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:
Figure 1 is a schematic diagram of a cascaded organic rankine cycle with a topping
cycle and a bottoming cycle;
Figure 2 is a TS-diagram for the bottoming cycle;
Figure 3 is a TS-diagram for the topping cycle; and
Figure 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
[0008] Figure 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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
[0016] Figure 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.
[0017] Figure 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.
[0018] Figure 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:
[0019] 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.
[0020] In Figure 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.
[0021] Figure 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 Figure 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 Figure 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.
[0022] 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 15F; 3 to 8C), 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
1. A cascaded Organic Rankine Cycle (ORC) system (20) comprising:
a topping cycle (22); and
a bottoming cycle (24) in thermal communication with said topping cycle (22) through
a condenser/evaporator (32) in which a bottoming cycle (24) 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 (20) as recited in claim 1, wherein said working fluid for said topping
cycle (22) is Siloxane MM.
3. The system (20) as recited in claim 1 or 2, wherein said working fluid for said bottoming
cycle (24) is R245fa.
4. The system (20) as recited in any of claims 1 to 3, wherein said bottoming cycle (24)
includes a recuperator (46).
5. The system (20) as recited in any preceding claim, wherein both said bottoming cycle
fluid and said topping cycle fluid in the condenser/evaporator (32) are saturated
over approximately 40% of said total heat transfer.
6. The system (20) as recited in any preceding claim, further comprising a hot oil circuit
(36) in thermal communication with said topping cycle (22) through an evaporator (34).
7. The system (20) as recited in any preceding claim, further comprising a cooling circuit
(42) in thermal communication with said bottoming cycle (24) through a condenser (44).
8. A method of operating a cascaded Organic Rankine Cycle (ORC) system (20) in which
a bottoming cycle (24) is in thermal communication with a topping cycle (24) comprising
maintaining a percent saturation for a working fluid in the topping cycle (22) at
less than a percent saturation for a working fluid in the bottoming cycle (24).
9. The method as recited in claim 8, further comprising:
utilizing Siloxane MM as the working fluid in the topping cycle (22); and/or
utilizing R245fa as the working fluid in the bottoming cycle (24).
10. The method as recited in claim 8 or 9, further comprising utilizing a condenser/evaporator
(32) as the thermal interface between the bottoming cycle (24) and the topping cycle
(22).
11. The method as recited in any of claims 8 to 10, further comprising operating a condenser/evaporator
(32) as a condenser for the topping cycle (22) and as an evaporator for the bottoming
cycle (24).
12. The method as recited in any of claims 8 to 11, wherein a "knee" of the working fluid
in the topping cycle (22) flowing right to left lies to the left of the "knee" of
the working fluid of the bottoming cycle (24) flowing left to right in a normalized
enthalpy plot.
13. The method as recited in any of claims 8 to 12, wherein the working fluid in the topping
cycle (22) is at less than a 40% saturation for the working fluid in the bottoming
cycle (24).