The Potential for Rechargeable Electric Municipal Public Transportation
The vulnerability of electrically powered municipal transportation systems is their dependence on the electrical power grid during the critical AM and PM peak demand periods. In North America, both New York City and Toronto along with many other cities worldwide have experienced power outages during these critical time periods. Most public utilities experience minimal demand for electric power during the overnight hours between 11:00PM and 6:00AM with peak demand periods occurring between 6:00AM and 9:00AM and again between 4:00PM and 7:00PM.
Modern developments in energy storage technology can literally allow for public transportation operators to purchase large amounts of electric power at low cost during the overnight off-peak hours. This assumes that electric power is being sold at premium prices during AM and PM peak demand periods and at bargain prices during the overnight off-peak hours. Operators of steam-based thermal power stations prefer to operate at steady output while the demand of electrically based transportation systems involves massive swings of power when vehicles accelerate.
The operators of such power stations may prefer that the electric transportation purchase their power at a steady rate of demand that can match a steady rate of output from the power station. An energy storage system that can absorb massive amounts of power at a steady rate during the off-peak periods would achieve the objective of the power station. During AM and PM peak periods, the electric transportation system could operate on its stored energy and be disconnected from the main power grid.
An electric subway system, an electric suburban commuter train system, an electric light rail system along with streetcars, trolleybuses and trams would remain fully operational should a major power outage occur on the main grid during a peak demand period. During such an event, hundreds of thousands of people would be able to travel on the electrically powered mass transportation system, including emergency personnel whose services may be needed at various locations along the route of that transportation system. There is a drawback to operating an electrically powered mass transportation system on stored energy during a power outage occurring during a peak period. It is likely that the demand for service would greatly exceed the supply.
There are a variety of proven and evolving technologies capable of storing sufficient amounts of electrical energy to allow for the peak period operation of a tramline, a light rail system, a subway system, a suburban electric rail system, as well as a fleet of trams, trolleybuses or streetcars. While mobile storage batteries are used to provide propulsive power for plug-in vehicles that operate along several low-frequency services, there is much potential in stationary energy storage technology. There is little scope of adapting much of the stationary technology to mobile operation.
Energy Storage Technologies:
Stationary Batteries:
Several types of stationary application electrochemical storage battery can be adapted to provide power for municipal mass transit applications. The flow battery system stores energy in a liquid electrolyte that flows through spaces between the plates of large batteries to provide electric power. While there may be scope to adapt a flow battery to mobile operation, the technology was developed around and is best suited for stationary applications.
Researchers in Japan have developed flow battery that uses a uranium-oxide based electrolyte that offers the highest storage density of all flow batteries. A bank of uranium-oxide flow batteries that is housed inside a building could literally provide several megawatt-hours of electric power, sufficient to power an electric light rail system during peak periods. The electrolyte of all flow batteries can be recharged using steady, constant input during which time the bank of batteries may be providing electric power for a transit line.
The molten sodium-sulfur battery is a competing design of stationary storage battery that still being developed and refined. It has to be housed inside an insulated building as it heats up to 300 C. When in operation, the largest molten sodium-sulfur batteries can store up to 250-megawatt-hours of electrical energy, enough to provide power to an electric transit system that would require up to 20MW of electric power for up to 8-hours every week day.
Like all batteries, the above examples will best provide electric power at steady output. The storage and supply system will have to include strategically placed banks of flywheels and ultra-capacitors to supply the sudden and brief demands for surges of electric power from electric vehicles under heavy acceleration. The flywheels and ultra-capacitors would also absorb surges of electrical energy caused by regenerative braking during deceleration. A computer management system could regulate the transfer of power from the stationary batteries into the flywheels and ultra-capacitors as well as allow for provision for energy from regenerative braking.
Compressed Air Storage:
Compressed air storage systems are being developed worldwide where suitable underground cavities exist in the earth's crust. Natural gas companies developed such storage technology to store compressed natural gas. Pillars of salt called salt domes can be found almost worldwide in the earth's crust. Salt domes have a dome roof and can measure up to 1500m in diameter by up to 10,000m high. They usually occur deep down at depths below 600m in hard, impervious rock.
It can take up to 3-years to flush the rock salt from a salt dome, after which it can be used to hold compressed air or natural gas under extreme pressure. Compressed air would be pumped into the cavity during the overnight peak period. During peak periods, air would flow to an intermediate tank that is kept at constant pressure and then through power turbines that drive electrical generation equipment. At some locations, there may be capacity to preheat the compressed air prior to expansion in the turbines. Much research is underway in high-temperature thermal energy storage, including a system that involves a mixture of thorium fluoride and thorium hydroxide contained inside finned cylinders made of silicon carbide.
A small salt dome that has been flushed of rock salt may be able to hold enough compressed air to drive air turbines and provide enough power to sustain the operation of an electrically powered mass transportation system during AM and PM peak periods. The air power system and its turbines would adjust more easily than a steam-based power system to the rapidly fluctuating power demands of such a transit system. During the overnight hours, the same design of piston compressors that pump natural gas are used to pump air into the underground cavity. Despite being able to easily adjust to rapidly fluctuating power demands, a compressed air based energy storage system will still require flywheels and ultra-capacitors to absorb the energy of regenerative braking.
Pumped Hydraulic Storage:
The latest word in pumped hydraulic storage is underground storage using a reservoir located some 600-metres below water surface. This alternative replaces an earlier method of building a reservoir in the valley of a mountain located next to a lake or ocean coast. Environmentalists oppose the destruction of plant life and animal habitat in all valleys with the possible exception of valleys in coastal deserts. There are many coastal cities around the world where electrically powered mass transit systems operate. Below is a partial list of such cities:
The ideal underground reservoir would be a small salt dome at a depth of some 600-metres and located in the strata of hard impermeable rock. Its proximity to the surface may disqualify it from being used for compressed air storage, courtesy of an incident in Western Canada where a small salt dome literally "blew its stack" while holding compressed natural gas. Hard, impermeable rock usually lies below the strata of porous rock of most cities. It would be ideal for an underground reservoir where seepage would be less than the evaporation rate from an above ground reservoir.
Engineers undertaking preliminary research work on future deep-level subway trains for New York City discovered the existence of hard, impermeable rock at those depths. If a suitable cavity is unavailable at a suitable location in the impermeable rock under a coastal city, a cavity may be excavated deep in that rock. The depth of the reservoir would generate extreme pressure and require minimal water volume flow rate to generate up to 1000MW for up to 8-hours. A reservoir of such capacity could sustain the peak period operation of an extensive electrically powered mass transportation system that would include subway trains as well as suburban electric trains.
Hydraulic and hydroelectric power generating systems are capable of responding to the rapidly fluctuating power demands of electric transit. While the stored energy system is in operation, energy from regenerative braking could activate one or more propellers that would push water uphill to the lake or ocean. The high water pressure at a depth of 600-metres may greatly reduce the onset of cavitation that could otherwise damage propellers. An alternative option would be to use banks of flywheels and ultra-capacitors to absorb the energy of regenerative braking.
On-site Power Generation
There have been numerous advances made in decentralized, small-scale power generation. Companies such as NuScale Power and Toshiba have developed mini-nuclear reactors of 10MW output that could operate in conjunction with energy storage technology that would be recharged during the overnight off-peak period. One research group in the USA is working on radiation-free boron-fusion technology, where a proton is fused into the chemical microstructure of boron that subsequently emits massive quantities of heat as the boron transforms into Beryllium.
There may be an economic case to be made for the off-grid operation of a large-scale, electrically powered mass transportation system. Every few years after the mini-nuclear power generation system expires, the manufacturer exchanges the spent reactor for a new or a refurbished reactor that has also been refueled to sustain it through several more years of operation. Several thermally based power technologies have been miniaturized and can deliver the energy efficiency and service life of their large-scale counterparts. In the future, several large-scale electrically powered mass transportation systems may operate their own power generation and energy storage technologies.
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