Rankine Cycle Waste Heat Recovery

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Abstract: Heat rejected from an internal combustion engine, such as through the EGR cooler or the tailpipe, reflects a significant loss of efficiency. Some of the wasted heat may be recovered using the Rankine cycle, where an intermediate heat transfer loop is added that contains a working fluid whose properties allow it to pass through an expander to capture some of the exergy of the waste heat sources. The design of the system requires a careful selection of the working fluid, and proper matching of the fluid to the hardware. Organic Rankine cycle systems have been used on several truck prototypes under the US DOE SuperTruck program.

Rankine Cycle

A Rankine cycle is a closed-cycle system where a working fluid circulates through a minimum of an evaporator, turbine, condenser and a pump to convert heat into work, Figure 1. The evaporator can incorporate or be followed by a superheater if the working fluid/heat source temperature allow it. The conventional working fluid for Rankine cycle plants is water.

In a subcritical Rankine cycle, the temperature and pressure of the working fluid remains below the critical temperature and pressure, Figure 1. In a supercritical Rankine cycle, Figure 2, the fluid pressure is raised above the critical pressure and heat addition continues until the critical temperature is exceeded; heat rejection remains subcritical. Supercritical operation avoids the iso-thermal portion of the heat addition of the subcritical cycle, thus raising the average temperature during heat addition and reducing the irreversibility of the heat transfer process. Supercritical Rankine cycles are sometimes referred to as transcritical cycles. Owing to the fact that heat rejection in a Rankine cycle is via condensation from a gas to a liquid, heat rejection returns the working fluid to a subcritical state. Cycles in which the working fluid remains in the supercritical state during heat rejection would be possible with a Brayton cycle.

The decision to use a supercritical Rankine cycle depends on the characteristics of the working fluid and the temperature of the heat source. Supercritical Rankine cycles provide more benefit when the heat source is at a higher temperature. In one study, supercritical operation was beneficial when a dry working fluid was used in combination with a recuperator and the heat source was over 240°C while wet fluids and no recuperator showed a benefit over supercritical operation at heat source temperatures over 360°C [3764]. Dry and wet working fluids are discussed further below.

Rankine cycle systems are commonly used to recover waste heat in a variety of applications. Depending on the application, the exergy and energy of the waste heat streams as well as the ability to reject heat to the sink can vary significantly. This has a significant influence a number parameters in a Rankine cycle WHR system including working fluid selection and equipment sizing and selection.