Wednesday, 6 July 2011
Kalina Cycle
Following the ASX announcement by Wasabi Energy to join the consortium to develop
a new 4.5MWe Kalina plant in Taufkirchen, Germany, I have been meaning to write
something about it. A Kalina cycle is of course a mixed fluid cycle where the
mixed fluid is ammonia and water. While the Kalina cycle if offered over a wide
temperature range, its advantages are more pronounced for lower temperatures.
To see why this Kalina cycle is thermodynamically superior to a regular Rankine
cycle, let us remember that the best efficiency that one can achieve in a power
cycle is the Carnot efficiency which is given as
where TC is the condenser temperature and TH is the turbine
inlet temperature. Consider a typical Rankine pure-fluid cycle where the brine
heat is used to evaporate a suitable cycle fluid. We are ignoring the superheat.
The brine and cycle fluid temperatures along the heat exchanger would then look
like this:
Because boiling takes place at constant temperature (at TX), the
turbine inlet temperature (TH) is much lower than what it could have been (TH')
if the cycle fluid were able to increase its temperature while it is receiving
heat from the brine. If this were possible than the cycle fluid would reach
TH' by following the dashed line.
It is possible to do this in a supercritical cycle where there is no phase
change. It is also possible to do so in a Kalina cycle where the boiling temperature
keeps increasing. The boiling point is not constant in a Kalina cycle because
as more ammonia is evaporated than water, the concentration of water in the
liquid increases and the boiling point increases. This is a so-called gliding
cycle and is the basic reason why Kalina cycle is better than Rankine cycle,
especially at lower temperatures where heat exchanger irreversibilities have
a larger influence on the cycle efficiency and the power generation.
There was a hiatus in the Kalina cycle development and this probably was due
to the the IP issues. While the original patent by Alex Kalina was issued in
1982 and must have expired by now, there have been a great many number of patents
since then addressing various aspects of the technology and it was not clear
what the IP was and who owned it. With the consolidation of the IP with Wasabi
Energy, a renewed interest in Kalina cycles was being expected for some time.
The decision by the Taufkirchen consortium to favour a Kalina cycle plant for
their CHP (Combined Heat & Power) project fulfils this expectation.
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