DEVELOPMENT OF THE TRILATERAL FLASH CYCLE SYSTEM .1. FUNDAMENTAL CONSIDERATIONS

The world market for systems for power recovery from low-grade heat sources is of the order of L1 billion per annum. Many of these sources are hot liquids or gases from which conventional power systems convert less than 2.5 per cent of the available heat into useful power when the fluid is initially at a temperature of 100 degrees C rising to 8-9 per cent at an initial temperature of 200 degrees C. Consideration of the maximum work recoverable from such single-phase heat sources lends to the concept of an ideal trilateral cycle as the optimum means of power recovery. The trilateral flash cycle (TFC) system is one means of approaching this ideal which involves liquid heating only and two-phase expansion of vapour. Previous work related to this is reviewed and details of analytical studies are given which compare such a system with various types of simple Rankine cycle. It is shown that provided two-phase expanders can be made to attain adiabatic efficiencies of more than 75 per cent, the TFC system can produce outputs of up to 80 per cent more than simple Rankine cycle systems in the recovery of power from hot liquid streams in the 100-200 degrees C temperature range. The estimated cost per unit net output is approximately equal to that of Rankine cycle systems. The preferred working fluids for TFC power plants are light hydrocarbons.