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Heberle, Florian ; Jahrfeld, Thomas ; Brüggemann, Dieter:
Thermodynamic Analysis of Double-Stage Organic Rankine Cycles for Low-Enthalpy Sources based on a Case Study for 5.5 MWe Power Plant Kirchstockach (Germany).
2015
Veranstaltung: World Geothermal Congress
, 19.04.-25.04.2015
, Melbourne, Australien.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung
,
Paper
)
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Abstract
Geothermal power generation by organic Rankine cycle (ORC) is examined in a case study for the 5.5 MWe double-stage power plant in Kirchstockach, Germany. The power plant consists of two structurally similar modules operating at high (HT) and low pressure (LT) level. In both units R245fa is chosen as working fluid. A simulation model is validated based on operational data of the power plant and shows a good agreement. The calculations are performed using the fluid property databases RefProp and StanMix. A significant deviation of about 10 % between the databases is observed for the enthalpy difference at expansion due to deviation in the prediction of the slope of the saturation curve. Considering operational data and actual estimations in literature the isentropic efficiency of the turbine calculated by StanMix seems to be more reliable. Furthermore, sensitivity analyses and comparisons to alternative power plant concepts are performed based on the validated model. A variation of the split ratio of the geothermal fluid at the outlet of the LT-evaporator shows a maximum of the second law efficiency for a ratio of 3:1 (HT:LT). A comparison of the double-stage concepts to single-stage ORC using pure fluids and zeotropic mixtures shows the highest gross second law efficiency for the single-stage concept and R134a/R245fa as working fluid. However, ORC units using R134a, R600a or these fluids as mixture component lead to high power requirement regarding the feed pump. As result the investigated double-stage concept lead to the highest net efficiencies of the considered optimization strategies.