Waste heat recovery
This project was initiated by questions from the industrial partners in the vehicle analysis steering group about if the use of Waste heat recovery (WHR) will influence a hybrid power train or vice versa. I.e. what synergies and conflict there might be. The reason to investigate this is that we need to know what aspects of WHR we should include when designing hybrid powertrains.
To analyse how WHR systems influence and inter-act with a hybrid powertrain.
• Make a litterature review of WHR tech-nology and its use in road vehicles.
• Develop a simplified simulation model for WHR which can be used to analyse the power flow in hybrid electric power trains combined with WHR.
• Analyse possible hardware synergies based on the simulation model and make a very simple cost benefit analysis for different vehicle types.
• Vehicle integration or market aspects are mostly excluded from the analysis.
The project has identified that a well designed and well controlled WHR to a large part can be included in a HEV simulation using very simple quasi steady state models for many factors, like the engine ex-haust temperature and exhaust mass flow rate. The part of the WHR system which should be given most attention is the heat exchanger and its dynamics. The heat exchanger is the component which has the greatest influence on the dynamics of the power generated by the complete WHR and thus will determine when the WHR produce power and how much. There are also some other dynamics in the system but they are generally much quicker and have only second order effects on the whole system. I.e. in simple concept analysis they can mostly be ignored. However, they are big challenges when designing the control system for WHR, but in our study we have assumed that the controlsystem is already well designed.
The report will discuss the system behaviour based on simulation of different parts in the system, and spend some time explaining the complex dynamics in the heat exchanger.
In the end a simple cost benefit analysis is made for both WHR and for HEV. It can be shown that the two systems are best suited for vehicles with different type of use (different driving cycles), i.e. they will often not occur on the same vehicles. However, there are some types of vehicles where they can be expected to both be used at the same time on the same vehicle. Typically, that is long haul trucks where a frequently varying speed or frequent gradients may make a HEV system profit-able.
Some synergies between WHR and HEV:
• WHR benefits a little from the possibility to store energy for later use
• WHR may benefit from the possibility to use the electric machine of the HEV system for feeding back recovered WH to propel the vehi-cle. This may be easier to integrate than feed-ing back the recovered WH mechanically.
• Control of WHR may be simpler if combined with a HEV since the battery make it possible to control the WHR only aiming for optimal in-ternal function of the WHR system, not need-ing to bother about the driving cycle .
One possible conflict between the systems:
• A HEV powertrain reduce the time the engine is ON, i.e. slightly reducing the amount of WH possible to recover by the WHR-system.
The energy savings by combining the two systems seems to be rather small, i.e. the energy saving syn-ergies does not seem to be an important reason to combine the systems. The integration of the WHR system, in which the EM of the HEV system can be used, is perhaps a reason to combine them, but to determine if that is the case requires more detailed studies of the WHR system and its integration in the vehicle.