Plazo razonable

The WHR systems can be divided in two main families and an overview is presented in Figure 6-51. The classification depends on the form in which the harvested heat is converted:

 Heat to Heat systems, that transfers the heat from exhaust gas or engine coolant to another fluid (oil, coolant, etc.); the main technologies based on this principle are:

o Fast Warm-up system for engine, trasmission and cabin fast heating.

o Adsorption Heat Pump (AHP), that converts heat to energy for cabin cooling.

 Heat to Power systems, where the heat is converted in mechanical or electric power; the main technologies are:

o Thermo Electric Generator (TEG), based on Seebeck effect the heat is transformed in electric energy.

o Turbo-compound, that generates mechanical or electric power by means of a turbine driven by engine exhaust gas.

130 o Rankine Cycle (RC) system, that generates mechanical or electric power by means of a

proper working fluid in a closed thermodynamic cycle.

o Stirling Engine, that generates mechanical or electric power.

The main features for each technology are ilustrated following, higlighting the working principle, the main benefints and technical challenges.

Figure 6-51 Engine Heat Recovery systems: Heat to Power technologies highlighted in blue and Heat to Heat in orange

Fast Warm-up system - An heat exchanger (see Figure 6-52), installed typically downstream the catalyst, transfers heat from exhauts gas to a fuid, oil or coolant for engine, transmission and vehicle cabin. The exchanger is bypassed when the warm up is finished or to reduce exhaust back pressure at high engine exhaust mass flow rate.

The technology is suitable for Hybrid Vehicles [27,28], reducing engine utilization to maintain coolant temperature (thermal management) and improving the cabin comfort in winter conditions (faster coolant warm-up).

The technology constraints are the managing of exhaust pressure drop and the avoiding of fluid boiling.

The Fast Warm-up technology is considered key also in this study and its benefit will be shown in the case study in Chpater #12.1.

131 Figure 6-52 Description of Fast Warm-up operating modes

Adsorption Heat Pump (AHP) – The cooling cycle is performed without the mechanical compressor, which is repalced by a “ thermal compressor”, the BED1 and BED2 in the Figure 6-53, that alternatively exchange heat with cabin and engine (exhaust gas or coolant). The working fluid is typical water or NH3

and the adsoprtion material used in the two beds are Silica Gel or Zeolite.

The capability to work also with waste heat at low temperature below 100℃ is a promisng feature for automotive applications. The system can be used for the cabin air conditionig or as a refrigerant. The main challenge are the system inertia and the maturity of the technology. The Volumetric Cooling Power (VCP) achievable is around 1000 W/Liter, comaprable with electric compressor cycle, and the Coefficient of Performance (COP) is in the range 0.4÷0.6, as reprted in [29, 30].

Figure 6-53 Scheme of an AHP system

132 Turbo-compound - An additional turbine, installed downstream turbocharger turbine, generates power output either electrical or mechanical (Figure 6-54, case a). It can be used as power boost or as a fuel saving component.

In an alternative layout an elctric moto-generator is integrated on turbocharger shaft (Figure 6-54, case b), named typically e-Turbo. In this case, in addition to the energy recovery, the compressor assist is possible, allowing turbo lag compensation. The technical challenges of this system are:

 the engine back-pressure, that reduces the advantage of the energy recovery;

 the lowering of exhaust gas temperature, in case of the preferred installation upstream the catalyst, with negative impact on the catalyst performance;

 the complexity of system in case of mechanical solution;

 the relailabilty of electric motor, mainly due to high speed and thermal stress.

Figure 6-54 Main layouts for the Electric Turbo-compound

This technology allows fuel saving up to 4÷6 % in standard cycles, if it is used only for the energy recovery. This achivement is confirmed by some studies [33], among these one [38] of the thesis author.

In case of architecture (b), the air boosting assist enables an higer degree of engine downisizing, that allows a fuel economy improvement up to 10%.

The design and integrtaion of motor/generator with the turbocharger are the key aspects of the electric turbo-compund. In Table 6-9 a comparison of the possible elctric machines is presented, indicating a measurement of their features versus the technical requirements. In the published studies and demostrators the preferred solutions are the Brushles Direct Current (BLDC-PM) motor [240, 241] and the Varible Reluctance motors [107], both syncronous and switched: the first one for its performance and lower noise, the second ones for their benefit/cost ratio and relialablity.

133 Table 6-9 Comparison of the electric machines for Electric Turbo-compound

Thermo-Electric Generator (TEG) – It works using an heat exchanger with thermoelectric generators layers between exhaust gases and engine coolant. The temperature difference between plates produces electron circulation (electric current), according to Seebeck effect. The electric production is performed with only one component and no moving parts, that is the main advantage of the technology. The used materials are different and in continuous evolution, the preferred ones are Bi2Te2 (Bismute Tellurure), Mg2Si and CoSb3 (skatterudite). The efficiency depends on a generating performance index of the material ZT according the following equation

(6.2)

For the typical materials ZT has a value between 0.2 and 1.4, depending on the temperature. The back pressure introduced in the exhaust line is negligible compared with the turbo-compound system. The technology challenges of TEGs are:

• the efficiency, low with actual materials (<8% thermal efficiency);

• the heat exchanger sizing (wall thermal resistance and conductance) and packaging;

• the material costs, that lead to 1÷1.5 €/W for the over all system.

Figure 6-55 TEG packaging and principle scheme of Seebeck cell

134 This technology allow fuel saving up to 1÷5 % in standard cycles, as confirmed by some studies [31,32,33], among these one [34] of the thesis author.

The costs and the limited power density are the main barriers for this technology application in mass production. But many researches on new material can improve significantly the TEG systems.

Rankine Cycle (RC) System – The system works with thermodynamic cycle based on Rankine process.

The system (see Figure 6-56) is composed of the following parts:

 the Pump, that feeds the evaporator with high pressure liquid;

 the Evaporator, that generates high temperature vapor with exhaust heat;

 the Expander, that expands vapor to produce mechanical work;

 the Condenser, that turns low pressure vapor back into liquid.

High fuel saving can by achieved up to 10% in real conditions. The back-pressure introduced by the evaporator in the exhaust line is lower compared with the turbo-compound system and it can be installed downstream the catalyst. These are the key advantages of this technology, that is very promising for ICE based powertrain, espcially elctric hybrid ones.

The technical challenges to be managed are the proper matching of the system with the engine (heat exchange sizing, working fluid and expander), the system complexity and mainly the packaging. For the high potential benfits and synergy with electric hybrid powertrian a deep analysis of the technology is presented following.

Figure 6-56 Rankine Cycle system components [130]