Aug 18 2017

A New Option for Grid Storage

Last year I wrote about the various grid energy storage options, adding a newly proposed option – chilled air. I am now writing to add another new option to the list – concrete gravity trains. But first, let me review the background.

We need massive grid storage for two main reasons. The first is peak shaving. Energy demand peaks in the early evening, which means we need to have the capacity to meet all of peak demand, even though demand the rest of the day is lower. Peak power is less efficient and clean, because the more efficient energy is used preferentially for baseload production. We dip increasingly into the less efficient energy options when necessary to meet peak demand.

Grid storage could shift energy produced during low demand and then draw upon it during peak demand. Optimally enough grid storage could completely flatten power production, increasing overall efficiency.

The second reason for grid storage is the increasing use of renewable energy sources like wind and solar which are intermittent and not on-demand. This varies by season and location, but generally peak demand occurs just after sunset, when the sun is not shining. No matter how much solar and wind energy we produce, without grid storage we would need to maintain all of our peak capacity. You can encourage people to shift their energy use to times when energy is produced, but it’s hard to get everyone to do something.

The bottom line is that if we ever want to get to significant renewable energy production we will need significant grid storage. 

Grid Storage Options

What are the features that make for good grid storage options? The most important is probably efficiency, sometimes called round trip efficiency – how much energy is lost when converting generated electrical energy to its stored form and then back to usable electricity? Other features are also important: how many cycles can the system last, what is it made out of (does it use rare, expensive, or toxic material), are there any safety issues, how big and heavy is it, and are there any limitations as to location or terrain, are there any environmental issues?

Batteries – Many people first think of batteries as an option for grid storage. They are potentially a decent option. Their efficiency is 60-70%, which is not the best but is good enough. Their capacity is relatively low, however, and their main limitation is charge-discharge cycles. Use of rare or toxic materials is also an issue. For grid storage options size and weight are not an issue.

Pumped Hydro – This is currently the most efficient option in widespread use, at around 80% efficiency. Essentially hydroelectric plants will pump water from lower to higher elevation during times of excess energy production and then use that water to generate electricity during high demand. This is the best option for locations that have good water reservoirs and terrain that allows for significant elevation differences. This is not an option for flat or very hot and dry areas (evaporation is a source of inefficiency).

Compressed air – Use energy to compress air in a chamber or tank, and then decompress the air when needed to turn a turbine and generate electricity. The big disadvantage of these systems is their low efficiency, around 45%. This is a deal-killer, in my opinion, despite the fact that other features are good. There is no limit of cycles and no special materials are needed.

Flywheel energy storage – Use energy to spin up a flywheel to high rpms (like 50,000), then use the kinetic energy to generate electricity when needed. Systems using mechanical bearings have a 50% efficiency, which is not great. Optimal systems using magnetic bearings in a vacuum claim 85% round trip efficiency, higher than pumped hydro. Efficient systems require use of material with high tensile strength to get up to high rpms. They get more inefficient over time, as the wheel slows down and loses stored energy. They can be charged and discharged quickly but overall capacity is relatively low. In general it seems these systems are useful on a relatively small scale but it will be difficult to use them for massive storage.

Hydrogen – You can store energy in hydrogen by using energy to split water into hydrogen and oxygen, then getting the energy back by burning hydrogen with oxygen. This is basically a hydrogen fuel cell. The big disadvantage for hydrogen is that is has a 30% round trip efficiency, which is very low.

Chilled air – You can use energy to chill the nitrogen in air to liquid nitrogen, which is then stored. When energy is needed you let the liquid nitrogen heat up, it then turns into a gas and the expanding gas can be used to turn turbines and generate electricity. This is a novel idea that uses air as its main component, but has an efficiency of about 50% which is not great.

Concrete Gravity Train – This brings us to the new option – a California company, Advanced Rail Energy Storage System (ARES) built a test system in which they store energy by rolling a train up a track. The train is mostly a huge block of concrete. When energy is needed you let the train roll back down the track, and use regenerative braking technology to generate electricity.

They claim 80% efficiency, which is the same as pumped hydro and is great for grid storage. The current system uses a 9.2 kilometer track with a grade of 7.2%, which they found to be optimal. The full system can provide a maximum of 50 MW of power, but only for a short period of time (on the order of 15 minutes) or less power for longer.

One limitation of this system is that you need a location that allows for a long track at a relatively steady grade, so it is highly location specific (like pumped hydro). This will be a viable option in some locations, such as Nevada where the current system is being developed. It is still not an option for locations with a flat terrain.

I did have a similar idea for energy storage and I wonder if it is practical from an engineering and efficiency point of view. Imagine a vertical tower with cables or poles up which heavy plates of iron or lead are lifted using excess energy. When energy is needed the heavy plates are allowed to fall, generating electricity as they do. Their descent is slowed either mechanically by turning a turbine or electrically by regenerative braking. Such a system could be built anywhere, out of common materials, and could have a high capacity. The only unknown to me is the potential efficiency.

Of course, this is a simple idea and many other people must have thought of it also. I did a search to find out what is out there that uses a similar idea. I found a gravity power energy system that uses a similar idea – hydraulics are used to raise a very heavy piston up a shaft. When power is needed the piston falls, which forces water through a turbine and generates electricity similar to pumped hydro. Rather than using a tower for the vertical height they use a deep open pit mine. This system is similar in that is uses a vertical system to raise a heavy metal weight against gravity to store potential energy.

They claim the same efficiency as pumped hydro, around 80%. If so this may become my new favorite option – it is efficient, has high capacity, can be build anywhere, and has a low environmental profile. They currently have a demo plant under construction in Bavaria. We’ll see how it goes.


Note: The article was corrected to the proper total energy storage of the system.

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