I have written previously about Mechanical Electric Inc.'s plans of developing an energy storage system which employs elevated weights as the energy storage medium. For reasons which I gave in the previous posting I do not think that this system is likely to be economically competitive (Just as an aside I note that the Mechanical Electric web site has vanished, so possibly their plans are no longer extant). Another possibility for such mechanical energy storage is to dig a deep shaft in the ground in which a large weight can move up and down. Excavating such a shaft is likely to be cheaper than erecting a very tall free standing structure. A company call LaunchPoint Technologies Inc. has recently spun off a subsidiary called Gravity Power LLC which is planning to develop such underground energy storage facilities. Their patent portfolio includes purely mechanical system with weights and cables, but the technology they are pushing is a hybrid system employing a large solid weight and water in a new form of underground pumped hydro storage(I have discussed underground pumped hydro previously here).

Their web site contains relatively scant information:

Gravity Power, LLC, a spin-off of LaunchPoint Technologies, Inc., is developing a revolutionary grid-scale electricity storage system. The company’s new Gravity Power Module (GPM) exploits the established principles of pumped storage hydropower, but extends the concept in a new direction: Down.

Pumped storage hydropower (PSH) is the only large-scale electricity storage technology widely used today, with over 120,000 megawatts of capacity worldwide. However, a new PSH installation typically takes 11-15 years to develop and an investment exceeding one billion dollars before a single watt of power is produced. It also has severe siting limitations due to the need for two large reservoirs at different elevations and the resulting environmental disruption.

In contrast, the Gravity Power Module can be quickly installed underground with virtually no environmental impact. The modular, closed system has a very small footprint that can be sited almost anywhere electricity storage is needed. With a combination of proven technologies and ground-breaking system architecture, the GPM has better operating characteristics and economics than conventional pumped storage, compressed air energy storage, or batteries.

A more extensive description of their proposed technology was given in an article published International Water Power and Dam Construction Magazine in March of 2010. I summarize some salient features from this article below.

The proposed storage system with water and a large piston made out of concrete will be completely enclosed and sealed. The schematic diagram given below (this is my own drawing) shows the system in a fully charged state, a half discharged state and a full discharged state. The top of the large cylinder should be imagined as being level with the surface of the earth.

The circle with the X in it is indicates the hydro electric turbine. When the weight descends water is forced through the turbine and electricity is generated. When electricity is used to run the turbine in reverse water is pumped into the excavated shaft below the piston and the piston is raised. I have depicted the concrete weight or piston as filling half the shaft volume. It can easily be demonstrated that having the piston fill half the shaft results in maximum energy storage per unit of excavated volume. The potential energy difference between the fully charged and the full discharged state can easily be calculated. If M is the mass of the concrete piston, h is the height the shaft and r is the ratio of the density of the piston to the density of water the potential energy difference is given by:

ΔU = ½Mgh(r-1)/r

This formula can be used to determine several things about this storage scheme. One interesting question is how much water is required relative to the more traditional (although nonexistent in a real world application) underground pumped hydro scheme. Let us compare a traditional pumped hydro storage system with a piston and water system under the supposition that same excavation depth is used for each system. I will suppose that the top of the underground reservoir for the traditional pumped hydro scheme is ¼h above bottom of the reservoir which is at depth h. In this case the water from the surface reservoir is elevated a distance ^{7}⁄_{8}h above the lower reservoir, and the potential energy difference is given by:

ΔU_{T} = ^{7}⁄_{8}mgh

Where m is the mass of the water used for storage. If we set the potential energy difference for the two storage schemes equal to each other we find:

^{7}⁄_{8}mgh = ½Mgh(1-r)/r

which implies that:

rm/M = v/V = ^{4}⁄_{7}(r-1)

Where v is the volume of water in the traditional pumped hydro system and V is the volume of the piston in the hybrid system. Concrete is 2.4 times as dense as water which implies the v/V=0.8; Therefore the volume of water required for the traditional pumped storage system is less than for the concrete piston hybrid system. Furthermore the total volume of material which has to excavated is less by more than a factor of two since for the traditional system you only have to excavate space for the water and not for the concrete piston.

So what advantages might a hybrid piston system have if it does not requires less water or less excavation? Here are some possibilities:

- Negligible water loss due to evaporation.
- Negligible water loss due to leakage into the water table.
- Can be excavated in soft rock since there is no concern about water leakage.
- The hydro turbine is at the surface where it will be easy to repair and maintain.

If we divide the potential energy difference for the piston system and multiply by the round trip efficiency η and by the number of cycles N we get the energy delivered per unit of mass:

E/kg = ½ηNgh(r-1)/r

I will assume that the round trip efficiency is 0.8 and the number of cycles is 5000 cycles (This corresponds to about a 14 year lifetime at 1 cycle per day). Gravity Power is planning to build its initial commercial installations at a depth of 500 meters. Plugging in these numbers and converting to kWh I find Energy/kg = 1.6kWh/kg. Concrete costs about US $70 per cubic yard which works out to US $0.038/kg. Therefore over 5000 cycles of the system the contribution of the concrete to electricity cost is US $0.024 per kW/h.Of course you have to add to this the cost of the extra electricity which has to be generated because the storage efficiency is less than 100%. If the base generation cost is B the efficiency costs is given by:

Efficiency Cost = B(η-1)/η

So, for example if the cost of base generation is US $0.06 and η=0.8 then the efficiency cost would be US $0.015 bringing the total up to $0.039/kWh plus interest if you borrow to buy the concrete.

Of course the system will have other costs besides the cost of concrete such as excavation costs, the cost of building the water proof container and the cost of the hydro turbines. I suspect excavation costs may dominate. In the Water Power article the company heads admits as much and says that finding sites with deep layers of soft rock such a limestone, chalk, or gypsum is the key to a reasonable cost structure.

In my previous posting on traditional underground pumped hydro I went through a calculation to determine how much water would be required (assuming a head of 600 meters) to store one day's worth of electricity for Santa Clara County California. I concluded that it would require an amount of water equal to 14% of the county's total reservoir capacity. For the gravity power system a somewhat larger amount of water would be required. Of course since the system is sealed the amount of new water required every year might be relatively small. However, in a relatively dry place like the Bay Area there might be some difficulty getting the system loaded up with water in the first place. If you want to store a week's worth of electricity then of course the amount of water required would increase proportionally. Digging deeper shafts is another way to lower the water requirements since the storage energy is proportional to the depth of the shaft.

I do not think that Gravity Power's system is going to provide dirt cheap storage. Like all utility scale power storage systems it will be difficult to get systems up and running because of a chicken and egg problem. The system will have high up front costs that will be recovered only after quite a few years of reliable operation. Power providers will be reluctant to incur these costs with proof that the system will work reliably, and proof that the system works requires that somebody puts up the bucks to build them. Government intervention may be required to jump start this kind of storage system.

October 14, 2010

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