Analysis of binary cycle efficiency using Redlich–Kwong equation of state

Abstract

Coal, natural gas, and nuclear power plants operate using various forms of Rankine cycle. A variant of Rankine cycle known as binary cycle has been implemented in power plants; yet, the optimization of binary cycles remains relatively unexplored. This article is concerned with efficiency maximization of binary cycle by investigating various working fluid materials for different operating conditions. We introduce a novel analysis approach by employing Redlich–Kwong equation of state to derive a simple, effective way to express the efficiency of binary cycles suitable for most working fluids. Five alkali metals and mercury are investigated for the topping cycle and two fluids for the bottoming cycle. The results show that for operating conditions of 1.25 MPa, 811 K, and 458 kPa, 589 K for the topping cycle and 10.6 MPa, 589 K, and 1.71 kPa, 289 K for the bottoming cycle, the mercury/ammonia combination provides the highest efficiency. For operating conditions of 1.25 MPa, 1123 K, and 458 kPa, 823 K for the topping cycle and 29 MPa, 823 K, and 4 kPa, 295 K for the bottoming cycle the mercury/ammonia combination also provides the highest efficiency. These binary cycle efficiencies are significantly higher than comparative Rankine cycle efficiencies.