The performance of different types of organic Rankine cycles (ORCs) and of the Kalina cycle is investigated and optimized for low-temperature (100-150 degrees C) geothermal heat sources. A variety of configurations is worth considering. The ORCs can be subcritical or transcritical and can have one or more pressure levels. Each cycle can be a simple one, have recuperation or turbine bleeding and up to 80 different working fluids can be used. Comparison of these cycles to each other and to an optimized Kalina cycle concludes that the transcritical and multi-pressure subcritical cycles are the best ones. Exergetic plant efficiencies of above 50% can be achieved when the brine is allowed to cool down as much as possible. A constraint on the brine outlet temperature causes a strong decrease in the mechanical power output of the cycle. Due to the low heat source temperatures, a low condenser temperature and small temperature differences in the heat exchanger are very important. (C) 2012 Elsevier Ltd. All rights reserved.