be used to determine the primary heat exchanger duty and mass flow rate of geofluid required. is the rate of heat added to the working fluid within the primary heat exchanger, kW is the specific enthalpy of the working fluid at the primary heat exchanger inlet, in kJ/kg is the mass flow rate of geofluid, in kg/s & are the specific enthalpy of the geofluid at the primary heat exchanger inlet and outlet respectively, in kJ/kg Efficiency There are a number of different definitions of efficiency that may be considered; these are discussed below. First law efficiency The first law efficiency (from the First law of thermodynamics) is a measure of the conversion of the heat provided to the cycle into useful work. When accounting for real life losses and inefficiencies, real binary cycle geothermal plants have a first law efficiency of between 10-13%. Carnot efficiency The Carnot efficiency gives the efficiency of an ideal thermodynamic cycle, operating between two reservoirs of different temperatures, as such it provides a theoretical maximum to the efficiency of any heat engine. For this reason, a geothermal power plant producing hot geofluid at 180°C (≈450 K) and rejecting heat at 25°C (≈298 K) has a maximum efficiency of just 34%. & are the hot and cold absolute temperature respectively, in K Second law efficiency The second law efficiency (from the Second law of thermodynamics) is a measure of the utilisation of the ideally maximum work available and conversion into useful work. is the exergy rate of geofluid, in kW. , & are the specific enthalpy, in kJ/kg, the specific entropy, in kJ/kg/K and the absolute temperature, in K, of the geofluid at the local reference condition. This could be local ambient, wet-bulb or reinjection conditions. Working fluid selection The working fluid plays a pivotal role in any binary cycle and must be selected with care. Some criteria for selecting a suitable fluid are given below. A critical temperature and pressure above the cycle maximum temperature and pressure - most of the heat is transferred at the maximum temperature, increasing efficiency. A saturation dome that resembles an inverted U - this prevents liquid drop out in the turbine, which reduces efficiency, damages the turbine blades and thus reduces the turbine's lifetime. High thermal conductivity - improves the heat transfer in the primary heat exchanger and the condenser, reducing the total heat transfer area required and