Note: Descriptions are shown in the official language in which they were submitted.
CA 02798770 2012-12-13
CASCADED ORGANIC RANKINE CYCLE SYSTEM
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.
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CA 02798770 2012-12-13
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:
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
Figure I 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.
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
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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.
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.
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.
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.
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.
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
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is achieved. Matching of the working fluids and the pressures thereof
facilitates this
interaction.
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
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.
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.
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:
ri2A Cl/A hA out) = thE (hBout 49 zn)
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.
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
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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.
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.
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
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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.
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.
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
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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.
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