Hybrid vehicle range extenders: goodbye pistons
2015年3月13天 Dr Peter Harrop
Hybrid vehicles are already big business from hybrid cars, buses and trucks to outdoor forklifts, even hybrid tugboats. Indeed, many hybrid aircraft will soon appear. An increasing majority of hybrid powertrains are series hybrids where the engine only charges the battery: it never drives the wheels. As explained in the IDTechEx report, Range Extenders for Electric Vehicles Land, Water & Air 2015-2025, the conventional engines with pistons that are used as range extenders are now being questioned because they are not a good starting point in making a lowest cost, weight and size power source that is very reliable and lasts the life of the vehicle.
Development of these "reciprocaurs", as opponents describe them, is being questioned because many simpler alternatives are beginning to look feasible. That said, the hold-up of such piston engine advances as the single cylinder Polaris range extender and the in-line free piston engines that directly provide electricity seems to be more a reluctance to invest than a technical problem.
One example of the potential escape from pistons originates from the fact that jet airliners have Auxiliary Power Supplies APU that are used on fairly steady load in the tails of jet aircraft for hotel facilities etc not flight. Derivatives of such Capstone turbines have had modest success as range extenders in buses and trucks, Total Cost of Ownership being one of the limitations.
Bladon Jets in the UK is developing tiny jet engines you can hold in your hands, cost reduction being promised from miniaturisation, having only one moving part and selling them in volume for home and office electricity supplies in countries such as India. The blades and shaft are spark eroded as one piece.
In an alternative approach, Monash University in Australia, in collaboration with Safran Microturbo of France, has made a small jet engine using 3D printing. Being additive, this wastes very little material, potentially taking costs of gas turbine range extenders even lower. Indeed Rolls Royce in the UK is already making jet engine parts this way that will fly later this year. They are both achieving more complex shapes and showing how to reduce costs. Monash uses laser 3DP but Rolls Royce used electron beam 3DP.
Microturbo is a member of the Safran group and a subsidiary of Turbomeca and it designs, develops and manufactures low-power gas turbines. Microturbo has become a global reference in small robust, reliable turbines and turbojets. It works on two lines of products. They are on-board APU for airplanes, helicopters and ground applications and starting systems and secondly turbojet engines for missiles and targets.
Change of course with fuel cells
Fuel cells are another "new" range extender if we ignore the fact that they have been around for 175 years and concentrate on the fact that they are yet to be successful in vehicles. Enthusiasts keep talking about five years from now, when the problems of cost, maintenance, size, charging infrastructure and so on start to be overcome to the point of volume production being contemplated, for example with Toyota expecting to produce up to tens of thousands of fuel cell cars.
Fuel cell materials developer ACAL Energy points to, "a number of breakthrough technologies that will be bought into the second generation fuel cell vehicles between 2020 and 2025 that overcome all of these issues and ACAL Energy's liquid catalysts will be part of this. ACAL liquid catalyst technology addresses the problems of:
- Instantaneous response to load
- Removal of all cathode degradation mechanisms
- Ruggedness - ability to operate with pin-holes in the membrane
- Simplified balance of plant
- And a 50% reduction on catalyst cost when compared with the Pt cost target required to get to <$35/kW system cost.
"Battery/Supercapacitor technology is complementary to FC powertrains as all the auto OEM's we are engaged with plan some form of hybrid drive system".
Proton Motor has developed fuel cell vehicles for 20 years with its own fuel cell stack and system design know how. It is currently developing a range extender fuel cell bus system with a fuel cell performance of 25kW and a battery on board between 80 to 120 kWh, depending on the drive cycle. This concept will allow a 12m 18 tonne city bus to run at a two shift operation with a refuelling time of hydrogen in 10 minutes. Cost for the bus is in line with the EU target for 2020, they told IDTechEx.
The advantages are fast refuelling, full range of operation, less weight and higher payload compared to a pure battery bus, less batteries on board (reduction of resources and problems to recycle large amount of batteries), low maintenance compared to a bus with a large fuel cell system, long lifetime of the fuel cell based on optimized operation mode and longer lifetime of the battery as it can be operated in an optimized SOC. Will cost be competitive? The BYD B9 is this size of bus and it is selling in many thousands already but its recharging time is 30 minutes or more.
Certainly more fuel cell cars are now appearing albeit in tiny quantities, though trials of fuel cell buses peaked ten years ago and the architecture is hybrid with the fuel cell as range extender, there being a large NiMH or Li-ion battery or a supercapacitor with conventional battery, these doing start-up, accepting energy harvesting electricity and managing load variations. Also see the IDTechEx report Electric Buses 2015-2025
Given the slowness of improvements and growing competition from improved pure electric vehicles, which are greener, attitudes have changed this year, with developers realising they must be open and accept help from any quarter. At the recent World Smart Energy Week in Tokyo, GM, which did a partnership with Honda for fuel cell development out of necessity to reduce risks and required investments, now called for suppliers to approach them.
Toyota recently threw its fuel cell patents open for a limited time to try to accelerate progress - Its Mirai fuel cell car being sold in very limited quantities presumably due to cost. In Japan, Toyota told IDTechEx that they can warranty 200,000 km of fuel cell durability - big progress with one former problem - but they do not know how the car resale market will evolve. Very low resale prices for battery electric cars have inhibited sales and the same will probably be true of fuel cell cars for some time.
Volkswagen said their position on fuel cells is that they are not fully convinced that fuel cell cars will be widely deployed. They will not invest highly and will use a petrol car platform for their fuel cell car development. They are concerned about how zero carbon fuel cells cars will ultimately be, given hydrogen from fossil fuels and the complexity of manufacture. Indeed, a hydrogen fuel cell typically emits one kilowatt of heat for every kilowatt of electricity used.
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Top image source: Bladon Jets