U.S. patent application number 15/426577 was filed with the patent office on 2017-09-07 for supercritical fluid heat engine.
This patent application is currently assigned to ROLLS-ROYCE PLC. The applicant listed for this patent is ROLLS-ROYCE PLC. Invention is credited to Paul FLETCHER.
Application Number | 20170254229 15/426577 |
Document ID | / |
Family ID | 55858957 |
Filed Date | 2017-09-07 |
United States Patent
Application |
20170254229 |
Kind Code |
A1 |
FLETCHER; Paul |
September 7, 2017 |
SUPERCRITICAL FLUID HEAT ENGINE
Abstract
A supercritical working fluid closed cycle heat engine includes,
in fluid flow series: a heat source heat exchanger configured to
raise the temperature of a working fluid; an expander configured to
extract work from the working fluid; a condenser configured to
exchange heat between the working fluid and the cooling medium; and
a separator configured to separate liquid phase working fluid from
gaseous phase working fluid and deliver gaseous phase working fluid
to a gaseous compressor and liquid phase working fluid to a fixed
displacement liquid pump, the compressor and pump being arranged in
parallel. Each of the compressor and fixed displacement pump are
configured to return fluid to the heat source heat exchanger. The
compressor comprises a variable inlet guide vane and a variable
outlet diffuser vane.
Inventors: |
FLETCHER; Paul; (Rugby,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROLLS-ROYCE PLC |
London |
|
GB |
|
|
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
55858957 |
Appl. No.: |
15/426577 |
Filed: |
February 7, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01K 25/103 20130101;
F02C 6/14 20130101; F02C 1/105 20130101; F02C 6/00 20130101; F01K
23/10 20130101; F01K 11/02 20130101; F02C 6/18 20130101; F05D
2260/611 20130101; F01K 7/165 20130101; F02C 6/16 20130101; Y02E
20/16 20130101; F02C 1/10 20130101 |
International
Class: |
F01K 25/10 20060101
F01K025/10; F01K 7/16 20060101 F01K007/16; F01K 11/02 20060101
F01K011/02; F01K 23/10 20060101 F01K023/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2016 |
GB |
1603699.8 |
Claims
1. A supercritical working fluid closed cycle heat engine
comprising, in fluid flow series: a heat source heat exchanger
configured to raise the temperature of a working fluid; an expander
configured to extract work from the working fluid; a heat exchanger
configured to exchange heat between the working fluid and a cooling
medium; and a separator configured to separate liquid phase working
fluid from gaseous phase working fluid and deliver gaseous phase
working fluid to a gaseous compressor and liquid phase working
fluid to a fixed displacement liquid pump, the compressor and pump
being arranged in parallel; each of the compressor and fixed
displacement pump being configured to return fluid to the heat
source heat exchanger; wherein the compressor comprises a variable
inlet guide vane and a variable outlet diffuser vane.
2. An engine according to claim 1, wherein the heat source
comprises an exhaust gas of a gas turbine engine comprising a core
compressor, a combustor and a core turbine.
3. An engine according to claim 2, wherein the heat source
comprises a heat exchanger configured to exchange heat between
exhaust gasses from the gas turbine engine and the working fluid of
the heat engine.
4. An engine according to claim 1, wherein the engine comprises a
recuperator heat exchanger configured to exchange heat between
working fluid downstream of the expander and working fluid upstream
of the heat source heat exchanger.
5. An engine according to claim 1, wherein the engine comprises a
working fluid storage vessel configured to control engine cycle
working fluid mass via a control valve.
6. An engine according to claim 1, wherein the compressor comprises
a centrifugal compressor.
7. (canceled)
8. A method of controlling an engine according to claim 1, the
method comprising the steps of: determining a temperature
difference across the heat source heat exchanger; and controlling
the gaseous compressor rotational speed such that the temperature
difference matches a target value.
9. A method according to claim 5, wherein the method further
comprises determining a gaseous compressor inlet pressure, and
controlling the working fluid storage vessel control valve to
maintain gaseous compressor inlet pressure at a target value.
10. A method according to claim 9, wherein the method comprises
determining a gaseous compressor outlet pressure, and the target
gaseous compressor inlet pressure is determined in accordance with
a schedule relating compressor pressure ratio and the temperature
difference across the heat source heat exchanger.
11. A method according to claim 8, wherein the method comprises
controlling the compressor variable inlet guide vane and a variable
outlet diffuser vane on the basis of a schedule relating vane angle
and one or more of compressor speed, compressor inlet mass flow,
compressor inlet temperature, and compressor inlet pressure.
12. A method according to claim 8, wherein the method further
comprises determining a gaseous compressor inlet pressure, and
controlling the working fluid storage vessel control valve to
maintain gaseous compressor inlet pressure at a target value.
Description
[0001] The present disclosure concerns a heat engine comprising a
supercritical working fluid.
[0002] Heat engines employing supercritical working fluids as part
of the thermodynamic cycle are known. One example comprises
supercritical carbon dioxide (sCO.sub.2). A supercritical fluid is
a gas which is held at a temperature and pressure above a critical
point for that fluid where distinct liquid and gaseous phases do
not exist. A supercritical fluid therefore behaves like a gas, in
that it will occupy all available volume, but has a density similar
the liquid phase of the fluid. For example, supercritical CO.sub.2
occurs where pure CO.sub.2 is held at a temperature greater than
30.degree., and pressures greater than 70 atmospheres.
[0003] One known supercritical engine cycle is described in
"Initial Test Results of a Megawatt-Class Supercritical CO.sub.2
Heat Engine" by Timothy Held, presented at the 4th International
Symposium of Supercritical CO2 Power Cycles held in Pittsburgh,
Pa., USA on Sep. 9-10 2014. This describes a system in which
CO.sub.2 is utilised in a Brayton cycle heat engine, wherein the
CO.sub.2 is in a supercritical phase during at least part of the
cycle. sCO.sub.2 is pumped around a closed loop cycle, in which the
working fluid is in turn heated in a temperature raising heat
exchanger, expanded in a turbine and cooled by a condenser, before
being pumped back around by the pump. Work from the turbine powers
the pump, and also a generator. CO.sub.2 (which may be in a liquid,
gaseous or supercitical phase) is held in a storage vessel, and
removed or added to the cycle in order to maintain system pressure
in response to fluctuations in process temperature. Such a system,
in which working fluid mass can be altered during operation, is
known in the art as "inventory control". Operational control of the
cycle is provided by throttle valves, which control working fluid
flow rates to maintain temperatures and pressures within an
operating range.
[0004] Such a system is highly efficient and compact. However, such
systems are relatively inflexible, since the system efficiency and
available power will be dependent on the temperature of the cooling
fluid used to cool the working fluid in the condenser. If this
temperature varies (as will be the case where atmospheric air or
sea water is used as the coolant), then the condenser may not cool
the CO.sub.2 sufficiently to produce liquid or supercritical
CO.sub.2, which will then require a compressor to raise the
pressure of the working fluid. A compressor must necessarily use
more power to pump a given mass of fluid to a given pressure than a
liquid phase pump, resulting in lower cycle efficiency. On the
other hand, where a compressor is provided, such devices generally
have a relatively narrow band of operating flow at a given level of
pressure ratio (i.e. the ratio between the inlet and outlet
pressures) under which they can operate without stalling.
Furthermore, variation in heat input to the working fluid in the
temperature raising heat exchanger will also affect the efficiency
of the system, and the condenser will only be able to handle a
fixed volume of working fluid. Furthermore, such systems are
generally slow to react to increased or reduced power demand due to
thermal inertia.
[0005] Consequently, it is desirable to provide a supercritical
working fluid heat engine and a method of operating such an engine
which is highly efficient at different operating temperatures, and
is capable of varying its output power quickly.
[0006] According to a first aspect of the invention there is
provided a supercritical working fluid closed cycle heat engine
comprising, in fluid flow series:
[0007] a heat source heat exchanger configured to raise the
temperature of a working fluid;
[0008] an expander configured to extract work from the working
fluid;
[0009] a heat exchanger configured to exchange heat between the
working fluid and the cooling medium; and
[0010] a separator configured to separate liquid phase working
fluid from gaseous phase working fluid and deliver gaseous phase
working fluid to a gaseous compressor and liquid phase working
fluid to a fixed displacement liquid pump, the compressor and pump
being arranged in parallel;
[0011] each of the compressor and fixed displacement pump being
configured to return fluid to the heat source heat exchanger;
[0012] wherein the compressor comprises a variable inlet guide vane
and a variable outlet diffuser vane.
[0013] Consequently, the heat engine is capable of operating over a
wide range of heat source and cold source temperatures, while
providing high efficiency and high reliability. By pumping as much
liquid as possible, and compressing the gaseous phase of the
working fluid separately, the power absorbed to raise the pressure
is reduced. On the other hand, since the relative amounts of liquid
and gaseous phase working fluids will change depending on cycle
pressure and temperatures, the volume of gaseous phase working
fluid will vary considerably. The invention solves this problem by
providing a gaseous phase compressor having a variable inlet guide
vane and a variable outlet diffuser vane, which is therefore
capable of accepting a wide variation in gaseous fluid flow
volume.
[0014] The heat source may comprise an exhaust gas of a gas turbine
engine comprising a core compressor, a combustor and a core
turbine. Alternatively or in addition, the heat source may comprise
one of a reciprocating internal combustion engine, a nuclear
reactor, and other heat sources. The heat source may comprise a
heat exchanger configured to exchange heat between exhaust gasses
from the gas turbine engine and the working fluid of the heat
engine.
[0015] The engine may comprise a recuperator heat exchanger
configured to exchange heat between working fluid downstream of the
expander and working fluid upstream of the heat source heat
exchanger.
[0016] The engine may comprise a working fluid storage vessel
configured to control engine cycle working fluid mass via a control
valve.
[0017] The compressor may comprise a centrifugal compressor. The
pump may comprise a fixed displacement pump.
[0018] According to a second aspect of the present invention there
is provided a method of operating a heat engine in accordance with
the first aspect of the present invention, the method comprising
the steps of:
[0019] determining a temperature difference across the heat source
heat exchanger; and
[0020] controlling the gaseous compressor rotational speed such
that the temperature difference matches a target value.
[0021] The method may further comprise determining a gaseous
compressor inlet pressure, and controlling the working fluid
storage vessel control valve to maintain gaseous compressor inlet
pressure at a target value.
[0022] The method may comprise determining a gaseous compressor
outlet pressure, and the target gaseous compressor inlet pressure
may be determined in accordance with a schedule relating compressor
pressure ratio and the temperature difference across the heat
source heat exchanger.
[0023] The method may comprise controlling the compressor variable
inlet guide vane and a variable outlet diffuser vane on the basis
of a schedule relating vane angle and one or more of compressor
speed, compressor inlet mass flow, compressor inlet temperature,
and compressor inlet pressure.
[0024] The skilled person will appreciate that except where
mutually exclusive, a feature described in relation to any one of
the above aspects of the invention may be applied mutatis mutandis
to any other aspect of the invention.
[0025] Embodiments of the invention will now be described by way of
example only, with reference to the Figures, in which:
[0026] FIG. 1 is a schematic view of a heat engine in accordance
with the present invention; and
[0027] FIG. 2 is a schematic view of a component part of the heat
engine shown in FIG. 1.
[0028] With reference to FIG. 1, a heat engine is generally
indicated at 10. The heat engine uses a working fluid which is
supercritical during at least part of the cycle.
[0029] The cycle typically uses CO.sub.2 as its working fluid,
though it will be understood that other supercritical working
fluids could be employed.
[0030] The heat engine 10 comprises a heat source in the form of a
gas turbine engine 12. The gas turbine engine comprises an open
Brayton cycle heat engine, compressor a gaseous phase core air
compressor 14, a combustor 16, and a gaseous phase core turbine 18.
In operation, air is directed into the compressor 14 at a
compressor inlet temperature T1G and a compressor inlet pressure
P1G where it is compressed and directed into the combustor 16. Fuel
is added and combusted in the combustor 16, thereby raising its
temperature. The combustion products are directed to the turbine 18
at a turbine inlet temperature T4G and turbine inlet pressure P4G,
where the gas is expanded, thereby reducing the pressure and
temperature of the combustion gasses to a turbine outlet
temperature T5G and pressure P5G, and driving the turbine 18. The
compressor 14 and turbine 18 are interconnected by a shaft 20, such
that rotation of the turbine 18 drives the compressor 14.
[0031] The gas turbine engine 18 is open cycle, such that exhaust
gasses from the turbine 18 are ultimately vented to the atmosphere.
However, the exhaust gasses from the gas turbine engine are at a
relatively high temperature, and so, prior to venting to the
atmosphere, exhaust gasses are passed through a waste heat recovery
heat exchanger 32, which thereby heats a working fluid of a closed
cycle heat recovery engine 30. The closed cycle heat recovery
engine 30 includes the cold side of the waste heat recovery heat
exchanger 32, a turbine 34, a recuperator 36, a working fluid
storage container 40 and controlling valve 38, a cooler/condenser
42, a gaseous phase compressor 44, a liquid phase pump 46, a
generator 48 and a shaft 50. The operation and interconnection of
each of these components is described in further detail below.
[0032] Heated working fluid from the waste heat recovery heat
exchanger 32 at a temperature T4C and pressure P4C is delivered to
an inlet of the turbine 34 where it is expanded, thereby reducing
the temperature T5V and pressure P5C and driving the turbine 34,
and thereby also the gaseous phase compressor 44 via the
interconnecting shaft 50. A load such as an electrical generator 48
is also coupled to the shaft 50, and so the heat engine provides
useful output power.
[0033] Cooled working fluid downstream of the turbine 34 is passed
to a hot side of a recuperator heat exchanger 36, which is thereby
cooled. Cooled working fluid is then passed to a cooler heat
exchanger 42, which further cools the working fluid.
[0034] The cooler heat exchanger 42 is shown in further detail in
FIG. 2. The cooler heat exchanger 42 exchanges heat between the
working fluid and a cooling medium (for example cold water) to
thereby cool the working fluid. The cooling medium may comprise
water from an environmental source such as sea water, which may be
at a variable temperature. The cooler heat exchanger 42 comprises a
working fluid inlet channel 52, which is in thermal contact with a
coolant channel 53 to cool the working fluid. Downstream of the
area of thermal contact with the coolant channel 53, the working
fluid is cooled to a low temperature and pressure, such that the
working fluid is no longer supercritical, i.e. it has distinct
liquid and gaseous phases, which may be mixed together. The working
fluid is separated into its gaseous and liquid phases by a
separator comprising a grid 54 comprising a plurality of apertures
to allow liquid phase working fluid to enter a liquid phase outlet
channel 56. Gaseous phase working fluid continues, and exits
through a gaseous phase outlet channel 58.
[0035] The gaseous phase working fluid is directed through the
channel 58 to the gaseous phase compressor 44. The compressor 44 is
configured to compress the gaseous phase working fluid, to thereby
raise its pressure prior to delivery downstream. The compressor 44
comprises variable geometry features, including a variable area
inlet guide vane 60, and a variable area outlet diffuser 62. The
compressor typically comprises a centrifugal impellor. The variable
area inlet guide vane 60 is controllable to control the area of the
compressor inlet, to thereby control mass flow and inlet pressure
to the compressor 44. Similarly, the variable area outlet diffuser
is controllable to vary the outlet pressure of the compressor 44.
In conjunction, the guide vanes 60 and diffuser 62 control the
pressure ratio and mass flow of the compressor 44. The compressor
14 is configured to raise the temperature and pressure of the
working fluid to above the supercritical temperature and pressure
of the working fluid.
[0036] The liquid phase working fluid is directed through the
channel 56 to the liquid phase pump 46. The pump 46 is configured
to operate on the liquid phase working fluid, to thereby raise its
pressure prior to delivery downstream. The pump 46 comprises a
fixed displacement pump, which is typically driven by a separate
shaft to the compressor 44, and may be driven by an electric motor
for example. Typically, for a given mass flow rate, the work
required to pump the liquid flow through the pump 46 is typically
lower than the work required to compress the working fluid in the
compressor 44. The pump 46 is configured to raise the temperature
and pressure of the working fluid to above the supercritical
temperature and pressure of the working fluid.
[0037] Downstream of each of the compressor 44 and pump 46 is a
common manifold 64, which re-joins flows from the output of the
compressor 44 and the pump 46. The working fluid flows downstream
of the common manifold to a cold side of the recuperator 36, where
the working fluid is re-warmed, prior to flowing back through the
cold side of the waste heat recovery heat exchanger 32, where the
cycle is repeated.
[0038] The working fluid (CO.sub.2) inventory 40 is provided
between the recuperator heat exchanger 32 hot side and the cooler
heat exchanger 42. The inventory 40 comprises a pressurised
container which stores working fluid to maintain a desired total
pressure and mass within the heat engine cycle. Flow to and from
the inventory 40 is controlled by a valve 38, which may also pump
working fluid to and from the inventory. Under normal operation,
working fluid may bypass the inventory 40, a portion of the working
fluid may be withdrawn from the cycle, or additional working fluid
may be added to the cycle.
[0039] The operating cycle of the above described heat engine is
subject to various external factors which govern the power
generated and operability of the engine. Firstly, the heat input
into the hot side of the waste heat recovery heat exchanger 32 from
the heat source (in this case the gas turbine engine 12) is
dependent on the gas turbine engine turbine 18 exhaust mass flow
rate W5G and temperature T5G. Each of these is in turn dependent on
fuel flow and compressor inlet temperature T1G, which is in turn
dependent on environmental air temperatures. Consequently, heat
input to the waste heat recovery heat exchanger 32 is ultimately
dependent on air temperature and fuel flow, which equates to power
demand of the gas turbine engine 12. Similarly, the heat removal
from the hot side of the cooler heat exchanger 42 is dependent on
the temperature and flow rate of the coolant on the cold side of
the heat exchanger. Where this coolant comprises an environmental
fluid such as air or sea water, this temperature may vary
considerably in operation, particularly where the system is used on
a ship, which may travel between hot and cold geographic locations.
Meanwhile, it is necessary that the cycle can operated efficiently
and reliably throughout a wide range of input and output heating
conditions.
[0040] The above described engines can accommodate such variations
due to aspects of the design. The varying heat sink and cold sink
temperatures of the cycle will result in many cases to only part of
the working fluid being condensed to a liquid phase. By separating
the working fluid into its two constituent phases, the gaseous
phase can be compressed back to a supercritical state, while the
liquid phase can be pumped, thereby using less power, while
avoiding both pump cavitation and compressor stalling. Due to the
varying volumes of gas and liquid state working fluid entering the
compressor 44 and pump 46 respectively, the compressor is provided
with variable area inlet guide vanes 60 and outlet diffuser vanes
62, to thereby accommodate the variable mass flows and pressure
ratios provided over various temperature conditions at the heat
exchangers 32, 42. Consequently, maximum use can be made of both
low cold sink temperatures and high heat source temperatures and
mass flows without encountering operational difficulties in other
regimes. The provision of a liquid phase pump 46 also provides a
means for starting the cycle using relatively little motive
power.
[0041] In order to provide further flexibility in the operation of
the system, the optional inventory 40 and valve 38 provide control
over working fluid mass in the cycle, and so, for a given working
fluid temperature, the pressure level in the system can be altered.
Increasing the pressure level by adding working fluid from the
inventory 40 via the valve 38 will tend to increase the mass flow
of liquid phase working fluid to the pump 46, and reduce mass flow
of gaseous phase working fluid to the compressor 44 inlet. Removing
working fluid will have the opposite effect. Consequently, the
valve 38 provides additional control over the operational
cycle.
[0042] One proposed method of operation of the engine is as
follows.
[0043] The gas turbine engine 12 is controlled using fuel injector
actuators to control fuel flow, and optionally, compressor variable
inlet guide vanes to control compressor compression ratio. The gas
turbine engine 12 is controlled in a conventional manner as would
be understood by the skilled person to obtain a desired power
output and gas turbine entry temperature T4G. The turbine exhaust
gas will be at a corresponding varying outlet temperature T5G.
[0044] Alternatively, the system may monitor the power output from
the gas turbine engine turbine 18 and the heat recovery engine 34
to determine the current power output. The system may increase fuel
flow until the total power generated is equal to the desired power.
As the heat recovery turbine 34 power output would be expected to
lag the gas turbine engine turbine 18 power output, this may result
in an initially high output level from the gas turbine engine,
followed by reduced operation as the heat recovery turbine 34 power
output increases.
[0045] The temperature differential (T5G-T4C) across the waste heat
recovery heat exchanger 32 is controlled by varying the gaseous
compressor 44 (and possibly also liquid phase pump 46) rotational
speed, to thereby control compressor outlet pressure P3C, and
therefore working fluid density at the heat exchanger 32 inlet. In
general, high mass flow rates will result in a higher temperature
differential than a low mass flow rate for a given gas turbine
outlet temperature T5G in view of the higher resultant density, and
therefore higher available heat absorption capacity. Consequently,
the waste heat recovery cycle can be controlled to accommodate
increased or reduced heat input by increasing or reducing
respectively heat exchanger mass flow rates.
[0046] In order to control compressor 44 inlet pressure P1C in
order to maximise efficiency and minimise surge risks, the
inventory control valve 38 is operated to add or remove working
fluid from the cycle in accordance with a schedule of overall
pressure ratio P3C/P1C (i.e. the ratio of the compressor outlet
pressure to the compressor inlet pressure) vs gas turbine outlet
temperature to compressor inlet temperature ratio (T5G/T1C).
Consequently, the compressor overall pressure ratio can be
controlled in accordance with heat exchanger temperature
differential. As will be understood, the thermodynamically ideal
compressor pressure ratio will be dependent on the temperature
ratio across the cycle.
[0047] The variable inlet guide vane (60) and a variable outlet
diffuser vane (62) may be modulated on the basis of a schedule
relating vane (60, 62) angle and one or more of gaseous compressor
(44) speed (NC), compressor (44) inlet mass flow (W1C), compressor
(44) inlet temperature (P1C), and compressor (44) inlet pressure
(T1C).
[0048] Simultaneously, in order to control gaseous phase compressor
44 surge margin SM1C, compressor variable inlet guide vane angle
ANG1 and diffuser angle ANG3 are controlled in accordance with a
schedule versus one or more of shaft rotational speed N1C, gaseous
compressor working fluid mass flow W1C, gaseous compressor inlet
temperature T1C, and gaseous compressor inlet pressure P1C.
[0049] It will be understood that the invention is not limited to
the embodiments above-described and various modifications and
improvements can be made without departing from the concepts
described herein. Except where mutually exclusive, any of the
features may be employed separately or in combination with any
other features and the disclosure extends to and includes all
combinations and sub-combinations of one or more features described
herein.
[0050] For example, the liquid phase pump and/or gaseous phase
compressor may comprise a flow recirculation line to accommodate
increased pressure ratios. The system may utilise a different heat
source. The system may be suitable for land based power generation
or aircraft power generation. The heat source could comprise a
plurality of gas turbine or reciprocating engines. Different
control methods could be employed. Multiple parallel or series
gaseous phase compressors could be used. The gaseous flow
compressor and turbine could be geared relative to one another.
Where multiple series stage compressors are used, an intercooler
heat exchanger could be provided.
* * * * *