EP3757359A1

EP3757359A1 - Parallel regenerative cycle in organic rankine cycle with convective heat source

Info

Publication number: EP3757359A1
Application number: EP19182579.3A
Authority: EP
Inventors: Wolfgang Klink; Peter Simon Rop
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2019-06-26
Filing date: 2019-06-26
Publication date: 2020-12-30

Abstract

An Organic Rankine Cycle system (1) is provided, the system (1) comprising, arranged one behind the other in the direction of flow of an organic fluid in an Organic Rankine Cycle (2), a turbine (3), a regenerator (4) with a first side (5), a condenser (6), a feed pump (7), the regenerator (4) with a second side (8) and a heat recovery unit (9) with first heating surfaces (10), wherein the Organic Rankine Cycle (2) branches out between the condenser (6) and the regenerator (4) and reunites between the regenerator (4) and the heat recovery unit (9), forming first and second branches (11, 12), wherein the first branch (11) includes the regenerator (4) and the second branch (12) includes a second heating surface (13) arranged in the heat recovery unit (9) behind the first heating surfaces (10) in the direction of flow of a flue gas through the heat recovery unit (9). is provided.

Furthermore, a method for operating an Organic Rankine Cycle system (1) is provided.

Description

FIELD OF THE INVENTION

The present invention relates generally to an Organic Rankine Cycle system and, more particularly, to an improved overall energy conversion of an Organic Rankine Cycle (ORC) in a convective heat source, like a gas turbine + ORC combined cycle power plant.

BACKGROUND OF THE INVENTION

Gas turbine combined cycle power plants based on a water-steam bottoming cycle are a well-known technology. The Organic Rankine Cycle is a similar technology like water-steam but the ORC uses an organic working fluid which can be selected based on the available heat source temperature and the application.
The main disadvantage of the ORC bottoming cycle for gas turbines is the so called "dry fluid" behavior of the fluid, which means that during the expansion in the ORC turbine the fluid is still in the superheated region. As a consequence, the bottoming cycle efficiency (and also the overall combined cycle power plant efficiency) is very bad due to the comparatively large heat loss in the condenser.
In an ORC bottoming cycle, according to the state of the art, the liquid working fluid is therefore pumped via a heat exchanger (called regenerator) into a waste heat recovery unit (WHRU). The regenerator is a heat exchanger between the ORC turbine outlet and the condenser inlet to utilize the superheated energy at the turbine exhaust to heat-up the liquid working fluid before entering the WHRU. The liquid ORC working fluid will be heated up to approximately 125°C at the outlet of the regenerator before entering the waste heat recovery unit.
The disadvantage is that since this liquid temperature at the WHRU inlet is already relatively high, the hot flow from the convective heat source (like the exhaust of a gas turbine) cannot be cooled any further. The stack temperature remains quite high and the heat recovery rate of the WHRU is limited compared to a traditional water-steam application. This limits the overall energy conversion.
So, no matter which variant you choose, with or without regenerator, the overall energy conversion is not satisfactory. Either one loses heat in the condenser or in the stack.
It is therefore a goal of the present invention to provide an Organic Rankine Cycle system, which overcomes the above-mentioned disadvantage. A further goal of the invention is to provide a method for operating an Organic Rankine Cycle system.

SUMMARY OF THE INVENTION

The object of the invention is achieved by the independent claims. The dependent claims describe advantageous developments and modifications of the invention.
In accordance with the invention there is provided an Organic Rankine Cycle system comprising, arranged one behind the other in the direction of flow of an organic fluid in an Organic Rankine Cycle, a turbine, a regenerator with a first side, a condenser, a feed pump, the regenerator with a second side and a heat recovery unit with first heating surfaces. The invention is characterized in that the organic Rankine Cycle branches out between the condenser and the regenerator and reunites between the regenerator and the heat recovery unit, forming first and second branches, wherein the first branch includes the regenerator and the second branch includes a second heating surface arranged in the heat recovery unit behind the first heating surfaces in the direction of flow of a flue gas through the heat recovery unit.
The invention is for so called direct heat exchange cycles where the convective heat source is used in a Waste Heat Recovery Unit (WHRU) to evaporate the ORC working fluid. The essential idea of the present invention is that the ORC bottoming cycle design is changed in that way that the liquid ORC working fluid will be split into two streams.
The first stream is the "normal" way to the regenerator to heat-up the liquid working fluid as done in the standard cycle design in an ORC bottoming cycle;
The second stream is routed without pre-heating directly to a separate, additional economizer of the Waste Heat Recovery Unit to cool down the flue gas further before leaving the stack.
It is advantageous when the heat recovery unit is connected downstream of a gas turbine in the direction of flow of an exhaust gas.
It is still advantageous, when a split ratio of amounts of organic fluid flowing through the first and second branches is adjustable. The split ratio can be adjusted based on the specific needs to optimize the heat recovery and overall energy conversion. The split ratio is an additional optimization parameter in the design of the combined cycle power plant with gas turbine and Organic Rankine Cycle.
In an advantageous embodiment a separator is arranged between the heat recovery unit and the turbine. There the evaporated portion is separated from the non-evaporated portion of the organic fluid. The evaporated portion is fed to the downstream turbine and drives it. The non-evaporated portion is returned to the inlet of the WHRU.
In the method according to the present invention a liquid organic fluid is circulated to first heating surfaces of a heat recovery unit, where heat is introduced to the fluid in order to convert it to vapor, with the vapor then passing through a turbine, with the resulting cooled vapor then passing through a first side of a regenerator and a condenser one after the other.
The method according to the present invention is now characterized in that before the organic fluid flows through a second side of the regenerator a stream of the organic fluid is divided into first and second partial streams, wherein the first partial stream passes through the second side of the regenerator and the second partial stream passes through a second heating surface of the heat recovery unit, wherein the second heating surface is arranged in the heat recovery unit behind the first heating surfaces in the direction of flow of a flue gas through the heat recovery unit.
It is advantageous, when the step of introducing heat to the fluid in order to convert it to vapor is accomplished by directing exhaust gas of a gas turbine onto first and second heating surfaces in the heat recovery unit.
It is still advantageous, when a split ratio of the amounts of first and second partial streams is varied in order to optimize heat recovery and overall system conversion.
Finally, it can also be appropriate, when after having left the heat recovery unit unvaporized organic fluid is separated from the vaporized fluid and only the vaporized fluid is fed to the turbine.
The advantages of this method largely correspond to the above-mentioned advantages of the Organic Rankine Cycle system.
The Organic Rankine Cycle system according to the invention as well as the corresponding method utilize the hot flow from the convective heat source (like a gas turbine exhaust) much more compared to the standard ORC cycle design without increasing too much the heat loss in the condenser which will result into a higher power output of the ORC bottoming cycle and thus results into a higher overall energy conversion (CCPP efficiency).
As already pointed out, the only way to recover more heat out of the gas turbine exhaust heat is either to minimize or skip the regenerator or try to find other ORC working fluids which have a lower inlet temperature of the WHRU.
Minimizing or omitting the regenerator will result in a higher heat recovery rate but on the other hand the bottoming cycle efficiency drops since a part or the entire superheated energy at the ORC turbine outlet will be lost into the condenser. This is not a viable solution.
Evaluation of different working fluids is ongoing but so far, all fluids have a negative impact on the overall efficiency.
The parallel regenerative cycle utilizes the available heat from the convective heat source much better compared to prior art ORC by reducing the stack outlet temperature without increasing too much the loss in the condenser. In other words, the parallel regenerative cycle is the optimum in both: partially regenerate to limit the loss in the condenser, partially further extract heat from the convective heat source.
With that parallel regenerative cycle design the stack outlet temperature is like the traditional water-steam bottoming cycle and therefore the ORC disadvantage in heat recovery is eliminated.
The parallel regenerative cycle design can be used with different ORC working fluids and is not limited to a single working fluid. The behavior is the same, regardless of the ORC working fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1: shows a prior art Organic Rankine Cycle system and
Figure 2: shows an Organic Rankine Cycle system according to the invention.

The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements may be provided with the same reference signs.

DESCRIPTION OF THE DRAWINGS

Figure 1 shows a well-known closed Organic Rankine Cycle system 1 commonly used for the purpose of producing electrical power. The embodiment of Figure 1 comprises an Organic Rankine Cycle 2 and a gas turbine 14 as the source of heat to the heat recovery unit 9 with first heating surfaces 10.
The closed Organic Rankine Cycle 2 comprises a heat recovery unit 9 for the evaporation of the organic fluid, a turbine 3 fed with vapor from the heat recovery unit 9 to drive the generator 16 or other load, a condenser 6 for condensing the exhaust vapors from the turbine 3 and a feed pump 7, for recycling the condensed fluid to the heat recovery unit 9.
The Organic Rankine Cycle 2 shown in Figure 1 further comprises a regenerator 4, which is a heat exchanger between the turbine outlet 17 and the condenser inlet 18 to utilize the superheated energy at the turbine exhaust to heat-up the liquid working fluid before entering the heat recovery unit 9. A first side 5 of the regenerator 4 is therefore arranged in the Organic Rankine Cycle 2 between the turbine 3 and the condenser 6, whereas a second side 8 of the regenerator 4 is arranged between the feed pump 3 and the heat recovery unit 9.
Further, a separator 15 is arranged between the heat recovery unit 9 and the turbine 3 in order to separate the liquid from the vapor phase of the organic fluid.
Figure 2 shows an Organic Rankine Cycle system 1 according to the invention. It distinguishes from the prior art by a change in the ORC bottoming cycle design. It is changed in that the liquid ORC working fluid is split into two partial streams.
A first branch 11 for a first partial stream is the "normal" way to the regenerator 4 to heat-up the liquid working fluid as done in the standard cycle design in an ORC bottoming cycle shown in Figure 1.
The second partial stream is routed via a second branch 12 without pre-heating directly to a second heating surface 13 arranged in the heat recovery unit 9 behind the first heating surfaces 10 in the direction of flow of a flue gas through the heat recovery unit 9. The second heating surface 13 acts like a separate, additional economizer of the heat recovery unit 9 to cool down the flue gas further before leaving the stack. A split ratio of amounts of organic fluid flowing through the first 11 and second branches 12 is adjustable.

Claims

Organic Rankine Cycle system (1) comprising, arranged one behind the other in the direction of flow of an organic fluid in an Organic Rankine Cycle (2), a turbine (3), a regenerator (4) with a first side (5), a condenser (6), a feed pump (7), the regenerator (4) with a second side (8) and a heat recovery unit (9) with first heating surfaces (10), characterized in that the Organic Rankine Cycle (2) branches out between the condenser (6) and the regenerator (4) and reunites between the regenerator (4) and the heat recovery unit (9), forming first and second branches (11, 12), wherein the first branch (11) includes the regenerator (4) and the second branch (12) includes a second heating surface (13) arranged in the heat recovery unit (9) behind the first heating surfaces (10) in the direction of flow of a flue gas through the heat recovery unit (9).
The Organic Rankine Cycle system (1) according to claim 1, wherein the heat recovery unit (9) is connected to a gas turbine (14).
The Organic Rankine Cycle system (1) according to one of claims 1 to 2, wherein a split ratio of amounts of organic fluid flowing through the first (11) and second branches (12) is adjustable.
The Organic Rankine Cycle system (1) according to one of the preceding claims, wherein a separator (15) is arranged between the heat recovery unit (9) and the turbine ((3).
A method for operating an Organic Rankine Cycle system (1), wherein a liquid organic fluid is circulated to first heating surfaces (10) of a heat recovery unit(9), where heat is introduced to the fluid in order to convert it to vapor, with the vapor then passing through a turbine (3), with the resulting cooled vapor then passing through a first side (5) of a regenerator (4) and a condenser (6) one after the other, characterized in that before the organic fluid flows through a second side (8) of the regenerator(4) a stream of the organic fluid is divided into first and second partial streams, wherein the first partial stream passes through the second side (8) of the regenerator (4) and the second partial stream passes through a second heating surface (13) of the heat recovery unit (9), wherein the second heating surface (13) is arranged in the heat recovery unit (9) behind the first heating surfaces (10) in the direction of flow of a flue gas through the heat recovery unit (9).
The method according to claim 5, wherein the step of introducing heat to the fluid in order to convert it to vapor is accomplished by directing exhaust gas of a gas turbine (14) onto first (10) and second heating surfaces (13) in the heat recovery unit (9).
The method according to one of claims 5 to 6, wherein a split ratio of the amounts of first and second partial streams is varied in order to optimize heat recovery and overall system conversion.
The method according to one of claims 5 to 7, wherein after leaving the heat recovery unit (9) unvaporized organic fluid is separated from the vaporized fluid and only the vaporized fluid is fed to the turbine (3).