VOLUMETRIC EXPANDER VERSUS TURBINE – WHICH IS THE BETTER CHOICE FOR SMALL ORC PLANTS?

asme-orc2015 Tracking Number 22

Since the steam turbine replaced the steam reciprocating engine by the end of 19th century it has been the only expander type in CRC and ORC power plants at least above 1 MWel. Positive displacement expanders like scroll- or screw-machines has often been applied for smaller units – in particular below 100 kWel. One reason for this is that in cooling or compressed air technology those machines are cheaply available as compressors which can be converted “easily” to expanders. In contrast, up to now small turbines are rather seldom in market. One goal of this paper is to discuss whether there are others than the already mentioned reasons to justify technically or economically the choice of volumetric expanders for small ORC plants? Or may small turbines provide benefits which couldn´t have been used in the past just due to a lack of appropriate machines? However, is the choice between volumetric or turbine expander finally made there is still a big variety of working principles and possible shapes for both expander types which have to be considered in order to get the best final expander design. Therefore, the paper introduces briefly the working principles of positive displacement and turbine expanders and evaluates them concerning their application in small ORC-plants. For the author, the advantages of turbines outweigh their disadvantages. Nevertheless, in the following the decision between e. g. impulse or reaction type, axial or radial, single or multistage turbine has to be made. The paper discusses and explains the “pro and cons” of all these turbine types regarding different applications like e. g. low/high temperature ORC, small or bigger units etc.. In summary the paper recommends turbine expanders even for small ORC-Units and provides a brief overview and comparison of different turbine types to facilitate the decision making concerning the best turbine for a given task.