Power Plant Engineering

(b) Gas Turbines. Some larger district heating plants have based their heat and power produc-
tion on gas turbines. They can be regulated less than gas engines, and as they by mean of their size
presuppose a large heat demand there will not be space for many new in the future. There are simply not
that many cities with a sufficiently large heat demand. Apparently there is neither any product develop-
ment-taking place to increase the power efficiency, as it is the case for gas engines.
Combined heat and power production based on steam
The Danish effort to increase the use of biomass mainly straw and wood as fuel in combined heat
and power production increasingly draws the attention towards steam engines and steam turbines.
The steam engine is a well-known technology, but for different reasons it hasn't been developed
for several years. One of the problems has been the contact between lubricating oil and steam. This
problem has been solved with a new design of the steam generator, which is manufactured in Denmark
and is just ready for the market.
The advantage of this cogeneration technology is that biomass can be combusted directly in the
steam boiler and obtain the wanted steam pressure of 20-30 bars.
The disadvantage is that power efficiency will hardly exceed 15%. Therefore it is a question if
the steam engine is able to compete with cogeneration based on gasified biomass in the longer term.
There seem to be better possibilities for steam turbines with a combination of direct stoking of
biomass in the boiler, and superheating of the steam with natural gas. A Danish district heating plant is
preparing a test plant based on this technology. Its advantage is significantly higher power efficiency
than the steam engine.
(c) The Stirling Engine. The Stirling engine is a hot-air engine, named after the Scottish priest
Stirling who invented it in 1817. Since then it has been designed and manufactured in a vast number of
designs.
In spite of intensive and expensive research it is nearly without importance, as the research has
been aimed at developing a car engine, which it is not suitable for.
On the other hand there are large perspectives in viewing it as a stationary combined heat and
power plant. There is a growing understanding of this that has resulted in new research and production
aimed at this. About 150 pieces have been made in batch production in India. This is a simple low-
pressure design with a power efficiency of about 10%.
The Stirling engine has many advantages. In principle it is a very simple technology—also in the
advanced version with helium instead of air and high mean pressure. Furthermore a big variety of fuels
can be used, including concentrated solar heat and clean exhaust from e.g. a gas engine. With the mate-
rials used today, it demands about 700°C as optimum working temperature. And the hot air must be that

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POWER PLANT ENGINEERING
clean, that coating does not occur at the heating surface. Finally, it is nearly noiseless and probably very
stable in operation.
The description of the Stirling engine is to a high degree based on the research carried out at the
Technical University of Denmark. A 10 kW power model with helium as medium and a mean pressure
of 50 bar has been tested in summer 1992. The results are very promising and it is specially interesting
that the power efficiency of this engine is about 30%.
The perspective of heat and power production based on the Stirling engine is that it can probably
be produced in a range of 1 kW power to 150 kW power in the nearby future; in the longer term may be
with an even higher output.
Such small cogeneration units can give the Stirling engine a tremendous distribution and have a
revolutionary influence on our energy supply.