ATAC i system - Energimyndigheten

Efficient steam turbines for small-scale energy conversion plants
Project number: P12457-2
Project leader: Prof. Torsten Fransson
Project members: Jens Fridh, doktorand (100%)
Project’s duration: 2001-07-01 to 2003-03-31
Granted funds: 1 960 000 kr
The reference group: Ulf Rådeklint, ALSTOM Power (fadder); Lars Atterhem,
Skellefteå Kraft (fadder); Lars Hedlund , ALSTOM Power;
Andrew Martin, KTH; Per Almqvist, KTH.
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Project description:
The Swedish government has decided to implement an "energy transfer" from nuclear to
renewable energy resources in the next decade or so. Various kinds of basic and applied
research activities are needed for this to be achieved. The main focus of the current project is
to investigate the possibilities of increasing the generation efficiency, within economically
reasonable boundaries, of small-scale steam turbines coupled to biomass boilers. Here the
term small-scale refers to turbines with inlet volumetric flows up to 0.7m 3 /s and electric
outputs less than 25 MW. The steam turbine is an important part of small-scale combined heat
and power (CHP) and combined cycle (CC) plants for future de-centralized power generation,
and it has not been thoroughly investigated in detail with the prerequisites of today. Decentralization
is an attractive scenario in the perspective of the Swedish transfer of electricity
generation from nuclear to other options. Furthermore, the effects on the efficiency via
advanced steam conditions and partial admission need to be clarified. In order to meet the
demands of relevancy, the study originates from the available 'state-of-the-art' technologies
for small-scale steam turbines and complements with further investigations and judgements.
To keep the study at a reasonable size, the main work is aimed towards aero-thermodynamic
calculations and experiments for efficient steam turbines.
The small physical size of the turbines applied in small-scale heat and power plants has a
great deal of importance for the isentropic turbine efficiency. The dimensions of turbine
bladings and flow channels are primarily a function of the volumetric flow rates passing
through the machine; which are consequently reduced for small turbines. In an ideal machine
where clearances, blade thickness and surface roughness could be held at a constant ratio to
other geometric scaling parameters, the small size would have very little impact on turbine
performance; according to similarity rules, only the Reynolds number decrease may affect the
entropy generation. However, these ratios cannot presently be upheld due to construction
difficulties, consequently the entropy generation to energy input ratio becomes large for small
machines. One way to maximize performance at small scale is to increase the blade heights
and allowe partial volumetric flow admission, which thereby decreases the entropy generation
to energy input ratio. In fact, partial admission is routinely applied at part loads, which is
common for small-scale turbines and especially for district heating turbines and backpressure
turbines in industrial applications (where the heat demand largely dictates the turbine load).
An improved understanding of losses induced by small volumetric flows and partially
admitted flows are also needed due to lack of open literature regarding this topic.