SOLAR FIRED COMBINED CYCLE WITH SUPERCRITICAL TURBINE

Abstract

Mechanical work for electric power generation is obtained from thermal energy in a plant arranged for introduction of solar energy, available intermittently, by reflecting and concentrating solar radiation to directly heat a flow medium such as the exhaust gas from a combustion turbine directed into a steam generating boiler/evaporator. Steam generators and staged turbines recover and extract energy optimally at particular temperature, pressure and flow parameters in a closed thermodynamic cycle. Solar energy that is available intermittently is injected into the cycle to elevate the energy of the flow medium, in particular to produce supercritical steam. A steam turbine optimized for expanding supercritical steam is deployed during periods of available solar radiation by a controllable clutch and other switching and valve arrangements. The exhaust from the supercritical steam turbine can be coupled to downstream staged turbines optimized for successively lower pressures and higher flow rates.

Description

FIELD OF THE INVENTION

This invention provides methods and apparatus wherein concentrated solar energy is used, when sunlight is available, to heat the exhaust gas from a gas turbine to temperatures high enough for supercritical steam production in a heat recovery steam generator. The additional supercritical steam is expanded in a steam turbine for electric power generation at the highest possible thermal efficiency.

BACKGROUND OF THE INVENTION

Like many renewable power generation technologies, Concentrated Solar Power (CSP) suffers from intermittence and unpredictability, which can render the technology difficult to justify and to deploy efficiently for meeting a constant demand for power. There are possible means to alleviate the discontinuous nature of energy supplied by an intermittent source, such as to provide thermal energy storage facilities, but that can add to plant size and cost appreciably. Another remedy is to supplement the solar thermal energy input by an on-demand energy source, for example by burning a fossil fuel when solar radiation is unavailable, but that runs against the purpose of deploying renewable energy technologies in the first place. A current industry practice is to dispatch renewable power when it is available and to bring fossil fuel fired generation capacity on line (primarily Gas Turbine (GT) based simple or combined cycle power plants in spinning or non-spinning reserve mode) when renewable power sources are not available but there is still a power demand to meet (e.g., at night, during periods of cloud cover, no wind, etc.).

A known solar-fossil hybrid technique uses solar thermal as a supplementary energy input. A well-known example is the Integrated Solar Combined Cycle (ISCC) power plant. In this concept, high pressure or intermediate pressure feed water from a Heat Recovery Steam Generator (HRSG) is sent to a CSP boiler (e.g., on a solar tower). The steam that is generated is sent back to the HRSG. Another example is using a solar tower or a parabolic trough heat collection system to heat feed water or to supplement the coal-fired boiler steam in a Rankine steam turbine power plant. In all these concepts, solar thermal energy accounts for only a small fraction of the total generation capacity (for example 10%) at a considerable expense and complexity. A CSP plant may have a large plant footprint, where hundreds or thousands of heliostats or parabolic troughs are distributed across a solar field that may encompass hundreds of acres. The plant cost is further increased by the need for equipment to handle complex integration issues such as a switching and control philosophy during solar thermal start and shutdown, among other challenges.

All existing ISCC concepts include making steam in a solar-powered “boiler.” Herein, the term boiler should be understood as a generic heat transfer system that converts water to steam. The most proven CSP technology deployed in ISCC applications is the parabolic trough, which comprises units with cylindrical parabolic reflectors having 4 to 5 mm thick silvered-glass mirrors. Solar energy concentrated by the trough is transferred to a synthetic-oil based Heat Transfer Fluid (HTF) flowing in a glass tube located at the focal point of the trough. For 1 MW thermal capacity, one needs a solar field of 4 to 5 acres containing rows of solar troughs with HTF tubes connected in loops. Hot HTF flows into a steam generator where pressurized feed water is converted to steam. A main drawback of such solar trough technology is that the maximum achievable steam temperature is limited to about 745° F. by HTF availability.

A recent CSP technology is the solar tower, which can generate high pressure steam at temperatures commensurate with modern steam turbine technology (i.e., 1,000° F. or higher). A solar tower system comprises a tall tower (several hundred feet high) with a boiler on top that receives concentrated solar radiation from a field of heliostats, which are dual-axis-tracking mirrors. In this form of boiler, high-temperature saturated or superheated steam is generated directly without an intermediate heat transfer medium.

The thermodynamic principle governing the optimal combination of solar and thermal steam generation in an ISCC is a direct manifestation of the second law of thermodynamics. Specifically, it is the principle of maximum exergy. Exergy (also known as availability) is a readily calculable fluid property that translates the Kelvin-Planck statement of the second law into practical engineering calculations. In order to understand this concept within the context of a solar-thermal hybrid power plant, specifically ISCC, the reader is referred to FIG. 1.

As shown in FIG. 1, the interaction between the Gas Turbine Combined Cycle (GTCC) and the CSP plants can be simplified into two water/steam streams:

• • Boiler feed water from the HRSG to the solar steam generator at pressure and temperature, PFW and TFW, respectively; and,
  • Saturated or superheated steam from the solar steam generator to the HRSG at pressure and temperature, PSTM and TSTM, respectively.

It can be shown that an optimal ISCC design, as dictated by the exergy maximization principle, can be based on high pressure steam generation in the CSP plant. (See S. C. Gülen, 2013, “Second Law Analysis of Integrated Solar Combined Cycle Power Plants,” GT2014-26156, to be presented in ASME IGTI Turbo 2014, Düsseldorf, Germany, Jun. 16-20, 2014.) In order to appreciate this principle, consider that

• • 1. The total energy input from the CSP plant to the GTCC plant, Q_IN, is a fraction of the solar power incident normally on the solar field when in sunlight.
  • 2. The maximum additional power that can be generated from Q_IN is equal to that which can be generated in a hypothetical Carnot engine operating between the same hot and cold temperature reservoirs.
    • a. This maximum (hypothetical) power is denoted by E_IN.
    • b. Actual additional power generated is a fraction of E_IN, i.e., W_ADD=EFF2×E_IN, where EFF2 is the conversion effectiveness (to be interpreted as a second law efficiency).
  • 3. The low temperature reservoir is at the existing ambient temperature, TAMB.
  • 4. The high temperature reservoir is at the Mean-Effective Heat Addition Temperature, METH, which is the ratio of water and steam enthalpy and entropy differences at the boundary between the CSP and the HRSG in FIG. 1.
  • 5. E_IN is a function of METH; the higher is METH, the higher is E_IN.
  • 6. EFF2 is also a function of METH; similarly, the higher is METH, the higher is EFF2.
  • 7. Thus, the higher is METH, the higher is W_ADD.

The current invention accomplishes its objects via increasing the mean-effective heat addition temperature, METH, much beyond that is possible with the existing technology shown in FIG. 1. This will be apparent in the paragraphs below when the details of exemplary embodiments are disclosed.

A mechanism for increased CSP contribution is increasing high pressure or throttle steam flow (also referred to as the main steam flow) through the Steam Turbine (ST). In order to accomplish this, high pressure feed water from the high pressure economizer is sent to the CSP plant steam generator and the high pressure steam is returned to the HRSG at the high pressure evaporator exit.

For a ST with a fixed volume swallowing capability, this is accompanied by an increase in throttle pressure, which can go up to about 2,400 to 2,500 psig. Higher throttle pressures can force the steam generation pressure in the HRSG high pressure evaporator to supercritical (above 3,200 psia), which requires a once-through design (with no separate steam drum). This is one of the two limiting factors in solar power augmentation; the other is the increase in the condenser pressure with increasing low pressure turbine exhaust steam flow (a typical alarm limit is 5 inches of mercury). This is a particularly critical item for plants with air-cooled condensers, which are very expensive in terms of capital expenditure and parasitic power consumption (power consumed by large fans driving the airflow through the condenser cells).

While the condenser pressure limitation can be overcome (e.g., by adding extra air cooled condenser cells to be deployed when low pressure exhaust flow is increased), the high pressure throttle pressure limit is not so straightforward to accommodate. It requires the following system design features:

• • 1. Enough additional energy to sustain supercritical steam generation (supplementary or duct firing, solar steam augmentation);
  • 2. Evaporator (boiler) section suitable to subcritical and supercritical steam production (once-through Benson design);
  • 3. Thick-walled balance of plant and steam turbine components to withstand very high pressures (well above 3,000 psia) at high temperatures (as high as 1,112° F. or even higher): valves, pipes, turbine casings (costly alloys); and
  • 4. An efficient steam turbine high pressure section at (relatively) low volumetric flows (reaction design) driven by high steam density.

One object of the current invention to provide solutions for the problem presented by the last item in that list.

SUMMARY OF THE INVENTION

It is an object of the current invention to provide a different way to introduce solar energy into the bottoming cycle of a GTCC plant, so as to accommodate the intermittent availability of solar energy while allowing the elements of the plant to be configured primarily for operation in steady state conditions as opposed to very different levels depending on whether solar energy is available or not. Another object is to generate power with high solar conversion effectiveness by operating power conversion turbines and other apparatus at pressure, temperature and flow rates that are optimized for performance whether solar energy is available or not. In particular,

• • 1. Solar energy is added to the GTCC plant directly at the HRSG (instead of at a receiver on a solar tower in a collector field, at a substantial distance from the GTCC); and,
  • 2. Solar energy is added to the GT exhaust gas (instead of at a water/steam flowpath in the steam generator inside the solar tower receiver).

Using solar radiation in a solar concentrator located between the gas turbine exhaust and the HRSG inlet, gas turbine exhaust gas (between 1,100° F. and 1,200° F. for F class advanced machines) can be heated up to 1,600° F. (870° C.), which is a typical upper limit for duct-fired HRSGs. Duct firing (also known as supplementary firing) is a widely adopted method for hot day power augmentation when gas turbines rapidly lose output due to lower compressor airflow at a time when power demand is high (especially in the U.S. with widespread use of air-conditioning in residential and industrial areas).

At this level of gas temperature, supercritical steam at very high pressure and temperature can be generated in a once-through boiler section in the HRSG (e.g., at least 3,750 psia, 1,112° F.). The supercritical steam from the HRSG is expanded in a steam turbine, which is connected to a standard GTCC steam turbine via a SSS (Synchro-Self-Shifting) clutch and, if necessary, a gearbox or torque converter.

Supercritical steam turbine technology is known in coal-fired power plant technology to generate electric power at high efficiency using turbine structures and mechanisms that are optimized for high temperature and pressure. In general, the “critical” point in steam is a level of high temperature and pressure at which the separate gas and liquid phases of water merge into one continuous phase.

Deployment of solar energy in the manner proposed herein provides a surprising improvement in efficiency compared to current technology. For example,

• • 1. Solar conversion efficiency is 5+ percentage points higher (incremental steam turbine power output divided by solar thermal input);
  • 2. The solar fraction (SF) of power input to the plant can be increased substantially, for example to up to nearly 25% (the fraction of solar thermal input to the total fuel energy input);
  • 3. Higher combined cycle efficiency (total power output divided by fuel lower heating value (LHV) input) by about 3 percentage points; can be as high as 70% net (85° F.-45% “hot” day basis with and air-cooled condenser at full load).

The foregoing objectives are achieved in an apparatus for generating mechanical work from thermal energy, primarily for coupling torque to an electric generator. The apparatus includes at least a first power generation component operable to produce at least one flow medium with an elevated energy state from which energy can be extracted, and at least one energy extraction component operable to extract mechanical work from the flow medium, extraction of such energy reducing the flow medium to a less elevated energy state. A flowpath is defined for the flow medium from the first power generation component to the energy extraction component. When solar radiation is available, a solar concentrator directs solar heat energy into the flow medium along the flowpath, thereby increasing the elevated energy state of the flow medium as a function of the extent of solar energy that is available.

The first power generation component, such as a gas combustion turbine, is operated whether the solar radiation is available or not. The gas turbine is arranged to produce steam at elevated temperature and pressure. The steam turbine is coupled to at least one energy extraction component such that mechanical work is extracted in conjunction with expansion and cooling of the steam.

In order to achieve an acceptable efficiency, the energy extraction component is sized and configured for optimal operation at particular flow, pressure and temperature characteristics. During availability of solar radiation, the additional thermal energy that is available would elevate the temperature and pressure out of the ranges in which the energy extraction component operates most efficiently. It is an aspect of the invention that the solar radiation, when available, is used to produce supercritical steam, that is steam at a sufficiently high temperature and pressure that the gas (steam) and liquid (water) phases of the flow merge into one phase. The supercritical steam is expanded in a turbine optimized for the supercritical steam (high pressure and temperature with relatively low volume flow rate as compared to other turbines in the plant). The exhaust from the supercritical high pressure steam turbine can be coupled to further turbines staged for successively lower pressures and temperatures and higher volume flow rates as the steam is expanded. The supercritical high pressure turbine is deployed only when solar radiation is available. Controllable mechanical shaft couplings and flow couplings permit the supercritical high pressure turbine to be coupled when needed and decoupled in the absence of solar radiation sufficient to operate the supercritical high pressure turbine.

According to one aspect, the foregoing provisions are part of an electric power generation system having at least one steam generator coupled in a recirculating flow path to supply a production flow of steam at nominal production pressure and temperature to a main steam turbine, the main steam turbine being mechanically coupled to an electric generator for generation of electric power by extracting energy from expansion of the production flow of steam. At least one additional steam turbine is coupled to the recirculating flow path and operated at a second production pressure and temperature that are one of a higher energy state than the nominal production pressure and temperature. A solar collector is configured to apply concentrated solar radiation to at least one zone along the recirculating flow path during periods of sufficient available solar energy, the solar collector increasing a pressure and temperature at said zone to the second production pressure and temperature. A controller coupled to requisite pressure, temperature and flow sensors assesses the condition of steam from the HRSG and operates at least one valve and/or mechanical coupling for selectively deploying the additional steam generator as a function of the availability of solar radiation.

The additional steam turbine, such as the supercritical steam turbine, can be mechanically coupled to contribute torque to turn the electric generator, through at least one of a self-synchronizing slip clutch operated by the controller, a gear coupling and/or a torque converter.

The solar radiation can be applied directly by concentrating the reflections from an array of heliostats, directly on the flow, particularly the exhaust of a gas turbine. In one embodiment, the exhaust is passed through a solar concentrator on which concentrated solar energy is reflected by lenses and/or mirrors. The additional turbine is configured to expand steam that has been boosted in pressure and/or temperature by the solar collector, especially to a supercritical phase state. The solar collector can include tracking movable reflectors configured to concentrate the solar radiation at the zone where solar radiation is thermally coupled to the flow of exhaust gases and/or to a once-through boiler for steam.

Where the solar collector boosts the steam pressure and temperature to a supercritical level, it is advantageous to employ a steam turbine that is optimized to expand supercritical steam. In one embodiment, the pressure is at least 3,750 psia and the temperature is at least 600° C. (1,112° F.). In another embodiment, the gas turbine produces a nominal output between about 585 and 700° C. (1,000 to 1,200° F.) and the solar collector is configured to increase the temperature up to 920° C. (1,600° F.) at maximum solar radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

For purposes of illustration, nonlimiting examples of certain embodiments according to the invention are described below and are shown in the drawings, wherein:

FIG. 1 is a schematic depiction of an ISCC power plant having a GTCC part and a CSP part comprising a solar field and solar steam generator, illustrated generically.

FIG. 2 is a schematic depiction of an exemplary embodiment of the present invention, characterized by a configuration wherein a CSP collector directs heat energy directly to raise the temperature in the exhaust flowpath of a gas turbine for producing supercritical steam that is in turn passed through steam turbines that are configured for the supercritical steam temperature, pressure and flow parameters.

FIG. 3 is a schematic diagram of a practical aspects of an embodiment showing flowpaths and including valves that are operated by a controller (not shown) to complement changes in operational conditions such as the availability of sunlight.

FIG. 4 is a schematic diagram showing an alternative embodiment; and,

FIG. 5 is a schematic diagram of the embodiment substantially as in FIG. 4, provided with flowpaths and controls.

FIG. 6 is a schematic showing a further alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the elements of a GTCC power plant having an associated solar power collection portion. Solar radiation is collected by multiple collector elements distributed over an area, so long as sunshine is present and incident on the collector elements. When solar radiation is present, it is used to heat a heat transfer medium used for extracting mechanical energy. Generally, the heat transfer medium produces or elevates the temperature of steam, through a heat exchanger. The flow of steam is passed through a steam turbine, resulting in a reduction in temperature or pressure (that is, enthalpy) in exchange for extracted mechanical energy. Typically the mechanical energy provides torque to a drive shaft coupled to an electric generator. In this disclosure, it should be appreciated that the term “coupled” denotes an operational connection of two or more elements wherein the connection may be direct or indirect, such as a connection through intervening elements.

For purposes of efficiency, it is generally appropriate to pass the flow of steam or other flow media through a series of energy transition stages that may result in use of the available heat energy at different thermodynamic conditions. For example, the flow through the system may pass plural steam turbines configured for progressively lower pressures and temperatures and progressively greater flow volumes. Re-heating flow paths can be used to recover heat energy for useful applications. The flow of steam can be recycled or potentially can end in a phase change from steam to liquid water. Likewise energy may be extracted by exploiting a temperature difference between the steam or water and a lower temperature energy sink.

According to the embodiment shown in FIG. 2, rather than coupling a solar field of heat collection elements coupled by a heat transfer fluid path to a steam generator (or other variation of a boiler), a CSP system is configured to reflect and concentrate the solar radiation incident on the solar field directly to the exhaust gases from a gas turbine. This arrangement of direct solar concentration from reflectors and concentrated heating of the gas turbine exhaust is preferably exclusive of use of a heat transfer fluid, although it is conceivable to provide for both techniques for solar power collection or for other combinations of techniques exploiting solar, combustion, geothermal, wind and other energy sources in various configurations of combined cycle plants.

As shown in FIG. 2, when there is no solar field contribution or insufficient solar contribution, such as at night or in overcast daytime conditions, the system produces electric power as a standard GTCC system. In this arrangement, the GTCC system can comprise a horizontal, Three-Pressure Reheat (3PRH) or Two-Pressure Reheat (2PRH) HRSG with a Once-Through Boiler (OTB) or once through evaporator section, with the steam flowpath coupled to a standard Steam Turbine Generator (STG) configured for efficient operation at the temperature, pressure and flow conditions that are present. In one embodiment, the HRSG generates high pressure (HP) steam at subcritical conditions, e.g., 1,800 psia and 1,050° F.

The gas turbine can be an advanced F, G, H or J class unit from original equipment manufacturers such as General Electric (e.g., Frame 7 or 9), Siemens (e.g., SGT6-8000H or SGT5-8000H), Alstom (e.g., GT24 or GT26) or Mitsubishi (e.g., 501J or 701J). These units burn combustion fuels and provide torque as well as a high temperature exhaust gas flow.

The HRSG has a “once-through” HP section. An example is the Benson technology of Siemens. The remaining sections can have a conventional drum-type design.

The steam turbine can be an advanced unit capable of high pressures and temperatures at the upstream steam flow path, progressively lower pressure turbine paths to accommodate the expanding steam and ending with multiple-flow LP turbine with long last stage buckets (LSB) commensurate with low condenser pressures. Examples are General Electric's D series, Alstom's STF series or Siemens SST series units. Each element in the series is configured for extracting energy at a progressively lower energy state of the steam.

The solar field can comprise a plurality of reflectors (e.g., several thousand) called heliostats. The heliostats focus incident solar radiation on a “beam-down” reflector at the top of a tall tower as described in U.S. Pat. No. 5,578,140-Yogev et al., hereby incorporated by reference. The beam-down reflector applies the concentrated solar energy to the exhaust path of the gas turbine.

The solar concentrator can be based on volumetric receiver technology where the concentrated radiation is absorbed within a porous structure. Prototypes of volumetric air receivers have been developed and tested on a limited scale (a few thermal megawatts), e.g., Jülich, Germany. See, Koll, G., et al., “The Solar Tower Julich—A Research and Demonstration Plant for Central Receiver Systems.”

The technology using such volumetric receivers is primarily based on heating gas turbine combustion air, which is also known “Solar Gas Turbine” or “Solar Hybrid Gas Turbine”. Yet another configuration using such volumetric receivers concerns mixing gas turbine exhaust gas (525° C.) with air that has been heated in a volumetric receiver atop a tower (“solar air tower”). For example if air heated in the tower to reaches 680° C., the mixture with 525° C. gas turbine exhaust may produce a final mixed temperature of 615° C. See, Horn et al., 2004, “Economic analysis of integrated solar combined cycle power plants; A sample case: The economic feasibility of an ISCCS power plant in Egypt,” Energy, 29, pp. 935-945.

Such a volumetric receiver is only one method of heating a gaseous fluid by concentrated solar rays. Other devices are conceivable to achieve solar concentrator heating of a gas flow as in the current invention, in particular to increase the temperature of the exhaust flow of a gas turbine.

According to the embodiment of FIG. 2, the invention comprises two distinct components, namely (1) a solar concentrator between the gas turbine exhaust and the heat recovery steam generator, and (2) a supercritical high pressure turbine.

The solar concentrator is placed directly into the path of the gas turbine exhaust gas. The solar concentrator receives the solar rays reflected from tower-mounted “beam-down” parabolic mirror to further heat the exhaust gas, for example from about 1150° F. (620° C.) up to 1,600° F. (870° C.). The Supercritical High Pressure Turbine (SCHPT) can be coupled to the main STG via a Synchro-Self-Shifting (SSS) clutch and a gearbox (or torque converter). The SCHPT generates mechanical power by expanding the supercritical steam (typically at least 3,750 psia and 1,112° F. (600° C.)) generated in the HRSG to pressures and temperatures suitable to the configuration of the main STG. By providing the self-synchronizing clutch and gearbox coupling, the SCHPT can be mechanically disengaged when solar radiation is absent or unavailable in some predetermined minimum amplitude to justify operation of the SCHPT. When operating, the exhaust from the SCHPT can be coupled through to further staged turbines and other energy extraction mechanisms that operate at lower pressures and temperatures, and higher volume flow rates.

According to the alternative embodiments shown, the invention is configurable or can be controllable to operate when one or the other of the two distinct components are not available, namely the solar concentrator to heat the gas turbine exhaust gas, or the supercritical high pressure turbine. In one alternative embodiment, shown in FIGS. 4 and 5, instead of a tower-mounted “beam-down” type reflector and solar concentrator, a central solar receiver system is utilized to transfer solar thermal energy to the GTCC system via additional HP steam generation either directly or using a heat transfer fluid (the standard ISCC configuration). However the additional solar thermal energy is applied to produce supercritical steam as described above and energy is extracted using the supercritical steam turbine configuration that is decoupleable in the absence of solar radiation.

In another alternative embodiment shown in FIG. 6, additional steam heating by “solar firing” is accommodated by the main STG. There is no separate, clutched SCHPT as described above. This embodiment is analogous to HRSG supplementary or duct firing where fuel burners are replaced or supplemented by the solar concentrator.

FIG. 2 shows a partly schematic depiction of an exemplary embodiment that employs the full complement of solar collector and concentrator with direct heating, plus a supercritical HP steam turbine that is coupled to contribute energy when solar energy is available and is decoupleable when solar energy is unavailable or insufficient. FIG. 3 is the flow diagram associated with FIG. 2.

Referring to FIGS. 2 and 3, in hybrid solar-thermal operation mode, the solar concentrator can heat the gas turbine exhaust gas up to about 1,600° F. (870° C.) by application of the concentrated solar beam from the solar field onto the gas turbine exhaust. Inasmuch as the flow of exhaust gases is necessarily confined, a volumetric receiver technology is appropriate where the concentrated radiation is absorbed within a porous structure through which the exhaust gases flow. High pressure steam at 3,750+ psia and 1,112° F. (600° C.) is generated in the OTB and sent to the SCHPT. The SCHPT is connected mechanically to the main STG via a SSS clutch. The SSS clutch engages mechanically when the SCHPT is operating (hybrid mode) and disengages when the SCHPT is not operating (thermal mode). This equipment and operating mode of engagement/disengagement is based on shaft speed matching (hence the term “synchronizing” in the name or designation SSS). Synchronizing couplings of this type are commonly utilized in single-drive-shaft GTCC systems. Depending on the nominal speeds of the SCHPT and the drive shaft, a speed-reduction gearbox or torque converter also can couple the SCHPT and the main STG as described more particularly below.

In fundamental engineering terms, the impact of the invention is a substantial increase in the mean-effective heat addition temperature, METH, which is the logarithmic average of the GT exhaust gas temperature at the inlet and exit of the solar concentrator. In numerical terms, consider that METH for the current technology shown in FIG. 1 is at its highest with HP steam generation around 650° F. With the current invention, with 1,150° F. at the solar concentrator inlet (for a typical advanced F class gas turbine) and 1,600° F. at the solar concentrator exit, METH is more than 1,300° F. Such a dramatic increase in METH and the commensurate increase in maximum power denoted by E_IN open the door to maximizing ISCC potential. This is the theoretical foundation of the current invention.

Merely increasing METH would waste exergy (i.e., useful work generation potential quantified by E_IN) if the flow at increased temperature was passed through a steam generator that has been configured for other conditions, such as the standard lower pressure and temperature conditions for which the main steam turbine was presumably optimized. Currently, the best available (and proven) technology for Rankine cycle steam power generation is represented by supercritical steam turbines, which are deployed in fossil power plants with coal-fired boilers. In the most recent and advanced versions of those plants, steam is generated at pressures up to 4,350 psia (300 bar) or higher at temperatures up to 1,112° F. (600° C.).

The first advanced supercritical demonstration plant, Eddystone in USA, was designed in the late 1950s and had a maximum steam temperature of 650° C. (1,200° F.). Due to commercial reasons, steam temperatures stayed around 550° C. through the 1970s and 1980s. Recently, environmental considerations associated with carbon dioxide emissions and the greenhouse gas effect, and other factors, have forced a change to higher temperatures. Ultra-supercritical steam technology with pressures up to 4,700 psia (325 bar) and 630° C. steam (1,166° F.) may become the commercial state-of-the-art within the next two decades assuming some progress with available materials (e.g., mixtures of ferritic and austenitic steels). Research is underway to reach 700° C. steam temperatures with Ni-based alloys (e.g., European Community's THERMIE 700 project), which presents significant challenges in component design (specifically high-pressure steam valve chests and turbine casings with thick walls).

According to the present invention, gas turbine exhaust gas is heated to 1,600° F. in the solar receiver for production of supercritical steam in a once-through HP boiler section in the HRSG with pressures of 3,750+ psia and temperatures up to 600° C. (1,112° F.) in the SCHPT. In fact, supercritical steam cycles have been considered for Rankine bottoming cycles of GTCC plants. One such system is described in U.S. Pat. No. 7,874,162-Tomlinson et al., hereby incorporated by reference.

At supercritical steam conditions, the density of motive (high pressure) steam is very high. Consequently the specific volume, v=1/ρ, and the volumetric flow rate are lower than typically encountered in standard 3PRH subcritical systems (e.g., as much as 50% lower for the same steam mass flow rate). In the HRSG of a combined cycle power plant, steam generation is limited by the GT exhaust energy. For a 1×1 GT-HRSG train, even with the larger advanced F class machines, supercritical steam generation is typically not at a level commensurate with the requirements of large supercritical steam turbines. For a feasible design with reasonable bucket height and stage efficiencies, the turbine rotational speed should be increased to above 3,000-3,600 rpm for direct electric generation at 50 and 60 Hz, respectively.

Steam turbine stage efficiency is strongly dependent on volumetric flow, {dot over (V)}, e.g.

$\eta \propto \frac{\stackrel{.}{V}}{U·r^2}$

where r is the mean diameter of the turbine bucket and U is the rotational speed at r, which is given by

$U=\omega ·r$ $\omega =\frac{2\pi ·N}{60}$

where ω is the angular speed and N is the number of revolutions of the turbo-generator shaft (3,000 or 3,600 rpm). Hence, combining the formulas above

$\eta \propto \frac{\stackrel{.}{V}}{N·r^3}$

Since {dot over (V)} is proportional to the product of r and blade height, h, a reduction in the “swallowing capacity” of the ST commensurate with the reduction in {dot over (V)} can be achieved by a reduction in r and/or h at the same proportion. Shorter blades are detrimental to ST performance due to magnified leakage and profile losses. For a 50% reduction in volumetric flow, without an unfavorable reduction bucket heights, maintaining the same level efficiency can be achieved by decreasing the mean diameter by about one-third and doubling the shaft speed, e.g., to 7,200 rpm for a 60 Hz system. This results in a smaller ST circumference (blade-hub-blade) than a comparable 3,600 rpm unit. The smaller, high-speed, high-efficiency supercritical turbine is connected to the main STG shaft, which is rotating at the grid-determined rate of 3,600 rpm, via a speed reduction (2:1 ratio) gearbox. The point is that the structural configuration and operational parameters of a turbine such as a steam turbine generator are closely associated with the temperature and pressure of the steam or other gas that is flowing through the turbine. An aspect of the invention is that provisions are made for efficient operations when using available solar energy to boost the gas turbine exhaust temperature sufficiently to produce supercritical steam, because the provisions include a steam turbine generator that is optimally configured for supercritical steam pressure and temperature conditions. That steam turbine generator is selectively coupleable mechanically to the main power generation train, comprising the main steam turbine and its generator, when solar radiation is available. The main steam turbine generator is optimized for steam at lower temperature and pressure, namely for the steam conditions that are present when solar radiation is not available. These can also be the conditions present at the output from the high pressure turbine, where the temperature and pressure have been reduced to below critical conditions. In this way, the high pressure and main steam turbines can be staged to extract energy from the steam at progressively lower temperatures and higher volume flow rates.

According to the present invention, the SCHPT is only operational when solar energy is generating supercritical HP steam in the HRSG. Since the solar energy injection per the present invention is analogous to supplementary firing (without the concomitant fuel consumption), gas energy is sufficient to generate supercritical steam at high pressure and temperature in adequate quantities. The SCHPT is connected to the main STG via the SSS clutch, which engages when the SCHPT speed (with or without a speed reduction gearbox) reaches the main steam turbine generator speed. In this way, solar energy contributes to power generation when the amount of solar energy is at least sufficient for the SCHPT to come up to the nominal speed of the main STG

In a multiple GT configuration (i.e., 2×1 or 3×1, meaning 2 or 3 GT-HRSG trains) steam production in the HRSGs can be high enough to facilitate a feasible SCHPT design with 3,000 or 3,600 rpm nominal speed (i.e., the same nominal speed as the main STG, depending on electric power grid frequency), in which case the speed-reduction gearbox can be eliminated. The connection between the SCHPT and the main STG in that case is via the SSS clutch only.

FIG. 3 illustrates the invention with reference numerals for the elements identified as follows. The solar concentrator (2) is located between the GT (1) and HRSG (3). Exhaust flow (4) from the GT enters the concentrator and heated by the concentrated solar beam (7) from the solar field (6) to a very high temperature. Heated exhaust gas (5) enters the HRSG (3) and generates steam at a pressure and temperature above the critical point of the H₂O in the steam substance. Supercritical high pressure steam (23) is coupled to the SCHPT (8) via a throttle valve (18). When SCHPT (8) is operational and turning on its shaft support (11), it is mechanically connected to the shaft of the main STG (12) via a SSS clutch (9) and, if necessary, a speed-reduction gearbox (10).

Supercritical steam expands as it passes through the SCHPT (8), producing torque, and is exhausted to the HP section (13) of the main STG (12). Thereafter, the process is the same as in a conventional GTCC steam turbine. Steam expands through the intermediate pressure IP section (14) and the LP section (15) in a staged manner, and is exhausted to the steam condenser (17). Shaft power from steam turbines (8) and (12) is converted to electric power in the generator (16) connected to the STG (12). When there is no solar energy available (e.g., at night), exhaust gas streams (4) and (5) are at the same temperature. Steam generated in the HRSG is subcritical. Thus, the SCHPT throttle valve (18) is closed and the main STG HP throttle valve (19) is opened. The SCHPT (8) slows down and the SSS clutch (9) disengages so that no power is contributed by the SCHPT. During this process, the bypass valve (20) can be opened to slow down the SCHPT and divert steam to the HP section (13) of the main STG (12) in a controlled manner. During some operational conditions (primarily during startup or shutdown or at low loads) part or all of the SCHPT exhaust can be diverted to the IP section (14) of the main STG (12) via the exhaust valve (21). The exhaust valve (22) can be partially or fully closed as needed to accomplish the transition and operation in a controlled manner.

FIG. 4 illustrates an alternative embodiment wherein integration of CSP and GTCC functions are achieved with additional high pressure steam generation in the receiver via direct steam generation DGS or with a solar steam generator SGS (for example in a molten salt based system). The clutch-coupled SCHPT of the invention as described introduces key advantages, which differ from ISCC systems that are known. The advantageous differences include an ability to run the GTCC bottoming cycle at advanced steam cycle conditions, for example 2,200 to 2,400 psia and 1,100° F. steam at the steam turbine throttle without any contribution from concentrated solar power (i.e., no power boost); and also, an ability to go to very high (supercritical) steam pressures with CSP contribution for significantly improved overall efficiency of the ISCC facility (with and without CSP power boost).

In order to maximize solar contribution with the standard ISCC, it may be necessary to significantly degrade the non-augmented steam cycle, e.g., to pressures of 1,600 psia or even lower, say, 1,400 psia. According to the invention, as explained above, this is not necessary. However, this approach might be feasible within certain economic constraints. Even in that case, the invention per the alternative embodiment in FIG. 4 is advantageous. For example, the main STG can be designed for 1,400 psia and 1,000° F. throttle steam conditions for operation with no solar steam generation. The separate HP turbine can be designed for 2,500 psia and 1,100° F. throttle steam conditions to accommodate additional steam flow and temperature with solar steam generation. In exactly the same manner as the SCHPT, the subcritical HP turbine is decoupled from the main STG via the SSS clutch when there is no solar steam generation. This will ensure optimal ST performance when operating in hybrid and thermal modes.

Referring to FIG. 5, the solar receiver and/or solar steam generator is a part of the CSP plant (30). Exhaust flow from the GT (4) enters the HRSG (3) and generates steam at high pressure and temperature. Exemplary conditions are about 2,400 psia and 1,100° F. at the steam turbine inlet/throttle.

In solar-augmented mode, high pressure feed water (31) is provided to the concentrated solar power CSP plant (30) and high pressure steam (32) is returned to the HRSG (3). At sufficiently high solar thermal input, the mass flow rate of additional high pressure steam increases to a level to facilitate HRSG steam generation above the critical point of the H₂O substance. The critical point is generally the temperature and pressure at which the steam changes from separate gas and liquid phases into a continuous plasma-like phase.

The HRSG (3) advantageously is of a once-through design configured to generate steam at subcritical or supercritical temperatures/pressures. The main STG (12) need not be configured to withstand such pressures. Supercritical HP steam (23) is sent to the SCHPT (8) via the throttle valve (18). The SCHPT is configured for supercritical steam at high pressure, namely to expand steam from a supercritical pressure to a pressure commensurate with the design of the main STG (12). When the SCHPT (8) is operational, it is connected to the shaft of the main STG (12) via the SSS clutch (9) and, according to some embodiments, with a speed-reduction gearbox (10). The rest of the system is substantially the same as described with respect to FIG. 3.

The alternative embodiment of FIG. 6 can be envisioned as a straightforward supplementary or duct-fired HRSG having a volumetric receiver configured for absorbing solar heat and conveying the heat into the gas turbine exhaust. The amount of solar thermal input is limited in part by the swallowing capacity at the inlet of the steam turbine, along a steam conduit coupled in heat exchange relationship with the volumetric receiver.

Some of the benefits of the foregoing exemplary and alternative embodiments can be appreciated by considering and comparing parameters numerically. A baseline for comparison is a standard GTCC in a 2×1 configuration. The gas turbines can be advanced General Electric Frame 7 units. The steam turbine can be a state-of-the-art General Electric D class unit discharging to an air-cooled condenser. The steam cycle is 1,800 psig and 1,050° F./1,050° F. for main and hot reheat steam. The steam turbine maximum pressure is 2,500 psig. This unit is nominally rated as a 600 MW—56% net (ISO base load) machine. On a hot day (85° F. and 50% relative humidity), as is typical of GTCC power plants, the output drops by about 5% (roughly 30 MW) due to a reduction in gas turbine airflow.

Taking the hot day performance of the GTCC as the basis for comparison, the invention in its preferred and alternate embodiments is applied to solar power augmentation. A comparison can be made between solar power augmentation as described versus supplementary firing (a standard method of hot day power augmentation in addition to gas turbine inlet air chilling) and standard ISCC operation with high pressure steam generation in the solar steam generator of the central receiver system. The results are summarized in Table 1, which demonstrates substantially improved solar thermal energy utilization at a higher efficiency (both incremental Rankine cycle and net GTCC thermal efficiencies) vis-à-vis the standard ISCC. Specifically, with a base GTCC of 56% efficiency, 64.5% is possible in ISCC mode with the current invention with only 1,450° F. gas temperature at the exit of the solar concentrator. This result indicates that utilizing a state-of-the-art H or J class gas turbine with 60% base efficiency in GTCC mode, 68.5% is possible in ISCC. Similarly, with 1,600° F. gas temperature at the exit of the solar concentrator, with an H or J class GTCC, 70% efficiency is possible in ISCC mode. Careful system optimization is likely to take the ISCC performance beyond these figures.

	TABLE 1
	GTCC	ISCC	Solar Fired
	Fired	Unfired	FIG. 2	FIG. 4	FIG. 6
Ambient		Hot	Hot	Hot	Hot	Hot	Hot
Solar Thermal Input	MWth	NA	150	217	217	173	169
Solar Fraction			0.14	0.21	0.21	0.16	0.16
Solar Concentrator	F.	NA	Sat.	1,450	1,450	1,050	1,380
Exit
SC HPT Throttle P	psia	NA	NA	3,750	4,250	3,750	NA
SC HPT Throttle T	F.	NA	NA	1,111	1,111	1,022	NA
SC HPT Output	kW	NA	NA	18,561	24,707	18,172	NA
Δ CC Output	MWe	73	50	84	85	70	62
Incremental Rankine			33.4%	38.5%	39.3%	40.8%	36.5%
Cycle Efficiency
Δ Net CC Efficiency	%	−2.59	4.74	8.32	8.48	6.69	6.22
Condenser Pressure	in. Hg	4.48	4.32	4.56	4.54	4.32	4.30
Main STG Throttle P	psia	2,515	2,515	2,533	2,489	2,482	2,515
Main STG Throttle T	F.	1,037	854	989	944	898	1,110
Main STG Reheat T	F.	1,048	904	1,048	1,048	975	1,048

As provided herein, a gas turbine combined cycle apparatus includes a combustion turbine (another term for gas turbine) exhausting hot combustion gas products to a heat recovery system with at least one steam generator. Generated steam is expanded in a main steam turbine coupled to an electric generator for generating electric power. A gas heater is coupled along the flow path between the combustion turbine and the heat recovery system. The gas heater applies concentrated solar energy from a solar field to add heat to at least a portion of the hot combustion gas products from the combustion turbine.

The heat recovery system can have at least one once-through evaporator or boiler section configured to increase the energy of steam from the temperature obtained using only combustion gas products in the absence of solar radiation, to supercritical conditions when solar radiation is available. It is also possible to include at least one additional evaporator section configured to increase the energy of the steam to subcritical pressure levels. Plural steam turbines can be deployed or are deployable to extract mechanical energy from flows of steam at different temperature, pressure and flow conditions, at least one such flow is boosted in temperature and/or pressure by the solar radiation. The respective flows at different energy states can be staged through successive turbines that each operate to expand and extract energy at successively lower pressures and higher volume flow rates. A controller and associated valves responsive to the controller are configured to route steam carrying available energy to selected ones of the plural steam turbines or between appropriate stages. At least one controllable clutch responsive to the controller can selectively engage to and disengage from a shaft mechanically coupled, directly or indirectly, to an electric generator at least one of the plural steam turbines.

The invention also encompasses a solar concentrator section in combination with power generation plant apparatus defining a flow path for gaseous products where at least one energy extraction device exploits the gaseous products to product useful work. The solar concentrator has a conduit with inlet and outlet couplings configured to engage with the source and the energy extraction device. The conduit includes or is coupled to a heat transfer section operable to receive and transfer incident radiation to the gas flowing through the conduit.

According to the embodiments disclosed herein, the solar concentrator is configured to direct solar radiation, which is available intermittently and at varying intensity, directly into the hot flow medium (such as the gas turbine exhaust) that carries heat into the heat recovery steam generator (HRSG). It is possible, for example, to increase the exhaust temperature up to 1,600° F. (870° C.), leading to elevation of the temperature and pressure of the steam produced from the HRSG for driving steam turbines that apply shaft work (torque) to an electrical generator.

The HRSG is employed to produce steam for driving the steam turbines whether or not the solar heat energy is available at any given time. In the absence of solar radiation, the steam from the HRSG is preferably routed through a series of high, intermediate and low pressure turbines configured and optimized to extract the available energy efficiently from expanding the steam. When solar energy is available, the temperature and pressure of the steam from the HRSG becomes elevated in temperature and pressure above their levels in the absence of the solar radiation. It is an aspect of the invention that a steam turbine stage optimized for elevated pressure and temperature parameters achieved via solar radiation is selectively deployed by being coupled into the beginning (the high temperature/pressure end) of the steam path through the staged series of steam turbines when solar radiation is available, and taken out of when solar radiation is unavailable.

In an advantageous embodiment, the solar concentration and HRSG steam generation capacities are arranged such that when the added energy of solar radiation is available, the resulting elevated temperature and pressure at the HRSG can be such that the steam becomes supercritical (such as 3,750 psia and 1,100° F. or more). When solar radiation is unavailable, a subcritical steam energy level (e.g., 1,800 psia and 1,050° F.) may ensue. Advantageously, the steam turbine that is deployed and coupled into the steam flow path at the high energy end is optimized for expanding supercritical steam through that pressure difference, contributing torque to drive the generator, and passing along subcritical steam into the staged series of turbines at an energy level that is comparable to the level when solar energy is not available.

The selective insertion of a mechanically engageable and disengageable steam turbine stage at the high energy end of a steam expansion path to expand periodically available high energy steam, is particularly useful for solar energy, which is intermittent by its nature. The technique is advantageous where the higher energy steam and selectively deployable steam turbine are supercritical whereas the turbines otherwise deployed for lower energy steam are subcritical. In that situation, the higher energy steam turbine can be configured specifically for supercritical parameters, leading to improved efficiency in the extraction of available energy. The technique improves operability and efficiency if operations with or without the intermittent additional energy source is sufficient to produce a distinct phase. That is, the HRSG steam may be supercritical or subcritical when solar energy is available or not, as opposed to changing from subcritical to supercritical with the added solar energy, provided that the selectively inserted steam turbine stage is configured for the higher temperature steam. In one embodiment, an HRSG may produce steam, for example, at 1,400 psia in the absence of solar energy, and generate higher pressure but still subcritical steam when solar energy is present, e.g., steam at 2,500 psia. The selectively deployed steam turbine in that case is optimized to expand the 2,500 psia steam to 1,400 psia and to feed steam into a next steam turbine stage optimized to expand the steam from 1,400 psia and so on.

The invention has been disclosed in connection with several exemplary embodiments and alternative embodiments; however the invention can be embodied in other specific ways as will now be apparent to persons skilled in the art. Reference should be made to the appended claims instead of the foregoing discussion of embodiments and examples, to assess the scope of the invention in which exclusive rights are claimed.

Claims

1. An apparatus for generating mechanical work from thermal energy, comprising:

at least a first power generation component operable to produce at least one flow medium with an elevated energy state from which energy can be extracted;

at least one energy extraction component operable to extract mechanical work from the flow medium, extraction of such energy reducing the flow medium to a less elevated energy state;

a flowpath defined for the flow medium from the first power generation component to the energy extraction component;

a solar concentrator operable when solar radiation is available to direct solar heat energy into the flow medium along the flowpath, thereby increasing the elevated energy state of the flow medium.

2. An electric power generation system comprising:

at least one steam generator coupled in a recirculating flow path to supply a production flow of steam at nominal production pressure and temperature to a main steam turbine, the main steam turbine being mechanically coupled to an electric generator for generation of electric power by extracting energy from expansion of the production flow of steam;

wherein at least one additional steam turbine is coupled to the recirculating flow path and operated at a second production pressure and temperature that are one of a higher energy state than the nominal production pressure and temperature;

a solar collector configured to apply concentrated solar radiation to at least one zone along the recirculating flow path during periods of sufficient available solar energy, the solar collector increasing a pressure and temperature at said zone to the second production pressure and temperature; and,

a controller coupled to sensors assessing the condition of steam from the recirculating flow path and operating at least one valve for selectively deploying the additional steam turbine.

3. The electric power generation system of claim 2, wherein the additional steam turbine is mechanically coupled to contribute torque to turn the electric generator, and further comprising a synchronizing clutch operated by the controller.

4. The electric power generation system of claim 3, wherein the additional turbine is configured to operate at a said second production pressure and temperature that are higher than the nominal production pressure and temperature, and wherein an exhaust from the additional turbine is coupled into the recirculating flow path upstream from the main steam turbine.

5. The electric power generation system of claim 2, wherein the additional turbine is configured to expand steam boosted in pressure and temperature by the solar collector.

6. The electric power generation system of claim 5, wherein the solar collector comprises an array of movable reflectors configured to concentrate the solar radiation at said zone.

7. The electric power generation system of claim 6, wherein the concentrated solar radiation boosts the pressure and temperature of the steam to a supercritical level.

8. The electric power generation system of claim 7, wherein the pressure is at least 3,750 psia and the temperature is at least 600° C. (1,112° F.).

9. The electric power generation system of claim 7, wherein the zone at which the solar collector concentrates the solar radiation is along an exhaust path of a gas turbine in the flow path.

10. The electric power generation system of claim 9, wherein the gas turbine produces a nominal exhaust flow at about 585 to 700° C. (1,000 to 1,200° F.) and the solar collector is configured to increase the temperature up to about 920° C. (1,600° F.) at maximum solar radiation.

11. A gas turbine combined cycle apparatus comprising:

a combustion turbine exhausting hot combustion gas products along a gas flow path to a heat recovery system having at least one steam generator

a main steam turbine coupled to an electric generator for generating electric power;

a gas heater coupled along the gas flow path, wherein the gas heater applies concentrated solar energy from a solar field to add heat to at least a portion of the hot combustion gas products from the combustion turbine.

12. The gas turbine combined cycle apparatus of claim 11, wherein the heat recovery system comprises at least one once-through evaporator section configured to increase the energy of steam to supercritical conditions.

13. The gas turbine combined cycle apparatus of claim 12, further comprising at least one additional evaporator section configured to increase the energy of the steam to a subcritical pressure levels.

14. The gas turbine combined cycle apparatus of claim 11, comprising plural steam turbines operable to extract mechanical energy from steam at different temperature, pressure and flow conditions, and further comprising a controller and associated valves responsive to the controller, configured to route steam carrying available energy to selected ones of the plural steam turbines.

15. The gas turbine combined cycle apparatus of claim 14, further comprising at least one controllable clutch responsive to the controller for selectively mechanically engaging and disengaging a shaft between the electric generator and at least one of the plural steam turbines.

16. A heat transfer section configured to define a flow path in a power generation apparatus having at least one source of gaseous products and at least one energy extraction device producing useful work from the gaseous products, comprising:

a conduit with inlet and outlet couplings configured to engage with the source and the energy extraction device;

wherein the conduit comprises a heat transfer section operable to receive and transfer incident solar radiation to gases flowing through the conduit;

a solar energy collector and concentrator system having reflectors arranged to direct solar radiation onto the heat transfer section.