is made smaller, turbine and compressor tip leakage, boundary layer effects, and aerodynamic
friction become a larger part of overall loss, so efficiency of small turbines is lower than the
efficiency of large ones of the same design and materials. In addition, it appears that it will be
extremely difficult to manufacture a ceramic gas turbine with a recuperator as cheaply as a
conventional IC engine. For example, even in light aircraft, where the requirements are well suited
to a turbine engine, spark-ignition piston engines are preferred over turbines in virtually all
applications under 300HP because of their higher efficiency and far lower cost.
At this point, it appears unlikely that a ceramic gas turbine can compete with IC engines on the
basis of efficiency or cost. The turbine’s high specific power and power density, lack of vibration,
and low emission potential may, however, make it an attractive engine candidate in some
applications, especially in hybrids where its poor part-load performance is irrelevant. Although it
would probably be less efficient than a diesel, it would be smaller and lighter than a diesel of equal
power, and have substantially lower emissions. Some companies such as NOMAC are developing
“low” technology, low cost gas turbines that could potentially compete on costs at the expense of
efficiency.
Stirling EnginesStirling engines operate on a thermodynamic cycle that resembles the ideal heat engine cycle, or
the Carnot cycle. For any given maximum temperature limitation, the Carnot cycle represents the
most efficient cycle theoretically possible under the second law of thermodynamics. In addition, it
uses a continuous combustion process, which can have low emissions. Stirling cycle engines are
external combustion engines, that is, they have a working fluid that does not come into direct
contact with combustion, but instead is heated through a heat exchanger. Those Stirling cycle
engines built to date utilize hydrogen as a working fluid. Hydrogen is heated at constant high
pressure in a specially designed heater head, expanded through a piston expander, recompressed
and reheated in the head to complete the cycle.
DOE funded the development of Stirling engines from the late 1970s to the mid-1980s before
terminating its program. The engines proved to have both cost and reliability problems. For
example, hydrogen containment, especially at high pressure and temperature, requires
sophisticated seals, which are expensive and failure prone, in the piston compressors and
expanders. The heater head exposes the coils containing high-pressure hydrogen to high
continuous temperatures. Very-high-temperature-capable alloys containing rare earth materials
such as cobalt and vanadium are required, and the heater head is both complex and costly to
manufacture.^151 The Stirling engine also does not have high part-load efficiency, and requires a
long warmup time owing to the thermal inertia of the heater head. After nearly a decade of
development, prototype engines did not demonstrate fuel efficiency levels even equal to that of a
gasoline IC engine.
(^151) W.H. Haverdink,
“Assessment of an Experimental Stirling Engine POWered Automobile," paper presented at theAutomotive Technology
Development Contractors Coordination Meeting U.S. Department of Energy, October 1984.