Energy Storage News

Energy Storage in Elevated Weights

I have written previously about pumped hydro storage which uses the gravitational potential energy difference between two reservoirs at different altitudes to store energy. The same concept of gravitational potential energy as a storage medium could be used with other substances than water. A while back I ran across the web site of a company called Mechanical Electric Inc. which is proposing to build this type of energy storage system. Below are some extracts taken from their web site:

The MESA -- Mechanical Electric Storage Appliance -- is a device that stores electricity as mechanical potential energy. When electricity is needed, the mechanical potential energy is used to drive a generator to deliver the stored energy as electricity.

The MESA thus provides a mechanism for storing excess electrical power when there is no direct need for it, and for later use of the electricity when it is needed. By storing the electrical power as mechanical potential energy, we facilitate long-term storage with little or no loss over time. The MESA is therefore particularly well-suited for use with intermittent energy sources, such as solar power or wind, or for situations where electricity rates vary according to instantaneous demand. The MESA is also well-suited to storage of electricity for use in emergencies or disasters.

The MESA shows quite well against conventional storage technologies. Like hydroelectric pumped power plants, the MESA 'lifts material up' and 'lets it fall' to generate electricity, but we use a material that is 10x as dense as water, and so consumes substantially less space. Hydro plants can cost billions of dollars to construct, consume millions of acres, take years to build; the MESA is cheap to build, requires little maintenance, stores energy indefinitely without loss, and is easily scalable. Lastly, unlike hydro plants, which require special sites with significant elevation changes (which are rapidly diminishing), and are generally located far from demand points, the MESA can be placed close to demand, and requires no special siting at all.

Like batteries, the MESA scales well and is highly efficient. Unlike batteries, the MESA is inexpensive at scale; batteries are among the most expensive technologies to deploy at scale for utility applications. Further, the MESA has no toxics issues, and requires no special manufacturing or disposal process. MESA is cheap to build, run, maintain, and decommission.

It cannot be denied that this form of energy storage is conceptually simple and is likely to have high efficiency and an extremely long cycle life. Nevertheless I find Mechanical Electric's claims of 'cheapness' hard to believe.

Let us consider the case of utilizing the interior space of an existing building for such an energy storage system. Maximum energy storage is obtained if some portion of the upper half of the building is completely filled with storage weights. The total fall in this case will be half the building height = ½H. The energy stored (in Wh) per kilogram of mass would be:

EM = (1kg×9.8m/sec2×0.5H×Efficiency)/(3600Joules/Wh) = 1.36×10-3×Efficiency×H

If the storage system had a round trip efficiency of 85% and was installed in a 200 meter tall building, then the energy storage per kg would be 0.23Wh/kg. Over 5500 cycles (at one cycle per day this number of repetitions represents 15 years of use) 1kg of weight would deliver 1.27kWh of electricity. Ignoring for the moment the value of the interior space occupied by the storage system and simply considering the costs of construction we see that the total installed system cost must be under US $0.13/kg of suspended weight in order for the cost of storage to be under US $0.10/kWh. Considering that $0.13/kg is in the range of scrap iron prices I consider these costs a challenge to meet.

Admittedly the cycle life of such a system might be considerable greater than 5000. However, a very long investment horizon would be required to realize the lower costs associated with a very long lifetime. I personally think that our society needs to greatly expands its time horizons with respect to infrastructure investment, but I am not holding my breathe waiting for this change of perspective to occur.

In addition to construction costs for the storage apparatus we must also consider the value of interior space in tall urban buildings.

If we assume a floor spacing of 4.5m then each cubic meter occupied by the storage system takes up (1/4.5)=0.22m2 or 2.4ft2 of floor space. Again I will consider the case of a 200 meter tall building and a system efficiency of 85%. In addition I will assume that the density of the suspended mass is ten times that of water or 10,000kg/m3. Under these assumption the energy delivered by 1 cubic meter of storage over 30 cycles (1 months' worth of use) is given by:

Emonth=10,000kg×9.8m/s22×100m×0.85×30/(3.6×106Joules/kWh) = 69.4kWh

If we divide 69.4kWh by 2.4ft2 of floor space we find that 28.9kWh are delivered each year for each square foot of floor space sacrificed.

How valuable is floor space in urban centers? A quick Google survey reveals skyscraper rents in major urban centers in the range of US $20 to $80. If you saved US $0.10 per kWh as a result of having a storage system in your building your monthly savings per square foot of floor space sacrificed would be US $2.89. This does not seem like a winning proposition.

Once could try to get around high urban rents by building purpose built storage structures on cheaper real estate. Some aspects of such construction would be simpler and cheaper than building structures for human habitation. However, the total load on a structure the upper half of which is entirely fill with suspended mass which has ten times the density of water would be quite high. I have trouble believing that such construction would be cheap.

An alternative to building up is to dig down. That is one could suspend weights above a shaft dug into the earth. Although I do not claim that excavation is cheap I think that digging a thousand foot hole is likely to be less expensive than building a thousand foot tall free standing structure. In the calculation I posted above I assumed a 200 meter tall building. Deeper holes might offer better economics since they would give more energy storage per kilogram of suspended mass.

I came up with this idea independently, but I have discovered that I am not the first to have conceived of such a storage scheme. I recently discover a company that is proposing a similar energy storage scheme, although their proposal is a hybrid between mechanical storage with rigid weights and pumped hydro storage. I will publish a follow up post on their energy storage scheme at a later date.

August 2, 2010

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rogerkb [at] energystoragenews [dot] com