Patent Application
20120006024
This invention relates generally to power generation from heat sources, and more specifically to those that are of relatively moderate to low temperature and having the characteristic that the temperature of the heat source decreases as heat is transferred (“sensible heat source”). There is an abundance of medium to low temperature heat sources from which it is difficult, inefficient, or unfeasible to produce power using standard practices. A thermodynamic cycle that efficiently combines a sensible heat source and a heat sink to generate electrical or mechanical power is necessary to harvest this vast resource.
Thermodynamic cycles, that convert heat into power, have existed for hundreds of years. The Rankine cycle was used in early steam engines and is still the most common means of power production. The cycle consists of pressurizing a working fluid or liquid, heating it until it has boiled and is all vapor, expanding the high pressure vapor through a turbine to a lower pressure, and completing the cycle by condensing the low pressure vapor. The Rankine cycle was first used with water/steam but the cycle is not limited to this working fluid. The choice of working fluid depends on the operating temperature of the cycle. Using other working fluids, Rankine cycles have been used to produce power from lower temperature heat sources.
Rankine cycles have an inherent limitation when absorbing heat from a heat source. The working fluid undergoes a constant temperature boiling, whereas the heat source is most often a sensible heat source, i.e., the temperature steadily decreases as the heat source gives up energy. This prevents the Rankine cycle from utilizing all of the usable heat from the heat source, a condition which is magnified with low temperature heat sources.
Power cycles have been devised which utilize a mixture of components to produce variable temperature vaporization, enabling more heat energy to be usable for conversion. A two-component vapor expander (“TCVE”) cycle using ammonia an water is an example. Variable temperature vaporization is achieved, producing for example a mixture of ammonia rich vapor and water rich liquid. While more heat energy is available for conversion, the TCVE cycle and other cycles suffer from substantial complexity and high cost due solely to the requirement to efficiently provide 100% vapor for a vapor expander.
In order to fully utilize relatively moderate to low temperature heat sources for power production, it is necessary to develop a thermodynamic cycle that will not only efficiently generate power but that will eliminate the inherent limitations and complexity that are experienced by other cycles. In the present invention this result is achieved in a new cycle unexpectedly by using a liquid and gas (two-phase) expander in conjunction with a multi-component working fluid.
It is a major object of the invention to provide a solution to the above described problems and needs. A major object of the present invention is to provide a thermodynamic cycle to generate power efficiently from a sensible heat source and a heat sink.
A further object is to provide a thermodynamic cycle that allows power generation from sensible heat sources at a low cost.
A further object is to provide a thermodynamic cycle that utilizes an efficient expander to generate power from widely variable thermodynamic conditions of multi-component two-phase working fluid.
A yet further object is to provide a thermodynamic cycle that allows for the use of compact heat exchangers.
An added object is to provide a thermodynamic cycle that allows for the use of safe, common, and environmentally friendly working fluids.
Another object is to provide a thermodynamic cycle that allows for the use of low priced power generation equipment.
An additional object is to provide a thermodynamic cycle that allows for the use of standardized equipment.
These objectives are met in the present invention by using a thermodynamic cycle that employs a multi-component mixture that has a variable temperature boiling point and that also uses a two-phase expander, thereby eliminating the complex requirements imposed by the use of a vapor expander in other multi-component thermodynamic cycles.
The use of a two-phase expander enables power generation directly from the high pressure two-phase flow leaving the heat exchanger in the cycle. As a consequence, equipment otherwise required for phase separation, flow partitioning, heat recuperation and flow re-mixing is completely eliminated. In addition, the efficiency of energy conversion is improved and the control requirements reduced, relative to other multi-component cycles.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which objectives are described:
Representative temperature profiles for latent and sensible heat sources are shown in
Sensible heat is energy increase or decrease that causes a corresponding change in temperature. An example of a sensible heat source is hot water. As heat is extracted from the water, the water temperature decreases proportionally to the heat loss. Latent heat change is energy change that is associated with the state change of a substance, and does not cause a corresponding change in temperature. An example of this is boiling water. As heat is added to a pot of boiling water, liquid water is turned into water vapor and the temperature of water in the pot remains constant.
A multi-component cycle uses a working fluid having more than one chemical component to absorb heat from a heat source, convert a fraction of that heat energy into power, and discharge the remaining heat into a heat sink. Unlike the Rankine cycle, which absorbs most of the heat into the working fluid in the form of latent heat, a multi-component working fluid is provided to absorb heat into the working fluid in the presence of variable temperature boiling. Because the multi-component working fluid heating regime is a variable temperature boiling, the vaporizing fluid can more effectively match the temperature profile of sensible heat sources. A representative temperature enthalpy plot for a Rankine cycle and a multi-component cycle using a sensible heat source are shown in
A two-component vapor cycle used is the ammonia-water cycle with a vapor expander (“AWVE”). The AWVE cycle has some of the thermodynamic advantages represented above. However, the limitations of using a vapor expander greatly complicate the thermodynamic cycle, resulting in the need for costly equipment and degradation of the cycle efficiency. For example,
Stream 12 is heated by the separated hot liquid, 16, in a regenerative heat exchanger, 17. This step is required to recover the heat in the separated liquid from a separator, 18, which is required to provide liquid free vapor to a vapor turbine 19. The heated stream, 2, leaving the regenerative heat exchanger 17 has a vapor quality of 0.648. Streams 1 and 2 are mixed at and flow at 3 to a separator, 18. The separator must be provided because the turbine, 19, is a vapor turbine that requires pure vapor to operate. The separated vapor, 4, enters the turbine and is expanded to the exit pressure, 5. The expansion adds power to the turbine and rotates a shaft, 20, which can drive a load, 21, such as an electric generator.
The vapor stream, 4, is rich in ammonia. The original ammonia fraction of about 76% for example has been concentrated to 96.8%. The higher ammonia concentration would result in a high vapor pressure in the condenser 22 making the vapor turbine produce less power. The separated liquid, 6, is rich in water, having a concentration of about 59.3% water. Therefore, the separated liquid, 7, after transferring heat in the regenerator to stream 12, must be mixed with the vapor stream, 8, from the vapor turbine in s mixer 25, before entering the condenser 22. The mixed flows, 7 and 8, enter the condenser, 22. A cooling stream, 23 supplied to the condenser removes the heat of condensation at 24, resulting in liquid, 10a, having the original composition of 76% ammonia.
Table 1 summarizes the complex fluid conditions for an example of this cycle. The ammonia fraction and division and re-mixing of flows must be carefully controlled to achieve the performance shown.
The power and cycle efficiently are summarized in Table 2 for a 75% efficient vapor expander:
Complexity is unexpectedly eliminated, while improving the thermodynamic efficiency, when a multi-component two-phase cycle as disclosed herein is used. The process diagram for the multi-component two-phase cycle (“MCTP cycle”) is shown in
The mixture then flows directly to a two-phase expander, 41. In a Variable Phase Turbine type of expander, the two-phase mixture is expanded in a nozzle, 41a. The two-phase mixture is expanded to a low pressure, producing a high velocity jet. The two-phase jet impinges on an axial turbine assembly, 40. The turbine drives a shaft, 42, which rotates a load, 43, such as a generator. The two-phase flow leaves the expander at a lower pressure and temperature at 53. The flow at 54 enters the condenser, 33, where it is condensed back into a liquid. The heat is removed by a cold stream 56 of water or air, 14. Heated water, 57, can be cooled in a cooling tower. Heated air, 57a, can be exhausted to atmosphere.
To illustrate the MCTP cycle, a calculation was made of heat absorption from a hot water source at 209 degrees F. The two-phase expander efficiency was 75%. The results are shown in Table 3.
The MCTP power output and cycle performance is summarized in Table 4.
The unexpected result of using a two-phase expander in a multi-component cycle, instead of a vapor expander, is that the cycle is greatly simplified, while keeping the same thermodynamic advantages as for a more complex cycle, such as the AWVE cycle. It is also unexpected that while eliminating numerous expensive components and reducing the mixture control and partitioning requirements, the cycle efficiency is actually increased.
By adding a regenerative heat exchanger to the Multi-Component Two-Phase cycle, a further unexpected advantage occurs. Many heat sources have a limit on the minimum temperature in the heat exchanger due to scaling or corrosion. The minimum temperature for the examples given was 122 F, selected only to maximize the cycle net power from the heat source.
The multi-component two-phase cycle with a regenerator is shown in
The high pressure two-phase flow, at 70, then enters the two-phase turbine, 71. The flow is expanded in the turbine to a lower pressure at 72, turning a shaft, 73. The shaft can be connected to a load, 74, such as an electric generator, to generate power. The flow leaving the turbine has a higher temperature than the heat rejection temperature. The flow, 72, enters the regenerator, 64, and transfers heat to stream 62. After transferring the heat, the flow, 75, enters the condenser 76. It is therein condensed and flows, at 77, to the pump, 63, completing the cycle. The composition is constant throughout the cycle.
An unexpected result of using a two-phase regenerator is that a perfect uninterrupted “glide” effect is produced in the primary heat exchanger. This is illustrated in
Table 5, below, provides the state points for the two-component two-phase cycle, with a regenerator, locations or “state points” appearing in
Table 6, below, provides the power and performance of the binary two-phase cycle with a regenerator.
The use of a two-phase regenerator with the two-phase turbine improves the cycle efficiency to 8.1%, a 36% increase above the AWVE cycle for the same hot water flow conditions. The net power is 770 kW versus only 568 kW for the AWVE cycle with the same temperature limit of 144.5 F.
Unlike the Rankine cycle, the inlet to the expander is not dry vapor in the MCTP cycle. This means that the expander must be able to effectively expand two-phase mixtures. Two-phase expanders include axial impulse turbines, radial outflow turbines and positive displacement expanders. A Variable Phase Turbine, U.S. Pat. No. 7,093,503 incorporated herein by reference, shown in
The fluid leaves the blades to flow at 7′ in a generally axial direction with respect to duct 7a′, typically with some swirl remaining. A continuous, generally annular shroud 8′ is attached to outer extents of the blades to collect any centrifuged liquid, as for example where the fluid consists of liquid, or a liquid and gas mixture, and to minimize blade to blade leakage losses and windage losses. Liquid collected on the shroud leaves the shroud with a small swirl that causes it to flow to and collect on the wall 9′ of the end plate, ensuring that it leaves the area of the rotating blades without impinging on the blades or shroud which would produce frictional losses. Any liquid on the wall and gas leave the VPTRA through outlet 10′, of duct 7a′.
For components that are non-corrosive and electrically non-conductive, a hermetic generator can be used. In this case (which is not necessary to the use of the Variable Phase Turbine, as described in the preceding paragraphs) fluid 21′ in liquid state is introduced to the VPTRA through another inlet 21a′. The pressure is increased by a pump 20′ attached to the shaft 12′. An impeller 20′ on shaft 12′ increases the pressure of fluid 21′ above that at the inlet, causing the fluid to flow to zone 18′, and lubricate the bearings 17′. The fluid leaving the pump also flows to zone 19′ adjacent outer extent of the stator, and cools the electric stator 14′, and rotor 13′.
After cooling the electric parts and lubricating the bearing parts, the fluid flows through a passage 23′, and leaves the structure at 24′ through an outlet 24a′ after reception in plenum zone 122′, and end zone 122a′, to cool structure and lubricate the bearing 17′ closest to rotor 6′. An internal seal 11′ on shaft 12′ isolates the cooling liquid 21′ from the fluid 120′ flowing in the rotor area, i.e. the flow paths of the two fluids are disjunct. The casing 25′ encloses the parts of the VPTRA and has only static seals at 26′ and 27′ to contain the fluid. For the hermetic case, no external rotating seals are required, greatly increasing reliability and useful operating life.
A usable radial outflow turbine, the Dual Pressure Euler Turbine, disclosed in U.S. Pat. No. 7,244,095, designed for operation with liquid, gas or mixtures of liquid and gas, is shown in
The fluid at the exit 4″, of the nozzles flows in a generally tangential direction to a rotor structure 5″, and flows radially outwardly through vanes 6″, attached to the rotor structure. Metal projections 7″ are carried by the rotor structure, and seal against non-rotating abradable surface or surfaces 8″, restricting the amount of flow which could otherwise bypass the passage or passages 9″, formed by the rotor blades.
High velocity flow from the nozzles enters the rotor passages, the rotor rotational speed being selected to minimize the relative velocity between the stream and the moving blades and to minimize the absolute value of the velocity of the stream leaving the blades.
All liquid or solid particles are centrifuged out from the radially extending space 10″ between the nozzles and the rotor blades. The residence of particles is limited to a fraction of a rotor revolution. This is in contrast to radial inflow turbines where solid or liquid particulate matter tries to flow in a direction opposite the centrifugal forces, resulting in trapped particles which continue to impact the moving blades and nozzles causing extensive erosion damage.
The fluid leaving the rotating blades flows into the annular diffuser passage, 10″, which recovers the absolute leaving velocity as pressure. This enables the pressure at the exit of the moving blades to be lower than the process imposed pressure, increasing the power output. The stream then flows into an annular plenum, 11″, and subsequently to exit port 12″ of the turbine assembly, where it is returned to the process. Reference to the patent further explains operation and reference numbers.
A non-contact seal assembly, 12a″, is provided to reduce the leakage of stream between the stationary surfaces of the casing 13″, and the shaft 14″, to which the rotor is attached.
This invention may include multi-component apparatus characterized as performing a two-phase thermodynamic cycle, for conversion of heat energy to useful power comprises:
a) fluid means consisting of two or more chemical components to absorb heat energy,
b) said fluid means providing an increasing temperature and increasing fraction of gas phase as increasing amounts of heat are absorbed,
c) expander means operating to convert the enthalpy in a received mixture of gas and liquid, formed in said fluid means, as a result of the said heat energy absorbed by the said fluid means, to mechanical, shaft power,
d) heat exchanger means operating to transfer said heat energy to be absorbed by the said fluid means,
e) condenser means operating to reject the unconverted enthalpy, by said expander means, in said fluid means, thereby condensing any gas to convert the fluid means to liquid, and
f) pump means operating to pressurize liquid fluid means leaving the condenser means, in order to return the said fluid means to the said heat exchanger means, closing the thermodynamic cycle.
The expander means may typically comprise a variable phase turbine including:
a) nozzle means to maximize the conversion of enthalpy of a medium of liquid, supercritical fluid or a mixture of liquid and gas to kinetic energy in a directed stream of a mixture of gas and liquid, supercritical fluid or pure gas, said directed stream composition determined by the chemical composition of the fluid and thermodynamic conditions, said nozzle means directing the flow at moving blade means,
b) said moving blade means configured to maximize the conversion of the kinetic energy of said directed stream to torque, acting upon said blades,
c) rotor means to which said blades are attached to transmit the torque to a shaft to which the rotor and a load are attached,
d) casing means to confine and direct the fluid and which incorporates bearings and seals to enable the shaft to transmit the torque to a load,
e) shroud means operable the prevent liquid which has transferred kinetic energy to the blades from contacting the casing and being re-directed to contact the moving blades, causing losses in torque.
The expander means may comprise a radial outflow turbine means that includes:
a) a first, stationary, nozzle means wherein a gas and liquid mixture of said fluid means is expanded in a radial outward direction, accelerating the said fluid means and directing it towards a rotor structure,
b) said rotor structure having a second nozzle structure consisting of vanes which receive impingement of the accelerated fluid means from the first nozzle means, and which further expand the said fluid means in a radial outward and tangential direction, adding power to said rotor structure,
c) a casing means into which the fluid means leaving the second nozzle structure is discharged to be effectively removed from the expander structure,
d) a shaft means, attached to the rotor structure supported by bearing means, that transmits the power generated by expansion of the fluid means to a useful load means, such as an electric generator.
The described structure may include a second heat exchanger means to transfer heat from the fluid means leaving the expander to the fluid means leaving the pump means, thereby to reduce the heat required by the heat exchanger means.
The expander means may comprise a positive displacement expander.
Components of the fluid means may include any of the following:
