Exhaust from the small gas turbine enters the burner windbox and flows through the throat of the register-type burner.
Related terms:
An Overview of Gas Turbines
Meherwan P. Boyce, in Gas Turbine Engineering Handbook (Fourth Edition), 2012
Small Gas Turbines
Many small gas turbines that produce an output of less than 5MW are designed similarly to the larger turbines already discussed; however, there are many designs that incorporate centrifugal compressors or combinations of centrifugal and axial compressors as well as radial-inflow turbines. A small turbine will often consist of a single-stage centrifugal compressor producing a pressure ratio as high as 8:1, a single-side combustor where temperatures of about 1800°F (982°C) are reached, and radial-inflow turbines. Figure 1-44 shows a schematic diagram of such a typical turbine.
Figure 1-44. A radial-inflow turbine.
Air is induced through an inlet duct to the centrifugal compressor, which rotates at high speed and imparts energy to the air. On leaving the impeller, air with increased pressure and velocity passes through a high-efficiency diffuser, which converts the velocity energy to static pressure. The compressed air, contained in a pressure casing, flows at low speed to the combustion chamber, which is a side combustor. A portion of the air enters the combustor head, mixes with the fuel, and burns continuously. The remainder of the air enters through the wall of the combustor and mixes with the hot gases. Good fuel atomization and controlled mixing ensure an even temperature distribution in the hot gases, which pass through the volute to enter the radial-inflow turbine nozzles. High acceleration and expansion of the gases through the nozzle guide vane passages and turbine combine to impart rotational energy, which is used to drive the external load and auxiliaries on the cool side of the turbine. The efficiency of a small turbine is usually much lower than a larger unit because of the limitation of the turbine inlet temperature and the lower component efficiencies. Turbine inlet temperature is limited because the turbine blades are not cooled. Radial-flow compressors and impellers inherently have lower efficiencies than their axial counterparts. These units are rugged and their simplicity in design assures many hours of trouble-free operation. A way to improve the lower overall cycle efficiencies, 18–23%, is to use the waste heat from the turbine unit. High thermal efficiencies (30–35%) can be obtained, since nearly all the heat not converted into mechanical energy is available in the exhaust and most of this energy can be converted into useful work. These units when placed in a combined heat power (CHP) application can reach efficiencies, of the total process, as high as 60–70%.
The OPRA Turbine operates at a pressure ratio of 6.7:1 and produces 1910 kW of power at an efficiency of 26.9% and a heat rate of 12,732 BTU/kW h (13,433 kJ/kW h). The Dresser-Rand KG2-3E is a similar type turbine shown in Figure 1-45 and which used to be a Kongsberg gas turbine manufactured in Norway and has one-stage centrifugal compressor with a pressure ratio of 4.7 and single-stage radial-inflow turbine to produce a power of 1895 kW at an efficiency of 16.7% and a heat rate of 21,542 BTU/kW h (22,729kJ/kW h). It is used for standby power with a 99.3% starting reliability.
Figure 1-45. The Dresser-Rand KG2-3E turbine.
The small Kawasaki gas turbines use centrifugal compressors and, in many cases, two centrifugal compressors are used as shown in Figure 1-46, but they use several stages of axial-flow turbines producing a pressure ratio of 10.5 to produce up to 1685kW of power at an efficiency of 26.6% and a heat rate of 12,841 BTU/kW h (13,548 kJ/kW h). At the higher pressure ratio and higher firing temperature because of the use of axial-flow turbines, it has a higher efficiency than other smaller turbines that use a centrifugal compressor and radial-inflow turbines. The blades of axial-flow turbines can be cooled while it is very difficult to cool radial-inflow turbines, thus limiting the firing temperature.
Figure 1-46. Two centrifugal compressors.
URL:
https://www.sciencedirect.com/science/article/pii/B9780123838421000019
Biomass: Some Basics and Biogas
Jussara F. Fardin, ... Augusto P.F. Dias, in Advances in Renewable Energies and Power Technologies, 2018
1.8.1.2 Microturbine Technologies
Microturbines are small gas turbines that burn a wide variety of clean gaseous including biogas. The size range for commercially available microturbines is from 30 to 250kW. They can be used in power-only generation or in combined heat and power (CHP) systems.
In typical microturbines, a radial compressor compresses the inlet air that is preheated in the recovery using heat from the turbine exhaust. This heated air from the recovery is then mixed with fuel in the combustor so that the hot combustion gas expands in the turbine, producing rotating mechanical power to drive the compressor and the electric generator.
URL:
https://www.sciencedirect.com/science/article/pii/B9780128131855000012
Microgrid and hybrid energy systems
Muhammad Kamran, in Fundamentals of Smart Grid Systems, 2023
7.3.2.3 Microturbines
Microturbines are small gas turbines coupled to their generators. They are used to produce both electricity and heat and are available in a range of 25–500kW and efficiency ranges between 20% and 30%. The technology of microturbines is based on the technology of diesel engine turbochargers, automotive designs, and aircraft auxiliary power systems. In the past, microturbines were designed for small commercial and large domestic applications. With the advancement in the distributed generation technology, they are designed for distributed generation since they feed their electricity to the distribution network.
The construction of microturbines is identical to that of gas turbines, consisting of a compressor, combustor, turbine, and generator. Apart from the size of the turbine, they differ in the shaft. The compressor and the turbine are mounted to the same shaft of the electrical generator. The air is mixed with the fuel in the combustion chamber and the resultant hot gases are used to derive the turbine. The turbine, as a result, rotates both the generator and the compressor. The compressor sends the intake air to the combustion chamber at high pressure. Microturbines are mostly packaged with a coupled high-speed permanent magnet generator to be easily installed in the DG application. Despite being small in size, the rotational speed of the microturbine and the couples generators are very high, normally between 40,000 and 120,000 RPM, which needs to be reduced to get an output of 50–60Hz for the synchronization purpose. To get the required frequency and voltage, a power electronics inverter and rectifiers are used.
Microturbines can be with or without a recuperator. Microturbines without recuperators are less efficient since the exhaust heat is wasted into the atmosphere without reuse. A recuperator is a cross-flow heat exchanger placed at the exhaust position of the system to recover the heat from the exhaust gases and is used to preheat the discharge of the compressor as shown in Fig. 7.8. Microturbines are also used for CHP applications, which make them more efficient.
Fig. 7.8. Flow diagram of the microturbine.
URL:
https://www.sciencedirect.com/science/article/pii/B9780323995603000065
Microturbines
Paul Breeze, in Gas-Turbine Power Generation, 2016
Abstract
Microturbines are very small gas turbines with sizes as small as 1kW although the main commercial machines are in the range 30 to 500kW. These small turbines usually have a single stage compressor and a single stage turbine, with a generator mounted on the same shaft. Both compressor and turbine are normally radial rather than axial in design, as would be the case for their larger relatives. Rotational speed is extremely high, usually greater than 40,000rev/minutes and power electronics are used to match the output frequency to the grid. Efficiency is low compared to a large gas turbine but many of the more efficient microturbines use recuperators to improve performance. Others use waste heat to produce hot water in a cogeneration system.
URL:
https://www.sciencedirect.com/science/article/pii/B9780128040058000082
Gas Turbine Combined Heat and Power Systems
Paul Breeze, in Combined Heat and Power, 2018
Microturbine CHP
A microturbine is a small gas turbine designed for use in domestic and commercial installations. Identical in concept to standard gas turbines, these devices are much simplified with perhaps a single set of compressor blades and a single set of power turbine blades. Microturbines can be designed either for power generation alone or as CHP units. Although they are capable of burning a variety of fuels, most will be intended to operate with natural gas. The units are usually designed as a complete package for electricity and hot water production. All that is required is to connect the package to the electricity supply, a gas supply and to the hot water system.
Microturbines are available in sizes ranging from 30kW up to 400kW. Beyond that conventional gas turbines take over. There are, in addition, much smaller microturbines aimed at the domestic market. These have electrical generating capacities of 1kW to 10kW. All microturbines operate at extremely high speed, with rotational speeds often in excess of 60,000rpm. The smaller the turbine, the higher the speed. Electrical efficiency of these small machines is relatively low with a 30-kW machine typically capable of around 23% efficiency. Larger machines are slightly more efficient. CHP efficiency is much higher and a 30-kW microturbine might achieve 67% CHP efficiency.
URL:
https://www.sciencedirect.com/science/article/pii/B9780128129081000067
Centrifugal Compressors
Meherwan P. Boyce, in Gas Turbine Engineering Handbook (Fourth Edition), 2012
Publisher Summary
Centrifugal compressors are used in small gas turbines and are the driven units in most gas turbine compressor trains. They are an integral part of the petrochemical industry, finding extensive use because of their smooth operation, large tolerance of process fluctuations, and their higher reliability compared to other types of compressors. It is normal practice to design the compressor so that half the pressure rise takes place in the impeller and the other half in the diffuser. The diffuser consists essentially of vanes, which are tangential to the impeller. The blades should be designed to eliminate large decelerations or accelerations of flow in the impeller that lead to high losses and separation of the flow. Potential flow solutions predict the flow well in regions away from the blades where boundary-layer effects are negligible. The second Helmholtz law states that the vorticity of a frictionless fluid does not change with time. The machine must operate with a suitable margin to the left of where these curves begin their steep descent or tail-off, and in the resultant operating range, the curve for backward-leaning blades is steeper.
URL:
https://www.sciencedirect.com/science/article/pii/B9780123838421000068
Mobile applications: cars, trucks, locomotives, marine vehicles, and aircraft
Wenqian Chen, in Design and Operation of Solid Oxide Fuel Cells, 2020
12.3.2 Auxiliary power for aerial and water vehicles
The conventional APU in aircraft is a small gas turbine system that provides pneumatic and electrical power for various operations such as starting the main engines, lighting, and air conditioning. An SOFC-APU can overcome the problems of conventional aircraft APU including low efficiency (20% on land and 40% in air) and high level of noise and pollution. For long-range traveling the target power generation capacity is between 400 and 977kW [73].
Due to high cost the development of an aircraft SOFC-APU has been limited to simulations, which showed that the SOFC–GT system (Fig. 12.2A) could be a suitable configuration [55,74–78]. One of the simulation studies showed that the SOFC–GT system could achieve an overall-thermal efficiency of 62%. The model also showed that high turbine inlet temperature and low pressure ratio could lead to high thermal efficiency.
In another simulation study an SOFC–GT system (Fig. 12.12) was designed to produce a net electrical power of 440kW for a long-range aircraft with a capacity of 300 passengers [76]. The operations at sea level and cruise level (12,500m) were considered to account for the changes in the ambient temperature and pressure. The mass of the SOFC–GT system was significantly higher at sea level than at cruise level (1912kg vs 1396kg) because a larger SOFC was needed to meet the higher power requirement at sea level as the turbine could not expand the gas as much under ambient pressure. The system designed for operating at sea level also had a significantly lower thermal efficiency than at cruise level (42% vs 73%) as the pre-compressed air from the environmental control system could be utilized at cruise level.
Figure 12.12. Solid oxide fuel cell–gas turbine system as an aircraft auxiliary power unit [76].
Another simulation study investigated two configurations of an SOFC–GT system (Fig. 12.13) as the APU of a 300-passenger long-range aircraft [78]. Similar to the study mentioned above, the target electrical power requirement was 440kW. At similar operating conditions, configuration 1 resulted in a slightly higher cycle efficiency than configuration 2 (58% vs 54%) as the operating temperature of the SOFC in configuration 1 was higher than configuration 2 (944°C vs 832°C) and the performance of the SOFC increased with temperature. The higher operating temperature of the SOFC in configuration 1 was the result of enhanced fuel heating by the low mass flow rate steam (Fig. 12.13A). Sensitivity analysis (Fig. 12.14) shows that high overall heat transfer coefficient and low air flow rate could lead to high cycle efficiency.
Figure 12.13. Solid oxide fuel cell–gas turbine systems as an aircraft auxiliary power unit: (A) configuration 1; (B) configuration 2 [78].
Figure 12.14. Effect of air flow rate and overall heat transfer coefficient (UA) on cycle efficiency [78].
Although one prototype has been launched, the development of an SOFC-APU for marine applications is largely simulation-based [79–82]. One simulation study investigated the SOFC–GT tri-generation system where the power output of the SOFC–GT unit was 250kW [79]. Also known as combined heating, cooling and power, tri-generation is an integrated energy system where the exhaust provides heating and desiccant provides cooling. Modeling results show that the SOFC–GT system with a double effect adsorption chiller, desiccant wheel, heating ventilation, and air conditioning (HVAC) (Fig. 12.15) had the highest overall system efficiency (68%) among the different configurations. This system could produce 25%–47% more electricity than the SOFC–GT system with heating ventilation and air conditioning only. This improvement of performance is attributed to the absorption chiller, which can make use of the waste heat from the SOFC–GT system. Furthermore, more cooling is available by using a double effect absorption chiller and, hence, less electricity was used by the HVAC for cooling.
Figure 12.15. Solid oxide fuel cell–gas turbine absorption chiller, desiccant wheel, heating, ventilation, and air conditioning system [79].
In another simulation study a diesel-fueled SOFC system was designed to provide 120kW auxiliary power for a naval surface ship [80]. With a target electrical power output of 120kW, the net efficiency of the diesel-fueled SOFC system was significantly higher than the marine diesel engine (55% vs 25%). The CO2 mission of the SOFC system was less than half of that of the marine diesel engine (423gCO2/kWh vs 890gCO2/kWh). In addition, the SOFC system was significantly lighter than the marine diesel engine (520kg vs 3400kg) and produced much less noise (50dB vs 100dB).
One important breakthrough for SOFC-APUs is the 50kW prototype developed by sunfire GmbH in the SchiffsIntegration BrennstoffZelle (SchiBZ) project [81–83]. The initial simulation study suggests that the SOFC system (Fig. 12.16) is advantageous over other FC systems in terms of investment cost and system efficiency [81]. Later experiments demonstrated the robustness of a similar sunfire SOFC system with smaller power output (3.8kW) as the voltage degradation over 1000hours of operation was insignificant [83]. The 50kW prototype was installed on a test ship from Reederei Braren to provide 25%−50% of the onboard power [82]. The electrical and overall efficiencies of the prototype were claimed to be over 50% and 90%, respectively.
Figure 12.16. Diesel-fueled solid oxide fuel cell system [81].
URL:
https://www.sciencedirect.com/science/article/pii/B9780128152539000124
Microturbine systems for small combined heat and power (CHP) applications
J.L.H. Backman, J. Kaikko, in Small and Micro Combined Heat and Power (CHP) Systems, 2011
7.5.2 Turbec
Turbec has its roots in the development of small gas turbines for automotive use in the early 1970s, where a permanent magnet high-speed generator was designed (Malmquist, 1988). Turbec was founded in 1998 and has its head office in Italy. The first commercial T100 unit was delivered in 2000. The microturbine comprises a single-stage centrifugal compressor, a radial inflow turbine and a recuperator. The compressor pressure ratio is 4.5 (Turbec, 2010). The microturbine uses a combustor that can run on various fuels such as natural gas, diesel, ethanol and biogas. The electrical efficiency using natural gas is 33% with a 1 percentage point uncertainty. The shipped units of Turbec microturbines have more than 3 million operating hours (Turbec, 2010). Annual sales are predicted to be around 100 units (Soares, 2008).
URL:
https://www.sciencedirect.com/science/article/pii/B9781845697952500076
Refined Weight and Balance Estimate
Pasquale Sforza, in Commercial Airplane Design Principles, 2014
8.11 Auxiliary power unit group weight
The auxiliary power unit (APU) is a small gas turbine engine mounted in the tail cone of an aircraft to provide autonomous electrical and mechanical power for the following:
- •
Starting power for the main engines.
- •
Pneumatic power for cabin air conditioning systems.
- •
Shaft power for other pneumatic and hydraulic systems.
- •
Backup electrical and pneumatic power for in-flight operations and emergencies.
- •
Electric and pneumatic power for ground operations with the engines shut down.
The APU enhances capability by permitting the aircraft to carry out various functions requiring electric power on the ground without need for a ground-based generating unit to be available at each airport. A schematic diagram of a conventional APU is presented in Figure 8.16 which shows that inlet air is drawn into the APU and divided into two streams. One stream flows through a centrifugal compressor feeding high-pressure air to a combustor which burns fuel drawn from the main fuel tanks. The APU may use up to 2% of the fuel consumed during a typical flight. The hot combustion gases drive a two-stage axial flow turbine, producing shaft power and then exhausting through an exhaust nozzle typically located at the aft end of the fuselage tail cone. The other air stream passes through a second centrifugal compressor driven off the main power shaft. There the air pressure is raised to about 50psia and then exits to power pneumatic and cabin environmental control systems. The main power shaft also drives the electric generator which feeds electric systems when the electric power from the main engine-driven generators is unavailable.
Figure 8.16. Schematic diagram of a typical APU shown driving an electric generator while supplying pressurized air for pneumatic systems and cabin environmental control.
General characteristics of some modern APUs for typical airliners are described in Table 8.3. Equations for estimating the weight of an installed APU system have been proposed by several investigators.
Table 8.3. APU Weight and General Characteristics of Some Airliners
| Aircraft | Gross Weight Wg (lb) | Passengers Np | Electric Power (kW) | APU Weight (lb) | Weight of Engines neWe (lb) |
|---|---|---|---|---|---|
| Dash8-100 | 36,300 | 38 | 40 | 115 | 1872 |
| ERJ 145 | 46,275 | 50 | 37 | 120 | 3162 |
| B737-600 | 145,500 | 110 | 60 | 282 | 10,250 |
| A320-200 | 169,800 | 120–180 | 90 | 308 | 10,500 |
| B787 | 503,000 | 250 | 450 | 525 (est.) | 25,644 |
| B777-200ER | 656,000 | 300 | 120 | 730 | 33,288 |
| B747-8I | 975,000 | 467 | 180 | 835 | 49,600 |
| A380-800 | 1,234,600 | 525 | 240 | 950 (est.) | 59,200 |
8.11.1 Torenbeek’s correlation
Torenbeek (1982) suggests that the installed APU group weight is directly proportional to the weight of the APU itself, that is, Wapug=kapugWapu where the range of the proportionality coefficient is given as 2<kapug<2.5. The following correlation for the APU weight correlation is offered:
(8.21)
This result is based on , the weight flow rate of bleed air, in lb/min, which the APU compresses and supplies to the aircraft to support onboard pneumatic systems, particularly cabin environmental control. A value of 1.1lb/min per passenger is recommended and with Np denoting the number of passengers Equation (8.20) may be written as
(8.22)
Then the estimate for the APU group weight due to Torenbeek (1982) becomes
(8.23)
8.11.2 Modified correlation
Comparing the predictions for APU weight given by Equation (8.22) with the actual APU weights in Table 8.3 shows that the original Torenbeek suggestion increasingly underestimates the APU weight as the number of passengers increases. A modified correlation of the same form which gives better results is given by
(8.24)
To form the group weight of the APU we again multiply the APU weight by the proportionality constant kapug, that is, the likely APU group weight is approximately some multiple of the actual unit weight. Then the APU group weight is
(8.25)
8.11.3 Kroo’s correlation
Kroo (2008) also offers an approximation for the APU group weight based solely on the number of passengers as follows:
(8.26)
This approximation is simple yet may tend to overestimate the APU group weight as the number of passengers increases.
8.11.4 Relating APU group weight to aircraft gross weight
The fundamental variable for the design effort is the gross weight of the aircraft. However, Equations (8.23), (8.25), and (8.26) offer estimates of the APU group weight based on the number of passengers, which is related to the cabin volume and therefore to the size of the aircraft. A major job of the APU is providing air to the cabin for pressurization and environmental control making the size of the cabin a major consideration in sizing the APU group. Furthermore, because weight minimization is of paramount importance in aircraft design it may be reasonably expected that the overall density of the aircraft, weight per unit enclosed volume, is kept very close to a minimum. Therefore the gross weight of the aircraft should be a good indicator of the cabin volume, and accurate information on gross weight is usually the easiest to find. Similarly, the typical passenger capacity Np of an aircraft is usually readily available, as are the gross weight and the engine weight.
The APU dry weight for the aircraft listed in Table 8.3 may be found, or reasonably estimated, from information in the trade literature. The nominal value of Np for each aircraft, as taken from Table 8.3, may be used in the correlation Equations (8.25) and (8.26) given above to calculate estimates for the weight of the APU group. These results are plotted as open symbols on Figure 8.17 along with the results of an equation for the APU group weight based on gross weight given by
Figure 8.17. APU group weight as a function of aircraft gross weight according to various correlations. Weight data with open symbols are from Torenbeek and those with closed symbols are from Kroo (2008).
(8.27)
The value for kapug in Equation (8.25) as shown in Figure 8.17 is taken as 3. Also shown in the figure is the value for the APU group weight for the aircraft in Table 8.3 if we assume it to be three times the APU weight itself.
Sample weight statements presented in Kroo (2008) include APU group weights and these are also plotted as a function of aircraft gross weight Wg on Figure 8.17. The correlation equations show somewhat lower values for the higher gross weight aircraft than the sample APU group weights, which are for first- and second-generation airliners, not for more modern aircraft. It is expected that the correlations, which are based upon current APU technology, represent a conservative estimate for the APU group weight and are reasonable for use in the preliminary design study. Of course, if additional details for current APU installations become available the above results should be adjusted accordingly.
URL:
https://www.sciencedirect.com/science/article/pii/B9780124199538000085
Microturbines, Fuel Cells, and Hybrid Systems*
Claire Soares, in Gas Turbines, 2008
Applications and Case Studies*
Grassroots design development is expensive and this field of small gas turbine systems providing distributed power lends itself to several OEM joint ventures and component supplier arrangements. Capstone turbine, for instance, uses a recuperator developed by Solar Turbines under the U.S. DOE funding umbrella. Ingersoll Rand (IR) formed an alliance with a division of Siemens to supply power to projects like the one described as follows:
In September 2002, Ingersoll-Rand Company Limited (NYSE: IR) and Siemens Building Technologies, Inc. formed an alliance. Siemens thus integrated the Ingersoll-Rand 70-kilowatt and 250-kilowatt PowerWorks microturbine products to create a new offering—Microturbine Energy Systems. Recently adapted for end-user applications, Ingersoll-Rand's microturbine technology functions as an on-site, micropower generation plant that can complement or supplement the user's other sources of power. These systems utilize natural gas or other fossil fuels to produce small-scale electricity in the ranges of 70 kilowatts for a single unit, to 3 megawatts of capacity with multiple units. Exhaust heat from the microturbine can also be used to produce hot water or steam in a highly efficient combined heat and power (CHP) plant.
Case 1. Microturbine in a CHP Application*
A CHP application example (Oregon) follows. In 2004, Siemens contracted to design, build, operate, and maintain a Combined Heat and Power Plant (CHP) to serve Oregon Health and Science University's (OHSU) new River Campus in Portland, Oregon.
The CHP will provide OHSU with a clean, flexible, reliable and efficient source of heat and electric power. It will feature five natural gas-fired microturbines, which will produce all of the heat required by the facility, as well as 34% of its electric power.
The CHP will serve Building One, a 400,000 square-foot facility that will house medical offices, outpatient surgery, research laboratories, a wellness center, an imaging center, conference center, retail outlets, and parking. It is the first building being built in the university's expansion along the Willamette River in the new South Waterfront District.
According to the U.S. Department of Energy, a CHP design will reduce the amount of fuel consumed by almost 40% when compared to a traditional fossil fuel-fired utility power plant and customer-owned boilers. In addition, OHSU estimates that CHP will reduce its CO2 emissions by roughly 9 million pounds per year.
A chilled water production plant will serve the cooling needs of the building. Both the CHP and chilled-water plants will be integrated and controlled as a coordinated central utility plant.
OHSU and its design and construction team are utilizing the U.S. Green Building Council's LEED (Leadership in Energy and Environmental Design) rating system to gauge the sustainability of the project. OHSU's goal is to achieve a gold LEED rating. Innovative features of the new OHSU facility, such as using rainwater to flush public fixtures and a internal bioremediator to treat waste, might just earn it LEED's highest platinum rating.
Siemens is helping OHSU to secure financial incentives for the CHP. State tax credits from the Oregon Department of Energy, combined with financial incentives from an Energy Trust of Oregon pilot project, contribute to making the project an effective choice for OHSU.
Case 2. A Fuel Cell Application*
The target this DOE-sponsored partnership's effort (Rolls Royce Allison and stack manufacturer) was aimed at producing between 3 and 30 MW units at under $1500/kW uninstalled. Interim progress is discussed. Work in this field continues.
Rolls Royce is working with a molten carbonate fuel cell manufacturer to accelerate commercialization of a pressurized power generation package. The intent of the participants is to introduce stack and reformer improvements coupled with a new system integration approach to achieve a product with commercially competitive qualities. Cost-effectiveness and durability must be brought to fuel cell systems which already achieve high efficiency and low emissions.
A key element of the program will be endurance testing in base load mode for extended periods. Stack performance will be tracked in order to assess the maximum useful working life of this high investment component.
Load following generally requires careful management in fuel cell systems, typically because the stack/reformer system should be kept in a near-steady state environment. This will be demonstrated.
Existing systems have usually been designed somewhat similarly to a chemical process plant, using off-the-shelf components. This approach leads to a highly predictable but excessively complex and costly system. System simplification and cost reduction efforts have been initiated, with a goal of a 3:1 reduction in the cost of the balance-of-plant.
Preliminary results of this work are not available due to a delay in the start of the program. The cost reduction goal is to bring the plant uninstalled cost to less than $ 1500/kW. The projected costs of early commercial systems as currently estimated are almost twice this goal.
Project Description
The baseline system to be tested in the evaluation program for performance and durability consists of three distinct modules: the power module, the mechanical module, and the electrical module. This is the configuration selected for most exploratory fuel cell plants. The primary purpose of this first system is to demonstrate stack life, performance, and operational characteristics of the new stack and reformer. The prototype system test bed configuration represents a more flexible but more expensive system than that required for a commercial product.
A simplified schematic of the pressurized molten carbonate fuel cell (MCFC) test system is presented in Figure 16-5. The power module contains the fuel cell stack assembly, reformer, manifolds, and the anode (fuel side) recirculation ejector. The mechanical module includes all other major air and fuel gas management equipment including the turbo-machinery, gas desulfurization system, cathode recycle blower, valves, and interconnect piping. The third module, the electrical module, provides power conditioning, electronics, and system controls.
Figure 16-5. Simplified pressurized MCFC. [16-5]
High Temperature Fuel Cell Plants
The three fuel cell systems, studied by Rolls Royce Allison under the DOE High Efficiency Fossil Power Plants (HEFPP) conceptualization program, have the following major components in common:
- •
Fuel cell stack
- •
Turbogenerator
- •
Power conditioning
- •
Electronic controls
- •
Fuel desulfurization
In pressurized systems, the fuel cells acts in place of a combustor for the turbogenerator. In the unpressurized system, the fuel cell is placed in the turbogenerator exhaust stream but also supplies heat and residual fuel to a heat exchanger acting as an indirect combustor.
Additional major components for individual systems are:
- •
1100°F heat exchanger (pressurized SOFC)
- •
1600°F heat exchanger (1 atmosphere MCFC)
- •
1500°F exchanger/reformer (pressurized MCFC)
Though each system has distinctive merits and challenges, a common problem is that each module is of high cost.
The power module uses raw materials that are expensive and heavy, whether they are ceramic or nickel based. At present, each kilowatt requires 10–20 lbs. of active fuel cell and 10–20 lbs. of associated structure, or more than 15 tons of stack per megawatt. This ratio is considerably higher than for a simple cycle gas. The fuel cell active surfaces can provide a range of current density in which the delivered voltage (efficiency) drops as current rises but there is little scope for increasing the current per unit area at the high efficiency. It follows that the required route is to increase the active surface area in a given stack volume, a requirement very similar to that of producing compact high effectiveness heat exchangers. Figure 16-6 gives an indication of the comparative compactness of available fuel cells and heat exchangers.
Figure 16-6. Fuel cell vs. heat exchangers comparison. [16-5]
Stack pressurization is required in order to provide enough fuel and oxidizer gas to these compact active surfaces with low parasitic pumping loss. This is the primary reason for the belief that pressurized systems are the long term valid solution. Though more heat is released per unit volume of stack, it is carried away by proportionally more gas, thus avoiding excessive temperature. Pressurization also offers important secondary gains, both increasing the power that may be drawn at a given level of stack efficiency and making it easier to convert cycle heat into electricity in associated turbomachinery (accounting for 10–20% of total system power), without using more fuel.
A preliminary comparison of performance characteristics and rough order of magnitude cost of solid oxide and molten carbonate systems was conducted. The results indicate that near term (before 2010), the two offer a similar level of performance and technical challenges. The somewhat simpler system advantage of the solid oxide is offset by its often higher operating temperature. The lower cost of the balance-of-plant system for the SOFC tends to be offset by higher stack cost.
The MCFC system's pressurized reformer has to be supplied with an excess of steam in order to suppress carbon formation. This phenomenon poses a significant complication, and requires the continuous addition of treated water to the cycle.
The higher operating temperature capability and the flexibility provided by a planar solid oxide stack arrangement indicates that the solid oxide system will dominate in the longer term. Over long term, the SOFC stack offers the following potential advantages:
- •
the more durable chemistry
- •
compatibility of internal reforming with potentially high pressures at the higher temperature
- •
eliminating the need for excess steam.
An opportunity has been made available to partner with M-C Power in the pressurized MCFC programs. This opportunity represents the earliest possibility to launch a fuel cell hybrid product. Pressurized MCFC and SOFC systems will enable moving towards longer term aspirations of launching higher power density products. The intent is to introduce into operation the first in-service pressurized MCFC system. The plan is to participate in designing and developing the prototype systems, currently funded by the DOE MCFC Product Design and Improvement (PDI) program.
Pressurized MCFC System Design
M-C Power gained considerable insight into the design and functioning of pressurized MCFC systems. This valuable experience was gained by running the plant installed at the Miramar Naval Air Station (test units ranging in size from 75 kW to 250 kW). This system is not a full hybrid. Although turbomachinery is used to turbocharge the system to about 3 atmospheres, no attempt is made to generate bonus electrical power with it.
A plant design for the PDI program was developed by the M-C Power partnership based on the operating experience gained at the Miramar station. The PDI program incorporates a number of improvements:
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power increased to 450 kW (improved stack mechanical and thermal design, and materials modifications)
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temperature stable reformer/catalytic combustor
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pressurization increased
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turbogenerator used for about 10% power bonus
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once-through steam generator (unattended)
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reduced footprint
The proposed PDI system analysis made it apparent that the cost of the system was very high as designed. The first reaction is of course, that the stack and reformer are determined to be expensive because they are complex parts produced on a “once-off handmade” basis. This was expected. There is a coherent plan to decrease their cost by an order of magnitude. The electrical module cost was based on off-shelf power conditioning and power electronics components. The cost of this module represented a minor portion of the overall system cost. This is partly because good electronic control systems are very competitively priced and inverters are continuing to decrease in cost.
What came as a surprise, however, was that the collection of pipes and valves in the mechanical module, specified to service the system, is the major cost challenge. Unlike the stack and reformer, the mechanical module is an assembly of standard, volume produced, components which are not likely to reduce cost. The PDI plant is configured in a fairly straightforward “chemical plant” style, with pipe work and valves linking and controlling the major processes. The mechanical module assembly, manufactured as prototypes with soft tools and jigs, requires a large number of pipes and valves. The components of the mechanical module alone cost more than the intended early production cost of the whole plant. This high system cost problem is being attacked to resolve it by a different approach. The plan is to modify the overall system operational control, and greatly simplify system integration. The following integration changes have been proposed:
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put all critical pipe connections inside the pressure vessel that houses the stack and reformer.
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air delivered from the intake compressor is discharged into the pressure vessel.
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the turbine is driven by the exhaust gases as they emanate and then pass to the external exhaust heat recovery unit.
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all other flow manipulation is accomplished inside the pressure vessel, and can therefore be handled in lightweight ducts (with small pressure differences across the ducts).
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small leaks are acceptable and this opens the way to using slip joints if necessary.
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eliminate motorized air/gas valves. Select and use the turbomachinery and its electrical loading to master flow control, probably aided by a waste gate system.
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provide an integrated turbomachinery package. Replace the present large low speed industrial hot recycle blower and its variable frequency drive and drive motor by a small high speed fan. The fan is driven by a free power turbine, using the same hot gas stream as the turbogenerator turbine.
The resulting system package which is closely integrated is sketched in Figure 16-7.
Figure 16-7. Simplified MCFC system. [16-5]
Revised system cost estimates indicate that these changes decrease the total initial production plant cost (uninstalled) by 30% and the long term plant cost by 45%.
URL:
https://www.sciencedirect.com/science/article/pii/B9780750679695500211
