“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, September 26, 2015

Rocks on rails

Image credit: Advanced Rail Energy Storage North America
In an earlier post I covered a concept of utilizing a massive rock piston over pressurized water to store energy. Another firm uses a concept analogous to pumped hydro storage but, rather than massive amounts of water and pumps and turbines, they use large solid masses and motor/generators. That firm is ARES, an acronym for Advanced Rail Energy Storage.

First, the headline numbers: ARES claims that their technology allows storage facilities of from 200 MWh of energy that can be delivered at a rate of 100 MW (i.e., it can run at full power for two hours) to 16-24 GWh that can be delivered at a rate of 2-3 GW (i.e., it can run at full power for eight hours). It's claimed to have a round trip efficiency of 80% (or 85%, depending on which interviewee you're listening to). The claimed ramp-up time is on the order of 8 seconds, dramatically better than any fossil fuel plant or pumped or stored hyrdro system, the only storage system to better that number is electrochemical (battery) storage. Finally, ARES says that the cost of an Advanced Rail Energy Storage facility is about 60% of that of an equivalent pumped hydro installation. All this sounds pretty good.

OK, what actually happens? During times of plentiful generation by intermittent generators or of low electrical prices if arbitrage is the name of the game, rail cars full of rocks are transported by rail up inclines via axle mounted motor generators on the cars. Unfortunately, their technical page has scant information regarding the specifics of the system, that information must be gleaned from other articles.

Nevertheless, we can see that ARES envisions three classes of system:
  • Ancillary services: The system is used as a Limited Energy Storage Resource (LESR) for frequency stabilization, spinning reserves, VAR (volt ampere reactive) support, etc.
  • Intermediate scale: The system is used for ancillary services as above, as well as for short duration storage to facilitate intermittent generation integration. Such a system is envisioned as capable of delivering 50 to 200 MW and having a two hour capacity.
  • Grid scale storage as described above, with 200 MW to 3GW delivery for up to 16 hours.
While the system cannot compete with pumped hydro for systems requiring days of storage, it is far less complex to construct and appropriate siting is dramatically easier to locate, and should be far easier to shepherd through the myriad review and permitting processes. And many systems don't require several days of storage. William Peitzke, ARES Founder and Director of Technology Development is quoted as saying "Generally, the market for storage tends to be an 8 hour requirement and in fact a lot of the utilities we talk with really only require five to six hours of discharge.”
Image credit: ARES

The cars carry a mass consisting of concrete and rock, and utilize electric traction motors to lift the masses up inclines. The same motors then act as generators when descending. Complex, automated control systems enable quick adjustments to suit system requirements, and the system can have some cars ascending while others descend. Scale can be increased simply by adding more mass. Energy is received and delivered via electrified rails. The cars themselves are modified ore cars. ARES holds patents on the system, but the individual components and systems are mature technologies with no technological breakthroughs needed.

ARES has constructed a pilot system in Tehachapi at about 1:3.75 scale (see photo at right) but, according to various reports, in Pahrump, Nevada, the Valley Electric Association has agreed to work with ARES to implement a 50 MW system. The projected cost is $40M. The objective is actually to accomplish frequency stabilization for the California ISO (Independent System Operator, known as "Cal-ISO"). The planned system would use 34 cars on a 9.2 km track with approximately a 7% incline. The difference in elevation between the top and bottom will be approximately 640 meters. Each shuttle will transport a mass of 230 tons (209 tonnes). A quick calculation [(34 cars)*(209 tonnes)*(1000 kg/ton)*(9.8 m/s)*(640 meters)*(80%)/(3.6*10^9 joules/MWh)] shows that this system may be able to store and deliver a maximum of just under 10 MWh. However, this is an "Ancillary Services" installation and thus not designed for primary purpose of storage per se, but rather for the regulation goals mentioned above. Unfortunately, I'm not able to find recent information on progress to date. The Valley Electric Association web site is silent on ARES with the exception of a pdf magazine from October of 2014.
Image credit: www.gearedsteam.com

I'd not go so far as to say that rail energy storage is the silver bullet for solving the integration of intermittent renewables into the grid, but it certainly seems to have significant benefits and few drawbacks, assuming that it hasn't jumped the track.

Update: A great set of photos of the pilot project in Tehachapi can be found at gizmag.


Gabriel Grosskopf said...

This sounds like a great idea. It beats pumped storage with higher density than water, no evaporation or containment issues, etc. And the system scales very well, simply add more rail cars.

Do you have an idea what the ideal gradient is? Are steep cog rails better or worse than smooth rails?

Rob Ryan said...

The one in Nevada is 7% (average) but a spokesperson for ARES says that the cars could climb a 25% grade "without breaking a sweat." I have my doubts about that. I would assume that cog rail would have significantly greater dissipative losses but I could be wrong. I don't have a good handle on the efficiency of a gear train under that type of load. I'd have to do lots of research to refine my opinion on that.