U.S. patent application number 13/334994 was filed with the patent office on 2013-06-27 for cascaded organic rankine cycle system.
The applicant listed for this patent is Bruce P. Biederman, Frederick J. Cogswell, Lili Zhang. Invention is credited to Bruce P. Biederman, Frederick J. Cogswell, Lili Zhang.
Application Number | 20130160449 13/334994 |
Document ID | / |
Family ID | 47296978 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160449 |
Kind Code |
A1 |
Cogswell; Frederick J. ; et
al. |
June 27, 2013 |
CASCADED ORGANIC RANKINE CYCLE SYSTEM
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.
Inventors: |
Cogswell; Frederick J.;
(Glastonbury, CT) ; Biederman; Bruce P.; (Old
Greenwich, CT) ; Zhang; Lili; (West Hartford,
CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cogswell; Frederick J.
Biederman; Bruce P.
Zhang; Lili |
Glastonbury
Old Greenwich
West Hartford |
CT
CT
CT |
US
US
US |
|
|
Family ID: |
47296978 |
Appl. No.: |
13/334994 |
Filed: |
December 22, 2011 |
Current U.S.
Class: |
60/653 ;
60/671 |
Current CPC
Class: |
F01K 25/10 20130101;
F01K 23/04 20130101 |
Class at
Publication: |
60/653 ;
60/671 |
International
Class: |
F01K 25/04 20060101
F01K025/04; F01K 25/08 20060101 F01K025/08 |
Claims
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 in claim 1, wherein said working fluid for
said topping cycle is Siloxane MM.
3. The system as recited in claim 1, wherein said working fluid for
said bottoming cycle is R245fa.
4. The system as recited in claim 1, wherein said bottoming cycle
includes a recuperator.
5. The system as recited in claim 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 in claim 1, further comprising a hot oil
circuit in thermal communication with said topping cycle through an
evaporator.
7. The system as recited in claim 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 in claim 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 in claim 8, further comprising: utilizing
a condenser/evaporator as the thermal interface between the
bottoming cycle and the topping cycle.
11. The method as recited in claim 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 in claim 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 in claim 8, wherein the working fluid in
the topping cycle is at less than a 40% saturation for the working
fluid in the bottoming cycle.
Description
BACKGROUND
[0001] The present disclosure relates generally to Organic Rankine
Cycle (ORC) systems and, more particularly, to a cascaded organic
Rankine cycle.
[0002] 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.
[0003] 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.
[0004] 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
[0005] 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.
[0006] 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
[0007] 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:
[0008] FIG. 1 is a schematic diagram of a cascaded organic rankine
cycle with a topping cycle and a bottoming cycle;
[0009] FIG. 2 is a TS-diagram for the bottoming cycle;
[0010] FIG. 3 is a TS-diagram for the topping cycle; and
[0011] 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
[0012] FIG. 1 schematically illustrates a cascaded Organic Rankine
Cycle (ORC) system 20. The cascaded ORC system 20 includes at least
two Rankine cycles, where a relatively hotter topping cycle 22 is
cascaded with a relatively cooler bottoming cycle 24. In the
disclosed non-limiting embodiment, the topping cycle 22 uses
Siloxane MM as the working fluid while the bottoming cycle 24 uses
R245fa. It should be appreciated, however, that additional cycles
and other working fluids may additionally be utilized.
[0013] The topping cycle 22 generally includes a power producing
turbine 26 which is driven by the working fluid to drive a
generator 28 that produces power. A refrigerant pump 30 increases
the pressure of the working fluid from a condenser/evaporator 32.
The heat exchanger group that transfers heat from the topping cycle
22 to the bottoming cycle 24 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 the topping cycle 22, and preheating and superheating of the
working fluid in the bottoming cycle 24.
[0014] An evaporator 34 such as a boiler receives a significant
heat input from, for example, an oil circuit 36 to vaporize the
Siloxane MM working fluid with the vapor thereof passed through to
the turbine 26 to provide motive power. Upon leaving the turbine
26, the relatively lower pressure working fluid vapor passes to the
condenser/evaporator 32 and is condensed by way of a heat exchange
relationship with the bottoming cycle 24 such that the
condenser/evaporator 32 operates as a condenser in the topping
cycle 22 as well as an evaporator in the bottoming cycle 24.
[0015] In the disclosed non-limiting embodiment, the turbine 26 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/evaporator 32. The condenser/evaporator 32 actually
de-superheats the working fluid and ultimately condenses the
working fluid back to liquid for communication through the pump 30.
The condensed working fluid is then circulated to the evaporator 34
by the pump 30 to complete the topping cycle 22.
[0016] The bottoming cycle 24 generally includes a power producing
turbine 36 which is driven by the working fluid in the bottoming
cycle and in turn drives a generator 38 that produces power. A
refrigerant pump 40 increases the pressure of the working fluid
from a recuperator 40. The bottom cycle working fluid is in thermal
communication with a cooling system such as a water circuit 42
through a water cooled condenser 44.
[0017] By the nature of the proposed cycle, the vapor entering and
leaving turbine 36 is highly superheated. The energy potential of
the superheated vapor at the turbine exit is not wasted, but is fed
into a recuperator 46. The recuperator 46 transfers heat from the
low-pressure hot vapor from the turbine exit to the high pressure
liquid at the pump exit.
[0018] The recuperator 46 uses this superheat to preheat the liquid
working fluid downstream of the pump 40. 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 the recuperator 46 is an
internal heat exchanger. When the low pressure side of the topping
cycle 22 is de-superheated, it is essentially recuperated into the
bottoming cycle 24 which is where high superheat is achieved.
Matching of the working fluids and the pressures thereof
facilitates this interaction.
[0019] The recuperator 46 is only in the bottoming cycle 24. As the
topping cycle 22 is not recuperated, its waste heat is captured by
the condenser/evaporator 32. Both cycles are highly superheated yet
avoid heat-exchanger pinches to minimize the heat-transfer
temperature difference and minimize process irreversibility
[0020] FIG. 2 shows a TS diagram for the bottoming cycle 24. The
condenser/evaporator 32 receives nearly saturated liquid (a
temperature that is close to boiling) from the recuperator 46. The
condenser/evaporator 32 boils then heats the refrigerant from state
6 to 1. The state 1 condition is highly superheated. The exit state
from the turbine 36, state 2, is also highly superheated. The
recuperator 46 uses this heat (state 2 to 3) to heat the high
pressure working fluid (state 5 to 6). Sizing of the recuperator 46
affects state 6. A smaller recuperator 46, 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/evaporator 32, and a larger percentage of that heat in
the preheating and evaporating regimes.
[0021] FIG. 3 shows a TS diagram for the topping cycle 22. The exit
state of the topping cycle turbine 26 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 the topping cycle 22 results in the highly angled
saturation dome. As a result, the inlet state to turbine 26 is only
slightly superheated.
[0022] 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)}.sub.A(h.sub.A in-h.sub.A out)={dot over
(m)}.sub.B(h.sub.B out-h.sub.B in)
[0023] 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.
[0024] In FIG. 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 the recuperator 46 in the proposed cycle affects
the starting enthalpy (and therefore temperature) of stream B.
[0025] 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 the ORC system 20, the
condenser/evaporator 32 heat exchanger has two major regions. The
first (on the left in FIG. 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 in FIG. 4) is superheated and
temperature increases with enthalpy. That is, a percent saturation
for a fluid in the topping cycle 22 is maintained at 38 percent
saturation compared to a 40 percent saturation for the working
fluid in the bottoming cycle 24.
[0026] 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.
[0027] The effect of the recuperator 46 on the condenser/evaporator
32 in the proposed cycle is to change the inlet enthalpy, and
therefore temperature, of the colder fluid, B. By increasing the
size of the recuperator 46, 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 topping cycle 22 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 bottoming cycle 24 that occurs during a saturated
evaporation.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
* * * * *