Waste Heat Recovery

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Abstract: Waste heat recovery is the use of thermal energy that would otherwise be transferred to the environment to accomplish a useful function. Examples for internal combustion engines include the use of engine coolant for cabin heat, turbocharging to increase power density, bottoming cycles to produce additional work from exhaust gas, or integrated exhaust manifold to facilitate engine warm-up.

WHR in Internal Combustion Engines

Waste heat recovery (WHR) is the use of thermal energy that would otherwise be transferred to the environment to accomplish a useful function. In many cases, WHR avoids or reduces the need for additional fuel energy input that would be otherwise required to achieve this function. Examples for internal combustion engines include:

The main pathways for heat rejection in the internal combustion engine that are potential candidates for WHR include the hot exhaust gases discharged from the tailpipe, the engine coolant radiator, as well as the EGR and charge air coolers.

In many cases, the goal of WHR is to generate additional work. Higher quality heat sources allow a larger portion of the waste heat to be converted to work. The “quality” of a particular heat source for the purpose of WHR depends to a large degree on its temperature. The higher the temperature of the medium, the higher its entropy, which allows a larger portion of the heat to be converted to useful work (i.e., the efficiency is higher or its exergy is higher). For instance, a WHR system driven by heat from the EGR cooler in a high pressure EGR loop can be expected to have a higher efficiency than a similar system that recovers heat from the tailpipe exhaust gases.

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 [3706]. The usefulness of these heat sources for the purpose of WHR depends on:

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 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).

**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, kW		136	348
EGR	Temperature, °C	500	600
	Heat, kW	21	51
	Exergy, kW	13	33
Exhaust, post SCR	Temperature, °C	400	400
	Heat, kW	64	187
	Exergy, kW	35	101
Charge air cooler	Temperature, °C	100	200
	Heat, kW	14	68
	Exergy, kW	2	24
Engine coolant (less EGR heat)	Temperature, °C	90	90
	Heat, kW	21	34
	Exergy, kW	3	5
Total	Heat, kW	122	340
Total	Exergy, kW	53	163

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.

Some of the important technologies that are used and/or are under development for WHR are summarized in Table 2.

**Table 2**
WHR technologies for internal combustion engines
WHR Technology	Principle of Operation	Status
Heat exchangers	Direct heat transfer between two media.	Commercial (e.g., cabin heating using engine coolant and exhaust gas heat).
Turbo-compounding	Conversion of exhaust heat into mechanical or electrical energy using an exhaust driven turbine.	Mechanical turbocompounding is a commercial technology.
Bottoming cycle	A thermodynamic cycle, such as the Rankine or Brayton cycle, that involves heat recovery and rejection via a working fluid (air, steam or organic fluid) to recover waste heat and to drive a turbine to produce mechanical or electrical energy.	Commercial for large stationary and marine engines. Working Rankine and Organic Rankine Cycle prototypes developed by several engine manufacturers for heavy-duty applications (e.g., under the US DOE SuperTruck program). Brayton cycle WHR systems are less developed than those based on the Rankine cycle.
Thermoelectric generators	Solid state devices that convert heat directly into electric energy via the Seebeck effect.	Commercial applications for car seat heating and cooling. Under development for engine WHR.
Thermochemical recuperation	Use waste heat to carry out steam reforming of fuel to increase its LHV.	Under development.
Thermoacoustic conversion	Stirling cycle derived technology operated at high frequency to convert pressure pulsations in a working fluid into electrical power.	Under development.