[rollei_list] Re: OT - Turbines generating electricity
- From: "Siu Fai" <siufai@xxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 28 Jan 2005 10:34:36 +0100
> Do you understand what the characteristic operational distinctions are
> between the use of a turbine vs a piston engine? As in, why a turbine
> engine is not as good a choice as a piston engine in certain
> circumstances?
The smallest micro-turbine available is in the tens of kW while the piston
engine can be made in as small as you want. At present, turbine only makes
sense if you are operating in the MW region. In the kW region, you cannot
beat the price and efficiency(!) of a good diesel engine. The diesel piston
engines are actually quite good in the smaller region. So, the car we are
driving today are actually not that bad.
It is very challenging to built small turbine that is sufficient efficient.
It is a continuous process with complex fluid dynamics. In order to get a
high efficiency, you need high compression and high temperature. The
combustion temperature determine the available work (or exergy) of fuel, the
operating pressure determine the conversion from available work to motion.
The higher the operating pressure, the further you can expand the hot
gasses, the lower the exhaust temperature and more energy is converted into
motion. So you need the high compression. But if you compress gasses, you'll
reduce its volume , so for moderate power output you need tiny gas channels
and miniature vans to expand it. The technical challenge is to make and
optimize these tiny vans. In addition, the larger gas turbines (MW class)
are highly optimized process with multi stages compression and expansion to
optimize its (isentropic-)efficiency. So, it is a complex and elaborate
process which does not scale down well.
Therefore, IMHO, the turbine process is less suitable for small scale
application. The more efficient alternative process for these smaller power
output is the electrochemical combustion or fuel cell process. This a very
different kind of thing and far more complex than some compression and
expansion.
Siu Fai
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