With the price of energy from new wind or solar rapidly dropping below that of traditional fossil fuels, renewable energy feels like a no-brainer for Australia. Yet despite massive strides in efficiency and affordability, we still can’t generate solar energy while the sun isn’t shining, or run wind turbines while the wind isn’t blowing. What we can do, however, is store that energy while conditions are good, and save it for a rainy day.
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Here’s a simple fact: electricity itself cannot be stored. Even the battery in your phone is not storing electricity — it’s storing chemical energy and then converting that energy back into electricity when it’s needed by the phone.
It’s of the fundamental rules of physics, a concept you were probably introduced to in high school. It’s also fundamental to a type of technology that Malcolm Turnbull and Elon Musk agreed was the key to the electricity networks of the future, something that makes up an important part of Australian Chief Scientist Dr Alan Finkel’s blueprint for the Australian grid: energy storage.
When it comes to energy storage, most people will immediately go to batteries — from home-scale batteries like the 14kWh Powerwall to larger modules such as the mega 1.6MWh battery used in Tasmania’s King Island Renewable Energy project.
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While chemical batteries are rapidly dropping in price, they’re still far from the most efficient or cost-effective means of storing energy. Rather some of the most efficient methods of energy storage take advantage of the simplest laws of physics, switching between different forms of energy including electrical, heat, light, chemical, mechanical and potential.
Pumped hydro has been having a moment in the spotlight since the recent announcement of an ambitious new plan to revitalise the Snowy Hydro Scheme with more pumped hydro storage. At first glance using electricity to power giant pumps seems counter-productive to the Snowy Scheme’s renewable aspirations, though it makes a lot more sense when you see pumped hydro for what it really is – a giant battery.
Pumped hydro operates on an incredibly simple premise – energy can be stored for as long as needed in the form of gravitational potential energy.
First, excess electricity is used to pump water from a low reservoir into a higher reservoir. Then, in times of high demand for electricity, the water is released back down the slope and through a hydroelectric turbine. Energy, or rather water can be stored in the high reservoirs for as long as needed, without the cycle limitations of chemical batteries. The only potential energy loss happens through evaporation, though this too can be minimised by enclosing the reservoirs.
Pumped hydro is far and away the most prevalent form of energy storage worldwide, making up a huge 99 per cent of it. It makes sense. Pumped hydro is one of the most efficient methods of storing electricity, reaching up to 80 per cent round-trip efficiency. For comparison, Tesla’s 210kWh Powerpack boasts 88 per cent round trip efficiency on its two hour system batteries and 89 per cent on the four hour systems, but at a much higher relative price.
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Australia’s largest pumped hydro system is also its oldest: the 600MW Tumut 3 Power Station, built in the Snowy Mountains in 1973. Since that heyday of Australian hydro, very little large-scale pumped hydro has been built here in over 30 years. The proposed Snowy Hydro expansion hopes to change that, however, promising increases in capacity of up to 50 per cent.
At the moment, pumped hydro is mostly used to timeshift cheap electricity – running the pumps in off-peak hours when electricity is cheaper, then pumping that power back into the grid at peak times when it’s worth more. But the real potential in pumped hydro is as a way to shore up the reliability of renewable energy sources like solar and wind.
When paired with renewables, excess energy generated by the sun and wind can be used to run the pumps, storing clean energy for later. The Australian Renewable Energy Agency has recently been investigating the potential for small, off-river pumped hydro installations to be coupled directly to nearby solar or wind farms for this very purpose.
Of course, pumped hydro is not a perfect solution. Compared to alternatives like battery farms, the infrastructure takes a long time to deploy and without careful placement can risk disrupting sensitive environments. As with any kind of hydroelectric installation, drought can severely effect operation.
In the case of the Snowy Hydro, an upgrade would also have to include higher powered or augmented transmission lines to get the electricity where it’s needed, when it’s needed. This is especially important in NSW, if it’s going to have the capacity to help South Australia in the event of a future energy crisis. But pumped hydro is just one tool in a storage arsenal that could pave the way for a renewable-led grid in Australia.
After pumped hydro, the second most popular form of energy storage worldwide is compressed air energy storage, or CAES. CAES operates on a similar principle to pumped hydro, only using air instead of water. Instead of powering a pump, excess electricity is used to compress air, which is then stored under pressure in an underground reservoir. When electricity is needed again, the air is heated and driven through an expansion turbine.
The history of CAES goes much further back than you would think. Similar technology was used to deliver power to households as far back as the 1870s, around the same time as the invention of the light bulb. Compared to more modern techniques its efficiency can be a problem however, ranging from only 40 percent to over 70 per cent, depending on how it is deployed.
Because of this, the technology has rarely been used on a large scale. Only two large CAES plants are in operation at the time being – the 290MW Huntorf plant in Germany and the 226MW McIntosh plant in the USA.
Both of these plants run on a lower-efficiency ‘diabatic’ system, in which the heat created from the air compression process is simply discarded. When the electricity is needed again, the plants use natural gas stores to heat up the air again.
A more experimental technique is being explored in ‘adiabatic’ systems, where the heat generated from compressing the air is captured and used again later to reverse the process. These systems, which have the potential to be far more efficient than their diabatic cousins, could be a game-changer for large-scale CAES.
A number of adiabatic plants are currently in the works, as the tech has seen renewed interest in recent years. These include projects like the 200MW ADELE that’s currently under construction in Germany, a planned 317MW plant in Anderson County, Texas and a conceptual 100 per cent renewable, 800MW CAES plant proposed by the UK’s Storelectric. Unfortunately, despite the renewed interest, many CAES projects have been halted by the lack of investment in what some might think of as outdated technology.
Molten Salt Solar
While both pumped hydro and compressed air operate on an electricity in/electricity out basis, molten salt solar storage captures and keeps renewable energy in its original form of heat. Without the energy lost in multiple state transfers, molten salt storage is one of the cheapest and most efficient forms of large-scale energy storage, even beating out pumped hydro by some estimates. Molten salt storage can be deployed at as little as 10 per cent of the cost of large-scale batteries.
Solar thermal is different to the kind of solar power you get from rooftop panels. The latter, photovoltaic solar, relies on the light from the sun to create an electrical current. Solar thermal, on the other hand, uses the heat from the sun to boil water and run a conventional steam turbine.
While solar thermal was one of the earliest forms of large-scale solar generation, it has fallen out of favour due to rapid advances in photovoltaic technology. But when coupled with molten salt, solar thermal technology becomes far more valuable for its potential to provide reliable ‘baseload’ power, something that has thus far been the sole realm (and rallying cry) of fossil fuels.
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The world’s largest currently operating solar thermal plant, the Ivanpah installation in the USA, doesn’t have molten salt storage integrated — but the power plant that will soon take its crown does.
The Ouarzazate Solar Power Station is a behemoth on the fringes of the Sahara Desert in Morocco, which will eventually be able to produce 580 MW in peak times. The completed first phase of the project, Noor 1, has a molten salt storage capacity of three hours, but when the plant is completed it will run for 20 hours a day.
Australia has its own molten salt-augmented power station on the cards, with the government having pledged $110 million to a solar thermal project in Port Augusta in this year’s Budget. At this point the frontrunner is a 110MW proposal by US company SolarReserve, though we may also see a proposal from Vast Solar, the company behind a miniaturised 1.1MW pilot project in Jemalong, NSW.
Though small, the Jemalong project is quietly proving the value of solar thermal. Its integrated storage costs only $25/kWh, which you can compare to Elon Musk’s Twitter quote of $US250/kWh for Tesla’s Powerpack-based battery farms, itself a rare bargain for large-scale batteries. The Jemalong farm is also able to run 24 hours a day, according to a claim by Vast Solar’s chief technology officer, James Fisher.
Out of every method covered by this article, hydrogen energy storage is the least like a battery and the most like a resource. Beyond our own electricity needs, it’s a way Australia’s theoretical excess renewables could be bottled up and shipped across the world as we already do with coal and LNG.
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Hydrogen is created from water in a process called electrolysis, which uses electricity to split H2O’s hydrogen from its oxygen. From there, hydrogen can be used as a fuel for vehicles that’s almost on par with petrol or diesel. It can also be run through a fuel cell with oxygen, essentially reversing the process of electrolysis to produce electricity, heat and water.
Although hydrogen energy storage has seen renewed interest in recent years, it’s still far from a perfect solution. Hydrogen suffers from relatively low round trip efficiency at around 30 to 40 per cent, but it does have one main advantage over other energy storage technologies: its ability to be sold and exported across the world. If renewable energy is used for the electrolysis that creates the hydrogen, Australia could harness its renewable energy as an export, a role currently filled by coal and LNG.
It even looks like a market will be there one day soon, with hydrogen trials currently taking place in Australia and around the world. In the ACT the Actew AGL gas distribution network, in partnership with the Crookwell Wind Farm and the ANU, are investigating efficient ways to produce hydrogen from renewable energy. From there they’re also investigating how hydrogen can be introduced to the existing ACT gas network, or even used to provide support to the beleaguered electricity network.
One roadblock still exists for hydrogen as an export, however. The gas must be compressed into a liquid for transportation overseas, and the process required to do this is currently too energy intensive to be viable. It’s a problem that will one day be solved with further research and development – promising advances already happening with a CSIRO pilot program aiming to investigate the feasibility of converting hydrogen into ammonia for ease of transportation.
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Getting Stored Energy Into The Grid
It may seem like we’ve got the solutions right in front of us, but all the energy storage in the world won’t help if the framework isn’t there to integrate that energy with the grid.
Many of these issues were addressed in the recent Finkel Review Blueprint, a report looking into the problems facing Australia’s National Energy Market (NEM) – the framework that serves Australia’s five eastern states.
Under Finkel’s projections battery storage is key to the future of Australian energy, especially when planning a transition to a low-emission grid. Unfortunately for any of our lofty storage ambitions, our grid was designed in the twentieth century – when storage was still a pipe-dream and the grid was far more centralised. There are still many outdated systems that must be overhauled to make storage work within the NEM.
For example, the NEM currently operates under a 30 minute settlement regime, which puts storage at a distinct disadvantage to traditional generation.
When electricity is needed, generators bid offers at five-minute intervals, with a cap of $14,000/MWh and a minimum price of -$1000/MWh. Through the National Electricity Market Dispatch Engine, the Australian Energy Market Operator (AEMO) chooses which generators will be dispatched to produce electricity through a co-optimisation process.
Generators are typically dispatched from cheapest to most expensive, based on system normal conditions (as opposed to “System Black” events like the one South Australia experienced in September last year).
The highest accepted bid will then set the price all generators in a NEM region are paid for their electricity. But while this ‘dispatch price’ is determined every five minutes, the actual ‘spot price’ that will be paid to generators is based on a half hour average of all dispatch prices.
So what does this actually mean for energy storage systems? The half hour average gives an advantage to generation tech that takes a while to warm up — gas plants, for example — while stored energy that can be deployed at a moment’s notice does not get the full benefit of its near-instantaneous nature.
Fortunately, the Australian Energy Market Commission (AEMC) is currently considering a rule change proposal that would revise the settlement time period from 30 minutes down to five, with an anticipated Draft Determination due on July 4, 2017. Even if the rule change is successful however, the AEMC has indicated a minimum three-year transition period for the change to come into effect.
Another of the Finkel Review’s recommendations is a “regional reliability assessment”, which would “inform requirements on new generators to ensure adequate dispatchable capacity is present in each region.” ‘Dispatchable’ refers to electricity that can be sent out at any time, like stored energy or gas-fired plants. Under this kind of system, renewable generators may actually be required to build a certain amount of storage alongside their power plants.
While Finkel specifically calls out batteries and pumped hydro for managing reliability, the report acknowledges that our best course of action is to use “a mix of storage solutions” as they are needed. “With current technology, no single storage medium has the characteristics to meet all the requirements for energy that the grid demands,” the report concludes on energy storage.
As we look towards transforming Australia’s struggling electricity system and moving towards new, cleaner sources of generation, Australia may be getting a new host of storage not just in batteries, but in pumped hydro, compressed air, hydrogen and molten salts — and maybe even new storage technologies that have still yet to be invented.