The present disclosure relates to a volumetric fluid expander used for power generation in the Rankine cycle.
The Rankine cycle is a power generation cycle that converts thermal energy to mechanical work. The Rankine cycle is typically used in heat engines, and accomplishes the above conversion by bringing a working substance from a higher temperature state to a lower temperature state. The classical Rankine cycle is the fundamental thermodynamic process underlying the operation of a steam engine.
In the Rankine cycle a heat “source” generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the “working body” of the engine while transferring heat to the colder “sink” until the working substance reaches the lower temperature state. During this process, some of the thermal energy is converted into work by exploiting the properties of the working substance. The heat is supplied externally to the working substance in a closed loop, wherein the working substance is a fluid that has a non-zero heat capacity, which may be either a gas or a liquid, such as water. The efficiency of the Rankine cycle is usually limited by the working fluid.
The Rankine cycle typically employs individual subsystems, such as a condenser, a fluid pump, a heat exchanger such as a boiler, and an expander turbine. The pump is frequently used to pressurize the working fluid that is received from the condenser as a liquid rather than a gas. Typically, all of the energy is lost in pumping the working fluid through the complete cycle, as is most of the energy of vaporization of the working fluid in the boiler. This energy is thus lost to the cycle mainly because the condensation that can take place in the turbine is limited to about 10% in order to minimize erosion of the turbine blades, while the vaporization energy is rejected from the cycle through the condenser. On the other hand, the pumping of the working fluid through the cycle as a liquid requires a relatively small fraction of the energy needed to transport the fluid as compared to compressing the fluid as a gas in a compressor.
A variation of the classical Rankine cycle is the Organic Rankine cycle (ORC), which is named for its use of an organic, high molecular mass fluid, with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. As such, in place of water and steam of the classical Rankine cycle, the working fluid in the ORC may be a solvent, such as n-pentane or toluene. The ORC working fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds, etc. The low-temperature heat may then be converted into useful work, which may in turn be converted into electricity.
In general terms, this disclosure is directed to a volumetric energy recovery system with a three stage expansion system. In one possible configuration and by non-limiting example, the present disclosure relates to a method of generating mechanical work via a closed-loop Rankine cycle, the method comprising: heating a working fluid to at least a partial vapor state; generating useful work at a first expansion stage by expanding the working fluid as the working fluid passes through the first expansion stage; generating useful work at a second expansion stage by expanding the working fluid as the working fluid passes through the second expansion stage; generating useful work at a third expansion stage by expanding the working fluid as the working fluid passes through the third expansion stage; and condensing the working fluid to a liquid state.
Another aspect of the disclosure relates to a system used to generate mechanical work via a closed-loop Rankine cycle, the system comprising: a power plant producing a heat stream and having a heat outlet through which the heat stream exits; a heat exchanging device configured to transfer heat from the heat stream to a working fluid steam; a first volumetric fluid expansion stage configured to receive the working fluid stream from the first heat exchanger; a second volumetric fluid expansion stage configured to receive the working fluid stream from the first volumetric fluid expansion stage; and a third volumetric fluid expansion stage configured to receive the working fluid stream from the second volumetric fluid expansion stage. Each of the first, second, and third volumetric fluid expansion stages is configured to generate mechanical work from the working fluid stream.
Various embodiments will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the present disclosure. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the appended claims.
Referring to the drawings, a system is illustrated in which a plurality of volumetric fluid expansion stages 20 having dual interleaved rotors extracts energy from a waste heat stream from a power source, which is also referred to herein as a power plant, that would otherwise be wasted. As described below, the rotors can be configured to be either straight or twisted. The volumetric fluid expansion stage 20 may also be referred to herein as an expander, expansion device or volumetric energy recovery device. An energy recovery system can be formed by coupling components with the output of the volumetric fluid expansion stage that transfers energy back to the power plant directly or indirectly.
Volumetric Energy Recovery System with Three Stage Expansion System
In some examples, the system 100 includes an engine 52; a plurality of heat exchangers 18-1, 18-2, 18-3, and 18-4 (collectively designated as 18); a three stage expansion system having a plurality of expansion stages 20-1, 20-2, and 20-3 (collectively designated as 20); a condenser 25; and a fluid pump 16.
The engine 52 can be an internal combustion engine that operates on combustion of a chemical fuel, such as diesel fuel or gasoline, and produces a great quantity of heat and exhaust gases. In some examples, the engine 52 can include a supercharger or turbocharger 102 to use forced induction.
The plurality of heat exchangers 18 is configured to transfer heat to and from the working fluid 12 passing therein.
The expansion stages 20 are configured to receive the working fluid 12 and generate mechanical work. In operation, as the working fluid 12 passes through the expansion stage 20, the temperature and pressure of the fluid 12 drop. In general, the expansion stages 20 rely upon the pressure of the fluid 12 to rotate an output shaft, thereby creating mechanical energy. The mechanical energy can be used or stored in several ways. For example, the torque created by the expansion stages 20 can be used by the engine 52. Each of the expansion stages 20 can return the extracted energy back to the engine 52 via an output shaft 38 of the device 20 (
The condenser 25 operates to condense the working fluid 12 from its gaseous state to a liquid state by cooling it.
The fluid pump 16 is configured to pump the working fluid 12 from low to high pressure while maintaining the working fluid 12 in its liquid state.
Referring to
The exhaust gas 108 at temperature T3 enters a first heat exchanger 18-1. The heat exchanger 18-1 utilizes a fluid 14 at temperature T9 flowing therein as a cooling fluid. The temperature T9 is lower than the temperature T3. As discussed below, the fluid 14 is discharged from a first expansion stage 20-1. The first heat exchanger 18-1 circulates the fluid 14 through its coils, thereby cooling the exhaust gas 108 as it flows past the coils and simultaneously heating the fluid 14 to produce a fluid 13 at temperature T11 that is higher than the temperature T9. The heated fluid 13 at the temperature T11 then passes through a second expansion stage 20-2.
The second expansion stage 20-2 receives the fluid 13 at the temperature T11 and discharges a fluid 17 at temperature T12 that is lower than T11. Furthermore, the fluid 17 has a lower pressure than the fluid 12. While reducing the temperature and pressure of the fluid 13 into the fluid 17, the second expansion stage 20-2 generates mechanical work that can be used or stored in various ways as discussed above.
The fluid 17, with a lower temperature and pressure than the fluid 13, immediately flows through a third expansion stage 20-3 which is again used to create mechanical energy as it discharges a fluid 21 at temperature T4 that is lower than the temperature T12. Furthermore, the fluid 21 has a lower pressure than the fluid 17. Because the fluid 17 has a lower temperature and pressure than the fluid 13 as it enters the third expansion stage 20-3, the third expansion stage 20-3 cannot produce as much mechanical energy as the second expansion stage 20-2. As such, the fluid 21 exiting the third expansion stage 20-3 has a lower temperature and pressure than the fluids 17 and 13. In some examples, the fluid 21 exiting the third expansion stage 20-3 has a mixed phase fluid comprising of a mixture of gas and liquid.
The fluid 21 at the temperature T4 then passes through a second heat exchanger 18-2, which is typically referred to as a recuperator, before flowing into the condenser 25. The recuperator 18-2 is placed between the third expansion stage 20-3 and the condenser 25 to further reclaim waste heat from the fluid 21 released from the third expansion stage 20-3. A fluid 23 exiting the recuperator 18-2 has temperature T10 that is lower than the temperature T4.
The fluid 23 is then sent to the condenser 25 that is used to convert the fluid 23, which, in some examples, can be a mixture of gas and liquid, to a saturated liquid 31 at temperature T5. As shown in the Rankine cycle of
The fluid 31 at the temperature T5 is pumped from low to high pressure by the pump 16. In this process, the temperature T5 of the fluid 31 increases, as shown in the Rankine cycle of
The recuperator 18-2 utilizes the fluid 27 at the temperature T13 to take heat from the fluid 21, which is released from the third expansion stage 20-3. Accordingly, the recuperator 18-2 transfers heat from the fluid 21 at the temperature T4 to the fluid 27 at the temperature T13, thereby producing a fluid 33 at temperature T6 that is higher than the temperature T13. The fluid 33 flows to a third heat exchanger 18-3.
The third heat exchanger 18-3 transfers heat from the exhaust gas 108, which is released from the first heat exchanger 18-1, to the fluid 33, thereby producing a fluid 35 at temperature T7 that is higher than T6. The fluid 35 thereafter flows to a fourth heat exchanger 18-4.
The fourth heat exchanger 18-4 transfers heat from the exhaust gas 108 at the temperature T3, which flows through the fourth heat exchanger 18-4 from the engine 52, to the fluid 35 at the temperature T7. As a result, the fourth heat exchanger 18-4 produces a fluid 36 at temperature T8 that is greater than T7. The exhaust gas 108 is simultaneously cooled to a lower temperature than T3 as it flows through the fourth heat exchanger 18-4 and released to the atmosphere.
The fluid 36 at the temperature T8 is received by the first expansion stage 20-1 that discharges a fluid 14 at temperature T9 that is lower than T8 and generates mechanical work as described above. The fluid 14 has a lower pressure than the fluid 36. The fluid 14 leaving the first expansion stage 20-1 at temperature T9 flows directly to the first heat exchanger 18-1 where it is re-heated directly by exhaust gas 108 supplied from the engine 52. The entire process is then repeated in a cycle as described above.
As illustrated above, the second heat exchanger 18-2, which is also referred to as the recuperator, the third heat exchanger 18-3, and the fourth heat exchanger 18-4 are connected in series. In some examples, the second, third and fourth heat exchangers 18-2, 18-3 and 18-4 are replaced by one or two heat exchanger devices, which operate the same as the combination of the first, second and third heat exchangers 18-2, 18-3 and 18-4.
In this example, the system 100 removes the first heat exchanger 18-1. In the first example, the fluid 14 at the temperature T9 is discharged from the first expansion stage 20-1 and enters the first heat exchanger 18-1 before flowing into the second expansion stage 20-2. In contrast, in this example, the fluid 14 at the temperature T9 discharged from the first expansion stage 20-1 is directly delivered to the second expansion stage 20-2. Furthermore, in the first example, the exhaust gas 108 at the temperature T3 supplied from the engine 52 passes through the first heat exchanger 18-1 and the third heat exchanger 18-3 in series. However, in this example, the exhaust gas 108 at the temperature T3 flows directly from the engine 52 to the third heat exchanger 18-3.
In this example, the recuperator 18-2 is directly connected to both the third heat exchanger 18-3 and the fourth heat exchanger 18-4 while the third heat exchanger 18-3 and the fourth heat exchanger 18-4 are arranged in parallel. The system 100 can include a splitter valve 19 (also known as a distributor valve), which operates to divide the fluid discharged from the recuperator 18-2 to flow into both the third heat exchanger 18-3 and the fourth heat exchanger 18-4 at the same time. Therefore, the fluid 33 at the temperature T6 discharged from the recuperator 18-2 is drawn into both the third heat exchanger 18-3 and the fourth heat exchanger 18-4. The third heat exchanger 18-3 transfers heat from the exhaust gas 108 of the engine 52 to the fluid 33, and discharges the fluid 29 at temperature T14, which is greater than the temperature T6. The fluid 29 flows directly to the first expansion stage 20-1. Similarly, the fourth heat exchanger 18-4 transfers heat from the exhaust gas 108 of the engine 52 to the fluid 33, and discharges the fluid 36, which is then drawn to the first expansion stage 20-1.
Although this example describes that the two heat exchangers 18-3 and 18-4 are arranged in parallel through one splitter valve 19, more than two heat exchangers can be arranged in parallel through one or more splitter valves, provided that the heat exchangers discharge the fluid 36 that is drawn into the first expansion stage 20-1.
Additional examples are directed to a method of using a three stage expansion system in a Rankine cycle as described in
The working fluid 21 enters the second heat exchanger or recuperator 18-2 (208). At process 208, the temperature of the working fluid 21 is reduced to the temperature T10 by the recuperator 18-2. The working fluid 23 at the temperature T10 then enters the condenser 25, which liquidizes the fluid 23 and discharges the working fluid 31 at the temperature T5 (210). As shown in
In this example, the method 200 further includes a step of increasing the temperature of the working fluid at the first heat exchanger 18-1 between processes 202 and 204 (220). At process 220, the working fluid 14, which has passed the first expansion stage 20-1, is drawn into the first heat exchanger 18-1 to increase its temperature before entering the second expansion stage 20-2. As the working fluid 14 passes through the first heat exchanger 18-1, the temperature increases from T9 to T11. Thus, the first heat exchanger 18-1 discharges the working fluid 13 at the temperature T11, which subsequently flows into the second expansion stage 20-2 for process 204.
The expansion device 20 has a housing 22 with a fluid inlet 24 and a fluid outlet 26 through which the working fluid 12-1 undergoes a pressure drop to transfer energy to the output shaft 38. The inlet port 24 is configured to admit the working fluid 12-1 at a first pressure from the heat exchanger 18 (shown in
As additionally shown in
As shown, the first and second rotors 30 and 32 are fixed to respective rotor shafts, the first rotor being fixed to an output shaft 38 and the second rotor being fixed to a shaft 40. Each of the rotor shafts 38, 40 is mounted for rotation on a set of bearings (not shown) about an axis X1, X2, respectively. It is noted that axes X1 and X2 are generally parallel to each other. The first and second rotors 30 and 32 are interleaved and continuously meshed for unitary rotation with each other.
The first and second rotors 30 and 32 are interleaved and continuously meshed for unitary rotation with each other. With renewed reference to
The output shaft 38 is rotated by the working fluid 12 as the working fluid undergoes expansion from the higher first pressure working fluid 12-1 to the lower second pressure working fluid 12-2. As may additionally be seen in both
In one aspect of the geometry of the expander 20, each of the rotor lobes 30-1 to 30-4 and 32-1 to 32-4 has a lobe geometry in which the twist of each of the first and second rotors 30 and 32 is constant along their substantially matching length 34. As shown schematically at
In another aspect of the expander geometry, the inlet port 24 includes an inlet angle 24-1, as can be seen schematically at
Furthermore, and as shown in
Referring to
In another aspect of the expander geometry, the outlet port 26 includes an outlet angle 26-1, as can be seen schematically at
The efficiency of the expander 20 can be optimized by coordinating the geometry of the inlet angle 24-1 and the geometry of the rotors 30, 32. For example, the helix angle HA of the rotors 30, 32 and the inlet angle 24-1 can be configured together in a complementary fashion. Because the inlet port 24 introduces the working fluid 12-1 to both the leading and trailing faces of each rotor 30, 32, the working fluid 12-1 performs both positive and negative work on the expander 20.
To illustrate,
In generalized terms, the working fluid 12-1 impinges on the trailing surfaces of the lobes as they pass through the inlet port opening 24b and positive work is performed on each rotor 30, 32. By use of the term positive work, it is meant that the working fluid 12-1 causes the rotors to rotate in the desired direction: direction R1 for rotor 30 and direction R2 for rotor 32. As shown, working fluid 12-1 will operate to impart positive work on the trailing surface 32-2b of rotor 32-2, for example on surface portion 47. The working fluid 12-1 is also imparting positive work on the trailing surface 30-4b of rotor 30-1, for example of surface portion 46. However, the working fluid 12-1 also impinges on the leading surfaces of the lobes, for example surfaces 30-1 and 32-1, as they pass through the inlet port opening 24b thereby causing negative work to be performed on each rotor 30, 32. By use of the term negative work, it is meant that the working fluid 12-1 causes the rotors to rotate opposite to the desired direction, R1, R2.
Accordingly, it is desirable to shape and orient the rotors 30, 32 and to shape and orient the inlet port 24 such that as much of the working fluid 12-1 as possible impinges on the trailing surfaces of the lobes with as little of the working fluid 12-1 impinging on the on the leading lobes such that the highest net positive work can be performed by the expander 20.
One advantageous configuration for optimizing the efficiency and net positive work of the expander 20 is a rotor lobe helix angle HA of about 35 degrees and an inlet angle 24-1 of about 30 degrees. Such a configuration operates to maximize the impingement area of the trailing surfaces on the lobes while minimizing the impingement area of the leading surfaces of the lobes. In one example, the helix angle is between about 25 degrees and about 40 degrees. In one example, the inlet angle 24-1 is set to be within (plus or minus) 15 degrees of the helix angle. In one example, the helix angle is between about 25 degrees and about 40 degrees. In one example, the inlet angle 24-1 is set to be within (plus or minus) 15 degrees of the helix angle HA. In one example, the inlet angle is within (plus or minus) 10 degrees of the helix angle. In one example, the inlet angle 24-1 is set to be within (plus or minus) 5 degrees of the helix angle HA. In one example, the inlet angle 24-1 is set to be within (plus or minus) fifteen percent of the helix angle HA while in one example, the inlet angle 24-1 is within ten percent of the helix angle. Other inlet angle and helix angle values are possible without departing from the concepts presented herein. However, it has been found that where the values for the inlet angle and the helix angle are not sufficiently close, a significant drop in efficiency (e.g. 10-15% drop) can occur.
In the diagram 48 of
From stage 48-2 the working fluid is transferred to stage 48-3. During stage 48-3, the pressurized working fluid 12 enters and passes through the heat exchanger 18 where it is heated at constant pressure by an external heat source to become a two-phase fluid, i.e., liquid together with vapor. From stage 48-3 the working fluid 12 is transferred to stage 48-4. During stage 48-4, the working fluid 12 in the form of the two-phase fluid expands through the expander 20, generating useful work or power. The expansion of the partially vaporized working fluid 12 through the expander 20 decreases the temperature and pressure of the two-phase fluid, such that some additional condensation of the two-phase working fluid 12 may occur. Following stage 48-4, the working fluid 12 is returned to the condenser 25 at stage 48-1, at which point the cycle is then complete and will typically restart.
Typically a Rankine cycle employs a turbine configured to expand the working fluid during the stage 48-4. In such cases, a practical Rankine cycle additionally requires a superheat boiler to take the working fluid into superheated range in order to remove or evaporate all liquid therefrom. Such an additional superheating process is generally required so that any liquid remaining within the working fluid will not collect at the turbine causing corrosion, pitting, and eventual failure of the turbine blades. As shown, the ORC of
Additionally, a smaller size expander may be used in the system 100 to achieve the required work output. The efficiency will never be above the Carnot efficiency of 63% because that is the maximum Caarnot efficiency eff=1−Tcold/Thot. The working fluid will likely be ethanol which has a max temp of 350 c before it starts to break down. The expander efficiency will be less than the peak efficiency of a turbo but the efficiency islands are considerably larger over a greater flow range then than the turbo expander so an overall efficiency for a cycle is larger.
The various examples described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example examples and applications illustrated and described herein, and without departing from the true spirit and scope of the following claims.
This application is a Continuation of PCT/US2014/013393, filed 28 Jan. 2014, which claims benefit of U.S. Patent Application Ser. No. 61/757,533 filed on 28 Jan. 2013, claims benefit of U.S. Patent Application Ser. No. 61/810,579 filed on 10 Apr. 2013, and claims benefit of U.S. Patent Application Ser. No. 61/816,143 filed on 25 Apr. 2013 and which applications are incorporated herein by reference. To the extent appropriate, a claim of priority is made to each of the above disclosed applications.
This invention was made with government support under Contract No. DE-EE0005650 awarded by the National Energy Technology Laboratory funded by the Office of Energy Efficiency & Renewable Energy of the United States Department of Energy. The government has certain rights in the invention.
Number | Date | Country | |
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61757533 | Jan 2013 | US | |
61810579 | Apr 2013 | US | |
61816143 | Apr 2013 | US |
Number | Date | Country | |
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Parent | PCT/US2014/013393 | Jan 2014 | US |
Child | 14810712 | US |