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Waste Heat Recovery (WHR) is a very interesting opportunity since almost two-thirds of fuel energy is not converted into mechanically useful energy. Moreover, the integration with other thermal streams on board (cooling and lubricating mediums, EGR cooling) can add further value to the recovery opportunity as well as the concept of managing the engine thermal management which can produce a sensible contribution that is appreciated mainly during urban driving. A huge scientific effort is underway, and a great expectation is perceptible. More generally, the technological options that can achieve a reduction in overall fuel consumption and, thus, the improvement of global engine efficiency, are the most valuable when they can be introduced without massive changes to the engine layout. This happens in all the energy applications in which Internal Combustion Engines (ICEs) are involved since the recovery unit can be introduced in the exhaust line. The mechanical energy recovered can be easily transformed into electrical energy, so represents an interesting integration with the hybrid propulsion powertrains.


The Organic Rankine Cycle (ORC) is by far the most studied opportunity in waste heat recovery: it is very similar to the steam Rankine cycle but uses a working fluid of an organic nature (Figure 3a,b). This consents to the recovery from low-grade thermal sources [59]. An ORC-based recovery unit is composed of a heat exchanger acting as an evaporator (or Heat Recovery Vapor Generator, HRVG, from section 2 to 3), an expander or turbine (state points 3–4), a condenser (4–1) and a pump (1–2). Some other components can be introduced for efficiency optimization or plant operability (such as a tank/reservoir, a bypass branch for the turbine, an internal heat exchanger for the regeneration stage, etc.) [60].
It is quite commercial for different applications (industrial, geothermal) but it has been studied only in recent years for the transportation sector, in particular for heavy-duty applications. For light-duty and passenger cars, the higher variability of the waste heat temperature and amount (for instance, exhaust mass flow rate) makes it difficult to implement a high-efficiency recovery plant in operating conditions [61].
Figure 3. (a) base T-s diagram of an Organic Rankine Cycle bottomed to exhaust gas and using R245fa as working fluid (dashed line is the saturation curve of the fluid)); (b) base layout of ORC unit.
Trilateral Flash Cycle (TFC), or Organic Flash Cycle (OFC), is a bottoming thermodynamic cycle particularly suitable for low-temperature waste heat recovery, around 80–100 °C [62][63]. The name “Trilateral” highlights the shape of the cycle, which is almost triangular (Figure 4) and it is particularly suitable for that kind of upper thermal sources that do not match with a Rankine or Hirn cycle. So, it can be interesting for very low temperature sources and limited flows of fluid cooling, or for cascade cycle, bottoming to a main recovery unit to further increase the final energy produced. The shape of the T-s diagram at the saturated liquid side can add some additional interest to the exploitation of this technology. It is based on a Rankine cycle in which the vaporization is not realized, but a saturated liquid is expanded. Hence, it is composed of a pump, a simple heat exchanger used to heat up the high pressure working fluid to the saturated liquid state and so, a wet expansion within the two-phase region and a final condensation [64].
The difficulties to recover energy in the two-phase region of the fluid (wet expansion) suggest using volumetric machines as expanders and, preferably, with a variable volume ratio [65]. As shown in Figure 4, the possibility to increase the exergy recovered is great (i.e., the closer the value of the TFC to the upper and lower thermal source/sink), but only if the expander efficiency is close to the one of a conventional turbine; TFC can compete with ORC in terms of final energetic efficiency [66]. More recently, the use of fluid mixtures has also been proposed to increase the efficiency of the OFC [67].

Figure 4. TFC T-S diagram [68]: transformation 1–2 is a pressurization in the liquid phase, 2–3 heating of the liquid, 3–4 is the wet expansion, 4–1 the condensation. Dark blue line is the saturation curve of the working fluid (R1234ze(E)).
The selection of working fluids is one of the most investigated issues. It should respect several constraints and parameters: low flammability, no toxicity, low ozone depletion potential and global warming potentials, as well as other low environmental impacts. At the same time, the working fluid should have high thermodynamic performances in relation to the upper and the lower thermal sources available. Dry and isentropic fluids are suitable for ORC-based units [69]. The opportunity to be mixed with lubricating oil is an additional positive issue because volumetric expanders which are very suitable for small-size recovery units need to be lubricated to improve mechanical and volumetric efficiencies.
In this regard, the opportunity to use zeotropic mixtures (i.e., mixtures of fluids, which can vary their temperature during phase change) can have the opportunity to better approach the thermal sources and sinks during evaporation and condensation of the working fluid, thanks to the temperature glide during phase transition (Figure 5) [70].

Figure 5: T-s diagram of an ORC with mixtures of working fluids [71]. Inset zoom on the temperature glide in the evaporation phase. Similar to the condensation one. Dashed line is the saturation curve of the fluid mixture (R245fa/benzene 0.95/0.05)
When the temperature of the upper thermal source is enough high (>300 °C, for instance), only ORC is not suitable, since it introduces a high exergy destruction rate in the heat recovery vapor generator [72]. Indeed, the influence of the pinch point temperature on the energy recovered is significant and the differentiation of the layout can be proposed to increase the recovery efficiency [73]. In particular, the combination of more than one thermodynamic cycle can be used, in a cascade form [74], in order to match the upper thermal level with one recovery section, and the cold sink with a bottomed one, increasing the exergy efficiency of the overall recovery unit [75].
The use of the CO2 Brayton cycle is very popular, where the CO2 is performed in the supercritical phase (pc,CO2 = 74 bar). In particular, the CO2 is in a dense phase when it is close to the critical point, with a thermodynamic advantage in terms of fluid density (it has a density close to the one of a liquid and it has low viscosity like a gas, increasing the mass per unit volume without increasing pressure drops across ducts) and reduced viscosity with respect to a gas. Moreover, CO2 is a natural compound, stable, non-toxic and non-flammable, with a reduced environmental impact with respect to fluorinated gases. Thus, the supercritical CO2 (sCO2) topping cycle is bottomed by an ORC one, aimed to partially recover the thermal energy to be disposed of in the lower CO2 pressure side (Figure 6a, [75]), and a trade-off between the cycle efficiency and the heat recovery must be analyzed, [76]. Only a few sCO2 recovery plants have been manufactured and actually operated since this technology has not yet reached technical and commercial maturity [77]. Several configurations have been proposed in this regard (parallel, cascade [78], regenerative [79][80], re-heated and intercooled [81][82], recompression [83], dual expansion [84][85], etc.) in order to increase the net power output. The complexity of the recovery plant is shown in Figure 6b, where the two cascade sections are sketched. State points from 1 to 5 refer to sCO2 Brayton cycle, while 7 to 9 are referred to ORC one. The need for an additional heat exchanger (CO2 cooler after HRVG, from state points 5 to 1) is highlighted. The management of supercritical values of CO2 pressures can also represent a limiting factor, in particular for small-scale units, where the amount of recovery does not justify high-pressure components, piping, seals, etc., which in turn brings higher costs. In fact, the introduction of ORC as the bottoming cycle in combined recovery plants has been proposed also for medium-low temperature sources, to increase the overall energy recovered [86].

Figure 6. (a) example of T-s diagram of a combined energy recovery unit composed by supercritical CO2 and ORC [75]; (b) simple layout of sCO2+ ORC unit.
Even though the Stirling engine was discovered in the early 1800s, only recently it has been applied in several applications, especially in combination with renewable sources and as a waste heat recovery option in industry [87][88]. The Stirling cycle is a gas engine cycle composed of two isotherms and two isochore lines as an indicator diagram. A regenerator is interposed in between. Its main advantage is that the Stirling cycle does not replace the working fluid for every cycle. The working gas can be air or other gases. The overall amount of thermal energy is supplied externally from the engine, making possible the use of any kind of source. Stirling engines can achieve high thermal efficiencies, ideally the one of the Carnot cycle, since the heat exchange takes place at a constant temperature. In reality, the transformations are usually far from the ideal ones, and the real efficiency does not overcome 20–25% during operation [89].
Stirling engines could be an interesting solution for ICE thanks to their compact design, easiness of management, application flexibility and the possibility to be adopted for different energy sources [90][91]. The power recoverable can range from hundreds of watts to kW. Different solutions of Stirling engines can be found, such as alpha, beta, and gamma configurations [92], single or double-acting cylinders [93], or free-piston operational mechanism [94].
The thermal power recovered can be directly used for heating purposes, in particular for steady state applications in CHP mode. The first example usually considered, is cabin heating in automotive applications [95]: the heat removed from the engine by the coolant is partially used in a heat exchanger placed in the dashboard of the vehicle. This exchanger is crossed on one side by the hot coolant, exiting from the engine jackets, and by air on the second side, thanks to the controlled fan, which regulates the thermal power to be sent to the cabin interior. Often, the cabin heater lays in a branch of the cooling circuit in parallel with the radiator and the bypass branches, without undergoing thermostat control [96]. Additionally exhaust heat or lubricating heat can be used for this purpose, in order to optimize the thermal level [97][98]. Its integration with other thermal needs improves the overall efficiency of the system [99], for instance, accelerating the warming up of the lubricant oil, reducing frictions, and improving engine thermal management: the use of the exhaust to warm up the engine oil during cold phase demonstrated a fuel consumption reduction over 3% [100][101].
A different use of the thermal power to be disposed on board is represented by absorption chillers, which can improve the integration concept of thermal needs [102][103], feeding refrigeration, and cooling needs. Waste heat driven heat pumps can also allow low-temperature heating purposes [104][105]. The integration of absorption chillers on board can also be used for increasing the propulsion system efficiency, cooling the engine charge air [106], or optimizing the thermal management of electric devices [107][108]. In industrial applications, the integration of Rankine cycles with multiple-level refrigeration systems can meet the requirements of air-conditioning, refrigeration, and also cryogenic cooling, aiming at a full energy recovery of lost heat [109].
When applied to exhaust gases, the heat exchanger performance plays a crucial role: it should have high thermal efficiency to maximize the thermal exchange towards the working fluid of the recovery unit [110], and also in terms of backpressure increase on the engine, which should be significantly limited in order to avoid excessive overconsumption on the engine itself and tailpipe emissions [111]. The impacts of extra weight, additional cooling fan power consumption, transient control, effects on engine intake air management as well as exhaust after-treatment thermal inertia should be also considered in mobile applications [112].