US20130160449A1 - Cascaded organic rankine cycle system - Google Patents

Abstract

A cascaded Organic Rankine Cycle (ORC) system includes a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from the topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to the bottoming cycle fluid that occurs during a saturated evaporation.

Description

BACKGROUND
• The present disclosure relates generally to Organic Rankine Cycle (ORC) systems and, more particularly, to a cascaded organic Rankine cycle.
• The Organic Rankine Cycle (ORC) is a vapor power cycle with an organic fluid refrigerant instead of water/steam as the working fluid. The working fluid is heated in an “evaporator/boiler” by a source of waste or low quality heat. The fluid starts as a liquid and ends up as a vapor. The high-pressure refrigerant vapor expands in the turbine to produce power. The low-pressure vapor exhausted from the turbine is condensed then sent back to the pump to restart the cycle.
• The simple rankine cycle used for power generation follows the process order: 1) Adiabatic pressure rise through a pump; 2) Isobaric heat addition in a preheater, evaporator and superheater; 3) Adiabatic expansion in a turbine; and 4) Isobaric heat rejection in a condenser, although other cycle modifications are possible such as the addition of a vapor-to-liquid recuperator.
• A main thermodynamic irreversibility in organic Rankine cycles is caused by the large temperature difference in the evaporator between the temperature of the waste heat stream and the boiling refrigerant. The higher the waste heat stream temperature the greater this irreversibility becomes. One way to reduce this loss is to cascade two thermodynamic cycles together where a cycle operating at higher temperatures rejects heat to a cycle operating at lower temperatures.
SUMMARY
• A cascaded Organic Rankine Cycle (ORC) system according to an exemplary aspect of the present disclosure includes a bottoming cycle in thermal communication with a topping cycle through a condenser/evaporator in which a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from the topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to the bottoming cycle fluid that occurs during a saturated evaporation.
• A method of operating a cascaded Organic Rankine Cycle (ORC) system in which a bottoming cycle is in thermal communication with a topping cycle according to an exemplary aspect of the present disclosure which includes maintaining a percent saturation for a fluid in the topping cycle at less than a 40 percent saturation for a fluid in the bottoming cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
• Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows:
• FIG. 1 is a schematic diagram of a cascaded organic rankine cycle with a topping cycle and a bottoming cycle;
• FIG. 2 is a TS-diagram for the bottoming cycle;
• FIG. 3 is a TS-diagram for the topping cycle; and
• FIG. 4 is a plot of temperature profiles in the counter-flow heat exchangers of the de-superheating and then condensing topping fluid (Siloxane MM), and the evaporating and then superheating bottoming fluid (R245fa).
DETAILED DESCRIPTION
• FIG. 1 schematically illustrates a cascaded Organic Rankine Cycle (ORC)system20. Thecascaded ORC system20 includes at least two Rankine cycles, where a relativelyhotter topping cycle22 is cascaded with a relativelycooler bottoming cycle24. In the disclosed non-limiting embodiment, thetopping cycle22 uses Siloxane MM as the working fluid while thebottoming cycle24 uses R245fa. It should be appreciated, however, that additional cycles and other working fluids may additionally be utilized.
• Thetopping cycle22 generally includes apower producing turbine26 which is driven by the working fluid to drive agenerator28 that produces power. Arefrigerant pump30 increases the pressure of the working fluid from a condenser/evaporator32. The heat exchanger group that transfers heat from thetopping cycle22 to thebottoming cycle24 is referred to herein as the “condenser/evaporator”32, although it should be understood that it may also include desuperheating and subcooling of the working fluid in thetopping cycle22, and preheating and superheating of the working fluid in thebottoming cycle24.
• Anevaporator34 such as a boiler receives a significant heat input from, for example, anoil circuit36 to vaporize the Siloxane MM working fluid with the vapor thereof passed through to theturbine26 to provide motive power. Upon leaving theturbine26, the relatively lower pressure working fluid vapor passes to the condenser/evaporator32 and is condensed by way of a heat exchange relationship with thebottoming cycle24 such that the condenser/evaporator32 operates as a condenser in thetopping cycle22 as well as an evaporator in thebottoming cycle24.
• In the disclosed non-limiting embodiment, theturbine26 is a radial inflow turbine that expands the topping cycle working fluid vapor down to a lower pressure and generates power by the extraction of work from this expansion process. The vapor is still superheated so that its heat potential is utilized in the condenser/evaporator32. The condenser/evaporator32 actually de-superheats the working fluid and ultimately condenses the working fluid back to liquid for communication through thepump30. The condensed working fluid is then circulated to theevaporator34 by thepump30 to complete thetopping cycle22.
• Thebottoming cycle24 generally includes apower producing turbine36 which is driven by the working fluid in the bottoming cycle and in turn drives agenerator38 that produces power. Arefrigerant pump40 increases the pressure of the working fluid from arecuperator40. The bottom cycle working fluid is in thermal communication with a cooling system such as awater circuit42 through a water cooledcondenser44.
• By the nature of the proposed cycle, the vapor entering and leavingturbine36 is highly superheated. The energy potential of the superheated vapor at the turbine exit is not wasted, but is fed into arecuperator46. Therecuperator46 transfers heat from the low-pressure hot vapor from the turbine exit to the high pressure liquid at the pump exit.
• Therecuperator46 uses this superheat to preheat the liquid working fluid downstream of thepump40. That is, if a cycle is driven to high turbine inlet superheat, then turbine outlet superheat will be high. The availability of this heat is thereby captured to maintain cycle efficiency as therecuperator46 is an internal heat exchanger. When the low pressure side of thetopping cycle22 is de-superheated, it is essentially recuperated into thebottoming cycle24 which is where high superheat is achieved. Matching of the working fluids and the pressures thereof facilitates this interaction.
• Therecuperator46 is only in thebottoming cycle24. As thetopping cycle22 is not recuperated, its waste heat is captured by the condenser/evaporator32. Both cycles are highly superheated yet avoid heat-exchanger pinches to minimize the heat-transfer temperature difference and minimize process irreversibility
• FIG. 2 shows a TS diagram for thebottoming cycle24. The condenser/evaporator32 receives nearly saturated liquid (a temperature that is close to boiling) from therecuperator46. The condenser/evaporator32 boils then heats the refrigerant from state 6 to 1. Thestate 1 condition is highly superheated. The exit state from theturbine36,state 2, is also highly superheated. Therecuperator46 uses this heat (state 2 to 3) to heat the high pressure working fluid (state 5 to 6). Sizing of therecuperator46 affects state 6. Asmaller recuperator46, for example, results in less heat transferred and therefore a cooler more subcooled state at 6 which results in more heat transfer required from the condenser/evaporator32, and a larger percentage of that heat in the preheating and evaporating regimes.
• FIG. 3 shows a TS diagram for thetopping cycle22. The exit state of thetopping cycle turbine26 is highly superheated, but a recuperator is not used. Instead, the low pressure working fluid vapor is de-superheated as the bottoming cycle high-pressure working fluid is superheated. The choice of a heavy molecule such as Siloxane for thetopping cycle22 results in the highly angled saturation dome. As a result, the inlet state toturbine26 is only slightly superheated.
• FIG. 4 represents an idealized counter-flow heat exchanger. The x-axis is normalized enthalpy change of each fluid, and the y-axis is temperature. The x-axis is based on the First Law of Thermodynamics which can be written for a heat exchanger as:
• 
  {dot over (m)}_A(h_{A in}−h_{A out})={dot over (m)}_B(h_{B out}−h_{B in})
• Where the subscripts A and B refer to streams A and B respectively, m is the mass flow rate, and h is the enthalpy of the fluid.
• InFIG. 4, the warmer fluid (A) is shown to travel from right to left, and the colder fluid (B) to travel from left to right through the heat exchanger. Heat transfers from fluid A to fluid B; therefore, fluid A's enthalpy decreases while fluid B's enthalpy increases. For each section of the heat exchanger the above equation must be true. For example, the first 10% reduction in enthalpy of Fluid A must equal the last 10% increase of enthalpy of fluid B. If the fluids were simple fluids with constant specific heat, then each temperature profile would be a straight line. When the fluids are refrigerants, the temperature profiles have various non-linear shapes. When a fluid is saturated there is no change in temperature with change in enthalpy. The change in temperature with enthalpy is generally different for a fluid as a liquid than as a vapor; therefore, the choice of fluid and operational temperatures affect the shape of these curves. Furthermore, the choice of other system components will affect their shape. Specifically the choice of and the size of therecuperator46 in the proposed cycle affects the starting enthalpy (and therefore temperature) of stream B.
• FIG. 4 shows how each temperature profile relates to the other at each physical location along the heat exchanger. In order for heat to flow from Fluid A to Fluid B, Fluid A must always be warmer than Fluid B. If A gets too close to B this is referred to as a temperature “pinch” condition. This is undesirable because a large heat exchange area is required to exchange the enthalpy in this region. In fact, the entire size of a heat exchanger may be defined by a “pinch” condition. Where the temperature difference is large, the thermodynamic cycle will be less efficient since more entropy is generated by heat exchange through larger temperature differences. An ideal arrangement is when the temperature difference throughout the heat exchanger remains relatively constant. Since vapor heat exchange usually has a lower heat transfer rate than saturated, it may be desirable to maintain a somewhat higher temperature difference in this region, typically up to or equal to 1 to 2 times. For theORC system20, the condenser/evaporator32 heat exchanger has two major regions. The first (on the left inFIG. 4) is saturated for both fluids and the temperature profiles are flat. This section covers about 40 percent of the total heat transfer in the disclosed non-limiting embodiment. The second (on the right inFIG. 4) is superheated and temperature increases with enthalpy. That is, a percent saturation for a fluid in the toppingcycle22 is maintained at 38 percent saturation compared to a 40 percent saturation for the working fluid in the bottomingcycle24.
• The point where the temperature profile transitions from flat (saturated) to increasing (vapor) will be identified herein as the “knee.” For the above goals to be achieved, the “knee” of fluid A must lie equal to or slightly to the left of the “knee” of fluid B in the normalized enthalpy plot. If the “knee” lies far to the left then the saturated section may have a good heat transfer difference (typically 5 to 15 F; 3 to 8 C), but the heat transfer difference of the vapor section will be too large. If the “knee” lies too far to the right then a “pinch” condition will be created between the two fluids. Practically the temperature difference will increase and the saturated temperature difference will be too high.
• The effect of therecuperator46 on the condenser/evaporator32 in the proposed cycle is to change the inlet enthalpy, and therefore temperature, of the colder fluid, B. By increasing the size of therecuperator46, the enthalpy of the inlet of B increases by recovering heat from the turbine exit. This results in a smaller percentage of the total heat transfer for Fluid B occurring to the left of the knee, shifts the knee of B to the left and results in a pinch condition. Conversely, if the recuperator heat exchange is reduced or eliminated, this shifts knee of B to the right and therefore increases the temperature difference in the vapor section. That is, a percentage of total heat transfer from the working fluid in the toppingcycle22 that occurs during a saturated condensation is equal to or slightly less (within 10%) than a percentage of total heat transfer to the working fluid in the bottomingcycle24 that occurs during a saturated evaporation.
• The selection of a high superheat cascaded cycle with a condenser/evaporator heat exchanger transferring heat from the topping cycle to the bottoming cycle, and the selection of refrigerants for the topping and bottoming cycles and recuperator in the bottoming cycle allows for optimized heat exchanger temperature profiles.
• It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.
• It should be understood that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom.
• Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present disclosure.
• The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.

Claims (13)

What is claimed is:

1. A cascaded Organic Rankine Cycle (ORC) system comprising:

a topping cycle; and

a bottoming cycle in thermal communication with said topping cycle through a condenser/evaporator in which a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from said topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to said bottoming cycle fluid that occurs during a saturated evaporation.

2. The system as recited inclaim 1, wherein said working fluid for said topping cycle is Siloxane MM.

3. The system as recited inclaim 1, wherein said working fluid for said bottoming cycle is R245fa.

4. The system as recited inclaim 1, wherein said bottoming cycle includes a recuperator.

5. The system as recited inclaim 1, wherein both said bottoming cycle fluid and said topping cycle fluid in the condenser/evaporator are saturated over approximately 40% of said total heat transfer.

6. The system as recited inclaim 1, further comprising a hot oil circuit in thermal communication with said topping cycle through an evaporator.

7. The system as recited inclaim 1, further comprising a cooling circuit in thermal communication with said bottoming cycle through a condenser.

8. A method of operating a cascaded Organic Rankine Cycle (ORC) system in which a bottoming cycle is in thermal communication with a topping cycle comprising:

maintaining a percent saturation for a working fluid in the topping cycle at less than a percent saturation for a working fluid in the bottoming cycle.

9. The method as recited inclaim 8, further comprising:

utilizing Siloxane MM as the working fluid in the topping cycle; and

utilizing R245fa as the working fluid in the bottoming cycle.

10. The method as recited inclaim 8, further comprising:

utilizing a condenser/evaporator as the thermal interface between the bottoming cycle and the topping cycle.

11. The method as recited inclaim 8, further comprising:

operating a condenser/evaporator as a condenser for the topping cycle and as an evaporator for the bottoming cycle.

12. The method as recited inclaim 8, wherein a “knee” of the working fluid in the topping cycle flowing right to left lies to the left of the “knee” of the working fluid of the bottoming cycle flowing left to right in a normalized enthalpy plot.

13. The method as recited inclaim 8, wherein the working fluid in the topping cycle is at less than a 40% saturation for the working fluid in the bottoming cycle.

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