Rankine Cycle Waste Heat Recovery

Hannu Jääskeläinen

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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.

Potential of WHR in Internal Combustion Engines

Waste heat from a heat engine or power plant is rejected to the environment either through a heat exchanger or directly through the expulsion of the hot working fluid. In an internal combustion engine, both of these are used: hot exhaust gas, the engine’s working fluid, is exhausted directly to the environment and heat exchangers are used to reject heat to the environment from the engine coolant, EGR cooler, charge air cooler and oil cooler.

Figure 1 summarizes the main pathways for heat rejection in a heavy-duty diesel engine that are potential candidates for waste heat recovery (WHR) [3706]. The usefulness of these heat sources for the purpose of WHR depends on:

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Figure 1. Main sources of internal combustion engine heat loss

Figure 2 illustrates in more detail the temperature of various heat rejection streams shown in Figure 1 for a heavy-duty diesel engine as a function of engine power. Data was collected at 53 engine speed and load conditions and the variations in EGR and exhaust temperature represent speed/load effects not captured by the effect of engine power [3709].

Figure 2. Temperature of various waste heat streams in a heavy-duty diesel engine

Engine: 2011 12.8 L Mack MP8-505C 505 hp (377 kW) @1800 rpm/1810 ft-lb (2454 Nm) @1100 rpm. EPA 2010 emissions. HP EGR/DOC-DPF-SCR.

Figure 3 illustrates the proportion of fuel energy producing brake work and lost through the various waste heat streams for three power settings of the engine of Figure 2. Also shown are more details of the waste streams that are available for WHR including the proportion of exhaust heat remaining in the exhaust gas after the aftertreatment system and the amount of heat transferred from the EGR cooler to the engine coolant [3709]. Table 1 summarizes the energy and a first approximation of the exergy—based on the Carnot factor—of the different waste heat sources for two of the operating conditions of Figure 3 (exergy represents the amount of work that can be theoretically produced from an energy flow).

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Figure 3. Proportion of fuel energy lost through waste heat streams of Figure 2
Table 1
Energy and exergy of the waste heat sources for two operating conditions of Figure 3 assuming a heat rejection temperature of 36°C
Engine output, kW136348
EGRTemperature, °C500600
Heat, kW2151
Exergy, kW1333
Exhaust, post SCRTemperature, °C400400
Heat, kW64187
Exergy, kW35101
Charge air coolerTemperature, °C100200
Heat, kW1468
Exergy, kW224
Engine coolant (less EGR heat)Temperature, °C9090
Heat, kW2134
Exergy, kW35
TotalHeat, kW122340
Exergy, kW53163

Waste heat from the EGR cooler represents the highest temperature heat available and therefore a high priority for WHR. Over 60% of the EGR waste heat is available as exergy. In applications without high efficiency SCR systems, EGR flow rates can be higher and heat recovery from the EGR system more significant [3711]. Post SCR exhaust gas is also important and considering that exhaust flow is typically much higher than EGR flow, represents considerable energy and exergy flows. About 50% of the exhaust heat is available as exergy and thus is also a priority for WHR. Charge air cooling and engine coolant are at significantly lower temperatures and represent relatively low quality heat. However, at higher loads, the charge air still contains a significant amount of exergy.

Rankine Cycle

If instead of rejecting heat from the various sources directly to the environment, 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, an efficiency benefit can be realized. This is the essence of Rankine cycle WHR—heat is still rejected but through suitable system design, some of the exergy from the waste heat flows is turned into useful work.

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 4. 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.

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Figure 4. Basic Rankine cycle

In a subcritical Rankine cycle, the temperature and pressure of the working fluid remains below the critical temperature and pressure, Figure 4. In a supercritical Rankine cycle, Figure 5, 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.

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 from supercritical operation at heat source temperatures over 360°C [3764]. Dry and wet working fluids are discussed further below.

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Figure 5. Temperature–entropy (T–S) diagram for a typical supercritical cycle

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.