Method and apparatus for implementing a thermodynamic cycle with recuperative preheating

A method and apparatus for implementing a thermodynamic cycle with preheating, involves expanding a gaseous working fluid to a medium pressure to transform its energy into usable form. The expanded gaseous working fluid is split into two different streams. One stream is further expanded to a spent low pressure level to produce further usable energy. This stream is then condensed. The other of the two streams is used to preheat the condensed stream and is mixed with the condensed stream at a point upstream of the point of preheating. This decreases the irreversibilities in the preheating process and enables greater efficiencies to be achieved.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to methods and apparatus for transforming 
energy from a heat source into a useable form using a working fluid that 
is expanded and regenerated. This invention further relates to a method 
and apparatus for improving the heat utilization efficiency of a 
thermodynamic cycle. 
2. Brief Description of the Background Art 
In the Rankine cycle, a working fluid such as water, ammonia or freon is 
evaporated in an evaporator utilizing an available heat source. The 
evaporated gaseous working fluid is expanded across a turbine to transform 
its energy into useable form. The spent gaseous working fluid is then 
condensed in a condenser using an available cooling medium. The pressure 
of the condensed working medium is increased by pumping, followed by 
evaporation and so on to continue the cycle. 
The Exergy cycle, described in U.S. Pat. No. 4,346,561, utilizes a binary 
or multi-component working fluid. This cycle operates generally on the 
principle that a binary working fluid is pumped as a liquid to a high 
working pressure and is heated to partially vaporize the working fluid. 
The fluid is then flashed to separate high and low boiling working fluids. 
The low boiling component is expanded through a turbine, to drive the 
turbine, while the high boiling component has heat recovered for use in 
heating the binary working fluid prior to evaporation. The high boiling 
component is then mixed with the spent low boiling working fluid to absorb 
the spent working fluid in a condenser in the presence of a cooling 
medium. 
A theoretical comparison of the conventional Rankine cycle and the Exergy 
cycle demonstrates the improved efficiency of the new cycle over the 
Rankine cycle when an available, relatively low temperature heat source 
such as ocean water, geothermal energy or the like is employed. 
In applicant's further invention referred to as the Basic Kalina cycle, the 
subject of U.S. Pat. No. 4,489,563, relatively lower temperature available 
heat is utilized to effect partial distillation of at least a portion of a 
multi-component fluid stream at an intermediate pressure to generate 
working fluid fractions of different compositions. The fractions are used 
to produce at least one main rich solution which is relatively enriched 
with respect to the lower boiling component, and to produce one lean 
solution which is relatively impoverished with respect to the lower 
boiling component. The pressure of the main rich solution is increased; 
thereafter, it is evaporated to produce a charged gaseous main working 
fluid. The main working fluid is expanded to a low pressure level to 
convert energy to useable form. The spent low pressure level working fluid 
is condensed in a main absorption stage by dissolving with cooling in the 
lean solution to regenerate an initial working fluid for reuse. 
In any process of converting thermal energy to a useable form, a major loss 
of available energy in the heat source occurs in the process of boiling or 
evaporating the working fluid. This loss of available energy (known as 
exergy or essergy) is due to the mismatch of the enthalpy-temperature 
characteristics of the heat source and the working fluid in the boiler. 
Simply put, for any given enthalpy the temperature of the heat source is 
always greater than the temperature of the working fluid. Ideally, this 
temperature difference would be almost, but not quite, zero. This mismatch 
occurs both in the classical Rankine cycle, using a pure substance as a 
working fluid, as well as in the Kalina and Exergy cycles described above, 
using a mixture as a working fluid. The use of a mixture as a working 
fluid in the manner of the Kalina and Exergy cycles reduces these losses 
to a significant extent. However, it would be highly desireable to further 
reduce these losses in any cycle. 
In the conventional Rankine cycle the losses arising from mismatching of 
the enthalpy-temperature characteristics of the heat source and the 
working fluid constitute about 25% of the available energy. With a cycle 
such as that described in U.S. Pat. No. 4,489,563, the loss of exergy in 
the boiler due to enthalpy-temperature characteristics mismatching would 
constitute about 14% of all of the available exergy. 
The overall boiling process in a thermodynamic cycle can be viewed for 
discussion purposes as consisting of three distinct parts: preheating, 
evaporation and superheating. The quantity of heat in the temperature 
range suitable for superheating is generally much greater than necessary, 
or the quantity of heat in the temperature range suitable for evaporation 
is much smaller than necessary. A portion of the high temperature heat 
which would be suitable for high temperature superheating is used for 
evaporation in conventional processes. This causes very large temperature 
differences between the two streams, and as a result, irreversible losses 
of exergy. 
In accordance with another invention of the applicant, the subject of U.S. 
Pat. No. 4,604,867, a fluid may be diverted to a reheater after initial 
expansion in the turbine to increase the temperature available for 
superheating. After return to the turbine, and additional expansion, the 
fluid is withdrawn from the turbine and cooled in an intercooler. 
Afterwards, the fluid is returned to the turbine for additional expansion. 
The cooling of the turbine gas may provide additional heat for 
evaporation. Intercooling provides compensation for the heat used in 
reheating and may provide recuperation of heat available which would 
otherwise remain unused following final turbine expansion. 
In the past preheating of a working fluid is usually performed by 
extraction of part of the working fluid stream between turbine stages. 
This is followed by injection of the extracted stream or streams into the 
stream of feed water to the turbine. As a result heat of a lower 
temperature level may perform preheating, which occurs at relatively low 
temperature levels. Therefore, in general, this process increases the 
efficiency of the power plant. 
However, conventional preheating has a drawback, because the steam used for 
preheating has a temperature which is significantly higher than the 
temperature of the feed water into which it is injected. This steam may 
even have a temperature which is higher than the temperature of the feed 
water obtained after injection. This creates irreversibilities and lowers 
the potential efficiency of the power plant. 
It would be highly desirable to provide a process and apparatus which 
avoids the creation of these irreversibilities and thereby increases the 
efficiency of the power plant. 
SUMMARY OF THE INVENTION 
It is one feature of the present invention to provide a significant 
improvement in the efficiency of a thermodynamic cycle by permitting 
closer matching of the working fluid and heat source enthalpy-temperature 
characteristics during preheating. It is also a feature of the present 
invention to provide a system of preheating which decreases the 
irreversibilities and therefore increases the efficiency of the entire 
system. 
In accordance with one embodiment of the present invention, a method of 
implementing a thermodynamic cycle includes the step of expanding a 
gaseous working fluid to transform its energy into useable form. The 
expanded gaseous working fluid is then split into two streams. The first 
stream is expanded to a spent low pressure level to transform its energy 
into usable form. The first stream is then condensed. The first and second 
streams are mixed to form a mixed stream after the second stream is used 
to preheat at least a portion of the mixed stream. Then the working fluid 
stream is evaporated to form the gaseous working fluid. 
In accordance with another embodiment of the present invention, an 
apparatus for implementing a thermodynamic cycle includes a first turbine 
having a fluid inlet path and a fluid outlet path. The fluid outlet path 
is split into first and second lines. A second turbine is connected for 
fluid communication with the first line. A heat exchanger is connected for 
fluid communication with the second line and the first turbine. A 
condensing system has its output connected for fluid communication with 
the second turbine. A mixing chamber is connected for fluid communication 
with the output of the condensing system. The heat exchanger is arranged 
to transfer heat from fluid flowing from the first turbine to the mixing 
chamber to fluid flowing from the mixing chamber to the first turbine.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawing wherein like reference characters are utilized for 
like parts throughout the several views, a system 100, shown in FIG. 1, 
implements a thermodynamic cycle, in accordance with one embodiment of the 
present invention. The illustrated system 100 includes a series of three 
turbines 102, 104 and 106, a condensing subsystem 108, and heat exchangers 
110-124. 
The condensing subsystem 108 may be any type of known heat rejection 
device. In the Rankine cycle, heat rejection occurs in a simple heat 
exchanger and thus for Rankine applications, the subsystem 108 may take 
the form of a heat exchanger or condenser. In the Kalina cycle, described 
in U.S. Pat. No. 4,489,563 to Kalina, the heat rejection system requires 
that gas leaving the turbine be mixed with a multi-component fluid stream, 
for example, comprised of water and ammonia, condensed and then distilled 
to produce the original state of the working fluid. Thus, when the present 
invention is used with a Kalina cycle, the distillation subsystem 
described in U.S. Pat. No. 4,489,563 may be utilized as a system 108. U.S. 
Pat. No. 4,489,563 is hereby expressly incorporated by reference herein. 
Various types of heat sources may be used to drive the cycle of this 
invention. For example, heat sources with temperatures as high as, say 
1000.degree. F. or more, down to the low heat sources such as those 
obtained from ocean thermal gradients may be utilized. Heat sources such 
as for example, low grade primary fuel, waste heat, geothermal heat, solar 
heat or ocean thermal energy conversion systems may also be implemented 
with the present invention. However, the present invention is particularly 
suitable for use with heat produced by the burning of fuel in a fluidized 
bed or by the burning of municipal wastes or other low grade fuel. 
Normally in the burning of such fuel, to avoid corrosion, the combustion 
gases cannot be cooled below a temperature of 300.degree. to 400.degree. 
F. 
A variety of working fluids may be used in conjunction with the system 100 
depending on the kind of condensing subsystem 108 utilized. In conjunction 
with a condensing system 108 described in the U.S. patent incorporated by 
reference herein, any multi-component working fluid that comprises a lower 
boiling point fluid and a relatively higher boiling point fluid may be 
utilized. Thus, for example, the working fluid employed may be an 
ammonia-water mixture, two or more hydrocarbons, two or more freons, 
mixtures of hydrocarbons and freons or the like. In general, the fluid may 
be mixtures of any number of compounds with favorable thermodynamic 
characteristics and solubility. However, when implementing the 
conventional Rankine cycle, conventional single component working fluids 
such as water, ammonia, or freon may be utilized. 
As shown in FIG. 1, a completely condensed working fluid which has been 
slightly preheated and pumped to a high pressure, exits the condensing 
subsystem 108 and is combined with a returning stream from the pump 126. 
The fluid exiting the pump 126 is at a temperature, pressure, and mass 
flow rate relatively close to that of the fluid exiting the condensing 
subsystem 108. In an illustrative embodiment the pressure of the two 
streams are substantially the same before they are mixed. After the two 
streams from the subsystem 108 and the pump 126 are combined at point 128, 
the working fluid is divided into two streams 130 and 132. The stream 132 
is heated in the heat exchanger 122 in counterflow with the fluid in the 
line 134 returning from the turbine 102. The flow along the path 130 is 
heated by counterflow in the heat exchanger 124 with the returning stream 
from the turbine 106. 
The returning stream along the path 134 that exits from the turbine 102 is 
a medium pressure stream relative to the returning streams from the 
turbine 106. The medium pressure returning stream from the turbine 102 is 
pumped by the pump 126 as described previously. In the heat exchanger 122, 
the returning medium pressure stream is condensed, releasing heat of 
condensation, which heats the stream 132. 
The returning stream from the turbine 106, progressing along the line 136, 
is at a lower pressure than the stream from the turbine 102 which 
progresses along line 134. This returning stream 136 gives up heat in heat 
exchanger 124 to heat the fluid flow along the path 130 as described 
previously. 
At point 138, the streams progressing along the paths 130 and 132 are 
combined and then divided into three streams which pass through heat 
exchangers 116, 118 and 120 respectively. The stream passing through line 
140 is heated by the return stream in the line 136 which exited from the 
turbine 106. The fluid stream progressing along line 142 is heated by the 
medium pressure returning stream in line 134 which exits from turbine 102. 
Finally, the fluid flow through the line 144 is heated by an external heat 
source in the heat exchanger 116. As a result of the processes occuring in 
the heat exchangers 116, 118 and 120, each of the exiting flows along the 
lines 144, 142 and 140 is evaporated and slightly superheated. 
Each of these slightly superheated streams are combined and pass through a 
heat exchanger 110 with heating by an external heat source. The flow 
exiting from the heat exchanger 110 is sent into the high pressure turbine 
102 where it is expanded to a medium pressure to produce work. 
The flow exiting from the turbine 102 is divided into two streams. One 
stream progresses along the path 134 and the other stream progresses along 
the path 146. The fluid flow through the path 134 is cooled and condensed, 
as described previously, to provide heat for preheating. 
The stream progressing along the path 146 is reheated in heat exchanger 112 
and is then expanded in the intermediate pressure turbine 104 to produce 
work. Thereafter, the stream is reheated in the heat exchanger 114 by an 
external heat source and then expanded in the low pressure turbine 106 to 
produce work. The flow exiting from the turbine 106 is a relatively low 
pressure returning stream. This stream progresses along the path 136 to be 
cooled in the heat exchanger 120, providing heat for the stream 140 as 
described previously. Ultimately the stream passes to the subsystem 108. 
While the present invention has been described with two stage cooling of 
the stream progressing along the path 134 and two stage heating of the 
turbine 102 feed water, those skilled in the art will appreciate that the 
present invention can be implemented with single, double, triple or 
multiple stage heating of the feed water and cooling of the flow through 
the path 134. 
A Kalina cycle condensing subsystem 108', shown in FIG. 2, is 
advantageously used as a subsystem 108 in the system shown in FIG. 1. In 
order to condense the working fluid stream, a distillation-condensation 
subsystem is employed when the pressure of the incoming stream to the 
system 108 is substantially lower than the pressure necessary to provide 
condensation of the returning low pressure stream at normal ambient 
temperatures. 
The stream from the path 136 is sent into a heat exchanger 200 where it is 
cooled and partially condensed, releasing heat. Thereafter the stream 
passes through the heat exchanger 210, where it is further cooled and 
condensed. The stream is then mixed with a stream of lean solution at the 
point 212. As will become apparent subsequently, the lean solution is a 
solution which contains a higher proportion of a higher boiling 
temperature component than the stream exiting from the heat exchanger 210. 
The new stream, called the basic solution, has an increased content of the 
higher boiling component in comparison with the returning low pressure 
stream and for this reason can be completely condensed by a cooling source 
such as water. After complete condensation in the condenser 214, the basic 
solution is pumped by a pump 216. The basic solution is then sent into the 
heat exchanger 210 where it is heated by the returning streams from the 
heat exchangers 200 and 218. 
Usually the temperature of the flow heading from the heat exchanger 210 
toward the heat exchanger 218 is slightly below the boiling temperature of 
the fluid. The stream is divided into three separate paths 220, 222 and 
224. The fluid progressing along the path 222 is sent into the heat 
exchanger 200 where it is partially heated and partially evaporated. The 
stream progressing along the path 220 is sent into the heat exchanger 218. 
Thereafter, the streams 220 and 222 are recombined to form the stream 226. 
The stream 226, a vapor-liquid mixture, passes through a gravity separator 
228 where it is separated into lean stream 232 and rich stream 230. Both 
streams 230 and 232 are sent through the heat exchanger 218 counterflow to 
the stream 220. The rich stream 230 is enriched with the light (lower 
temperature boiling) component and is cooled and partially condensed in 
the heat exchanger 218. 
The partially condensed rich stream is combined with the flow from the path 
224 producing a working solution composition. The working solution 
composition passes through heat exchanger 234 where it is further cooled 
and condensed. From here it is finally sent into the condenser 214 where 
it is fully condensed by a cooling source. 
The condensate is pumped by a pump 236 to an intermediate pressure. 
Thereafter, it is sent counterflow through heat exchanger 234 where it is 
preheated. After preheating the stream is finally pumped to a high 
pressure by the pump 238 where it exits from the subsystem 108'. 
Returning now to the lean stream, which is enriched with the heavier 
(higher temperature boiling) component, exiting from the gravity separator 
228 along the line 232, the lean stream is cooled in the heat exchanger 
218. Then it is further cooled in the heat exchanger 210 providing heat 
for the output flow from the pump 216. Thereafter, the stream progressing 
along the path 232 is throttled by the throttle valve 240 and is mixed at 
212 as described previously. 
The parameters of flow at the various points indicated in FIGS. 1 and 2 are 
design variables that can be chosen in a way to obtain the maximum 
advantage from the system 100. One skilled in the art will be able to 
select the design variables to maximize performance under the various 
conditions and circumstances that may be encountered, while achieving a 
heat balance. The parameters of the various process points, shown in FIG. 
1, are subject to considerable variation depending on specific 
circumstances. 
In order to further illustrate the advantages that can be obtained by the 
present invention, a set of calculations were performed. In these 
calculations, an illustrative power cycle in accordance with the system 
shown in FIGS. 1 and 2 was selected wherein the working fluid was a 
water-ammonia mixture. The parameters for the theoretical calculations 
(assumed ambient temperature 60.degree. F.) which were performed utilized 
standard ammonia-water enthalpy-concentration diagrams. In the following 
table the points set forth in the first column correspond to the points in 
FIGS. 1 and 2. The column headed by the letter "G" shows the weight of the 
fluid at each point in proportion to the weight of fluid at the point 38. 
TABLE I 
______________________________________ 
NH.sub.4 Concen- 
tration (lbs 
Point 
Temp. Press. Enthalpy 
NH.sub.4 /Total 
G 
No. (.degree.F.) 
(PSI) (BTU/lb) 
Wt.) (lb/lb) 
______________________________________ 
1 60.00 21.10 -75.04 .4196 4.7337 
2 60.33 93.42 -74.72 .4196 4.7337 
3 118.41 72.42 -13.32 .4196 1.1421 
4 144.50 70.92 81.72 .4196 1.1421 
5 148.50 70.92 97.40 .4196 4.2459 
6 148.50 70.92 618.06 .9671 .5122 
7 118.41 73.42 -13.32 .4196 4.7337 
8 118.41 72.42 -13.32 .4196 .4878 
9 122.81 69.42 574.78 .9671 .5122 
10 148.50 70.92 25.98 .3445 3.7337 
11 121.68 69.42 287.88 .7000 1.0000 
12 126.64 60.92 2.84 .3445 3.7337 
13 99.13 68.42 213.40 .7000 1.0000 
14 60.00 67.42 -51.63 .7000 1.0000 
15 149.99 70.92 103.17 .4196 3.1037 
16 123.17 23.10 419.73 .7000 1.0000 
17 75.33 22.10 277.77 .7000 1.000 
18 84.53 22.10 29.51 .4196 4.7337 
19 86.13 22.10 -36.98 .3445 3.7337 
20 88.69 50.92 -36.98 .3445 3.7337 
21 60.60 186.88 -51.01 .7000 1.0000 
22 127.92 2560.00 23.43 .7000 1.0000 
23 52.00 -- -- WATER 25.84 
24 81.41 -- -- WATER 25.84 
27 118.41 72.42 -13.32 .4196 3.1037 
28 117.68 181.88 11.91 .7000 1.0000 
30 145.71 2560.00 43.57 .7000 1.4614 
31 145.71 2560.00 43.57 .7000 .3705 
32 145.71 2560.00 43.57 .7000 1.0909 
33 384.51 2460.00 362.57 .7000 1.0909 
34 384.51 2460.00 362.57 .7000 .3705 
35 384.51 2460.00 362.57 .7000 1.4614 
36 384.51 2460.00 362.57 .7000 .3002 
37 384.51 2460.00 362.57 .7000 .0482 
38 181.52 24.10 781.28 .7000 1.0000 
39 384.51 2460.00 362.57 .7000 1.1130 
40 697.62 2435.00 1008.57 .7000 1.1130 
41 697.62 2435.00 1008.57 .7000 .0482 
42 697.62 2435.00 1008.57 .7000 .3002 
43 697.62 2435.00 1008.57 .7000 1.4614 
44 1050.00 2400.00 1275.85 .7000 1.4614 
45 799.05 756.07 1124.30 .7000 1.4614 
46 799.05 756.07 1124.30 .7000 1.0000 
47 1050.00 731.07 1291.96 .7000 1.0000 
48 722.15 152.00 1084.92 .7000 1.0000 
49 1050.00 127.00 1297.04 .7000 1.0000 
50 732.62 30.10 1093.38 .7000 1.0000 
51 399.51 27.10 899.47 .7000 1.0000 
52 799.05 756.07 1124.30 .7000 .4614 
53 393.51 753.07 831.98 .7000 .4614 
54 175.52 743.07 77.76 .7000 .4614 
55 183.57 2560.00 87.23 .7000 .4614 
______________________________________ 
The cycle with the parameters as set forth in Table I was calculated to 
have a total net electrical output of 598.32 BTU with a total heat input 
of 1385.65 BTU. Thus, the net thermal efficiency was 43.2%. The calculated 
total pump work was 18.04 BTU. 
When the disclosed system is utilized in connection with low grade fuel 
such as municipal waste, these calculations indicate that efficiency could 
be improved as much as 25% or more over conventional systems. 
While the present invention has been described with respect to a single 
preferred embodiment, those skilled in the art will appreciate a number of 
variations and modifications therefrom and it is intended within the 
appended claims to cover all such variations and modifications as fall 
within the true spirit and scope of the present invention.