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Mechanical batteries could save Eskom

11 December 2019 7:15 PM
Did you know we can store energy in dams, blocks, trains and flywheels?

Almost all electricity is generated by spinning a coil of wire in a magnetic field called an induction generator. Typically we use steam to get those coils spinning via a turbine to get the most energy from the high pressure, superheated steam. Most power stations are effectively hot water boilers to create steam and use a variety of fuels to generate the heat.

Coal plants heat the water by burning the crushed coal in a boiler. Nuclear plants use superheated pressurised water to superheat lower pressure water to create steam. Gas plants use the superheated and pressurised air to spin the turbines to drive the generators.

Wind farms use the wind to turn the blades that connect to a gearbox to drive the generator.

Solar is different in that it uses the photovoltaic effect to release electrons that can do electrical work.

Hydro dams use high-pressure water under gravity to drive turbines connected to generators. They are effectively mechanical batteries in that we can use electrical energy to pump water into the dams when demand is low and then release it when demand peaks to provide the additional electricity when needed.

Battery storage is needed to reduce the need for baseload generation (which is the minimum needed to be generated consistently) and to use the off-peak times to store the energy that can be used during the peak times to effectively create a relatively flat demand to use the least amount of resources to generate the required amount of electricity.

In previous articles, I have mentioned using vanadium flow redox batteries (which Eskom is now busy testing) and lithium-ion batteries like the ones you have in your phone.

This article will look at the potential of four additional types of mechanical battery to the pumped water storage option.

Flywheels

The use of flywheels to help sustain engine power has been around for a long time. They converted steam engines unbalanced power cycles into smooth rotations and are still used to smooth the actions of pistons in combustions engines.

The battery form of a flywheel relies on the significant amount of energy needed to get a heavy wheel spinning, and that it is able to release quite a bit of energy when slowing down.

Assuming you use an electric motor to get it spinning you could use it as a generator when you need to convert the mechanical power back into electric power. Ever heard of a bus with a flywheel battery? They were used in the ’50s and would use the spinning flywheel to drive the bus’s electric motors along tram routes. At regular intervals, there would be overhead charging posts to add more energy to the flywheel. The bus was silent and had no emissions. If the electricity used to generate it was also renewable you would have a good public transport option with a very cheap form of battery that was less likely to burn like lithium-ion batteries.

Keeping those flywheels spinning with little wear proved to be the issue that saw combustion engines replace them despite the problems with pollution and climate damage.

But they work best as static storage options that capture excess electricity generated to be used when the power is needed suddenly. They may be expensive at first but with scaled production, the costs would come down and the quality would improve. Many could be built along the electricity grid both to make the grid more robust and to create electricity management options closer to towns and cities. When combined with local generation options too, it would make for a more stable and resilient grid allowing sudden spikes in demand to be supplied safely. While their capacity, for now, might be low, improvements might see bigger flywheels be used for larger capacity storage that could store renewable energy and supply it back over longer periods of time.

Gravity batteries

Dams used for hydroelectric power are a form of gravity battery with the weight of the water supplying the power to drive the turbines.

The challenge with pumped water options is that you need a lot of water. An alternative uses a very heavy train on an inclined track built in an area with natural hills. The weight of the train and the incline determine how much energy it takes to push the train up the hill which also determines how much energy you can get out of it when you let it come back down again.

Like other batteries, you would use excess energy when you wanted to charge them and draw it out again when you needed it. The few moving parts and relatively cheap and environmentally friendly building materials could see many constructed in parallel with some or all being released with energy is needed.

The experimental version is being tested in the US. Might we see a version of weights hung on the side of Table Mountain, that is raised to the top overnight and lowered during the morning or afternoon peak?

Then there is an option that creates a mini-mountain that can be built anywhere with large heavy block and cranes. When energy is available able the heavy blocks are raised up to 35 storeys high. When needed the blocks are lowered to reportedly recover 90% of the energy that was stored.

While they could be built anywhere, their height might not make everyone happy. The full-size version should be able to provide 4 MW of power and deliver it over a long period depending on how many blocks have been raised.

Might these six crane towers become a common sight to store electricity one day?

Building the blocks out of cement should see them last for the 40-year expected life span, but concrete does have an environmental impact so perhaps alternatives could be used.

If the concept proves functional, it could be incorporated into high rise buildings adding to their smaller energy footprint if the blocks were stored in a column in the centre of the building while packing them out across the full width of the roof.

Most of these used gravity to store power this next one uses the opposite, buoyancy. Take a tall column or cable in the sea. Attach large buoys to the surface and using energy when the demand is low, winch them down to the seafloor. This can be done slowly to use energy efficiently. Once fully submerged they can remain there ready to rise to the surface when needed generating electricity as it does. While the sea is a very tough environment for mechanical devices, the electrical parts can all be kept safely above the water with only the column or central cables and buoys needed to remain in the sea. Depending on the depth of the water and the number of buoys used you can create a relatively powerful and local power option to once again make the overall grid a little more stable and less prone to failure with generating nodes spread across the system. Many parts of False Bay are over 50 m deep and less than 20km from the shore. A grid of submerged buoys might be able to offer a way to capture the output of ocean-based wind farms built in the same area.

These are not sure-fire bets, but the world needs more energy as developing nations to look to provide electricity to their citizens. Burning fossil fuels is no longer an option, so many more renewable options will be needed.

Storing that output is still a challenge but hopefully one of these unusual options may prove to be a useful addition for energy storage.

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11 December 2019 7:15 PM

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