Parametric Optimization and Working Fluid Selection of Organic Rankine Cycle (ORC) for Engine Cooling Water Waste Heat Recovery

Abstract

In this paper, aimed to utilize engine cooling waste heat completely, thus promoting engine fuel economy significantly, organic Rankine cycle (ORC) system is proposed and investigated in detail. Considering environmental impact, four promising hydrocarbons, namely butane, isobutane, pentane and isopentane are taken into consideration. The results indicate that butane distinguishes among other fluids for the highest value of output power, and its output power is as large as 6.33kW, besides that its volumetric flow rate is relatively low as well. Pentane and isopentane can acquire a maximum thermal efficiency of 9.5% or more for the whole operating parameters. KEYWORD: waste heat recovery; engine cooling water; working fluid International Conference on Industrial Technology and Management Science (ITMS 2015) © 2015. The authors Published by Atlantis Press 1007 be no lower than 70°C, as a too small value penalizes ICE fuel economy. 2.1 Thermodynamic analysis of investigated ORC configurations The considered ORC layout configuration is shown in Fig.1. Conventional ORC consists of four main components, i.e. pump, evaporator, turbine and condenser. The low-pressure working fluid is pumped to high pressure. It then flows into the evaporator and heated by jacket water, after that, it converts heat into mechanical power by expansion in the turbine. Eventually it discharges unused residual heat through condensation. Fig.1 ORC layout configuration In order to simplify the model, some assumptions are made. Specifically, all processes operate in steady states. The pressure drop and heat rejection of the pipes are negligible. Based on the first thermodynamic law, energy investigation is conducted, and the corresponding mathematical model for each ORC configuration is collected in Table 1. While Table 2 gives the operating parameters for ORC. According to the Montreal Protocol [9], many refrigerants were or will be phased out in the near future due to the environmental impact, especially the depletion of the ozone layer, apart from that, global warming also attracts increasingly attention. Calm and Hourahan pointed out that hydrocarbons are promising candidates of the next generation of refrigerants for the characteristics of zero/low ozone depletion potential (ODP), low global warming potential (GWP) and high efficiency[10,11]. The main concern against the application of hydrocarbons is flammability which can be avoided through adequate safety precautions, and it’s noticeable that currently there are no working fluids available that completely meet all requirements including energy efficiency, safety, environment friendliness etc. In our work, butane, isobutane, pentane, isopentane are included for consideration. Their thermodynamic characteristics are collected in Table 3. Table 1 Mathematical equations for each thermal process Table 2 Operating parameters for ORC system