Design and off-design optimization procedure for low-temperature geothermal organic Rankine cycles

•Two-step optimization tool for design and off-design of binary geothermal ORCs.•Brine temperature and electricity price are most-influencing parameters.•Net power output varies from 1.95 MW to 4.74 MW for Tenv = 29.07 °C to −4.28 °C.•Data reduction and splines reduce off-design calculation time by a factor of 88.•Optimal ORC design for a given location, including off-design performance.