Optimizations and comparison of three two-stage Rankine cycles under different heat source temperatures and NG distribution pressures

•Three two-stage Rankine cycles with direct expansion are compared.•Fluids with high critical and low normal boiling temperatures are better.•A maximum efficiency of 30.54% is achieved by ammonia.•Two-condensation ORC is both the best thermodynamic and economic systems.•The systems could greatly reduce CO2 emission.