BACKGROUND OF THE INVENTION
The present invention relates generally to a power generation system capable of providing dual-mode (cogeneration) power demands, and more particularly to the use of a Rankine-cycle heat energy utilization subsystem in conjunction with a prime mover subsystem, wherein the otherwise unusable waste heat from the prime mover's exhaust stream is routed through the heat energy utilization subsystem for the production of supplemental mechanical or electrical power. Such combination yields a cogeneration system that can provide control over varying power demands and increase overall cycle efficiency, thereby reducing unwanted emissions.
Many commercial and industrial concerns, as well as residential users, consume widely disparate levels of electricity during the course of daily or seasonal operation. When such electricity is supplied over the grid, these concerns are often at the mercy of circumstances beyond their control, including emergency and planned service outages, as well as brownouts or blackouts stemming from heavy usage by others on the grid. In such circumstances, the electricity supplier (normally a utility company) must themselves purchase electricity from other suppliers on the grid, usually at a dramatically inflated price. This extra price is then typically passed on to the end user. In addition, even in periods where power is uninterrupted, the costs of the same quantum of electricity can be considerably higher during peak periods, which often coincides with normal business operating hours, thus rendering the option of operating during off-peak hours to get the lower electric rate unfeasible.
One way to meliorate the uncertainty of off-site electricity generation is for the using concern to generate the power on-site. The simultaneous production of electric power and useable heat from a common fuel or energy source is known as cogeneration, or combined heat and power (CHP). While large industrial entities have long been engaged in cogeneration through steam-producing boilers or reciprocating engines, the bulkiness, as well as the level of support and maintenance, of establishing such a system is prohibitive in smaller operations, such as private residences, restaurants, small commercial and light duty industrial businesses, or in geographic locations where the transmission and distribution infrastructure is inadequate or doesn't exist.
Even in circumstances where on-site generation is physically possible, the cost of installation and operation can be formidable, where new systems are extremely costly, and older systems require dedicated service and maintenance, often by skilled, highly paid specialists. In addition, the generation of energy can also carry with it hidden or hitherto unforeseen costs. Perceived impacts to the environment, in the form of gaseous, liquid and solid byproducts of the power generation cycle, such as SOX, CO, NOX, thermal pollution of cooling water sources, and increased ash (in the case of coal fired generators) have come under increasing levels of government and private scrutiny. Traditional power generation systems require additional effluent treatment equipment to bring air- and water-borne pollutants down to acceptable levels. The additional costs associated with installing and maintaining such equipment, as well as the cost of monitoring and compliance with strict pollution requirements, is manifest.
Microturbine technology is a relatively new field that finds its roots in conventional gas turbine engines for auxiliary power units and transportation applications. It is also part of a growing trend in electric power production, namely that of distributed generation (DG), which arose out of a need to provide alternatives to traditional grid-based power sources for small to medium-sized users. Microturbines are generally much more compact than steam-based, or even central gas turbine power units, and can provide cleaner, lower maintenance power than traditional reciprocating engines at a reasonable cost per kilowatt-hour. In addition, the relative compactness of microturbines, with or without a Rankine-cycle heat energy utilization subsystem, readily lends itself to increased system modularity, portability and upgradeability.
Configurationally, a microturbine has much in common with other gas turbine engines, including an engine housing, one or more rotating shafts, a generator, compressor, combustor, turbine, and exhaust duct. In some microturbines, the compressor, generator and turbine are coupled to a single shaft, and rotate as a unit. Normally, the shaft itself is mostly or entirely contained within and coupled to the housing, often through a bearing-mount-strut-frame arrangement well known to those skilled in the art. In a typical gas turbine system, ambient air enters through a generator section and into a compressor, which typically pressurizes the air from three- to ten-fold. From the compressor, it next goes into an optional recuperator, where the air can be preheated prior to entering the combustor to increase overall cycle efficiency. The preheating of the compressed air in the recuperator arises out of a heat exchange process with hot exhaust gas from the turbine discharge. Higher preheated air temperature leads to higher cycle efficiency, which can have dramatic impacts on life-cycle fuel usage. In addition, preheated air has the requisite temperature to facilitate a form of combustion, known as catalytic combustion (discussed below), which has been identified as a promising way to prevent the onset of NOX formation, which is becoming a major concern in urban airsheds. After the warmed, compressed air exits the recuperator, it enters the combustor, where the air mixes with high pressure fuel, with the resulting mixture burned in a combustion chamber. The hot gas next enters the turbine section and impinges on the turbine rotor, so that as the gas expands through one or more stages of the turbine section, it causes the rotor to spin, which in turn drives the compressor. A generator may also be driven by that rotor, or by a second turbine rotor driven from the exhaust of the first rotor. Upon leaving the turbine section, the hot exhaust gas gives up some of its excess heat in the aforementioned recuperator to heat up the incoming air. Finally, the exhaust gas is ducted through an exhaust into the atmosphere.
While microturbines are particularly well-suited to providing prime mover power in a cogeneration system, it is not necessary that the prime mover be a microturbine. For example, conventional gas turbines, steam boilers (powered by burners fired by natural gas, coal, oil, or possibly even nuclear reactors), diesels, fuel cells, thermoelectric, thermophotovoltaic and even renewable sources, such as solar energy and combustible biomass all provide viable alternatives. These cogeneration approaches are part of a larger class of power plants often referred to as “combined cycle”, where a higher-temperature thermodynamic cycle rejects its heat to a lower-temperature thermodynamic cycle that typically utilizes a different working fluid. The two cycles making up a combined cycle are typically known as topping and bottoming cycles, with the topping cycle often referred to as a prime mover subsystem, and the bottoming cycle as a waste heat recovery, or energy utilization, subsystem. Combined cycle operation is common in larger size units, such as systems with a power output of 10 megawatts (MW) or greater. These systems often employ a gas turbine topping cycle and a steam turbine Rankine bottoming cycle, where the high temperature exhaust gas from the prime mover subsystem is used to drive a waste heat steam turbine. However, when the prime mover subsystem is a smaller unit, such as a microturbine, the addition of a similar steam turbine system results in a degree of complexity that sacrifices many of the microturbine's modular features, as well as paving the way for significant cost growth, both in initial purchase price and the higher cost of maintenance. Moreover, the use of conventional steam based systems with which to exploit the waste heat's useful energy necessitates the use of high vacuum condensers and cooling towers. In addition, steam systems and many other heat energy utilization systems are not hermetically sealed, thus exposing the user to increased mess and maintenance issues. In either case, such configurations present unacceptable situations to small commercial and residential users.
Accordingly, there exists in the art a need for a system which can provide compact, clean, inexpensive, reliable, low maintenance, on-demand power with flexibility to tailor electrical or mechanical power output to the users' particular needs.
SUMMARY OF THE INVENTION
The present invention satisfies the aforementioned need by providing a means by which heat from a prime mover subsystem is used to drive a secondary subsystem that generates additional power.
According to an embodiment of the present invention, a heat energy utilization system includes a heat engine that is made up of at least a thermal circuit, a pump, a power module and a pair of heat exchangers. As defined herein, a “thermal circuit” is piping or ducting designed to carry fluid through a path that interconnects the various heat energy utilization system components. Similarly, the term “pump” includes any device that can be used to increase the fluid flow rate or pressure. The power module is itself made up of at least an expander and a load absorbing device. The load absorbing device can be another pump, gearing, generator or similar energy conversion apparatus. The heat energy utilization system is designed to be run as either a stand-alone power generation source, or as an optional bottoming cycle for a larger system such that it extracts heat from the heat stream of a prime mover such as an exhaust gas waste heat stream, or a dedicated heat producer. In either configuration, when the heat energy utilization system is operational, it generates useable work via vapor expansion of a working fluid through the thermal circuit. In the present context, the term “useable work” is that which is capable of producing a tangible mechanical or electrical effect, such as rotating or reciprocating motion in a shaft or related device against a resistance (in the case of mechanical work) or an electric current flow and potential (in the case of electric work). As such, the mere creation of any non-recoverable work is excluded from the instant definition of “useable work”, and that the operation of all thermodynamic cycles produces at least some heat that is non-useable. Nevertheless, the present inventors have discovered that the waste heat or exhaust gases emitted from a microturbine are capable of performing additional useable work, as they are well-suited to powering a Rankine-cycle subsystem to recover and reuse the exhaust gas energy as additional power, thus further enhancing the efficiency of the overall power generation cycle. The present inventors have also recognized that the while the heat energy utilization system can be of either open or closed variety, the preferred configuration is closed, comprising a continuous loop requiring no external fluid makeup, save that associated with normal system losses occurring over long periods. The chief advantage of the closed system is that it is self-contained, and therefore more adaptable to modular uses, as well as uses where maintenance and cleanliness/neatness issues are important. In the present closed cycle heat energy utilization system, the working fluid can be any number of compounds, such as organic refrigerants, water, ammonia, propane or N-butane. The pump pressurizes the working fluid, which is then routed to a first of the heat exchangers (such as an evaporator) that boils the working fluid by absorbing heat from an external heat stream. From the evaporator, the working fluid passes to an expander in the power module. The expanding working fluid can then impart work to the expander, which can then turn a coupled shaft to produce mechanical, or, if attached to a generator, electrical, work. After passing through the expander, the working fluid is cooled and condensed in the second heat exchanger (typically a condenser) so that it can return to the pump and start the cycle all over again.
The inventors have recognized that a primary advantage of the heat energy utilization system of the present invention is that key components, including pumps, expanders, heat exchangers and electric generators can be contained within individual hermetically sealed modules in the heat energy utilization system. This is especially relevant to a power module where the expander is coupled to an electric generator. Thus, for example, a “hermetically sealed” expander would have self-contained moving parts, including bearings, orbiting shafts, rotating shafts and disks, armature coils and optionally heat exchange and lubricant-circulating devices that are contained within a module shell so as to be sealed from the external environment. Thus, save fluid inlet and outlet ports, and possibly an access port through which additional working fluid or lubricant may be added to periodically replenish that lost during normal operation, and electrical connectors to carry electricity to or from the generator, the power module operates in complete autonomy, thus avoiding maintenance issues and the mess associated with lubricants, leaky seals and noisy machinery. In addition to permitting application in places where cleanliness is paramount, such as around people, foodstuffs, sensitive electronic equipment and damage-susceptible chattels, the system exploits its inherent modularity to permit it to be moved or upgraded as requirements demand.
One way the power module is able to remain hermetically sealed is through the use of a scroll expander. While hermetic operation is not unique to scroll configurations, the present inventors recognize that, by virtue of the low number of moving parts (with attendant reduction in maintenance) in a scroll device, its configuration is an especially good fit with the limited access inherent in sealed environments. In a scroll (also known as an involute spiral wrap) device, which can be operated as either an expander or compressor, one or more pairs of meshed axially extending involute spiral wrap members, one fixed to the housing, the other attached to and orbiting with a shaft, are axially meshed to define a plurality of crescent-shaped chambers which, by virtue of the orbital motion of one wrap member relative to the other changes the shape and size of the crescents, which in turn changes the pressure of the fluid contained therein. In an expansion mode, the fluid enters through a central port, and proceeds circuitously in a radially outward direction, causing the crescent chambers to move, which, through an anti-rotation mechanism (such as an Oldham link or a ball coupling ring assembly), consequently turns an eccentric linkage, coupled to the orbiting scroll. The linkage is attached to a rotating shaft with an offset functionally equal to the radial distance from the rotational axis of the shaft, thereby transforming the scroll orbital motion into rotating motion in the shaft. Conventional needle bearings can be placed in the eccentric aperture to reduce friction between the linkage and the orbiting scroll.
As mentioned above, the scroll expander could further incorporate two scroll pairs, each disposed on opposing ends of a common rotating shaft. Each pair is in turn made up of the aforementioned pair of meshed axially extending involute spiral wrap members for symmetric bearing loading, annular cooling channels, an optional external armature for supplemental electrical power generation, and axial compliance features to avoid thermal expansion mismatches. This dual scroll configuration is especially valuable in providing a third, hybrid operational mode, where one of the spiral wrap members can be operated in expansion mode while the other concurrently operates in compressor mode. This and additional features of the scroll device with an integral field-generating rotor are described in copending application, U.S. Ser. No. 09/681,363, INVOLUTE SPIRAL WRAP DEVICE, filed Mar. 26, 2001, by Sullivan et al., herein incorporated by reference. Regardless of being configured as a single or dual scroll device, the scroll of the present invention is a low maintenance device largely due to the rolling versus sliding contact of the scroll wall flanks, the elimination of dynamic seals, and the elimination of valves. The inventors have recognized that the use of a scroll expander in the present invention heat energy utilization system has significant advantages over traditional bottoming cycle devices. The small number of parts associated with the scroll design, coupled with its inherently simple motion ensures a low maintenance part that can be placed in an infrequently-accessed sealed container. The hermetic sealing unique to this approach facilitates an entirely integrated, modular power generation system. Additionally, the compact nature of the expander can be made even more diametrically compact through the use of dual opposed scroll wrap members, such as those described in the aforementioned copending application. Many significant advantages of the scroll machine have been proven with the successful use of scroll compressors in the refrigeration and air conditioning industry.
In the case where the desired power output is electrical, the load-absorbing device can be a generator made up of a field-generating rotor situated around the periphery of a rotating shaft. A stator coil in inductive proximity to the field rotor could be affixed to the outer portion of the scroll housing, but still within the module's larger hermetic seal shell. When the scroll device is operating in expansion mode, alternating current electricity could be passed from the generator, through the hermetic shell via electrical conductors, and to attached electrical connectors. Thus, power output can be effected without having to pass a shaft (and attendant sealing mechanisms) through the hermetic housing, thereby alleviating concerns over seal boundaries and leakage/contamination paths.
Another option to the heat energy utilization system is the inclusion of one or more process heat utilization modules that can extract heat from the thermal circuit for various process needs, while still providing supplemental power from the heat energy utilization system's power module. Preferably, the heat recovery modules include at least a low temperature unit to provide for lower temperature process requirements (such as warming air in dwelling spaces occupied by people, referred to as space heat), and a high temperature unit to provide higher temperature process requirements, such as domestic hot water or steam. This feature has the advantage of accommodating additional user needs, beyond just electricity requirements, to provide hot water, heated air or steam, among others.
Another option includes a heat energy utilization system quick-start module. The quick-start module permits the heat energy utilization system to either pre-start prior to the operation of an optionally attached prime mover, thus speeding up its response time, or to operate as the sole provider of power in dynamic (i.e.: rapidly fluctuating) or lower power modes where operating a prime mover would be impractical. All modern power generation systems, including microturbines, require initiation, or start-up, of their operating sequence. Typically, this is effected by a logic and control module that is capable of sending control signals to the various components within the system. An energy storage (or auxiliary power) unit, such as an electrical battery, of sufficient size is included to power the logic and control module and related equipment. A recharging module can be disposed between the load absorbing device and the energy storage unit such that extra power generated by the load absorbing device can be used to keep the energy storage device fully charged. The sequence of using the quick-start operation includes igniting an auxiliary burner and powering the pump to increase the pressure and temperature of the working fluid, which can then pass through the expander to generate power and thereafter render the system self-sustaining. The quick-start feature allows the less cumbersome heat energy utilization system to start with minimal stored energy, thus reducing the size of the energy storage unit. Once started, the heat energy utilization system can provide sufficient power to the prime mover to allow for a complete start, or, if necessary, as the sole provider of power in low or dynamic power situations, thus comprising a self-sufficient system rather than as a subsystem to a larger combination. An optional variant of the quick-start mechanism includes a high-pressure accumulator connected through one or more control and isolation valves between the evaporator and the expander. Upon cessation of normal power generation system operating conditions, the accumulator collects high thermal and pressure content working fluid. Under the quick-start mode, a control valve is opened, allowing the high pressure and temperature working fluid to boil off and enter the thermal circuit such that it can expend its excess energy in the expander. Optional pre-start activation of the auxiliary burner ensures that the working fluid will contain adequate thermal and pressure properties upon quick-start. An isolation valve can be used to direct heat from the auxiliary burner directly to the accumulator during the starting sequence. The chief advantage of the start-up module without the high pressure accumulator is in its simplicity. The optional high pressure accumulator, on the other hand, while requiring a separate function in the control module to synchronize pump, valve and burner sequencing, will result in a more rapid response from the expander, leading to shorter start-up sequences.
According to another embodiment of the present invention, a heat energy utilization subsystem is adapted to be coupled to a prime mover subsystem, where the output of the heat energy utilization subsystem is electric potential. The heat energy utilization subsystem includes a thermal circuit, pump, hermetically sealed power module with a plurality of scroll pairs and a coupled generator to produce the electric output, a throttle valve to regulate working fluid flow into the power module, and first and second heat exchangers. The prime mover subsystem can be any power source that includes some form of thermal energy in a heat stream. In this regard, prime movers that provide an exhaust gas from a combustion process (including gas turbines and their subset of microturbines), steam (from natural gas, coal, oil or nuclear powered devices), chemical reaction (including fuel cells) as well as solar, thermophotovoltaic and thermoelectric sources are all considered valid examples that can be coupled to the heat energy utilization subsystem.
According to another embodiment of the present invention, a power generation system for providing a primary and secondary source of output power is disclosed. The primary source of output power comes from a prime mover subsystem, and the secondary source of power comes from a heat energy utilization subsystem similar to that of the first embodiment. As with the first embodiment, the subsystem may include one or more scroll pairs, as well as features capable of providing heat energy utilization subsystem quick-start, such as a control and logic module, an energy storage device, an accumulator, or an auxiliary burner in thermal communication with the heat stream. An optional throttle valve may be included to regulate the flow of working fluid to the power module.
In addition to these and the other options associated with the earlier embodiments, two additional features that could be included in the present embodiment are a capacity control module that uses proportional integral differential (PID) logic, and a load splitting module that includes a fuzzy logic controller. The first, the capacity control module, permits the heat energy utilization subsystem to respond to changes in subsystem power levels based on the analysis of control signals coming to and going from the control module. Accordingly, the capacity control module, which includes a rapid response portion and a slow response portion, is used to determine power requirements of the heat energy utilization subsystem in response to loads set on it from elsewhere (such as from the below-described load-splitting module). The PID-based controller combines the instantaneous response of proportional control with the offset correction features of integral control and the rapid response to error signals of derivative control. In applications where both the primary and secondary power output is electrical, the inventors of the present device are not aware of any prior art that allows for splitting of the electrical load between the uniform and dynamic components to be applied to a combined multiengine thermodynamic system. Components making up the capacity control module include a speed sensor coupled to the expander, a feed-back controller operatively responsive to a signal from the speed sensor so as to actuate the valve that isolates the accumulator, a bypass valve disposed within the heat stream to control heat stream flow into the first heat exchanger module, a plurality of sensors disposed in the thermal circuit to measure the temperature and pressure of the working fluid, and a proportional integral differential logic controller to control the bypass valve, pump and auxiliary burner based on first heat exchanger sensor input signals. Additionally, the speed sensor and feed-back controller can also be incorporated within the power module hermetic shell.
The second, the load splitting module, can be used to isolate the prime mover subsystem from rapid-response dynamic loads by using a fuzzy logic controller to set loads for each of the two power generating subsystems. The load-splitting module analyzes electrical use requirements in order to set the load on each of the two power generating subsystems. The optional fuzzy logic controller is used to determine the substantially uniform load (also known as a “quasi-steady state” load, typically associated with the prime mover subsystem) component, and the dynamic load component (typically associated with the heat energy utilization subsystem). The practical applications of fuzzy logic have been on the rise in recent years, providing rule-based ways of determining continuous, intermediate truth values from vague or incomplete data sets such that a result, processable by digital computers, can be obtained. As such, fuzzy logic-based inference engines and controllers are well-suited to process-driven events, where quick, accurate monitoring of; and active feedback to, a dynamic environment can provide improvements in system response, efficiency and overall operability. Thus, with the fuzzy logic-based load splitting module, a composite electric generation profile, comprising component contributions from both the prime mover and heat energy utilization subsystems, can be produced based on an interactive controller such that the efficiency of the overall generation of electricity is maximized.
Optionally, the prime mover subsystem can be a microturbine, either without or with a recuperator. In the first instance, since the turbine exhaust gas does not have to give up its thermal content in a recuperator to preheat the compressed air going into the combustor, full exploitation of the exhaust gas can occur at the heat energy utilization subsystem's evaporator. Thus, the non-recuperated variant has the advantage of having the simplest interconnection and operation, as well as the smallest, least obtrusive footprint, thus maximizing its affordability. Specific power, a common metric expressed as the ratio of power output to either weight or displaced volume of the system, is also maximized in the non-recuperated subsystem. In the second instance, the microturbine-based prime mover subsystem employs a recuperator, which is essentially a dual-loop heat exchanger connected between the compressor discharge and the combustor inlet for the first loop, and between the turbine exhaust and ambient for the second loop. The turbine exhaust gas, after giving up its heat in the recuperator to raise the temperature of the air coming out of the compressor, will have a lower energy content than that for the non-recuperated device of the previous embodiment, and therefore will have less energy to give up to the Rankine-cycle heat energy utilization subsystem. To make up this difference, the recuperated subsystem variant can also include a separate prime mover auxiliary burner which could be included with, but external to, the modules of the heat energy utilization subsystem for situations requiring high efficiency. The prime mover auxiliary burner could be placed at various locations in or around the prime mover to optimize its effectiveness, such as either upstream or downstream of the recuperator, or in a mixing relationship with the fluid directly leaving the turbine exhaust. The benefits of incorporating the recuperated subsystem features include the aforementioned easy start-up due to the presence of low pressure pre-start components, as well as not requiring a high efficiency recuperator to achieve suitable overall system performance.
The higher prime mover combustor inlet temperatures made possible through the use of a recuperator would also permit a catalytic combustor to be utilized in place of the conventional combustor. The use of a catalytic combustor permits combustion byproducts that would otherwise be discharged as gaseous or particulate pollutants to be burned, or chemically altered to less objectionable species, thus providing the dual benefit of generating additional power while simultaneously reducing airborne pollutants. With a catalytic combustor, when exhaust gases and particulate come in contact with a noble metal coated ceramic core, chemical changes occur in the byproducts that permit them to ignite at relatively low temperatures, thus promoting more complete combustion, even in lower temperature operating regimes. To be effective, the air entering the catalytic combustor must itself be substantially preheated to promote the chemical reaction. The inventors of the present invention have recognized that a recuperator and a catalytic combustor can be placed in series with a supplemental, low pressure burner to reheat a turbine exhaust stream prior to introduction of that stream into the heat energy utilization subsystem's evaporator. In addition, multistaged turbines and compressors could be employed to add more flexibility to the design, effecting decisions on how much supplemental burner heating is necessary. From such a configuration, bleed or discharge ducts could route exhaust streams of appropriate pressure and temperature to any of several desired locations. The advantage of this feature is that the coupling of the catalytic combustor and recuperator, as part of this flexible embodiment, is particularly well-suited to extremely low emissions operation, and is in keeping with the overall system's flexibility features.
In accordance with still another embodiment of the present invention, the prime mover and heat energy utilization subsystems are integrated to provide both a primary and secondary source of power. In the present context, the term “integration” means more than the mere interconnection of disparate subcomponents, as true integration is an engineering solution designed around the proper interrelationship of these subcomponents, especially on how variations in the performance of one effects not just another, but the system as a whole. To that end, the system of the present invention includes, among other factors, considerations of size, load dynamics isolation and load splitting, heat energy utilization subsystem capacity control, durability, flow rates, quick-start sequencing, temperatures, pressures, acquisition and life-cycle costs, pollution minimization and aesthetics. The approach of this embodiment is especially beneficial when the prime mover is a microturbine, which can furthermore be either non-recuperated or recuperated. This allows system designers the flexibility of accommodating varying combustor and emissions requirements, such as the use of a catalytic combustor, into the overall power generating system. In addition to a microturbine prime mover, the system of the present embodiment includes a heat energy utilization subsystem, which in turn includes a closed loop thermal circuit, pump, first heat exchanger, hermetically sealed power module with scroll expander and load absorbing device, and a second heat exchanger. The invention described herein represents a practical and cost-effective approach to achieving a microturbine combined cycle (MTCC) at a much smaller scale than gas turbine combined cycle (GTCC) power plants currently in use.
The system of the present embodiment may be outfitted with the same options as that of the heat energy utilization system of the first embodiment, as well as the optional load-splitting module and the capacity control module of the previous embodiment. In the present embodiment, these additions are now integral features the combination of which can provide a total power generation package. The advantage of the integrated system is the resulting turn-key approach to providing solutions to a user's power requirements, including automated system operation modes. For example, the load-spitting module will continually monitor actual electrical load dynamic characteristics and adjust the load split between the prime mover and heat energy utilization subsystems through the use of optional sophisticated fuzzy logic that can mimic a variety of operational parameters without the need for user intervention. Similarly, the capacity control module will monitor various parameters (such as evaporator pressure and vapor superheat temperature) with a distributed network of pressure and temperature transducers. The capacity control module's smart controller automatically analyzes evaporator pressure and temperature dynamics to provide rapid response and control to the pump, valves, and auxiliary burner firing rate.
The use of the integrated approach to incorporating the heat energy utilization subsystem herein described is well-suited to situations involving low thermal energy heat streams from the prime mover, where, by adding a low pressure auxiliary burner to energize the prime mover heat stream, sufficient heat exchange can take place within the heat energy utilization subsystem's evaporator. This approach is especially useful in gas turbine prime movers, where recuperators can be used to heat prime mover incoming air. Similarly, in non-recuperated prime mover subsystems, where preheated air for the prime mover subsystem's main combustor is not required, the heat energy utilization subsystem can be run directly off the turbine exhaust of the prime mover, thus removing the need for a burner to reheat the exhaust gas. In such a system, higher initial compression of the air entering the prime mover could provide sufficient thermal content to abrogate the recuperator. The combination of portable, modular features inherent in both subsystems is further exploited to ensure that a complete power generation package is available to the user, and can be adapted to myriad parametric requirements. By tailoring the needs of the heat energy utilization subsystem with the capabilities of the heat stream provided by a prime mover, a system optimized for size, power and emissions output and cost can be effected.
In accordance with another embodiment of the present invention, a method for producing power by using a power generation system made up of a prime mover subsystem and a heat energy utilization subsystem comprises the steps of operating the prime mover to turn a first electric generator, arranging the components of the heat energy utilization subsystem such that at least an evaporator is in a heat exchange relationship with the heat stream generated by the prime mover, an exchange of heat between the waste heat stream and the evaporator such that heat is transferred to a working fluid flowing through a thermal circuit that maintains fluid communication between the components of the heat energy utilization subsystem, regulating the flow of the working fluid with a throttle valve, expanding the working fluid in an expander that is coupled to a second electric generator, condensing the expanded working fluid, and then pressurizing the working fluid.
Optionally, the method could also include additional attributes and steps. For example, a throttle valve can be included to help regulate the flow of working fluid to the expander. In addition, either or both of the load absorbing devices can be a generator to generate electricity. Both the expander and second load absorption device can be hermetically sealed, while the expander is preferably a scroll device (which itself can comprise single or dual scroll pairs). A lubricant pump and a lubricant droplet separator may be used in situations requiring separation of the working fluid from the lubricating fluid, and both the pump and separator can be disposed within the hermetically sealed power module. Another optional step could include operating a desuperheating heat exchanger such that the high temperature working fluid exiting the expander can pass through the heat exchanger; the heat exchanged therein could be used for a DHW or SH loop. As previously discussed, a microturbine can be used as the prime mover subsystem. Furthermore, at least one process heat utilization module can be placed in thermal communication with the working fluid such that heat can be extracted from the working fluid and directed to the process heat utilization module. Similarly, the process heat utilization module can be placed in thermal communication with the thermal circuit to provide process heat. Another step includes using an accumulator to receive and store elevated temperature and pressure working fluid such that the accumulator can smooth out system operation during certain conditions. For example, the accumulator can be used to provide alternate steps to initiate a start-up sequence in the heat energy utilization subsystem, as can an auxiliary burner. A load splitting module can be incorporated to coordinate steady-state and dynamic load requirements in the system, while providing a capacity control module will assist in promoting better response within the heat energy utilization subsystem. The load splitting module may further be based on a fuzzy logic controller that can sense various instantaneous and historical data to provide output instructions to the prime mover and heat energy utilization subsystems.
In accordance with yet another embodiment of the present invention, a method of operating a heat energy utilization system is disclosed. The method includes the steps of arranging at least a pump, first heat exchanger, expander and second heat exchanger to be in fluid communication with one another via circulated working fluid routed through a thermal circuit. In addition, an auxiliary burner, fuel supply and auxiliary burner exhaust line are arranged such that the auxiliary burner exhaust line is placed in thermal communication with the thermal circuit. Next, a start-up sequence is initiated in the heat energy utilization system by providing electric current to the control module so that it in turn can send start-up signals to one or more of the heat energy utilization subsystem components such that the heat produced in the auxiliary burner and routed through the auxiliary burner exhaust line exchanges its heat with the first thermal circuit such that a working fluid flowing through the thermal circuit enables the operation of the heat energy utilization system to be self-sustaining. The control of the power level in the heat energy utilization system is effected in the present method by regulating the flow of the working fluid to the expander with a throttle valve disposed within the first thermal circuit. After passing through the throttle valve, the working fluid goes through an expander such that the energy released by the expansion process turns the coupled generator, which in turn produces electricity. After passing through the expander, the working fluid is routed to a condenser for cooling, and a pump for circulating throughout the first thermal circuit. By this entire method, the heat energy utilization system is capable of sustained, stand-alone operation.
Optional steps in the method include hermetically sealing the expander and generator in a power module, as well as utilizing a scroll device in the power module's expander, as well as placing a lubrication system within the hermetic shell, and removing excess heat from the expander through a desuperheating heat exchanger, both of the last two in a fashion similar to that used in the previous method. Additional steps that may be embodied in the current method further include utilizing a microturbine as the prime mover system, and utilizing either or both high and low temperature heat recovery modules in thermal communication with the condenser of the heat energy utilization system, such that the heat recovery module can extract heat, thereby producing process heat in addition to, or in place of a secondary electric generation output. Also, in addition to operating the heat energy utilization system with quick-start features, the method can incorporate load-splitting and capacity control, both as described in conjunction with the previous embodiment.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims.