Enhanced Geothermal Systems show promise for meeting growing energy demands. The Organic
Rankine Cycle (ORC) can be used to convert low and medium-temperature geothermal energy to electricity,
but the working fluid must be carefully selected for the ORC system design. This paper compares
the system performance using R134a, isobutane, R245fa and isopentane for four typical geofluid
temperatures below 200 C. Three type (subcritical, superheated and transcritical) power generation
cycles and two heat transfer control models (total heat control model and vaporization control model)
are used for different EGS source temperatures and working fluids. This paper presents a basic analysis
method to select the most suitable working fluid and to optimize the operating and design parameters
for a given EGS resource based on the thermodynamics.