Microturbines and Micro Gas Turbines
Capstone C30 Microturbine Hybrid Electric Vehicle Model
The small gas turbine is a remarkable piece of machinery but also a comparatively
rare one. As prime movers go the ratio of gas turbines to piston engines around the
World must be less than one in a million! Small GTs are generally confined to aviation
and may be found in applications such as auxiliary power units (APU) in aeroplanes
and helicopters, turbo-
Small gas turbines may be divided in to four distinct categories with little overlap,
they are; aerospace developed and manufactured units, radio control model power plants,
turbocharger derived units and microturbines. They may be described as follows-
Aerospace; A small gas turbine engine of power output up to 500 hp (350 Kw) providing shaft, pneumatic, electrical or hydraulic power for use in aircraft and helicopters. Also as ground support and starting equipment.
KHD T312 Auxiliary Power Unit
The KHD T312. Klockner-
The T312 is a sophisticated single shaft gas turbine unit featuring a single stage
compressor (earlier variants also had a axial booster stage), annular combustor with
vaporising burners, a two stage axial turbine assembly and a hydro-
RC Model Power Plants; Developed over the past few decades, a miniature turbojet,
The KJ66 turbojet engine. An early generation engine designed for use in model aircraft. It was available commercially and in kit form. It could also be built from scratch by hobbyists. Many different models now exist today although all adopt the same basic simple layout.
The KJ66 Model RC Turbo-
Turbocharger; It is possible to construct a simple demonstration gas turbine from
a automotive turbo-
Jet Powered Go-
Microturbine; The microturbine may be considered a unique breed of small gas turbine engine power plant in its own right. In power generation very large gas turbines are normally operated producing megawatts of power, a small gas turbine producing only a few tens of kilowatts in comparison is of “Micro” size and hence the designation microturbine. Commercial microturbine models having been built in volume production, range in power output from just 2.6KW (probably the World’s lowest rated production gas turbine) to around 250Kw. Until now microturbine applications have consisted of installations providing combined heat and power (CHP) or where a supply of waste product fuel gas is available of sufficient quality to support combustion in a gas turbine. CHP is usually where the waste heat from the gas turbine after having produced electricity is used to heat a building as part of a heating ventilation and air conditioning (HVAC) installation. There may be occasions where the abundance of quality heat output may be used as part of an industrial process and where the electricity generated is used also. Now there is the prospect of these devices becoming used for power generation on board a vehicle in the form of a “range extender”. A range extender is basically a device to aid the recharging of an electric vehicle battery whilst on the move.
There are areas where these categories of power plant cross over for instance RC
turbines may be used in professional applications as propulsion for military of civilian
drones and un-
The microturbine engine working cycle is the same as all conventional gas turbines small and large. The component parts are similar with the addition of an extra device known as a recuperator.
The microturbine consists of a centrifugal compressor wheel and a turbine wheel mounted on a common shaft. This assembly rotates at high speed and is supported by special bearings. In the case of many microturbine designs (but not all) the shaft is also coupled directly to an electrical generator that also acts as the starter.
A simple illustration of the workings of a gas turbine engine
Simplified schematic of a microturbine power plant
The above diagram illustrates a typical microturbine gas turbine installation. Size and weight may not be the primary design criteria but if intended for automotive use then weight and compactness are important.
Component parts in detail.-
Compressor. Unlike larger aero or industrial engines, microturbines exclusively adopt
the use of the centrifugal type compressor wheel and surrounding diffuser. One microturbine
aspiring manufacturer Bladon Jets infer in some of their publicity material and
illustrations that a miniature axial type compressor has been produced and it surpasses
or supersedes other designs. No significant evidence has ever been seen by the author
to substantiate the claims that such a working unit actually exists. A photogenic
and aesthetic design has certainly been publicized and used in the promotion of the
concept hybrid electric super-
A centrifugal compressor consists of two major components the impeller wheel and diffuser. The impeller wheel consists of a flat cone or disc shape wheel with a number of radially disposed vanes mounted on it. Between the vanes channels are formed that carry air outwards to the edge of the wheel. The vanes may be split in to long and short types, the longer of the two emanating from close to the wheels cent re. Depending on the design of the wheel the vanes may be curved in the direction of rotation in order to alter the aerodynamic characteristics of the wheel. An additional component is added to the wheel in the form of a ring of fan like blades known as rotating inlet guide vanes. These additional vanes line up with the radial vanes and guide the air flow in to the wheel. Air is drawn in by the guide vanes towards the centre of the wheel and under the action of centrifugal force flows out to the edge of the wheel at high speed. The effect of this is to raise the static pressure and velocity of the air flow.
Around the circumference of the compressor wheel is mounted a ring of fixed radial passages formed between either curved vanes or wedge shaped blocks. These radial passages form diverging nozzles or simple venturi that slow down the fast moving air that is thrown out of the periphery of the compressor wheel. The combined action of the compressor wheel and diffuser raises the static pressure of the air by up to a ratio of 4 to 1. The air having been compressed as become heated to up to a temperature of 180 degrees C.
Compressor wheels are usually constructed from alloys of aluminium and may be CNC machined from solid or cast. Various corrosion resistant or finishing treatments may also be required.
Fact or Fiction? A working miniature gas turbine featuring an axial compressor
Sometimes a space is built around the diffuser to collect the air and pass it to
the next stage of the microturbine usually the recuperator, this is known as a volute.
The volute is snail shell in appearance and is a common part found in many turbo-
Combustor. The combustor or combustion chamber is the region in which heat is added to the working cycle of the gas turbine. Heat like in any other engine is the source of energy that is ultimately converted to mechanical work. Fuel is burnt either in gaseous form or a form originating from a liquid state i.e. as a vaporized gas and atomised mist.
In microturbines combustion chambers take the form of either a annular region wrapped
around the hot end of the engine or a single can type vessel mounted two one side
of the engine rotating axis. One or more injectors or nozzles are used to admit fuel
in to a zone within the combustion chamber with which a portion of the air incoming
from the compressor is mixed. The combustion chamber is lined with a perforated cylinder
that admits the air in to differing zones. Generally there is a zone close to the
injector (s) where the primary combustion takes place. Here a steady flame is held
in suspension by the action of re-
Fuel is injected in to the combustion zone by means of a nozzle or burner assembly.
Fuel may be gas or a liquid. In gaseous systems the gas may be pre-
The popular Capstone C30 microturbine employs a wrapped-
Other microturbines employing a single can combustor are fitted with a single burner. The advantage of a single burner is it may be removed and services as one complete unit with minimum disturbance of the rest of the microturbine engine assembly.
The working cycle is initiated by the starter often formed by the generator being
operated in reverse as a motor. As the shaft rotates the compressor wheel draws in
air that is compressed by the action of the wheel itself and a surrounding stationary
component known as the diffuser. In a basic engine design the air from the compressor
is passed to a combustion system. The combustion system is formed by a chamber that
mixes the air from the compressor with fuel that is burned. The fuel air mixture
is initially ignited during starting by an electrical spark igniter, once lit it
burns continuously until the engine is shut down. The resulting combustion introduces
heat in to the working cycle creating gases that expand as they get hot and exit
the combustion chamber with added energy. The hot gases are directed through another
stationary component a nozzle that directs streams of hot gases that impinge on the
turbine spinning it around. As the compressor and turbine rotate, they accelerate
under the combined action of the starter and the hot gas stream, as speed increases
the compressor discharge pressure also increases overcoming thermodynamic and mechanical
losses and a point is reached where the cycle is self-
The gas turbine in its simplest concept may be likened to a windmill or turbine connected to a fan or propeller. If this arrangement is rotated, wind from the fan would impinge on the turbine and drive the whole thing around. This would be perpetual motion and of course does not work! If heat is added (a source of external energy) between the fan and the windmill then the air expands and hits the windmill with increased ferocity creating a force that could assist in driving it around. In practice this still does not work as the losses (air escapes and leaks around components) are very high and the net forces at atmospheric pressure are very low and are not able to overcome mechanical losses and friction. The individual component efficiencies could be improved by enclosing them in a housing, making the fan in to an impeller shaped wheel, sculpturing the turbine and tailoring the gas flow in and out of it. If this arrangement is spun around by an external driving force (starter), and if it is spun fast enough with sufficient heat added (but not too much as to melt the hot components), then the process will become self supporting and the device runs on its own. This is the gas turbine principal, a device has been created that drives itself and expels hot gases in its exhaust. If a restricting nozzle is place in the exhaust, the gas velocity may be increased, a combination of the pressure drop across the nozzle exit diameter and the accelerated mass flow of gas though it creates a reaction on the whole engine assembly that is known as thrust. The gas turbine has become a simple jet engine!
The gas turbine working cycle is driven by the heat input to it between the compressor outlet and the turbine inlet. This heat normally comes from the combustion of fuel with a flame, it can come from other sources such as an external heat source via a heat exchanger or even an electrical heater. An electrically fired gas turbine could be produced but the heating intensity would have to be quite large (to mimic that that is normally produced by a steady flame) and or the combustor assembly made quite large also to fully heat all the air to the required turbine inlet temperature.
In an operating gas turbine considerable power is absorbed by the compressor and once started it is all provided by the turbine. When these components are optimized within the operating temperature range of the turbine material, additional power is available from the connecting shaft to drive an external load e.g. the generator. In a conventional gas turbine some two thirds of the power developed buy the turbine is used to drive the compressor.
A gas turbine featuring one shaft that connecting the compressor to the turbine and an additional load is known as a single shaft or single spool engine. An alternative arrangement is possible where by a second turbine wheel known as a power turbine is placed in the exhaust stream of the engine and is used to extract power to supply a load. This turbine is mechanically independent of the compressor turbine and may also be known as a free power turbine. An engine of this configuration is referred to as a two shaft or two spool engine.
Due to the nature of their applications microturbine normally feature a device known
as a recuperator. Gas turbines (compared to piston engines) naturally produce a large
amount of waste heat in the exhaust, it is possible to recycle this heat back in
to the working cycle of the engine by using a device called a recuperator. A recuperation
consists of a heat exchanger matrix fitted to the exhaust and compressor gas flows.
The exhaust (gases exiting the turbine) from the engine pass through one half of
the heat exchanger. Air leaving the compressor flows though the other half of the
heat exchanger before entering the combustion chamber. Heat from the exhaust stream
is passed to the combustion chamber supplementing that required from combustion,
this results in less fuel being burned for the same heat input to the working cycle.
The effect of this is to improve the fuel efficiency of the gas turbine. The penalty
for this is the size and weight of the machine is significantly increased for a given
size and power output. Part-
Centrifugal compressor assembly
Combustor Liner Can
Turbine. The next component that makes up a microturbine is the turbine wheel. The turbine wheel is used to extract mechanical energy from the gas stream emerging from the combustor. The turbine wheel drives the compressor part of the gas turbine engine and may also be used to supply power to an external load e.g. a generator.
The turbine wheel in its basic form is similar to the compressor wheel and operates in reverse. In the compressor mechanical work (supplied from the turbine) is carried out to raise the pressure of the incoming air and subsequently it is also heated. Gases passing through the turbine assembly lose pressure and also temperature as work is extracted from the stream and transmitted via the shaft to the compressor and an external load.
The turbine wheel is known as a radial inflow device and takes the basic form of
an impellor shape and is similar those that are found in automotive turbochargers.
There are in fact two types of turbine wheel found is gas turbine engines. A second
type of wheel is known as the axial flow turbine and consists of a hub surrounded
by small individual blades. This type of turbine is common in larger commercial gas
turbines either for aircraft power-
The radial turbine wheel is surrounded by a component known as a nozzle. A ring of individual converging orifices form nozzles surrounding the turbine wheel that direct the hot combustion gases in to it in the direction of rotation. The turbine and the nozzle must be made of a high temperature corrosion resistant steel alloy such as Nimonic. These metals are very tough and are required to withstand the high turbine inlet temperatures at high power outputs with long life. The hot combustion gases impinge on the turbine blades and pass between them toward the centre and are guided along the shaft axis by curved portions of the blade. The exhaust exits the turbine along the shaft axis.
The compressor turbine rotor group is supported by special high speed bearings. In
conventional small gas turbines ball race bearings are common place that are lubricated
by oil jets from a re-
In engines adopting oil lubrication, seals are required to prevent loss of oil in to the gas flows and exhaust. Common seals in small engines may be labyrinth types often requiring some sort of partial pressurization and spring loaded carbon face seals.
An unusual design is built by the Capstone Turbine company. Capstone adopt air bearings to support the gas turbine rotor shaft and the generator rotor. The Capstone is an ingenious design, no oil system is required reducing maintenance costs and complexity. There is also no seal components normally required to prevent oil contamination of the exhaust gases.
Air bearings operate by trapping air between the high speed shaft and a surrounding foil medium. A boundary layer effect holds the shaft in place, this only occurs at high speed so the shaft must be spun up quickly to “lift off” and also “grabbed” and halted quickly during shut down. Air bearings are sensitive to shock and so the engine assembly must be resiliently mounted to prevent penetration of external low frequency vibration.
Recuperator. The recuperator is an essential part of a microturbine generator, without it the efficiency of the unit is simply to poor and not able to justify the manufacturing, installation and running costs compared to a piston engine alternative. The recuperator is also a tricky component to produce and operate.
In the 1960s a special type of recuperator was built for automotive gas turbines that consisted of one or two rotating glass/ceramic disc matrixes through which the hot and cold gases were alternately passed. The gas sealing and durability of this component proved very difficult to achieve economically and partly contributed to the downfall of the automotive gas turbine during that era.
A static recuperator consists of a heat exchanger matrix not dissimilar to a automotive
Recuperator matrix viewed through engine exhaust port
Capstone remote control graphical user interface (GUI)
Control and instrumentation of many stationary micro-
Schematic showing turbine recharged electric vehicle (TREV) system
As well as standby power and grid connect applications micro-
The current generation of micro-
Faced with the prospect of fitting a micro gas turbine and associated hardware car
size represent more of a installation challenge. A complete system is more bulky
than the original piston engine set up. The batteries even if used for “peak” power
(e.g. acceleration) represent much weight and current micro-
Microturbines generally do not employ any kind of accessory gearbox unlike an aero-
An important part of any gas turbine engine is the ignition system. A high energy discharge system is common with many microturbine designs. Similar to aviation types a special capacitor discharge exciter is used to send a high energy pulse to a special surface discharge igniter plug. The exciter consists of a capacitor and inverter charging system that charges up to approximately 2000V. The action of a electronic circuit or sealed discharge gap fires when the capacitor is charged up and a fierce spark is created over the surface of the plug.
An alternative to the high energy discharge system is to use a glow plug system.
The innovative IHI (Ishikawajima-
There are two basic types of generator that may be found fitted to microturbines,
Microturbines employing a free turbine two shaft layout may use a reduction gearbox
to drive a normal two or four pole synchronous machine. The advantage of this is
a near conventional off the shelf alternator may be used but it will require a reduction
gearbox that is specially built to handle the high input speed from a turbine unit
typically 50,000 rpm. The gearbox will also require a re-
Microturbine engines may drive a generator directly at the running speed that may be up to 100,000 rpm. The advantage of this arrangement is the alternator is much smaller in size due to the much higher operating frequency. In any electrical generator, the electromotive force (EMF) or output voltage produced is proportional to the rate of change of the magnetic field inside the generator. If the alternator turns faster the rate of change increases, as the speed doubles (ignoring losses) the output voltage doubles also. For very high speed machines the size is greatly reduced. A generator fitted to a Capstone C30 30Kw unit is barely larger than a tea mug, weighs around 2 kg and is built in to the air intake. A conventional 50Hz two/four pole brushless alternator turning at 3000/1500 rpm may measure some 500mm x 400 mm and weigh 90kg!
The catch with high speed generators is they must be precisely manufactured and they require external electronics to process the electrical output in to a useful format. Very large forces may exist in high speed machinery, the rotors must be specially designed and very well balanced.
Capstone Permanent Magnet Generator (PMG)
Microturbines employ inverters to process the generator output current to suit the required application. The Capstone features a permanent magnet generator (PMG) that is connected to a special unit known as the Digital Power Controller (DPC). The PMG supplies a three phase high frequency current of up to 750V to the DPC where it is converted to a DC bus. It is then depending on the application back to a low frequency AC feed for connection to a load or the power grid. A special HEV version of the DPC is able to feed a DC current for the charging of vehicle traction batteries.
Capstone Digital Power Controller (DPC)
Starting of the Capstone micro-
The fall and rise (and fall again?) of Microturbines
There is a possibility that microturbines may become common place as range extenders in electric vehicles (EVs) installed in a Turbine Recharged Electric Vehicle (TREV). But how likely is this? A small rugged, economic, efficient power plant, also producing low emissions is a fantastically difficult thing to achieve! No company anywhere in the world has so far produced anything suitable for the adoption as a range extender in any type of vehicle. Several companies have built prototype vehicles using the Capstone microturbine but the C30 unit is bulky, heavy and relatively fragile. It also required compressed air as an externally supplied service and takes 10 minutes to shut down! Over the past decade trials have taken place with range extended super cars, SUV type vehicles and buses but have they caught on? No!
The mix of skills, experience and expertise needed to develop and manufacture a microturbine is a wide ranging one.
Recuperator; A robust long-
High Speed Alternator; A bespoke precision device is needed drawing on the experience of high speed drives.
Power Electronics; A manufacturer of AC drives, inverters or high speed motors to take up the challenge? All of this brought together under one roof with the common goal of a mass produced and integrated microturbine system.
A number of companies are making claims regarding microturbine products and publishing impressive CAD drawings and artistic illustrations but where is the substance? Where in 50 years of small gas turbine development by many experienced and “tooled up” companies are the products? Most of them have failed to become cheap enough, durable enough or cost effective?
Automotive and aerospace manufacturers have been producing small gas turbines for many years since the 1960s and very few survive today, in short it’s all be tried before. In static power generation, distributed generation and CHP manufactures have come and gone. What is different today is the potential appetite for a range extender type power plant now exists and the complimentary batteries and electronics continue to improve. Any company that truly develops such a device will make World automotive history. It will be an event more significant than the gas turbine Rover BRM completing Le Mans, the demonstration of the prototype Rover P6 and the Chrysler Turbine Cars, history in the making!
A Potted History Microturbines A to Z Click Here
Heat recuperator exchangers in small gas turbine systems pose a number of problems
particularly in terms of response times when changes in electrical load are experienced.
It is the function of the engine control system to ensure that a stable output power
supply is maintained and that the gas turbine and recuperator assembly are not subject
to unnecessary thermal transients. To deal with this problem there are a number of
1. A storage battery system may be used to “buffer” the flow of energy to and from the gas turbine. The heat exchanger may have a large thermal inertia and so the engine will be slow to accelerate and decelerate as load demand changes (As it absorbs and releases heat). The battery via an inverter may be used to temporarily take up the electrical load and the engine speed and load “ramped” up and down to “catch up” with the demand and stabilize with it.
The storage battery system takes a form similar to that of an uninterruptible power
supply unit (UPS). When the turbine output cannot meet the power demand then the
battery bank takes up the task maintaining continuity of supply like a UPS. The battery
is also used for starting and cool-
2. A breaking resistance inside the power controller system may be used to dissipate excessive electrical energy if the applied load is rapidly reduced. The control system will throttle back the turbine fuel supply (And hence the heat input to the working cycle) and it will decelerate reducing output as the heat in the recuperator is expelled and the system stabilizes at a lower power condition. Unlike a simple cycle gas turbine without a heat exchanger, this type of system cannot instantaneously respond to load changes and so a management system must be used.
3. The generator output may be fed into the electrical grid system that acts as a
load placed upon the turbine. The grid system may be considered an “infinite bus-
4. An additional way to rapidly dissipate unwanted energy in a micro-
5. In order to operate under light load a micro-
The modern microturbine engine is made possible by the addition of an sophisticated electronics. The electronics performs two significant tasks firstly to control the functioning of the gas turbine and secondly to convert the electrical power generated in to a useful format. A third task is to process the electrical power in reverse from an external source to start the engine.
A microprocessor is used to monitor and control the gas turbine engine. A number of sensors are fitted to the engine such as compressor inlet temperature, turbine outlet temperature, compressor speed, generator output and so on. Values from these sensors feed in to a software program that runs the engine within prescribed limits and ensures it is not overloaded. An operator control panel will also fed commands to the microprocessor and this may be achieved locally at the engine enclosure or remotely from a control centre far away.
A Introduction and Technical Guide