WO2014064673A2

WO2014064673A2 - Hybrid combined cycle system for generating electical power

Info

Publication number: WO2014064673A2
Application number: PCT/IL2013/000079
Authority: WO
Inventors: Roi HARPAZ
Original assignee: Newco2Fuels Ltd.
Priority date: 2012-10-22
Filing date: 2013-10-20
Publication date: 2014-05-01
Also published as: IL222613B; KR20150086279A; WO2014064673A3

Abstract

A system is provided for generating electrical power. The system comprises at least two turbine units among which a first turbine unit is configured to operate at a higher temperature than a second turbine unit and ingress means operative to enable introducing a working fluid being at an elevated temperature to the system. The system is characterized in that a) at least part of the incoming energy is received from a variable energy source and b) that it comprises: a controllable diversion means adapted to divert part of the working fluid before it arrives at the first turbine unit, and a controller operative to control the controllable fluid diversion means, and wherein the controller is operative to divert that part of the working fluid whenever thermal energy contained in the working fluid, exceeds a pre-defined threshold.

Description

HYBRID COMBINED CYCLE SYSTEM FOR GENERATING ELECTICAL

POWER FIELD OF THE INVENTION

The present invention relates in general to systems used for utilizing energy derived from an external energy source, and particularly to systems implementing combined cycles .

BACKGROUND OF THE INVENTION

The ever growing environmental problems of increasing energy demand and resources' shortages require new technologies for power plants that will on the one hand increase the overall percentage of renewable energies while on the other hand allow continuous energy supply. One of the relatively new technologies includes hybrid power plants which operate on various combinations of energy sources.

The combined cycle power plants are gas-steam power plants in which the principles of a gas turbine power plant and a steam power plant are combined. They are used for conventionally generating electrical energy from fossil fuels. In a steam power plant in which the fossil fuels are used for evaporating water, the thermal energy of the water vapor is transformed into electrical energy through a steam turbine which in turn drives a generator. In a gas turbine power plant, a gas turbine is operated with liquid or gaseous hydrocarbon based fuel like, for example, natural gas. The gas turbine itself also drives a generator for power generation. The exhaust gases of the gas turbine have high temperature and can therefore be used for additionally providing energy that will be utilized by the steam turbine, e.g. by heating a waste heat boiler in the steam power plant, so that the gas turbine, besides directly generating electrical energy from fossil fuels, is also used as a heat source for subsequent steam generation for the steam turbine.

In addition to using the waste heat of the gas turbine, the electrical power derived from a steam turbine can be further increased by supplying heat to the steam power plant. In a solar thermal power plant, the solar radiation energy is introduced into the power plant cycle through a receiver (also known as absorber or collector) and becomes the primary source for energy.

A solar operated unit with gas and steam turbine is disclosed in US 5,444, 972. In this unit, the solar heat is only being used in the steam cycle, and the solar heat is provided for use by the steam turbine in addition to the exhaust heat of the gas turbine.

US 5,417,052 describes a hybrid combined cycle power plant that includes a solar central receiver for receiving solar radiation and converting it to thermal energy. The power plant includes a molten salt heat transfer medium for transferring the thermal energy to an air heater, which uses the thermal energy to preheat the air from the compressor of the gas cycle. The exhaust gases from the gas cycle are directed to a steam turbine for additional energy production.

US 2011/185,742 discloses a solar hybrid combined cycle for gas-steam power plant including a solar unit, a gas turbine unit and a steam turbine unit. The gas turbine unit includes a gas turbine with a waste heat boiler arranged downstream, and a steam turbine with a feed water heater. The power plant includes a heat transfer medium cycle for transferring solar heat which is coupled to the gas turbine unit through a gas turbine heat exchanger and to the steam turbine unit through a solar boiler.

WO 11/077,248 describes a combined cycle solar power generation using a primary cycle based on a solar receiver, in which compressed air is heated by concentrated solar radiation, coupled with a secondary cycle based on a water/steam circuit driven by exhaust gas from the primary cycle. When the primary cycle is inactive, typically at night time, the secondary cycle can be driven by accessing a heat store of liquid or solid heat storage material, such as a molten salt or concrete blocks, which has been heated earlier during day time operation. The water/steam circuit is reconfigurable between first and second switching conditions, wherein in the first switching condition heat is transferred directly or indirectly from the primary cycle to heat the heat storage material, and in the second switching condition stored heat is transferred from the heat storage material to the water/steam circuit in order to generate steam.

However, one of the problems which has not yet been properly addressed and is associated with such a hybrid combined cycle of power generation, is how to efficiently operate a gas turbine which is primarily driven by hot gas heated by energy received from a variable external source (e.g. an external intermittent source) such as solar radiation, excess process heat or waste heat. If we take solar radiation as an example, the collectable energy changes throughout the day and throughout the year. For a gas turbine designed in conjunction with a certain pre-defined solar field size, the solar based energy that arrives at the gas turbine inlet may be one of the following three cases. It may be lower than the designed operating range, it may be higher than the designed operating range, or may be within the designed operating range. Obviously, the latter case does not present any problem. The typical solutions that are known nowadays in the art for the other two cases are the following. When the first case occurs, i.e. when the energy derived from the solar field is lower than the designed operational range, the typical solution is to use a combustor which runs on fossil fuel feedstock. This in turn creates a problem as one of the goals in implementing solar harvesting technology is to minimize the use of fossil fuel intake, by minimizing the use of combustor. In the other case, where the energy derived from the solar field is higher than the designed operating range, the typical solutions are either to direct part of the energy received to energy storage (which is a rather costly and not too efficient solution) and use it later on, when required, or to shut down the gas turbine cycle since the gas turbine is highly sensitive to overheating.

The present invention seeks to provide a solution that overcomes the problems associated with such variable energy sources .

SUMMARY OF THE DISCLOSURE

It is therefore an object of the present invention to provide means and apparatus for improving the operation efficiency of a solar hybrid combined cycle gas and stream power plant.

It is an object of the present invention to provide means and apparatus for reducing the amount of fossil fuel consumed in a solar hybrid combined cycle gas and stream power plant while preserving the gas turbine from being subjected to overheating. It is another object of the present invention to provide means and apparatus for improving the operation efficiency of a waste heat or excess process heat driven combined cycle gas and steam power plant.

It is another object of the present invention to provide means and apparatus for reducing the amount of fossil fuel consumed in a waste heat or excess process heat driven combined cycle gas and steam power plant while preserving the gas turbine from being subjected to overheating.

Other objects of the invention will become apparent as the description of the invention proceeds.

According to a first embodiment, there is provided a system for generating electrical power, wherein the system comprises at least two turbine units among which a first turbine unit is configured to operate at a higher temperature than a second turbine unit, an ingress means operative to enable introducing a working fluid being at an elevated temperature to the system,

wherein the system is characterized in that at least part of the incoming energy is received from a variable energy source and in that it comprises a controllable fluid diversion means adapted to divert part of the working fluid thermal energy before arriving at the first turbine unit, and a controller operative to control the controllable fluid diversion means, and wherein the controller is operative to divert (e.g. towards the second turbine unit) that part of the working fluid thermal energy, whenever the thermal energy contained in the working fluid exceeds a pre-defined threshold.

According to another embodiment, the pre-defined threshold is derived from a maximum allowable energy intake of the first turbine unit. In accordance with another embodiment, the system further comprises a at least one solar collector, which is adapted to collect solar radiation and convert the collected solar radiation into thermal energy being carried by a working fluid, and wherein the system is further characterized in that the maximal capacity of collectable solar radiation at the at least one solar collector is higher than the maximum allowable energy intake of the first turbine unit.

According to another embodiment, the thermal energy being carried by a working fluid is derived from waste heat or excess process heat, and wherein the system is further characterized in that the maximal capacity of energy derived from the respective heat source is higher than the maximum allowable energy intake of the first turbine unit.

By still another embodiment, the controllable diversion means comprises a controllable valve assembly, operative to enable diversion of part of the incoming working fluid flow before arriving at the first turbine unit whenever the thermal energy contained within the working fluid exceeds a pre-defined threshold.

According to another embodiment, the controllable diversion means is a heat exchanger, and wherein heat is removed from the working fluid (e.g. to a second working fluid used in the second turbine unit) whenever the thermal energy contained in the working fluid being introduced via the ingress means exceeds a pre-defined threshold .

By yet another embodiment, the system further comprises an auxiliary heating element operative to heat the working fluid being introduced via the ingress means in case the temperature of that working fluid is less than a pre-defined threshold. According to another embodiment, the auxiliary heating element is a combustion chamber, operated on fossil fuel (s) .

In accordance with another embodiment, the second turbine unit comprises at least one member of the group consisting of: a steam turbine and a turbine operated with organic operating media.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following detailed description taken in conjunction with the accompanying drawings in which simplified schematics of an example solar hybrid power plant are illustrated, wherein:

FIG. 1 - illustrates a prior art gas and steam power plant with a solar field associated therewith;

FIG. 2 - illustrates the gas and steam power plant according to an embodiment of the present invention; and FIG. 3 - illustrates the gas and steam power plant according to another embodiment of the present invention .

DETAILED DESCRIPTION

In this disclosure, the term "comprising" is intended to have an open-ended meaning so that when a first element is stated as comprising a second element, the first element may also include one or more other elements that are not necessarily identified or described herein, or recited in the claims.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It should be apparent, however, that the present invention may be practiced without these specific details .

FIG. 1 illustrates a simplified schematic of a system of a power generating plant 10 comprising a solar hybrid combined cycle. Air being the working fluid in this example is introduced to the system via compressor 15 and is directed to solar collecting unit 20 which typically comprises a field of solar collectors (e.g. heliostats) 25 and a receiver 30, where the air is heated to elevated temperatures. The hot air leaves receiver 30 via combustor 35 which is operated when the amount of energy carried by the hot air is less than the operating range of gas turbine 50. When the energy carried by the heated air is within the operating range of gas turbine 50, the air is brought to gas turbine 50 to generate electrical power in generator 55. The cooled air leaves gas turbine 50 via heat exchanger 60 and out of system 10. The energy still left in the air leaving turbine 50 (or at least part thereof) is used in heat exchanger 60 for heating (and/or superheating) the steam generated in steam generator 65. Steam then flows towards the second turbine 75 via combustor 70, which is operated when the amount of energy carried by the steam is less than the operating range of the second turbine 75, and an additional electrical power is produced at generator 80. The steam leaving steam turbine 75 flows through condenser 85 and circulated by circulating pump 95 to steam generator 65.

However, as was previously explained, there is an inherent problem with such a set up of solar hybrid combined cycle for power generation. As the gas turbine 50 is highly sensitive to overheating, the field of solar collectors 25 is designed so that under proper solar conditions, its maximum energy output will be such that for a given air flow, the air temperature at the inlet of gas turbine 50 will not exceed the allowable temperature operating range of the turbine. However, this leads to an obvious problem since the maximum capacity of the solar field at the best solar conditions (i.e. maximum irradiation conditions) will match the turbine's upper part of the inlet temperature range only at very specific conditions. The problem being that since solar radiation conditions change throughout the day and throughout the year, there will be too many occasions where the air reaching the gas turbine would be in a temperature which is less than the turbine's allowable temperature range. According to the prior art solution, once this problem arises, the way to overcome it is to further heat the air before it reaches gas turbine 50 by using combustor 35. However, such a combustor runs on fossil fuel feedstock, and this in turn creates a problem as one of the goals in implementing solar harvesting technology is to minimize the use of fossil fuel intake, by minimizing the use of combustor. Furthermore, it is known that for operating solar fields, there is typically an upper limit to the amount of fuel that can be burnt at the combustor, and if that limit is reached, the gas turbine will have to be shut down every time the hot air cannot be heated to the required temperature.

Therefore, it is clear that the prior art solution does not provide an adequate solution to the problem at hand .

FIG. 2 illustrates a simplified schematic system of a power generating plant 110 according to an embodiment of the present invention. The system comprises a solar hybrid combined cycle, which solves the problem described hereinabove, by having a designed solar collecting capacity which is higher than the maximum allowable operating conditions of the hot gas turbine. However, it should be understood that the system illustrated in the Fig. 2 as well as the one illustrated in Fig. 3 are merely examples of embodiments of carrying out the solution provided by the present invention. Therefore, it should be understood that any other source of energy having variable nature such as energy being in the form of waste heat or excess process heat, is encompassed by the present invention.

Now, in the system illustrated in Fig. 2, wherein the solar field is the source of variable energy, air being the working fluid is introduced to the system via compressor 115 and is directed to solar collecting unit 120 which comprises a field of solar collectors 125 and a receiver 130, where the air is heated to elevated temperatures. As explained above, the field of solar collectors 125 is designed so that under proper solar conditions, the solar field maximum energy output will be such that for a given air flow, the air temperature at the inlet of gas turbine 150 will exceed the allowable temperature inlet temperature of the gas turbine 150. Consequently, even when solar radiation conditions change throughout the day and throughout the year, there will be much fewer occasions where the air reaching the gas turbine would be at a temperature less than the turbine's allowable inlet temperature. Therefore, depending on certain design constrains, having an auxiliary heating element, such as combustion chamber 135 in this system, can become only an optional choice.

By implementing the above, the first part of the prior art problem is solved, as it allows a dramatic drop in the use of combustor 135, or even eliminate its use all together from the system. In order to overcome the second part of the problem, according to one embodiment of the present invention a controllable valve assembly 140 is installed before the inlet to the gas turbine unit. The controllable valve assembly 140 is operative to divert at least part of the flow of the incoming working fluid before it arrives at the first turbine unit whenever the thermal energy contained within the working fluid exceeds a pre-defined threshold. As to the diverted working fluid, it may be forwarded towards the second turbine unit, e.g. via a heat exchanger 145, where the energy carried by the diverted part of the working fluid will be transferred for the generation of super heated steam that will be used at the second turbine 175. The controller of controllable valve assembly 140, is operative to divide the flow into two parts, one (the major part) for the first turbine unit and one for the second turbine unit, based upon the flow and temperature of the incoming working fluid, thereby ensuring that the thermal energy of the working fluid that would reach the first turbine 150, does not cause the turbine to exceed its maximum allowable inlet temperature. Better control over the thermal energy of the incoming working fluid enables utilization of a first turbine with design optimized for a narrow range of working conditions, which, as known in the art, enables a design with higher working point efficiency which in turn increases the efficiency of the whole system.

Fig. 3 illustrates another embodiment of the present invention construed to overcome the overheating problem discussed above. According to this embodiment the system further comprises a heat exchanger 240 which is installed before the inlet to the gas turbine unit. This heat exchanger is operative to divert part of the incoming energy load being carried by the first working fluid entering the system, before that first working fluid arrives at the first turbine unit, by reducing the working fluid temperature without changing the flow whenever the thermal energy contained within the first working fluid exceeds a pre-defined threshold. The operation of heat exchanger 240 may be controlled for example by adapting the flow of a second working fluid being used to remove heat from the first working fluid, to match the excess of energy contained in the first working fluid. Valve 245 and optional valve 247 are operative to control the flow of the secondary working fluid through the heat exchanger 240. Thus, when the first working fluid leaves heat exchanger 240 to flow to the first turbine 250, it will be at a flow and temperature which are within the range of allowable operating conditions of the first turbine 250. Preferably, the heat removed from the first working fluid in heat exchanger 240 will be used in the generation of super heated steam which in turn will be used for electrical power generation at the second turbine 275. The improved control over the temperature of the incoming working fluid enables utilization of a first turbine with design optimized for a narrow range of working conditions, which, as known in the art, enables a design with higher working point efficiency which in turn increases the efficiency of the whole system.

It is to be understood that the present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art.

It should be noted that some of the above described embodiments describe the best mode contemplated by the inventors and therefore include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims. When used in the following claims, the terms "comprise", "include", "have" and their conjugates mean "including but not limited to" .

Claims

1. A system for generating electrical power, wherein the system comprises at least two turbine units among which a first turbine unit is configured to operate at a higher temperature than a second turbine unit and an ingress means operative to enable introducing a working fluid being at an elevated temperature to the system,

wherein the system is characterized in that at least part of the incoming energy is received from a variable energy source and in that it comprises a controllable diversion means adapted to divert part of the working fluid thermal energy before it arrives at the first turbine unit, and a controller operative to control the controllable fluid diversion means, and wherein the controller is operative to divert said part of the working fluid thermal energy whenever thermal energy contained in the working fluid exceeds a pre-defined threshold .

2. The system of claim 1, wherein the pre-defined threshold is the maximum allowable energy intake of the first turbine unit.

3. The system of claim 1, further including a solar facility comprising at least one solar collector which is adapted to collect solar radiation and convert the collected solar radiation into thermal energy being carried by the working fluid, and wherein the system is further characterized in that the maximal capacity of collectable solar radiation at the at least solar collector, is higher than the maximum allowable energy intake of the first turbine unit.

4. The system of claim 1, wherein the thermal energy being carried by a working fluid is derived from waste heat or excess process heat, and wherein the system is further characterized in that the maximal capacity of energy derived from said heat source is higher than the maximum allowable energy intake of the first turbine unit .

5. The system of claim 1, wherein the controllable diversion means comprises a controllable valve assembly, operative to enable diversion of part of the flow of the incoming working fluid before it arrives at the first turbine unit whenever the thermal energy contained within the working fluid exceeds a pre-defined threshold.

6. The system of claim 1, wherein the controllable diversion means is a heat exchanger, and wherein heat is removed from the working fluid whenever the thermal energy contained in the working fluid being introduced via the ingress means, exceeds a pre-defined threshold.

7. The system of claim 1, further comprising an auxiliary heating element operative to heat said working fluid in case the temperature of said working fluid is less than a pre-defined threshold.

8. The system of claim 6, wherein the auxiliary heating element is a combustion chamber, operating on fossil fuel.

9. The system of claim 1, wherein the second turbine unit comprises a turbine operating on a fluid being steam or an organic fluid.