temperature of said gaseous fluid to a level above its critical temperature.

4. A system according to claim 3 in which the purging means includes a water separator having a water feeder line connected to said distillate fuel source for replacing burned fuel with condensed water vapor and said water cooled condenser is provided with a CO.sub.2 feeder line connected to said oxygen source for replacing burned oxygen with condensed CO.sub.2 rendering said system completely closed with in-system storage of combustion by-products.

5. A method of raising the overall system efficiency of a supercritical thermal power system by using its composite combustion gas as its working fluid comprising:

extracting work from said combustion gas in an energy converter;

lowering the temperature of said combustion gas below the critical temperature of carbon dioxide;

condensing said carbon dioxide to a liquid form;

purging excess said liquid form of said carbon dioxide from said system;

raising the temperature of said liquid form of said carbon dioxide to a gaseous form;

mixing said gaseous form of said carbon dioxide in a combustion chamber with a hot combustion gas created as oxygen and distillate fuel burns in said combustion chamber said gaseous form being cooler than said hot combustion gas to cool the mixed combustion gas;

feeding the mixed combustion gas to said energy converter for preventing heat damage to said energy converter and to raise said overall system efficiency; and

maintaining said combustion gas at supercritical pressure levels throughout said system to further raise said overall system efficiency.

6. A method according to claim 5 further including:

storing said excess said liquid form of said carbon dioxide making said system closed and self contained.

7. A system according to claim 1 in which a heat transfer occurring as said combustion gas has its temperature reduced from said first temperature to said second temperature raises the temperature of the remainder of said liquid form of said combustion gas above the critical temperature of said liquid form of said combustion gas to ensure a change of state to said gaseous form.

8. A system according to claim 7 in which said first temperature is in excess of 1400.degree. F, said first pressure is in excess of 1475 PSIA, said second temperature is in excess of 100.degree. F, said second pressure is in excess of 1425 PSIA and said critical temperature is substantially 87.degree. F.