Publication

Multi-objective optimization of turbo-ORC systems for waste heat recovery on passenger car engines

Abstract

Waste heat recovery from passenger car internal combustion engines by means of an organic Rankine cycle (ORC) system is promising for reducing CO2 emissions. In this study, different cycle configurations capable of converting waste heat from both coolant and exhaust gases are investigated based on different working fluid categories. Radial-inflow turbines are considered as expansion devices and corresponding isentropic efficiencies are evaluated based on a preliminary design map accounting for the effect of the pressure ratio. Mechanical losses resulting from the use of a gas-bearing-supported rotor driving a permanent magnet generator are also evaluated. In order to identify the turbo-ORC system design tradeoffs, constrained multi-variable and multi-objective optimizations are performed using an evolutionary algorithm. It is found that the optimal cycle configuration and working fluid depend on the available space in the vehicle and that the condenser is the most critical component for the ORC system integration. In addition, the most suitable working fluids for this application are characterized by (1) a boiling point close to the heat sink temperature, (2) a high critical pressure, and (3) a high molecular weight. The resulting optimal radial-inflow turbines are 10e33 mm in tip diameter and operate at 80 e330 krpm.

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Related concepts (37)
Working fluid selection
Heat engines, refrigeration cycles and heat pumps usually involve a fluid to and from which heat is transferred while undergoing a thermodynamic cycle. This fluid is called the working fluid. Refrigeration and heat pump technologies often refer to working fluids as refrigerants. Most thermodynamic cycles make use of the latent heat (advantages of phase change) of the working fluid. In case of other cycles the working fluid remains in gaseous phase while undergoing all the processes of the cycle.
Mathematical optimization
Mathematical optimization (alternatively spelled optimisation) or mathematical programming is the selection of a best element, with regard to some criterion, from some set of available alternatives. It is generally divided into two subfields: discrete optimization and continuous optimization. Optimization problems arise in all quantitative disciplines from computer science and engineering to operations research and economics, and the development of solution methods has been of interest in mathematics for centuries.
Waste heat recovery unit
A waste heat recovery unit (WHRU) is an energy recovery heat exchanger that transfers heat from process outputs at high temperature to another part of the process for some purpose, usually increased efficiency. The WHRU is a tool involved in cogeneration. Waste heat may be extracted from sources such as hot flue gases from a diesel generator, steam from cooling towers, or even waste water from cooling processes such as in steel cooling.
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