RANKINE MICROTURBINE POWER PLANT Jon W. Teets -TMA Power, LLC tmapower@cox.net J. Michael Teets -TMA Power,LLC tmapower@bellsouth.net www.tmapower.com ABSTRACT A Rankine-Microturbine Power Plant is a combined cycle (gas turbine and rankine turbine) generator designed to produce high efficiency (>40%) electrical output. However, unlike large combined cycle power plant(s), having a clutch coupled gas turbine and rankine turbine, the Rankine-Microturbine incorporates a rankine turbine integral to the microturbine rotor to drive a common alternator rotor (Patent No.: US 7,211,906 B2). Typical microturbine generators (<500Kw) are nonsynchronous generators and some demonstrate 29% cycle efficiency with exhaust gas heat exchangers. For simple cycle, up to 15% efficiency for a low pressure ratio single spool and estimated 20% cycle efficiency for high pressure ratio (11:1) simple cycle unit that incorporates (2) rotor spools. The latter is the TMA 70SC and will produce 78Kw (sea level std conditions). This simple cycle microturbine unit will form the gas turbine engine core for the Rankine-Microturbine Power Plant. The high pressure ratio microturbine (TMA70SC) does not have a traditional gas turbine exhaust gas heat exchanger but operates with two rotor spools aero thermodynamically coupled. The #2 spool assembly (power producer) incorporates a high speed alternator integrated with a compressor rotor and turbine rotor. The #1 spool (with High Pressure Ratio Compressor, HPRC) acts as a turbo charger for increased engine thermal cycle efficiency. The Rankine-Microturbine with high electrical output power efficiency could be used for main power plants in [stand alone] communities, business, industrial or distributed energy (D.E.) INTRODUCTION The 142 Kw Rankine-Microturbine Power Plant is a combined cycle unit with exception that the rankine turbine drives a common high speed Permanent Magnet Alternator (PMA), integral to a two spool microturbine power producing spool assembly. In countries so dependent on fossil fuels for modes of transportation, industry and domestic electric power generation, it is imperative to conserve the use of that finite fuel supply. We may not have control on the price of fuel but we should do all we can to conserve. This product could prove to be the most efficient engine (less than 500Kw) in the world, for electric power generation. A two spool microturbine unit (TMA70SC), under development at TMA Power, LLC will be the core engine in the Rankine-Microturbine Power Plant. The high pressure ratio microturbine is a two spool simple cycle gas turbine engine estimated to have an efficiency of 20% @ 78 Kw electric power generation (LHV 18400 btu/lb). Reference table 1. Addition of a rankine turbine to the core unit (78Kw technology demonstrator currently in test) will create the combined cycle power plant unit referred to as the Rankine-Microturbine. The rankine turbine will be integral to the gas turbine rotor and drive a common high speed PMA. It is estimated to produce 142 Kw electric power generation with electrical power out estimated efficiency > 40%. The 78 Kw PM generator will be scaled up to produce the 142Kw power extraction from the rankine turbine and gas turbine engine combined. 1
Rankine-Microturbine Power Plant Features: Compact / small footprint Use of common rotor castings with shroud line changes for spool application cost considerations Dry Low Emissions (DLE), currently less than 30 ppm NOx and CO at all power levels (use of diesel & gasoline fuels). The design goal is less than 25 PPM Use of liquid or gaseous fuels Excellent rotor dynamics Ability to spool down rotor speed operation (50% N) without rotor or blade frequency issues. N =104000 Low rotor speeds for reduced power needs allow reduced fuel consumption at off design conditions Combustor design provides for low emissions, low flame pattern factor and high turndown ratio capability without fuel supply nozzle staging. Low cost fuel pump for liquid fuels No external oil plumbing Low speed operation to initiate Rankine cycle operation Figure 1 TMA70SC, Two-Spool-Microturbine (Technology Demonstrator) Common alternator assembly The air and gas flow through the Two-Spool Microturbine No engine start-up or shutdown limitation beyond is shown in figure 2, though not exhibited with rotors exhaust gas temperature for spool # 2 perpendicular as in the technology demonstrator. Ability to start the gas turbine using Rankine cycle Production version of TMA70SC unit will have parallel or air start for simplicity rotors configured for compactness, weight and cost considerations. TWO SPOOL-MICROTURBINE DEVELOPMENT STATUS The simple cycle high pressure ratio (11:1) two-spool Microturbine, (designed, engineered, manufactured, assembled and tested by the authors having experience in Turbomachinery) is under development test and engineering validation. The unit in test is defined as the TMA70SC technology demonstrator and is shown in figure 1. The gas turbine engine rotor spool assemblies are perpendicular in orientation for purposes of separate unit tests. Initial tests evaluated the air supply producer (turbocharger), defined as # 1 spool. Tests were performed to evaluate the compressor stage, turbine stage and rotor dynamics. This was accomplished by attaching an in-house slave combustor design, as well as hardware to replace the turbine scroll/volute to be used with final unit integration to the assembly that houses the # 2 rotor spool assembly. Spool #1 housing assembly is on the right hand side of figure 1. This unit also provided valuable information on an engine air start design, accomplished by impinging air jets to the compressor wheel blade tips. Figure 2 TMA70SC, Two- Spool- Microturbine (Production Configuration) 2
The gas turbine rotors are simple cantilevered wheels with the bearings and alternator assembly lubricated and cooled in a sealed housing. The two spool microturbine project was initiated by the authors in May 2003 (design, engineering and hardware procurement). The authors each have over 30 years experience in Turbomachinery and more specifically coinvented the Microturbine in 1994. In the subsequent years following up to year 2002 worked toward bringing that product to market. TMA Power,LLC is testing the integrated rotor spool assemblies that comprise the TMA70SC technology demonstrator configuration as shown in figure 1. Initial engine test was accomplished in October 2004. Estimated cycle state points for this unit are shown in Table 1. These estimated values are in process of validation. The TMA70SC production unit is estimated to be 200lbs (microturbine and power electronics). Each unit is about the size of a 2ft cube. The modular design, consisting of two assemblies provides for ease of shipping, handling and assembly. A unique feature on the TMA70SC, is the ability operate the rotor(s) at various speeds, based on power demand. In essence, if you do not need the electric power from spool # 2 it will operate at a reduced speed as well as #1 spool. Figure 3 # 1 Spool Rotor Assembly Module Spool # 1 produces the engine air flow along with the major portion of the pressure ratio. Reference figure 3 for # 1 spool rotor assembly and figure 4 for # 2 spool. The rotor speeds will vary to satisfy the customer demand. Maximum speed of both rotors will occur at maximum power demand. To date 12/08 have pulled 45 Kw at sea level 90F compressor inlet. This corrects to 60 Kw at sea level 59F compressor inlet. Laps-rate is 0.5Kw/F compressor inlet. The power producer spool assembly #2 needs to have changes to compressor stage and turbine stage. Also, the mass air flow will increase form 1.22 to 1.3 lb/sec. These changes will increase output by 18Kw to attain design point power (i.e.78kw) for 59F sea level standard day. Table 1 Estimated Simple Cycle State Points Stage Element Units State Point # 1 Compressor Mass Flow lb/sec 1.33 Inlet Temp F 59.0 Inlet Pressure psia 14.6 Pressure Ratio 5.0 Stage Efficiency % 77.0 Exit Temp F 443.0 Exit Pressure psia 73.5 # 2 Compressor Mass Flow lb/sec 1.31 Inlet Temp. F 440.0 Inlet Pressure psia 72.5 Pressure Ratio 2.2 Stage Efficiency % 74.0 Exit Temp. F 710.0 Exit Pressure psia 158.0 # 2 Turbine Mass Flow lb/sec 1.3 Inlet Temp F 1850.0 Inlet Pressure psia 155.0 Pressure Ratio 3.3 Stage Efficiency % 85.0 Exit Temp F 1350.0 Exit Pressure psia 46.0 # 1 Turbine Mass Flow lb/sec 1.29 Inlet Temp F 1350.0 Inlet Pressure psia 45.0 Pressure Ratio 3.1 Stage Efficiency 84.0 Exit Temp F 1050.0 Exit Pressure psia 15.0 Fuel Lower Heating Value btu/lb 18400 Fuel Flow lbm / hr 67.0 Power from # 2 Spool, horsepower 113.5 Power Losses # 2 Spool Bearings, horsepower - 1.5 Alternator Rotor Windage Loss, horsepower - 1.0 Power Available, to Drive Alternator horsepower 111.0 Power, Alternator(0.97) @ Term. Lugs kw 80.2 Power To Drive Fuel Pump kw - 0.15 Power To Drive Oil Pump kw - 0.37 Power Electronic Losses kw -1.2 Power Available to customer @ 60Hz kw 78.5 When integrated with Rankine cycle - Rankine Binary Fluid Pump Assembly kw 3 - Condenser Fan Motor Assembly kw 1 - Microturbine Power kw 74.5 3
squeeze film damping occurs at the bearing and bearing carrier to the housing. With the rotor bearings / housing squeeze film damped the rotor stability is enhanced to a log. dec. 1.96 compared to a log. dec. of 0.2 without. An undamped critical speed was performed for the # 2 rotor assembly and indicates that the third and forth critical speed will be well off from the operating speed range. The oil squeeze damping for the bearings and bearing housing to the engine housing as in the # 1 also show excellent results. Figure 4 # 2 Spool Rotor with Alternator Rotor and Bearings Power electronics will provide output electric power at 60 or 50 Hz. regardless of the power producing rotor spool # 2 speed. It is anticipated the two rotor assemblies in production will share the same compressor wheel and turbine wheel castings. However, wheel diameters and shroud lines will be different and distinct for their intended application. At design point (demonstrator unit) the # 1 spool operates at 108000 RPM and the # 2 spool at 104000 RPM. The ability to operate at various rotor speeds without rotor dynamic or blade frequencies running at or near destructive entities is paramount in turbomachinery. The unit has operated successfully from 50% to 110%N without issue on the # 2 spool and 30% to 110%N for the # 1 spool assembly. Rotor dynamic stability at various speeds is attributed to the modular rotor assembly design that incorporates oil squeeze film damping at the bearing support mounts to the housings. The rotor assemblies with the bearing mounts are installed into the engine housings. Endurance testing will be used to validate design. Reference figure 3 for the # 1 spool rotor assembly module and figure 4 for the # 2 spool rotor assembly with the alternator rotor. Results of rotor dynamic analysis on # 1 and # 2 rotor assembly(s) further support the effect of squeeze film damping (SFD). For # 1 spool, a second critical speed is in the 75600 to 108000 rpm, but it intersects the critical speed line in the sloping section. This makes the second critical speed well damped. In essence, this indicates there will be no adverse effect to the rotor dynamics due to the oil squeeze damping. This is further verified in the unbalance response and stability analysis performed. Oil Test results on the unit show, with an unbalance 0.0015 ozin per plane (two planes per rotor), that recorded G loads at various operating speeds to be less than 1G. The accelerometers used are located on the engine housing above the bearings (adjacent to the compressor wheels). COMBUSTOR and FUEL DELIVERY SYSTEM The current TMA70SC engine incorporates a TMA Power, LLC combustor design that is a Rich burn - quick Quench - Lean burn (RQL). This combustor is based on microturbine design experiences (patent pending). Initial engine tests, using diesel and or gasoline fuel, exhibited low (DLE) emissions with CO and NOx less than 30 parts per million (ppm) throughout the engine operating range (engine test results). Low pressure fuel injectors are incorporated. The staged annular type combustor has circumferentially spaced fuel / air mixing chambers with tangential air swirlers incorporated internally in the primary zone. The low delta pressure fuel injector design allows for the use of an automotive type fuel pump for simplicity and cost.. Also, of equal importance, is the ability of the combustor to have a high turn down capability (1.5 to 10.0 gal/hr) with no flame instabilities and good emissions at various engine power levels and rotor speeds (i.e. off design speeds). Through engine operating range (~50% to 100%N2) the combustor has exhibited excellent flame stability, low emissions and low flame temperature differential (based on <30F delta spread of thermal couples in the power turbine exhaust duct). Engine testing shows the combustor exhibits consistency of equivalence ratio throughout the power range in primary and secondary zones. A start and main fuel system is incorporated. Once the main fuel is on, fuel modulation is used to control speed. 4
PERMANENT MAGNET ALTERNATOR The high speed Permanent Magnet Alternator (PMA) is designed to produce 80 Kw (3667 Hz) at the output terminal lugs (3) located on the engine housing. Customer power available is estimated to be 78Kw (60 or 50Hz) at the output lugs (3) located on power electronics. Design specifics for the alternator assembly output @ 3667 Hz: Efficiency 97% Produce 80Kw load, V=447 VRMS, or V=635 Vpeak line-line At No load V=556 VRMS, or V=790 Vpeak line-line Tests to date, at no load condition and voltage output through use of a bridge rectifier for DC measurement, show 0.0068 DC volts / rpm. Thus at 110,000 rpm output voltage is 748 VDC which equates to 550 VRMS (design is 556 @ no load). Initial design of the PMG was for 110000 RPM. However, the RPM has been changed to 104000 for thermodynamic reasons. Engine loading to date is 45Kw at 90 F compressor inlet. This equates to 60 Kw on a 59F sea level condition. DC voltage attained at the 45 Kw point 560 DC volts with a current of 80.4 amps. AC Voltage would equate to 421 volts. With the turbomachinery operation at lower speed will have to provide design change to increase the voltage output (via magnet axial length and or winding in the alternator stator). The PMA design and analysis done by TMA Power, LLC, is based on the authors experience and is typical technology in the industry, with exception to restraint means of the magnets, incorporating a unique assembly process and tooling to attach the critical non-ferrous containment ring. The metal containment ring provides good stiffness in rotor dynamics. The # 2 spool assembly, that incorporates the alternator rotor assembly, was run to 115,000 rpm with no vibration issues. The containment ring is designed for 121000 rpm. The PMA unit is cooled via synthetic oil across the stator OD (via cooling fins), stator end turns and rotor ends. The limit on oil temperature is 190F to assure stator windings and magnet material keep well under life limiting temperature. OIL SYSTEM Oil system design is a sealed unit that operates at 30 psig, 2 gallon capacity, no external plumbing, oil change interval is estimated to be 15000 hrs with filter change time. This could be higher pending field tests. There is no source for contamination or oil breakdown, do to excessive temperature. A 3 micron filter is used along with a fine screen mesh to capture any particles, most likely from assembly and what may come from stator assembly / manufacturing at the vendor. The design of the sump minimizes the existence of bubbles (foam) and a means by which oil heat is removed through use of a finned sump base that accepts forced convection air flow. Oil flow is used for the rotor assembly bearings (4), remove heat from the alternator stator, alternator rotor and squeeze film damping. Oil is pumped by use of a gerotor pump powered by a 24 VDC electric motor, which is retained to the engine housing. AIR START SYSTEM The TMA70 microturbine has an air start system in place of the typical electric start, currently used on microturbines. The latter uses the high speed alternator as a motor to spool up. The air start system removes the need for battery dependence and the maintenance that goes along with battery use. Also, this approach will reduce complexity of the power electronics and for reduced cost and maintenance. However, in its place the need for 24 VDC for valves, ignition, and control system will be accomplished through 24VDC extraction from the high speed alternator at low rpm air start spool up via the power electronic design. Air start is accomplished by air impingement to the # 2 spool compressor wheel. To date the TMA70 microturbine air start system is used without issue. At the end of turbine engine use the air supply reservoir will have been auto recharged via #2 compressor discharge and ready for the next start. POWER ELECTRONICS The power electronics for this product will provide 60 or 50 Hz power out to customers. The output frequency will be maintained throughout the operating range of the engine at various power levels as well as black start capability for low speed 24CDC needs. The PE design requirements have been issued to vendors and estimates received to accomplish this task. At this time there is no need for PE system other than the bridge rectifier and DC load bank, now in use. 5
To date all tests, start, run and shutdown have been accomplished / controlled by the human factor (manual). An electronic fuel control has not been integrated at this time. proposed Rankine-Microturbine 142 Kw powerplant. RANKINE-MICROTURBINE The Rankine-Microturbine, introduced in this technical paper, represents a second phase program that will build on the two-spool-microturbine core engine once developed. The Rankine-Microturbine unit is estimated to provide a 142Kw electrical power plant at an electrical output efficiency of 40%. The rankine cycle analysis used a binary fluid of ammonia and water selected based on excellent heat transfer properties. Critical, in the study accomplished, is the ability to attain high enthalpy values for entry on the cold side of the economizer. This is a first cut and additional work is needed for this binary fluid evaluation along with other fluids currently used in the industry. The estimated cycle state points for the proposed Rankine Microturbine are shown in figure 6, along with assumptions. Engineering to be accomplished for this product will be the rankine turbine stage (ref. table 2 for a first cut), waste heat recovery boiler, economizer, condenser, control system & valves (with modulation control) to regulate the fluid / vapor flow. The Rankine-Microturbine Power Plant is a combined cycle engine with the exception the rankine turbine stage will be integral to the gas turbine rotor assembly and drive a common Permanent Magnet Alternator, reference figure 5. The new 142 Kw alternator will be scaled from the 78 Kw alternator assembly. Reference figure 7 for the RANKINE TURBINE Figure 5 Rankine-Microturbine Engine The rankine turbine, illustrated in figure 5, is a radial inflow back to back type. Though other types of turbines could be used, the radial was selected for its ruggedness, capability to have 4:1 pressure drop, and thrust balance to minimize the thrust reaction on the gas turbine rotor assembly when used. As the rankine turbine stage comes on line it is anticipated to occur at low rotor speed to minimize effect of mass flow droplets that could damage the blades. Damage would occur due to the baseball effect (i.e blade velocity is greater than the mass flow droplets). Estimated cycle state points, total power and system efficiency are shown in figure 6. Reference table 2 for preliminary turbine stage design. WASTE HEAT RECOVERY BOILER The waste heat recovery boiler will make use of existing type technology typical in the industry and customized for this application. At the design power point, waste heat from the gas turbine exhaust will be at 1050 F, with mass flow of 1.3 lb/sec., and the rankine cycle mass flow of 0.51 lb/sec. The superheated mass flow to the rankine turbine stage is estimated to be 740F. The Rankine Microturbine, superheater source could be other than or supplemental to the gas turbine exhaust gas waste heat, 6
through other external heat sources. Also, as a means to start the brayton cycle other than aforementioned air impingement, stored or external heat source could be used or the rankine cycle to drive the brayton cycle start rotation into self sustaining rotor speed requirement. ECONOMIZER Certainly one of the key components for the success of the Rankine-Microturbine is the economizer. This unit will optimize the ability to recover energy not extracted by the rankine turbine. The economizer capability to process the energy latent vapor mass flow, instead of a large loss through the condenser, allows for an increased mass flow (lbs/sec) hence more power extraction through the rankine turbine over a system without an economizer. This is key to the estimated system efficiency. The greater the enthalpy of the mass flow prior to entry into the waste heat recovery boiler the more mass flow that can be processed for the same waste heat from the gas turbine exhaust. Reference figure 6 for cycle state points. In the analysis / design process consideration will also be made for the low temperature side that will take fluid at an estimated temperature of 190 F. The 190F fluid will go from single phase at entry than thru two phase boiling prior to exit of the economizer. Likewise, superheated vapor from the turbine exducer will go through a phase change of superheated to saturated stage prior to exit and entry to the condenser. Operation of the economizer will have several different two-phase heat-transfer phenomena, all of which must be taken into account. DISCUSSION The novel Rankine-Microturbine Power Plant, described in this paper, offers the ability to produce electrical power with low emissions and higher cycle efficiency than current fossil fueled power plants. The ability to provide an electric power plant on a smaller scale than current electric power plants will allow for improved Distributed Energy (D.E.) without necessarily having any line ties. This system will provide the owner / customer the ability to generate reduced cost electrical power with fuel use, heat energy source, diversity. TABLE 2 RANKINE-MICROTURBINE OUT-PUT POWER & PRELIMINARY WHEEL DESIGN Rotor Speed Tip Diameter Mass Flow 104,000 rpm 4.8 inch 0.40 lbm/sec Stage Efficiency 83 % Inlet Turbine Stage Pressure: 1753 psia Inlet Turbine Stage Temperature: 740 F Cp (in) 0.76 BTU/lb-R CONDENSER / PUMP / FAN k 1.37 The condenser will take the mass flow from the economizer hot side and change it to liquid, to be able Inlet Enthalpy H 1147 BTU/lb pump back to economizer low temperature side. The media to be used for cooling will be air flow and is estimated to require a 1.0 kw fan motor. This is debited as Exit Turbine Stage Pressure: 438 psia accessory power, from the microturbine power available Exit Turbine Stage Temperature: 426 F shown in table 1 and listed in table 2. The use of air will minimize fowling of the condenser cold side. It is Exit Enthalpy 988 BTU/lb anticipated to use existing technologies in the industry but unique for this application. The pump for use will be a Rankine Turbine Power 67 kw piston type. The power to drive the electric pump motor is estimated to be 3 kw. This also is debited as accessory Power to drive pump motor * 3 kw power from the microturbine. Power to drive motor fan * 1 kw (* Power from simple cycle gas turbine output balance) 7
Figure 6 Rankine Cycle State Points 8
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