Supercapacitors, batteries and trying to go wireless
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There's a fashion for electrically powered buses; fuelcells enjoyed a vogue a few years ago, hybrids sell as political solutions to needing to be seen to be green and battery buses continue to find niches as temporary, often exhbition, installations. There is a perception that somehow avoiding building an overhead infrastructure is a good thing. Part of this thinking has to do with balking at being serious about cutting greenhouse gas emissions and reducing on-street air pollution.|
The latest 'fad' is the use of supercapacitors and/or li-ion batteries that are charged intermittantly, either at stops or at route terminii.
This idea, using supercapacitors, has been trialled over a number of years in Shanghai and extended to run to the huge Expo 2010 exhibition. The initial 2005 installation followed a circular trolleybus route in case of breakdowns, and indeed, trolleybuses had to be substituted. Any new route may need more substations than a trolleybus route, to cope with a considerable number of high power charge stations. Heavy dependance on supercaps alone will shorten their lives. By 2009, vehicles entered service in Shanghai that -
use lithium iron phosphate batteries and ultra-capacitors
require overnight battery charging of 3 to 5 hours
have a range of 160km with air conditioning
have a range of 250 - 300km without air conditioning
have a maximum speed of 130Km/h
Super or Ultra-capacitors are relatively long lived and can charge very quickly, making them ideal for storing regenerated energy obtained from braking. Many of the latest trolleybuses are being installed with them as they considerably reduce energy consumption. Like-for-like trials in Solingen found a reduction in power consumption from 2.4 euros/km to 1.8 euros/km, a 25% gain.
But their use to avoid installing trolleybus overhead is problematic. The Shanghai installation found that buses had to be recharged at every stop, whether they needed to pick-up passengers or not. They could also become stranded in adverse traffic conditions. Adding batteries should overcome this inflexibility, but batteries, although improving, are still expensive, loose efficency over their 5 year maximum life, and are slow charging. By using the terminal layover as a required charge period, the bus is tied to a new operational constraint and so added costs. The experience of battery powered trolleybuses in Rome, used to avoid installing overhead over a 3km section in the historic centre, is that air conditioning may need to be off to ensure making it between overhead sections and that battery replacement every 4-5 years is very expensive. The battery set costs 20-30k euros per vehicle, which during a 30 year life equates to approaching 200k euros or 20-30% added cost to the vehicle. Newer technology batteries, that charge faster, cost more (50-60k euros per set) and still have a service life of between a year to less than 5 years, based on a 10km route run 8 times a day. Avoiding total discharge can double the service life, but oversizing is costly, heavy and wasteful. Comparative running costs are 174 euros/day for a battery system, 162 euros/day for diesels and 54 euros/day for trolleybuses. Better to have a smaller dependance on batteries to provide only short distance running.
Attempts to induct electricity to the bus (or tram) from underground cables through necessarily complex but safe collector systems, have been tried in a number of designs and trials, such as the Bombardier Primove and Korean systems. The fundamental inefficency (around 75%) limits any induction system, as well as reliability issues related to complexity, weather and dirt. No system has been commercially installed.
The most elegant and efficient system remains the trolleybus. Taking cues from the lessons learnt, there may well be a case for using using trolleybuses with supercaps and batteries that maintain their 100% electric ability, are responsive to severe traffic hinderances and could mean an end to junction overhead complexity. There has been historic retinance to install trolleybus overhead in large city centres, not for aesthetic reasons but over concerns about the buses becoming imobilised and causing a traffic hazard. Perhaps these objections could be overcome by de-poling before junctions at speed and re-poling at the next stop after the junction. Complex overhead arrangements to allow for multi-route operations could be avoided by removing all or all but the single most important routing. Beijing uses such a system over routes adjacent to Tiananamen Square. Re-poling, even at speed, could be improved or made possible by using laser position sensing. Using trolleybuses with a reduced overhead requirement would be cheaper to install and maintain, although the vehicles would be more expensive but they would be playing to the inherent advantages of supercaps, batteries and direct electricity supply.
|More details of worldwide Tbus developments are here|