Energy Storage on a Mountain Side

I have written before here and here about energy storage in the form of the gravitational potential energy of a solid mass. A now defucnt startup called Mechanical Electric Inc. proposed to store energy in suspended weights in shafts in commercial buildings. I am not surprised the the company is defunct since as I pointed out in my previous posting the low energy density of this storage system and the high value of the interior space of buildings constructed for human use made such a system economically impractical.

A second company called Gravity Power has proposed an alternative system in which a solid mass would be suspended inside a shaft dug deep into the ground. Below the mass would be water which would be pumped through a reversible turbine when the weight descended to the bottom of the shaft, thus producing electricity. In the electricity storage mode the turbine would be reversed and the mass would be hydraulically lifted back to the top of the shaft. Gravity Power is still in existence although they have not yet broken ground on their first demo project. I think that this design for energy storage in a solid mass is more practical than Mechanical Electric's design but has yet to be proved in terms of capital cost and long term reliability.

A third possiblity for gravitational energy storage in a solid mass is the the use existing elevations in the earth's surface in the form of mountains and hills. At first though one might think that a vertical cliff would be necessary for such a storage system thus limiting the number of possible locations and presenting significant construction problems. However, one does not need a vertical cliff for such a system. One merely needs an extended slope and a low friction means of moving a mass up and down the slope.

Two energy storage companies, ARES (Advanced Rail Energy Storage) North America and Energy Cache, are developing storage systems based on this principle.

ARES proposes to store energy using a a reversible electric generator which will act as train engine in the storage mode, pulling a string of loaded cars up a mountain side. When electrical generation is required the string of loaded cars will roll down the mountain turning the generator/engine and generating electricity.

This energy storage concept is so simple that there is no question that it will work over many cycles with high reliability. It's economic practicality is less certain and obviously depends on total cost, including capital costs, operation costs, land use costs, and efficiency cost. The efficency cost is given by:

Efficiency Cost = [(1-e)/e]base generation cost

where e is the round trip fractional energy efficiency. Sinced Ares claims that the round trip effciency is 78.3% the effciency cost is given by:

Efficiency cost = [(1-0.783)/0.783]BGC = 0.28BGC

So if the base generation cost was $0.20/KWh then the efficiency cost would be $0.056/KWh.

On their web site ARES mention the possiblity of loading and unloading the freight cars at either end of mountainside rail in order allow greater storage capacity with a given number of cars. However, they do not decribe the method that will be used to effect the loading and unloading with low parasitic energy loss.

The company Energy Cache is pursuing a different option for mountainside energy storage. In their system a series of scoops attached to a ski lift like system move up and down the mountain side. A each end of the transport system the scoop travels over and under a hopper. When passing over the hopper the scoops can depoisit graven into the receptacle and when passing under the hoper they can recieve a load of gravel from the receptical. In the energy storage mode the scoops receive gravel at the bottom of hill and deposit gravel the top of the hill. In the energy production mode this sequence is reversed. The gravel lift is powered by a reverisible generator/motor as is the train engine in the ARES system. You can view a video that will explain the operation of this system far better than my verbal explanation.

Again this system design is so simple that there is little doubt that it will work reliably for many cycles, though potentially this design could be more affected by windy conditions than would a rail based system. Cost, rather than technolgy risk, is the main issue in whether this kind of storage system will find practical applications.

One aspect of cost that can easily be calculated in that of the storage medium itself: gravel. Without searching very hard on the internet I found a number of businesses offering gravel at $25/ton=$0.027/Kg. Over 5000 cycles moving 1 kg of gravel up and down 100 vertical meters at 78% round trip efficiency would deliver a total energy of:

E = 50009.811000.78 = 382,200Joues = 1.06kWh

Therefore the gravel contributes 2.7 cents per kWhr to the cost. This cost would go down as total vertical distance increases. At 1000 meters of head (to use a hydropower term) the cost would go down to a negligible 0.27 cents per kWhr. Other system cost components are likely to be more important than the storage medium itself.

Both ARES and Energy Cache are quoting costs for mountainside storage as 40% less than pumped hydro which is the current leading energy storage system in the world. Ares seems to be targeting the American Southwest for its energy storage system citing the abundance of sunlight and mountains making for a good renewable energy / energy storage synergy. The fact that mountain side energy storage will not put demands on the water supply of this relatively arid region gives it an advantage over pumped hydro storage or even over solar thermal generation with molten salt storage, since solar thermal uses large amounts of cooling water for its steam cycle. Solar PV with mountainside storage seems to be target combination of generation/storage.

Even if the 40% lower cost than pumped hydro is realized, the uptake of this technology may be slow. Most pumped hydro is an add on to existing hydro facilites rather than purpose build energy storage. In general hydro energy is built only through goverment funding because of the high up front costs and the very long time period of return on investment which is not attractive to short term oriented private credit markets. Whether or not 40% lower costs are sufficient to bring this new storage technology into the time horizon of private investment remains to be seen. Also issues of land permitting may turn out to be non-trivial.

May 16, 2014

Energy Storage News

rogerkb [at] energystoragenews [dot] com