The power generation technologies of the twenty-first
century will require that four primary factors be considered in any future
engine design:
Energy resource availability;
Emissions characteristics;
Engine efficiency; and
Engine cogeneration potential.
With respect to energy resource availability in North
America, leading energy experts, such as Robert Hefner Ref.1, have forecast that
domestic natural gas supplies could meet existing energy usage patterns for the
remainder of this century. Toward the end of the century, as natural gas
supplies decline, a shift to hydrogen as a primary fuel may well occur.
Because of increasing concerns about the effects of global warming and
acid rain on the biosphere, future power generation technologies must also have
extremely low emissions signatures, particularly with respect to sulphur dioxide,
oxides of nitrogen, oxides of carbon and volatile hydrocarbons.
The concerns about energy resource availability and emissions are
intimately related to the third factor, which is engine efficiency.
All else being equal, less fuel is used and fewer emissions are released
to the atmosphere by an engine that is more efficient in producing useful work
from a given fuel input.
One of the most efficient power generating systems
currently in widespread use is the combined cycle gas turbine.
Utility-grade combined cycle turbines Ref. 2 can have an electrical
efficiency approaching 55%. The key to their superior efficiency is that the
heat from the combustion of the fuel is used in two power generating cycles by
the engine. In the first cycle, the
heat of combustion is used to turn a gas turbine generator.
In the second cycle, the exhaust heat from the gas turbine cycle is used
to produce steam in order to turn a steam-powered generator.
Extremely high efficiency results because the heat of fuel combustion is
used in two engine power cycles, not just one.
However, the size and complexity of utility-grade systems make them
impractical for most combined heat and power (“CHP”) applications.
A relatively new type of turbine has been developed by
Quasiturbine Agence Inc. of Montréal, Canada Ref. 3, which is known as a
“Quasiturbine” (“QT”). The
QT is a relatively low RPM engine. Figure
1 illustrates the basic QT operating mechanism, which includes a rotor, stator
with or without the four carriages. The
Quasiturbine concept has resulted in a large family of potential engine
configurations, including combustion QT, a steam-powered QT and a Stirling QT.
Because the centre of the QT is hollow, an electric generator shaft can
be inserted directly into the core of the engine. Consequently, the QT is able
to directly drive conventional electric generators.
The QT has multi-fuel capabilities and can be operated in
internal combustion mode with a gaseous fuel, such as natural gas, syngas or
hydrogen. The ports and spark plug
of the combustion QT, as illustrated in Figure 2, can be oriented either
radially or axially. With its high
pressure compression ratios, the combustion QT is capable of operating in
photo-detonation mode without a spark plug.
Because the combustion QT is a high compression photo-detonation engine,
the combustion of natural gas results in extremely low emissions of oxides of
nitrogen and other greenhouse gases. The
QT can also function as a Stirling engine Ref. 4. A Stirling engine produces
power by virtue of a working “fluid”, which may be a liquid or a gas (such
as water or helium, for example) transiting between zones of high temperature
and of low temperature within the engine. The rotor of a Stirling QT engine is “pushed” by the
pressure created by the heated working fluid in the high temperature zone and is
“pulled” by the partial vacuum created by the cooled working fluid in the
low temperature zone.
The combined cycle Quasiturbine (“CCQT”) increases
engine efficiency by integrating a combustion QT with a Stirling QT.
During operation, the combustion QT produces both power (which is
delivered as torque to the rotating shaft) and hot combusted exhaust gases.
In a single cycle engine, the latent energy of the hot exhaust gases is
either vented to atmosphere or, in the CHP mode, is recycled for hot water or
space heating. In contrast, the
Stirling QT utilizes a portion of the energy of the hot exhaust gases to
generate more engine power, thereby increasing overall engine efficiency.
A thermal recycling unit delivers the hot exhaust gases to the Stirling
QT's “hot temperature zones”. As
the temperature of the working fluid in the hot temperature zone increases,
additional power is delivered to the shaft.
As the shaft rotates, the heated working fluid is transferred to an
adjacent “cold temperature zone” and is cooled.
The fuel for the combustion QT travels through the cold temperature zone
of the thermal recycling unit, cooling the working fluid. The heated fuel is
then combusted in the combustion QT. This
technique is commonly known as “process fuel cooling” and may be
supplemented with other standard cooling techniques.
Mechanical efficiencies of approximately 50% are anticipated with a CCQT
operating on natural gas. Like
other CHP systems, the heat not used by the CCQT to generate power can be
directed to hot water or space heating uses.
The CCQT is also a potential alternative to the automotive
internal combustion engine. Because
operation in CHP mode is not practical for a vehicle, “on-board” engine
efficiency is the primary measure of a vehicle's overall efficiency.
The Stirling QT's “on-board” engine efficiency can be substantially
increased by using a very cold fuel, such as liquefied natural gas (-160C) or a
refrigerated gas like natural gas or hydrogen (-50C to -75C).
The Stirling QT is unique in that it is able to recover a portion of the
energy used to refrigerate or to liquefy the fuel, thereby further increasing
the CCQT's “on-board” efficiency. Because
of its efficiency, a CCQT, operating on a very cold fuel, should more than
double a vehicle's average miles per gallon (gasoline equivalent), while
simultaneously reducing emissions of oxides of carbon, oxides of nitrogen and
volatile hydrocarbons to a fraction of that produced by a gasoline engine.
With hydrogen as the fuel, the CCQT would function as a high efficiency
“zero emissions” engine. Development and testing of QT prototypes are being
conducted by Quasiturbine Agence Inc. under the direction of Dr. Gilles
Saint-Hilaire.
References
1. Hefner, Robert, 27 International Journal of Hydrogen Energy 1 (2002).
2. Flavin, Christopher, Power Surge, p.101 (W.W. Norton & Company 1994).
3. http://quasiturbine.promci.qc.ca
4. http://quasiturbine.promci.qc.ca/QTStirling.html
