Is This Energy Storage Idea Dumber Than A Box Of Rocks?


Energy storage solutions come in many varieties. Some are very high tech, like grid storage batteries. Some are rooted in technology that would have been considered ancient way back in the 19th century. Yesterday, we introduced you to Electric Mountain in Wales. There, millions of gallons of water are pumped up uphill every day, then allowed to flow downhill again, turning hydroelectric turbines along the way.

ARES rail car energy storage system

Nevada is attracting lots of high tech investment these days. Tesla is building its Gigafactory in the desert outside of Reno. Faraday Future and Hyperloop One have both selected Las Vegas as their new home. Now a start-up company based in southern California wants to go back to the future by filling boxcars full of rocks. It proposes to run the train up a mountain near Pahrump, Nevada, then let it roll back downhill, generating electricity from regenerative braking.

The impetus for the idea is the demand by regulators, particularly in California, for more renewable energy on the electrical grid. California wants utilities to use 50% renewable energy by 2030, but how to store it until its needed? That’s where Advanced Rail Energy Storage comes into play.

Francesca Cava, ARES operations manager, says the company plans to have a demonstration model of its train full of rocks idea up and running by 2019. The 9,600 ton train will run along a 5.5 mile long track covering 106 acres of desolate nothingness in the middle of nowhere. Nevada has an abundance of land like that.

The ARES plan is not cheap. $55 million has been budgeted for the first phase, which will have a power capacity of 50 megawatts. “Fifty megawatts doesn’t get us to economies of scale,” ARES CEO James Kelly tells UtilityDive. “We are more efficient as we get larger.”

sisyphusThe rail line has to be capable of handling heavier than normal boxcars. Then there are all the electronics and transmission lines needed to make the project work. Andy Lubershane, a senior analyst with IHS Emerging Energy Research, says the setup is more expensive per kilowatt-hour “than almost anything else on the market today.” Ravi Manghani, a senior energy storage analyst at GTM Research, urges a wait and watch approach. “Any new technology has to go through some big hurdles,” he says.

Could moving thousands of tons of rocks up and down a mountain prove to be cheaper than buying grid storage batteries from Tesla Energy and other companies? That remains to be seen. But if the company has one iota of humor, it will include Sisyphus in its official logo.

Source: Wired  Graphics credit: ARES


About the Author

I have been a car nut since the days when Rob Walker and Henry N. Manney, III graced the pages of Road & Track. Today, I use my trusty Miata for TSD rallies and occasional track days at Lime Rock and Watkins Glen. If it moves on wheels, I'm interested in it. Please follow me on Google + and Twitter.
  • Seems like a solution that simply ignores all sense to me.

    I way prefer the Welsh hill idea.

    • Semper Gumby

      The Welsh hill idea is a really good one, but one advantage of this system is that it doesn’t require millions of gallons of water to work. This can work anywhere there is a good sized slope. Both systems operate on the same principle, converting electrical energy to kinetic energy and back again.

      • I dunno.

        It seems like there is a lot of engineering and a lot of potential for things to go wrong.

        What is the maximum gradient that it will work on, and the minimum too ?

        Scaling seems to be problematic.

        I would guess that anywhere there is hydro electric power you could increase the storage with little more than a pipe and a pump.

        Feels really rather scalable to me.

        • Bob_Wallace

          The slope of the rail is limited by engine wheel slippage. “For freight trains, gradients should be as gentle as possible, preferably below 1.5%.” – Wiki

          They’d be asking these trains to start on a slope, something that I think isn’t done with trains.

          Or perhaps they have no intention of stopping on the slope but either pulling all the way to the top of traveling on all the way to the bottom.

          • Calamity_Jean

            The video rather explicitly showed the cars stopping mid-slope. I suppose there could be a flat area there.

  • Ash45

    Would you really get more energy out of it than was put into it though? Seems to me like they’d probably have a better business case by using those rails for delivering products more efficiently than simply trucking it and winding through the state roads.

    Just loading it up with rocks seems kind of wasteful to me.

    • Steve Hanley

      No idea. You would think before someone spent $55 million on a trial program, they would crunch the numbers pretty carefully.

      The pumped hydro system in Wales is said to be 72% efficient, which is about double the efficiency of an internal combustion engine. I could not find any stats on this idea that would clarify its efficiency.

      • athbr

        You can double the capacity of the system by adding a siding at the top. It seems that efficiency is not the driving factor here.

    • LafayetteCoboll

      They can’t get more out than they put in. It’s an energy storage system. They just need to get out some economic percentage of the energy put into making the train go uphill, out again when it comes downhill.

    • Rick

      Would you get more energy out of a battery than was put into it though?

  • Ken

    This is my absolute favourite method of energy storage. It takes much less construction and land area compared to a hydro dam and reservoir. Electric motors/generators are essentially the same for trains and pumped storage. The train does much less environmental damage compared to the double reservoirs which have huge water level changes making both shorelines a wasteland. This can be deployed anywhere not just parts of the world with lots of water.

    So lower cost, lower negative impact, applicable anywhere, not impacted much by weather and most likely much higher storage efficiency.

    • athbr

      The added benefit is that large scale solar farms are best suited for deserts … which don’t have large bodies of water at their disposal.

      • Steve Hanley

        Good point. Put the storage near the supply. Brilliant!

        • athbr

          Wouldn’t that allow you to reuse the same grid infrastructure you build out for the plant? Otherwise, your grid will need different lines for day and night…and grid lines cannot just be shut down at a moment’s notice whenever the sun stops shining. This way, you can maintain the supply and just send a few rail cars (or release the water) to make up for the deficit at night and on cloudy days.

      • GregS

        However this system does require large elevation changes which limits the system a bit, unless you’re willing to build a mountain to run your train up and down

        • athbr

          Well, that would defeat the purpose! This would only work where there are natural grades available. Outside of the great plains, that is usually not that hard. I’m more of a biker than a driver and you really know what the grade is. I can tell you that over here in new england, we go uphill both ways!

    • Radical Ignorant

      Your points are valid. However there are two main questions unanswered: 1 – How efficient is that?
      2 – How costly is that?
      1. Someone here should know regenerative breaking efficiency. It’s basically 95% (electric motor efficiency) times regen efficiency. But my short search shown everything from 20 to 90% for regenative brake efficiency. My limited knowledge of physics tells me it theoritacly could go all the way to almost 100. So I don’t know but I believe it could be very efficient.
      2 – how much maintaince cost. Seems that rails needs more care than pumped hydro but that is just guess. However piece of railroad seems to be cheaper than huge engineering effort to build pumped hydro.

  • kevin mccune

    I live three ridges away from the largest pumped storage facility in the world,it was expensive to build and requires a bit of maintenance,but the owners claim it makes money hand over fist,I am a bit skeptical ,but that is what they claim. The water levels fluctuate so much between the upper and lower reservoirs that you cannot use those pools for recreation(they solved that problem by constructing a large rec pond below the outvert .)People say it generates electricity which is not really the case ,it stores the power in the upper reservoir and converts the stored kinetic energy back into electricity(it functions more as an accumulator,because the 6 large turbines can only use the water they pump up the mountain during off peak,like a huge storage battery ,I dont know what the supposed efficiency is) The 500 KV transmission line runs about a quarter of a mile from my house(now the concerned citizens wont let wind generators anywhere near this county)(currently they are battling a proposed Natural Gas line that is supposed to be partially routed in this county)

    • Steve Hanley

      Attitudes are so critical to the transition to renewables. How people can oppose wind turbines but be OK with oil trains that occasionally explode, leveling entire towns, is a great mystery to me.

      • kevin mccune

        They claimed it messed up the “viewshed” but some of these same people are alright with “security” lights and several acres of grass desert to mow with the John Deere riding mower ,plus the requisite SUV ,ATV and large 4×4 pickup and sometimes an airplane threw in the mix ,go figure.My old home county is steadily turning into a gated community and the well off want to keep it that way .

  • MattyBumpo

    Contrary to the claims, this system appears to be far more environmentally intrusive, as well as far more expensive, than pumped storage. If this demo’s 100 acre footprint for12.5 megawatt-hrs of storage is representative of larger scale designs, then it would require tearing up much more land than pumped hydro. A closed loop pumped hydro plant with 2500 mwh of storage might require 200 acres of reservoir area. Do the math on the construction footprint. Also consider the wear and tear; pumped storage plants last 70-100 years. How often do you reckon the whole rail infrastructure will need to be replaced with frequent cycling?

    • super390

      This isn’t a replacement for pumped storage; there’s just not enough places where pumped storage is enough. For instance, in Nevada the dams are already under pressure because of the regional drought and immediate electricity needs. Some places have old railroads in place, but in the US they’re rarely electrified. As long as the railroad is already there, it’s not using any more land. The problem is whether there’s still any other traffic on the rail line.

      • MattyBumpo

        There are enough places, particularly for closed-loop projects. Projects on the scale of 400-700+ megawatts. Near transmission. There are enough excellent pumped storage sites across the western interconnection region to take care of most of the transmission-side storage need. Batteries can handle the rest: distribution-side, short-term (15-30 minute) storage. The primary obstacle is not lack of sites. It’s development timeline and cost associated with that development.

        • Bob_Wallace

          It’s also that we don’t yet need more large scale storage. We’ve got plenty natural gas and coal to shut down first. It’s going to take a few years to build our way to needing a lot of storage.

          Batteries are likely to make big inroads in the short term. They seem to have become cost competitive with gas peakers for short term storage/grid smoothing.

          • MattyBumpo

            As a baseload technology, coal is not pumped storage’s competitor. In fact, historically, coal and pumped storage went hand in hand. Of course, if and when renewables penetration is high enough, then the renewables + pumped storage combo can replace the next assumed generation of baseload technology, which is combined cycle gas. Or, demonstrating that the pumped storage + renewables combo can be a part of the coal retirement plan, one doesn’t necessarily wait until the coal is retired. Keep in mind that since a pumped storage project takes at least 7-8 years to license and build, the horizon for beginning development of the best projects in the areas that have the greatest need begins today, not 2025.

  • Bob_Wallace

    Dumb? I’ll suggest that it could be a lot smarter.

    The slope of the rail is limited by engine wheel slippage. “For freight trains, gradients should be as gentle as possible, preferably below 1.5%.” – Wiki

    Wheel slippage is likely to be a lot more of a problem with an engine stopped part way up a sloped track.

    If they used stationary winches (generators when run in reverse) at the top of the rise then the slope could be very much greater, significantly shortening the track length. (Possible in the neighborhood of 25%, a 45 degree slope.)

    The amount of energy stored is limited by the amount of weight each engine can pull up the slope. Freight engines are not capable of starting a tow of all cars at the same time (large number of cars). The train is stopped with some ‘slop’ between each car so that the first car gets underway before the second is ‘grabbed’, the second gets underway before the third….

    The amount of weight (number of rock filled cars) on the slope at any one time could be limited to what the winch was able to start uphill in one clean jerk.

    If they used winches and created track space on which to store extra cars at the top and bottom of the slope they could store enough energy to operate for days, non-stop. As one car reaches the end of the sloped track move another into place.

    The cable system would probably work best as a continuous loop, similar to San Francisco trolley cables.

    • Volt Owner

      Why, you could even make the track a closed loop. Gentle grade up, steep grade down…

      • Bob_Wallace

        Could, but using a stationary engine rather than one on wheels makes a lot more sense to me. Much shorter track for the same amount of storage capacity. The ability to store much more energy.

    • Calamity_Jean

      “The amount of energy stored is limited by the amount of weight each engine can pull up the slope. Freight engines are not capable of starting a tow of all cars at the same time”

      I got the impression from the video that each car had its own motor-generator and operated independently.

      • Bob_Wallace

        If that’s the case then they are drastically overbuilding. One top of the hill winch could power hundreds of cars. Just run them up/down a very steep slope one at a time and park as many as desired extra cars at the top (energy waiting to be used) and at the bottom (opportunity for more energy to be stored).

        • Calamity_Jean

          I dunno, independently powered cars makes more sense to me. It would allow greater variability in energy uptake and release because any number of cars could be moving at once, from one to dozens, and the number of cars that were moving could be changed instantaneously and repeatedly.

          • Bob_Wallace

            You have the expense of putting a motor, drivetrain, and control system in each car. You have to electrify the tracks in order to supply the cars.
            Steel on steel is low friction. Can’t start a loaded car on a steep slope. That means more real estate and more track to achieve the same height.

            I don’t know how the longer tracks but more cars on it would work out. That’s math to be done….

  • kevin mccune

    There is a system engineered in Europe (I believe) that uses some kind of a giant piston and fluid displacement ,it actually seems rather viable considering its small footprint .

    • Bob_Wallace

      It’s a purposed system. I don’t think there’s been any attempt to build one.
      Pump-up hydro storage works. It’s worked for 100 years. In the US we’ve got thousands of existing dams that could be converted to PuHS. Abandoned rock quarries and open pit mines, even subsurface mines are usable. We’ve got 1,000 abandoned rock quarries on federal lands alone. And there are thousands of other places where PuHS could be built.

      And flow batteries may turn out to be cheaper than PuHS. Like PuHS flow batteries can serve for short term storage as well as store large amounts of energy for the ‘deep storage’ we’ll need during longer periods of low wind and solar. Just just larger tanks.

      • kevin mccune

        The problem is head,to make anything like this worthwhile you need enough “head ” to have a velocity of the water that gives a good generating capacity or you need a tremendous reservoir,pumped storage pretty well closes the bodies of water for recreation purposes(even though in some instances it can help serve for flood control)
        Of course there are the proposed super flywheels,compressed air systems and what have you .I am surprised there is not a hydrolysis system that produces hydrogen for combustion during peak demand (sounds fairly dangerous )
        The real earth saving deal would be to satisfy present and future demand with existing powerplants ,EVs of course would strain the credulbility of being able to do this.The technology exists for us to do much better with our electricity consumption.(not to just build more largely automated powerplants because this nation can afford too )

        • Bob_Wallace

          The thousands of dams I mentioned have adequate head. We’ve got about 80,000 existing dams in the US. About 2,500 of those dams are electricity producers. Based on a survey of dams on federal lands over 10% of the remaining ~77,500 should have adequate head and be reasonably close to transmission lines.

          Lots of head or lots of flow. Either works.

          Living in the west where reservoirs shrink over the summer fluctuating water levels is something one learns to live with. The largest issue, I would imagine, would be the need to reposition any floating marinas. But most PuHS sites are too small for that sort of infrastructure.

          BTW, never been below a producing dam when they release the turbines? One’s boat can rise fairly rapidly. They typically blow warning sirens.

          Flywheels don’t seem to be catching on. There are some online but I’m seeing no real excitement over installing more.

          Air compression (CAES) hasn’t caught on either. There’s the problem of inefficiency via heat loss during compression.

          Hydrogen is a very inefficient way to store energy. Very inefficient.

          Sorry, not existing power plants. Coal and gas must go. Nuclear is simply too expensive. Many of our reactors are old and aging out. We’re not seeing them being replaced with new nuclear. (Plants closing is running almost 2x that of plants being built.)

          EVs will not strain the system. Cars spend about 90% of the time (almost 22 hours a day) parked. They need to charge, on average, about three hours per day. That makes them an excellent dispatchable load. With smart charging EVs can charge on solar and wind input peaks and drop out when supplies are strained.

          EVs will make it easier to add more wind and solar to our grids, lowering our electricity costs.

          • kevin mccune

            I finally developed a “Strangelove ” attitude on the nukes after it was carefully explained to me what went on,it was incredible and the unwarranted hype was incredible also,frankly a lot of the publics fear stem from the early AECs enviromental testing of atomic bombs .sure its around a long time ,but the real bad stuff leaves rather quickly.The nukes need more respect because they can fill the gap till more renewables come onboard

          • MattyBumpo

            A couple of points, Bob. First, it’s not really “head or flow, either work.” Low head = high flow = big tunnels, bigger reservoirs, bigger powerhouses. Cost gets prohibitive. Head is a tremendous advantage. Fortunately, there are many high-head sites. Now, to your point about CAES and efficiency: too many people have been misled by the efficiency issue because they confound the electrical and the thermal efficiency dimensions of CAES. The fact is that the round-trip electricity-in to electricity-out efficiency (if you’re comparing apples and apples) of existing CAES technology is equivalent to that of pumped storage: 70-80%. It’s definitely the best long-duration storage option IF you have the geology. And there’s the rub: so few places have the right geology for CAES.

  • Steve Hanley

    This has turned into one of the best and most fascinating discussion I can remember here at Gas2. Thanks to all who have contributed. Lots of good ideas and sensible discussion.

  • med

    Great idea
    but there may be some changeling issue when discharging the energy,
    1- the speed control
    2- the acceleration
    3- the breaking energy vs the weather conditions
    mean- what if there will be a use of extra energy basically not breaking but make an extra distance inclined upper and self breaking
    4- size the the weight so that they can be placed on different place vi a vi of train gravity center, mean when pushing up, the weight could be placed on the upper side, but when going down the weight could be placed on the lower part of the train

    Note, the water gravity in existing dam, is still the best storage solution by any standards , because you can use the existing infrastructure but you add a pumping to be powered when the combination of electricity is in excess (negative power cost) and water level is down.

  • kevin mccune

    Hope that works.the website chopped part of it off, Try Bath County pumped storage project -Mtn .Grove ,VA

  • kevin mccune

    Hey Steve I forgot to mention this,the local electric co op was starting a buy into solar sharing project and the locals rejected it ,so instead of being on the old Elementary campus it was moved to a neighboring county,I somehow get the feeling that there is an above average population of Luddites where I reside,about the Pumped Storage facility,it probably couldnt be built today,despite the the temporary economic boom it afforded this poor rural county .

    • Steve Hanley

      The Luddites are increasing everywhere, Kevin. In my little corner of the world, there are two protest movements going on at the moment — one to ban a proposed new natural gas “peaker” plant and another to ban wind turbine project,

      I can see being against one or the other, but not both!

      • kevin mccune

        Where will we be ,when the sky is dark and the lights go out ? I grit my teeth when I hear the expression ” Clean Coal ” (ask the Folks on the Dan River about “clean coal ” .

        • Calamity_Jean

          Clean coal is even less real than the Tooth Fairy.

          • kevin mccune

            Have to agree.

  • MattyBumpo

    Yes, capturing the heat with recuperators (which I think is standard in the newer designs?) is ideal. But more generally: there is a generally agreed equation for electrical input, fuel input, and electrical output for today’s CAES technology:

    .7 kWh in (the range is usually between .67 and .74) + ~3,800-4,200 Btu natural gas
    = 1 kWh out

    This alone is sufficient for calculating the costs and benefits of CAES. Now, it seems generally agreed that calculating overall round-trip thermodynamic efficiency is not an accurate reflection of electrical round-trip efficiency. If one converted a “source-neutral” electrical input into joules (a fundamental unit of energy), and the above gas quantity into joules, then divide the electrical output in joules by the input, the resulting figure is in the range of 53-65% (an Arizona State University figure uses 64%). This is, however, an erroneous method for calculating electrical storage efficiency because in reality, joules of electrical energy are “worth more” than joules of natural gas energy that has yet to be converted into electrical energy. Specifically, one Btu is considered equivalent to 3,412 kWh based on 100% thermal-to-electric efficiency; the reality is that such efficiency is only in the range of 30% to 50% (simple cycle to combined cycle range).

    There is no universally applied method for calculating CAES round-trip electrical efficiency; it is
    dependent on context and method. The most common method that I’ve seen cited in the literature focuses on the equivalent electrical value of the natural gas component. Specifically, if the 4,000 Btu of natural gas were consumed in a conventional gas turbine with a heat rate of 9,000 Btu to produce electrical power, it would be worth the equivalent of .44 kWh. If we add this to the .7 kWh of renewable energy input, we get 1.14 kWh in per 1 kWh out, or an effective efficiency of 88%.

    Figures in the literature based on the latter methodology usually range from 66% to 82%. Some put it as high as 88%. Because the electrical input to compression is used on the other
    end to displace natural gas combustion and dramatically increase the efficiency (i.e., lower the heat rate) of a gas turbine, it is fair to transfer the credit for that efficiency gain to the electrical input, even in the absence of a simple efficiency formula. It is thus a valid approach if the context is the conservation of environmental credit associated with a renewable energy input.