Safety aspects of Organic Rankine Cycles (ORC) with combustible working fluid and sub-ambient condenser pressure

ORCs are often designed with the lower pressure limited by the ambient pressure, to avoid air leaking into the working fluid. Safely lowering this limit will be beneficial for power-production efficiency. First, cycle operations with air inleak are investigated. Exemplary cases with n-pentane and benzene are used. The investigation combines tools from thermodynamics of cycle working fluids, pump operation and analysis and combustion science. It appears that pumps customarily used in ORC will forward only liquid, and air mixing into the working fluid at the high-pressure side will not be sufficient for flammable conditions. Next, combustion properties of the air/working fluid mixture in the condenser are evaluated. Transient zero-dimensional and one-dimensional steady state laminar premixed flames are applied for the modelling. Detailed chemical mechanisms from the CRECK and TDTVT groups are used for benzene and n-pentane. It is found that some substances, like pentane, will not make flammable mixtures within the conditions of the power cycle. However, fluids that at the relevant temperatures have saturation pressure much lower than the ambient, might give flammable mixtures with air inleak. For potential sub-ambient use of these, ignition energy sources and quenching geometries have to be considered carefully.