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Business Unusual

Hot rocks as a way to make renewables sustainable

22 September 2021 7:15 PM
Digital technology

The options for grid storage keep improving, but there is not a clear winner yet.

Once upon a time the gadgets that had the most variation and innovation was the mobile phone. It unlocked the latest leap in the information age including big data and unfortunately the start of the surveillance age.

But the rate of innovation has slowed. Improvements are incremental rather than revolutionary.

Battery technology once looked pretty staid thanks to our dependence on fossil fuels but as we shift to renewables and need to do so quickly, we need to find a way to store the excess when the demand is low and release it when the peak hits.

The Duck Curve

The catch with most renewables like wind and solar is that they tend to provide the peak output during the day while most consumption happens in the early morning and evenings.

The two curves when plotted create an outline that can be described as a duck. What we need is to shift the supply and demand lines so they match.

In a previous edition of Business Unusual we have looked at Lithium ion batteries which are good for lots of power that can be delivered instantly, but they are quite expensive when scaled up and currently still have lifespan issues.

The show has also looked at flow batteries which can be very big and last a long time, but can also be pricey and have some chemical parts that can be dangerous.

Then we looked at a range of mechanical batteries like Flywheels, Gravity storage and even floats. Each had benefits for where they may be used and typically could scale and be affordable but are still untested for reliability.

Hot rocks

Now there is another system that relies on the work done to use compression and heat as a store. The company is Stiesdal and it takes it name from its founder Hendrik Stiesdal who helped create the modern wind turbine.

Compressing air heats it. Storing heat (or cold) can be used to generate power when pushed through a turbine or heat exchanger. By finding the optimum pressures, temperatures and temperature difference creates the best efficiency and the makers claim the system is 60% efficient which is adequate for a system that is both able to scale to store a good amount of energy and be able to discharge it over a long period of time.

It is a two stage process that uses two temperature and pressure control units that are filled with air and stones. Using electricity one cylinder is heated to over 600C while the other tank is chilled to -30C.

When the energy is needed the super heated and pressurised air is put through as heat exchanger and turbine to produce electricity. The air then moves to the very cold tank at about 75C while the temperature main tank drops to just under 400C before the process begins again.

Why stones

Air can conduct heat and flow but can’t store as much heat as liquids and solids. Liquids can flow but can’t be compressed and solids can’t be compressed or flow.

A great profile and explanation of how the new battery storage system works from the YouTuber Dave Borlace's channel Just have a think.

Combining the flow and compression of air with the heat storage of a solid gives the best of both. By using stone, you can use a material that is typically available wherever the battery needs to be built, it is relatively cheap, it is mostly inert and a good heat sink. Some types of stone are better than others and in time the best type and size will hopefully be determined.

Power storage and capacity

Batteries have a few factors that determine how suitable they would be for grid scale storage.

How much power it can supply is the first issue, the higher the power rating the more power it can supply.

For capacity it determines for how long the battery can supply the power. Ideally it should be able to work long enough to flatten the demand peaks or while a plant repair can be done.

The next aspect is how quickly the battery can respond to the need for power. Faster is critical to keep the grid stable. These heat storage types are not as quick as the more common Lithium Ion batteries.

Lithium Ion batteries are the most common at the moment and offer the speed to respond and the power output, but they typically are not that good for long periods or rather they would become very expensive if they did need to last longer than a few hours.

The heat cycle batteries offer the potential for good power capacity although not very responsive, but they do offer much longer run times. Many hours to potentially many days.

The ideal is to have enough Lithium Ion to handle faults and sudden surges but only while the slower battery times can be engaged to take over and then handle the demand until it subsides or a base load supply takes over.

The options to use a combination of battery types from pumped water (dams), chemical (Lithium ion, Redox flow) mechanical (like this one) should make renewables which are getting better and cheaper and can be built in many more varied locations than traditional power stations.

This means the energy generation can take place closer to where it is needed and potentially reduce the cost and the losses to distribute energy over long distances.

This also helps transition those that have been invested in fossil fuel generation to switch their skills to work on renewable systems and potentially to get more working in the industry.

It is a little too optimistic to believe that this can take place soon or that it will cost less than fossil fuel options like coal for now.

South Africa may be one of the largest generators and users of electricity on the continent but there is still a lot of work to get electricity to all those that need access and to do so at a price they can afford.

I had hoped that South Africa would see the opportunity not just to address our own energy needs but to assist our neighbours improve their capacity too. The disastrous last decade at Eskom and the many decades prior that first did not cater for all South Africans then failed to maintain those plants makes it hard for South Africa to become a leader without some very hard work.

That is not to say South Africa can’t become more competitive, but it will not be easy. Perhaps the good work by South African universities and the innovation and partnerships with entrepreneurs may see us create a more robust network while still working to reduce our reliance on coal.

It may not exclude the use of energy like nuclear and may even benefit from short term fixes from gas plants, but pushing for nuclear now and signing up for 20 year gas contracts will turn out to be another decade of mistakes.

It would be better to make the most of the rapid improvements in renewables and to help find the best options for storage to get the costs down and the output up.

This article first appeared on 702 : Hot rocks as a way to make renewables sustainable

22 September 2021 7:15 PM
Digital technology

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