As it is not feasible to create battery electric vehicles with the same range as it is given e.g. by diesel driven vehicles, alternative energy solutions need to be investigated. By extend an electric vehicle by different types of range-extender solutions the final customer can decide for the daily work if the higher energy is needed or if the energy of the base vehicle battery only is sufficient. The costs of the basis vehicle can be reduced, and the use-case can be optimized depending on the chosen technology.
As electrical drives are already state-of-the-art in the automotive industry by today, in agriculture industry combustion engine driven vehicles are used almost exclusively. One of the main reasons are individual work applications, which have a correspondingly high energy requirement due to the very high load of the entire drive train like traction drive, hydraulics and PTO. In such applications, today's battery-electric concepts, have a significant disadvantage compared to conventional diesel-powered vehicles due to the lower energy density of even the modern lithium-ion batteries compared to liquid fuels as diesel. Basically, there is no economic added value for the end user if the base vehicle covers all use-cases with the energy source installed. Assuming there is enough installation space given this high energy would mainly be necessary for peak loads.
In addition to the lower energy density also the charging of the battery needs much more time e.g. compared to the refilling process of a similar diesel driven vehicle. To reduce the impact on the working process and to preserve the main battery the charging process should be shifted to non-productive time frame at night. Instead, a modular approach of an optionally adaptable range extender is more effective. The basic vehicle is equipped with a battery capacity designed for sufficient extent respectively satisfactory range and usable for the standard applications, which usually work with not more than 30% - 60% of the vehicle's nominal power.
For applications with increased power requirements or when there is a general need for an extended range the basic vehicle can be expanded modularly by a range-extender, which directly supports the work process at high loads or can even recharge the vehicle's internal battery. There are various approaches for implementing such a range extender.
Types of range-extenders based on different technologies
The approach with the lowest technical effort can be realized as a diesel-powered generator. A conventional diesel-engine with less than 56kW (due to limited efforts for the exhaust gas aftertreatment system) is connected to an electric motor via a transmission unit to provide electric power to the main vehicle. By running the diesel-engine in constant speed and load the optimal operation point can be realized with optimized efficiency.
As a diesel-engine seems not to be that eco-friendly and future-proof solution the same concept can be powered by hydrogen instead of diesel. The main difference is the tank system which needs nearly 10 times the installation space of a diesel driven system. At the same time, it is expected that the availability of ‘green’ hydrogen is given. With ‘blue’ hydrogen such a system is not the future-proof from environmental point of view as the efficiency of the total system is affected by the combustion engine and the production of hydrogen, too.
By still working with hydrogen as energy but by using a fuel-cell to generate the needed electrical power, the system efficiency can be optimized. An optimization of the total system efficiency of around 20% compared to the hydrogen combustion engine reduces the fuel consumption and decreases the required installation space. But that fact is opposed by today’s availability of components which results in high system costs. The availability of hydrogen or the availability of fuel cell systems is to be expected much better in the range 2025 – 2030. By looking on the market entry of such systems by this time those systems to be seen as a real future readiness technology.
To have same functionality in-between it is also feasible to work with additional swappable battery-packs. Total system costs can be decreased by cheaper battery technologies as Lithium-Iron-Phosphate as potentially a higher installation space is given as it is in the main vehicle battery. The additional battery pack can be charged by alternative sources as photovoltaic or wind-energy and installed when the main vehicle battery gets empty. Furthermore, such battery extension systems can be used as an enabler for Vehicle-To-Grid technology because it is not needed for powering the vehicle.
It is obvious that it is possible to focus on one technology as the fuel-cell as most promising technology is not that ready to the market as it would be needed by now. By using the three other concepts it is technically feasible to realize a range-extender by today with different cons. As the diesel-engine should be only a basis, the technologies of the next years should be seen in hydrogen-engines and swappable battery backs. As hydrogen engines are typical in higher power classes by today a hydrogen engine in this lower power class is not to be seen as an off-the-shelf component but more as a development based on an existing diesel motor. With a more environmental focus both concepts, hydrogen engines and swappable battery packs, need enablers as ‘green’ hydrogen or second-life battery solutions.
Dr.-Ing. Thomas Woopen
Development Engineer Vehicle
AVL Tractor Engineering Germany GmbH
Dr.-Ing. Stephan Hammes
Skill Team Leader Vehicle Development - Off-Road
AVL Tractor Engineering Germany GmbH
Design Engineer Distributed Systems, AVL Tractor Engineering Germany GmbH