Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
RINK CYCLE POWER PLANT WIT IMPROVED
ORGANIC WORKING FLUID
TECHNICAL FIELD
This invention relates to a Ranking cycle power
plant operating with an improved organic working fluid.
BACKGKOUNV ART
Ranking cycle power plants operating with an
organic working fluid are known in the art. Such a
power plant comprises a boiler for vaporizing the
working fluid, a turbine responsive to vaporized
working fluid produced by the boiler for expanding the
vapor and producing work, a generator coupled to the
turbine for converting the work produced thereby into
electrical energy, and a condenser for condensing
expanded vaporized working fluid exhausted from the
turbine and producing condensate that is returned to
the boiler either by pump or under the influence of
gravity. A power plant ox this type, hereinafter
referred to as a power plant of the type described, is
commercially available through Format Turbines Ltd. and
is described in the Patent literature, in, for example,
U.S. Patent No. 3,0~0,528.
Power plants of the type described are in current
use throughout the world for supplying electrical
energy or telecommunication relay stations, for
example, and other installations where the power
required is in the 300-3000 W range, and reliability is
critical. Reliability is enhanced in a power plant of
the type described by utilizing an air cooled
condenser, by mounting the turbine and generator on a
common shaft (turbo generator) and hermetically
enclosing these components in a canister, by diverting
a small portion of the condensate from the condenser to
the bearings of the turbo generator in order to effect
long term operations without wear, and by controlling
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the voltage of the generator by on/off operation of the fuel supplied to the boiler.
Conventionally, the working fluid is a fluorinated
hydrocarbon such a Freon, trichlorobenzene, etc.; and
the turbine operating conditions for trichlorobenzene
are about 160C and below atmospheric pressure while
the air cooled condenser operates at 70C and much less
than atmospheric pressure. Under these conditions of
temperature and pressure, conventional working fluids
are stable with time in the presence of copper,
stainless steel, low carbon steel, aluminum and brass,
or metals that are used in the construction of the
power plant of the type described. In addition, these
conventional working fluids have thermodynamic
properties which permit the working fluids to be used
advantageously in a Ranking cycle power plant of the
type described.
In order to up-size power plants of the type
described by an order of magnitude or more so as to
utilize low grade heat sources, such as waste heat,
geothermal heat, and solar heat, a larger turbine
; operating at higher pressures and temperatures must be
utilized if the size of the turbine is to be kept to
reasonable dimensions. Conventional working fluids
such as fluorinated hydrocarbons prove to be unstable
with time in the presence of the usual metals found in
power plants when the operating temperature is in the
range 300-400C. Moreover, as the capacity of power
plants of the type described increases, the bearing
loads on the turbine also increase; and it is not
always practical to construct both the turbine and
generator as a single unit enclosed in a hermetically
sealed canister as is done conventionally on small
capacity power plants.
As a consequence, a 750 ow or larger power plant
of the type described (such as would be operable to
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generate power from a waste heat or a geothermal
source, for example) would normally have a single stage
turbine mounted in its own housing with the rotor
journal Ed in bearings mounted in a housing, and a
separately housed generator coupled to the output shaft
of the turbine. thus, efficient seals are required,
particularly if the turbine housing is under a vacuum,
which would permit ambient water vapor to leak into the
housing. With many types of conventional working
fluids, water vapor chemically reacts with the working
fluid in the temperature range utilized and produces
constituents that are corrosive to the various metals
used in constructing the power plant. Increased
maintenance and expense in operation result.
Another disadvantage with many conventional
working fluids is their relatively high freezing point.
For example, the freezing point of commercial
trichlorobenzene is about 10C which would render the
power plant incapable of "cold-starting" in many places
in the world. While expedients are known by which
mixtures of different working fluids can be used in
order to suppress the freezing point of the
combination, this approach to solving the problems
encountered in using power plants of the type described
in cold regions is not always satisfactory.
It is therefore an object of the present invention
to provide a new and improved organic working fluid
suitable to a power plant of the type described wherein
the working fluid is more stable and provides improved
results
BRIEF DESCRIPTION OF INVENTION
According to the present invention, the working
fluid is a compound selected from the group consisting
off hi cyclic aromatic hydrocarbons, substituted
bicyclic aroma-tic hydrocarbons, heterobicyclic aromatic
hydrocarbons, substituted heterocyclic aromatic
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hydrocarbons, bicyclic or heterobicyclic compounds
where one ring is aromatic and the other condensed ring
is non-aromatic, and their mixtures.
Compounds in such group are inherently stable in
the temperature range of interest and have good
thermodynamic properties. 'one molecular weight of such
compounds is less than the molecular weight of many
conventional working fluids and this results in a lower
Mach number at the turbine exit, thereby increasing the
efficiency of the turbine.
The present invention is particularly useful for
power plants of the type described wherein the heat for
operation is obtained from solar heating of the working
fluid. The worming fluid circulates in a primary solar
collector. Part of it is vaporized by flashing to
obtain high pressure and high temperature vapor for the
turbine of the power plant in the range of 300-400C
and 3-6 atmosphere pressure. The cool condensate
produced by the condenser is preheated in a secondary
solar collector before being returned to the sup of
the flash chamber from which liquid is returned by pump
to the primary solar collector.
The liquids belonging to this group are known to
be stable under conditions of radiation within nuclear
power plants and are therefore suitable as cooling
liquids in nuclear reactors operating at temperatures
under 400C. The game liquids can be used in nuclear
power plants both as cooling liquids and/or working
fluids for the turbines. In this way, the need for
very expensive heat exchangers could be eliminated.
The advantage of this is that the power plant would
then operate at relatively low vapor pressure,
approximately 3-7 elm. In conventional nuclear
reactors the operating pressure can be over 100 elm.
Thus, in the present invention, the cost of
construction, installation and safety devices is
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reduced. In power plants operating above 150C, it is
normal practice to use water as a working fluid and the
pressure obtained may exceed 100 elm. In addition,
when using water as a working fluid, intensive
superheating by way of heat exchangers is required.
Low pressure nuclear reactors cooled with organic
fluids have been operated in the past, but with a
liquid unsuitable for driving a turbine. The advantage
of the fluids in the present invention is that they can
be used as a working fluid to drive a power plant,
eliminate the expensive high pressure devices, the
boiler and superheater heat exchangers and the need for
corrosive conditioning
In a further aspect of the invention, a binary
Ranking cycle power plant is provided in which the
condenser of the high temperature and pressure turbine
is cooled by a different working fluid which is
vaporized thereby and supplied to a low temperature and
pressure turbine. The different working fluid may be
an aliphatic hydrocarbon such as one of the hotness,
or it may be water.
DESCRIPTION OF DRAWINGS
Embodiments of the present invention are shown in
the accompanying drawings wherein:
Fig. 1 is a block diagram of a power plant of the
type described into which the present invention is
incorporated;
Fig. 2 is a block diagram of the modification of
the block diagram of Fig. l;
Fig. 3 is a portion of a temperature-entropy
diagram for tetralin, which is a bicyclic compound
wherein one ring is aromatic and the other condensed
ring is non-aromatic.
Fig. 4 is a chart listing the enthalpy, pressure
and volume of tetralin at the various states shown in
the diagram of Fig. 3; and
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Fig. 5 is a chart comparing the vapor pressure of
some of the compounds of the present invention with
that of water.
DETAILED DESCRIPTION
Referring now to Fig. 1, reference numeral 10
designates a first embodiment of a power plant of the
type described according to the present invention.
Power plant 10 is in the form of a binary Ranking cycle
power plant comprising high pressure portion 13 and low
pressure portion 48, each having different associated
working fluids. The heat source for this power plant
is constituted by a pair of solar collectors, the
primary solar collector being designated by reference
numeral 12 and the secondary solar collector being
designated by reference numeral 14. Collectors 12 and
14 are conventional in nature and serve to collect the
focus solar energy on a series of tubes containing
liquid working fluid. Heated working fluid produced by
solar collector 12 is piped to flash chamber 16 through
control valve 40 and then through trotting valve 18
which produces A pressure drop that flashes the heated
working fluid into vapor within chamber 16. That
ration of the liquid not flashed into vapor collects
at the bottom of flash chamber 16 in sup 20.
Vaporized working fluid passes through inlet
conduit 22 into the nozzles (not shown) of high
temperature turbine 24 wherein expansion occurs causing
the turbine to do work by rotation of shaft 260
Generator 28 coupled to shaft 26 converts the worn
produced by turbine 24 into electrical energy.
Cooled and expanded working fluid exhausted from
turbine 24 it transported by exhaust line 30 to closed,
counter-flow heat exchanger 32. A second working fluid
is also applied to heat exchanger 32 for the purpose of
cooling the vaporized first working fluid. As a
consequence, condensate of the first working fluid is
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collected in conduit 34 and is applied by pump 36 to secondary solar collector 14 wherein solar energy
reheats the cooled condensate to a temperature close to
the temperature of the liquid in sup 20 of flash
chamber 16 and returns the heated condensate to sup
20. Pump 38 returns liquid in sup 22 to primary solar
collector 12 enabling the working fluid cycle 'co
repeat.
Valve 40 in the line connecting primary solar
collector 12 to flush chamber 16 is normally in a
position that effects the transfer of hot working fluid
into the flash chamber and to block flow into conduit
42 which is connected to the exhaust manifold 30 of
turbine 24. Likewise, valve 44 at the outlet to
secondary collector 14 is also normally in a position
to permit heated condensate to enter sup 20 instead of
being diverted through conduit 46 directly to the input
side of pump 38.
Low temperature, low pressure portion 48 of the
power plant is constituted by one side of heat
exchanger 32 within which the low temperature working
fluid circulates for the purpose of condensing the high
temperature working fluid contained in portion 13 of
the binary power plant. Heat exchanger 32 thus
converts the liquid working fluid in portion 48 of the
power plant into a vapor which is applied to low
temperature turbine 50 which, preferably, is also
coupled to shaft 26 enabling the work extracted from
the second working fluid by turbine 50 to be converted
into electrical energy by generator 28.
Cooled working fluid in the vapor state is
exhausted from turbine 50 through conduit 52 and
applied to condenser 54 wherein the exhaust vaporized
fluid is condensed. The resultant condensate produced
by condenser 54 is applied via pump 56 to heat
exchanger 32 for completing the cycle of the operating
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fluid.
In one embodiment of the invention, the working
fluid in portion 13 of the power plant is tetralin
which is a bicyclic compound wherein one ring is
aromatic and the other condensate ring is non-aromatic.
Tetralin (which chemically is tetra-hydro-naphtalene)
has a freezing point of -35C and is suitable for many
cold weather applications.
Liquid tetralin contained in solar collector 12 is
typically heated to about 302C and a pressure of about
5.8 bar by the collector. Flash chamber 16 will
typically have a pressure of about 5 bar producing a
liquid in sup 20 at about 2g7C. Tetralin vapor
entering high temperature turbine 24 will be at about
290C with a pressure of about 5 bar. The vaporized
tetralin expands in the turbine 24 and exhausts a
pressure, typically of about 0.2 bar. Condensation of
the tetralin exhausted from turbine 24 occurs in heat
exchanger 32 wherein the condensate has a temperature
of about 150C arid has a pressure of about 0.2 bar.
Pump 36 pumps the condensate to collector 13 which
heats the condensate to a temperature of about 297C
and returns the heated condensate to sup 20.
Portion 48 of the binary power plant contains
Newton as the working fluid. The operation of heat
exchanger 32 vaporizes the Hutton and supplies
vaporized Hutton at about 140C and 3 bar pressure to
the inlet of low temperature turbine 50 wherein the
Hutton expands to a pressure of about 0.1Z bar and a
temperature of about 40C. An air cooled condenser is
envisioned for condenser 32, and the cycle repeats.
when the flow rate of tetralin in the primary cycle 13
is about 5 kg/sec, the Ross power produced by the
turbines 24 and 50 will be about 735 ow. The pumping
power required is such that the net power produced by
the power plant it about 715 ow.
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By utilizing flash chamber 16, all the working
fluid in solar collector 12 will remain in a liquid
state allowing the collector to operate most
efficiently. The pressure drop in chamber 16 due to
throttle valve 18 is only about 0.5 bar; and this
arrangement provides for efficient utilization of a
solar collector.
Valves 40 and 44 function as bypass valves which
are operated in the event that the solar input to
collectors 12 and 14 is inadequate to properly operate
portion 13 of the power plant. When ambient conditions
thus warrant, valves 40 and 44 are operated to bypass
the flash chamber 16 and turbine 24. In such case, the
primary working fluid is circulated serially through
both solar collectors 12 and 14 and into heat exchanger
32 so that the low temperature portion 48 of the power
plant can continue to operate.
The preferred working fluid for high pressure and
high temperature stage 13 of the power plant is
tetralin whose T-S diagram is shown in Fig. 3. State A
occurs at the input to solar collector 14, here the
entropy of the tetralin is about 1.76 KJ/KG/K. Solar
collectors 12 arid 14 raise the temperature from about
149C to about 300C and the state is changed to state
B. Expansion in flash chamber 16 produces a change in
state from B to C; and expansion through turbine 24
produces a change in state from C to D.
The T-S diagram for tetralin is shaped with a
negative slope along the saturated vapor line so that
expansion of the working fluid in turbine 24 occurs in
the superheater region. Thus, the turbine blades are
not subjected to droplets of condensed working fluid.
Fig. 4 summaries the indicated physical properties of
tetralin in the various states shown in Fig. 3.
A stability test on tetralin at temperatures in
excess of 300C in the presence of metals commonly
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found in power plants, such as aluminum, brass,
steel, and stainless steel as well as low carbon
steel has shown that the working fluid at this
temperature has no effect on these metals, and they
have no effect on the working fluid. Decomposition
of tetralin could not be detected at these temper-
azures during -the test. At a temperature of about
400, the decomposition rate of tetralin was low.
The present invention contemplates that
the fluid which acts as a moderator, coolant and/or
working fluid is a compound selected from the group
consisting of bicyclic aromatic hydrocarbons, sub-
stitu-ted bicyclic aromatic hydrocarbons, hotter-
bicyclic aromatic hydrocarbons, substituted hotter-
basilic aromatic hydrocarbons, bicyclic or hotter-
bicyclic compounds where one ring is aroma-tic and
the other condensed ring is non-aromatic, and -their
mixtures. The elude which acts as a moderator may
be deuterated. Russ group is sometimes hereinafter
referred to as working fluids of the present
invention. An example of a suitable bicyclic hydra-
carbon is napthalene having a freezing point of
~0.5C and l-methyl-napthalene with a freezing point
of -22C, and 2-methyl~napthalene with a freezing
point of 35C. An example of substituted
heterocyclic aromatic hydrocarbons is quinoline,
with a freezing point of -50C and benz-thi.ophene.
Tetralin its a bicyclic compound where one ring is
aromatic and the other condensed ring is non-
aromatic.
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According to the invention, a mixture of
fluids may be used wherein the overall mixture has a
freezing point which its lower than the freezing
point of the compound of the mixture having the
highest freezing point.
The compounds in the group described above
have high stability in the presence of aluminum,
grass, steel, stainless steel and low grade carbon
steel up to 400C. Their high Ranking cycle effi-
Chinese at a given temperature, and their relatively low melting point
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allow them to be used in almost ambient conditions when
the condenser is air cooled.
Further improvement can be obtained by adding low
molecular weight compounds, e.g., methanol, in order to
reduce the Mach number at the turbine exit. By the
addition of about 0.6% by weight of methanol to
quinoline, the boiler will operate at 2~0C, and the
Mach number of the mixture is reduced from 3.6 to 2.9
when the condenser temperature is 50C. The addition
of methanol to quinolene reduces the average molecular
weight, thereby resulting in the improvement of Mach
number. The composition of the vapor is about 20% by
weight of methanol and about 80% by weight of
quinolene.
The thermodynamic efficiency of the cycle can be
improved by the use of mixtures. For example, by
combining quinoline, which is a "dry" type of fluid
whose saturated vapor line in the T-S diagram has a
negative curvature, with methanol, which is a "wet"
type of fluid with a T-S diagram like that of water,
the resulting mixture will have a T-S diagram wherein
the saturated vapor line is almost perpendicular to the
entropy axis.
A saturated mixture of methanol with tetralin will
reduce the melting point from about -35C to lower than
-45C and will be suitable for many arctic conditions.
Mixtures of working fluids of the present invention
permit selection of still lower freezing points and
intermediate thermodynamic properties. For example, a
mixture of 75~ tetralin and 25% methyl naphtalene has a
freezing point of -70C; tetralin alone has a -freezing
point of ~35C; and methyl naphthalene alone has a
freezing point ox -10C.
Another feature of the present invention arises in
the ability of the compound selected from the group
identified above to be used with continuous heat
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sources such as nuclear reactors and waste heat in the
form of stack gases or exhaust gas turbine and or
diesel engines. When the heat source is a nuclear
reactor, naphthalene, which has a relatively high
freezing point, can be used advantageously because
"cold starting" is not a requirement. Saturated
naphthalene vapor at 300C has a pressure of about 5
bar, and these conditions eliminate the need for
special piping to withstand high pressures, as well as
the need for heavy and costly containment buildings
usually associated with nuclear reactor power plants.
The embodiment of the invention in Fig. 2
illustrates a low-pressure nuclear reactor power plant
utilizing naphthalene as the working fluid. Power
plant 16 is a binary cycle plant having high pressure
portion 61 and low pressure portion 70. Heat source 62
in high pressure portion 61 is a nuclear reactor which
produces naphthalene vapor at the temperature and
pressure indicated above. Vaporized naphthalene is
applied to high temperature turbine 64; and the
naphthalene expands in the turbine causing the latter
to produce work which it converted into electrical
energy by generator 66. Naphthalene vapors exhausted
from turbine 64 are applied to counterfoil heat
exchanger 67 where the naphthalene vapor condenses.
The condensate is applied by pump I to the heat source
62 and the cycle repeats. The heat exchanger will
operate, typically, at about 150C, where its vapor
pressure is about 0.16 bar. No high pressure piping
for the working fluid is required; and by reason of the
low pressures involved, no special containment for the
nuclear reactor heat source is required.
Heat exchanger 67 heats another working fluid,
such as water, in low pressure loop 70 of the power
plant shown in jig. 2. When water is the working fluid
in portion 70 of the power plant, heat exchanger 66
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will produce steam at about 40C and a pressure of
about 3.6 bar. This steam is applied to low
temperature turbine 72 where the steam expands, causing
the turbine to produce work which is converted into
electrical energy by generator 66 in the manner
indicated previously. Steam exhausted from low
temperature turbine 72 will be at about 0.07 bar and
condenser 74 will condense this steam into liquid
water. In this case, a liquid cooled condenser can be
used and the condensate exits from the condenser at
about 40C- Pump 76 returns the condensate to heat
exchanger 66 and the cycle repeats.
The advantage gained in using the compounds
identified above in a power plant where the heat source
is a nuclear reactor can be illustrated quite simply by
comparing the vapor pressure of various fluids at two
convenient operating temperatures. As seen in the
chart of Fig. 5 to which reference is now made, the
vapor pressure of water is many times the vapor
pressure of both tetralin and naphthalene and l-methyl
naphthalene. Thus, these compounds are ideally suited
for a power plant where the heat source is a nuclear
reactor.
Working fluids of the present invention are
particularly suitable for power plants based on nuclear
reactor heat sources because such fluids can perform
multiple functions in a reactor. In addition to
constituting the worming fluid for the power plant, the
working fluids of the present invention can also
function as the reactor moderator fluid and cooling
fluid. This eliminates the need for heat exchangers
and minimizes problems with leakage which is of great
concern in heat exchangers.
The advantages and improved results furnished by
the methods and apparatus of the present invention are
apparent from the foregoing description of the various
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embodiments of the invention. Various changes and
modifications may be made without departing from the
spirit and the scope of the invention as described in
the claims that follow.
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