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Comment by Osiris30 | Hacker News Reader

Correct. But in large enough grid there are many non-fossil ways available using existing, deployable tech to integrate a high-penetration of non-dispatchable renewables.

For example - utility scale storage (pumped hydro); distributed storage (home batteries and Vehicle-to-grid battery Ev cars); demand-side management; power-to-gas (hydrogen); and geographic grid integration.

See more at

http://www.nrel.gov/analysis/re_futures/

https://www.raponline.org/featured-work/teach-the-duck-to-fl...

http://cleantechnica.com/2016/03/02/base-load-power-is-a-myt...

http://cleantechnica.com/2014/08/08/rmi-blows-lid-baseload-p...

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But stored solar energy no longer undercuts coal for most of these storage technologies.

Pumped hydro is probably the best bet, but is very geography dependent. Adding transmission is a non-trivial investment.

> Pumped hydro is probably the best bet, but is very geography dependent.

You could build surface level reservoirs as two tank hydroelectric dams, and then use the water as an energy reserve as well as freshwater reserve.

Rail storage is the cheaper, more versatile version of pumped hydro.

https://www.greentechmedia.com/articles/read/First-Grid-Scal...

https://news.ycombinator.com/item?id=11595195

The physics don't make sense for that one. Simple back of the envelope example: How much mass does a train need and/or how far does it need to fall to provide the same kinetic energy as a lake?

We have a pumped hydro solution where I live. It's huge and only supplements a small power plant.

Why doesnt your back of the envelope calculation (did you do it?) match ARES analysis?

Could they perhaps achieve high enough mass in a small volume by using eg. lead?

Could it be that a train is more efficient at converting potential energy than a water turbine?

Could it be that they have a longer drop?

You gotta show your assumptions if you're arguing through back of the envelope calculation.

Did they provide any calculations?

> the same kinetic energy as a lake

Not all locations have a lake (or 2) conveniently located near a large elevation drop.

The article says "They move up and down an 8-degree slope with an elevation change of about 3,000 feet", that's a lot of energy.

No it isn't. A Canadian bulk train 3km long and massing 20 000 tonne at an elevation of 1000m has a potential energy of 20M kg * 9.8 m/s^2 * 1000 m = 196GJ = 54MW-hour. In the winter my house uses about 50kW-hour per day so that will keep about a thousand such houses running for one day. (130m^2, 1950's house, Norway). yes I know my house is not very energy efficient.

A reservoir 10m deep and 45m square at the same elevation has the same potential energy. If you can find a convenient 1000m plateau I think it would probably be cheaper to build a reservoir on top than to build the tracks and trains.

That's just over four miles of track (21,346 ft.) for the run length of the railcars down that slope. (Thanks, trigonometry!)

Now I wonder how fast these railcars move.

http://www.utilitydive.com/news/first-of-its-kind-rail-energ...

"One of the striking things about the ARES project is its size: 50 MW of power capacity and 12.5 MWh of energy. That would be large for a battery storage project, but for ARES, it is on the low side. “Fifty megawatts doesn’t get us to economies of scale,” CEO James Kelly said. “We are more efficient as we get larger.”

Kelly said ARES projects can be sized anywhere from 50 MW to 1 GW. “If we had a 500 MW project, we could double the capacity and it would only increase capital costs by 20%,” he said."

Interesting. Thanks for the link.

Until they build one, it's only potentially cheaper.

This is one of the most insane yet brilliant ideas I have heard of. I love it.

I wonder how hard it is to scale though.

True - but it is also following the same cost reduction curve as Solar.

See this for a comprehensive annual survey by Lazard on storage technologies and costs

http://blog.rmi.org/blog_2016_01_21_how_much_does_storage_re...

Not sure what you mean by "cost reduction curve". Can you expand?

Every doubling of production tends to result in a percentage decrease in cost. I see numbers for batteries and other technologies of 15-25%

>Adding transmission is a non-trivial investment.

Non-trivial in terms of size and politics, but an extremely solid one that will reduce the cost of electricity.

Molten Salts seems to be the way many solar plants are going (concentrated Solar ones versus Photo voltaic).

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