NoWaste Project

Engine Waste Heat Recovery And Re-Use

Project Coordinator: Federica Bettoja

State of the art

There are several processes using waste heat to generate power: thermodynamic processes (Joule, Rankine, Stirling), thermoelectric processes and processes using the high pressure and temperature device like a turbo charger.

Turbocompounding

Turbocompounding is a technique in which a combustion engine works together with one or more exhaust gas turbines. Thereby not only from the engine, but at least from one of the turbines power output is taken. Instead of expelling wasted energy via the exhaust pipe, the heat is extracted from the exhaust gases by a second exhaust turbine downstream to the turbocharger.

The second turbo (the turbocompound turbine) spins at 55,000 r/min. This motion is passed through turbine gears and a hydraulic coupling, then through the timing gears to the crankshaft. Stepping down the revs produces a useful boost in torque, which when reaching the flywheel adds momentum. This extra driving force do not increase expenditure on fuel.

The economical green band on the rev counter provides a wide range of economical engine speeds. The engine exhibits great flexibility. The rotation of the crankshaft benefits from the constant extra drive coming from the turbocompounding process, helping to even out the rhythmic pressures induced by combustion. So the engine runs more smoothly.

First air-craft engines with turbocompound were used already during the II World War. Then they were used in marine engines. Scania Company has been producing trucks equipped with turbocompound engines since 1991 and Volvo since 2001. Turbocompounding in short terms will be state of the art for most suppliers. In the USA works are carried out on electrical systems turbocompound.

Caterpillar is focusing its research efforts on the development of an electric Turbocompound system for heavy-duty on-highway truck engines consisting of a turbocharger with an electric motor/generator integrated into the turboshaft. The generator extracts power at the turbine, and the electricity it produces is used to run a motor mounted on the engine crankshaft. The electric turbocompound also provides more control flexibility as the amount of power extracted can be varied. This allows the control of engine boost and thus of the air/fuel ratio. To improve vehicle driveability, the system can be run in turbo assist mode to accelerate the turboshaft.

CO2 Benefit: 8%

Additional Cost: MEDIUM

Technology maturity: HIGH

Advantages: the system does not require a specific heat rejection system

Drawback: only part of the available waste heat can be re-used

Thermo-Electric Generation

Thermoelectricity (TE) is one of the simplest technologies applicable to energy conversion. It has long been discovered that a few materials can generate electric power from heat, and use electricity to function as heat pumps providing active cooling or heating: such materials are said to be thermoelectric.

Small pieces of thermoelectric material (TE elements) are connected as shown in figure to form a Peltier couple. One element is of p-type, the other one of n-type; they are connected in series electrically, but thermally in parallel. A multitude of couples forms a TE-module. When a heat source is applied to the hot plate, and the cold plate is maintained at a cold temperature, a voltage difference ΔV is created between the cold and hot plates, that can induce a current I in an external circuit: the Peltier couple is an electric generator.

Thermo-Electric Generation

The efficiency of a TEG depends on the material ZT as well as on the temperature drop Th - Tc across the device. The only TEG application on board vehicles described in the literature is the Hi-Z/Mack Truck Generator. This system utilizes the waste heat of a class 8 Diesel Truck to produce about 1 kW electric power.

CO2 Benefit: >5% (state of the art materials)

Additional Cost: HIGH

Technology Maturity: LOW

Advantages: the system has almost no moving parts

Drawbacks: only part of the waste heat can be re-used and the materials should still be developed.

Rankine cycle systems

The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water or an organic fluid as the working fluid. generation from wasted heat. This RCS can be operated on the exhaust gases of the Internal Combustion Engine. The system is composed of four/five different components: Exhaust gas heat exchanger, expander, (+generator), pump, and the condenser.

The ideal Rankine cycle consists of the four following evolutions:

  • The thermodynamic fluid enters the pump at state 1 as saturated liquid and is compressed to the operating pressure of the boiler.
  • The fluid enters the boiler as sub-cooled liquid at state 2 and leaves as superheated vapor at state 3.
  • Heat is transferred to the fluid at constant pressure.
  • The superheated vapor at state 3 enters the turbine where it expands isoentropically and produces work by rotating a shaft connected to an electric generator. The temperature and pressure of the fluid drop during this process at state 4, where it enters the condenser.
  • The fluid is condensed at constant pressure in the condenser, by rejecting heat to the atmosphere (for vehicle applications) and leaves the condenser as saturated liquid and enters the pump

Rankine cycle

CO2 Benefit: >15%

Additional Cost: MEDIUM

Technology Maturity: MEDIUM-HIGH

Advantages: almost the waste heat can be re-used

Drawbacks: increases the load of the heat rejection system

Progress beyond the state of the art

The plan for the NoWaste project foresees theoretical and numerical studies for different technologies to improve the Rankine system efficiency and integration. Within the Rankine cycle architecture development phase the best compromise resulting from the theoretical pre-studies shall be included in the design.

Position of the evaporator on the exhaust line

In conservative designs of mobile Rankine cycles recovering waste heat recovery from the exhaust line the evaporator is mostly situated behind the after treatment system of the exhaust gas. This configuration has the advantage that the after treatment of the exhaust gas won’t be disturbed. On the other hand the exhaust gas temperature decreases by 50° K to around 100° K in the after treatment system.

Another disadvantage of the conventional configuration is the possible corrosion and damage of the Rankine evaporator due to soot and ammonia from a possible SCR system.

This study shall investigate e a possible configuration of the Rankine cycle evaporator between the turbocharger outlet and the EATS inlet and determine the gain in energy recuperation by simulations.

The research content will mainly concentrate on the possibility of an adapted after treatment system to lower exhaust gas temperatures. The advantage of this configuration would be founded in better heat exchanger efficiency due to higher heat source temperatures which leads to reduced packaging needs. The minimum exhaust gas temperature at the EATS inlet will limit the efficiency of waste heat recovery. A good adaptation of the EATS configuration to low exhaust temperature could lead to a higher overall efficiency of heat recuperation and a gain in after treatment efficiency since the after treatment system inlet temperatures can be controlled.

Further the development of the right control strategy for the Rankine system will be crucial in order to let the after treatment system work correctly at every moment the engine is working. A bypass of the Rankine system evaporator is necessary in all cases.

Indirect heat exchange and heat storage

The main drawback of the Rankine cycle is the need of heat rejection during the condensation phase. For full charge engine working points, especially at a high environmental temperature the cooling capacities of the vehicle won’t be sufficient to cool the engine and the Rankine cycle which leads to a necessary energy costly fan engagement.

The indirect heat exchange uses a caloric fluid or the working fluid of the Rankine cycle itself to recover heat from the exhaust gas and transfers the heat to the Rankine cycle. To use a specific caloric fluid can give advantages in heat recovery and heat storage efficiency but also increases the complexity of the overall waste heat recovery system.

A heat storage device permits to decouple the Rankine cycle from the internal combustion engine and to produce energy when the vehicle thermal management permits it. Another advantage of an independent Rankine cycle is the possibility to develop an independent control strategy for the waste heat recovery cycle.

The main drawbacks of this technology are increased system complexity, weight and a loss in direct waste heat recovery efficiency if a caloric fluid is used.

In a first step a possible architecture with or without a caloric fluid shall be identified and the steady state waste heat recovery potential shall be identified. This phase also includes investigations on efficient heat storage systems.

In the second phase the fuel economy potential shall be analysed performing road cycle simulations with a Rankine cycle model including indirect heat exchange. Only with this approach the overall benefit towards a direct Rankine cycle can be determined.

If the numerical studies show an advantage using modified cycle architecture with heat storage this technology can be taken into account in the final architecture definition phase.

Supercritical Rankine Cycles

Sub critical Rankine cycles are often constrained by low evaporation pressures, especially for organic fluids or heat exchanger temperature pinch limits between the hot and cold source due to very high latent evaporation energy needed as it is the case for the water Rankine cycle.

The supercritical cycle offers a good potential of overcoming these drawbacks and maximising the Rankine cycle efficiency by using high pressure differences between the condensation and evaporation side.

In order to adapt the cycle to the vehicle heat rejection potential and packaging needs using a supercritical cycle can permit to increase the condensation pressure and temperature of the working fluid without losing cycle efficiency.

The drawback of a supercritical cycle is the need of a high pressure and temperature resistant heat exchanger. Furthermore the expansion device needs to be adapted to high pressure ratios and inlet pressures.

The research shall concentrate on possibilities for supercritical heat exchange. In first steady state simulation models the potential of a supercritical Rankine cycle shall be identified using different architectures and working fluids. In a second phase the supercritical cycle shall be simulated in a 1D physical Rankine cycle simulation model.

Optimal heat management and heat recovery from multiple heat sources

Beside the exhaust gas the engine environment offers various heat sources as the EGR, the engine coolant or the charge air after turbo compression. These media need to be cooled down by the EGR cooler, the front radiator and the charge air cooler.

The engine coolant as well as the charge air offers a good heat potential for preheating the Rankine cycle working fluid, for organic Rankine cycles with condensation temperatures lower than 70°C. The EGR can be used for pre-and superheating. These additional heat sources can lead to higher energy recuperation whereas the necessary external cooling power for charge air, engine coolant and EGR decreases and the Rankine cycle condensation heat rejection increases.

The study concentrates on the determination of optimal cycle architecture for the given boundary conditions and the estimation of necessary heat exchanger dimensions.

Heat Rejection

The waste heat recuperation means to increase the heat load of the vehicle heat rejection system proportionally to the recuperated heat.

So, for each 100 kW of recuperated heat, assuming a conversion efficiency of about 20% (very high), the heat rejection load will increase of at least 80 kW. This means that the vehicle heat rejection system should be the focus of a deep review.

The most simple solution is to add to the front module an heat exchanger devoted to the heat recuperation system, but this approach risks to have a negative effect of the vehicle cooling drag and to reduce the effectiveness of the existing heat exchangers.

On the other hand the vehicle heat rejection system is normally designed to comply with sever condition (very high thermal load, low speed high external temperature), in other world in almost all the operating conditions there is a heat rejection capacity available.

The approach that will be adopted within NoWaste is based on two main points:

  • the use of the existing heat rejection capacity when available (almost all the operating conditions)
  • increase of the heat rejection capacity using part of the body panels as heat exchangers (flat heat exchangers) functionalising the aerodynamic under-body and of part of the side body panels. The panel will be channelled and transformed in heat exchangers, but keeping their original functions (aesthetics and aerodynamics). This solution allows to increase the overall vehicle heat rejection capacity and the effectiveness increases increasing the vehicle average speed so in coherence with the amount of recuperated heat

Other features

Whatever will be the Rankine cycle architecture it is crucial to develop a suitable and optimised control strategy in order to overcome transient losses and to maximise energy recuperation while influencing the engine environment as little as possible.

Inputs to strategies come from the engine environment, the Rankine cycle itself and the vehicle environment. Control parameters within the waste heat recovery cycle can be described with bypasses, engine and pump speeds or specific valve characteristics for the energy sources or the bottoming cycle itself.

This project includes control strategy development tasks and work packages in almost all phases of the project in order to develop a highly efficient and robust strategy.

Social impact

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