02.19.10Harry Valentine, Commentator/Energy Researcher,

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The official worldwide history of electrically powered public transportation began in the year 1890 when electric trams and electric streetcar replaced horse drawn trams and streetcars. Electrically powered subway trains subsequently replaced steam hauled subway trains in London, UK. The construction and introduction of New York City's electrically powered subway trains soon followed. Electrically powered trams, streetcars, suburban rail systems, light rail systems and subway trains plus electric trolleybuses provide essential services on a daily basis to millions of people worldwide.

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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Reader's Comments

Date	Comment
Barry Alexander 2.23.10	I wonder what the relative loss and cost is for each of the alternatives. And of course time based pricing is not yet in effect in every area of the country. Otherwise a very good article.
William Higdon 2.23.10	I don't think that too many people will want uranium oxide or molten sodium near their homes or workplaces. Some of these other technologies may be very expensive and objectionable due to environmental impact. There will be significant breakthroughs in the storage efficiency (i.e. low float loss) for kinetic energy storage systems in the near future. These devices will have no toxic chemicals or "moon rocks", and so be cost competitive and environmentally acceptable.
bill payne 2.23.10	1. 1 kWh = 3412.14163 BTU. 2. Second law of thermodyamics: In all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state. This is also known as the law of entropy. Google 'bloom box 60 minutes' for grins.
bill payne 2.23.10	"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." 1. 1 kWh = 3412.14163 BTU. 2. Second law of thermodyamics: In all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state. This is also known as the law of entropy. lol
Len Gould 2.24.10	Bill. It's wierdly not as hard a law as you claim, since the value of a unit of energy depends not only on its quantity but also its quality. eg. 1 kwh of potential heat as natural gas burned in a furnace might provide a house with .93 kwh of useable space heating (eg. stays within your law of entropy) but the same kwh of gas burned in a CC power station may provide .57 kwh of electricity which after 10% transmission losses may then be used to power a COP 4 heat pump which will provide (.57 - .057) x 4 = 2.05 kwh of useable space heating, which clearly defeats your law of entropy. It applies to a lot of other places as well, often not quite so dramatically but still. A matter of being smart about investigating all possibilities without limit or fear of contradiction, which Harry does well.
Len Gould 2.24.10	And I know, the law of entropy is not really defeated by the heat pump since it actually greatly increases the entropy of the surroundings where it pumps the heat up from. Still, not quite so simplistic.
Don Hirschberg 2.24.10	I suggest it’s much simpler and easier to understand a heat pump without invoking entropy. The working fluid (refrigerant) gets heat transferred from an environmental source thus increasing the working fluid’s ENTHALPY, and thereby decreasing the enthalpy of the environment by the exact same amount. (It’s he reverse of an air conditioner where heat is transferred to the environment.)
Jim Beyer 2.25.10	Len, Cool formula!! If you burned the methane locally with a Bloom Energy fuel cell at the same 57% efficiency, used 90% of the waste heat, and applied the full 0.57 to the COP 4 heat pump, you'd have: (.43)x(.9)+(.57)X4 = 2.667 kWh. I dunno if that's economically practical anytime soon, but I found this exercise interesting in that it points out the validity of both CHP and heat pumps. I guess most people know that, but that would argue for smaller generation units, not bigger ones.
Don Hirschberg 3.1.10	In 1899 Akron had a battery operated police wagon that carried 12 passengers. It could go 30 miles on a charge at 16 mph. But at that time it’s hard to imagine where they could find street surfaces smooth enough for that speed on solid tires - even using wheels that look to be maybe 54” in diameter. (Later Akron became the pneumatic tire capital.)
Len Gould 3.2.10	Jim, agree totally regarding small distributed units. That's clearly the path to the future for any use of natural gas for space heating since the latest generation of very efficient heat pumps and the arrival "any day now" of rationally economical micro-generation. That's what my IMEUC proposal was all about. Enabling an even and fair two-way market in electricity and fuels for every customer. I'm not sure yet that Bloom has the fuel cell technology needed since I've seen somewhere that their core stack lasts (I think) only a couple of years but they do appear to have defeated the platinum catalyst barrier which was holding up everyone else. If so, they've worked their way into a gold mine, though naturally all the big competition must now sit back and block them until their patents run out. Too bad.
Len Gould 3.2.10	"all the big competition must now sit back and block them until their patents run out." -- That last has got me thinking about some sort of solution The key problem is that the market provides incumbent producers of technologies no signal at all to invest in substitutes for eg. petroleum fuels, until the supply of the petroleum fuels has dwindled to such a critical level that the increased cost of it threatens to criple our economies. Current incumbents may choose to forcast shortages into the future and begin to prepare, but in doing so they are basically competing against themselves, while the real replacement technology developers don't often work well in such large bureaucratic structures. Asking nimble mice to dance ballet with elephants knowing that elephants fear and will kill mice at every opportunity. Present strategy to deal with the obvious market failure regarding introduction of new technologies is to subsidize the end-user purchase of the needed replacement technologies, but that still leaves the development burden in the bureau's of the large old-tech market incumbents who are presently collecting the market's provided benefits for solving the problem (eg. heating homes and providing them electricity), clearly a conflict of interest. Most often, the needed new technologies are developed by small independents whose primary aim is to patent and license the new developements at maximum gain to incumbents. (Bloom FC, EEstor capacitors, etc. etc.) Perhaps its time society considered switching from subsidizing the output end user, which is mostly far too late to benefit the original developers, to offering / negotiating to flat out purchase the patent rights to essential and proven new developments and then making the technology available "on some basis" to organizations capable of rapidly implementing it. Perhaps "giving" rights to two or three domestic manufacturers who already have supply chains set up in the business? Not sure of that....
Jack Ellis 3.2.10	Storage is still pretty expensive. Vanadium flow batteries run around $3500/kW and sodium sulfur around $1500/kW. Absent some form of subsidy, huge energy price differentials are required to make storage cost-effective. A sodium sulfur battery that can provide 6 hours of discharge capacity requires something on the order of a 7 cent per kWh differential on average to recover it's capital costs, and that's without considering losses or maintenance. 7 cent differentials are rare events in any North American wholesale market, let alone differentials of that magnitude on a consistent basis. I would encourage everyone to take a careful look at the Bloom box. From my rough calculations, there's less than meets the eye unless they can drive down the capital cost significantly. Apparently early versions are selling for $7,000/kW before rebates and subsidies. After the subsidies, the cost of delivered power with $1/therm natural gas (retail price) is around 11 cents, not including the cost of maintenance and stack replacement. that's cheaper than grid power in places like California and Hawaii, so the devices may be attractive to end users if they can avoid wires and public benefit charges. At full price, however, the busbar cost is closer to 16 cents. Point of fact - fuel cells have efficiencies comparable to state-of-the-art combined cycle plants and cost 3-6 times as much per kW of installed capacity.
Len Gould 3.2.10	Agreed Jack. Present stated prices per kw for the Bloom FC are way off the charts for rational economics. Only reason I give them a second glance is because they "may have" overcome the platinum catalyst problem, and if so, taken away the hard floor on future price reductions due to further development.
Jim Beyer 3.3.10	I also agree with you Jack. Bloom deserves some credit for making a 100 kW fuel cell (in any fashion) that doesn't have a huge pool of molten electrolyte. But costs are too high. Modifying Len's formula to something reasonable (Capstone turbine at 30%) we'd get: (.7)*(.9) + (.3)*(4) = 1.83 kWh. Interesting. Better off with a heat pump than local fuel burning. At least for the time being. How much are COP 4 heat pumps?
Jeff Presley 3.4.10	Harry, a small correction is in order on your 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. under the Pumped Hydraulic Storage heading. Adding water to a salt dome makes salt water and eliminates said dome. In fact slurry water methods are one of the ways to mine salt here's a site that talks about it In fact I'd like to hear more about how an underground reservoir of any kind can generate power. Is the plan to have a generator underneath that reservoir, or are we doing something to gravity twice? Hydro-power works for us because of head pressure, which is provided free of charge by gravity. Here's a picture of the normal method, yours I can't quite visualize, perhaps you have a link?
Don Hirschberg 3.5.10	My second generation heat pump (with air exchange) will heat my house down to about 20 F ambient, where the I^2 R chimes in. The resistance heating contribution COP is of course 1.0. At 20 F the COP of my heat pump might be 2 and decreasing with every drop in temperature below 20 F. We sometimes have sub-zero temps. I am on a lake so it would be possible (at great initial expense) to exchange with lake water (never below 32 F) rather than ambient air and increase my COP, but only during quite cold weather. I see where COP’s of 4 are cited above. Sure, I get great COP when the ambient temps are 40 F and then the load is small so who cares, but I wonder under what circumstances can one realize a COP of 4 year to year?
Len Gould 3.7.10	Don: In checking for above comment I found numerous manufacturers (often in europe or asia) claiming COP's up to 6.0. I didn't do enough investigating to determine test conditions, as am not presently planning a purchase. That said, I suspect you're correct, that such high COP's depend on stable thermal source reservoirs at temperatures well above cold northern winter air. My impression is the technology has been improving quite rapidly lately. The following article from Enercan - Govt. Canada provides a useful graph of typical COP vs. outdoor temp ranges in Figure 5. Their recommendation is still to restrict the installs to southern Canada, and size it only slightly larger than A/C requirements, due to high up-front costs. "At 10°C, the coefficient of performance (COP) of air-source heat pumps is typically about 3.3. This means that 3.3 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. At –8.3°C, the COP is typically 2.3. The COP decreases with temperature because it is more difficult to extract heat from cooler air. Figure 6 shows how the COP is affected by cooler air temperature. Note, however, that the heat pump compares favourably with electric resistance heating (COP of 1.0) even when the temperature falls to –15°C. "
Len Gould 3.8.10	An obvious benefit in A/C dominated climates would be to install an air source heat pump unit sized for peak A/C load, and use the hot side for both domestic water heating (readily available configuration) and for swimming pool and hot tub heating (minor hot water circulation and HX install).
Malcolm Rawlingson 3.11.10	A thought provoking article Harry as always but I don't envisage any of these ideas being adopted. Most transit systems can hardly afford to replace their aging fleets of transit vehicles let alone invest billions in storage technologies. Electricity would need to be extremely expensive to make this worthwhile and it isn't. Also most electricity grid systems have high levels of reliability and it is rare for there to be a complete blackouts although they have occurred as we all know. Transit operators must balance the probability of a rare event against the high cost of adopting your proposals to over come it. My guess is that they would rather use the cash to lower fares and increase ridership than buy batteries to store electricity. Also I think you're getting confused between the purpose of natural has storage and the purpose of compressed air storage. In the case of natural gas the compression of the gas allows larger volumes to be stored in a given underground cavity. The purpose is not to store the energy of compression but to store as much of the chemical energy of the methane as you can in the smallest space. Compression of gases releases heat which is not recoverable and makes the compression process as a means for storing energy very inefficient. - the reason you have an intercooler on a 2 stage air compressor is to get rid of this heat. It is not recoverable and is lost to the surroundings. The chemical energy of the natural gas IS recoverable of course (by combustion with oxygen). Using a straight compressed air system would be very inefficient, costly to build and costly to operate. I cannot envisage any transit operator putting money into a system like that. In Toronto the transit commission is having a hard time holding down fare increases so the notion of them spending millions and millions of dollars on electric energy storage is almost to the point of comedy. Good ideas but not very practical. Malcolm.

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