Performance Benefits for Organic Rankine Cycles with Flooded Expansion

An organic Rankine cycle (ORC) is often used in waste heat recovery applications. These are typically small-scale applications where cycle thermal efficiency is low, and the benefits of traditional cycle enhancements (such as reheat stages or feed-water heaters) do not typically outweigh the costs required to implement them. An ORC with flooded expansion and internal heat regeneration is an alternative enhancement that provides comparable benefits at reduced cost and complexity. The improvement in efficiency for the ORC with flooded expansion and internal regeneration is analyzed for several working fluids and for two flooding media: water and Zerol 60 compressor lubricant. It is shown that internal regeneration alone provides most of the efficiency enhancement for dry working fluids (R600a, n-Pentane, and R245fa). n-Pentane is shown to offer the most efficient cycle even without flooded expansion in most cases. A quantitative comparison is given between the proposed cycle and the reheat and feedwater heater cycles with internal regeneration. In applications where a hydrocarbon may not be appropriate as a working fluid, R245fa and R717 show promise as alternatives. R717, which shows the most benefit from flooded expansion and internal regeneration, requires this enhancement in order to be competitive with the dry working fluids.