U.S. patent application number 14/902224 was filed with the patent office on 2016-06-02 for use of highly efficient working media for heat engines.
The applicant listed for this patent is Jens BUSSE, Muhammad IRFAN, Joern ROLKER, Gregor WESTPHAL. Invention is credited to Jens BUSSE, Muhammad IRFAN, Joern ROLKER, Gregor WESTPHAL.
Application Number | 20160153318 14/902224 |
Document ID | / |
Family ID | 50972704 |
Filed Date | 2016-06-02 |
United States Patent
Application |
20160153318 |
Kind Code |
A1 |
BUSSE; Jens ; et
al. |
June 2, 2016 |
USE OF HIGHLY EFFICIENT WORKING MEDIA FOR HEAT ENGINES
Abstract
The invention relates to a heat engine for performing an organic
Rankine cycle (ORC) which comprises an evaporator, an engine, a
condenser and a circuit comprising a fluid working medium, wherein
the working medium has a critical pressure (p.sub.c) between 4000
kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa, the
working medium has a critical temperature (T.sub.c) between 450 K
and 650 K, preferably between 460 K and 600 K, the working medium
has a molar mass between 50 g/mol and 80 g/mol, preferably between
60 g/mol and 75 g/mol, and the gaseous working medium partially
condenses out during adiabatic expansion. The invention further
relates to the use of a working medium having a critical pressure
(p.sub.c) between 4000 kPa and 6500 kPa, preferably between 4200
kPa and 6300 kPa, having a critical temperature (T.sub.c) between
450 K and 650 K, preferably between 460 K and 600 K, and having a
molar mass between 50 g/mol and 80 g/mol, preferably between 60
g/mol and 75 g/mol, in a heat engine, wherein the gaseous working
medium partially condenses out during an adiabatic expansion in an
organic Rankine cycle (ORC).
Inventors: |
BUSSE; Jens; (Bochum,
DE) ; ROLKER; Joern; (Alzenau, DE) ; IRFAN;
Muhammad; (Erlensee, DE) ; WESTPHAL; Gregor;
(Muelheim an der Ruhr, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BUSSE; Jens
ROLKER; Joern
IRFAN; Muhammad
WESTPHAL; Gregor |
Bochum
Alzenau
Erlensee
Muelheim an der Ruhr |
|
DE
DE
DE
DE |
|
|
Family ID: |
50972704 |
Appl. No.: |
14/902224 |
Filed: |
June 16, 2014 |
PCT Filed: |
June 16, 2014 |
PCT NO: |
PCT/EP2014/062516 |
371 Date: |
December 30, 2015 |
Current U.S.
Class: |
60/531 |
Current CPC
Class: |
Y02E 20/14 20130101;
F01K 25/08 20130101; C09K 5/04 20130101 |
International
Class: |
F01K 25/08 20060101
F01K025/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 2013 |
DE |
10 2013 212 805.3 |
Claims
1. A heat engine for performing an organic Rankine cycle (ORC), the
heat engine comprising an evaporator, an engine, a condenser and a
circuit comprising a fluid working medium, wherein the working
medium is methyl formate, and the heat engine is operated with a
heat source at a temperature ranging from 80.degree. C. to
150.degree. C.
2. The heat engine according to claim 1, wherein, during adiabatic
expansion during the organic Rankine cycle (ORC), 1% to 30% of the
mass of the working medium condenses out.
3. (canceled)
4. The heat engine according to claim 1, wherein the heat engine is
an expansion machine.
5. The heat engine according to claim 1, wherein a pump is disposed
between the condenser and the evaporator in the circuit of the heat
engine, said pump allowing the fluid working medium to be conveyed
from the condenser to the evaporator.
6. The heat engine according to claim 1, wherein the circuit of the
heat engine does not comprise a recuperator.
7. The heat engine according to claim 1, wherein an erosion rate of
the working medium unalloyed steel is less than 0.05 mm/a at
150.degree. C. and/or an erosion rate of the working medium towards
alloyed steel (1.4571) is less than 0.005 mm/a at 150.degree.
C.
8. The heat engine according to claim 1, wherein the working medium
exhibits no endothermic or exothermic reactions or first or second
order phase transitions in the temperature range between 70.degree.
C. and 200.degree. C. when subjected to temperature changes over
time.
9. A process, comprising operating a heat engine with methyl
formate as a working medium, wherein the heat engine is operated
with a heat source at a temperature ranging from 80.degree. C. to
150.degree. C.
10. The process according to claim 9, wherein, during adiabatic
expansion during the organic Rankine cycle (ORC), 1% to 30% of the
mass of the working medium is condensed out.
11-12. (canceled)
13. The process according to claim 9, wherein the heat engine is
operated with an organic Rankine cycle (ORC).
14. The process according to claim 9, wherein an expansion machine
is used as the heat engine.
15. (canceled)
Description
[0001] The invention relates to a heat engine for performing an
organic Rankine cycle (ORC) which comprises an evaporator, an
engine, a condenser and a circuit comprising a fluid working medium
and to the use of a working medium for a heat engine.
[0002] There is a great demand in the chemical industry for using
low energy waste heat streams generated at a temperature range from
80.degree. C. to 250.degree. C.
[0003] To optimize existing site integration systems, and with a
view to improving energy efficiency and reducing CO.sub.2
emissions, one promising option is the conversion of these not as
yet utilized waste heat streams into electricity through the use of
combined heat and power (CHP). This employs heat engines such as
are disclosed in DE 10 2009 024 436 A1, DE 10 2011 076 157 A1 and
EP 1 016 775 A2 for example. The latter two heat engines employ
water/steam as the working medium. The disadvantage of these is
that they operate at relatively high temperatures.
[0004] The problem of high operating temperatures of steam
processes has been overcome by the use of ORC technology since this
technology employs organic fluids rather than steam as the working
medium.
[0005] ORC stands for organic Rankine cycle "organischer
Rankine-Kreisprozess" bedeutet. An ORC process is a thermodynamic
cycle for converting heat into mechanical work using an organic
working medium.
[0006] An ORC process is a simple thermodynamic cycle in which the
working medium is evaporated and optionally superheated by
supplying heat at a high pressure level. The superheated vapour
undergoes expansion cooling to a lower pressure in an expander (in
particular an engine such as a piston engine or a turbine) thus
performing work. The work may be directly mechanically utilized or
is converted into electrical current using a generator. The vapour
exiting the expander may still be in the superheated state or may
already be decompressed to such an extent that it occupies the wet
vapour region so that some of it is already in the liquid state.
Complete liquefaction takes place in the condenser. Here, the
electricity-generating cycle is operated not with water but with an
organic working fluid which can utilize the heat generated at a low
temperature level with greater thermodynamic efficiency.
[0007] The working medium employed thus has a key role since the
optimal interaction between the working medium and the process
configuration has a determining influence on the efficacy and thus
on the efficiency of the entire process. For example, the working
medium influences the plant configuration. Optimal selection of a
working medium can enhance the utilization of the heat source and
the efficiency of the plant.
[0008] Suitable working media for ORC processes include especially
(hydro)chlorofluorocarbons and hydrocarbons and also mixtures of
fluids (hydrocarbons and water, (hydro)fluorocarbon mixtures) and
organic silicon components. The existing industrially realized
prior art employs not only hydrocarbons such as pentane, but also
siloxanes such as octamethyltrisiloxane or chlorinated hydrocarbons
such as R134a or R245fa (Quoilin, S., Lemort, V., Technological and
Economical Survey of Organic Rankine Cycle, 5th European Conference
Economics and Management of Energy in
[0009] Industry, Vilamoura, Portugal, 14.04.-17.04.2009). A heat
engine utilizing such ORC technology is disclosed, for example, in
EP 1 174 590 A2 where pentane is used as the organic working fluid,
i.e. as the working medium.
[0010] The disadvantages of the prior art working fluids include
possible hazards to the environment (CFCs: harmfulness to the ozone
layer and global warming) and to workplace safety (hydrocarbons:
flammability, explosion prevention) and also thermodynamic
limitations due to insufficient optimization of plant design and
fluid properties.
[0011] For certain vapour-expansion engines (piston engines) there
are no optimized working fluids yet in existence that may be
employed in the temperature range from 80.degree. C. to 250.degree.
C.
[0012] The fluorinated hydrocarbons are some of the most
extensively described working media. A substantial advantage of
these substances lies in their physical properties. For instance
these substances are generally not flammable and nontoxic. The
disadvantage of such substances is that the boiling point of
fluorinated hydrocarbons is generally very low since said
substances were usually developed as coolants and are thus of only
limited suitability for use in an ORC system at relatively high use
temperatures.
[0013] A further large group of ORC working media are hydrocarbons,
for example toluene, pentane and isobutane. Hydrocarbons are very
well known as suitable ORC working media and are employed in ORC
engines. However, when utilizing these media their properties must
be taken into account. The main disadvantage of these substances is
that they are usually flammable and hazardous to the environment.
Said substances generally also have a highly deleterious effect on
climate.
[0014] As an example of a prior art ORC application, ethanol is
currently used in an ORC vapour engine from DeVeTec GmbH as the
most efficient working medium in a temperature range starting at
about 250.degree. C.
[0015] However, since industrial waste heat streams are often at a
temperature level between 80.degree. C. and 250.degree. C. an
ethanol-based ORC process cannot be operated economically here.
[0016] In the light of this prior art the problem addressed by the
invention is that of providing a working fluid for an organic
Rankine cycle (ORC) comprising a vapour-expansion engine using
waste heat streams from DeVeTec GmbH in extended temperature ranges
between 80.degree. C. to 250.degree. C., in particular from
80.degree. C. to 200.degree. C., particularly preferably from
80.degree. C. to 150.degree. C. This broad temperature range is a
result of the different temperature levels of the waste heat
streams. While offgases from biogas, biomass or mine gas combustion
are present at temperatures in the region of 450.degree. C., the
industrial sphere is host to many lower temperature streams in the
range from 100.degree. C. to 200.degree. C. which can no longer be
utilized in many chemical sites but whose potential can be enhanced
via an ORC cycle. Different working fluids are thus utilized
depending on the application.
[0017] In addition to suitable thermodynamic properties (inter glia
thermal stability, enthalpy of vapourization, vapour pressure and
heat capacity) the working medium must meet further requirements
such as low toxicity and low environmental impacts (for example
with regard to innocuousness towards the ozone layer and climate)
and must not be flammable nor corrosive towards components of the
heat engine.
[0018] A further problem addressed by the invention is that of
providing a working medium employable with heat engines at low
temperatures with a high degree of efficiency. The working medium
shall simultaneously exhibit good environmental compatibility, in
particular in terms of harmfulness towards the ozone layer and
climate. The working medium should further effect as little attack
and corrosion as possible on the components of such a heat engine.
The working medium shall moreover be as nonhazardous as possible in
its application, i.e. should exhibit the lowest possible
flammability and present no risk of explosion.
[0019] Further problems addressed by the present invention and not
mentioned explicitly will become apparent from the overall context
of the following description, examples and claims.
[0020] These and other problems not explicitly mentioned but
readily derivable or discernible from the above context discussed
in the introduction hereof are solved by a heat engine having all
the features of claim 1 and by a method having all the features of
claim 9. Protection for advantageous developments of the inventive
method according to claim 1 is sought in subclaims 2 to 8.
Protection for an advantageous development of the inventive heat
engine according to claim 9 is sought in subclaims 10 to 15.
[0021] The problems addressed by the present invention are solved
by a heat engine for performing an organic Rankine cycle (ORC)
which comprises an evapourator, an engine, a condenser and a
circuit comprising a fluid working medium, wherein the working
medium has a critical pressure (pc) between 4000 kPa and 6500 kPa,
preferably between 4200 kPa and 6300 kPa, the working medium has a
critical temperature (Tc) between 450 K and 650 K, preferably
between 460 K and 600 K, the working medium has a molar mass
between 50 g/mol and 80 g/mol, preferably between 60 g/mol and 75
g/mol, and the gaseous working medium partially condenses out
during adiabatic expansion.
[0022] It may be provided that upon adiabatic expansion during a
work cycle of the ORC process 1% to 30% of the mass of the working
medium condenses out, preferably 10% to 20% of the mass of the
working medium condenses out.
[0023] These property ranges of the working medium ensure good
functioning of the ORC process and the heat engine with a high
degree of efficiency.
[0024] It may further be provided with particular preference
according to the invention that the working medium is cyclopentene
or at least one alkyl formate or a mixture thereof, preferably
methyl formate and/or ethyl formate.
[0025] These substances are particularly suitable as working media
for the intended use as is shown in detail hereinbelow.
[0026] A development of the invention proposes that the heat engine
is an expansion machine which preferably comprises a vapour
expansion engine comprising pistons as the engine or which
comprises at least one turbine as the engine.
[0027] In the context of the present invention the engine may thus
be realized either as a piston engine or as a turbine. Other types
of heat engines may also be employed as the engine provided they
are capable of converting the expansion work of the working medium
into mechanical work utilizable outside the process. It is thus
also possible to employ a rotary engine.
[0028] A vapour expansion engine having reciprocating pistons is
particularly preferred in accordance with the invention since the
wet behaviour of the working medium makes it possible to eschew a
recuperator and the conversion of the ORC process may thus be
carried out in particularly cost-effective fashion.
[0029] The mechanical work delivered by the engine may be directly
mechanically utilized or converted into electrical current using a
generator.
[0030] It may also be provided that a pump is disposed between the
condenser and the evapourator in the circuit of the heat engine,
said pump allowing the fluid working medium to be conveyed from the
condenser to the evapourator.
[0031] This ensures that the ORC process may be readily started
up.
[0032] A particularly preferred embodiment of the invention may
provide that the circuit of the heat engine does not comprise a
recuperator.
[0033] The eschewal of a recuperator (heat exchanger) is made
possible by the working media according to the invention. This
makes the heat engine simpler and more cost-effective to set
up.
[0034] It may also be provided with preference that the erosion
rate of the working medium towards unalloyed steel is less than
0.05 mm/a at 150.degree. C. and/or that the erosion rate of the
working medium towards alloyed steel (1.4571) is less than 0.005
mm/a at 150.degree. C.
[0035] This ensures that long-term operation of the heat engine
with the working medium is possible.
[0036] It may further be provided that the working medium exhibits
no endothermic or exothermic reactions or first or second order
phase transitions in the temperature range between 70.degree. C.
and 200.degree. C. when subjected to temperature changes over time,
preferably not even when subjected to tenfold repetition of a
temperature/time profile between 70.degree. C. and 200.degree.
C.
[0037] Such phase transitions might disrupt the ORC process.
[0038] The problems addressed by the invention are also solved by
the use of a working medium having a critical pressure (pc) between
4000 kPa and 6500 kPa, preferably between 4200 kPa and 6300 kPa,
having a critical temperature (Tc) between 450 K and 650 K,
preferably between 460 K and 600 K, and having a molar mass between
50 g/mol and 80 g/mol, preferably between 60 g/mol and 75 g/mol, in
a heat engine, wherein the gaseous working medium partially
condenses out during adiabatic expansion within a cycle of the ORC
process.
[0039] The problems addressed by the invention are preferably
solved by the use of alkyl formates or cyclopentene or mixtures
thereof as the working medium in a heat engine.
[0040] It may be provided that methyl formate and/or ethyl formate
are employed as the alkyl formate, preference being given to
employing methyl formate or ethyl formate as the working medium in
the heat engine.
[0041] The process according to the invention is easy to implement
and thus cost effective in its realization.
[0042] As a further criterion the use of mixtures may be highly
advantageous for reducing the energy losses during heat transfer
since the evaporation thereof does not occur at constant
temperature.
[0043] Uses according to the invention may preferably provide that
the heat engine is operated with an ORC process. The substances and
substance classes at issue are particularly suitable for ORC
processes.
[0044] It may also be provided that the heat engine employed is an
expansion machine, preferably a vapour expansion engine comprising
pistons or at least one turbine as the engine.
[0045] It may finally also be provided that the heat engine is
operated with a heat source in a low-temperature range between
80.degree. C. and 200.degree. C., preferably between 80.degree. C.
and 150.degree. C.
[0046] The working media intended for use are particularly suitable
for the low temperature range.
[0047] One fundamental finding of the is that working media having
suitable physical properties in terms of critical pressure,
suitable boiling point and suitable behaviour during adiabatic
expansion, namely partial condensation, may be used to carry out an
ORC process in a heat engine with which low-temperature offgas
streams too may be utilized for conversion into electricity without
the occurrence of other deleterious effects.
[0048] Accordingly, endeavours in the context of the present
invention led to the development of novel, efficient working
fluids/working media for a heat engine.
[0049] In order to achieve the objective of efficient utilization
of waste heat, endeavours in the context of the present invention
led to the identification and development of working media (i.e.
working fluids) for low temperature applications which not only
achieve maximum thermodynamic efficiency but are also optimal from
safety and environmental aspects.
[0050] Of central importance for the suitability of a chemical
substance as a working medium are in particular the following
material data/measured parameters which are characterizable by the
derivable parameters and relationships that they intimate.
[0051] 1. Vapour pressure: [0052] characterizable by the
temperature and pressure range of the process (low- or
high-temperature) [0053] derivation of the gradient of the
saturated vapour line in the T-S diagram from .DELTA.h.sub.Lv,
C.sub.p, (2 methods) (wet or dry fluid, condensation during
adiabatic expansion) [0054] large enthalpy of vaporization (large
pressure ratio of upper to lower process pressure) [0055]
derivation of optimal process conditions
[0056] 2. Heat capacity: [0057] derivation of the gradient of the
saturated vapour line in the T-S diagram from .DELTA.h.sub.Lv,
C.sub.p, (heat transfer area capital expenditure costs)
[0058] 3. Thermal and chemical stability: [0059] high thermal and
chemical stability (in contact with steel, lubricants, seals, air,
water)
[0060] 4. Viscosity: [0061] general applicability, pump work, heat
transfer (heat exchanger capital expenditure costs)
[0062] 5. Corrosivity: [0063] low propensity for corrosion
[0064] 6. Criticality data: [0065] critical temperature, critical
pressure and critical volume
[0066] 7. Thermal conductivity: [0067] heat transfer
[0068] 8. Density: [0069] heat transfer [0070] apparatus
dimensioning (high vapour density.fwdarw.low specific
volume.fwdarw.small streams)
[0071] 9. Molar mass: [0072] It is a tendency that: the greater the
molecules the higher the critical volume of the critical
temperature and the poorer the high-temperature resistance
[0073] .DELTA..sub.Lv is the enthalpy of vaporization at constant
volume, c.sub.p is the heat capacity at constant pressure,
T.sub.c,Fluid is the critical temperature of the working medium,
T.sub.process is the process temperature, T is the temperature and
S is the entropy.
[0074] One particular advantage of a heat engine filled with a
working medium according to the invention (for example the piston
expansion engine from DeVeTec GmbH) is that so-called "wet" working
fluids, which may be decompressed into the wet vapour region, may
be employed. Recuperation is not necessary for such a fluid and the
engine for performing the process may therefore be markedly
simplified.
[0075] Hereinbelow, exemplary embodiments of the invention and
diagrams relating to the invention are elucidated by reference to
eight schematically represented figures and diagrams without any
intention to restrict the invention. Dabei zeigt:
[0076] FIG. 1 shows a simplified schematic representation of an ORC
process/a heat engine for implementing a process according to the
invention;
[0077] FIG. 2 shows an ideal-type representation of the changes of
state for wet, dry and isentropic fluids in the ORC process in a
temperature-entropy diagram;
[0078] FIG. 3 shows a schematic representation of a setup for
determining the vapour pressure of suitable working media;
[0079] FIG. 4 shows the temperature/time profile for a calorimetric
measurement (DSC) for analyzing suitable working media;
[0080] FIG. 5 shows a vapour pressure/time diagram for determining
the thermal stability of 1-propanol at 195.degree. C. to
180.degree. C.;
[0081] FIG. 6 shows a vapour pressure/time diagram for methyl
formate at 150.degree. C.;
[0082] FIG. 7 shows a vapour pressure/time diagram for ethyl
formate at 150.degree. C.;
[0083] FIG. 8 shows cyclic differential thermal analysis diagrams
(DSC curves) for ethyl formate.
[0084] FIG. 1 shows a simplified schematic representation of an ORC
process for implementing a process according to the invention, i.e.
an ORC process, such as is carried out in a heat engine according
to the invention.
[0085] The ORC process depicted is a simple thermodynamic cycle in
which a working medium is evaporated and optionally superheated at
a high pressure level by supplying heat. The superheated vapour
undergoes expansion cooling to a lower pressure in an engine (for
example a turbine or piston engine) thus performing work. The
vapour exiting the expander may still be in the superheated state
or may already be decompressed to such an extent that it occupies
the wet vapour region so that some of the working medium is already
in the liquid state. Complete liquefaction takes place in the
condenser. Here, the electricity-generating cycle is operated not
with water but with an organic working fluid which can utilize the
heat generated at a low temperature level with greater
thermodynamic efficiency.
[0086] A parameter of central importance is the vapour pressure of
the components which firstly permits general classification for the
low- or high-temperature range. Efficient working fluids make it
possible to realize, for a given temperature of the heat source and
the heat sink, the greatest possible pressure ratio between the
upper and lower process pressure. This requirement may readily be
shown in a logarithmic representation of the vapour pressure via
the negative reciprocal absolute temperature as is shown in FIG. 2.
Since the gradient of the vapour pressure curve in the Raoult
diagram is proportional to the enthalpy of vaporization in
accordance with the Clausius-Clapeyron equation, working media
having large enthalpies of vaporization promise advantages on
account of the greater expected pressure ratio in the expander.
Together with the heat capacity there are also methods of
estimation that allow predictions to be made regarding the fluid
type (wet, dry or isentropic).
[0087] The changes of state of the working fluid in the cycle may
be depicted in the temperature (T) entropy (S) diagram. FIG. 2
shows the advancement of the process for different fluid types in
the T-S diagram with the simplification that the fluids are
decompressed in isentropic fashion. The working fluids may be
categorized according to the path of the saturation line and the
dew line into wet (negative gradient dew line), dry (positive
gradient dew line) and isentropic (vertical dew line) working
fluids. The substantial difference when using these different fluid
types in the ORC process lies in the state of the vapour after the
decompression. For wet and isentropic fluids the vapour is in the
superheated state only to a very limited extent, if at all, after
the decompression, i.e. the fluid is decompressed into the wet
vapour region so that liquid droplets are already present. In the
case of the dry fluids a superheated vapour is present which is at
a temperature higher than the condensation temperature. Depending
on the proportion of heat in the superheated steam it may be
necessary in the case of turbine utilizations to use this
unutilized heat for warming the cold fluid after the pressure
increase in order to achieve improved efficiencies for the process.
Process costs may simultaneously be increased by about 30% due to
the use of the additional heat exchanger.
[0088] In certain cases using wet fluids as ORC media is
advantageous and thus preferable since said fluids make it possible
to eschew a recuperator (heat exchanger). The further required
properties (see above) only come into play after this fundamental
requirement has been met but are then no less important. The most
important requirements include thermal and chemical stability, low
viscosity, no corrosivity, no toxicity, easy handleability
(explosion limits outside operating conditions, no
flammability).
[0089] In order to operate the ORC process in economic fashion,
preference among the potential working media is given to
wet/isentropic behaviour in order that a recuperator may be
eschewed. A medium is referred to as a wet fluid when the gradient
of the dew line in the T-S diagram is negative (FIG. 2). This
results in the formation of wet vapour upon isentropic
decompression starting from the dew line. When the dew line is
vertical the medium is referred to as isentropic and when the
gradient is positive the medium is referred to as dry.
[0090] In order to evaluate the thermodynamic suitability of new
working media in the ORC process a model of the cycle was
constructed in the "Aspen Plus" computer simulation program which
allows the thermal efficiency to be calculated as a function of the
medium employed and the temperature of the available heat
source.
[0091] The following boundary conditions derived from the
apparatuses employed by DeVeTec apply to the simulation: [0092]
efficiency of the pump: 65% [0093] maximum pressure: 35 bar [0094]
efficiency of the expansion machine: 88% [0095] final conditions of
the expansion: either 1.1 bar or 35.degree. C. [0096] total
condensation without supercooling
[0097] The maximum temperature in the evaporator is accordingly a
degree of freedom. The simulations were performed for various
temperatures: 100.degree. C., 150.degree. C., 200.degree. C. and
250.degree. C. The thermal efficiency of the process was evaluated
for the various conditions.
[0098] The efficiency is generally defined as:
.eta. = Q useful Q supplied ##EQU00001## [0099] .eta.--efficiency
[0100] Q.sub.useful --useful energy [0101] Q.sub.supplied--supplied
energy
[0102] In the case of the organic Rankine cycle process (ORC
process) the utility is the output of the expansion machine. The
input is composed of the power of the pump and the supplied
heat.
[0103] Evaluation of the simulations makes it possible to compile a
list of the theoretically achievable efficiencies for the various
operating conditions. Ethanol was defined as the reference medium.
The particularly suitable working media found in the context of the
present invention were compared with the working medium ethanol for
various temperatures. In general terms it should be noted that the
choice of working medium is dependent on the heat source available.
Depending on the evaporator temperature certain working media are
more or less suitable for use as the working medium in a heat
engine.
TABLE-US-00001 TABLE 1 Efficiency at the following maximum
temperatures 200.degree. C. 150.degree. C. 100.degree. C. methyl
formate 22.65 19.82 13.72 2,3-dihydrofuran 20.98 16.50 9.46
tetrahydrofuran 19.42 14.60 7.03 cyclopentene 20.76 16.78 10.30
ethyl formate 20.46 16.19 9.34 ethanol 18.20 13.02 4.75
[0104] Compared to ethanol there is a marked improvement in
efficiency at lower use temperatures. Further investigations were
carried out for the use of the selected particularly preferable
substances. In particular, the stability of the substances at the
use temperature was analyzed.
[0105] The vapour pressure is the pressure established when a
vapour is in thermodynamic equilibrium with the associated liquid
phase in a sealed system. The vapour pressure increases with
increasing temperature and is a function of the substance/mixture
present. When the vapour pressure of a liquid is equal to the
ambient pressure in an open system the liquid begins to boil.
[0106] The vapour pressure is one of the crucial substance
properties for the design and operation of an ORC plant. Due to the
operating conditions defined for the vapour engine the vapour
pressure of a suitable liquid should be below 35 bar.
[0107] The vapour pressures of the working media are determined in
a sealed and temperature-controlled high-pressure autoclave. This
comprises heating the liquid and measuring the pressure at the
particular temperature setting. The more accurate the measurement
of these two values the better the determined vapour pressure data.
Calculations may be performed with "Aspen Plus" for comparison with
the literature values. In the case of deviations in the data,
in-house measurements of the vapour pressure may then be
performed.
[0108] Specific heat capacity indicates the amount of heat that
needs to be supplied to a kilogram or a mole of a particular
substance to raise its temperature by 1 Kelvin.
[0109] These substance-specific data are necessary in particular
for the design of the heat engineering components of an ORC system.
Experimental determination of the data is performed in a
calorimeter. Heat capacity is generally measured using DSC
(differential scanning calorimetry).
[0110] Viscosity is a measure of the resistance of a fluid to
deformation and influences heat transfer and pump performance in an
ORC system. For comparison at 20.degree. C. water has a viscosity
of about one mPas, edible oils have a viscosity of about 100 mPas
and honey has a viscosity of about 1000 mPas. The lower the
viscosity the more mobile a liquid and the quicker said liquid can
flow under constant conditions. Suitable ORC working media should
therefore have a low viscosity of less than 10 mPas at 20.degree.
C.
[0111] The chosen working media all have a rather low viscosity
which is comparable to the viscosity of water (about 1 mPas at
20.degree. C.). In the region above about 100.degree. C. which is
of interest for an ORC system the viscosities of the preselected
working media hardly differ from one another anymore.
[0112] One of the further important substance properties for the
design of a thermodynamic cycle is the density of the liquid and
gaseous phase of the working medium.
[0113] The density of the working media is essential to the design
of the circulation pumps. Volume flow is converted into mass flow
using the density of the substances.
[0114] The data for the cited physical parameters of the various
substances are obtainable from the literature and/or from databases
concerning the working media analyzed.
[0115] The enthalpy of vaporization is the amount of heat required
to effect the transition of a liquid from the liquid into the
gaseous state. The converse process in which the gaseous medium is
reliquefied gives off the heat of condensation. Both parameters are
of great importance for a thermodynamic cycle in which a liquid is
continually evaporated and recondensed.
[0116] The enthalpy of vaporization may be obtained from the
literature or, similarly to the heat capacity, measured by
calorimetric methods (for example by DSC).
[0117] The vapour pressure is one of the most important physical
substance properties of a working medium. Designing an ORC system
and validating the simulation data require accurate knowledge of
the vapour pressure curve. Equipment allowing accurate measurement
in an absolute pressure range from 0 bar to 100 bar and at
temperatures from 20.degree. C. to 400.degree. C. was constructed
for the experimental determination of said curve. Since accurate
measuring means for such a large measurement range are not
available the equipment was divided into three measurement regions.
Table 2 which follows summarizes the permissible operation data for
the individual autoclaves.
TABLE-US-00002 TABLE 2 Design parameters for the vapour pressure
measuring apparatus autoclave 1 autoclave 2 autoclave 3 temperature
range 20-150.degree. C. 20-250.degree. C. 20-350.degree. C.
pressure range 0-2 bar 2-50 bar 50-100 bar volume 100 ml 100 ml 100
ml
[0118] Measurement accuracy was enhanced by using pressure sensors
(from Endress & Hauser) calibrated for the relevant pressure
and temperature range. The autoclaves were heated using an electric
heating collar. Temperature control was effected by measuring the
temperature in the individual autoclave and in the heating collar
using precise Ni-Cr temperature sensors and comparing these
temperatures with one another. The autoclaves were sealed using
special copper washers and copper paste. The apparatus and the
conduits were fully insulated to reduce heat losses and achieve
improved controllability. The integrated vacuum pump makes it
possible to obtain measurements under high vacuum. The vacuum is
also required in particular when changing the fluids for cleaning
purposes and for purging the measuring means with nitrogen for
avoiding explosive atmospheres. Readings were acquired using an
automatic data acquisition means with a sampling rate of one second
for the entire duration of the test. A basic schematic construction
of the measuring means is shown in FIG. 3.
[0119] Startup and calibration of the measuring means was carried
out with ethanol and water, ethanol being suitable for the pressure
range up to 60 bar. The vapour pressure of water was measured at
pressures up to 100 bar. The two substances were also chosen
because the data for the substances are well known and may be
consulted for validation of the apparatus. It was found that the
deviation is below 1% of the absolute value and the method of
measurement is therefore suitable for the further investigations.
For the high-pressure range too the measuring means was
sufficiently validated with the data for water.
[0120] The actual suitability as a working medium depends not only
on the maximum obtainable efficiency but also to a substantial
extent on the long-term stability of the substances when in use.
Thermal decomposition of the substances can result in undesired
byproducts which can lead, for example, to a reduction in the
vapour pressure or to corrosion of the materials employed in the
heat engine. In the first screening operation the working media
were subjected to short-term stress and analyzed in terms of a
plurality of criteria. Four substances were selected therefrom for
further tests. The second test phase comprised carrying out
extensive corrosion and material compatibility tests. The third
test phase comprised carrying out long-term tests.
[0121] The working media were subsequently tested in a heat engine
under realistic conditions.
[0122] Knowledge of the thermal stability of a substance is
generally indispensable. An untested substance may suffer a loss of
quality and give rise to unforeseeable hazards due to excessive
temperatures during production, storage and transport. It is an
important feature of the working media sought that no undesired
decomposition products are generated during use which could
endanger the operation of the plant.
[0123] Thermal stability was determined using the following
principle of measurement:
[0124] The working media were charged into an autoclave at room
temperature and inertized with nitrogen. The temperature of the
medium was subsequently increased up to a maximum use temperature
and sustained for a prolonged period. The vapour pressure of the
substance was initially determined at room temperature and compared
with literature values. This was followed by continuous
determination of the vapour pressure as a function of temperature
and long-term measurement at the maximum temperature. On completion
of the test the working medium was cooled and analyzed by gas
chromatography.
[0125] The gas chromatograph (GC) allows the composition of
substance mixtures to be determined. This results in a chromatogram
in which all substances are unambiguously assigned. The measurement
is performed for an untreated laboratory-tested substance. This
makes any decomposition products formed unambiguously determinable.
The measurement makes it possible to determine not only the type of
byproducts but also the percentage fraction thereof.
[0126] A further method for determining thermal stability is
differential scanning calorimetry (DSC). This method was used to
determine stability over a plurality of cycles.
[0127] DSC comprises heating two sealed crucibles (first crucible
containing about 10 mg of sample and second empty crucible as
reference) at a predetermined heating rate (10 Kelvin/minute in
this case) up to a target temperature (up to 200.degree. C. in this
case). Both crucibles are subjected to the same temperature
program. The energy absorption or decrease is analyzed during
heating. The energy balance changes in comparison to the empty
sample depending on the the heat capacity of the sample or
exothermic and endothermic processes in the sample such as melt or
vaporization. Once heating is complete the sample is held at a
constant maximum temperature. For a thermally stable substance no
energy changes occur during this time. Decomposition of the
substance is observed via a change in the energy absorption or
energy decrease.
[0128] FIG. 4 shows the employed temperature/time profile for the
DSC. Over the time period from 0-20 minutes the temperature is
increased as a constant heating rate and energy is correspondingly
absorbed. In the range between 20-50 minutes the temperature is
kept constant. For a stable medium no absorption or emission of
energy occurs. Between 50-70 minutes the sample is cooled down
again and the temperature is reduced with a corresponding energy
decrease.
[0129] The reproducibility of the measurement was confirmed by
carrying out a plurality of cycles per medium. This is because the
decomposition products may also arise only after a prolonged
operating time and a plurality of cycles.
[0130] Since the selected working media could exhibit corrosive
behaviour towards the employed materials of the ORC engine,
extensive investigations into corrosion behaviour were carried out.
To this end, both metallic and nonmetallic (largely elastomeric
materials of the seals) materials were defined and investigated in
conjunction with the individual media.
[0131] For metallic materials samples having defined dimensions
were prepared. To determine the erosion rates the metal strips were
weighed and completely submerged in the respective fluid in an
autoclave. The autoclaves were sealed, inertized and brought to a
defined temperature and held at this temperature over a prolonged
period. The metal strips were subsequently removed again, cleaned
and weighed to determine the erosion rate. To determine any local
corrosion the individual samples were examined by microscopy.
[0132] To investigate the corrosion behaviour of the working media
towards nonmetallic materials the following tests were
conducted:
[0133] Long-term thermal stability is crucial for trouble-free
operation of an ORC system. However, working media may be
decomposed by use at high temperatures. The stability of a novel
working medium must therefore be established prior to its use. The
relevant tests were carried out in a high-pressure autoclave with
the objective of determining the maximum use temperature of each
medium. The test temperature and test pressure were measured.
Decomposition of the fluid also results in a change in vapour
pressure. This change may in turn be observed by reference to the
measured values. The decomposition products were analyzed in a gas
chromatograph and compared with the starting product.
[0134] FIG. 5 depicts the investigation of the thermal stability of
1-propanol at 195.degree. C. and 180.degree. C. The measured vapour
pressure (upper curve) increases with time at constant temperature
(lower curve) at various temperatures between 195.degree. C. and
180.degree. C. This shows that 1-propanol is not stable at these
use temperatures. Below 180.degree. C. the vapour pressure becomes
too low (less than 20 bar) to be usefully employable as the working
medium in a heat engine. In the experimental setup of FIG. 6 methyl
formate was stored at a temperature (upper curve) of about
150.degree. C. The vapour pressure (lower curve) remains constant
and the fluid may therefore be described as stable at this
temperature.
[0135] All potential working media were investigated for use
temperatures of from 150.degree. C. to 200.degree. C. in this
fashion. Ethyl formate also exhibits similar behaviour to methyl
formate (FIG. 7). At a use temperature of 175.degree. C. this fluid
undergoes slight decomposition over time. At a use temperature of
150.degree. C. (upper curve in FIG. 7) it remains stable, i.e. the
vapour pressure (lower curve in FIG. 7) does not increase.
[0136] The working media methyl formate, ethyl formate and
cyclopentene are particularly advantageous on account of these
investigations for example. The extended investigations tested the
thermal stability of the preselected working media in a longer test
of two months in duration.
[0137] The working media tested were stored in high-pressure
autoclaves at an operating temperature of 150.degree. C. After the
test the decomposition rate of all samples was investigated by GC
analysis to determine thermal stability. The results of this
analysis are summarized in table 3. The maximum decomposition of
the working media methyl formate, ethyl formate and cyclopentene
was about 2% and is therefore in an industrially acceptable
range.
TABLE-US-00003 TABLE 3 Degree of purity and decomposition of the
working media after subjection to thermal stress for 2 weeks.
before test after test percent decomposition methyl formate 98.62%
97.21% 1.43% ethyl formate 97.28% 95.16% 2.18% cyclopentene 98.51%
98.31% 0.20%
[0138] The following investigations sought to test the extent to
which the working media employed exhibit corrosive behaviour
towards the typical materials employed in heat engines. The
following materials were tested in the corrosion tests: unalloyed
steel (P265GH) and alloyed steel (1.4571) including a weld seam.
The materials were employed in the form of sheet-metal (90
mm.times.10 mm.times.6 mm). The test specimens were weighed in a
materials engineering laboratory and characterized by optical
microscopy. The test was then carried out in the abovedescribed
apparatus for measuring vapour pressure. Once the samples were
removed evaluation was once again performed in the materials
engineering laboratory. The results of the first corrosion
investigation are shown in table 4.
TABLE-US-00004 TABLE 4 Test results and evaluation of the corrosion
tests working erosion rate microscopy material medium [mm/a]
findings unalloyed steel (P265GH) methyl formate 0.0301 no findings
alloyed steel (1.4571) 0.0024 no findings unalloyed steel (P265GH)
ethyl formate 0.0384 no findings alloyed steel (1.4571) 0.0034 no
findings unalloyed steel (P265GH) cyclopentene 0.0317 no findings
alloyed steel (1.4571) 0.0013 no findings
[0139] Although the optical microscopy evaluation found no local
corrosion and cracks a crack test was additionally performed on
material 1.4571 using the penetration method. This also found no
cracks. The technical stability limit for metallic materials is
given by an erosion rate of .ltoreq.0.1 mm/annum. There must
moreover not be any instances of local corrosive attack since these
preclude technical stability of the materials. The two material
classes tested must accordingly be ranked as having technical
stability towards the preferred working media methyl formate, ethyl
formate and cyclopentene under the cited test conditions at
150.degree. C., i.e. the three working media are fundamentally
suitable.
[0140] Cyclic stability tests carried out with the described DSC
method showed no deterioration/change in the working media (methyl
formate, ethyl formate and cyclopentene). By way of example FIG. 8
shows the cyclic DSC curves of ethyl formate, with methyl formate
and cyclopentene also showing similar curves. The upper curve once
again shows the employed temperature profile for the DSC
measurement. The lower set of curves represents the results of the
DSC measurement. All three working media therefore show sufficient
long-term storage.
[0141] Measurements of the efficiency of the cycle with the
selected working media (methyl formate, ethyl formate and
cyclopentene) and the abovementioned tests determined that fluids
are particularly suitable for use in the heat engine when the
critical pressure p.sub.c is between 4000 kPa and 6500 kPa, in
particular between 4200 kPa and 6300 kPa, particularly preferably
between 4700 kPa and 6000 kPa, the fluids have a critical
temperature (T.sub.c) between 450 K and 650 K, preferably between
460 K and 600 K, particularly preferably between 475 K and 510 K,
and the fluids have a molar mass between 50 g/mol and 80 g/mol,
preferably between 60 g/mol and 75 g/mol. Such fluids are also
usable with a high degree of efficiency at low temperatures of the
offgas to be utilized/at a low temperature of the evaporator. It
has been found that to simplify the construction of the heat engine
the use of a recuperator (heat exchanger) may be eschewed when a
"wet" working medium is employed. The working medium is referred to
as a "wet" working medium when the gaseous working medium undergoes
partial condensation upon adiabatic expansion.
[0142] These criteria are well met by the preferred working media
methyl formate, ethyl formate and cyclopentene. Thus, the critical
temperature (T.sub.c) of methyl formate is 487 K, that of ethyl
formate is 508 K and that of cyclopentene is 507 K. The critical
pressure p.sub.c of methyl formate is 5998 kPa, that of ethyl
formate is 4742 kPa and that of cyclopentene is 4820 kPa. The molar
mass of methyl formate is 60 g/mol, that of ethyl formate is 68
g/mol and that of cyclopentene is 74 g/mol. All three of these
working media undergo partial condensation upon adiabatic expansion
and it is therefore possible to eschew a recuperator in the circuit
of the ORC.
[0143] For the simulation conditions the efficiency of the
inventive working media in a heat engine at an offgas temperature
(evaporator temperature) between 80.degree. C. and 200.degree. C.
is superior to prior art working media for heat engines, for
example ethanol. These results were confirmed by experiment for
methyl formate in an ORC engine (piston expansion engine) from
Devetec GmbH. The working media according to the invention thus
achieve an improvement in the efficiency of the heat engine at
temperatures between 80.degree. C. and 200.degree. C., in
particular between 80.degree. C. and 150.degree. C.
[0144] The features of the invention disclosed in the above
description and in the claims, figures and exemplary embodiments
may be essential to the realization of the invention in its various
embodiments either individually or in any desired combination.
* * * * *